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ELEMENTS
PHYSICS;

/-
NATURAL   PHILOSOPHY,
GENERAL  AND MEDICAL
WRITTEN FOR
UNIVERSAL  USE,
    PLAIN OR NON-TECHNICAL LANGUAGE;

                AND CONTAINING

NEW DISQUISITIONS AND PRACTICAL SUGGESTIONS.
         COMPRISED IN FIVE PARTS,

1. SOMATOLOGY, STATICS AND \ 3. PNEUMATICS, HYDRAULICS
  DYNAMICS.               AND ACOUSTICS.
2. MECHANICS.           ' 4. HEAT AND LIGHT.
        5. ANIMAL AND MEDICAL PHYSICS.
COMPLETE IN ONE VOLUME.
BY NEIL  ARNOTT, M.D.,
       OF THE RO'YAE'TOLLEGE OF PHYSICIANS.
A NEW EDITION. REVISED AND CORRECTED FROM THE LAST ENGLISH EDITION.

             WITH  ADDITIONS,______________

          BY ISAAC HAYS, Il{. D. LI B R A R V
      PHILADELPHIA:
LEA  AND  BLANCH
            1845.
SURGEON GENERAL'S OF"

    SEP,-30--t>3
A^WDr^ 
•*\ • * **  '   *  .  • ♦      '* • .V
qc
    Entered according to the Act of Congress, in the year 1841, by
               LEA  AND  BLANCHARD,
in the Clerk's Office of the District Court of the United States, in and for
               the Eastern District of Pennsylvania.
T. K. & 1* (i Collins, I'uinteks 
ADVERTISEMENT
                     OF THE

AMERICAN   PUBLISHERS.
  The very valuable and popular work  of Dr. Arnott has passed
through several editions in this country, in the form in which it was
originally published by the author, in separate parts.  A new edition
being now called  for, the work has been  carefully revised  and cor-
rected, and the whole condensed  into one volume.  In this form it
cannot fail to be more acceptable  to the public, and  rendered  more
convenient and useful for the purposes of instruction in the various
Colleges and Seminaries of Learning that have adopted it as a  Class
Book for their pupils.  This volume embraces all  that has been pre-
pared or published by the author. 
 

INTRODUCTION.
  To appreciate the importance of Physics or Natural Philosophy, as an
object of study not only to all persons engaged in scientific  pursuit, but, in
the present day, to all who pretend to a moderately good education, we must
take  a rapid glance at the nature of human knowledge generally, and at its
bearings on the existing condition of mankind.
   While the inferior races of animals on earth seem to  have changed as
little in any respect since the beginning of human records, as  the trees and
herbs of the thickets which  give many of them shelter, the condition of man
himself has fluctuated, but, on the whole, progressed  in a very remarkable
manner. The inferior animals were formed by their Creator such, that within
one life or generation they should attain all the perfection of which their na-
ture was susceptible.  Their wants were either  immediately provided for—
as instanced in the clothing of feathers to birds, and of furs to quadrupeds;
or were so few and simple, that the supply was easy to very limited powers
—except in a few cases where considerable art was required, as by the bee
in making its honey-cell, or by the bird in constructing its beautiful nest, and
there, a peculiar aptitude or instinct was bestowed.  Thus, a crocodile which
issues  from its egg in the warm sand, and never sees its parent, becomes as
perfect and knowing as any crocodile that has lived before or that will appear
after it.—But how different is the story when we turn  to man!  He comes
into the world the most helpless of living beings, long to continue so; and'if
deserted by parents at an early age, so  that he can learn only what the expe-
rience  of one life may teach him,—as to a few individuals has happened who
yet have attained maturity in woods  and deserts,—he grows up in  some
respects inferior to the nobler brutes.   Now as  regards many regions of the
earth, history exhibits the early human inhabitants in states of ignorance and
barbarism, not far removed  from  this lowest possible grade, which civilized
men may shudder  to contemplate.  But these countries, occupied  formerly
by straggling hordes of miserable savages,  who could scarcely  defend them-
selves  against the wild beasts that shared the  woods with them, and the
inclemencies of the weather, and the consequences of want and fatigue, and
who to each other were often more dangerous than any wild beasts, unceas-
ingly warring among themselves, and destroying each other with every spe-
cies of savage, and even cannibal cruelty—countries so occupied  formerly,
are now become  the abodes of peaceful, civilized  and friendly men, where
the desert and the impenetrable forest  are changed into cultivated fields, rich
gardens and magnificent cities.
   It is the strong intellect of man, operating with the faculty of language as
a means, which has gradually worked this wondejful change.  By language,
fathers communicate  their gathered  experience and reflections  to their chil-
dren, and these to succeeding children, with new accumulation: and when,
after many generations, the precious store had grown until simple memory
could retain no more, the arts of writing, and then of printing, arose, making
     2 
VI
introduction.
language visible and permanent, and enlarging illimitably the repositories of
knowledge.   Language thus, at the present moment of the world's existence,
may be said to bind the whole human race of uncounted  millions into one
gigantic rational being, whose memory reaches to the beginnings of written
records, and retains imperishably the important  events that have occurred;
whose judgment, analyzing the treasures of memory, has discovered many of
the sublime and unchanging laws of nature, and has built on them all the arts
of life, and through them, piercing far into futurity, sees clearly many of the
events that are to come; and whose eyes and ears, and observant mind at this
moment, in every corner of the earth, are watching and recording new pheno-
mena, for the purpose of  still better  comprehending the  magnificence and
beautiful order of creation, and of more worthily adoring its beneficent Author.
   It might be very interesting to show here, in  minute detail, how the arts
and civilization have progressed in  accordance with the gradual increase -of
man's knowledge of the universe;  but to do so would lead too far from the
main subject.  We deem  it right, however, to make evident to the student
the arousing truths, that the progress is not yet  at an end ;  that it has been
vastly more  rapid in recent times than ever; and that it seems still to pro-
ceed with increasing celerity:—and we know not where the Creator has fixed
the limits of the change!  Although  there are thousands of  years on the
records of the world, our Bacon,  who first taught the true way to investigate
nature, lived but  the other day.  Newton followed him, and illustrated his
precepts  by the most sublime discoveries which one man has ever made.
Harvey detected the circulation of the blood  only two hundred years ago. ■
Adam Smith, Dr. Black and  James Watt were friends, and  the last, whose
steam-engines are  now changing rapidly the  condition of  empires, may be
said to be scarcely cold in his grave.  John Hunter died not  long  ago;
Herschel's accounts of newly-discovered planets, and  of the sublime struc-
ture of the heavens, and Davy's account of chemical discoveries not less im-
portant to man, are in the late  numbers of our scientific journals ;—illustrious
Britons these, and  who have  left worthy successors treading in their steps.
On the continent of Europe, during the same period, a corresponding constel-
lation of genius has shone; and  Laplace was lately the  bright star shining
between the future  and the past.
   But there is a change going on in  the world, connected  closely with the
progress of science, yet distinct from it, and more important than a great part
of the scientific discoveries;—it is the diffusion of existing knowledge among
the mass of mankind.   Formerly, knowledge was  shut up in convents and
universities, and in books written in the dead languages—or in books which,
if in the living languages, were so abstruse and artificial, that only a few per-
sons had access to their meaning; and thus, the human  race being considered
as one great intellectual creature, a small fraction only of its intellect was
allowed to come into contact with science,  and therefore into activity.   The
progress of science in those times was correspondingly slow,  and the evils of
general ignorance prevailed.   Now, however, the strong barriers which con-
fined the stores of wisdom have been thrown down, and a flood is overspread-
ing the earth; old establishments are  adapting themselves to the spirit of the
age; new establishments  are arising; the  inferior schools are introducing
improved systems of instruction; and good books are rendering every man's
fireside a school.   From all these causes there is growing up an enlightened
public opinion, which quickens  and  directs the progress  of every art and
science,  and  through  the  medium  of a  free  press, although  overlooked by  '*
many, is now rapidly becoming  the governing influence in all the affairs of 
INTRODUCTION.
Vll
   man.   In Great Britain, partly perhaps as a consequence of its insular situa-
   tion, which lessened among its inhabitants the dread of hostile invasion, and
   sooner formed them into a united and compact people, the progress of enlight-
   ened public  opinion had been more decided than  in any other state.   The
   early consequences were more free political  institutions ; and these gradually
   led  to greater and  greater  improvements, until Britain became an object of
   admiration among  the nations.  A colony of her children, imbued with her
   spirit, now occupies a magnificent territory in  the new world of Columbus; and
   although it has been independent  as yet for only  half a century, it already
   counts more people than  Spain, and will soon be second to  no nation on
   earth.  The example  of the Anglo-Americans  has aided in rendering their
   western hemisphere the cradle of many other gigantic states, all free, and
   following, although at a distance, the like steps.   In the still more recently
   discovered continent of Australasia, which is nearly as large as Europe, and
   is empty of men, colonization is spreading with a  rapidity never before  wit-
   nessed; and that beautiful and rich portion of the earth will soon be covered
   with the descendants  of free-born and enlightened Englishmen.  Thence,
   still onward, they  or  their institutions will naturally spread over the  vast
   archipelago of the  Pacific  Ocean,  a track studded with islands of paradise.
   Such, then, is  the  extraordinary moment of revolution, or  transit, in which
   the world at  present exists!  And  where, we may ask again, has the  Creator
   predestined that the progress shall cease?   Thus far at  least we know,  that
   he has made our hearts rejoice to see the world filling with happy human
   beings, and to  observe that the increase of  the sciences can make the same
   spot maintain thousands in  comfort and godlike elevation of mind, where with
   ignorance even hundreds had found but a scanty and degrading supply.
     The progress of  knowledge  which has thus led  from  former barbarism to
   present civilization, has gone on by certain remarkable steps, which it is easy
   to point out; and which it is  very useful to consider, because we  thereby
   discover the  nature  of  human knowledge, with  the relations and importance
   of its different branches; and we obtain great facilities  for studying science,
   and  for quickening  its farther progress.
     The human mind, when  originally directed to the almost infinity of objects
   in the universe around it, must soon have discovered that there were resem-
'   blances among them ; in other words, that the  infinity was only a repetition
   of a  certain number of kinds.   Among animals, for instance, it would distin-
   guish the sheep, the dog, the  horse; among vegetables, the oak, the beech,
   the pine; among minerals, lime, flint, the metals, and so forth.   And becom-
   ing aware that by studying an exemplar of  each kind, its limited power of
   memory might acquire a  tolerably correct  knowledge of the  whole,  while
   this  knowledge would  enable the possessors  more  easily to obtain what was
   useful to them, and to avoid what was  hurtful, the desire for such knowledge
   must have arisen with  the first exercise of reason.   Accordingly, the pursuit
   of it has been unremitting, and the labour of ages has at last nearly completed
   an arrangement of the constituent materials of the universe, under three great
   classes of Minerals, Vegetables, and Animals ; commonly called the three
   kingdoms of Nature, and of which the minute description is termed Natural
   History: and museums of natural history have been formed which  contain
   a specimen of almost every object included  in  these classes, so that now, a
   student, within the  limits of an ordinary  garden, may be said to be  able to
   examine the whole  of the material universe.
     While men were  examining the forms and  other qualities of the bodies
   around them, they could not avoid noticing also the  motions or changes going 
Vlll
INTRODUCTION.
on among bodies;  and here, too, they would soon make the grand discovery
that there were resemblances in the multitude.   Self-interest, as in the  case
of the bodies  themselves, having prompted  to careful classification, in the
present day, as the result of countless observations and experiments made
through  the series of ages, we are enabled to say, that  all the motions, or
changes, or phenomena (words synonymous here) of the universe, are merely
a repetition and mixture of a few simple manners or kinds of motion or
change, which are as constant and regular in every case  as where they pro-
duce the returns of day and night, and of the seasons.   All  these phenomena
are referable to four distinct classes, which we call Physical, Chemical, Vital
and Mental.   The simple expressions which describe them are denominated
General Truths or Laws of Nature, and as a body of knowledge, they  con-
stitute what is  called Science or  Philosophy, in  contradistinction to Natu-
ral History,  already described.  Now as man cannot,  independently  of a
supernatural revelation, learn any thing but what respects,  1st, the moment-
ary state, past  or present, of himself and the objects around  him ; and 2d, the
manner in which the states have  changed : Natural History and  Science, in
the sense now  explained, make up the whole sum of his knowledge of nature.
  To exemplify the process by which a general truth  or law of nature is dis-
covered, we shall take the physical  law of gravity  or attraction.   1st. It
was observed that bodies, in general, if raised from  the earth, and left unsup-
ported, fell towards it; while flame, smoke, vapours, &c, if left free, ascended
away from the earth.   It wns held, therefore, to be a very general law, that
things had weight; but that there were exceptions  in such  matters, as  now
mentioned, which were in their nature light or ascending.   2d. It was dis-
covered that our globe of earth is surrounded by an ocean of  air, having nearly
fifty miles of altitude or depth, and of which a cubic foot, taken near the sur-
face of the earth, weighs about an ounce.  It was then perceived that flame,
smoke, vapour, &c, rise in the air only as oil rises in water, viz., because not
so heavy as  the fluid by which they are surrounded; it followed, therefore,
that nothing was known on earth naturally light, in the ancient sense of the
word.   3d.  It was found that bodies floating in water, near to each other,
approached  and feebly cohered;  that any contiguous hanging  bodies were
drawn towards each other, so as not to hang quite perpendicularly; and that
a plummet suspended  near a hill  was drawn towards the hill with force only
so much less than that with which it  was drawn towards the earth, viz., the
weight of the  plummet, as the hill was smaller than the earth.   It was then
proved that weight itself is only an instance of a more general mutual attrac-
tion, operating between all the constituent elements of this globe ; and which
explains, moreover, the fact of the rotundity of the globe, all the parts being
drawn towards a common centre, as also  the form of  dew-drops, rain-drops,
globules of mercury, and of many other things;  which, still farther, is the rea-
son why the distinct particles of which any solid mass, as a stone or piece of
metal, is composed, cling together as a mass, but which,  when overcome by
the repulsion of heat, allows the same particles to assume  the form of a liquid
or air.  4th. It was farther observed, that all the heavenly  bodies are round,
and must, therefore, consist  of material obeying  the  same law.   5th.  And
lastly, that these bodies, however distant, attract each other; for that the tides
of our ocean rise in obedience to the attraction of the moon,  and become high
or spring-tides, when the moon and sun operate in the same direction.  Thus
the sublime truth was  at last made evident, and by the genius of the immor-
tal Newton, that there is a power of attraction connecting together the  bodies 
INTRODUCTION.
IX
of this solar system at least, and probably limited only by the bounds of the
universe.
  Acquaintance with the laws of nature has been very slowly obtained, owing
to that complexity of ordinary phenomena, which  is produced  by several
laws operating together, and under great variety of circumstance.   With re-
spect to many laws of Chemistry and Life, men seem to be yet little farther
advanced than they were with respect to the physical law of attraction, when
they knew only that heavy things  fell to the earth.   But we have  learned
enough to perceive that the great universe is as simple and harmonious  as  it
is immense;  and that the Creator, instead of interposing separately, or mira-
culously, in the common sense of the word, to produce every  distinct pheno-
menon, has willed  that all should proceed according  to a few general laws.
There is  nothing in nature so truly miraculous and adorable as that the end-
less and beneficent variety of results which we see, should spring from such
simple elements.  In times of ignorance, men naturally regarded every occur-
rence which  they  did not understand, that is to  say, which  they could not
refer to a general law, as arising from a direct interference of supreme power;
and thus, for many ages, and among some nations still, eclipses  and earth-
quakes, and many  diseases, particularly those of the mind, and the winds and
weather,  were or are accounted miraculous.  Hence arose, among heathens,
many ceremonies,  and sometimes even barbarous sacrifices, for propitiating
or appeasing their offended  deities;  but founded on expectations no  more
reasonable than if we should now pray to  have the day or  the year made
shorter, or to have a coming eclipse averted.  They had not yet risen to the
sublime conception of the one God, who said, " Let there be  light,"  and the
light was; and who gave to the whole of nature permanent laws, which  he
allows men to discover for  the direction of their conduct in life—laws so
unchanging, that by them  we can  calculate  eclipses backward or forward for
thousands of years, almost without erring,  by the time of one beat of a pen-
dulum; and as our knowledge of nature  advances, we can  anticipate and
explain other events  with equal precision.  Even the wind and the  rain,
which, in common  speech, are the types of uncertainty and change, obey laws
as fixed as those of the sun  and moon; and already, as  regards many  parts
of the earth, man can foretell them without fear of being deceived.   He plans
his voyages to suit the coming monsoons, and he prepares against the floods
of the  rainy seasons.
  The general laws of nature, divisible, as stated above, into the four classes
of, 1st. Physics, often called Natural Philosophy; 2d. of Chemistry; 3d. of
Life, commonly called Physiology; and 4th. of Mind, may be said to  form
the pyramid of Science, of which Physics is the ba.«e, while  the others con-
stitute succeeding layers in the order now mentioned ; the whole having cer-
tain mutual relations and dependencies well figured by the parts of a pyramid.
We must describe  them more particularly, to show  these relations.
  Physics.—The  laws of Physics govern every phenomenon  of nature in
which  there is any sensible  change of place, being concerned  alone in the
greater part of these phenomena, and regulating the remainder which origi-
nate from chemical action, and from the action of life.—The  great physical
truths, as comprehended in the present day by man, are reduced to four, and
are referred to by the words atom,  attraction, repulsion and inertia.  It
gives an astonishing, but true idea of the nature and importance of methodical
Science, to be told that a man, who understands these words, viz., how the
atoms of matter by mutual attraction approach and cling together to  form
masses, which are solid, liquid, or aeriform, according to the  quantity or 
X
INTRODUCTION.
repulsion of heat among them, and which,.owing to their inertia or stub-
bornness, gain and lose motion, in exact proportion to the force of attraction
or repulsion acting on them,—understands the greater part of the phenomena
of nature;  but such is the fact!   Solid bodies existing in conformity with
these truths, exhibit all the phenomena of Mechanics; JAquids exhibit those
of Hydrostatics and Hydraulics;  Airs, those of Pneumatics; and so forth,
as seen  in the table of heads  given below, at  page xii.  And the  whole of
this work is merely a list of the most interesting physical phenomena, arranged
in classes under these heads.
   Chemistry.—Had there been only one kind of substance or matter in' the
universe, the laws of Physics  would have explained all the phenomena;  but
there are iron, and sulphur, and  charcoal, and  about fifty others, which, to
the present state of science, appear essentially distinct.  Now these, when
taken singly, obey the laws of Physics; but when two or more of them ate
placed  in contact  under certain circumstances, they  exhibit a new order of
phenomena.  Iron and sulphur, for  instance, brought together and heated,
disappear as individuals, and unite into a yellow metallic mass, which, in most
of its properties, is unlike  to either:—under other new circumstances, the two
substances will again separate,  and assume their original forms. Such changes
are called  chemical, (from an Arabic word signifying to .burn, because  so
many of them are effected by means of heat,) but during  the changes, the
substances  are not withdrawn  from the influence of the physical laws,—their
weight or inertia,  for instance, is  not altered; and indeed the phenomenon is
merely a modification of  general  attraction  and repulsion.   Many chemical
changes, besides, are only the beginnings of purely  mechanical changes, as
when the new chemical arrangement produced by heat among the intimate
atoms of gunpowder, causes the mechanical or physical motion of the sudden
expansion  or explosion.  And all the manipulations of Chemistry, as  the
transferring of gases from vessel to vessel, the weighing of bodies, pounding,
grinding, &c, are directed to  Physics alone.   Chemistry,  then, is truly, as
figured above, a superstructure on Physics, and cannot be understood or prac-
tised by a  person who is ignorant of  Physics.—The chief departments of
study involving the consideration of Chemical in conjunction with Physical
laws, are enumerated in the table below, under the head of  Chemistry.
   Life.—The most complicated state in which matter exists, is where, under
the influence of life, it forms bodies with a curious internal structure of tubes
and  cavities, in which fluids  are  moving and  producing incessant internal
change.  These are called Organized Bodies, because of the various distinct
parts or organs which they contain; and they form two remarkable classes,
the individuals of one of which are fixed to the soil, and are called  Vegetables;
and  of the other, are endowed with power of locomotion, and are called Ani-
mals.   The  phenomena  of growth, decay, death, sensation, self-motion and
many others, belong to life, but from occurring in material  structures which
subsist in obedience to the laws of physics and chemistry, the life is truly a
superstructure on the other two, and cannot be studied independently of them.
Indeed, the greater part of the  phenomena of organic life are merely chemical
and  physical phenomena, modified by an additional principle.—The science
of Life is divided  into animal and vegetable Physiology, (see the table below.)
   Mind.—The most important part of all science  is the knowledge which
man has obtained of the laws governing the operations of his own mind.
This department  stands  eminently distinct  from the others, on several  ac-
counts.   Unlike that of  organic  life,  which could  not be  understood until
physics and chemistry had been previously investigated, this had made extra- 
INTRODUCTION.                         XI
ordinary advances in a very early age, when the others, as methodical sciences,
had scarcely begun to exist.   In proof of this assertion we need only refer to
the writings of the Greek philosophers.  The most brilliant discoveries and
applications, however, were  reserved for the moderns, as will occur to many
readers, on perusing, in the table below, the several divisions of the subject,
and recollecting the  honoured names which are now associated with each.
It is truly admirable to see the modern analysis, deducing from a few simple
laws of mind all the subordinate departments, just as it deduces mechanics,
hydrostatics, pneumatics, &c, from the laws of physics : and let us hope that
sound  opinions on  this subject, ensuring human happiness,  and therefore
beyond comparison more important than any other knowledge, will soon be
widely spread.—The crowning science of Mind, although in certain respects
independent of the science of Matter, is still closely allied to them in the fol-
lowing ways.   The  faculties of the mind  are originally awakened or called
into activity solely by the impressions of matter or  external nature: all the
language used in speaking of mind and its operations, is borrowed from mat-
ter; and many mental emotions are entirely dependent on bodily conditions.
The science of Mind,  therefore, cannot be studied until  after knowledge
acquired of an external nature; and cannot be studied extensively until that
knowledge be extensive.
   Quantity.—To express most of the facts and laws of physics, chemistry
and life, terms of quantity are required, as when we speak of the magnitude
of a body, or say, that the force of attraction between two bodies diminishes,
in a certain proportion, as their distance increases.   Hence  arises the neces-
sity of having a set of fixed measures or standards, with which to compare
all other quantities.  Such measures have been adopted ; and they are, for
numbers, the fingers, or fives and tens; for length, the human foot, cubit,
pace, &c; and  lately the  seconds' pendulum and  the French metre, (taken
from the magnitude of our globe); for surfaces, the simplest forms of circle,
square, triangle, &c, compared among themselves by the lengths of their
diameters or other suitable lines; and for solid bulk, the corresponding sim-
ple solids, of globe, cube, pyramid, cone, &c, similarly  compared by the
lengths of diameters or of other lines of dimension.  The rules  for applying
these standards to all possible cases, and for comparing all kinds of quantities
with each other, constitute a body of science, called the Science of Quantity,
the Mathematics.  It may be considered as a subsidiary department of human
science, created by the mind itself, to facilitate  the study of  the others.
   Supposing description  of particulars, or Natural History, to be studied
along with the different  parts of the System of Science sketched in the table,
there will be included in  the scheme the whole knowledge of  the universe
which  man  can acquire by the exercise of his own powers:  that is to say,
what he can acquire independently of a supernatural Revelation.  And on
this knowledge all his arts are founded,—some of them on the single part of
Physics, as that of the machinist,  architect,  mariner, carpenter, &c;  some
on Chemistry,  (which includes Physics,) as that  of the miner,  glass-maker,
dyer, brewer, &c; and some on Physiology, (which includes much of Physics
and  Chemistry,) as that of the scientific gardener or botanist,  agriculturist,
zoologist, &c.  The business of teachers of all kinds, and of governors, advo-
cates, linguists, &c. &c, respects chiefly the science of Mind.   The art of
medicine requires  in its professor  a  comprehensive knowledge of all  the
departments. 
Xll
INTRODUCTION.
    TABLE OF  SCIENCE AND ART.
1.  Physics.                       2. Chemistry.
Mechanics,
Hydrostatics,
Hydraulics,
Pneumatics,
Acoustics,
Heat,
Optics,
Electricity,
Astronomy,
  &c.
Simple substances
Mineralogy,
Geology,
Pharmacy,
Brewing,
Dyeing,
Tanning,
  &c.
3. Life.
Vegetable Physiology,
Botany,
Horticulture,
Agriculture,
  &c.

Animal Physiology,
Zoology,
Anatomy,
Pathology,
Medicine,
  &c.
4. Mind.
Intellect.
Logic,
Mathematics,
  &c.
Motives to action.
Emotions and Passions,
Morals,
Government,
Political Economy,
Theology,
Education.
  In the first stages  of education, viz., during the years of childhood and
youth, the learning acquired is necessarily of the most mixed kind, and much
of it is determined by what is called accident; but from the mutual depend-
ence of  the different  departments of science, as  explained  in the preceding
paragraphs, it follows that with a view to complete erudition, the order exhi-
bited in " The Table,"  is that in which they should afterwards be studied, so
as to prevent repetitions and anticipations,  and to diminish, as much as pos-
sible, the labour of acquirement.
   Every man may be said to begin his education, or acquisition of knowledge,
on the day of his birth.   Certain objects, repeatedly presented to the infant,
are,  after a time, recognized and distinguished.   The number of objects thus
known gradually increases, and from the constitution of the mind, they are
soon associated in the recollection, according to their resemblances, or obvi-
ous  relations.   Thus, sweetmeats, toys, articles  of dress, &c, soon form
distinct classes in the memory and conceptions.  At  a later age, but still very
early, the child  distinguishes readily between a mineral mass,  a vegetable,
and  an animal; and  thus his mind has already noted the three great classes
of natural bodies, and has acquired a certain degree of acquaintance with Na-
tural History.   He  also soon  understands the phrases "a fallino- body,"
"the force of a moving body," and has  therefore a perception of the great
physical laws of  gravity and inertia.   Then having  seen sugar dissolved  in
water, and wax  melted round  the wick of a burning candle, he has learned 
INTRODUCTION.
Xlll
some  phenomena of Chemistry.   And  having  observed the conduct of the
domestic animals, and of the persons about him, he has begun his acquaint-
ance with  Physiology and  the  science of Mind.  Lastly, when he  has
learned to count his fingers and his sugar-plums, and to judge of the fairness
of the division of a cake between  himself and brothers, he has advanced into
Arithmetic and Geometry.  Thus, within a year or two, a child of common
sense  has  made a degree of progress in all the  great departments of human
science; and in addition has learned to name objects, and to express feelings,
by the arbitrary sounds of language.  Such, then, are the beginnings  or foun-
dations of knowledge, on which  future years of experience, or  methodical
education, must rear the superstructure of the more considerable attainments
which befit the various conditions of men in a civilized community.
  In the course of the  preceding  disquisition, we have seen  that Physics or
Natural Philosophy, the subject of the present volume, is  fundamental to
the other parts, and is therefore that of which a knowledge is indispensable.
Bacon truly calls it " the root of the sciences and arts."  That its import-
ance has not been  marked by the place which it has held in common sys-
tems of education, is owing chiefly, 1st, to the misconception that a know-
ledge  of technical mathematics was a necessary preliminary; and, 2d, to an
opinion, also erroneous, that the degree  of acquaintance with Physics which
all  persons acquire by common  experience, is sufficient for common pur-
poses : now it is true, that the toys of childhood,  as the windmill, ball, syphon,
tube,  and a hundred others, furnish so many exemplifications of  the laws of
Physics, and may well be called a philosophical  apparatus; but they give
information which is exceedingly vague, and not at all such  as is absolutely
requisite in the practice of many  of the arts.  If, then,  the study of Physics
be so  easy as now appears, and  so important as we shall  try still farther to
show, there can be no excuse for neglecting it.
  The greatest sum of knowledge acquired with the least trouble is, perhaps,
that which comes with the study of the few simple truths of Physics.  To
the man who understands these,  very many phenomena, which, to  the unin-
formed, appear prodigies,  are only beautiful illustrations of his fundamental
knowledge, and this he carries about with him, not as an oppressive weight,
but as a charm supporting the weight of other knowledge, and enabling  him to
add to his valuable store every new fact of importance which  may offer itself.
With such a principle of arrangement, his information, instead of resembling
loose  stones or rubbish thrown  together in  confusion, becomes as a noble
edifice, of correct proportions and  firm contexture, and  is acquiring greater
strength and  consistency with  the  experience of every  day.  It has been a
common prejudice, that persons thus instructed in general  laws had their
attention too much divided, and could know nothing perfectly.  But the very
reverse is  true ; for general knowledge renders all particular knowledge more
clear  and  precise.  The ignorant man may be said to have charged  his hun-
dred books of knowledge,  to use  a rude simile,  with single objects,  while the
informed man makes each support a long chain, to which thousands of  kin-
dred and useful things  are attached.  The laws of Philosophy may be com-
pared to keys which give admission to the most delightful gardens that fancy
can picture ;  or to a magic power, which unveils the face of the universe, and
discloses endless charms of which ignorance never dreams.  The informed
man,  in the world, may be said to*be always surrounded by what is known
and friendly to him, while the ignorant man  is as  one in a land of strangers
and enemies.   A man reading a thousand volumes of ordinary books  as agree-
able pastime, will  receive  only vague  impressions;  but he who studies the 
XIV
INTRODUCTION.
methodized Book of Nature, converts  the great  universe into a simple and
sublime history, which tells of God, and may worthily occupy his attention
to the end of his days.
   We have said already, that the laws of Physics govern the great natural
phenomena of  Astronomy, the tides,  winds, currents, &c.   We will now
mention some of the artificial purposes to which man's ingenuity has made
the same laws subservient.
   Nearly all that the civil engineer accomplishes, ranges under the head of
Physics.  Let us take, for instance, the admirable specimens  scattered oyer
the British Isles:—the numerous canals for inland  traffic;  the docks to receive
the riches of the world, pouring towards us from every quarter; the many
harbours offering safe retreat to the storm-driven mariner; the magnificent
bridges which everywhere facilitate intercourse ;  hills bored through to open
ways for commerce by canals, common roads and rail-roads, the canals  in
some places being supported, like the roads, on arches across valleys or above
rivers, so that here and there the singular phenomenon is seen of one vessel
sailing directly  over another; vast tracts of swamp or fen-land drained, and
now serving for agricultural;  the noble light-house,  rearing* its head  amidst
the storm, while the dweller within trims his lamp in  safety, and guides  his
endangered fellow-creature through the  perils of the night, &c. &c.
   In Holland, great part of the country has been won and is now preserved
from the sea, by the same almost creating power; and  now rich cities and
an extended garden  smile, where, as related by Caesar, were  formerly only
bogs and a dreary waste.
   As a general  picture, it  is interesting to consider,  that in many situations
on earth  where  formerly the  rude savage beheld the cataract falling  among
the rocks, and  the wind bending the trees of the forest, and sweeping  the
clouds along  the mountain's brow, or whitening  the face of the  ocean, and
regarding these phenomena with awe and terror, as marking the agency of
some great but  hidden power, which might destroy him  ; in the same situa-
tions now, his  informed son, who works with the laws  of nature, can lead
the waters of the cataract, by sloping  channels, to convenient spots, where
they are  made to turn his mill-wheel, and to do  his multifarious work;  the
rushing winds,  also, he makes his  servant, by rearing  in their  course  the
broad-vaned wind-mill, which then performs a thousand offices for its mas-
ter, man; and the breezes which whiten the ocean are caught in his expanded
sails, and are made  to waft their lord and  his treasures  across the deep, for
his pleasure or  his profit.
   In Architecture, also, Physics is supreme, and has  directed the construc-
tion of the temples,  pyramids, domes and palaces, which adorn the earth.
   In respect  to machinery, generally, Physics is the guiding light.   There
are, for instance, the mighty steam-engine; machines for spinning and weav-
ing, and  for moulding other bodies into various shapes, yea, even iron itself,
as if it were  plastic clay;  wind-mills and water-mills, and wheel carriages;
the plough, and implements of husbandry; artillery and the furniture of war:
the balloon,  in  which  man rides  triumphantly  above the  clouds, and  the
diving-bell, in which he penetrates the secret caverns of the deep; the imple-
ments of the intellectual arts, of printing, drawing, painting, sculpture, &c;
musical instruments; optical and mathematical instruments, and  a thousand
others.
   But Physics is also an important foundation of the healing art.  The medi-
cal man, indeed, is  the engineer pre-eminently; for  it  is  in the animal body
that true perfection and the greatest variety of mechanism are found.  Where, 
INTRODUCTION.
XV
to illustrate Mechanics, is to be found a system of levers  and hinges, and
moving parts, like the limbs of an animal  body; where such  an hydraulic
apparatus, as in the heart and blood-vessels; such a pneumatic apparatus,  as
in the breathing chest; such acoustic  instruments, as in the ear and larynx;
such an optical instrument, as in the eye;  in a word, such variety  and per-
fection, as in the whole of the visible anatomy?  All these  structures, then,
the medical  man should understand, as a watchmaker knows the parts of a
time-piece about which he is  employed.   The watchmaker, unless he can
discover where a pin is loose, or a wheel injured, or a particle of dust adher-
ing, or oil wanting, &c, would ill succeed in repairing an injury; and  so
also of the ignorant medical man in respect to the human body.   Yet will it
be believed, that there are many medical men who  neither understand me-
chanics, nor  hydraulics, nor pneumatics, nor  optics, nor acoustics, beyond
the merest  routine; and that systems of medical education are put forth  at
this day which do not even mention the department of Physics!  That such
is the case, furnishes an illustration of what is stated in  the beginning of this
essay; viz., that the sciences and arts are progressive, and that perfect methods
of education must arise gradually, like all other things of human contrivance.
It is within the recollection of persons now living, that political economy was
discovered to be a grand foundation of the art of government, indicating means
of security against many national misfortunes common in former times, yea,
even against famine and war.   And the day is not distant, when the members
of  the medical profession generally will understand  how much the correct
knowledge of animal  structure  and function,  and of many  remedies, must
depend on precise acquaintance with  Physics.—Besides  the  more strictly
professional  matters contained in the medical sections of the present  work,
there are many others scattered through it which greatly interest the medical
man; such are the subjects of meteorology, climate, ventilation and warming
of dwellings, specific gravities, &c. &c.
   The laws  of Physics  having an influence  so extensive  as appears from
these paragraphs, it need not excite surprise that all classes of society  are at
last discovering the deep interest they have to understand them.  The lawyer
finds that in many of the causes tried in his courts, an appeal must be made
to Physics,—as in cases of disputed inventions; accidents in navigation,  or
among carriages, steam-engines, and  machines generally; questions arising
out of  the agency of winds, rains, water-currents, &c: the statesman is con-
stantly listening to discussions respecting bridges, roads, canals, docks, and
the mechanical industry of the nation:  the clergyman finds ranged among
the beauties of nature, the most intelligible  and striking proofs of God's wis-
dom and goodness; the sailor  in his ship has to deal with  one of the most
admirable machines  in existence:  soldiers, in using their  projectiles,  in
marching where rivers are to be grossed, woods to be cut down, roads to  be
made,  towns to be besieged, &c, are dependent chiefly on their knowledge
of Physics : the land-owner, in making improvements on his estates, building,
draining, irrigating, road-making, &c; the farmer equally in these particu-
lars, and in all the machinery of agriculture ;  the manufacturer, of course ;
the merchant who selects and distributes over the world the products of  manu-
facturing industry—all these are interested in Physics; then also the man
of letters, that he may not,  in  drawing his  illustrations from  the material
world, repeat the  scientific  heresies and absurdities which  have heretofore
prevailed, and which,  by shocking the  now better-informed public, exceed-
ingly lower the estimation in which  such specimens of the Belles Lettres
are held, and lessen their general utility:  and, lastly,parents of either sex, 
XVI
INTRODUCTION.
whose conversation and example have such  powerful effect on the character
of their children, who, when grown up, are to fill all the stations in  society;
all should study Physics, as one important part of their education.
  And it is for such reasons that Natural Philosophy is becoming  daily more
and more a part of common education.   In our cities  now, and even in an
ordinary dwelling-house, men are  surrounded  by prodigies of mechanic art,
and cannot submit to  use  these, regardless  of how they are  produced, as a
horse is regardless of how the corn falls into his manger.  A general diffu-
sion of knowledge, owing  greatly  to the increased commercial intercourse of
nations, and therefore  to the improvements in the  physical  departments of
astronomy, navigation, &c, is changing everywhere the condition  of man,
and elevating the human character in all ranks of society.  In remote times
the inhabitants of  the earth were generally divided into small states or socie-
ties,  which  had  few  relations  of amity  among themselves, and  whose
thoughts and interests were confined very much within  their own  little terri-
tories and  rude habits. In  succeeding ages, men found themselves belonging
to larger communities, as when the English  heptarchy  was united;  but still
distant kingdoms and quarters of the world were of no interest to  them, and
were often totally unknown.  Now, however, every one feels that  he is  a
member of one vast  civilized society, which covers the face of the earth;
and no part of the  earth is indifferent to him.  In England,  for instance, a
man of small fortune  may cast his  looks around him, and say  with truth and
exultation, " I  am  lodged  in a  house which affords me conveniences and
comforts which, some centuries ago, even a king could not command. Ships
are crossing the seas  in  every direction, to  bring me what is useful to me
from all parts of the  earth.  In China, men are gathering the tea-leaf lor
me;  in America, they are planting cotton for me; in the West Indies, they
are preparing my  sugar and my coffee ; in Italy, they are feeding silk-worms
for me; in Saxony, they are shearing the  sheep to  make  me clothing; at
home,  powerful steam-engines are  spinning and weaving for me, and making
cutlery for me, and pumping the  mines  that minerals  useful to me may be
procured.   Although my patrimony was small, I have post-coaches  running
day and night, on  all the roads  to  carry my correspondence; I have roads,
and canals, and bridges, to bear the  coal for  my winter fire; nay, I have
protecting fleets and armies around my happy country, to secure my enjoy-
ments and repose.  Then I have editors and printers, who daily send me an
account of what is going on throughout  the world, among all these people
who serve me.  And in a  corner  of my house I have Books! the  miracle
of all my possessions, more wonderful "than the wishing-cap of the  Arabian
Tales ; for they  transport me instantly, not only to all places,  but to all
times.  By my books I can conjure  up before  me, into vivid existence, all
the great and good  men of  antiquity; and for my individual satisfaction I'can
make  them act over again the most renowned of their  exploits;  the orators
declaim for me; the historians recite ; the poets sing ;  and from the equator
to the pole, or from the beginning  of time until now, bv my books, I can be
where  I please.    This picture  is  not overcharged, and might be  much  ex-
tended, such being God's goodness and providence, that each individual of
the civilized millions dwelling on the earth, may have neatly the same enjoy-
ments as if he were the single lord of all.
  Reverting to the importance of Natural Philosophy as a general study  it
may be remarked  that there is no occupation which so much strengthens and
quickens the judgment.  This praise has usually been bestowed on the Ma
thematics, although a knowledge of abstract Mathematics existed with all the 
INTRODUCTION.
XV11
absurdities of the dark ages; but a familiarity with Natural  Philosophy,
which comprehends  Mathematics, and gives  tangible and pleasing illustra-
tions of the abstract  truths, seems incompatible  with the  admission of any
gross absurdity.   A  man whose mental faculties have been sharpened by ac-
quaintance with these exact sciences in their combination, and who has been
engaged, therefore, in contemplating reed relations, is more likely to disco-
ver truth in other questions, and can better  defend himself against sophistry
of every kind.   We cannot have clearer evidence of this than in the history
of the sciences, since the Baconian method of reasoning by induction  took
place of the visionary hypotheses of  preceding times.   Until then, even
powerful minds did not recoil from the most absurd theories on all subjects.
Astronomy was mixed with Astrology ; Chemistry  with  Alchemy ; Physi-
ology with the singular hypotheses which preceded the discovery of the cir-
culation of the blood; Politics with the errors  of monopolies,  prohibitions,
balance of trade,  &c.  Even Religion itself, in various  ages and countries,
has felt the influence of the state of the public mind as to solid  attainments.
To a man with the knowledge of nature which  we now  possess, the fables
and licentious abominations of the Greek and Roman theologies are shocking
indeed; as are the religions of the  God  of Fire in  China, of Vishnoo in
India, of Mahomet's imposture and pretended miracles, &c.  But the enlight-
ened  Christian minister earnestly recommends the study of nature;  first,
because from contemplating the beauty of creation,  with the  wisdom  and
benevolent design manifest in all its parts, there spring  up in every unde-
praved mind  those feelings of admiration and gratitude, which constitute the
adoration of  natural  religion, and which form, as shown  by many estimable
writers on Natural Theology, a fit foundation for the sublime doctrine of
immortality,  and  secondly, because a Revelation being probable only by the
miracles occurring at its establishment; to enable men to distinguish between
miracles and  the usual course of nature, a perfect knowledge of that course,
or of Natural Philosophy, is essential: all the false religions  of antiquity
were founded on, and upheld by pretended miracles.  As regards the ques-
tion of immortality,  even independently  of Revelation, no man who con-
templates the order and beauty of the material world, and then thinks on the
hideous deformities of the moral world—where vice so often triumphs, and
modest virtue pines and  dies—can for a moment believe that they are the
work of the  same author, unless there be a hereafter  of retribution;  and
feeling thus that eternal justice  requires another state for  man,  he embraces
with delight  the cheering  promises of immortality.   There have been, how-
ever, at various times, even among Christians, sincere, but weak-minded or
ill-informed men, who decried the study of the natural sciences,  a* inimical
to true  religion;  as  if God's ever-visible  and  magnificent revelation of his
attributes in  the  structure of the universe could be at  variance with  any
other  revelation.  But such  prejudices are now quickly  passing away.
Wherever considerable knowledge of nature exists, debasing and gloomy
superstition must  cease.   It is not the abject terror of a slave which is in-
spired by contemplating the majesty  and  power of our  God,  displayed in
his works, but a sentiment akin  to the lender regard which  leads a  favoured
child to approach with confidence a wise and indulgent parent.
   It remains for the author now only to say a few words with respect to the
present work.  He was  originally led to  the undertaking with the view of
supplying the desideratum  in  medical literature, of a  treatise on Medical
Physics ; but soon perceiving that the preliminary investigation of  General
Physics,  necessary to adapt the work  to medical readers, would require to 
XV1U
INTRODUCTION.
 be  nearly as extensive as it  would for general readers,  and reflecting  that
 every person of liberal education must now possess such a book, not to be
 read once and then thrown aside as a novel is, but to be frequently consulted
 as a manual, he determined to make his book as complete and as extensively
 useful as possible.  He has been encouraged, during his labour, by the belief
 that the growing light of science, which now exhibits more clearly the natu-
 ral  relations of the°different departments of study, as attempted to be por-
 trayed in the preceding pages, might enable him to avoid some of the defects
 of former elementary treatises, and to add features of novelty and improve-
 ment to his  own.  The sections on Animal Physics were, of course, written
 for medical  men; and a  great service will be  rendered  by the  work, if it
 only awakens them to a just  sense of the  importance of  Physics as one of
 the foundations of  their  art.   But  even for general readers there  are few
 parts of these sections which the author would  exclude.   There is nothing
 more admirable in nature than the structure and functions of the human body,
 and there are many reasons why no liberal mind  should be careless of the
 study.   The details here given are not more anatomical than the illustrations
 from the animal  economy contained  in  the common treatises on Natural
 Theology.   From the attempt in this work to compress into the  smallest
 possible space the greatest possible sum  of scientific information,  few his-
 torical details have been admitted, whether relating to the distinguished men
 who have benefited the world as authors or inventors, or to the history of
 the  progress  of  science :—such details form  an  interesting, but  distinct
 branch of study.
  The author must not conclude without observing, that no treatise on Natu-
 ral  Philosophy can  save, to a person  desiring full information on  the  sub-
ject, the necessity of attendance on experimental lectures or demonstrations.
 Things that  are seen, and felt, and heard, that is, which operate on the exter-
 nal senses, leave on the memory much stronger and more correct  impres-
 sions than where the conceptions are produced merely by verbal description,
 however vivid.  And no man has ever been remarkable  for his  knowledge
 of Physics, Chemistry, or Physiology, who has not had practical familiarity
 with the objects.  With reference to  this familiarity,  persons who take a
 philanthropic  interest in the affairs  of the world,  must observe, with much
 pleasure, the now daily increasing  facilities of acquiring useful knowledge,
 afforded by  the scientific institutions formed and forming, not  only  through
 this kingdom, but through most civilized nations.
  Bedford Square, 1st March, 1827. 
ELEMENTS
OF
NATURAL   PHILOSOPHY.
              SYNOPSIS,  OR GENERAL REVIEW.

  If it excite our admiration that a varied edifice,  or even a magnificent city
can be constructed of stone from  one quarry, what must our feeling be to
learn how few and simple the elements are  out of which the sublime fabric
of the universe, with all its orders of phenomena, has arisen, and  is now
sustained !   These elements are general facts and laws which human saga-
city is able to detect, and then to  apply to endless  purposes of human advan-
tage.
  Now the four words, atom, attraction, repulsion, inertia, point  to  four
general truths, which explain the greater part of  the  phenomena of nature.
Being so  general, they are called physical truths, from  the Greek word signi-
fying nature, as  also " truths of Natural Philosophy," with the same mean-
ing, and sometimes " mechanical truths," from their close relation to ordinary
machinery.  These appellations distinguish them from the remaining general
truths, namely, the chemical truths, which regard particular substances, and
the vital  and mental truths,  which have relation only to living beings.   And
even in the  cases where a chemical or vital influence operates, it modifies,
but does not destroy, the physical influence.  By fixing the  attention, then,
on these four fundamental truths, the studen obtains, as it were, so many
keys to unlock, and lights to illuminate the secrets and treasures of nature.
  1st. Atom.  Every material mass in nature is  divisible into very minute
indestructible and unchangeable particles,—as when a piece of any metal is
bruised, broken, cut, dissolved, or otherwise transformed, a thousand times,
but can always be exhibited  again as perfect as at first.  This truth is con-
veniently recalled by giving to the particles the name atom, which is a Greek
term, signifying  that which cannot be farther cut or divided, or an exceeding
minute resisting particle.
  2d.  Attraction.  It is found  that the atoms above referred to, whether
separate or already joined into masses, tend towards all other atoms or masses,
—as when the atoms of which any mass is composed are, by an invisible
influence, held together with a certain degree of force; or when a block of
stone is similarly held  down to the earth on which it lies; or when the tides
on the earth rise towards the  moon.  These facts are conveniently  recalled
by connecting with  them the word Attraction (a  drawing together)  or  gra-
vitation.
  3d. Repulsion.  Atoms  under certain circumstances, as of heat  diffused
among them, have their mutual attraction countervailed or resisted, and they 
20
SYNOPSIS.
tend to separate;—as when ice heated melts into water, or when water heated
bursts into steam, or when gunpowder  ignited  explodes.   Such tacts  are
conveniently recalled by the term Repulsion (a thrusting asunder.)
   4th. Inertia.  As a fly-wheel made to revolve, at first offers resistance to
the force moving it, but gradually acquires speed proportioned to that force,
and then resists, being again stopped, in proportion to its speed, so all bodies
or atoms in the universe have about them, in regard to motion, what may be
figuratively called a stubbornness, tending to keep them in their existing state,
whatever it may be—in other  words, they neither acquire motion, nor  lose
motion, nor bend their course in  motion, but in exact proportion  to some
force  applied.   Many of the motions  now going on in the universe with such
regularity—as that turning of the earth which produces the  phenomena of
day and night—are motions which began thousands of years ago, and continue
unvarying in this way.  Such facts are conveniently  recalled by the term in-
ertia applied to them.
   A person  comprehending fully the import of these  four words, that is to
say, having  present to his mind  numerous  good  types or exemplars of the
facts referred to them, may predict or anticipate correctly, and may control
very many of the facts and phenomena which the extended  experience of a
life can display to him;  and such a  person is commonly said to know the
causes or reasons of things and events.  Now it is important here to observe,
that when a person gives a reason or  explanation  of any fact, other than that
it  is a fact, or than that the Creator has willed it, he  is merely, although he
may not be aware of  this, showing its resemblance  to many other facts, no
one of which he understands better than itself—and what he  calls a general
truth, or law, or principle, is merely  an expression for the observed but un-
accountable resemblance of the facts.  Thus,  when a man says that a stone
falls because of attraction or gravitation,  he only uses a word which recalls
thousands of instances which  he has witnessed of one body approaching
another;  but by any cause of the approach, other than that God has willed
it, is to him utterly  unknown.  Should  men, in the progress  of their re-
searches, discover that the phenomena now classed by them under the heads of
attraction and repulsion, although apparently opposite, are really as closely
allied  as they already know the rising of a balloon and the falling of a stone
to be (the balloon rises like a cork in  water, being pushed up by the fluid air
around it, heavier than it, and seeking to descend,) they will not have disco-
vered  a new cause, but a new resemblance, (new to them) among phenomena,
and will only have advanced one step farther in perceiving the simplicity of
creation.   In accordance with  these views, it will be found that this volume
is chiefly an extensive display of the most important phenomena of nature
and art, classified so as to be explained by the four physical truths, and mutu-
ally to illustrate one another.   They will be distributed under the following
heads  or divisions:

                               PART I.

             CONSTITUTION of masses, motions and forces.
  The four fundamental  truths extensively examined, and used  to  explain
generally, in
Section
   1. The nature or constitution of the material masses which compose the
       universe; (a department technically called Somatology, from Greek
       words signifying a discourse on body.) 
SYNOPSIS.
21
  2. The motions or phenomena going on among the masses ;—a department
      including  the common divisions  of Statics (things stationary or at
      rest,) and Dynamics (what relates to force or power.)

                             PART II.

                        phenomena of solids.
  The four truths explaining the peculiarities of state and motion among solid
bodies:—a  department called, in  a restricted  sense,  Mechanics, (from  the
Greek word signifying a machine.)

                             PART III.

                        phenomena of fluids.
  The truths explaining  the peculiarities of state and motion among fluid
bodies:—a department called Hydrodynamics (from Greek words signifying
water and force.)
Section
  1. Hydrostatics (water at rest or in  equilibrium.)
  2. Pneumatics (airphenomena.)
  3. Hydraulics (water or fluid in motion.)
  4. Acoustics (phenomena of sound and hearing.)

                             PART IV.

                phenomena of imponderable substances.
  The truths aiding to explain the more recondite phenomena of Imponde-
rable Substances, under the  heads of
Section
  1. Heat or Caloric.
  2. Light or Optics.

                              PART V.

                     ANIMAL AND MEDICAL PHYSICS.
  In this part will be ranged the most interesting illustrations afforded by the
animal economy, constituting—Animal and Medical physics.

  As no man can well understand a subject of which he does not carry a dis-
tinct outline in his mind, it is recommended to the reader of this work to
study the general synopsis, and the analysis placed at the heads of the chap-
ters and sections, until the memory be well impressed with them.
3 
22                    CONSTITUTION OF MASSES.
                           PART   I.

THF FOUR FUNDAMENTAL TRUTHS MINUTELY EXAMINED, AND USED TO EXPLAIN
  GENERALLY FIRST, THE NATURE OR CONSTITUTION OF THE MATERIAL MASSES
  WHXCrcOMPOSE THE UNIVERSE, AND SECONDLY, THE MOTIONS OR  PHENO-
  MENA GOING ON AMONG  THEM.
SECTION L—THE CONSTITUTION  OF MASSES.
ANALYSIS OF THE SECTION.
The visible universe is built up of very minute indestructible atoms called
  matter, which, by mutual attraction, cohere or cling together in masses
  of various form and magnitude.  The atoms are more or less approxi-
  mated, according to the quantity or repulsion of heat among them, and
  hence arise the three remarkable forms in the masses, of solid,  liquid and
  air, which mutually change into each other with change in the quantity
  of heat.   Certain modifications of attraction and repulsion produce the
  subordinate peculiarities of state called crystal, dense, hard, elastic, brittle,
  malleable, ductile and tenacious.
                    " Minute Indestructible Atoms."*

That the smallest portion of  any substance which the human  eye can per-
  ceive, is still a mass of many ultimate atoms or particles, which may be
  separated from each other, or newly arranged, but which  cannot individu-
  ally be hurt or destroyed, is deduced from such facts as the following:
  A particle of powdered  marble, hardly visible to the naked  eye, still ap-
pears to the microscope a block susceptible of indefinite division; and, when
it is broken by fit instruments, until the microscope can  hardly discover the
separate particles of the fine  powder, these  may be yet farther divided, by
solution in an acid;  the whole becoming then absolutely invisible, as  part of
a transparent liquid.
   A small mass of gold may  be hammered into thin leaf, or drawn into fine
wire, or cut into almost invisible parts, or liquefied in a crucible, or dissolved
in an acid, or dissipated by intense heat into vapour; yet, after  any and all of
these changes, the atoms can  be collected  again to form the original mass of
gold, without the slightest diminution or change.   And all the  substances or

  * The different heads or titles, which appear thus, throughout the work, between  inverted
 commas, are the successive portions of the Analysis, detached for separate consideration.
 The reader is particularly requested to re-peruse the analysis at the several interruptions,
 that he may have constantly before him that clear view of the general relations among the
 different parts of the subject, which is essential to a perfect understanding of it. 
CONSTITUTION op masses.
23
elements of which our globe is  composed, may thus be  cut, torn, bruised,
ground, &c, a thousand and a thousand times, but are always recoverable as
perfect as at first.
   And, with respect to delicate combinations of these elements, such as exist
in animal and vegetable bodies, although it be beyond human art, originally
to produce, or even closely to imitate many of them—for we cannot build up
a feather or a rose—still, in their decomposition and apparent destruction, the
accomplished chemist of the present day does not lose a single atom.   The
coal which burns in  his apparatus, until only a little ash remains behind, or
the wax-taper which seems to vanish altogether  in flame, or the portion of
animal flesh which putrefies, and gradually dries up and disappears—present
to us phenomena which  are now proved to be only changes  of connection
and arrangement among the indestructible  ultimate atoms ; and the chemist
can offer all the elements again, mixed or separate, as desired, for any of the
useful purposes to which they are severally applicable. When the funeral piles
of the ancients, with their charge of  human remains, appeared to be wholly
consumed, and left the idea with  survivors that no base use could be made, in
after time,  of what had been the  material dwelling of a noble or beloved spi-
rit, the flames had only, as it were, scattered the  enduring blocks of which a
former edifice had been constructed,  but which were soon to serve again in
new combinations.
   Facts, to be stated under the heads  of " chemical composition" and " crys-
tal," will prove,  that the ultimate particles  of any substance must be, among
themselves, perfectly similar.

                "Minute." (Read the Analysis page 22.)
The following are interesting particulars in the arts or in nature, helping the
   mind to  conceive how minute  the ultimate atoms of matter must be.
   Goldbeaters, by hammering, reduce gold to  leaves so thin, that  360,000
must be  laid upon one another to produce  the thickness of an inch.  They
are so thin, that if formed into a book, 1,800 would occupy only the space of
a single leaf of common paper;  and an octavo volume  an inch thick  would
have as many  pages as the books of a well-stocked ordinary library contain-
ing 1,800 volumes of 400 pages each; yet those leaves are perfect, or free
from holes, so that one of them laid upon any surface, as in gilding, gives the
appearance of solid gold.
   Still thinner than this is the coating of gold,  upon the silver wire of what
is called gold lace ; and we know not that such coating is of only  one atom
thick.   If  we place a piece of this wire in nitric acid, so as to dissolve the
silver within, the gold coating remains as a  metallic tube of exquisite tenuity.
   Platinum can be drawn into wire much finer than human hair.
   A grain of blue vitriol or carmine, will tinge a gallon of water, so that in
every drop the colour may be perceived.
   A grain  of musk will scent  a  room for  twenty years, and will have lost
but little of its weight.
   The carrion crow  seems to smell its food at a distance of many miles.
   The  thread of the silk-worm is  so small,  that many folds  have  to be
twisted  together to form our finest sewing  thread;  but that of the  spider is
smaller  still, for two  drachms of it by weight would reach from London to
Edinburgh, or 400 miles.
   In the milt  of a cod-fish, or in water in which certain vegetables have
been infused,  the microscope discovers animalcules, of which many thou- 
24
CONSTITUTION OF MASSES.
sands together do not equal in bulk a grain of sand; yet these have their
blood and other subordinate parts like larger animals; and, indeed, nature,
with a singular prodigality, has supplied many of them with organs as com-
plex as those of the whale or elephant.  Now the body  of an animalcule
consists of the same elementary substances, or ultimate atoms, as  the body
of man himself.  In a single pound of matter, it thus appears, that there
may be more living creatures than of human beings on the face of this globe.
What scenes  has the microscope laid open to the admiration of the philoso-
phic inquirer!
   Water,  mercury, sulphur, or, in general, any substance, when sufficiently
heated, rises as invisible vapour or gas:  in  other words, is made to assume
the aeriform  state.  Great heat, therefore, would cause the  whole of the ma-
terial  universe to disappear, the previously most solid bodies becoming  as
invisible and  impalpable as the air  we  breathe.  Utter annihilation  would
seem  but one stage beyond this.

                              " Matter:'
   The inconceivable  minuteness of ultimate atoms, as shown above, has led
some inquirers to  doubt  whether there  really be matter;  that is to  say,
whether what we call substance or matter have existence or not.  In answer
to this, it  has been usual to adduce, besides  the weights of the substances,
and the proofs of indestructibility already mentioned, which seem conclusive,
the fact that every kind or portion of matter obstinately occupies some space
to the exclusion of all other matter  from that particular space.  This occu-
pancy of space is the  simplest and most complete idea which we have of ma-
terial existence.  The awkward word impenetrability has  been used to ex-
press it, with reference of course  to the individual atoms.  The following
are elucidations:
   We cannot push one  billiard-ball into the substance of another, and then
a second and then a third, and so on; or the material of the universe  might
be absorbed in a point.
   A mass of iron on a support will resist the weight of thousands of pounds
laid upon it  and pressing to descend into  its place; and  although  a very
great weight might crush or break it into pieces, still one  particle would not
be annihilated.  In a  forcing-pump, or in Braham's water-press, millions of
pounds cannot push the piston down, unless the water below it be allowed
to escape.
   A weight laid upon bladders full of air, or on the piston handle of a closed
air-pump, is supported in the same manner.
   A quantity of air escaping  from a vessel under water ascends throuo-h the
water as a bubble, displacing  its bulk of water in its way.
   A glass tube, left  open at  bottom,  while  the  thumb  closes the top, if
pressed from air into water,  is not filled with  water, because the air con-
tained m it resists;  but if the air  be  allowed to  escape by  removing the
thumb from the top, the tube  becomes filled immediately to the level of the
water around it.  In a goblet or basin  pushed  into  water, with the mouth
downwards, the entrance of water is resisted for the like reason ;  and if the
goblet be inverted over a floating lighted taper, this will  continue to float
under it, and to burn in the contained air, however deep in the water it may
be carned-exhibiting the curious  phenomenon of light  below water, and
being an  emblem of the living inmate of a diving bell, which is  merely a
larger goblet holding a man instead of a candle. 
GENERAL ATTRACTION.
25
           "Mutual Attraction" (See the Analysis, page 22.)
  Any visible  mass of  matter, then, as of  metal, salt, sulphur,  &c,  we
know to be really a collection of dust, or minute atoms, by some cause made
to cohere or cling together;  yet there are no hooks connecting them, nor
nails, nor glue; and the connection may be broken  a  thousand times, by
processes of nature or art, but is always ready to take place again ; the cause
being no more destroyed  in  any case  by interruption, than the  weight of a
thing is destroyed by frequent lifting from the ground.  Now the cause we
know not, but we call it attraction.  The phenomena of attraction and its
contrary, repulsion, particularly when  occurring between bodies at consider-
able distances  from each other, are as inexplicable as any subjects  which
the human mind has to contemplate; but the manner or laws of the  pheno-
mena are now  well understood.  The general nature and extensive influence
of attraction may be judged of from the following facts:
  Logs of  wood floating in  a pond, or ships in calm water,  approach each
other, and afterwards remain in contact.   When the floating bodies are very
small, or can approach very  near to each other at the water's edge—as glass
bulbs in a teacup—an additional force  is called  into play, as will  be  ex-
plained under the head of "  capillary attraction."
  The wreck of a ship, in a smooth sea after a storm, is often seen gathered
into heaps.
  Two bullets or plummets suspended by strings near to each other, are
found  by the delicate test of the torsion balance (which  will be described
afterwards) to attract each other, and therefore not to hang quite perpendicu-
larly.
  A plummet suspended  near the side of a mountain inclines towards it, in
a degree proportioned to its magnitude;  as was  ascertained by the well-
known trials of Dr. Maskeleyne near the mountain Schehallion, in Scotland.
  And the reason why the plummet in such a case tends much more strongly
towards the earth than towards the hill, is only that the earth is larger than
the hill.
  At New South Wales, which is situated on our globe  nearly opposite to
England, plummets hang and fall towards the centre of the  globe, as they
do here ; so that in respect to England, they are hanging and falling upwards,
and the people there, like flies on the opposite side of a pane  of glass, are
standing with  their feet towards us,—hence  called our antipodes.  Weight,
therefore, is merely general  attraction  acting everywhere.
   But it is owing to this  general attraction that our earth itself is a globe:—
all its parts being drawn towards each other, that is, toward a common centre,
the mass assumes the spherical or rounded form.
   And the moon also is round, and all the planets; nay, the glorious sun, too,
so much larger than these, is round;—suggesting the inference  that all must
at one time have been to a certain degree fluid, and that all are subject to the
same law.
   Descending  again to the earth and observing minuter masses, we have
many interesting instances of roundness from the same cause; as—the parti-
cles of a mist or fog floating  in  air—these, mutually attracting and coalescing
into larger drops, and  so forming rain—dew-drops—water  trickling on a
duck's wing—the tear dropping from the cheek—drops of laudanum—glo-
bules of mercury, like pure  silver beads, coalescing when near, and forming
larger ones—melted lead  allowed to rain down from an elevated sieve, and 
26
CONSTITUTION OF MASSES.
by cooling as it descends so as to retain the form of its liquid drops, becom-
ing the spherical shot-lead of the sportsman, &c.
  The cause of this extraordinary phenomenon which we call attraction, acts
at all  distances.—The  moon, though 240,000 miles  from the earth, by her
attraction, raises the water of our ocean under her, and forms what we call
the tide.—The sun, still farther off, has a similar influence; and when the
sun and moon  act in the same direction, we have the spring tides.—The
planets, so distant that they appear to us little wandering points in the heaven,
yet, by their attraction, affect the motion of our earth in her orbit, quickening
it when she  is approaching them,  retarding it when she is receding.

The attraction is greater the nearer the bodies are to each other; as the light
  of a taper is more intense near to the taper than  at a distance.

  A board of a foot square, represented in fig. 1 by A B, at a certain distance
from a light, supposed  at C, just shadows a board of two feet square, as E
D, at  double distance;  but a board with a side of two feet has four times as
much surface as a board with a side of one foot, for it is not only twice as
high or  long, which would make it double, but twice as broad  also,  which
                                 Fig. 1.
                                            E
makes it quadruple—as a globe of two feet in diameter requires just four
times as much paper to cover it as a globe of  one foot,—and the corner, or
fourth part, E F, of the larger square here  shown is just equal to the whole
of the smaller square  A B.  Light,  therefore, at double distance from its
source, being spread over four times  the space, has onlv one-fourth  of the
intensity;  and for a similar reason, at thrice the distance it has only a ninth
part, at four times a sixteenth part, and so  on.   Now light, heat, attraction,
sound, and, indeed, every influence from a central point, are found to decrease
in the proportion here illustrated, viz., as the surface of squares which shadow
one  another increases.   The technical expression  is,  "the intensity is in-
versely as the square of the distance;" (the distances being estimated from
the centres of attraction or radiation)  or one-fourth part as strong at double
distance, four times as strong at half distance, and in a corresponding manner
for all other distances.
  Accordingly, what weighs 1,000 lbs. at the sea-shore, weighs five lbs. less
at the top of a mountain of a certain height, or when raised in a balloon—as
is proved  experimentally by a spring balance, or other means;—and at the
distance of the moon, the weight, or force towards the earth, of 1,000 lbs., is
diminished to five ounces, as is proved by astronomical tests.

Attraction has received different names as it is found acting under different
  circumstances.  The chief distinctions are  Gravitation, ^Cohesion, Capil-
  lary and* Chemical attractions.

Gravitation is the name given  to it when acting at sensible distances, as  in
  the cases of the moon lifting  the tides—the sun and earth attracting each 
COHESIVE ATTRACTION.
27
   other—a  stone falling, &c.  Most of the facts enumerated at  page 25,
   belong to this head.

Cohesion is the  name given, when it is acting at very short distances,  as in
   keeping the atoms of a mass together.
   It might appear at first sight that it cannot be the same cause which draws
a piece of iron to  the earth with the  moderate force called its weight, and
which maintains the constituent atoms of the iron in such strong cohesion;
but when we recollect that attraction is stronger as the substances are nearer
to each other, the difficulty is  met.   Atoms very nearly in contact may be a
million times nearer to each other than when  only a quarter of an inch apart,
and therefore when the  heat among the atoms of any mass allows them to
approach very near, they should attract mutually  with great force.
   If,  then, the surfaces of the bodies were not in general so very rough and
irregular,  that,  when  applied  to each  other, they can touch only in a few
points of the million, perhaps, which  each surface contains,  bodies would
be invariably sticking together or cohering by any accidental contact.  The
effect of  artificially smoothing the  touching surfaces  is seen in the following
examples:—we may remark,  however, that  besides irregularity of surface,
there is another reason,  explained a little farther on, which prevents the
cohesion.
   Similar portions being cut off with a clean knife from two leaden bullets,
and the fresh surfaces being brought  into contact with  a slight turning  pres-
sure,  the bullets cohere, almost as if they had been originally cast in one
piece.
   Fresh-cut surfaces of  India-rubber or caoutchouc cohere in a similar  way.
We may hence make elastic air-tight tubes, by cutting off the edges of a strip
of India-rubber and bringing the cut surfaces into contact by winding the strip
spirally round any small  rod or cylinder, and fixing  it there for a time  with
tape or cord.
   Two pieces of perfectly smooth plate glass or marble, laid upon each other,
adhere with great force: and  so indeed do most well-polished flat surfaces.

Cohesion between a solid and liquid, and between the particles of a liquid
   among themselves,  is seen in the following instances.

   A flat piece of glass, balanced at the end of a weighing  beam, and  then
allowed to come into contact with water, adheres to the water,  and  with
much more  force than the weight of water remaining upon it when again
forcibly raised.   If there were not cohesion  or  attraction of the water par-
ticles among themselves, as well  as to the  glass, the latter could only be
held  down  by the weight of the  water which directly adhered  to it. In
pouring water from a  mug or bottle-lip, the water does  not at once fall per-
pendicular, but runs down along the  inclined outside of the vessel; chiefly
in consequence of the attraction between this and  the water; hence the dif-
ficulty of pouring from a vessel which has not a projecting lip.
   The particles of water cohere among themselves  in  a degree which
causes small needles gently laid on the surface to float:—the weight of the
needles is not sufficient to overcome the cohesion of the water surface.
   For the same  reason many light insects  can walk upon the surface of wa-
ter without being wetted.
   It is chiefly  the  different force  of the attraction of cohesion in different 
28
CONSTITUTION OF MASSES.
liquids that causes their drops or gutts from the lip of a phial to be of different
magnitude.  Sixty drops  of  water  fill the same measure as 100 drops of
laudanum from a lip of the same size.
  In a larger  mass of liquid, the  attraction which, if acting alone, would
draw  the particles into the form of a distinct  globe, yields to  that which
draws them towards the centre of the earth, and therefore the liquid assumes,
more  or  less completely,  what is called  the level  surface, that  is to say, a
surface corresponding with the general surface of the globe of the earth.

Attraction is called capillary  when it acts between a liquid and  the interior
  of a solid, which is tubular or porous.

  When an open  glass tube is partially immersed in water, the water within
it stands  above the level of that on the outside; and the difference of  level
is  greater as the tube is  less, because in small  tubes, the glass  all round
being nearer to the raised water, attracts it more powerfully.
  Between two plates of glass standing near to  each other, with  their lower
edges in  water, a  similar rising of water  will occur; and if they are closer
atone  perpendicular edge than at  the other, the surface  of the suspended
water will be  higher there.  The two plates of glass  in such a case are
found to be drawn towards each other by  the interposed waters with a certain
force as happens also to glass beads, or other small bodies, floating in water
with their surfaces so near to  each  other  at the water's edge, that the water
may rise between them,—and the nearer  they approach, the higher the wa-
ter rises, and the more strongly it attracts.
  Water, ink  or oil, coming  in contact with the edge of a book, is rapidly
absorbed far inwards among the leaves.
  A piece of sponge or a lump of sugar touching water by its lowest corner,
soon becomes moistened throughout.
  The wick of a  lamp  lifts the oil to supply the flame, from two to three
inches below it.
  A mass of cotton thread hanging over the edge of a glass  from the water
within it will empty it as a syphon would.  A  towel  will empty a basin of
water in  the same way.
  Dry wedges of wood driven into a groove formed round a pillar of stone,
on being moistened, will swell so as to rive off the portion from the block.
In some  quarries  of Germany, mill-stones are thus cut from the rock.
  An immense weight or mass suspended by a  dry rope may be raised a
little way, by merely wetting  the rope;—the moisture imbibed  by capillary
attraction into  the substance of the rope causes it to swell laterally, and to
become shorter.
  At one time, the small vessels of  vegetables were supposed to raise the sap
from the roots, by capillary attraction ; but  this  is  known now to be chiefly
an action of vegetable life.

Attraction has  received the name of chemical attraction,  or affinity, when
  it unites the atoms of two or more distinct  substances into one  perfect
  compound.

  There are about fifty substances in nature which appear, in the present
state of science, distinct from each other, and are therefore called  kinds of
matter:  such as the various metals, sulphur, phosphorus,  &c;  but whether
these are in truth, originally and essentially different or are only one simple 
CAPILLARY ATTRACTION.                    29
primordial matter, modified by  circumstances  as  yet  unknown  to us, we
cannot  at present positively determine.   Diamond and  pure black  carbon
are the  same substance only with different arrangement of atoms ; and steel,
which in the soft state the graver cuts as it would copper or silver, is exactly
the same  substance  as  when, after being tempered by heating and sudden
cooling, it has become as hard nearly as diamond itself.  Yet these differences
are more  striking than appear between some substances, which we now ac-
count essentially distinct.
   It is found, however, that the  atoms of what we call different  substances
will not cohere and unite indifferently, to form masses, as atoms of the same
kind do,—there being singular  preferences and dislikes  among  them, if it
may be so expressed, or affinities as the chemists term it; and when atoms
of two  kinds do combine, the resulting compound generally loses  all resem-
blance to either of the elements.—Thus:
   Sulphuric acid will unite with copper and form a beautiful translucent blue
salt;  with iron it will form a green salt; and if a  piece of iron be thrown
into a solution of the copper salt, the  acid will  immediately let fall the cop-
per, and take up or dissolve the iron.—Sulphuric acid will not unite with or
dissolve gold at all.—Quicksilver and sulphur unite in certain proportions and
form the paint called vermilion :  in other proportions they form the black mass
called Ethiops Mineral.—Lead with oxygen absorbed from the atmosphere or
other source, forms what  is called red lead, used by painters.—Sea-sand or
flint, and the substance called soda, when heated together, unite and form that
most useful substance called glass.—Certain proportions of  sulphur and of
iron combine and produce those beautiful cubes of  pyrites or  gold-like metal
which are seen in slate.  Chemical attraction operating thus, does  not, in the
slightest degree, interfere with general attraction or gravity, for  every chemical
compound weighs just as much as its elements taken separately.
   The  history and classification of such facts  connected  with the combina-
tions and analysis of different substances, constitute the  science of chemistry,
so attractive and so useful.  It explains how the fifty kinds of matter above
alluded  to, by variously combining, form the endless diversity of bodies which
constitute, as far as  it has yet been explored, the mass of our globe.   The
reasons  of these various  modifications of attraction are  yet much  hidden
from us.
   It is a remarkable  truth, that when different substances combine in the way
now described, the proportions  of the ingredients are always uniform, and
such as  to lead to the conclusion,  that for every atom  present, of one  substance,
there is exactly one, or two, or  three, &c. of the other;  so that, if there be
ten atoms of one substance, there are exactly ten, or twenty, &c. of the other,
but never  an intermediate number, as 13 or 23 to  10, for then a  particle of
the compound would consist of one atom of the first, and of  one  and three-
tenths, or  two and three-tenths, &c. of the second  substance,  an absurdity if
the atom  be indivisible.   For instance, a certain number of atoms of quick-
silver, which weigh twenty-five grains, combine with a certain number of atoms
of sulphur, weighing two  grains, and form a black compound  called Ethiops
Mineral, or black sulphuret of mercury;  and if a little more of either ingre-
dient be added, it lies as a foreign mixture in the sulphuret of mercury; but if
just as much more sulphur be added as at first, so that there may be  two atoms
of it, instead of one, in every particle of the compound, a perfect combination
of the whole will take place, and a new substance will appear which we call
vermilion.   Many elementary substances will only unite  with each other in
one proportion, so that any two  such substances form only one compound : 
30
CONSTITUTION OF MASSES.
but others unite in several proportions, so that several distinct compounds
arise out of the same two elements.
  It thus appears, that although we do not know the exact number of atoms
in a given  quantity of any substance,—whether, for instance, a grain of sul-
phuret of mercury has more or less than a million of them ; still, as we know
that in that grain there are just as many atoms of sulphur as of mercury, and
that the weight of the whole sulphur to that of the whole mercury is as two
to twenty-five, we know that the single atoms must have the same relation,
or that the atom of mercury  is 12^ times as heavy as that of sulphur.
  Tables have been formed exhibiting the relative weights of the  atoms of
different substances ; and the number standing opposite to each substance is
called its chemical equivalent,—that is to say, the weight of its atom in rela-
tion to the weight of the atom of  some other  substance chosen as a standard.
The equivalent of a compound substance depends of course both on the equi-
valents of  the  ingredients, and on the number of atoms existing in one inte-
grant  particle of the compound.
  There is  no such thing as an atom of vermilion, or of any other  com-
pound, for the  ultimate  molecule or particle must contain at least one atom of
the respective  ingredients.
  The facts  of the peculiarities and constancy of chemical unions are among
the strongest arguments for the existence of similar ultimate atoms.

Besides the  simple cases of  attraction now explained, there are two curious
  modifications, called   electrical  and magnetical  attractions,  which, from
  their peculiarities, are reserved for consideration in a future division of
  this work.
"Atoms are more or less close, according to the  quantity or Repulsion of
  heat among them; hence the forms of solid, fluid, air, 4*c."   (Read  the
  Analysis,  p. 22.)
  Were there in the universe only atoms  and attraction, as  hitherto  ex-
plained, the whole  material of creation would rush  into close contact, forming
one huge solid mass of stillness and death.   But there is also heat or caloric,
which counteracts attraction,  and singularly modifies the  results.  It has been
described by some as a most subtile fluid, pervading all  things, somewhat as
wafer pervades a sponge; others have accounted it merely a vibration among
the atoms.  The truth is, that we know  little more of heat as a cause of
repulsion than  of gravity as a cause of attraction : but we can study and clas-
sify most accurately the phenomena of both.
  When a continued addition of heat is made to any body, it  gradually in-
creases the mutual distance of the constituent atoms, or  dilates the  body.   A
solid thus is first enlarged and softened; then melted or  fused, that is to say,
reduced to J.he state of liquid, as  the cohesive  attraction is overcome; and
lastly, the atoms are repelled to still  greater  distances,  so that the substance
is converted into elastic fluid  or air.  Abstraction of heat from such air causes
return of states in the reverse order.
  Thus ice when  heated becomes water, and the water when farther heated
becomes steam : the steam when cooled again  becomes water as before and
the water  when cooled becomes  ice.  Ice, water and  steam, therefore, are
three  forms  or states of the same substance—one of the most common in
nature, being the material of the ocean.
   Other substances are similarly affected  by heat, but  as all have different
relations to it,  some requiring much for liquefaction, and some very little, we 
LIQUID AND AIR.
31
have that  beautiful variety of solids, liquids  and airs, which constitutes our
external nature.
   Dilatation.—A rod of iron, which, when cold, will pass through a certain
opening, and will lie lengthwise between two fixed points, when heated, be-
comes too thick and too long to do either.—For accurate mensuration, there-
fore, rods or chains used as the measure, must either be at a given temperature,
or due allowance must be made for the difference.
   The walls of a building, under the pressure of a heavy roof, had begun  to
bulge out so as to threaten  its stability.   No force tried was sufficient to re-
store them to perpendicularity, until the idea occurred of using the contracting
force of cooling iron.  The opposite walls were then connected by a number
of iron bars, passing through both, and having nuts to screw close to the wall,
upon their projecting ends, of which bars one-half were heated at a  time,
viz., every second or alternate  bar, by lamps placed under them,  and while
lengthened in consequence,  and projecting farther beyond  the wall, their nuts
were again screwed close up ; so that, on cooling and contracting, they pulled
the wall in a degree back to its place.   The nuts of the  second set of bars
being then screwed home, the other were again heated, and advanced the
object as much as the first; and so on, until  the object was accomplished.
   The iron rim of a coach  wheel, when heated, goes on loosely and easily,
but when afterwards cooled, it binds the wheel most tightly, giving remark-
able firmness and strength.
   Iron hoops on masts and casks are made to bind in a  similar manner.
   The common thermometer for measuring degrees of heat, is a glass bulb,
filled with mercury or other fluid, and  having a narrow tube rising from  it,
into which the fluid, on being expanded  by heat, ascends, and so  marks the
degree.
   A bladder not quite full of cold air, on being heated, becomes tense, and if
weak, may even be burst.
   Liquid and Air.—A piece of gold, lead,  pitch, ice,  sulphur, or of  other
thing, if sufficiently heated, melts or becomes liquid;  each substance,  how-
ever, requiring a different degree of heat—gold requires 5,000 degrees, lead
600, ice 32, and so  forth ; and if the heating be afterwards  continued, most
things at certain higher temperatures suddenly expand  again to many times
the liquid volume, and become aeriform fluids.
   The conversion of water into steam is familiarly known to all.  One pint
of water driven off as steam from the boiler of a low-pressure steam-engine,
fills a space  of nearly 2,000 pints, and raises the piston through this, with a
force of many thousands of pounds : it immediately afterwards appears  again
in the cold condenser as  a pint of water.
   Six times as much heat is required to convert a pint of water into steam,
as to raise it from an ordinary temperature to that of boiling; but the steam,
by occupying nearly 2,000 times  the space of the water, proves  that  heat
merely produces a repulsion among the particles, and  by no means fills up
the interstices.   The steam rising from boiling water does not appear to the
thermometer hotter than the water itself;  and  hence it was that Dr. Black,
whose genius shed so much light on this part of knowledge, gave the excess
of heat the name  of latent heat.
   The latent heat of common air is made sensible in the match syringe.   In
this, which is close  at the bottom, the piston is driven  down quickly and
strongly, so as to compress very much  the  air which is underneath  it, and the
heat then condensed with the air is sufficiently intense to  light a small piece
of tinder attached to the bottom  of the  piston. 
32
CONSTITUTION OF MASSES.
   Not only are spirits, aethers, oils, &c, convertible, as water is into aeriform
fluid, but also sulphur, phosphorus, mercury, and, indeed, all the metals and
elementary substances ;—some of them, however, requiring heats of great
intensity.
   The varieties of form, then, in the bodies on the face of this earth, may be
considered accidental, as dependent on the temperature of the earth, and do
not mark the permanent nature of the substances.
   In the planet Mercury, which is near the sun, resin, tallow, wax,  and many
vegetable substances deemed by us naturally solid, would all be liquid, as oil
is with us ; and a certain mixture of tin, zinc and lead, which with us is solid
at common temperatures, but melts in boiling water, would there be always
liquid like our quicksilver.   Our water, oils, and spirits, would there be in a
state of steam or air,  and  could not be  known as liquids, except by cooling
processes and compression, such as we have lately learned to use for reduc-
ing our different airs to  the form of liquids.
   Again, in the cold  planet Herschel, which is  nineteen times farther from
the sun than our earth is, water, if it exist, can be known only as rock crys-
tal, which fire would have to melt as it does glass with us: our oils would be
as butters or resins, and quicksilver might be hammered as  lead or silver is
with us.
   On our own earth, near the equator,  common sealing-wax will not retain
impressions ; butter is oil in  the day, and a soft solid at night; and  tallow
candles cannot be used.   And near our pole, in winter, the quicksilver from
a broken thermometer is solid metal; water must be melted by fire for use ;
oils are solid, &c.
   To judge, then, of the constitution of nature  aright, we must always take
extended surveys, and not allow prejudice to mislead us, as it did that Eastern
potentate, who put a traveller to  death for saying he had visited remote
northern countries, where water was sometimes  to be seen solid like crystal,
and sometimes white  and fleecy, like feathers.—The ancients believed that
there were just four elements concerned in forming our globe, with all upon
it, viz., earth, water, air and fire.   What a contrast between former and pre-
sent knowlege!


                    Repulsion without sensible Heat.
   As we stated in a former paragraph  that, besides general attraction, under
the names gravitation,  cohesion, capillary and chemical attraction, there are
modifications which have the names of electrical and magnetical attractions;
so we have now  to remark, that, besides  the general repulsion of heat just
described, there are peculiarities which  we call electrical and magnetical re-
pulsions.  Whether these depend  altogether on  different causes, or are only
modifications of effect from the same cause, we cannot yet positively decide.
   And it is a curious fact connected with the subject, that there seems to be a
film of repulsion, so to express it, covering the general surfaces of all bodies,
and preventing their meeting in  absolute contact, even when they appear to
the human eye so to meet.   Were it not for this, things would be  constantly
approaching so closely to each other, that they  would stick or cohere, in a
way to disturb the common operations of nature.  The followino-  facts'illus-
trate this superficial repulsion, and the  means which art uses to overcome it
for particular purposes.
   Newton found that a  ball of glass, or a watch-glass, laid upon a fiat  surface 
REPULSION OF SURFACES.
33
of glass, does not really touch it and cannot be made to touch it by a force of
even 1,000 pounds to the inch.
   In like  manner, when glass, stone, porcelain, or" indeed almost any body
is  broken, we cannot make the parts cohere again by simply pushing them
together in their former position.  Where a'union. therefore, between separate
masses is  desired, we are compelled to have recourse to various artifices.

A  few cases in which cohesion is  easily affected, were enumerated at page
   27:  the following are other instances of a different kind.
   Gold leaf laid  upon clean steel, and then  forcibly struck by a hammer,
coheres to the steel and gilds it permanently.
   But iron can be made to cohere to iron, only by rendering both pieces red
hot before hammering:—the process is called welding.   Iron and platinum
are the only metals that can be welded.
   Tin and lead, in sheets, pressed together between the strong  rollers of a
flatting-mill, cohere.
   The other metals require to be melted before the superficial repulsion gives
way so as to allow separate quantities to  cohere or run into one mass.  It is
thus, for instance, that gold, silver, lead, &c, are treated.
   In many cases  the substances are not such as can be melted (wood or mar-
ble, for instance,) and then it is necessary to  use some soft glue or cement.
Cements  must have strong attraction for both substances, and, when dry or
cool, must be tenacious in themselves ; solder, paste, common glue, mortar,
&c, are the principal substances of this kind.

"  Certain modifications of attraction produce the subordinate states, called
   crystal, porous, dense, fyc"  (Read the Analysis, page 22.)
   It is a  remarkable  circumstance, that  attraction, in causing the atoms to
cohere so as to form  solid masses, seems not to act  equally all around each
atom, but between certain  sides or parts of one, and corresponding parts of
the adjoining one; so that when  atoms  are allowed to cohere  according to
their natural  tendencies, they always assume a  certain regular arrangement
and form, which we call crystaline.  Because in this circumstance they seem
to resemble magnets, which attract each other only by their poles, the fact
has been  called the polarity of atoms.  It is the cause of several of the pecu-
liarities above enumerated, as elasticity &c.
   " Crystalization" is exemplified in the following particulars :
   Water beginning to freeze, shoots delicate needles across the  surface;
these thicken and interweave until the whole mass has become solid, but the
crystaline arrangement always remains.  In most substances, this arrange-
ment is remarkably proved, by the forms of the surfaces left, when  the mass
is broken.
   Moisture, freezing on the window-pane in winter, exhibits a beautiful vari-
ety of arborescence.
   A flake of snow, viewed in the microscope, is seen to be as symmetrically
formed as a fern  leaf or a swan's feather.
   If a piece of copper be thrown into  a solution of silver in nitric acid, it is
preferred by the  acid to the silver, and is dissolved accordingly:  the silver in
the mean time, during its precipitation or separation, assumes the form of  a
singularly beautiful shrub or tree, resting on the remaining copper as its root.
 This appearance is called the arbor Dianae.
   Any metal which has been melted, when allowed to cool again, slowly and 
34
CONSTITUTION OF MASSES.
 at rest, becomes solid first on the outside of the mass.   If, before the cooling
 be completed, the remaining liquid be poured from within, a curious internal
 crystaline  structure, like grotto work, is seen.   What is called  the grain of
 a metal is  the result of this crystalization.
   Saltpetre, glauber salt, copperas (to use popular names,)  or any other of
 the many neutral  salts, being dissolved  in water, and  the water being then
 allowed slowly to evaporate, reappears in beautiful regular crystals, each salt
 having its  peculiar forms, bounded by perfectly plane and polished surfaces.
 If any such crystal be broken in any part, the broken  surface appears to  the
 microscope as if regular layers of particles had been disturbed, (as we see on
 a larger scale in a broken stack of bricks, or broken pile of shot in a battery
 yard,) and  the defect of the crystal will be exactly filled up by replacing it
 in the evaporating solution—proving that the ultimate  particles are all of the
 same size.
   All the precious stones are crystals, and can be well cut  only parallel to
 their natural faces.
   The basaltic pillars  of the Giant's Causeway in Ireland, and of the Isle cf
 Staffa,  which appears  like a garden supported on magnificent columns in the
 midst of the ocean, are natural crystaline arrangements of particles, equaling
 in regularity and beauty any human work, and in grandeur so far surpassing
 even the Egyptian pyramids, that superstitious conjecture naturally supposed
 them the work of giant architects.
   It would be endless to go on enumerating crystaline masses, for nature's
 forms generally, in the inanimate creation,  as well as in organized bodies, are
 regular and symmetrical;  and what we  see on earth  of broken continents,
 and islands, and rocks, and wild  Alpine scenery, are the effects of subsequent
 convulsions, which have deranged a primitive and natural order.
  Much ingenuity has been employed to  account for the specific forms which
 different crystaline bodies assume; but  the subject is not yet reduced to a
 state fitting it to be a part of this elementary study.  A familiarity with the
 various figures  which the exact science of measures treats of, is required in
 the person who expects to pursue it with pleasure or advantage.  The facts are
 extremely curious, and the scientific investigation of them may ultimately give
 important information respecting the intimate constitution of material nature.
  "Porous."—The crossing of the constituent crystaline needles or plates
 in bodies, causes  them to be porous or full  of small vacant spaces.  In some
 cases these are visible  to the eye, in many  more cases,  they are visible to the
 microscope, and in all, they are  to be proved in some way.
  Owing to the porosity arising from the new arrangement of atoms of solidi-
 fying, water and a very few  other substances become more  bulky in the
 change from the liquid to the solid state.   Water then dilates  with such force
 as to burst the strongest vessels which art can provide,  and in winter to split
 even rocks, where it has been retained in  their crevices;—freezing water thus
 curiously  producing effects which  surpass those  of exploding gunpowder
 This agency of water contributes to the gradual breaking down of our Alpine
 summits, and the  falling of their  destructive fragments into the valleys
  The stone called hydrophane (agate) is opaque, until dipped into water,
 when it absorbs into its pores one-sixth of  its weight of the water and after-
 wards gives passage to light.
  Into crystalized sugar, and various stones, much water will enter without
 increasing the bulk.
  A kind of sandstone, suitably shaped, forms an excellent filter or strainer
for water. 
DENSITY.
35
  Pressure will  force water through the pores of the most solid gold:—as
was seen in the famous Florentine experiment, where a hollow, thick, golden
ball, being filled with water and squeezed, to try the compressibility of water,
was found to perspire all over.

The examples of porosity in animal and vegetable bodies, are, however, the
  most remarkable.
  Bone is a tissue of cells and partitions, as  little solid  as a heap of empty
packing-boxes.
  Wood is a congeries of parallel tubes, like bundles of organ pipes.—It has
lately been proposed to prepare wood for certain purposes, as for making the
great wooden pins or nails used in  ship-building, by squeezing it to half its
lateral bulk between very strong rollers, and  thus making  its density approach
to that of metal.
  A piece of wood sunk to a great depth in  the  ocean, and exposed to the
pressure there, has its pores soon filled with water, and becomes nearly as heavy
as stone.  Thus it was with the boat of a whale-fishing ship, which had been
dragged far under water by a whale, and which, on being afterwards drawn
up, was supposed by  the crew to be bringing a piece of rock with it.
  A piece of cork in a strong close glass vessel nearly full of water, may be
seen floating at the top; but if more water  be then forcibly pumped into the
vessel, the cork will be squeezed and reduced in size, until at last it becomes
heavier than water and sinks.  On water being afterwards allowed to escape,
the cork will resume its bulk and will rise.   A cork  sunk 200 feet  under
water, will never rise again of itself.
  A bottle of fresh water, corked  and let  down thirty or  forty feet into the
sea, often comes  up again with the water  saltish, although the cork be still
in its place:  the explanation  being,  that  the cork,  when far  down, is  so
squeezed as to allow the water to pass in or out by its sides, but on rising,
resumes its former size.

"Density"  or the quantity of atoms which exist in a given space, is very
  different in different substances.
  A cubic inch of lead is forty times heavier than the same  bulk of cork.
Mercury is nearly fourteen times heavier than an equal bulk of water.
  The density must depend on, first, the  size or weight of the individual
atoms; secondly, the degree of porosity just now  explained; and thirdly, the
proximity of the atoms in the more  solid parts  which stand between the pores.
  From many circumstances it appears, that the atoms even of the most solid
bodies are no where  in actual contact, but are retained in their places  by a
balance between attraction and repulsion—thus,
  A body  dilates  or  contracts, according  as heat is added or taken away
from it.
  A weight placed on any upright rod or  pillar,  shortens it and lessens  its
bulk, and if suspended from the bottom, lengthens it and increases its bulk,—
the rod in both cases returning to its former dimensions when  the weight  is
removed.
  When a plank or rod is bent, the atoms on the concave side are, for the
time, approximated, and those on the convex side are drawn more apart.   It
is remarkable  in  solid bodies, not  only how precisely the balance between
attraction and repulsion determines the relative position of the particles, but
also how strongly; for  any farther separation of the particles  is resisted  by 
36
CONSTITUTION OF MASSES.
all the force which we call the tenacity or cohesion of the substance, and any
nearer approach by all the force which we call the hardness or incompressi-
bility.
  Tin and copper, when melted together, to form bronze, occupy less space
by one-fifteenth than when separate:  proving that the atoms of the one are
partially received into what were vacant spaces in the other.   A similar con-
densation is  observed in many other mixtures.  A pound of water  and  a
pound of salt, when mixed, form two pounds of brine, but which has  much
less bulk than the ingredients apart.   So also of a pound of sugar dissolved
in a pound of water.
  Water and liquids generally resist compression very powerfully, but yield
enough to show that  the  particles  are not in contact.  It  is found that at
1,000 fathoms down in the sea the water is compressed by  the superincum-
bent water so as to have bulk about a hundredth part less than it would have
at the surface.
  In aeriform masses  the atoms  are very distant, and  hence the masses are
more easily compressed.  A pint of water, on assuming the  aeriform state,
in which it is called steam, under ordinary pressure,  acquires nearly  2,000
times its former bulk.   A hundred pints of common air may be compressed
into a  pint vessel, as in the chamber of an air-gun; and if the pressure be
much farther increased, the atoms will at last collapse and form a liquid.   The
heat which was contained in such  air, and gave it its form, is squeezed out
in this operation, and becomes sensible all around.
  From  these proofs of the non-contact of the atoms, even in  the  most solid
parts of bodies;  from the very great space obviously occupied by pores—the
mass  often having  no more solidity than a heap of empty  boxes, of which
the apparently solid parts may still be as  porous in a second degree, and so
on; and  from the great readiness with which light  passes  in all  directions
through  dense  bodies like glass, rock crystal, diamond, &c,  it has been
argued that there is so exceedingly little  of really solid matter, even in the
densest mass, that the whole world, if the atoms could be brought into abso-
lute contact, might be received into a nut-shell.   We have as yet no means
of determining exactly what relation this idea  has to truth.

The comparative weights of equal bulks  of different bodies are called their
  specific gravities.

  In thus comparing bodies, it was necessary to choose a standard;  and water,
as being  the substance most easily procurable at all times and in all places,
has been generally adopted.
  The metal called platinum, the  heaviest of known substances, is  about
twenty-two times as heavy as an equal bulk of water, and  is therefore said
to have specific  gravity of 22—gold is nineteen times as heavy—mercury
thirteen and a half—lead eleven—iron eight and a half—copper eio-ht—com-
mon stones about two and a half—woods from half to one and a half—cork
one quarter, &c.

"Hardness" iss  not proportioned, as might be expected, to the density of the
  different bodies, but to the polarity of the atoms in them, that is, to the force
  with which the atoms hold  their places in some particular arrangement.

  Hardness is measured generally by the circumstance  of one body  bein*
capable of scratching another—It  is  here worthy of notice, however, thai 
DENSITY.
37
the powder or dust of a softer body  will often, through an  effect of motion
to be described below, aid in wearing down or polishing one  that is harder.
   Gold, though  soft, is  four times heavier  than  the hard diamond ; and
mercury, which is fluid, is nearly twice as dense as the hardest steel.
   Diamond is the hardest of known substances.  It cuts or scratches  every
other body, and is generally polished by means of its own dust.
   Glass-cutters  use  a point  of  diamond  as  a glass-knife for  dividing and
shaping their panes.
   Common flint also cuts glass, as is proved by the frequent  scribblings on
windows.
   It is  remarkable, that the  preparation of iron, called steel, may  either be
soft like pure iron,.or from being heated and suddenly  cooled,  in the process
called tempering, may become nearly as hard as diamond.  The discoverv
of this fact is, perhaps, second in importance to few discoveries which man
has made; for it has given him all the edge tools and cutting instruments by
which he now moulds every other substance  to his wishes.  A savage will
work for twelve months, with fire and sharp  stones, to fell a great  tree, and
to give it the shape of a canoe;  where a  modern carpenter, with  his tools,
could accomplish the object in a day or two.
   The project has lately been realized of engraving on plates of soft steel
instead of copper, and afterwards tempering the steel to such  hardness, that
it may be used  as a type or die to make its impression, not on paper, but on
other plates of soft steel or of copper; each of which  is then  equal in value
to an original and distinct engraving.  By this means the beautiful produc-
tions of art, instead  of being limited  to  a  comparatively small  number of
copies and  of persons, may be multiplied almost to infinity,  becoming  the
cheap delight of all.

" Elasticity" is  present in a  mass when the  atoms, cohering in a particular
   arrangement only, yield, however, to a certain extent,  when force is applied,
   but move back or regain their  natural  positions on the  force being  with-
   drawn.
   Elastic bodies vary much as to the extent to which they  yield without
breaking, and as to the  degree of perfection with which, after the  bending
or displacement of atoms, they regain their  former state.   India rubber is
extensively elastic, for it yields far;  but it is not perfectly elastic, for  when
stretched much  or  often, it becomes perfectly elongated.   Glass,  again, is
perfectly elastic, for  it will retain no permanent bend; but, unless in very
thin  plates indeed, or in fine threads, it will not bend far without breaking.
   All hard bodies are elastic, as steel, glass, ivory, &c, and  many soft ones,
as caoutchouc,  silk, a harp  string, &c.   The aeriform bodies are all per-
fectly elastic, as is rudely seen in a bladder filled with air, when squeezed,
and allowed to expand again;  and they will change volume to a  very great
extent.   Liquids also are perfectly elastic, but to a small extent.
   A  good steel  sword may be bent until its ends meet,  and yet  when allowed
will return to perfect straightness.
   A  rod of  bad steel, or of other metal, will  be broken in bending, or will
retain a bend.
   An ivory ball, let fall on a  marble slab, rebounds, owing to the  great elas-
ticity of both bodies, nearly to the height  from which  it fell, and no mark is
left on either.   If the slab be  wet, it is seen that the ivory or marble, or both
had yielded considerably at the point of  contact,  for a circular surface of
     4 
38
CONSTITUTION OF MASSES.
some extent on the slab is found dried by the blow.  The sudden expulsion
of air from between the meeting surfaces might contribute to the effect, but
the result is very nearly the same when the experiment is made in a vacuum.
Billiard-balls scarcely lose even their  polish by  long  wear, although the
touching parts yield at every stroke.
  A marble chimney piece long supported by its ends, is found  at last to be
bent downwards in the middle;  and the bend is permanent.
  A steel watch spring, although so much and so constantly bent, resumes
its original form whenlreed at the end  of a century; but occasionally, with-
out evident cause, while in action, it will suddenly give way.
  Elasticity is  a property of bodies of great utility to man, as in his time-
pieces, carriage-springs, gunlocks,  <fec. &c.

" Briltleness" designates that constitution of a body where, with hardness,
  and elasticity perfect as far as it goes, the cohesion among the atoms exists
  within such narrow limits that a very slight change  of position  or increase
  of  distance among them is  sufficient to  produce a  rupture.   A compara-
  tively slight force, therefore, if sudden, breaks them.   It belongs  to most
  very hard bodies.
  Glass scratches  an iron hammer, proving that it is  harder than iron—yet
glass  is  the very type of fragility;  yielding  to the stroke of soft wood, or,
indeed, of almost any thing which can give a blow.
  Steel, when  tempered so as to be very hard, becomes brittle also.   The
steel chisels and tools with which artificers now shape the stones and metals
as they formerly did wood, require, of course, to be exceedingly hard; but
they thereby lose  in regard to the  extent of their elasticity, and hence are
frequently broken.  Cast  iron,  which is  much harder than  malleable or
wrought iron, is very brittle, while soft iron and steel are  the toughest things
in nature.

" Malleable," or  reducible into  thin plates or leaves by  hammering.  This
  property, in  opposition to elasticity and  britlleness,  belongs to  bodies
  whose  atoms cohere equally  in  whatever relative situations they happen
  to be, and therefore yield to force, and shift about among each other, with-
  out fracture or change of property, almost like the atoms of a  fluid.
  Gold is remarkably malleable, for it  may be reduced to leaves of the  thin-
ness of 360,000 to the inch, or of 1,800 to a sheet of common paper.  For
gold-beaters the metal is first formed into rods,  these are  afterwards rolled or
flattened into ribbons; the ribbon is cut into portions, which are extended by
hammering to  great  breadth and thinness, and which, being  again divided
into portions, are hammered and extended to  the thinness described.
  Silver,  copper and tin may also be hammered until very thin.   Most other
metals crack or are torn before the operation is carried  far ; and some on
being struck, are broken at once, almost like  glass.

"Ductile" or susceptible of being drawn into wire.  One might expect mal-
. leability and ductility to belong  to the  same substances and in the same
  degrees—but they do not.  In  ductile  substances, as  in  malleable, the
  atoms seem to have no more fixed relation  of position than in a liquid, but
  yet they cohere very strongly.

   One end of a rod of iron,  or other ductile  metal, being reduced in size so
as to pass through an opening in a plate of steel, is seized by strong nippers 
DENSITY.
39
 on the other side of the plate, and  the whole rod  is drawn  through.   It is
 thus reduced of course, to the size of the opening, and is lengthened in a like
 proportion.   By repeating the operation through smaller holes successively,
 a wire may at last be obtained of the size  of a hair.
   Dr. Wollaston's ingenuity produced platinum  wire finer  than  spider's
 thread.   He filled  a space in the axis of a silver wire with  small platinum
 wire.  He  then drew or  reduced the compound piece to the  smallest wire
 possible, and on dissolving the silver from the outside, he exposed to view the
 delicate filament of platinum.
   The order in which metals may be ranged  according to their ductility is,
 platinum, silver, iron,  copper, gold, &c.
   Melted glass has great ductility.  The workers draw or spin it into threads
 by merely attaching a  point, pulled out from  the mass, to the circumference
 of a turning-wheel.   A uniform thread then  continues to  be drawn out and
 wound upon the wheel, at a rate of 1,000 yards or more per hour.   This
 glass thread, when lying together in quantities, resembles beautiful white hair,
 and when cut in bunches, it serves as an ornament to the female head, wav-
ing in the air like the  delicate plume of a bird of paradise.

 "Pliant."   In bodies  distinguished by this  title, the cohesion  is not destroyed
   by considerable change of direction among the particles, but there is little
   elasticity, and unlike  what happens in  a  ductile mass, the same  atoms
   always remain together.
  Of all pliant things, the chief are animal and  vegetable fibres and membranes
 —as silk, bladder, lint, hemp, &c. &c.

 "Tenacity" means  the force of cohesion among the atoms of any mass.  It
   belongs more or less to all solids,  and even to liquids.
   This property varies much in different substances.  Iron and its modifica-
 tion called steel possess it in the most remarkable degree.
   The following table shows the comparative tenacity, or strength to resist
pulling, of certain metals and woods.  Supposing similar wires or rods of each
to  be used, and of such a size that the surface of a broken end or cross-section
would be the one-thousandth of a square inch, the  weights supported would
be nearly as follows:
METALS.		
Cast Steel .		. 134 lbs.
Best wrough	t iron	. 70
Cast iron		. 19
Copper		19
Platinum		16
Silver .		11
Gold .		9
Tin .		5
Lead .		2
WOODS.		
Teak .		13
Oak .		12
Beech .		12 1-2
Ash .		14
Deal .		11
 
40
CONSTITUTION OF MASSES.
   Iron, compared in this way, is five or six times stronger than oak.
   Steel wire will support about 39,000 feet, that is, 7  1-2 miles of its own
length.
   Certain animal substances have great tenacity; as—the silk-worm's thread,
which is our strongest connecting or sewing material, and  has such flexibility
united with its strength—the ligaments and tendons of the animal body, pos-
sessing  at once such admirable strength, elasticity and pliancy:  these, when
dried, and  otherwise prepared, constituted the tough bow-strings of our re-
mote fore-fathers—the hair or  wool of animals, twisted, into threads, and
worked into strong and beautiful textures of the loom—strips of animal intes-
tine prepared and twisted, forming the cords of harp and violin, and in strength
and uniformity rivaling the steel wires of keyed instruments.
   The gradual discovery of substances possessed of strong  tenacity and which
man could  yet easily mould to his purposes, has been of great importance to
his progress in the arts of life.  The place of the hempen  cordage of European
navies is still held in China by twisted canes and strips of bamboo; and even
the hempen cable of Europe, so great  an  improvement on former usage, is
now rapidly giving way to the  more complete and commodious security of
the iron chain—of which the material to  our remote ancestors existed only
as a useless stone or earth.   And what a  magnificent spectacle is it, at the
present  day, to behold chains of tough iron stretched high  across a channel
of the ocean, as at  the  Menai  Strait, between Anglesea  and England, and
supporting there an  admirable  bridge-road of safety along which crowded
processions may pour, regardless of the deep below, or of the storm;  while
under it, ships with full  sails spread pursue their  course, unmolesting and
unmolested! 
APPENDIX
TO PART  I. —SECTION I.
BY THE AMERICAN EDITOR.
  If the reader has studied the preceding section with attention he is prepared
to understand the following propositions.
  Prop.  1.—Matter is endowed with properties.
  Prop.  2.—The properties of matter are distinguishable into two classes,
first, those which are general or belong to all kinds of matter, and second, those
which axe peculiar or belong only to particular kinds of matter.
  Prop. 3.—The general properties of matter are, indestructibility (p. 22;)
extension or the property of occupying a portion of space (p. 24;) divisibility
(p. 23;)  impenetrability (p. 24;) and inertia, (p. 42.)
  Prop.  4.—Every particle of matter, and also all masses, have a mutual at-
traction for one another, or endeavour to get near each other; and this attrac-
tion is inversely as the squares of the distances.
  Attractions may be primarily distributed into two classes ; one consisting
of those  which  exist between  the molecules or constitutent parts of bodies,
and  the other between the bodies themselves.   The former are called mole-
cular or atomic attractions, the later gravitation  (p. 26:) of the former there
are several  varieties, 1st, cohesion (p. 27;) when  this variety of molecular
attraction is exhibited by liquids pervading the interstices of porous bodies,
ascending in crevices or in the pores of small tubes, it is called capillary at-
traction,  (p. 28.)  The other varieties of molecular attractions are affinity or
chemical attraction (p. 28,) and electric and magnetic attraction, (p. 30.)
  Prop.  5.—Attraction of gravitation, or that force by which all the masses
of matter tend  towards each other, is exerted at all distances.
  Prop.  6.—Attraction of cohesion acts only within certain limits, and where
its sphere of attraction ends, a  repulsive force begins.
  Prop.  7.—Repulsion, except when dependent on electricity or magnetism,
is owing to the  presence of heat, which latter pervades all matter.
  Prop.  8.—The particles of matter are more or less close, according to the
quantity  of heat, among them ; but they are never in actual  contact (p. 30—
31,) and hence porosity is usually considered  as  one of the properties of
matter.
  Prop 9.—The peculiar properties of  matter are  density (p. 35,) hardness
(p. 36,)  elasticity (p. 37,)  brittleness (p. 38,) malleability (p. 38,) ductility
(p.  38,)  pliability (p. 39,) tenacity,  (p. 39,) &c. 
42
MOTIONS AND FORCES.
    SECTION II.—THE MOTION OR PHENOMENA OF THE
                            UNIVERSE.*

                      ANALYSIS  OF THE SECTION.

The bodies or masses composing the universe may be at rest or in motion,
  and to change any present state, force proportioned to  the  quantity of
  the body and to the degree of change, is equally required, whether to give
  motion, to take it away, or to bend it:—a truth expressed by saying that
  matter has inertia, or figuratively, a  stubbornness.  Uniform  straight
  motion, then, is as naturally permanent as rest.  And the motion in any
  body, measured by its velocity quantity of matter and direction, is the
  measure of the amount and direction of any single force or of any com-
  bination of forces, ivhich has produced  it, as also of the force or momen-
  tum which the body can exhibit again when opposed or made to act itself
  as a cause of some  new motion.
The great forces of nature, referred to by the two words attraction and
  repulsion, acting upon inert  matter, produce the equable, accelerated,
  retarded and bent motions  which constitute the great phenomena of the
  vniverse.—Tides, currents, winds, falling bodies, fyc, exemplify attrac-
  tion.—Explosion, steam collision, fyc,  exemplify  repulsion.  And as in
  every  case of attraction or repulsion two  masses  at  least must be con-
  cerned, there is no motion or action in the universe, ivithout an equal and
  opposite motion or re-action.
Motion"
   Is the term  applied to the  phenomenon of the  changing of place among
bodies.
   Were there no  motion in  the universe it  would be dead.   It would be
without the rising or setting sun, or river-flow, or moving winds, or sound,
or light, or animal  existence.
   To understand the nature and laws of the motions or changes which are
going on around him, is to man of the greatest importance, as it enables him
to adapt his actions to what is coming in  futurity, and often to  interfere so
as to control futurity, for his special purposes.
   Motion, in any particular case, is described by referring to certain objects
to mark place, and to some other motion chosen as the standard  of velocity.
—A man  sitting on the deck of a sailing ship,  has common  motion with
the ship:  if walking on the  deck, he  has  relative  motion to the ship; but
if he be walking towards  the stern,  just as  fast  as the  ship advances, he
is at rest relatively to the bottom or shore.  A ship  sailing  against the tide,
just as fast as the tide runs, is as  much at  rest relatively both  to the  earth
and water as if she were at anchor.   Absolute motion is  that which is rela-
tive to the whole universe, or rather to the space in  which the universe ex-
ists.   We have no means of ascertaining  such: for  although we know how
fast our globe whirls upon  its axis and wheels round the sun,  we have no
measure of the motion  of the sun himself—revolving possibly round some
* The reader should here re-peruse the title and Analysis at pa°-e 22. 
MOTION.
43
more distant centre, but  almost  certainly  having a progress in  space, and
carrying all the planets along with him.
   Motion is called rapid, as  that of lightning—slow, as that of the sun-dial
shadow ; both terms having reference to the ordinary intermediate velocities
observed upon earth.  It is called straight or rectilineal, in the apparent path
of a falling body—bent, or curvilinear, in the track of a body thrown ob-
liquely—accelerated, in a stone falling to the earth—retarded, in  the stone
thrown upwards while rising to  the point  where it stops  before again de-
scending.

" Owing to the  inertia of bodies, force is equally required to impart motion
          and to take it away"   (Read again the last Analysis.)

   If a man put  his hand to the crank of  a heavy fly-wheel or grindstone, to
turn it, he  experiences a certain resistance, which, however, gradually yields
to his effort, and he leaves the wheel whirling with  velocity  proportioned to
the effort.   If he then puts out his hand  again to stop the wheel,  he experi-
ences an opposite but similar  resistance, which, however, as before, gradually
yields, and he brings the  wheel to  rest.  In the second case the effort re-
quired of him is less than in the first, by reason of the friction of the turning
axle, and the resistance  of the air in which the wheel moves,—obstructions
which,  when he was giving motion, opposed him, but when  taking it away
assisted him.  That these obstructions cause the whole difference in  such a
case, and that they are  the great reasons why all ordinary  motions on earth
seem to tend of  themselves to cease, will be shown in subsequent pages.   It
is the resistance overcome in moving the wheel or in stopping it, and occa-
sioning an  expenditure of force proportioned to the  mass and to the degree
of change of state, which is  called the inertia of the mass, or the vis iner-
tix, and sometimes, to help the conception of the student, the stubbornness,
sluggishness, or inactivity ;  but no one "of these words can originally sug-
gest to the  mind all that is intended to be conveyed.
   An exact measure of  the amount of inertia is contained in the familiar fact
that a body let fall near the  surface of the earth, falls rather more than  16
feet  in the first second of time,—the well-known  weight  of the  body,  or
force of terrestrial attraction  acting upon it for one second, being just suffi-
cient to overcome its inertia to the extent stated.  Were the inertia of matter
only half of what it is, a body near the earth  would fall 32 feet in the second,
instead of 16,  as it  equally would,  if, with present  inertia, the attraction  of
the earth were  doubled.  And were there  no inertia, it would fall or pass
through any height, however great, in one  instant.   As the  amount of iner-
tia thus determines  the  amount of other force  required to  give motion to a
mass, so does it  determine the amount of force  required to destroy motion
in a mass.   A  heavy cannon-ball, if wanting inertia, might be dispatched
with the speed of lightning by the slightest force, but then the stiffness of a
stalk of corn would suffice to arrest it; and while the ball, with the inertia
now existing, takes the force of  pounds of gunpowder to  give it  its usual
motion, it may not be stopped, even by the  cohesion of a block of granite,
which accordingly it shivers  to pieces.  The  numerous examples now  to
follow  will  prove the  immense importance  of inertia in the general opera-
tions of nature.
   When the sails of a ship are first spread  to receive the force or impulse  of
the wind, the vessel does  not acquire  her full speed at once, but slowly, as
the continuing force gradually overcomes  the  inertia of her mass.  When the 
44
MOTIONS AND FORCES.
§ails  are afterwards taken in, she  does not lose her motion at once, but
slowly again, as the continued resisting force of the water destroys it.
   Horses must make  a greater effort at first to put a carriage in motion  than
to maintain the motion afterwards.   And a strong effort is required to stop a
moving carriage.   When a carriage, of which  the body hangs from  springs,
is first moved, the body appears  to  fall back, and a person within seems to
be suddenly forced against the back cushion.   When the carriage is stopped
again, the body swings forward, and if the stoppage be very sudden, a care-
less passenger may unwittingly pop his head through a front glass.   These
particulars prove the inertia, first of rest, and secondly of motion.
   A man standing carelessly at the stern of  a boat, when the boat begins to
move, falls into the water behind; because his  feet are pulled forward,  while
the inertia of his  body keeps it where it was,  and therefore behind its sup-
port.   The stopping of a boat, again, illustrates  the opposite inertia of motion,
by the man's falling forward.
   An awkward rider on horseback may be left  behind, when his horse starts
forward suddenly ; or  may be thrown off on one side by the horse starting to
the other.   A horse at  speed, stopping suddenly, often sends his  cavalier
over his ears—as was  mortifyingly experienced by a coxcomb, who, on an old
cavalry horse, chose to canter along  a foot-path, to the annoyance of the com-
pany, and whose horse, on hearing  the word halt loudly addressed to it by a
waggish officer of the regiment, who  happened to be there  and to recognize
it, suddenly stood, and got rid of its load.   The mind  or  will of the  beau
had sinned against the law of propriety, but  his  body  very  perfectly obeyed
the laws of inertia and gravity, by  shooting  forward in a parabolic curve to
the earth.
   A young man not yet accustomed to the whip, drove his phaeton against
a heavy coach on the road, and then to his father foolishly excused his awk-
wardness, in a way which  led to a  prosecution of the coachman for furious
driving.  At the trial, the youth and his servant both deposed that the shock
of the coach was  such as to throw them over their horses' heads, and thus
lost the cause, by  unconsciously proving, that the faulty velocity was  their
own.
   A man jumping from a carriage at speed is  in great danger of falling for-
ward, when his feet reach the ground; for  his  body has as much  forward
velocity as if he had been running with the speed of the carriage; and unless
he advance  his feet like a.running  man, to support his advancing body, he
must  as certainly be dashed to the  ground, as  a  runner  whose feet  are sud-
denly arrested.  A  man  racing who receives  a  signal to  stop, and a man
jumping from  a flying vehicle, must check their  motion nearly in the same
way.
   A person wishing to leap over a ditch or chasm, first makes a run, that the
motion thereby acquired  may help  him over.   A standing leap falls  much
short of a running one.
   An African traveller saw himself  pursued by a tiger, from which he could
not escape by  running; but perceiving that the animal was  watching an op-
portunity to seize him by its usual  spring or  leap, he artfully led  it to where
the plain terminated in a  precipice  hidden by brush-wood, and he had  just
time to transfer his hat and cloak to a bush, and to retreat a few paces when
the tiger sprung upon the bush, and by the motal inertia of its body  was
carried over the precipice, and destroyed.
   From a  glass of water suddenly  pushed forward on a table, the water  is
spilt or left behind; but if the glass be already in  motion, as  when carried by 
MOTION.
45
a person walking, and if it then be suddenly stopped by coming against an
impediment, the water is thrown or spilt forward.
  A servant carrying a tray of glasses or china in the dark, and coming sud-
denly against an obstacle, hears all his freight slipping forward and crashing
at his feet: and a too hurried departure with such a load causes equal destruc-
tion, on the opposite side.
  The actions of beating a coat or a carpet with a cane, to expel  the dust; of
shaking the snow from one's shoes, by kicking against a door-post; of clean-
ing a dusty book by knocking it against a table, or shutting  it violently—all
illustrate the same principle.
  If a guinea be laid on a card which is already balanced on the point of the
finger, a small  fillip or blow to the edge of the card will cause it to dart  off,
but the guinea, owing to its inertia, will remain  resting on the finger,—its
inertia being greater than the friction on it of the card passing from under-
neath it.
   When we desire a person, with suspected disease of the brain, to shake
his head, and tell whether and where he feels pain, we are doing nearly as if
we touched the naked brain with  the finger to find  the tender part; for the
inertia of the brain, when the skull is moved, causes a momentary pressure
between it and the skull, almost equivalent, for the purpose desired, to such
a touch.
   This kind of pressure is sufficient  to break and destroy tender wares—as
glass or eggs—in packages which are too suddenly moved or stopped.
   A weight suspended by a spring on ship-board  is  seen vibrating up and
down as the ship pitches with the waves.   It seems to fall as the ship rises,
and to rise as the ship falls:  but the  motion  is really in the ship, and  the
comparative rest is  in the weight.  A heavy weight so  supported, and con-
nected with a pump-rod, would work the pump.
   Like the weight  last mentioned, the mercury of a common barometer on
ship-board is seen  rising and  falling in the tube;  and  until the important
improvement was lately  made, of narrowing one part of the tube to prevent
this, the mercurial barometer was useless  at sea.   The  explanation is,  that
the tube rises and falls with the ship, from  being connected  with it;  but the
mercury, which plays freely in the tube, and is supported by the  atmospheric
pressure, tends, by its inertia, to remain at rest, and thus makes the motion
of the ship apparent.                                             %■
   What happens to the mercury  in the barometer-tube on  ship-board, indi-
cates what happens to the blood  in the vessels of animals under similar cir-
cumstances.   In any long vein below the heart, when  the body falls, the
blood, by its inertia and  the supporting action of the vessels, does not  fall so
fast, and therefore really  rises in the vein: and  as there are valves in  the veins
preventing return, the circulation is  thus  quickened without any muscular
exhaustion on the part of the individual.  This helps "to explain the effect of
the movement of carriages, of vessels at sea, of swings, &c, and of passive
exercise generally, on the circulation, and leaves it less a mystery why these
means are often so useful in certain states of weak health.
   If- a cannon  ball were  to break to pieces in its flight,  its  parts would  still
advance with the previous velocity.   And  thus, in the deadly contrivance of
the Shrapnell-shell, which is in a case containing hundreds of musket bullets,
when these are scattered at the desired distance from the devoted body of
men, they retain the forward velocity of the shell, and spread death around
 like the near discharge of a whole battalion of musketry.
   On the awful occasion of a ship in rapid motion  being suddenly arrested 
46
MOTIONS AND  FORCES.
by a sunken  rock, all things on board, men, guns, and furniture, start from
their places and dash forwards;  while the onward inertia or motal obstinacy
of the hinder parts of the ship, suffices to crush her bow against the rock.

                "Motion as naturally permanent as rest"
   From the instances now given, it is seen  that a body at rest would never
move if force were not applied, and that a body piit in motion retains motion,
at least  for a time, after the  force has ceased; but there is a feeling from com-
mon experience, that motion is  an unnatural or forced  state of bodies, and
that all  moving things, if left to  themselves, would  gradually come to rest.
It is recollected that a stone projected comes to rest, or a wheel left moving,
or a bowl rolled on the green, or the waves heaving after a storm—and, in a
word, that there is no perpetual  motion on earth.
   On more attentive consideration, however, it may be  perceived that there
are prodigious differences in the  duration of motions, and  that the  differences
are always exactly proportioned  to evident causes of retardation, and chiefly
to friction and the resistance of the air.
  Friction is  the resistance  which bodies  experience when rubbing or sliding
upon each other; and however much it  may be diminished by art, it can in
no case  be annihilated.   Air-resistance, again, to motions  going on in air, is
of the same nature as water-resistance to motions going on in water, only less
in degree : and as advancing science has shown the true nature of our atmo-
sphere,  the amount of this resistance is perfectly ascertained.
  A smooth ball rolled on  the grass  soon stops—if rolled on a green cloth
over a smooth  plank it goes longer—on the bare plank, longer still—on a
smooth  and level sheet of ice, it  hardly suffers retardation from friction, and,
if the air be moving with  it, will  reach a distant shore.
  Two  little  windmill-wheels set in motion together with equal velocity, but
of which  one has the flat sides of the vanes turned  to their course, and the
other the edges, if moving  in the air, will stop at very different times, but if
tried in  a vessel from which the air has been  removed, they will both go
much longer, and will then stop  exactly together.
  As it  is  to facilitate the motion  of fishes in  the water, that they are of sharp
form before and behind; so it is to facilitate the motion of birds in the  air that
they have somewhat of a similar form.
  A large spinning top, with a fine hard point, set  in motion in a vacuum,
and on a hard, smooth surface, will continue turning  for hours.
  A pendulum  moving in a vacuum has only to overcome  slight  friction at
its point of suspension, and, therefore, if once put in motion, will vibrate for
a day or more.

But it is in the celestial spaces  that we see motions completely freed  from
  the obstacles of air and friction—and there they seem eternal.
  Had  the human eye, unassisted, been able to descry  the four beautiful
moons of  Jupiter, wheeling around him  for these thousands of years  with
such unabated regularity, and which now form,  to the telescope of the'astro-
nomer,  a  perfect and magnificent time-piece in  the sky, or had science lon<*
proved that the velocity imparted to our  globe, when first launched  into ill
present  orbit, still wheels it along as swiftly  as in the days  of the first man
this error  or prejudice, that motion is always tending  to rest, would never
have arisen.
  Indeed, had these, and other such truths, been long familiar to the common 
MOTION UNIFORM.
47
mind, the opposite prejudice might as well have obtained, that motion is the
natural  state, and rest a forced or unknown  state.   We know of nothing
which is absolutely at rest.   The earth is whirling round its axis .and round
the sun; the sun is moving round  its axis and round the centre of gravity of
the solar system, and, possibly, round some more remote centre in the great
universe, carrying all his planets and comets about his path.
   If there were any natural tendency in moving bodies to stop, a thing float-
ing in a trough of water, on board a sailing ship, should always be found at
the end of the trough nearest the stern; and in all the seas and lakes  of the
earth, the floating things should be accumulated on the western shores, because
the surface of the earth is always turning to the east.   We know that neither
of these suppositions is truth.   A man on board a moving ship can throw
any body just as far towards the bow as towards the stern; although in the
two cases the velocity, as regards the earth, is so different.
   Ignorance of the law  of motal inertia led a story-telling sailor to assert, as
a proof of the speed of his favourite ship, that  when a man one day fell from
the mast head, the  ship had passed from under him before he reached the
deck: the fact, in such a case, being, that he  must have fallen on the same
part of  the  deck, whether  the  ship were  in motion  or at rest, because his
body had just the motion or rest which  belonged to the ship.
   Another equally sapient  man, reflecting that the  earth turned round once
in twenty-four  hours, proposed rising  in  a balloon, and waiting aloft, until
the country which he desired to reach should be passing under him.

   "Motion naturally uniform."   (See the Analysis.)

It is only repeating that  a body can neither acquire  motion nor lose  motion
   without a cause, to say that free motion must be uniform.
   The  perfect uniformity  of undisturbed  motion is proved by every  fact
observed in the universe.  If any continued motion, as of a planet, for in-
stance, be found at one  time to have certain  relative velocity to some other
continued motion, the same relation is found always  to hold:  or deviations
from perfect uniformity are exactly proportioned to the disturbing  causes.
Thus we  can foretell the exact time of an eclipse, a  thousand years before
its occurrence.
   Had motion  not been in  its  nature uniform, man could have formed no
rational conjecture or anticipation  as to future events; for it is  by assuming,
for instance, that the  earth  will continue to turn uniformly on its axis, that
he speaks of to-morrow and of next week, &c, and that he makes all his
arrangements for future  emergencies: and were the coming day, or  season,
or year, to arrive sooner or later than such anticipation,  it would throw such
confusion into all his affairs, that the world would soon  be desolate.
   To calculate futurities, then, or  to speak of past events, is merely to take
some great uniform motion  as a standard with which to compare all others;
and then to say of  the  remote event, that  it coincided or will  coincide with
some described state  of the standard  motion.  The most obvious and best
standards are the whirling of the earth about its axis, and its great revolution
round the sun.   The first is rendered very sensible to man by his alternately
seeing and not seeing the sun, and it is called a day;  the second is marked by
the succession of the seasons, and it is called a year.   The earth turns upon
its axis nearly 365 times while it is performing one circuit round the sun, and
thus divides the year into so many smaller parts, and the day is divided into
smaller parts, by the progress of the earth's whirling being so distinctly marked, 
48
MOTIONS AND FORCES.
in the constantly varying direction of the sun, as viewed from any given spot
on the face of the earth.—When advancing civilization made it of importance
for men to be able  to ascertain with precision the very instant of the earth's
revolution, connected with any event, various contrivances were introduced for
the purpose; as,—sun-dials, where the shadow travels progressively round the
divided circle;—the uniform flux of water through a prepared opening;—the
flux of sand in the common hour-glass, &c.   But the great triumphs of modem
ingenuity are those astronomical clocks and watches,  in which the counted
equal vibrations of a pendulum, or  balance-wheel, have detected periodical
inequalities even in the motion of the earth  itself, and have directed attention
to unsuspected  disturbing causes, important to be known.
   It is the natural uniformity of undisturbed motion which  causes any num-
ber of bodies moving together, as  the furniture of a sailing ship, to appear
among themselves as if at rest,—no one  tending  to  pass  before, or to fall
behind, or to move to one side of another.   For  the same reason a person
who is moving with such  bodies is  absolutely insensible of his uniform pro-
gression, and knows it  only by reasoning  from such  facts as  the changing
appearances of other objects around which do not share the motion, the rush-
ing of  the waves or wind, &c.  When a ship is becalmed at sea, she may,
as numberless sad accidents  have proved, be carried by rapid currents in any
direction, without one of the crew suspecting that she has motion at all;  and
if the suspicion do arise, the truth  can  be come at only by such means as the
sounding line, where the bottom can be reached, or careful observation of the
heavenly bodies where it cannot.   A man in the hold  of a ship in a river or
tides-way cannot say whether  the  rushing of water,  which he  hears from
without,  be a rapid tide  passing the  ship at  anchor, or the effect of the ship's
advance  in the river.   A man in a balloon going  80  miles an hour, knows
not in what direction he is moving,  nor, indeed, that he is  moving at all, but
by observing the objects below.
   This explains why men are not sensible of the motion of the earth itself,
which they know, however, to be turning round its axis once in  twenty-four
hours, and therefore to have  its surface near the equator moving with a speed
of more than 1,000 feet per second; and as in the  case of a ship or balloon,
there would be  no difference of sensation whether the speed were of one mile
per hour  or of 10 or 100, so in the case of the earth, there would be none
whether  it turned  as now, once  in twenty-four hours;  or, like  the planet
Jupiter, once in ten.   A  hunter  among the hills, who, during  the heat of
noon, rests and contemplates around  him  a  sublime  scene of solitude and
silence, may little think  that  if, amidst  that apparent repose of nature, he were
for a moment lifted up from the earth and  held at rest above its surface, he
would see its face of hill and dale sweeping  past beneath him at the prodi-
gious rate of 1,000 miles  an hour,  on account solely of the whirling of the
earth.
   The fact that a cannon ball can be shot just as far upon the surface of the
earth, eastward, in the  direction of  the earth's motion, as westward, against
it, illustrates  the  truth,  that whatever common motion objects may have, it
does not  interfere  with  the effect of a force producing any new relative mo-
tion among them.  All the motions seen on  earth are really  only slight differ-
ences among the common  motions:  as in a fleet of sailing ships, the°apparent
changes  of place among them are in reality  only slight alterations of speed or
direction, in their individual courses.
   A man continuing  to throw upwards a ball or orange, or several of them
at once,  and to catch and return them alternately, uses no difference of art as 
MOTION STRAIGHT.
49
regards them, whether he be  standing on the earth  and whirling with it, or
on a sailing ship's deck, or in a moving carriage, or on a galloping horse's
back.   He and the oranges  have always the same forward common motion.
And when a man, standing on  a galloping  horse, leaps  through a hoop held
across his course, he does not leap forward—for this would throw him over
the horse's ears—but merely jumps up, and allows his motal inertia to carry
him through.
   The reason that a lofty spire or obelisk stands more securely on the earth
than even a short pillar stands on the  bottom  of a moving wa^on, is, not that
the earth  is more at rest  than the wagon, but that its motion is uniform.—
Were the  present rotation  of our globe to  be  arrested but for a moment,
imperial London, with its thousand spires  and turrets,  would, by the motal
inertia, be swept  from its valley towards the eastern ocean, just as loose snow
is swept away by a gust of wind.

                   "Force is required to bend motion"

If a body moving freely cannot vary its velocity without a cause, neither can
   it vary its course without a cause;  and free motion, therefore, is straight
   as  well as uniform.
   A ball shot directly up or down gives men their simplest idea  of straight
motion.
   A bullet or arrow, projected horizontally, is gradually drawn downwards
by the attraction  of the earth, but it deviates neither  to the right hand nor to
the left.
   William Tell,  trusting to the natural  straightness of motion, obeyed  the
tyrant's order, and shot an apple placed on his child's head.
   And the right eye of Philip of Macedon is said to  have been destroyed by
an arrow which brought  a label on it, telling its destination.
   Riflemen shooting at a target, hit the very spot they choose to aim  at.
   A stone in a sling, the moment it is set at liberty, darts off as straightly as
an arrow from the bow-string or a bullet  from a gun-barrel, and it is only
because the point of its circle, from which it should depart, cannot in prac-
tice be accurately determined, that the same sure aim cannot be taken  with it.
   A  body  moving in a circle, then, or curve, is constrained  to do what is
contrary to  its inertia.   A person, on first approaching this  subject, might
suppose  that a body, which for a time  has  been constrained to move in a
circle, should naturally  continue  to  do so  when set  at liberty.  But on
reflecting that a circle is as if made up of an infinite  number of little straight
lines, and that the body moving in it has its motion bent at every step of  the
progress, the  reason is seen why constant force  becomes necessary  to keep
it there, and force just equal  to the  inertia with  which
the body  tends,  at  every point of the circle, rather to        Fig. 2.
pursue the straight line, called a tangent, of which  that
point, as seen in  fig. 2, is the commencement, than the
circle itself.  The force required to keep the body in the
bent course, is called centripetal or centre-seeking force;
while the inertia  of the  body tending outwards, that is,
to move in a straight line rather than in a curve,  is called
the centrifugal or centre-flying force;  and  the term  cen-
tral forces is applied to both.
   A sling-cord is  always tight while the stone is whirling: and its tension is
of course the measure both of the centripetal and centrifugal force.  A means,
D 
50
MOTIONS AND FORCES.
then, of measuring the tension of a sling-cord would experimentally demon-
strate the amount of centrifugal force;  and such a means we possess in the
contrivance  called  the " whirling  table," upon which a leading sling, or any
mass with a string attached to it, may be placed to revolve, at any desired
distance  from the centre,  and  with any desired velocity, while  the string
passing over a pulley at  the  centre, is made to lift weights proportioned tu
the outward dragging of  the revolving mass.   By this apparatus it is found,
as would be expected, that centrifugal force—in other words, the  force  with
which  the inertia  of moving matter resists the bending of its course from
straight to circular, is proportioned, first, to the quantity of matter moved—
every separate particle  having  its own  inertia; second, to the size of the
circle or orbit described in  the same time—a body moving in a circle of
double diameter  for instance, having to be forced  inwards from  the tangent,
at every  departure, twice as  far in a given time;  third, that with a double
revolution in the same time, the centrifugal force is  not double but quadruple
(a corresponding proportion existing for  other velocities,) because, not only
are there twice as many bendings or angular departures  from the  tangent for
the two circles as for one, requiring,  as may be said, twice as  many tugs  or
impulses of the centripetal force, but every  impulse must be  made with
double energy, for it has  to drive the mass inwards  through the required dis-
tance in half the time; and twice as many impulses, every one  being twice
as strong,  make a quadruple amount of force on  the  whole; fourthly and
lastly, it  is  found, agreeing with  the relation between inertia and terrestrial
gravity described at page 43, that a body revolving, for instance, in a circle
of four feet diameter,  that it may have centrifugal force just  equal to  its
weight, requires to complete  its revolution in one second and a half of time.
This and similar facts will be more particularly considered  when we come to
treat of the  motions of the planets round the sun.   This analysis of central
forces will suffice to excite in the student a due interest touching the kindred
phenomena  now to be described.
   Bodies  laid  on a whirling horizontal wheel, are readily thrown off.
   In a corn-mill, the grain, after being admitted between the stones  through
an  opening in the centre  of the upper stone, is  then kept  turning round
between them, and is, by its  centrifugal force, always tending and travelling
outwards until it escapes as flour from the circumference.
   A man, if he lie down on a turning millstone with his head near the edge,
falls asleep, or dies  of apoplexy, from  the  new pressure of blood on the
brain.
   A wet mop, or bottle-brush, made to turn quickly on its handle as an axis,
throws the water off in all directions, and soon dries itself.
   Sheep, in wet weather, thus  discharge the water from  their fleeces, by a
semi-rotatory shake of the skin.   Water-dogs, on coming to land, dry them-
selves by the same action.
   A tumbler of water, placed in a sling, may be made to vibrate like a pen-
dulum with gradually increasing oscillation, and at last to describe the whole
circle, and continue revolving about the hand, without spilling a drop:—the
water, by its inertia  of straightness, or centrifugal  force, tending more away
from the  centre of  motion towards the bottom of the tumbler, even when that
is uppermost,  than toward the earth by gravity.
   As solid bodies laid on a whirling table are thrown off, so water in a vessel
caused to spin round in any way, as on the centre of a  horizontal wheel,
instead of lying at the bottom, is  raised up all around, against the sides of
the vessel. 
CENTRIFUGAL FORCE.
51
   Water, poured  obliquely into a  funnel, runs round the interior of it, and
 often leaves  an open passage of air all the way down through it, as if  there
 were merely a lining of water to the funnel.  The centrifugal force of the
 turning water  is a chief reason of this phenomenon:—another reason will
 be considered  farther on, under the head of atmospheric pressure.
   Great whirlpools at sea, and smaller ones, or eddies in rivers, occur when-
 ever a current  is obliged suddenly  to bend, as in rounding a point of land or
 a rock, or in meeting and mingling with a contrary current.   The water, by
 tending to continue its straight motion, falls in  behind the obstruction, reluct-
 antly as it were, and leaves there a  pit surrounded by a liquid revolving ridge.
 Charybdis, in  the Mediterranean, and  the great whirlpool off the Norwegian
 coast, are noted examples.
   It is owing to the  centrifugal force in any bending part of a stream of
 water, that is to say, to the tendency away from  the centre of the curvature,
 that when a bend has once commenced, it increases, and is soon followed by
 others, until that complete  serpentine winding  is  produced, which charac-
 terizes most rivers  in their course across extended plains.  The water being
 thrown by any cause to the left side, for instance, wears  that into a curve or
 elbow, and, by its centrifugal force, acts constantly on the outside of the bend,
 until rock or higher land  resists the gradual  progress ;  from  this limit being
 thrown back again, it wears  a similar bend to  the right hand, and after that"
 another to the left,  and so on.
   Carriages are often overturned in quickly rounding corners.  The inertia
 carries the body of the vehicle in the former direction,  while the wheels are
 suddenly pulled round by the horses into a new  one.   A loaded stage-coach
 running  south, and turning suddenly  to the  east or west, strews its  passen-
 gers on the south side of the road.  Where a sharp  turning in a carriage-road
 is unavoidable, the  road towards the outside of the bend should alvvays be
 made higher than at the inside, to prevent such accidents.
   A man or a horse turning a corner at speed, leans much inwards, or  towards
 the corner to counteract the centrifugal force, that  would throw him away
 from it.
  In skating with  great velocity,  this leaning inwards at the turnings be-
 comes very remarkable,  and  gives  occasion  to the fine variety of attitudes
 displayed by the expert; and if a skater, in running, finds his body inclined
 to one side and in  danger of falling, he  merely makes  his skate describe  a
 slight curve  towards  that side, when the  tendency of his  body to move
 sraightly, or its centrifugal force, refusing to follow  in the curve, allows the
 foot  to push itself again under the body, and to restore the perpendicularity.
 Skating becomes to the intelligent man an intellectual as well as a sensitive
 or bodily treat, from its exemplifying so pleasingly  the laws of motion.
  The last example explains, also,  why a hoop rolled along the  ground  goes
 so long without falling:  if it incline to one side, threatening to  fall, by that
very circumstance,  the part touching the  ground is made to  bend its course
to that side, and as in  the case  of the skater  who turns his foot, the  sup-
porting base is  again forced directly under the mass  of the body.
  A coin dropped on the table or floor often  exhibits the same phenomenon.
It is  said to run and hide itself in the corner.   Just  before falling, if not ob-
 structed,  it describes several  turns of a decreasing spiral,  the minute exami-
 nation of which is a pleasing mathematical exercise.
  The reason  also why a spinning top stands, will be understood here.
 While the top is quite upright, the  extremity of its  peg, being directly under
its centre, supports it steadily, and although turning so rapidly,°and with much 
52                       MOTIONS AND FORCES.
friction, has no tendency to move  from the place;  but if the top incline at
all, {he edge or side of the peg, instead of its very print, is  in contact with
the floor, and the peg  then  becoming as a turning litlle  roller,  advances
quickly,  and describes  a curve somewhat as  a skater's foot does, until it
come directly under the body of the top as before.   It thus appears that the
very fact of the top inclining, causes the point to shift its place, and to con-
tinue moving until it comes again directly under the centre of the top.  It is
remarkable that even in  philosophical treatises of  authority the  standing of
a top is still vaguely attributed to centrifugal force.  And some persons be-
lieve that a  lop  spinning in a weighing scale,  would be found lighter than
when at  rest; and others most  erroneously hold that the centrifugal force of
the whirling,  which of course  acts  directly  away  from  the  axis, and quite
equally in all directions, yet becomes, when the top inclines, greater upwards
than  downwards, so as  to counteract the gravity  of the top.  The  way in
which centrifugal  force really  helps to maintain  the spinning  of a  top is,
that when the body inclines or begins to  fall in one direction, its motion in
that direction continues until the point describing its curve, like  the  foot of
a skater,  has forced itself under the  body  again.
   By reason of centrifugal force also, it is  easier to do feats of horsemanship
in a small ring as at our theatres, than if the animal were running on a straight
road.  We see the man and the horse  always inclining  inwards, to counter-
act centrifugal force, and  if the rider tend to fall inwards, he has merely to
quicken the pace; if to  fall outwards, he  has  to slacken it,  and  all is right
again.
   If  a pair of common  fire-tongs,  suspended by  a cord from  the top, be
made to turn by the twisting or untwisting of the cord, the legs will separate
from  each other with force dependent on the speed of rotation, and will
again collapse when the turning ceases.   Mr. Watt adapted this fact most
ingeniously to the regulation of  the speed of his steam-engine.   His steam-
governor may in truth be described as a  pair  of tongs  with heavy  balls at
the ends, to make  their  opening  more energetic,  attached  to some turning
part of the machine. / If the engine move  with  more than the assigned speed
the balls  open or fly asunder  beyond their middle  station, and by a  simple
contrivance  are then made to act on  a valve which  contracts the steam tube;
on the contrary, with too  slow a motion, they collapse and open the valve.
   A  half-formed vessel  of soft clay,  placed in the centre  of  the potter's
table,—which is made  to whirl  and is  called his wheel,—opens  out or
widens merely by the force of its sides, and thus assists  the  worker in giving
its form.
   A ball  of soft clay, with a spindle fixed  through  its centre, if made to turn
quickly,  soon  ceases to  be a  perfect ball.   It bulges  out  in  the middle,
where the centrifugal force is great,  and becomes flattened towards the ends,
or where the spindle issues.
   This change of form  is exactly  what  has  happened to  the ball  of our
earth. It has bulged out seventeen miles  at the equator, in  consequence of
its daily  rotation, and is  flattened at the poles  in a corresponding degree.—
A mass of lead that weighs one thousand  pounds  at our pole, weio-hs about
five pounds less at the equator, by reason of the centrifugal force. °
   In the planets Jupiter and Saturn, of which  the  rotation is much quicke!
than  of our earth, the middle or equator bulges  out still more__even so as to
offend an eye which expects a perfect sphere.
   If  the   rotation  of  our earth were seventeen times faster  than it  is, the
bodies or matter at  the  equator would have  centrifugal  force equal to their 
QUANTITY OF MOTION.
53
gravity, and a little more velocity would cause them to  fly off altogether, or
to rise and form a ring round the earth  like  that which  surrounds Saturn.
Saturn's double  ring seems to have  been formed- in  this way, and is now
supported chiefly by the centrifugal force of  the parts.   Were it to crumble
to pieces, the pieces might still revolve, as so many little satellites.  His true
satellites are only more distant masses sustained in the  same manner.  And
our earth and  the other primary  planets have  the same  relation  to the sun
that these  satellites  have to Saturn—all being sustained by an  admirable
balance between centrifugal force and  gravity.

" The quantity of motion in a body measured by  the velocity and quantity
                              ofmatter."
   If a single atom of matter were moving at the rate of one foot per second,
it would have  a definite quantity of motion expressed by  these words ;  and
if it were moving ten feet per second  it would have ten times the  quantity.
Again, in a mass consisting of many atoms, the quantity of motion  would be
still as much greater as there were more atoms in it than one.
   By experiment it is found, that if  a  ball of soft clay of one pound, sus-
pended by a cord as a pendulum, be allowed  to fall, with a velocity of ten feet
per second, against a ball of nine pounds suspended in  the same way, but at
rest, the two,  after contact, will  start together at the  rate of one  foot per
second, the original quantity of motion being  then diffused through  ten times
the quantity of matter,  and therefore exhibiting only one-tei.th of the velocity.
   A cannon-ball of a thousand ounces, moving one foot per second, has thus
the same quantity of  motion in it as a musket-ball  of one ounce, leaving the
gun-barrel with a velocity of a thousand feet in the second.

" The quantity of motion in a body is the measure of  the force  which pro-
                               duced it."
  The experiment of the balls of clay mentioned above furnishes one instance
of this truth.  Again, a body falling for ten seconds, acquires ten times as
much velocity as  by falling for one second;   its motion thus measuring the
force  of gravity which has been exerted upon it.
   When a large body or mass of many atoms falls, it of course has as much
more motion than a smaller body, as  there are more atoms in it than in the
smaller: but as gravity acts  equally on  every atom, the force causing either
body to fall is still exactly indicated by the quantity of  motion in it.
  A large body or mass of many atoms falls, where there is no impediment,
with the same velocity as a smaller body or a single atom; for gravity pulls
equally at each atom, and must  overcome its inertia equally, whether it be
alone or with others.
  This remark contradicts the popular opinion, that a large and heavy body
should fall to the earth much faster than a small and light one; an opinion
which has arisen from our constantly  seeing such contrasts, as the rapid fall
of  a gold coin, and the  slow descent of a feather.   The true cause of the
contrast is, that the atoms of the feather are much spread out,  so as to be
more resisted by the air  than those of the gold.  If the two be let fall toge-
ther in  a vessel from which the air has been extracted—as in the common
air-pump experiment, they arrive at the bottom in exactly the same time:
and even in the air, if  the coin be  hammered out  into  gold leaf, it will fall
still more slowly than the feather.  One brick dropped  from a height, be-
cause its motion is not much affected by the air, reaches the earth very nearly
     5 
54
MOTIONS AND FORCES.
as soon as ten bricks let fall near it, whether they be  connected or separate
—as a single horse mav reach the goal as soon as ten horses galloping abreast.
   A man's force will move a small skiff quickly, a loaded barge very slowly,
and a large ship in a degree scarcely to be perceived. In each case, however,
the quantity of motion may be  the same, and a true  measure  of the force
which produced it.
   A ball of one pound  weight, impelled by a given force, moves twice as fast
as a ball of two pound  impelled  with the  same;  yet, although the velocities
are different, the quantities of motion, as ascertained by the rule already given,
are equal, and indicate  an equality of producing force.

" The quantity of motion in a body is the measure also of the force or mo-
      mentum ivhich it can exhibit again"  (See the Analysis 42 )
  Bodies, owing  to their  inertia, may be  regarded as passive reservoirs of
force or motion, always ready to  return as much as they have received.   Mo-
mentum is the name given  to the motion in a body,  with reference to the
production by  it of new motions  or the overcoming of resistances, and is but
another term for the quantity of motion.
  A cannon ball, according  to the quantity of motion in it, may have  only
the force or momentum that will bruise a plank, or it may have enough to
penetrate a tree, or even to shoot its rapid way through a block of the hardest
stone.
  A block of wood, floating against a man's leg with moderate  velocity,
would be little felt; but a loaded  barge, coming at the same rate, and pressing
it against the quay, might break the bones ; a large ship, again, although mov-
ing no faster, would crush his body against any fixed obstacle ; and an island
of ice, opposed in its approach to another, even by a first-rate man-of-war,
would destroy it,  as meeting barges destroy a floating  egg-shell.
  A hail-stone falling,  strikes rudely; a stone rolled from a height, as of old,
by the besieged  against besiegers, may carry death  with it  to many;  an
avalanche, breaking from its hold on a  mountain  steep, may  sweep away a
village.

To meeting bodies, the shock is the same, whether  the  motion be shared
  between them or be  all in one.

  If a running man come against a man who is standing, both receive a cer-
tain shock.  If both be running  at the same rate  in opposite directions, the
shock is doubled.   In  some such cases, as  where swift skaters have met, the
shock has proved  fatal.
   The meeting fists of boxers not unfrequently dislocate or break bones.
   A man's skull is fractured as certainly by its being dashed against a tree or
beam while he is  on a galloping horse, as by the blow of a similar beam coming
upon  him with the velocity of the horse.
   When two ships in  opposite  courses  meet at  sea, although each may be
sailing at a moderate rate, the destruction is often as  complete to both as if
with a  double velocity they had struck on  a rock.   Many melancholy in-
stances of this kind are on record.  In the darkness of night a large shin has
met one smaller and weaker, and in the lapse of a few seconds, have followed
the shock of the encounter, the scream of the surprised victims  and the hor-
rible silence when the  waves had again closed over them and  their vessel for
ever.—In November, 1825, on the coast of Scotland, the Comet steamboat
was thus destroyed, and carried  to the bottom with her about seventy passen 
DIRECTION OF FORCES.
55
gers, into whose ears  the  drowning water rushed  before the sounds of ar-
rested music and joy  had  died away.

            "Direction of the force or forces producing motion."
When only one force acts on a body, the body obeys in the exact direction
  of the force.
  A ball floating in water, or lying on  smooth  ice, is driven exactly south
by a wind blowing to  the south.  A bullet issues  from the mouth of a cannon,
in the direction of the axis of the cannon—which is, as the force impels it.

When two or more forces, not in the same direction, act upon a body at the
  same time, as it  cannot move two ways at once, it holds a middle course
  between the directions.   This course is called the resulting direction, viz.,
  resulting from the composition of the forces.
  A ball or ship moving south by  a direct wind, may, at the same time, be
carried east, just as fast,  by a tide or current moving east; every instant,
therefore,  it will go a  little south and a little east, and really will  describe a
middle line pointing south-east.
  These particulars may be well represented on paper, as by fig. 3: where
b is the  original place of the ball or ship, e the east, s the
south, and b a the middle line pointing to the south-east,      Fl£- 3-
and showing the true  course of the vessel.   This figure is
called the parallelogram of forces, and is an important help  £
to the understanding of many facts in natural philosophy.
The minute investigation of the subject belongs to  the sci-
ence of  measures, or  technical mathematics; but the gene-
ral  truths  are  quite intelligible to  common  sense, or the
mathematics of common experience.                         s          a s
  When two  forces act upon a body, like  the wind and
tide in the last example, the result is the same, whether they act together or
one after the other.   For  instance, if the wind drive a vessel one mile south,
as from b  to s, fig. 3,  and  immediately afterwards  the tide drive it one mile
east, as  s to a, the vessel will be in the same place at last, viz., at a, as if she
had been driven at once southeast, in the line b a, by the simultaneous action
of the two.  Therefore, by drawing  the lines b s and  b e  to represent the
force and  direction of the  two causes of motion, and  by then adding  one of
them, or an equivalent, to  the end of the other, as s a to b s, or e a to b e, the
square or  parallelogram is sketched, of which the middle line or diagonal,
as it is called, shows  the  resultant of the forces, and the true course  of the
body obeying them.
  What is thus  true of the effect  of continued forces like wind and tide  is
true also of momentary impulses, like the blows of  clubs simultaneously strik.
ing a ball, or of two billiard-balls striking a third.
  When the forces exactly cross each other, and are equal, as in the case of
 
56
MOTIONS AND FORCES.
 the ship above supposed, the figure becomes a square, as at fig. 3; but if one
 of the forces be greater than the other, the figure  becomes oblong, as at fig.
 4;  if the forces cross obliquely, the figure becomes as at fig. 5; and  if they
 cross in an opposing direction, it will be  as  at  fig.  6.   In all  the  cases,
 however, the diagonal still shows the result.  It is evident that the same line
 may be the diagonal of many figures, as seen in b a at fig. 7; and therefore,
 that very different degrees and  directions of combined forces may produce
 the same residt.
  Forces crossing each other so obliquely as to be represented by lines drawn
 in almost opposite directions, would  form a parallelogram having scarcely
 any breadth, that is to say, the diagonal would approach to nothing; showing
 thus, that opposing  forces neutralize or destroy each other.  In fig. 6, by
 reason of this crossing, the  resultant is less than  either of the constituents.
 And for the same reason, when forces  cross so acutely as to advance nearly
 parallel to each other, the  resultant is  longer than either, as seen in fig. 5.
 Forces directly opposed, or entirely agreeing in direction, give as their result-
 ant their difference or their sum.
  Forces crossing each other directly, or at right angles, as is  true of the
 exactly eastward force b e, and the exactly southward force b s, in figures 3
 and 4,—do not in the slightest  degree neutralize  or alter each other,  for the
body, when arrived at a, is just as far east as it would be at e, and as far south
 as it would be at s.   This explains why the progressive motion of the  planets
in their orbits  is not  at all affected by the directly crossing centripetal force
of gravity which keeps  them at their due distances from the sun.
  In all cases  where  the two crossing forces are equal, with whatever obli-
quity they cross, the resulting direction must be midway between them.—-
Thus a boat impelled by oars, goes straight, although the direction in which
the  oars  act is  constantly changing; because the  changing obliquity of the
force is always the same on both sides.—This  explains also why a bird fly-
ing, or a man swimming, holds a perfectly straight course, although in both
cases the direction of the impelling  forces is  constantly varying.—And it
explains why a body suspended, as a plummet, or falling to the earth as an
apple does from a tree, is always in a  line towards the centre of the earth:
for, while the  part of  the earth immediately  under the body is pulling it
               straight  down to the centre, the action of parts on any one
               side of the perpendicular  is exactly counterbalanced  by the
               action of corresponding parts on the opposite side; and the
               perpendicular is still the diagonal or middle line of every pair
               of attracting  parts.   In fig. 8 b a represents the common dia-
               gonal.   In speaking of the attraction of our earth, therefore,
               which really  is the united attraction of all the individual atoms,
               we may always consider it as a single force acting towards the
               centre of the earth.
  When a body is carried below the surface of  the earth, its weight becomes
 less, because the matter then above it is drawing it up, instead of down, as
 before.   A descent of a few hundred feet makes  a  sensible difference, and
 at the centre of the earth,  if man could reach it, he would find things to have
 no weight at all; and there would be neither up nor down, because bodies
 would be attracted equally in all directions.
  When more than two forces act on a body, the resulting direction may be
 found, first  of two, and then of the last  resultant with each  of the   others
 successively:—or the forces may be  represented on paper  by lines  tacked
 together, of which one  denotes  the strength and direction of each-  the ex-
 
DIRECTION OF FORCES.
57
Fie. 9.
tremity of the last  line will mark the place of the  body after  being acted
upon by the combined forces.  A sailor, to know the true place of his ship
and the course which she has steered, considers, first, the forward progress
as found  by the  log, then the leeway  or sideward  motion produced  by a
cross wind,  and  then the effect of any tide  or  current in which he may be
sailing.

Resolution of Forces is a phrase  pointing to another  important use of  such
  parallelograms or figures as have just been described, viz., the enabling us,
  when force or motion is given, to find the forces or motions in any other
  directions of which it may be the  resultant, and those into which  it may
  itself be resolved.

  Thus, if a line b a (in  any of the preceding figures  4, 5, 6,  &c.) repre-
sent a force  or motion, and the line b s represent one of two  elements com-
posing it, we have but to complete the parallelogram b s a e to obtain the other
line, b e representing the  only other force or motion which, combined with
the first element, can produce the given resultant.—If a ship pass from  b to
a (fig. 5) while sailing through the water eastward, a distance  expressed by
b e, she must at  the same time have been carried by a tide current to the  dis-
tance and in the  direction marked by the line b  s.
  Again, if  a line be given  representing  a single
force, or motion, as b a, and if it be  desired  to know
how much there is  in this capable of acting in another
direction, as 6 d ; it is only necessary to draw a line
in the direction b d, from the commencement of b a,
and to cut such line by another drawn directly  upon
it—or at right angles to it, as the term  is, from the
other end of  b a: the length of b d,  so  cut off, viz.,
b s, shows the proportion required.
  It is thus  that a sailor who  knows how far he has
sailed in any oblique direction, finds out how much
he has gone north  and  east or south and  west; in
other words, finds out the difference in  latitude and  longitude between his
present place and a  former one.  In the  above figure, b a may represent the
course and distance sailed,  b s the difference of latitude, and b e the differ-
ence of longitude.
  Thus  again, if a ball  b strike a  table a  c,
with  velocity and  direction, both represented
by the line  b c; and if the ball be  supposed
afterwards with  the same velocity to approach
the table in the oblique  direction  e c, it will
then strike with  as much less force than before,
as the line e a is shorter than e c.   For e a  is
found, according to  the rule for decomposing a
force, given  above;  and, to  common sense, it
is  obvious,  that if  the  whole velocity  of the
ball be represented  by e c, the rate of approxi-
mation towards the  table, or merely downward
velocity, and therefore the downward force is marked by the line e a.   The
body only falls through the distance e a while moving all the way from e to c.
  Figure  10, explains the important  cases of the force of wind upon ships'
sails, windmill vanes,  &c; and the force  of water  upon float-boards,  wa-
ter-wheels, &c; showing that the moving  mass exerts force upon a sur-
Fiar. 10.
 
5S
MOTIONS AND FORCES.
face, not in  proportion to the speed with which it may be passing along or
near the surface, but to the rate of perpendicular approximation.  It explains
also, why the slanting blow of a club or  ball is so  slight, compared  with
the direct blow.

" The  two great forces of Nature are Attraction and Repulsion."  (Read
                              the Analysis.)
  A person, on first approaching this subject, is far from  supposing that the
beautiful and almost endless variety of  phenomena exhibited in the universe
around, are all referrible  to the  two principles, attraction and repulsion, exa-
mined  in the first section :—but such is the truth.—It will  first be  shown
here, how the great classes of accelerated, retarded, and bent motions  arise
from them.
  Attraction.—Until Newton  said, that what we call weight of bodies is
merely an instance of that universal attraction of matter which  diminishes
with increasing distance, it was never  suspected that weight was less,  high
up in the air than on  the ground;  or  on a lofty mountain than  on  the sea-
shore.   But this we now know to be the case.   However, in studying what
goes on in obedience to gravity near the surface of the earth, except in few
very nice cases, gravity  may be  considered as  a uniform  power ;  for man
has neither approached the centre of the  earth in mines, nor receded  from
it in balloons, by more than about a thousandth part of his distance from it;
and weight has relation to the distance from the  centre, not to the distance
from the surface.

                  "Accelerated Motion from Gravity."
  Owing to the  inertia of  matter, any  force continuing  to act  on a  mass
which  is free to  obey it, produces  in the mass a quickening or accelerated
motion: for as the motion given in the first  instant, continues afterwards
without  any farther force, merely on account of the inertia, it follows, that
as much more motion is  added  during the second instant, and as  much again
during the third, and so on.  A falling  body, therefore, under the influence
of attraction, is,  as it were, a reservoir, receiving every instant fresh velocity
and momentum.
  It is said that Newton's sublime genius read the nature of attraction in the
simple incident of an apple falling before him from a lofty branch in his gar-
den.—The eye which perceives an apple beginning to fall, can follow it  for a
time and mark the  gradual acceleration of  its descent, but soon sees its path
only as a shadowy line.
  A boy letting  a ball drop from  his hand, can catch it again in  the first in-
stant, but after a little delay his hand pursues it in vain.
  A fragment of rock, detached from  the  brow of  a hill by  the lightning
stroke, begins its motion slowly; but once fairly launched, it gathers  fresh
speed  and  momentum with every instant, and bounds from steep to steep
driving every obstacle before it.
  Any liquid falling from a reservoir, forms a descending mass or stream, of
which  the bulk diminishes  from  above downwards, in the same proportion
as the  velocity of the particles  increases.   This truth  is well exemplified in
the pouring out of molasses or thick syrup : if the height of the fall be consi-
derable, the bulky sluggish mass, which first  escapes, is  reduced before it
reaches the  bottom, to a small  thread :  but the thread  is moving  proportion-
ately faster, and  fills the receiving vessel with surprising rapidity.   The  same
truth is exhibited on  a vast scale in the Falls of Niagara; where the broad 
MEASURE OF ATTRACTION.
59
river is seen first bending over the precipice a deep slow moving mass, then
becoming a thinner and a thinner sheet as it descends, until at last, surrounded
by its foam or mist, it flashes into the deep below, apparently with the velo-
city of lightning.
  When velocity becomes  considerable in any case of falling;  it cannot be
measured accurately by the eye, but its effects  ascertain it.   A man  leaps
from a chair with impunity, from a table with a shock, from a high window
with fracture of his bones, and in falling from a balloon his body is literally
dashed to pieces.
  The force of gravity or  general  attraction is such at the surface of this
earth, that, in the first second of time, it gives  to a body allowed  to fall a
velocity  of 32 feet nearly  per second, that is, a velocity which, remaining
uniform  from  the end of the second, would carry it, without farther action
of gravity, through 32 feet in the next second.   Yet  the body  falls only 16
feet in the first second;  and the reason is, that the velocity of 32  feet pos-
sessed at the end of the second is  gradually acquired, the body having only
half of it at the half second, and as much less than  half at any distance be-
fore  that time, as it has more than half at the same distance afterwards; and
the average, therefore, is only half of the 32, or 16 feet in the whole second.
In the next  second, it falls of  course  through  the  whole 32  feet, with 16
additional, from the new action  of  gravity,  in all three times as much as in
the first  second ;  and in two seconds, therefore, it falls altogether four times
as far as in one second.  At the end of two seconds the velocity is doubled,
or is 64  feet per second, so that in the third second  the  body falls 64, and
other new 16, in all, five times as much as in the first second ; and in three
seconds, therefore,  it has descended nine times as far as in one second, &c.
Knowing this progress, the velocity acquired by a falling body, and the dis-
tance through which it falls, in any given time, are easily calculated ; and the
height of a precipice, or the depth of a well, may be ascertained by marking
the time required for a body to fall through the space.
   The doctrines of falling bodies are of such importance in the minute exami-
nation of many of the phenomena of nature, that much attention has been
bestowed upon them.   Mr. Atwood's ingenious contrivance  by which the
motion of falling bodies may be  retarded in any desired degree, without the
character of the motion  being otherwise altered, has  enabled experimenters
to render evident to the senses all that abstract calculation had anticipated.  A
pound weight, left quite free, falls towards the ground, sixteen feet in the first
second,  proving that  attraction  of one pound is  just sufficient to overcome
the  inertia of one pound  at that rate.  But if  the  inertia were doubled, or
tripled,  or increased in any other degree, the fall of  course would  be just so
much slower.   Now Mr.  Atwood's machine in effect increases
it, by causing falling weights to overcome  not only their own    Fig. 11.
inertia, but also that of other weights ; fig. 11.   Thus, a and b,     /"—\p
being weights of two pounds each, balancing each  other over    [ * \
the  very easily turned pulley c,  are moved  by  a weight of one      s—'
pound d, hooked to one of them ; and gravity in pulling this
down, with force of one pound,  has to overcome, not the inertia
of one pound, but of five,  for the other two weights  must move
 as fast as the one pound does ;  and thus, the velocity being re-
 duced to one-fifth of what is natural to a falling body,  the descent
 can be  minutely observed.  The  experiments  with Atwood's  aT_\  Qb
 machine may be varied exceedingly, and  they are most  inte-    "x ,
 resting. 
00
MOTIONS AND FORCES.
                   "Retarded Motion," from gravity.
  What has been said of the  changing velocity of  a  falling body, from gra-
vity, is exactly true, in a reversed way, respecting a rising body exposed to
the same influence.
  A bullet shot directly upwards, every instant loses a part of its velocity,
until at last it comes to rest in the sky,—where a soaring eagle might see the
messenger of death  motionless and harmless for a moment by his side :—the
ball then descends again, and so that, at corresponding points of- the ascent
and descent, but for the  resistance of the air,  the velocities would be equal;
and, on reaching the ground, it would have acquired exactly the velocity with
which it first departed.
  It is explained in  a preceding paragraph, that a body falls four times as far
in two seconds as in one, although the  velocity at the end of two  seconds is
only doubled.   For the same reason, a body shot upwards with double velo-
city, rises four times as far as if shot with a single velocity; if shot with triple
velocity, it rises nine times as far, and so forth.
  In aiming for amusement at bodies thrown up into the air,  it is easy to hit
them near their point of turning, and more difficult  always as they are nearer
to the ground, whether rising or falling.
  An upward jet of water is small below, where it  issues from  the pipe with
great velocity, but  it becomes more bulky as the water loses velocity in
ascending,  and at the top, it often spreads a little  like  a palm tree, and any
light round  solid will continue supported and  playing upon its summit.
   The rise of a pendulum from the bottom of its  arc,  is an exact copy, re-
versed, of its previpus descent to that point.

                           "The Pendulum"

exemplifies well both accelerated and retarded motion.   The name is appli-
cable to any  body so suspended,  that it  may swing freely backwards and
forwards.   When such  a body is made of certain form and length, although
so simple,  it is one of the most admirable contrivances  of man's ingenuity.
   Galileo having observed the hanging chandeliers of lofty ceilings to con-
tinue vibrating long and with singular  uniformity,  after any accidental cause
of disturbance, was led  to investigate  the laws of the phenomenon; and out
of what, in  some shape  or other, had been  before  men's eyes, but uselessly,
from the beginning of the world, his powerful genius  extracted the  most
important results.   Independently of the light which  the theory of the pen-
dulum has thrown on various branches of physics, the instrument itself, with
a few wheels  attached,  to record its vibrations, has now become the perfect
time-keeper, regulating  many of the affairs of men.
   A common pendulum consists of a  ball, fig. 12, as a suspended  by  a rod
                                from a fixed point as b, and made to swing
            Fig- 12-             backwards and forwards, or to  vibrate un-
               <&                der this point.  Being raised to c, and then
                                set at liberty, it  falls back to a with an ac-
                                celerating motion like a ball rolling down
                                a slope, and when arrived there, it has just
  t£                            acquired momentum enough to carry it to
                          yC    d, at an elevation on the other  side; from
                                this  it falls back again, again to rise; and
                                would so go on for ever, but for the impedi-
                                ments of air and friction.—The pendulum
                                is strictly an object of mathematical study;
 
                              PENDULUM.                           61

but we shall give a general  idea of its important characteristics in common
language.
   1. The times of the vibrations of a pendulum are very nearly equal, whe-
ther it be moving much or little, that is to say, whether the arc described by
it be large or  small.  This  remarkable property  is what makes  it a time-
keeper.  The reason that a  large vibration is performed  in the same time as
a small one, in other words, that  the pendulum always moves faster in pro-
portion as its journey is longer—is, that in proportion as the arc described is
more extended, the steeper are its beginning and ending, and the more rapidly,
therefore, the  pendulum falls  down  at first, sweeps along the intermediate
space, and  stops at last.  It is evident, for instance, that  the portion c e of
the arc (fig. 13) is much more steep than the equal portion e c.—A pendulum
made to vibrate in the curve called a cycloid,  which, in the central part, very
nearly coincides with a circular arc, but towards the extremity rises a little
more steeply, has its beats perfectly isochronous, or in equal times, whatever
their extent.
   A common clock is merely a pendulum with wheel-work attached to it, to
record the number of the vibrations, and with  a weight or spring having force
enough to counteract the retarding effects of friction and the resistance of the
air.  The wheels  show how many swings or beats of the pendulum have
taken place, because  at every beat, a tooth of the last wheel  is allowed to
pass.   Now if this wheel has sixty teeth, as is common, it will just turn
round once for sixty beats of the pendulum, or seconds, and a hand fixed on
its axis projecting through the dial-plate, will be the second hand of the clock.
The other wheels are so connected with the  first, and the numbers of teeth
on them so proportioned, that one turns sixty times  slower than the first, to
fit its axis to  carry a minute  hand, and another by moving  twelve  times
slower still, is fitted to carry an hour hand.
   2. The length of a pendulum influences the time of  its vibration.—Long
pendulums vibrate more slowly than short ones, be-
cause, in corresponding arcs or paths, the hob or ball         Fig. 13.
of the  long  pendulum has a greater journey to per-
form, without  having a  steeper line of descent. If a                ,
pendulum 6 a be twice  as long as another reaching              /
from b to e, it  has twice as much to fall in its descend-         ^(A-.....
ing arc c a, as the other in  its arc d e, while in cor-         /"'".....
responding parts of the two paths, the slope  or inch-        /
nation is always equal:—the ball of the long pendulum   ^/
may be considered as having rolled twice as far down   J3)........................
a given slope as the ball of the short pendulum.  Now      "'•-...
as a body falls four times as far, either directly or on
any uniform slope, in two seconds, as in one, a pendu-                 Y
lum must be four times as long, to beat once in  two
seconds, as to beat every second.   A pendulum of a little more than 39 inches
beats seconds; one of four times the length is required to beat double seconds,
and one of  one-fourth the length to beat half seconds.—As a pendulum to
answer its  purpose must be of invariable length, one which beats seconds
constitutes an  easily  found standard of measure.
  Because the smallest  change in  a length  of a pendulum  alters  the rate of
going of the clock, it is important to be able to counteract the dilatation or con-
traction of pendulums caused  by the changing heat of the seasons; and for
this purpose various ingenious means have been contrived.  One of the best
of these is the  gridiron pendulum, as it is called, from consisting  of various 
62
MOTIONS AND FORCES.
          rods of metal.   It renders the different dilatability by heat of two
 Fig- 14.   metals composing it, the cause of unchanged length in the whole.
    o      The adjoining sketch may show that if the  central rod of brass
  rl]      represented by the  strong line  from b to c, dilate alone  just as
    $     much as the two rods of steel, represented by the weaker lines on
          either side of the other, dilate together (the expansion of brass by
          heat is about double that of steel,) it will exactly counteract  the
          lengthening of these, and will keep the ball d always at the same
          distance from the point  of suspension a.    Some astronomical
          clocks in the present day are so perfect that they do not err one
          beat of the pendulum in  a  year.  Common clocks are regulated
  W     by a screw which lifts or lets down the ball of the pendulum, and
  c\      so changes the  effective length, that  is, the distance between  the
  0<^   point of suspension and what  is called the  centre of oscillation,
          treated of in the next chapter.
  3. The force of gravity, of course, is what determines how long the pen-
dulum  shall be in falling to the bottom of  its arc, and how long in rising, for
the  ball of the  pendulum, as already stated, may be considered as a body
descending by its weight on a slope; a change in the force of gravity, there-
fore, would at once alter the rate of  all the clocks on earth.   At the equator
of our  earth,  where the gravity of bodies is counteracted in a small degree
by the  centrifugal force arising from the earth's  motion  (as explained atpage
53,) a  pendulum vibrates more slowly than  elsewhere, and must therefore be
made shorter to answer the same purpose.   Corresponding results take place
when a pendulum  is carried to a mountain top, and therefore farther away
from the centre of the earth, which  is the centre of attraction—or when  car-
ried to the bottom of a mine, where it is attracted by the matter  above it, as
well as by the matter beneath.
  The popular  prejudice  refuted at page  53,  that a large  or  heavy body
should fall to the earth, even in a vacuum, more quickly than a small or light
body,  attaches itself also  to the case of a heavy and  a  light  pendulum.
Now there is no  difference  for  pendulums  of the same  length, whatever
their weight  or material, but what depends on  the resistance of the air.  It
is a very remarkable fact thus  proved, that in all substances the gravity and
inertia perfectly agree.
                    There is a small pendulum  called a metronome, used
    Fig. 15.       by  musicians for marking  time; which, although very
                  short, may  still be made to  beat whole seconds, or even
                  longer  intervals.   The reason of its slow motion is,  that
                  its rod is prolonged beyond  its axis of support, at a, up-
                  wards, to b, and  has a ball upon the top  at b, as well as
                  on the  bottom at c; which upper ball prevents the under
                  one from moving so fast as it otherwise would, just as a
                  small weight attached  to one end  of a weighing-beam,
                  prevents a  greater weight attached  to the other end from
                  falling so fast as it would if there were no counterpoise.
                  The rate of motion changes with any change  in the dis-
                   tance of the ball b from the  centre  of motion a; and  to
                   allow of such change, the ball b is made to slide.
   A pocket-watch  differs from a clock in having a vibrating wheel instead
 of  a vibrating pendulum ; and  as, in a clock, gravity is always  pulling the
 pendulum down to the bottom of its arc, which is its  natural  place of rest,
 but does not fix it there, because  the momentum  acquired during its fall
At 
PENDULUM.
63
from one side is just sufficient to carry it up to an equal height on the other
—so in a watch, a spring, generally spiral, surrounding  the axis of the ba-
lance-wheel is always forcing this towards a middle position of rest, but does
not fix it there, because the momentum acquired during its approach from
either side to the middle position, carries it just as far past on the other side,
and the  spring has to begin its work again.  The balance-wheel at each
vibration allows one  tooth of the  adjoining wheel to  pass, as the pendulum
does in a clock, and the record of the beats is preserved by the wheels which
follow, as already explained for the clock.  A  main-spring  is used to keep
up the motion of a  watch,  instead of the weight used in a clock; and as a
spring acts equally, whatever be its position, a w7atch keeps time although
carried in the pocket or in a moving ship.
  As the rate of a clock is  influenced by the length of its pendulum, so is
the rate of a watch by the size or  diameter of its balance-wheel; and heat,
which retards  the motion of a common clock by lengthening the pendulum,
retards the motion of a common watch by dilating the balance-wheel.  Inge-
nuity, however, has  found  a  remedy for the latter case as for  the former,
viz., the contrivance called the expansion balance-wheel.  Of this  the circum-
ference, instead of being a  continuous ring, is made  up of two half-rings,
each attached by one end only, to a cross bar, and which half-rings being of
brass on  the outside and of steel within, bend or curl inwards  by heat—as
a sheet of damp paper bends when held  to the fire—and thus diminish the
size of the wheel at their  loose extremities, so as just to counterbalance its
increase by the expansion of the cross bar.
  As the motion of a pendulum has relation to the force of gravity, so has
the motion  of  a balance-wheel fo  the stiffness of the  balance-spring;  and
the regulator of a watch is  merely a  pin which  bears  against the balance-
spring, and by sliding  backwards  or forwards, so as to  shorten or lengthen
the part of the spring left  free to act, changes  the degree of its stiffness.
A change  produced by the  variation  of temperature is compensated  for by
the expansion-wheel described above.
  It would be exceeding the limit marked out for this general work, to speak
more particularly here of those admirable watches which have been pro-
duced within the last thirty years under the name of chronometers, for the
purpose  of ascertaining the longitude at sea ; but the  author may  perhaps
be excused for mentioning a moment of surprise  and delight which he expe-
perienced, on  first  seeing  their singular perfection actually proved.  After
months spent in a passage from South America  to Asia, his pocket chrono-
meter, with others on board, announced  one morning that a certain  point of
land was then bearing east from the ship at a distance  of fifty miles ; and in
an hour  afterwards,  when a mist had  cleared away, the looker-out  on  the
mast gave the joyous  call of " Land a-head!" verifying the report of the
chronometers almost to a mile after a voyage of thousands.  It is natural, at
such a moment, with the dangers and uncertainties of ancient navigation be-
fore the mind, to exult in contemplating what man has now achieved.   Had
the rate of the wonderful little instrument in all that time been changed even
a little, its  announcement would  have been worse than useless,—but in the
night and in the day, in storm  and in calm, in  heat and in cold, while  the
persons around it were experiencing every vicissitude of mental and bodily
condition, its steady beat went on, keeping exact account of the rolling of
the earth and of the  stars; and in  the midst of  the trackless waves it  was
always ready to tell its magic tale of the  very spot of the globe  over which
it had arrived.   The mode of using a chronometer for so valuable a purpose
will be explained in the section on  astronomy. 
64
MOTIONS AND FORCES.
  Bent or curvilinear  motion from attraction.—This  takes place  when-
ever attraction is acting across the path of any existing free  motion.  The
flying cannon-ball or stone, drawn down by gravity, is an example,  for the
projectile  force ceases  with the first impulse,  but the bending force is  acting
every instant, and by every instant producing a new effect, causes a curvili-
near path.
  An oblique jet of  water is to the eye a permanent exhibition of the curve
described  by a body thus projected.   The particles of the liquid move in
the line which they would describe if projected singly, and  the  continued
succession of them marks the line of situations through which each  passes
in its course to the earth.
  A cannon  or musket-ball, shot quite  horizontally over a level plain, will
touch the  ground or  plain just as soon as another  ball  dropped  at the same
instant directly from  the  cannon's mouth; for the  forward or  projectile mo-
tion does not, in such case, at all interfere with the action of gravity.   This
result, which most persons, before consideration, would be disposed to doubt,
makes strikingly sensible the extraordinary speed of the cannon-ball; viz.,
that it has already moved, perhaps, six hundred feet forward, during the half
second that a ball dropped from the  hand of  a standing person  requires to
reach the  earth only four feet beneath.   This  fact also explains why, for a
long range, the gun must be pointed  more or less upwards.
  A dozen marbles  swept horizontally from off a table by a stick, all reach
the floor at the same instant, how different soever the distances to which they
may respectfully be  driven.
  The particular study of the subject projectiles is very important to military
engineers  ; and we know how successfully they have pursued  it, by the pre-
cision with which they now direct  their shot  and  shells  to objects at very
great distances.
                              A cannon-ball shot horizontally from the  top
                           of  a lofty mountain,  would go  three or  four
                            miles.  (The mountain is here  represented on
                           an enlarged scale, as standing on  the globe b, c,
                           d, at a.)   If there were no atmosphere, to resist
                           its  motion, or if  the  mountain top were above
                           the surface of the atmosphere, the same original
                           velocity would carry  it thirty or  forty miles be-
                           fore it  fell, as to b:   with more force  still, it
                           would reach to c, and with still more to d.  And
                           if it could be dispatched with about ten times
                           the velocity of a  common cannon-shot, it would
                           not have approached nearer to the earth  than at
                           first, even when it had again reached round e
or  to a;  and its velocity being undiminished, it  would perform a  second
similar tour,  and then  a  third, and so forth : it would,  in fact, have become
a little satellite, or planetary body, revolving round the earth.   In the suc-
cessive ranges represented  in the figure, it is  seen that the centrifugal force
of  the ball, or us tendency to move in a straight line  becomes more  and
more nearly a counterbalance to gravity, and at last is exactly equal to it.  If
the force given to the ball were more than sufficient to brino-  it round again
to the level of a, it would for a time fly off, or increase its distance from the
earth, acquiring somewhat of the eccentric motion of a comet.  There may
really be  such revolving masses above our atmosphere, although  invisible
to us, owing to their smallness.  It  has been supposed by some, that the
 
PROJECTILES.
65
meteoric stones, which fall to the earth every now and then, come from such
bodies, or are the entire masses, having become entangled in our atmosphere,
so  as to lose their  forward  velocity.   The four little planets  discovered
lately beyond the orbit of Mars, are not larger than a six-thousandth part of
our earth.
  Repulsion,—produces accelerated, retarded, and bent motions, like attrac-
tion, but it acts only at minute distances, while  attraction  draws from  the
sun, or from the very limits of the universe ; repulsion acts, for instance, be-
tween the adjoining  atoms of an elastic fluid.  Yet repulsion plays a part in
the economy of nature, not at all inferior to its  sister attraction.   We have
already seen, when considering the  constitution of masses  in section first,
that repulsion prevents or modifies the  contact of the atoms of all  bodies ;
that with increase of temperature, it causes these atoms to  separate, and of
a solid forms a liquid, or even an air;  that it  operates around all masses as
if it were a film or covering, preventing their mutual cohesion, &c. &c.
  Accelerated motion from repulsion is seen when the atoms of gunpowder
explode and propel the bullet from the bottom of a piece to  the muzzle with
such  rapidly increasing  velocity.   The strength  of this repulsion  of gun-
powder is so much greater than the strength of gravity or common attraction,
that its action on a bullet, during the passage along a barrel of five or six feet
in length, may  not be overcome  by gravity, during an ascent of a  mile  or
more.
  A visible retarded motion from repulsion is exemplified by a moving body
coming against a spring or a bladder full of air, or against the piston-handle
of an air-syringe, so  as to compress the air beneath it.
  Any elastic body striking  against  another body and recoiling, exhibits  in
conjunction the  phenomena of retardation, acceleration, and often also  of
bending, chiefly from repulsion ;  for instance :
  An ivory ball driven forcibly against a marble slab, does not stop at  the
instant that apparent contact takes place, but still advances and compresses
that part of the substance which is against the marble,—as  is proved by  the
facts mentioned  at page 37.   While this compression of the  ivory is going
on,  the resistance made by the increasing repulsion of the particles gradually
retards, and ultimately destroys the forward motion of  the ball;  and at the
instant of its final arrest, the parts in contact, both of the ball and of the mar-
ble, being in their greatest degree of compression, act on the ball, and repel
it again with gradually accelerating motion, until it leaves  the marble with
the  same velocity which  it had on approaching.   The retardation and acce-
leration take place here within so small a space, and in  so short a time, that
they are not apparent to sense, but the mind perceives the nature of the phe-
nomenon as distinctly  as if the ball had rolled against the end of a long steel
spring.—If the  ball strike the marble obliquely, as from a to c, in a path form-
ing  the angle a c d with a perpendicular line, it does not rebound in the same
line by which  it approached, but just as obliquely to-
wards the other side, viz. from c to b; and it then exhibits        Fig. 17.
a bent motion from repulsion. This case illustrates also          erl
the  " resolution  of motions," for the oblique descent a c
being composed of a direct downward motion  from a to     j,
the  table, and a horizontal or  forward motion from a to-
wards  the  perpendicular, the table destroys the  down-
ward  motion and converts it into an opposite directly   ___
upward motion, but  it does  not affect the forward mo-
tion, which immediately combines  again with the  up-
/ 
66
MOTIONS AND FORCES.
ward and  carries the ball as far beyond the  perpendicular at b as it was dis-
tant from  it at a.   The important law in  physics, of which  this case is  an
example, is usually expressed—" The angles of incidence and ot reflection
are equal."   It applies to  all reflected bodies, as balls, waves, sound, light,

  If the ivory ball and marble, in the above case, were supposed to be both
perfectly hard, and  without elasticity, still the repulsion which surrounds  all
bodies, as a  thin covering, preventing their cohesion (see page  32,) would
act exactly as the real  elasticity of the ivory, and would cause a retarded
motion  until perfect rest came, and then an accelerated motion back again,
until the ball recovered its  primitive velocity.
  Collision between hard  bodies  always exhibits more or less of the  truth
now described:  when it occurs between soft bodies, as lumps of lead or  of
moist clay, the approaching parts  mutually displace each other, and there is
no recoil.
  When a straight  steel plate,  of  which the end is fixed in a block, is bent,
as by a ball rolling  against it, the particles on the side which becomes con-
cave are made to approximate,  and there is a resistance or  repulsion gradu-
ally  increasing among  them ;  the particles on the convex  side, again, are
drawn a little more from each  other, and  are therefore exerting attraction to
return:  the recoil of the spring is thus owing to both  forces trying to replace
the particles in their former relative situations.

   "Tides, Winds, fyc, exemplify Attraction."   (Read the Analysis,
                               page 42.)

Until we  reflect attentively on this subject, we are far from perceiving that
  all the phenomena of nature are  only instances of attraction and repulsion,
  acting under a variety of circumstances.

  Attraction.—Tides are raised by the attraction of the moon and sun, and
fall again  by the general attraction  of the  earth ; producing in many of  the
shallower  parts  of the ocean  very rapid horizontal  currents.   They do a
great deal of work  for man.  They carry his ships along the coasts, and  up
and down the rivers ; they turn water-wheels for him ; they fill his docks and
canals at convenient times ;  they rise to receive his ships, launched from ele-
vated building-yards, &c.    What a busy scene is a great sea-port river, dur-
ing the rising and  falling of the tide—with the thousands of people  along its
banks, borrowing assistance in  their various occupations !
   Winds are produced chiefly by the fluid atmosphere seeking its  level, in
obedience to the attraction  of the earth, after the action of disturbing causes,
such as the heat of the sun, &c.   They help  man  in the important business
of navigation ;  they turn his windmills, &c.
   The currents of rivers are water constantly descending on slopes, that is,
regaining its level, in obedience to the earth's attraction.   Water-mills  and
inland navigation are among the advantages which they afford to man.
   All falling and pressing bodies exhibit attraction in its simplest form.
   Repulsion—is instanced in explosion, steam, the action of springs, fyc.
   Explosion of gunpowder is repulsion among the particles when°assuming
the form  of air.
   Steam, by the repulsion among its particles, moves the piston of the steam-
engine.   In our days it performs half the  labour of society.
   Accidental explosions of fire-damp, or hydrogen  in mines,  and the  tie- 
PRODUCTION OF GREAT VELOCITIES.
67
mendous evolutions of elastic fluid  in  volcanoes and earthquakes, are other
instances of the same class.
  Elasticity, as seen in springs, collision, &c, belongs chiefly to repulsion ;
as seen in  india-rubber, and  other  substances resuming their usual length
afier extension, it belongs chiefly to attraction.
  A spring is often, as it were,  a reservoir of force, kept ready charged for a
purpose ; as when a gunlockis  cocked, a watch wound up, &c.
  It will be remarked, with respect to many of the phenomena now and here-
after to be  mentioned, that it is not the original Attraction or Repulsion which
man uses as his servant, but the momentum gradually accumulated in masses
by the exertion  of such attraction or repulsion ; in other words, the inertia is
used as a great  working power or force.
  Electrical, galvanic, magnetical, and optical phenomena, are also in great
part peculiar attractions and repulsions, as will be seen in the chapters devoted
to the explanation  of them.   And even the actions  of animals, so infinitely
varied, are  all results  of a shortening of the fleshy threads called  muscular
fibres, which is  produced by the mutual attraction of their component parti-
cles ;—just as the varied motions of a telegraph, or of a ship's yards, are pro-
duced by the shortening of certain ropes of connection.
  However closely allied the last-mentioned particular attractions and repul-
sions may be to the general attraction and  repulsion formerly treated of, it is
found convenient to consider them apart.

In the remarkable  phenomena of nature and art, all the motions being caused,
  as now shown, by Attraction and Repulsion, these forces do not operate by
  a single impulse, but through a  repetition of impulses, or a continued
  action, of which the effect is gradually accumulated in the inertia of matter.
  Thus all great velocities and momenta are the terminations of an accele-
  rated motion.

  Meteoric stones, falling from great heights, bury themselves deep  in the
earth by the force of their gradually acquired  velocity.
  When the wood-cutters among the Alps launch an enormous tree from high
on the mountain side, along  the smooth wooden trough or  channel prepared
for  it, and  in fewer minutes than it traverses miles, it is seen  plunging into
the lake below; it acquires its frightful velocity, not at once, but through  the
action of gravity continued during the whole of its  descent.
  The shock or blow of the ram of a pile-engine, is not the effect of momen-
tary attraction  between it and the earth, but  of that attraction accumulating
motal inertia or power, during the descent of the ram through a space of
twenty or  thirty feet.
  A common hammer, in its instantaneous shock, has the condensed effect
of the arm  and  of gravity, as accumulated through its whole previous course;
and when a powerful blow is intended, the  hammer, or hatchet, or  club, or
fist in boxing, is lifted high, or carried  far back,  that there may be  time and
space for imparting greater power.

The inferior animals, by many  of their actions, illustrate the same truth, and
  prove their experimental or instinctive acquaintance  with it.

  Sea-birds carry  shell-fish  up into the air, and drop them on smooth stones
to break them,  and to obtain the food.   It is related  in  Grecian story, that a
bird once  mistook the venerable bald head  of a sage meditating on the sea- 
6S
MOTIONS AND FORCES.
shore for a smooth stone, and by the same act killed an oyster and the philo-
sopher.
  There are some long-necked birds, that fight and kill their prey by a blow
of their beak.   They draw back  the  head,  bending  the  neck like a swan or
serpent, and then dart it forward, with  a continued  effort, until the strong
wedge-like beak reaches  its  destination, almost with the velocity of a pistol
bullet.  One snake in darting its fangs at another passing swiftly across its
coil, has been known to miss its aim and inflict a mortal wound on its own
flesh.
  Bulls, rams  and goats, in  fighting, alternately recede and  run at each
other, that the shock may be great when  their  foreheads  meet.
  A horse in kicking, from  the great length of his leg,  and the consequent
space through which he can be adding velocity to his foot, drives it at last
against the object almost like a cannon  shot.
  A bow-string propelling an arrow, follows it  through a considerable space,
and so gives the great velocity at last produced.
  A sling gives to the hand the power of adding velocity to the stone through
a long path; for the hand moves in a small circle while the stone moves in a
larger, and the  hand being kept always somewhat in advance of the stone,
pulls at it  without intermission, until the moment of discharge.
  The battering-rams of the ancients allowed those about them to accumulate
in them the efforts of many  hands, and of a considerable duration of action,
so as  to give at last one great and sudden shock.
  Even the gentle action of  the human  breath, exerted for a time on a pea or
small hard ball of clay while passing through a long smooth tube, gives a velo-
city which will inflict a sharp and painful stroke on a distant animal.   In Bor-
neo and others of the Eastern Islands, poisoned arrows are thrown in this way
with great force and precision.
  The action of gunpowder on bullets, although appearing so su'dden, is still
not an instantaneous, but a gradual, and therefore accelerating action ; and ac-
cordingly we find the effect to depend much on the length of the piece along
which the force pursues the  ball.  A small  fast sailing  vessel with  a single
long gun, has  often  compelled a very superior  vessel, whose guns  were
shorter, to yield.

For the same reason that all great velocities require continued action or re-
  peated  impulse to produce them, so do  they also to destroy them; the
  inertia of motion and of rest being exactly equal.

  A vast  mass of rock suspended  like a pendulum, and allowed  to sweep
down its curve  from  a considerable elevation, would arrive at the bottom like
a battering-ram, with force  sufficient to shake a thick wall or rampart to its
foundation.  The continued action  of  gravity would have  given this force,
and if, instead of the solid resistance supposed, and which would scarcely be
sufficient to take the whole momentum  away, the mass were merely allowed
to continue its  course as a pendulum, and  to  ascend on the  other  side, the
continued action of gravity then opposing its motion, would  bring it to pow-
erless rest again, by  the time when it had reached an elevation  equal to that
from which it  fell.
  Soft air expanding gives  gradually the death-carrying velocity to the can-
non-ball ;  and soft air, or cotton, or wool, resisting in a close strono-  tube,—if
the bullet  could be directed  exactly into it—would again  gradually annihilate
the motion.   Were the  attempt made,  however, to  stop the ball suddenly, 
PRODUCTION OF GREAT VELOCITIES.
69
by a block of the hardest granite, the block would instantly be riven by its
force.
   Bales of cotton or thick masses of cork, attached round a ship, will receive
cannon-balls,  and bring them to rest, without themselves suffering much,
while the naked firmer side of the ship would be penetrated.   The cotton or
cork offers an increasing resistance through a considerable space, while the
oak opposes its hard front at once, and must instantly suffice or be destroyed.
A hard body,  that it may at once destroy such a motion as we are supposing,
must be able to oppose  as much force in perhaps the space of one-hundredth
of an inch, that is, in the extent to which its elasticity  will let it yield without
breaking, as  the  moving cause gave, through a much greater space (a  plate
of steel will thus  oppose a pistol-bullet;)  and when it  cannot do this, it  must
be broken or penetrated by the moving body.  It is  to be remarked,  how-
ever, that the continued opposition of a thick mass of wood, stone, or earth, to
an entered bullet, brings it to rest at last as any elastic unbroken opposition
would.  Gunners have ascertained  the  exact depth  in each substance to
which  a ball will penetrate;  and they call buildings bomb-proof or ball-proof,
which  have a  thickness or depth exceeding that.
   A hempen or silken rope supporting the scale of a weighing beam, would
resist a greater weight falling into the scale than would be resisted by  an
iron chain which  were even  stronger than the rope for the purpose of  bear-
ing a quiescent weight: because the hemp or silk would yield by  its elasticity,
and continue its resistance  through a considerable space and time, and thus
would  at last gradually overcome the momentum;  while the iron, by scarcely
yielding at all, would require  to be strong enough to stop the mass suddenlv
or would break.
   Yet  for the same reason that iron is weakest in such a case as the last, it
is stronger than hemp or rope when  used as a cable for a ship, to  withstand
the sudden force of waves.
   This will be understood on considering, that the chain by its weights hangs
as a curve or inverted arch in the water, while the rope, being nearly of the
weight of water, is supported in it almost as a straight line from the anchor to
the ship;  therefore, when a great wave dashes against the ship, the bent chain
will  yield until it  be drawn nearly straight, by which great extent of yielding,
and consequent length of resistance, it will withstand a great shock ;  whereas,
the straight rope, as it can yield only by the  elasticity of its material, and
comparatively, therefore, a little way, will resist much less.
  A heavy ship moving quickly with the tide or wind, could not be stopped
instantly by a short tope or chain of any magnitude: if the attempt  were
made to destroy at once so vast  a momentum, something would  certainly
give way; but a rope of very moderate size, kept tight between  the shore
and the ship,  and from  time to time allowed to slip a little round a wooden
block, when the tightness threatened its breaking,  would accomplish the end
very soon and  easily.

The following are farther proofs that forces are to be  measured  as much by
  the time or space through which they act, as by their difference of intensity
  or momentary  power.

  A door standing open, and  which would yield readily on its hinges to the
gentle  push of a finger, is not moved by a cannon-ball piercing through it.
Now the ball really overcomes the  whole force of  cohesion among the atoms
of tough wood:  but that force is allowed to act or resist for so short a time,
     6 
70
MOTIONS AND FORCES.
owing to the rapid  passage of the ball, that it is not sufficient  to affect the
inertia of the door, in  a degree to  produce sensible motion.  The cohesion of
the circle in the door, cut out by the ball, would have borne a weight of more
than a hundred pounds laid quietly upon it,  but supposing  the  bullet to fly
twelve hundred feet in a second, and the  door to be one inch thick, the cohe-
sion being allowed to  act for only  the 14,400th part of a second, its influence
is  not perceived.  The following  are other examples of the same kind.
   A leaden bullet pressed slowly against a  pane of glass, breaks it irregu-
larly, where the  strength happens to be least; but the same bullet shot at  it
from a pistol, makes  only a small round hole.   It  has been amusingly said
of such a case, that the particles  struck and  carried away, have not  time to
warn their neighbours of what is  happening.
   A cannon ball, having very great velocity, passes through a ship's side, and
leaves but a little mark ; while one with less  speed  splinters and breaks the
wood to a considerable distance around.  A near  shot thus often injures a
ship less than one from a greater distance.
   A sheet of paper standing edgeways on a  table, is  not driven down by a
pistol-ball fired through it.
   The truth at present under consideration explains, with respect to gun-shot
wounds, why the man often remains  ignorant for  a time of his misfortune,
and why a rapid bullet only kills the parts which it touches, while a spent
ball may bruise and injure all around.   In many cases of injury, popularly
attributed to the wind of a ball, the ball itself has really touched the  part.
   A man lying down  and receiving the blow of a great hammer on his chest,
would be killed by  it; but if a  heavy anvil be first laid upon the chest, and
the blow  then received  upon the anvil, the man bears it with impunity.
Here the quantity of  motion in  the hammer  being diffused through the great
mass of the anvil, produces but a  trifling velocity, which  the elasticity of the
chest, in its  slow yielding, easily overcomes.
   A circular plate of soft iron,  made to turn  with extreme rapidity, will cut
through the hardest steel file, almost as  a knife cuts through a carrot.  In
cases where  a soft powder  suffices to polish  a hard body, it acts partly like
this plate, by the motion or velocity given to  the wearing particles.


"TJiere is no motion  or action in the  universe, without a concomitant and
         opposite action of  equal amount."  (See the Analysis.)
   This truth has otherwise been expressed—"action and reaction are  equal
and contrary."—It  is  evident, that if no  action or  movement takes place on
earth but  in consequence of either Attraction or Repulsion,—and this has
now been shown—there must always  be two objects  or masses  concerned,
and each must be attracted  or repelled just  as much  as  the other, although
one will have less velocity than the other, as  it may be itself greater, or fixed
to another mass.
   If a man  in one boat pull  at a rope attached to another, the two boats will
approach.   If they be of equal size  and load, they will both move at the
same rate, in whichever of  the boats  the man  may  be; and  if there be a
difference in the sizes, and resistances, there will be a corresponding differ-
ence in the velocities, the smaller boat  moving the fastest.
   A magnet and a piece of iron attract each  other equally, whatever dispro-
portion there is between the masses.  If either be balanced in a  scale, and
the other be then brought within a certain distance  beneath it, the very same
counterpoise will be required to prevent their approach, whichever be in the 
ACTION AND REACTION EQUAL.
71
scale.  If the two were hanging near each other as pendulums, they would
approach and meet;  but the little one would perform more of the journey in
proportion to its littleness.
  A man in a  boat  pulling a rope  attached to a large ship, seems  only to
move the boat: but he really moves the ship a little, for, supposing the
resistance of the ship to be just a thousand  times  greater than that of the
boat, a thousand men in a thousand boats, pulling simultaneously in the same
manner would make the ship meet them half way.
  A pound of lead and the  earth attract each other with equal force, but that
force makes the lead approach sixteen  feet  in a second towards the earth,
while the contrary motion of the earth is of course as much less than this as
the earth is weightier than one pound,—and is therefore unnoticed.   Speak-
ing strictly,  it is true, that even a feather falling lifts the earth towards it, and
that a man jumping kicks the earth away.
  A spring unbending between two equal bodies, throws them off with equal
velocity;  if between bodies of different magnitudes, the velocity of the
smaller body is greater in proportion to  its smallness.
  On firing  a cannon, the gun recoils with even more motion or momentum
in it than the ball has, for it suffers  the  reaction of the expelled gunpowder
as well as of the ball; but the momentum in the gun being diffused through a
greater mass, the velocity is small, and easily checked.
  The recoil of a light  fowling-piece will hurt the shoulder, if the  piece be
not held close to it.
   A ship in chase, by firing her bow  guns, retards her motion; by firing
from her stern she quickens it.
   A ship firing a broadside, heels or inclines to the opposite side.
   A vessel of water suspended  by a cord hangs perpendicularly:  but  if a
hole be  opened on  one side, so as  to allow the  water to jet out there, the
vessel will be pushed to the other side by the reaction of the jet, and will so
remain while it flows.  If the hole be oblique, the vessel will constantly  turn
round.
  A vessel of water placed upon a  floating  piece of plank, and allowed to
throw out a jet, as in the last case, moves the plank in  the opposite direction.
  A steamboat  may be driven by making the engine pump or squirt water
from the stern, instead of making it, as  usual,  move paddle-wheels.   There
is a loss of power, however, in this mode of applying it, as will be explained
under the head of " Hydraulics."
   A man floating  in a small boat, and blowing  strongly with a  bellows
towards the  stern, pushes himself onwards with the same force with which
the air issues from the bellows-pipe.
  A sky-rocket ascends, because, after it is lighted, the lower part is always
producing a large quantity of aeriform  fluid,  which,  in expanding, presses
not only on  the air  below, but also  on  the rocket  above, and  thus lifts it.
The ascent is aided also by the recoil of the rocket from the part of its  sub-
stance, which is constantly bursting downwards.
  He was a foolish man who thought he had found the means of command-
ing always a fair wind for his pleasure-boat, by erecting an immense bellows
in the stern.  The bellows  and sails acted against each other, and there was
no motion: indeed, in a perfect calm, there would be a little backward motion,
because the  sail  would not catch  all the wind from the bellows.
  A man supported on a floating plank, by walking  towards  one end of it
gives it a motion in the direction opposite. 
72
MOTIONS AND FORCES.
  A man using  an  oar, or a  steam-engine  turning paddle-wheels, advances
exactly with the force that drives the water astern.           • iv •  u   j
  A swimmer pressing the water downwards and backwards with his hands,
is sent forwards and "upwards  with the same  force, by the reaction of  the

  And a bird flying, is upheld  with exactly the force with  which it strikes
the  air in the opposite direction.
  A man  pushing  against the  ground with a  stick,  maybe considered as
compressing  a spring  between the  earth and  the end of his  stick, which
spring is therefore pushing him up as much as he pushes down; and  if, at
the  time, he were balanced in the scale of a weighing beam, he would find
that he weighed just as much less as he was pressing with his stick.
  Thus an invalid,  on  a spring  plank or chair, who, by  a trifling  downward
pressure of his hand on a staff*  or on a table, causes his  body to rise and fall
through a great range, and thus  obtains the advantage of almost passive  exer-
cise, is really lifting himself while he presses downward.
  When a boy cries, on  knocking his head against a table or pane of glass,
he is commonly told, and truly, that he has given as hard a blow as he has
received;  although his  philosophy probably,  looking chiefly to  results,
blames the  table for his head hurt, and his head for the glass broken.
  The difference of momentum acquired in a  fall of one foot or of several,
is well known : the corresponding  intensities  of reaction are unpleasantly
experienced by a man who sits  down in  an  easy chair, or who, in sitting
down where he supposed a chair to  be, unexpectedly reaches  the floor.
  What motion the wind has given to a ship it has itself lost, that is to say,
the ship has  reacted on the moving air: as is  seen when one vessel is  be-
calmed under  the lee of another.
  When one billiard-ball strikes directly another ball of equal size, it stops,
and the second ball proceeds with the whole velocity which the first had—
the action which imparts the new motion being equal to the reaction which
destroys the old. Although the transference of  motion, in such a case, seems
to be instantaneous, the change is really progressive, and as follows.  The
approaching ball, at a certain  point of time, has just given half of its motion
to the other equal ball, and if both were of soft clay, they would then  proceed
together with  half the original velocity; but, as they are elastic, the touching
parts at the moment supposed are compressed like a spring between the balls,
and by then  expanding,  and  exerting  force equally  both ways, they double
the velocity of the  foremost  ball, and  destroy altogether the motion of that
behind.
  If a billiard-ball  be propelled against the nearest one of a row of balls
equal to itself, it comes to rest as in the last case described, while the farthest
ball of the row darts  off with  its  velocity,—the intermediate balls having
each received and transmitted the motion in a  twinkling, without appearing
themselves to move.
  As farther illustrative of the  truths, that action and reaction are equal  and
contrary, and that in every case of hard bodies striking each other, they may
be  regarded  as  compressing a very small  strong spring between them, we
may mention, that  when  any elastic body, as a billiard-ball, strikes  another
body larger than itself, and rebounds, it gives to that other, not only all the
motion which it originally possessed, this  being done at the  moment when
it comes to rest, but an additional quantity, equal to that with which it recoils
—owing to  the equal action in  both directions of the repulsion or spring 
ACTION AND REACTION EQUAL.
73
which causes the recoil.   When the difference of size between the bodies is
very  great,  the  returning velocity  of the smaller is  nearly as great as its
advancing motion  was, and thus it gives a momentum to the body struck
nearly double of what it originally  itself possessed.  This phenomena con-
stitutes the paradoxical case of an effect being greater than its cause, and has
led persons, imperfectly acquainted with the subject, to seek from the prin-
ciple, a  perpetuum mobile.  A  hammer on rebounding from an anvil has
given a blow nearly double the  force which  it had itself, for the anvil felt its
full original force while stopping it, and then, equally with itself, was affected
by the repulsion which caused its return.
  Many other interesting facts might be adduced as examples of equal action
and reaction, but these will suffice.
  This  second section of the work  has now explained the nature of inertia
in matter, and has shown that  the  infinitely varied phenomena of motion,
which the universe  exhibits, are only  attraction and repulsion, acting on
inertia of atoms separate or conjoined, under diversified  circumstances.—
And such is the  sublime simplicity of the whole  scheme of nature. 
74                            APPENDIX.
     APPENDIX

TO  PART  I.—SECTION  II.
BY THE AMERICAN EDITOR.
  The attentive perusal of the preceding section will prepare the reader to
understand the following propositions.

                              Definitions.
  Prop.  1.—When a body is successively changing its place it is said to be
in motion, p. 42.
  The idea of motion involves those of space, time, velocity, direction, the
quantity  of matter and momentum.
  Prop.  2.—The  space described is the  distance passed over by a body
during its motion;  and is measured  by the number of units of length, as a
foot, a yard,  a mile, &c. contained in this  distance.
  Prop.  3.—The time consists of a certain number of units of time adopted
as its measure, as  a second, a  minute,  &c, which have  elapsed during the
motion of a body.
  Prop.  4.—The  velocity of a body is the rate at which  it  moves, or the
number of these assumed  units of space that it passes  over during  the as-
sumed unit of time.
  All the above  measures  may be represented  graphically by lines that are
proportioned to them, p. 65.
  Prop.  5.—The  direction  of a body may be  straight or  curved; when
straight or rectilinear, it  is the  angle which its path makes with any straight
line in the same plane, adopted as an axis;  when the path of a  body is a
curve, its direction at any point is the angle which the tangent to the curve
at the point makes with  the fixed axis.
  Prop.  6.—The momentum of a body is its  quantity of motion, both the
mass and velocity  being taken  into consideration, and its  proper measure is
the product of the mass into the velocity,  pp. 53, 54.
  Prop.  7.—A  body is said to have  a uniform motion when its velocity
remains constant, that is, when it describes equal spaces  in equal  successive
intervals  of time, p. 47.
  Prop.  8.—Every motion that is not uniform  is said to be  varied, and is
called accelerated or retarded as the velocity increases or decreases.
  Prop.  9.—When the  velocity constantly  increases  or  decreases  in the
direct ratio of the  time that  the body has been moved, the motion is  said to
be uniformly accelerated or retarded, pp. 43, 58, 59, 60.
  Prop.  10.—Whatever is capable of producing or destroying the motion of
a body is called/orce. 
APPENDIX.
75
   Prop. 11.—A  force that produces its  effect instantaneously, and  then
ceases to act, is called an impulsive force.
   Prop. 12.—A  force that acts continually and equally is termed a constant
force.
   Prop. 13.—When the constant force  acts in lines  directed towards  a
single  point or centre, it is called centripetal, and the path of the body its
orbit,  p. 59.
   Prop. 14.—That part of the impulsive  force which tends to make a body
move directly from the centre, is termed the centrifugal force, p. 49.
   Prop. 15.— A force  that is capable of destroying motion without being
able, under any circumstances, to produce motion, is termed a passive force.
   Prop. 16.—The state of rest produced  by the action of opposite forces is
termed equilibrium.
   Prop. 17.—When a body is struck, its  particles yield to  the impulse, and
the form of the body is  changed.   When the  body possesses the inherent
power, when thus changed, of restoring  its  form,  it is said to  be elastic;
when  it has not this power, it is called non-elastic, p. 37.
   Prop. 18.—A  body oscillating  below a point to  which it is in any way
attached, is termed a pendulum, p. 60.

                            Laws of Motion.
   Prop. 19.—1st.  If a body  be  at rest it will continue at rest,  and  if in
motion, it will continue  to advance uniformly in a right line, unless  com-
pelled to change  its state by some external force, pp. 47, 49.
   Prop. 20.—2d. The motion of a body  is in the direction of the force that
produces it and is proportional to that force, pp. 53, 55.
   Prop. 21.—3d. Action and reaction are always equal and opposed to each
other;  or when a body  communicates motion to another, it  loses of its own
momentum as much as it gives to  the other body, pp. 70, 72.

               Of Impulsive force and Rectilinear motion.
   Prop. 22.—The effect of an impulsive force is  to produce uniform rec-
tilinear motion, p. 49.
   For during the moment of its action on any body, it must set it in motion
with a certain velocity; and by the first law of motion, the  body must con-
tinue to advance  in a straight line with that velocity.
   Prop. 23.—In rectilinear motion the space  is as the velocity multiplied
into the time.
   For if a body move with the velocity of three feet per  second, it is evi-
dent, that it will  move over 6 feet in two  seconds, i. e. x32 ; and 9 feet in 3
seconds, i. e. X-33, and 12 feet in 4 seconds,  &c. &c.
   Prop. 24.—The time is as  the space divided by  the velocity.
   For if a body  passes over 12 feet for instance, when its velocity is 3 feet
per second, it is evident, that in order to find the number of seconds, which
the  body has employed  in passing over 12 feet of space, we  need  only
divide  12  by 3,  (i. e., the space by the velocity) and the quotient 4, is the
time sought.
   Prop. 25.—The velocity is as the space divided by the time.
   For  if a body move over 12 feet in 4 seconds, its velocity is  evidently 3
 feet per second or  12-T-3.
   The velocities of two bodies may be compared, in the same manner: the
velocities of  two bodies A and B, for  instance, of which A moves over 54 
76
APPENDIX.
feet in 9 seconds, and B, 96 feet in  6 seconds; their velocities  will be  as
6 (54-T-9) to 16 (96-4-6.*)

           Of a constant force and uniformly accelerated motion.
  Prop. 26.—The effect of a constant force acting upon a body, is to pro-
duce in it a uniformly accelerated motion, p. 58.
  For since the effect of force is to produce velocity,  a constant force must,
in successive instants of  time, afford continual and equal additions  to the velo-
city of the body it has set in motion; that is, the velocity will increase  in
the direct  ratio that  the body has been moving, which  is the definition of
uniformly accelerated motion.
  Prop. 27.—In uniformly accelerated motion the space described  is as the
square of the time, pp. 58, 59.
  Thus it is found by experiment, that if a body move with a gradually and
constantly  increasing  velocity that would  carry it through a mile in one
minute, that at the end of this time it has acquired such a velocity as would
carry it through two miles the next minute, if the force  that communicated
its  motion  ceased to act at the end  of the first minute; but if the force con-
tinues  to act, it acquires  a velocity that would carry it over an additional mile,
so that it will pass over  three miles the second minute, or four miles in two
minutes.   At the end  of the second  minute it has acquired a velocity that
will carry it over double  the  space  in  the third minute, that it moved over in
the first two minutes, or a velocity of 8 miles in 2  minutes,  or 4 miles a
minute.   But the force still continuing to act, it will  move a mile farther or
five miles in the third minute.   Hence, if a body acted upon by a continued
force move a mile the first minute, it  would move 3 miles  the second, 5 the
3d, 7 the 4th, 9 the 5th, &c.
  Thus the spaces described in  successive equal parts of time, by uniformly
accelerated  motion, are  always  as the odd numbers  1,3,  5, 7, 9,  &c, and
consequently the whole spaces are  as the squares of the times or of the last-
acquired velocities.  For the continued addition of the odd numbers  yields
the squares of all numbers from unity upwards.   Thus 1  is  the first odd
number and the square of 1 is 1  ; 3 is the second odd number, and this added
to one makes 4, the square of 2;—5  is the  third odd number and this added
to 4 makes 9, the square of three;  and so on for ever.  Since, therefore, the
times  and  velocities  proceed evenly  and  constantly as 1,2, 3, 4,  &c, but
the spaces described in equal times  are as 1,3, 5, 7, &c, it is evident that the
space described,
    In 1 minute will  be                              1 =  square of  1
    In 2   "       "         -        -        1+3=4=     "     2
    In 3   «       "         -        -     1+3 + 5=9=     "     3
    In 4   «       "         -        1+3 + 5 + 7=16=     «     4  &c.

  * For the benefit of those who are acquainted with algebra, we subjoin the followin"
equation, which expresses all  the circumstances of uniform motion
  Let t = the time of motion,
     s = the space described in the time t,
    v = the velocity :
                  Then, s = vt from which we oblain
                            s

                            t
                            s
                    and t = —
                            v 
APPENDIX.
77
                               Of Gravity.
    Prop. 28.—The force  which causes bodies to fall to the earth is of the
 kind named constant, and is called gravity, p. 58.
    Prop. 29.—The direction of gravity is in lines perpendicular to the earth's
 surface.
    Prop. 30.—The force  of gravity is directly proportional to the mass of
 the body.
    For however small the parts into which we divide a body, we find them all
 affected by gravity, since this force must act upon all the particles of a body.
    Hence, in an unresisting medium, all bodies setting out from a state of rest,
 fall through the  same space in the same time, because the force of gravity
 acting upon them increases in proportion to the mass to be moved.
    Prop. 31.—The force of gravity decreases, as  the square of  the distance
 from the attracting body increases.
    This is proved by astronomical observations.

                     Motion produced by joint forces.
    Prop. 32.—When a body is acted upon at the same moment by a plurality
 of forces, each of these forces produces its full effect; and the place of the
 body at the end of any given time  is the same as  it would have  been if the
 forces had acted in succession each during that time, pp. 55, 56,  57.
    Thus  let  A B represent the direction of  a
 force that would move a body, A the distance
 from A to B in a cerlain interval of time, (a
 second for example,) and A C, the direction of
 a force that would  propel the same body from
 A  to C in the same interval of time.  Suppose
 the first force  acted alone, it would  move the
 body from A to B in one second; if the force A C then acted at B, by drawing
 B  R equal and parallel to A C, B R will represent the direction and velocity
 of the  force A C, and  R the position in which the body would be  in at  the
 end of the second interval of time.  Unite A  and  R and the line A R  will
 represent  the course of the. body A if acted upon at the same  moment by
 the two  forces  A B and A C, and  R  the  position  of the body at the end of
 the first interval of time.
   In the same  manner the action of any num-              Fig. 19.
 ber of forces may be represented.   Thus  let
 A B, A C, A D, A E, represent the separate
 effects of four  different forces  acting in the
 same plane, capable  of moving a  body the
 distances A B, A C,  A D, A E, in a  given
 interval of time.   Draw B c, c d, d R, equal
 and parallel to  A C, A D, A E respectively,
 and join A R, A B c  d R,  will represent the
 path of the body  if these forces had acted
 successively each during one interval of time,
 and A R the  path of the body if they all act
 together, and R the position of the body at the end of the first interval of time.
  Prop.  33.—The line AR in the figures given to illustrate the preceding
proposition represents the direction and measure of a single force, equivalent
to all the others in each figure; and hence  the process by which  it  is deter-
 mined is called the composition of forces, pp. 55, 56, 57.
 
78
APPENDIX.
  Prop.  34.—Any force may be decomposed into any number of other forces,
that shall be equivalent to it, by the reverse of the foregoing operation.   This
process is called the Resolution of forces, p. 57.
  Thus the force A R fig.  18,  may be separated into two forces A B, A C,
and the force A R, fig. 19,  into four forces, A B, A C, A D  and A E.
  Prop.  35.— When  the forces act in the same  right line, we have only, in
order to ascertain  the spaces described by their  combined  action, to add or
subtract the spaces which  would  be described by  their separate  action, ac-
cording as these forces act, in the same or opposite directions.

                              Equilibrium.
  Prop.  36.—A body acted upon by a plurality of forces, in opposite direc-
tions, will remain at rest, or in equilibrio; when these forces were supposed
to act in  succession each during  the same interval of time,  the body would
arrive at its point of departure.
  The simplest and most evident case of equilibrium is that in which a body
is acted upon by two equal and opposite forces.

          On the joint action of an impulsive and  a constant force.

            A.   Wlien these forces act in the same right line.
  Prop.  37.—When the  forces  act in the same direction,  the place of the
body at the end of any given time, may be determined, as in the  problem of
the composition of forces,  by supposing, first, that the impulsive  force acts
during that time, and then that the action of the constant  force commences
and  acts  alone during the same time:  the spaces added altogether will give
the space passed over by the joint action of these forces during the assumed
time.
  Prop.  38.—When  the forces act  in opposite directions,  the place of the
body may be  ascertained  by a similar  process; in this case, however, the
spaces are to be subtracted one from the other, pp.  58, 59.
  When a constant force is acting in a direction  contrary to that of a moving
body set  in motion by an impulsive force,  the  retardation  that the  former
produces  may be determined by comparing the  motion with that of a body
moved by the same force.
  The degrees by which an  ascending body loses its motion, are  the same
as those  by which it is again  accelerated at the  same points, when it has
acquired  its greatest height and again descends, for the velocities at the corre-
sponding parts of  the ascent and descent are equal.  Thus we may calculate
to what height  a  body will  rise when  projected  upwards  by an  impulsive
force, gunpowder, for instance, and retarded by  the force of gravity.   Since
the force of gravitation produces or destroys a  velocity of  32 feet in every
second, a velocity of 320 feet will be destroyed in 10 seconds ; and according
to what has been premised, a body  will fall in 10 seconds through a hundred
times 16 feet or 1600 feet, which is  therefore the  height to which a velocity
of 320 feet in a second will  carry a ball projected, without resistance from
other cause than gravity, in a vertical direction,  p.  60.

             B. When these forces act in different directions.

     *  When the successive directions of the constant forces are parallel.
   Prop.  39.—If the constant  force be that of  gravity, the successive direc- 
APPENDIX.
79
Fig. 20.
tions of which are  assumed  to be  parallel, the  investigation of the effects
produced, constitutes the doctrine of projectiles;  a projectile  being  a body
thrown in any direction  by an impulsive force and  at  the same time acted
upon by  the force of gravity,  pp. 59, 60.
  Prop.  40.—The place of  a projectile at  the end of any given time may
be determined, as in the problem of the composition offerees, by supposing
first that  the impulsive force alone has acted during  that time, and then that
the action of gravity commences,  and  acts alone during the same time.
  Thus let A H represent a hori-
zontal plane, and A B, the initial
direction and velocity of a body
projected from  the point A in
the same plane.   If the impul-
sive force alone acted  on  the
body it would describe  the path
A B B' B"B" &c. with uniform
velocity.   But as  the  force  of
gravity acts from the moment of
projection,  the  body  will  be
drawn downwards from  the line
A B'" so as to be found after the
successive intervals of time, at
the points g g1 g", &c, and as
the force of gravity produces a
velocity  which increases as  the
squares of the distances, if the distances A  B, B B', B' B", B" B'" be equal,
B o-, B' g', B" g", B'" g'", &c, will be as the squares of these distances, and
the3path of the projectile through the  points g g' g" g'" will be  a curve, and
this curve mathematicians  have called a parabola.

**  When the successive directions  of the constant force tend to a common
                                 centre.

   Prop. 41.—This case constitutes the doctrine of central forces, see prop.
13. p.  75.
   Prop. 42.—The place of  the body at the end of any given  time may be
determined here also by the problem  of the resolution of forces.
   Thus, suppose A represent  a body impelled
towards  H with such a force, as by  itself, would
enable it to run over the equal spaces A B, B F,
F G, &c, in equal  portions of time: suppose like-
wise that it is acted upon the same time by con-
stant force which would enable it to pass over the
unequal  spaces A I, I K, K L,  &c.  in the same
equal  portions of  time.   It is  evident, that the
joint action of both these forces would compel the
body A to pass over  the curvilinear path A N 0
P, &c.  Through  B draw the line B C, (viz. in
the centre of attraction ;) through I draw I N paral-
lel to A  B; and at the end of the first  portion of
time the body will be found at N, whence it would
proceed  in the straight direction  N R, (by the first
law of motion) if the constant force  then  eeased
 
80
APPENDIX.
 to act.   But as this force continues to act, the body at the end of the second
 portion of time will be found in O ; for the like reason, at the end of the third
 portion of time, it will be found in P and so on.   The course then A N O P,
 is not straight  but consists of the lines A N, N O,  O P, forming certain angles
 with each other.  Now  it will not be difficult to conceive  that, because  the
 attractive force acts  not by intervals but constantly and unremittedly, the real
 path of the body must be a polygonal  course, consisting of an infinite number
 of sides; or more  justly speaking, a continuate curved line, which passes
 through the points  A, N, O, P, &c. as is shown by the  dotted line.
   Prop. 43.—Should the action of the centripetal force  cease at any instant,
 the body would proceed straight forward, p. 49.
   The portion of the impulsive force by which this is affected is called  the
 centrifugal, prop. 14.
  Prop.  44.—Whilst the distance from the centre remains unchanged, as
 when the body moves in a circular orbit, the centripetal and centrifugal forces
are equal.

                         Laws of Central forces.
  Prop.  45.—When bodies revolve in equal circles,  their centrifugal forces
 are proportional to the squares of their velocities.
  Prop. 46.—When two bodies revolve with equal velocities at different dis-
 tances, the centrifugal forces are inversely as  the distances.
  Consequently (prop. 45, 46,) the centrifugal forces are in all cases, directly
 as the squares of the velocities, and inversely as the distances.
  Prop. 47.—When two bodies revolve in equal times at different distances,
 their centripetal forces are simply  as their distances.
  In general the centripetal forces are as the distances  directly  and as  the
 squares or the times of revolution inversely.
  Prop. 48.—When the forces vary inversely as  the squares of the distances,
as in the case of gravitation, the squares of the times of revolution are pro-
 portional to the cubes of the distances.
  Thus,  if the distance of one  body be four times as great as that of another,
 the cube of 4 being 64, which is the square of 8, the  times of its revolution
 will be 8 times as great  as that of the first body.
  Prop. 49.—Where the orbit  deviates more or less from  a circular form, a
 right line joining the revolving body and its centre of attraction, always  de-
scribes equal areas in equal times, and the  velocity of the body is therefore
always inversely as  the perpendicular drawn  from the centre to the tangent;
and the velocity at any point less than three-eighths, greater than that neces-
 sary to make the body describe a circle.
  Prop. 50.—To propel a body in an elliptical orbit, the force directed to its
 focus must be inversely as the  square of the distance.
  This is proved by astronomical observations, but we have no other proof of it.
  The motion of the planets round the sun in the solar system is governed
by the laws of central forces,  the  centripetal force in this case beino- that of
gravity.                                                          °

              On the joint effect of active and inactive forces.
                 A.  When they have opposite directions.
  Prop. 51.—The effect of passive forces is to restrain and modify the action
of other forces so as to confine the motion of a bodv to a particular course or
 path, and the direction of the passive force  affecting a body at any moment 
APPENDIX.
81
 is the line perpendicular to that part of this path at which the body is found
 at this moment.  If the direction of the active force be also  perpendicular to
 this path, the body must evidently remain  at rest, since no part of this force
 can be resolved into the direction of the path in which alone  the body can
 move.

                 B.  When they have different directions.

                              General rule.
   Prop.  52.—Resolve the active  force into two, one perpendicular, and the
 other a tangent to the path of the body, the effect of the former force  will be
 entirely destroyed (prop. 51,) and the body will advance by  the latter  alone.

      *  On the motion of a  body impelled obliquely against a plain.
   Prop.  53.—Let M N  represent the plane,
 and A B  the direction  and  velocity from  the
 impulsive force, resolve A  B into the forces
 A C perpendicular to the plane and C B in its
 direction, then  by the general rule (prop.  52)
 the body  will  move along  the plane  with  a
 velocity of which C B is the measure.

  ** On the motion of a body impelled obliquely against  a curved surface.
   Prop.  54.—Let M N  represent  the curve and
 A B the direction and velocity from  the  impul-
 sive force.  Resolve A  B into two forces, C  B
 perpendicular to the curve at B, and B D  (equal
 to A C) a tangent to the curve at the same  point.
 Then B D will represent the velocity at the point
 B.
   Prop. 55.—If the curve be interrupted  at any
 point, or change the direction of its concavity, the body will advance with its
 last velocity in a tangent to the curve at that point.

           ***  On the descent of a body along an inclined plane.

  Prop. 56.—Let M N represent  an in-
 clined plane and A B (perpendicular to the
 horizontal base H N) the  force of gravity
 as measured by the distance which it would
 cause a body to descend in the first second
 of time.   Resolve A B into two, A C, per-
 pendicular to the plane, and C B in its di-
rection, then the body will be urged down
the plane  by a constant force  measured by
 C B.
         Laws of the descent of bodies doion inclined planes.
Prop. 57.—1st.  The motion of a body down an inclined plane is uni-
rmly accelerated.
Prop. 58.—2d. The velocity acquired is proportional to the perpendicular 
82
APPENDIX.
descent, so that a body falling from  M to II has the same  velocity at H as
one descending the whole length of  the plane at N.
  Prop. 59.__3d. The  times of descent down plains  of the same heights
are  as their lengths.
  Prop. 60.__4th.  The times of descent down all planes which  are cords
drawn to the lowest point of the same circle, are equal.
                       Thus, if the balls A, B, C, be placed at different
        Fig. 25.       points of the circle  and suffered  to descend at  the
                     same instant  along as many planes which meet at the
             "X     lowest point of the circle, they will arrive there at the
                \«   same time.
                7\      Or it may  be enunciated  in the following terms: the
               /  I   times of descent down all the cords  drawn from the
              /  /   same point or circumference of a circle will be the
             '   /    same.
           ^/        This will be made evident by supposing the above
          d           figure inverted, D being made the upper point and the
                      balls allowed to fall from that point to A, B, and C*

       **** On the descent of a body down the vertical curved line.
  Prop. 61.-—The times of descent down the cords of different circles are
to each other as the square roots of  their diameters.
  Prop. 62.—If a body fall from a  state of rest down a curve,  the velocity
acquired is equal to  that which it would have  by falling through the same
perpendicular height.
  For if the curve be considered as  made up of an infinite number of conti-
guous planes, it is evident that  the  angle of inclination of any two of these
adjacent planes is infinitely small, or nothing, and consequently there  is no
velocity lost  by a  change of direction in passing  from  one to  the other.
Therefore, as the effect of gravity is not impeded,  the truth of the proposi-
tion becomes  evident.
  Prop. 63.—If a body be projected up a curve, the perpendicular height to
which it will  rise is equal to that through which it must  fall to acquire  the
velocity of projection.   For the body in  its ascent  will be retarded in  the
same degree that it was accelerated in  its descent.
                         Thus let  B A B' be a curve in which the lowest
                       point is A,  and the parts A B,  A B'are similar;
                       a body in falling down B A will acquire  a velocity
                       that will carry it to B', and since the velocities in all
                       equal altitudes in the ascent and descent  are equal,
                       the times of ascent and descent are equal.
                         The foregoing proposition  is equally true whe-
                       ther the body actually move over a solid surface or be
retained in its path by a string which is in every part perpendicular to it.

                       Of the simple Pendulum.
  Prop. 64.—The  simple pendulum is  conceived  to  be a mere material
point suspended by an imponderable and inextensible thread, p.  60.
  Prop. 65.—If the  simple pendulum  vibrates  through very small arcs,
these  may,  without   sensible error, be conceived  to  coincide with their
chords, and we may derive  from this consideration the  following  theorems:
  1st.  As the times  of  descent of the body  down  different chords of the
Fig. 26.
 
APPENDIX.
83
same vertical circle  are equal (prop. 60.) the  vibrations of the same pendu-
lum, although  performed through unequal  arcs, will be very nearly equal,
p. 61.
   2d. The times of vibrations of different pendulums will be to each other
as the square roots of the lengths of these pendulums, or, which is the same
thing, their lengths  are proportioned to the squares of the times of vibration,
p. 61.
   The times of descent down the chords of different circles are the  same as
would be occupied in descending vertically through their diameters, and are
consequently proportional to the square roots of these diameters.

                        Of the impact of bodies.
   Prop.  66.—When a body in motion strikes  directly  another body,  it
always communicates motion to the  second body, and  loses part of its own,
and from the third law of motion it is evident that the momentum gained by
the second body is exactly equal to that lost by the first.
   Prop.  67.—When one non-elastic body strikes against another, the  two
bodies will move on together since there is no force to separate them ; and as
one of the bodies gains all the momentum  which the other loses, the momen-
tum after impact will be equal to the sum of the momenta before impact.
   Prop.  68.—When an  elastic body strikes against another, the second  is
impelled forward with double the momentum  which it would have  received
under the same circumstances if non-elastic.
   For at the moment of impact the form of the body struck is changed by a
force equivalent to the momentum which it receives from  the striking body,
and if this body  be  perfectly elastic,  its form will be restored to it  by a force
exactly equal  to that by which it was changed, and  this  force  (which we
have just seen to be equal to the original impulse,) will be exerted in driving
the body forward.   The body thus  receives, besides its original impulse, the
equal force of  the re-bound.
   Prop.  69.—The striking body when elastic,  is also  acted upon by the
re-bound, and  loses twice as much  momentum as it would have lost if non-
elastic.
   In this case, as in the former, the sum of the  momenta is the same after
impact as before it; but the bodies after impact do not move on together.
   Prop.  70.—If an elastic  body strike against a firm plane, the angle of
reflection will  be equal to the angle  of incidence, p. 66. 
S4
MECHANICS.
                          PART   II.

                    PHENOMENA OF SOLIDS.

THE  FOUR FUNDAMENTAL TRUTHS  USED TO EXPLAIN THE  PECULIARITIES  OF
  STATE  AND MOTION WHICH DEPEND ON  THE SOLID FORM  OF BODIES:  A
  DEPARTMENT  COMMONLY CALLED MECHANICS.
                      ANALYSIS OF THE CHAPTER.*

A force, which moves part of a solid body, must affect the whole or break
  off the part.                                       .
If the force be directed towards a  certain central point in the  mass, it will
  affect the whole equally, whether simply to support the  mass, or to move
  it or to stop it when in motion.   The point, according to  circumstances,
  is called the centre  of gravity of  inertia,  or of action.
In solid bodies moving  about an axis, as exemplified in a wheel or iveigh-
  ing beam, the  various parts describe circles or move through spaces which
  are greater in proportion to their respective distances from the centre of
  motion.  Hence forces differing as  to speed,  may still, through a solid
  medium, be brought exactly to  co-operate or to  oppose one another;  a
  slow force counter-balancing  or  being equivalent  to a  quicker one, pro-
  vided that it be more intense in  proportion as  it is slower.   The simple
  MACHINES, Or  MECHANICAL  MEDIA,  called LEVER, WHEEL AND AXLE, PUL-
  LEY, inclined plane,  wedge, screw, fyc, are so many arrangements
  of solid parts, by which forces of different velocities and intensities may
  be thus connected or opposed, or  may be conveniently substituted  one for
  another.
By solid connecting parts also the  direction of any existing motion  or force
  may be changed, as  when  the straight motion of running water is con-
  verted into the rotary motion of a water-wheel, Src.  Hence arises an end-
  less variety of complex machines.
In all  machines, an  important circumstance to be considered is  the resist-
  ance among moving parts which  arises from friction :—and  in solid
  structures generally, the forms and positions of parts have to be adjusted
  to the strength of  the materials, and to the strains ivhich  the parti
  have to bear.

  " Solid"  is the term applied to a mass  in which  the mutual attraction  of
the atoms is so strong, that the mass may be moved about as one body, with-
out the relative positions of the component parts being thereby disturbed.

"Force moving part of a solid must affect the whole or break off  the part."
  This is a necessary consequence of the description or definition of a solid
just given.   And it follows that in  all cases of  breaking, the cohesion of the
     * The reader should here re-peruse the general table or synopsis at page 19. 
CENTRE OP GRAVITY.
85
 atoms at the fractured patt must have been less strong than the weight of the
 remaining mass, or its inertia resisting the degree of change attempted, or the
 force fixing it to its place, or than some combination of these particulars.
   The sharp blow of a hammer given to an ivory ball, causes it to dart off
 swiftly, but  does not  injure it, because the cohesion among the atoms struck
 is stronger than the opposing inertia of the mass, even under a rapid change :
 but the blow of the hammer on a large elephant's tusk indents or breaks the
 part, because the opposing  inertia of the larger mass  is stronger  than the
 cohesion of the atoms which receive the blow.
   A  vessel of pottery-ware may be safely suspended  by its handle; proving
 that the cohesion which fixes the handle to it is stronger than the weight of
 the vessel; but if the attempt be  made to lift  the  vessel quickly, the handle
 may rise and leave the body behind; because then the weight and inertia are
 acting together to destroy the cohesion.   Thus servants attempting to lift too
 quickly the loaded stone-ware dishes at a  dinner-table, often break off the part
 by which  they take hold.
                       Centre of Gravity or Inertia.
   If  any uniform beam  or rod be supported by its middle, like a  weighing
 beam, the  two ends will just balance each other.   This is in accordance with
 the general truth or law of attraction already explained; for as there is just
 as much similarly situated matter on one  side of the support as on the other,
 there will  also be just as much attraction, and  therefore no reason why the
 matter on  one side should  overpower that on the other.  If equal weights be
 afterwards attached in corresponding situations on the two arms of the beam,
 the balance will not be thereby disturbed ;  and the operation of adding weights
 that counterpoise, above and below, and near and far from the centre may be
 continued, until a bulky mass is built up  upon the beam—and instead of a
 beam a wheel may be used—yet the whole will remain perfectly supported
 and in equilibrium about the original centre.   In the  pages now to  follow, it
 will be shown that, in every body or mass, or  system of connected masses,
 in the universe, there  is a point of this  kind about which all the parts balance
 or have equilibrium, and it is this point which is called the centre of gravity
 or of inertia.  Although in any mass,  therefore, every atom has its separate
 gravity and inertia, and the weight and  inertia of the whole are really diffused
 through  the whole, still by supporting this one  point, either from  above or
 from  below, the whole mass  is equally supported; by lifting it, the  whole is
 lifted ; by stopping it, the whole is brought to rest; and when it rises or falls,
 the general mass is really rising or falling.  Thus, for many purposes, a body,
 however large, may be considered as compressed into or existing only in the
 single point called its  centre of gravity or of inertia.
  This centre in a mass of regular shape and of uniform substance, as a ball
 or cube of  metal, is easily found, because it is the evident centre of the form ;
 but in bodies that are  irregular, either as to density or form, it must  be found
 by rules  of calculation hereafter explained.
  To say that the centre of gravity will always take the lowest situation which
the support of the body will allow, is only to repeat, that bodies tend by their
gravity towards the centre of the earth.  In a suspended body, therefore, as
the lowest  situation which the centre of gravity can find is, when it  is imme-
diately under the point of suspension, all bodies hanging freely  must have
their centre of gravity directly under that  point.   A plummet is an  interest-
ing example of this ; and the truth furnishes, in many cases of irregular masses,
a very simple practical mode of finding the centre,
      7 
86                            MECHANICS.
27.
s/o
Fie. 28.
  Thus if an irregular piece of plank or of pasteboard, represented here by
                 the figure a e b d, be suspended from  any point, as a, and
                 the cord of a plummet a g be attached at the same point, the
                 centre of o-ravity  of the board  must be somewhere in the
                 direction of the plummet, and a chalk line left on the board
                 where  the cord touched  it, must pass over the centre of
                 gravity.  If the board be then suspended by another point,
                 as (/, and another chalk line d e be made in the same man-
                 ner, the place c, where the two lines cross or cut each other,
                 will indicate the centre of gravity ; and the board when  sup-
                 ported  by a cord attached there, will hang evenly balanced.
                   The following cases farther illustrate the truth, that the
                 centre of gravity always seeks the lowest place.  They seem
                 at first to be exceptions to the law; but when more fully con-
sidered, are interesting proofs of it.
                        A wooden cylinder or roller e d c, placed on  a slope
                     or inclined plane a  b,  will naturally descend, because
                     its centre of  gravity is thereby approaching the earth;
                     but if there be a heavy mass of lead c introduced at one
                     side, which must rise before the roller can descend, the
                     rise of the mass being  contrary to gravity, the  motion
                     will be arrested.   Indeed, if the roller were placed on
                     the plane with the lead in the position d, the lead would
fall  down  to the position c, and so would move  the roller towards b, exhibit-
ing the singular phenomenon of a body rolling up hill by the action of its weight.
  If a billiard-ball be  placed upon the small ends of two billiard sticks or  cues
                                        a b and c d, laid on  a table  with
                  g'                    their points  c and a in contact, but
                                        with the larger ends b and d so far
                                        apart that there may be just room
                                        for  the ball to touch the table be-
                                        tween them, the ball will roll along
                                        between the cues, sinking  gradu-
                                        ally from  its high situation  near
                                        their points, to its lower situation
                                        near b.   To a careless observer, it
would  then have  the appearance of rolling upwards, because the cues on
which it rests are  thicker towards the ends d and  b; but it  would really be
descending in obedience  to gravity.  If a  double cone, as represented at/,
were substituted for the ball, it  would similarly roll from c to e, and with still
more of the fallacious appearance of rolling upwards, because its ends would
always be resting on the upper and rising surfaces of the cues.
                              The board  or stick c d resting on the  edge of
                            the table a b would naturally fall if left to itself,
                            because more than half of it is beyond  the  edge
                            of the table;  but strange to  say, an  additional
                            weight e attached to its  projecting part as atfi
                            by the cord b e,  instead of pullinglt down  fast-
                            er, shall fix or steady it  on the table, provided
                            the weight be pushed inwards a little  by a rod
                            d e resting against it and against a niche in the
                            stick at d.   It is evident that  the  stick c  d, in
                            falling, must  turn round the edge of the  table at
Fig. 30.
 
CENTRE OP GRAVITY.
S7
b;  but  in so doing, after the arrangement now  supposed, it  must lift the
weight e along the path ef—which rise, as the weight is heavier than the
stick (that is to say, as the common centre of gravity of the connected objects
is near e,) gravity forbids, and therefore the stick and weight will both remain
supported by the table.   An  umbrella or walking cane, hanging on the edge
of a table by a crooked handle, is another instance of the same kind.   And
the common toy of a  little  man standing on tiptoe upon the top of a  pillar,
and supporting two  leaden bullets by wires descending from  his hands, is
another combination of parts which places the centre of gravity of the  whole
below the support, making the combination a kind of pendulum.
  By attending to the centre of gravity of the bodies around us on earth, we
are enabled  to explain why, from  the influence of gravity, some of them are
stable or firmly fixed, others tottering, others falling.
  If we find that a body, from its form or position, cannot be overturned with-
out its centre of gravity being lifted,—knowing now that the general mass is
then lifted in the same degree, we see why a weak cause cannot effect the
change.  The rise of the centre of gravity, or body, in any case of falling
over, where  the centre of gravity is over the middle of the sustaining base, will
be proportioned to  the breadth of the base of the body, compared with the
height of the centre of gravity above the base.  This is  shown in the annexed
figures,  in which the two particulars of  base and height are  combined in a
series of proportions.   In the figures, the dot c marks the place of the centre of
gravity, and  the curved line beginning  from the dot marks the path of the
centre of gravity, when the body is  overturned.  This curved line is a por-
tion of a circle which has the edge or extremity of  the base (b, in fig. A) as
a centre, because the body in  turning must rest upon such extremity or cor-
ner as the centre of its motion.  The farther  inwards, therefore, from this
extremity that the  centre of gravity is, as marked by where a plumb-line as
p, hanging from it, crosses the base, the farther, of course, is the centre of
gravity from the top of the circle which it has to describe in moving, and the
steeper, consequently, will be its commencing  path; and  as in  the  case of
bodies made  to roll up slopes, the steeper the ascent, the greater will be the
force necessary to give motion.—The line of a plummet hanging from the
centre  of gravity is called the line of direction of the centre, or that in which
it tends naturally to descend to the earth.
  In fig. A, which has a broad base and little height of the centre of gravity,
we  see that the centre must rise almost perpendicularly before it can fall over,
and the resistance to overturning  is  therefore  nearly  equal to  the whole
weight of the body.   Hence the firmness of a pyramid.
  In figures B, C and D, progressively, the commencing path of the centre is
less steep, because  the base is narrower, and hence  the bodies are so much
the  less stable.  B may represent an ordinary house, C a tall narrow house,
and D  a lofty chimney. 
ss
MECHANICS.
  Fig. E shows a tottering position, for the centre of gravity being directly
over abase which is a mere point, the least inclination places it on a descend-
ing slope, and the body must fall.
                                 Fie. 32.
                                                          6^

  In F the position is tottering  on one side, and stable on the other.  This
explains how the least inclination of a standing body virtually narrows, in one
direction, its sustaining base.
  In G, which represents a ball upon a level plane, the whole mass is sup-
ported on a single  point as in E, yet  the  body has no  tendency to move,
because, in any other  possible  position, the centre would still be  as far from
the sustaining plane.   In moving, the centre describes the straight  level  line
a b.
  In H the ball is on an inclined plane,  and  rolls down, the  centre of gravity
describing the oblique line b a.
  In  I, which is an  oval body  resting on  a level plane, when the body is
moved to either  side, the centre of gravity must rise, as in the case of a
pendulum.  Hence an oval body on a level will rock or vibrate like a pendulum.
  K is a true  pendulum whose  centre  of gravity describes  the  curve  here
shown, as explained in Section II, at page 60.

The importance of the subjeat of the centre  of gravity will be farther judged
  of by the facts  which are now to be reviewed.
  A cart loaded with  metal or  stone may go  safely along a road of which one
side is higher than the other,  as here shown,  but were the same cart loaded
with wool or hay it would be overturned;  because, although the sustaining
base be the same in the two cases, the  line  of direction falls much within it
from the low centre of gravity of the metal at c, but falls very near the wheel
                    at P, or altogether on the outside, from the high centre
                    of the wool at a, and in  the  latter case the centre  has
                    offered to it a descending path.
                      This explains why lofty stage coaches or vans are so
                    dangerous, and particularly when heavy luggage is placed
                    on the top,  and why lofty gigs and curricles have led to
                    so many fatal accidents.   As regards any  of these, a
                    defect of  smoothness or of level in the road, or even, in
                    a case of quick driving, a slight lateral bend,  often suf-
                    fices to produce the catastrophe.   The safety-coaches of
                    late times are made with the  wheels far apart to give a
                    broad base, and with the luggage receptacles and seats
for outside passengers placed  low down before and  behind the body of the
carriage, instead  of on the top as formerly.
   The feet of tripods are  generally expanded below to  give a broad base.
The same  is true of our common chairs;  but  a thoughtless child often leans
so far over the back of a chair,  that he causes the line of the o-eneral centre
of gravity to fall beyond the base, and  the chair with its  load is overturned.
 
CENTRE OF GRAVITY.
89
The small lofty chairs made to raise children to  the  parent's elbow at the
dinner-table, are very dangerous if the feet are not made to spread much.
Pillar-and-claw tables, candle-sticks, table-lamps, and many other articles of
household furniture, have stability given in the same manner.

The least inclination of a standing body virtually narrows the supporting base.
   This truth is explained by fig. F.   It shows the necessity of building the
thin walls and tall chimneys of modern houses perfectly upright.  And hence
the extreme importance and utility of that simple instrument, the plummet or
plumb-line, which, when applied to a body, is a visible indication of the line
of its centre of gravity.   The mason and many other workmen cannot pro-
ceed a step without their guiding plummet.
   The brick  walls of ordinary houses are so  thin, that, to have standing
strength,  they require to rest against one another; and hence they occasion-
ally exhibit the kind of stability which belongs to a child's house built of cards.
As contrasted with the masses of masonry which remain to us from antiquity,
resting on firm-spreading basements, they are examples of what is truly ephe-
meral, in  comparison with  that which has partaken of the permanency of
nature's own  works, covering  regions with mighty ruins.  What magnifi-
cent illustrations of strength and durability dependent on proportions, are
those  ancient pyramids  and temples, which still give such interest to the
banks of the Nile, and to the valleys and plains of Asia!
   There  are many remarkable structures on earth which lean or incline  a
little: yet so  long as the line of their centre of gravity remains within the
base, and the  parts of the mass have tenacity among themselves sufficient to
hold together, the structure  will stand.  The famous tower of Pisa was built
intentionally inclining, to frighten and surprise :  with a  height of one hundred
and  thirty feet, it overhangs its  base sixteen feet, and assumes nearly the air
of fig. F,  in page 88.
   The tall monument near  London  Bridge inclines so much, that in  high
winds from a particular quarter, timid minds have doubted of its stability.
   And many of the most lofty and beautiful of our cathedral spires or towers,
as that of Salisbury, have lost something of their perpendicularity.
   An oval body on a flat level  surface, as  already explained by fig. I,  page
88,  oscillates  somewhat like a  pendulum, because, when  disturbed  from its
middle position, its  centre  of gravity has risen and seeks to return.  The
same is true of any regular  slice or portion of a solid globe,  which will con-
sequently always come to rest withits plane face turned directly upwards.
   The rocking-horse of children and the common cradle are exemplifications
of the same class.
   But perhaps the  most curious instances are those rocks called Loggan or
Laggan stones, of which there are several among the picturesque barriers of
the British coast.  An immense mass, loosened in  some convulsion of nature,
is found with a slightly rounded base resting  on a flatter  surface  of  rock
below; and is so nearly balanced, that the force of a man suffices to move it.
Some of  these have been objects of much superstitious veneration  to  their
neighbourhood.
   There is an amusing Chinese  toy, made in obedience  to the same  prin-
ciple.  It  has the appearance of a little fat laughing man, sitting on the ground
with his feet concealed under him;  but where  the  feet should be, there is
only a rounded smooth surface, with heavy lead ballast placed in it, so  low,
as always, when allowed, to raise the body to  the erect or  sitting attitude. 
90
MECHANICS.
A child pushes the little  fellow down again and again, and would  persuade
him to be still, but is surprised to see him always up the moment after, shak-
ing about and as lively as ever.
  The vibratory motion of a pendulum, as dependent upon the circumstance
of the centre  of gravity having been moved from its lowest place which it
again constantly seeks, was  so fully considered in the  last  chapter, that it
need not be again dwelt upon here;  but we have to enumerate the following
phenomena as being of the same class.
—The vibrations of a common swing.
—The rocking of a balloon when it first ascends.
—The spontaneous shutting of those gates or doors of which the upper hinge
overhangs or  projects  beyond the lower, causing the gate, when in the shut
position, to have its lock lower than when in any other.  Such a gale always
returns of itself, from either side, to the shut position, just  as a pendulum
returns  to the lowest part of  its arc:—the gate in fact is but a sloping  pen-
dulum.
  Of the same nature also is the  rocking  or rolling of a ship, in particular
states of wind and sea.  When the centre of gravity of a ship is too low,
owing to all the heavy load being placed near the keel, this pendulum-motion,
in rough weather, becomes excessive and dangerous.

The actions and postures of animals, and particularly  of man, illustrate beau-
  tifully the observations made above with respect to the centre of gravity.
  A body, we have seen, is tottering in proportion as it has  great altitude
and narrow base—but it is the noble prerogative of man to be able to support
his  towering  figure with great firmness, on a very narrow base, and under
constant change of attitude.   This faculty  is acquired slowly because of the
difficulty.  A child does well  who walks at the end of ten or twelve months;
while the young  of quadrupeds, which have  a broad supporting  base, are
able to stand  and even to  move about almost immediately after birth.
  The supporting base of a  man is the space  occupied  by  and  included
between the  feet.  The  advantage  of turning out the toes  is, that without
taking much from the length of the base, it adds considerably to the breadth.
  If there be much art in walking on two  perfect feet, there is still more in
walking on two slender  wooden legs,  with rounded  extremities:—which,
however, we  often see done, by mutilated soldiers and sailors.
  All the ladies of the  empire of China have to acquire nearly the  same
talent as these victims of  war; for barbarous custom has crippled them, by
confining their feet for life, in such shoes as fitted them in infancy.
  But surpassing in difficulty any of these  instances is the practice, which is
general among the inhabitants of the sandy plains called the Landes, in the
south-west of France,  of walking on stilts.   The Landes afford  tolerable
pasture for sheep; but during one portion of the year are half  covered with
water, and during the remainder are still very unfit walking ground, by
reason of their deep loose sand and thick furze.  The natives meet the incon-
veniences of all seasons by doubling the length of their natural legs, through
the  addition to  them  of the stilts  mentioned, which they call  des echasses.
Mounted on  these, which are wooden poles, put on and off as regularly as
the  other parts of dress, they appear to strangers a new and extraordinary
race of long-legged beings, marching over the loose sand, or through the water,
with steps of eight or ten feet in length, and with the speed of a trottino- horse;
their moderate journeys being of thirty or forty miles in a day.  While watch- 
CENTRE OF GRAVITY.
91
ing their flocks, they fix themselves in convenient stations by means of a third
staff which  supports them behind, and then with  their rough sheep-skin
cloaks and caps, like thatched roofs over them, they appear like little watch-
towers, or singular lofty tripods, scattered over the face of the country.
  Still beyond the art of walking on stilts is that which some persons attain
of walking  and dancing on a single  rope  or wire; or even of keeping the
centre of  gravity above the base, while standing  on  the movable  support of
a galloping horse.—A rope-dancer usually carries a long pole in his hand, to
balance him : it is  loaded  at each end, and when he inclines, he throws it a
little towards the side required, that the reaction may restore his perpendicu-
larity.
   Much art of the same sort is shown in the attitudes and  evolutions  of the
skater; in the amusements of supporting a stick upright on the  end  of the
finger; and in many other feats of'a like kind.
   Attitudes generally depend on  the necessity of keeping  the centre of
gravity of the body over the base under variety of circumstances, as  in the
straight or upright port of a man who carries a load on his head ;—the leaning
forward of one who carries it  on his  back; the  hanging backwards of one
who  bears it  between his arms;—the leaning to one  side of him who is
carrying  a weight  on the other side;—the habitual  carriage of very  fat
people, whose head  and shoulders  are thrown back, giving a  certain air of
self-satisfaction,—an air which belongs also to the expectant mother,  and
even  to the dropsical patient, although producing in the latter  so  sad  an  in-
congruity.
   When  a man walks or runs, he inclines forward,  that the centre of gravity
may overhang the base:  and  he must then be constantly advancing his feet
to prevent his falling.  He makes his body incline  just enough to  produce
the velocity which he desires.
   A man, in pulling horizontally at a load, is  merely  causing his  body to
overhang its base, so that  its tendency to fall may become  a force or  power
applicable to the work.
   When  a man rises from a chair, he is seen first to bend the body forward,
or to  draw the feet backward,  so as to bring the feet or base under the centre
of gravity, and then  he lifts the body  up.  If he lifts too soon, that is, before
the body  be sufficiently advanced, he  falls back again.
   A man standing with his heels close to a perpendicular wall, cannot with-
out falling, bend forward  sufficiently to pick up any  object that  lies  before
him on the ground;  because the wall prevents him from throwing part of his
body backward, to  counterbalance  the head and arms which must  project
forward.  A person little versed in such matters, might agree to  give ten
guineas for permission to  possess himself, if he could, of a purse  of twenty,
laid on the ground before  him: he of course would  lose his stake.
   When  a man walks at  a moderate  rate,  his centre of gravity comes alter-
nately over the right and over the left foot.  This  is  the reason why the body
advances  in a waving line, and why persons walking arm in arm shake each
other, unless they make the movements of their feet to correspond, as sol-
diers do in marching.

   Sea Sickness is  a subject closely related to the present.  Man requiring,
as now explained, so strictly to maintain his perpendicularity, that is, to keep
the centre of gravity always over the supporting part of his body, ascertains
the required position  in  various ways, but chiefly by comparing the per-
pendicularity, or other known position of things about him, with his  own 
92
MECHANICS.
position.   Vertigo  and sickness are the consequences of depriving him of
his standards of comparison, or of disturbing them.
   Hence on shipboard, where the lines of the masts, windows, furniture, &c.
are constantly changing, sickness, vertigo and  other affections of the same
class are common to persons unaccustomed to ships.  Many persons experi-
ence similar effects in  carriages, and in swings;  or on looking from  a lofty
precipice, where known objects being distant, and viewed under anew aspect,
are not so readily recognized; also in walking on a wall or roof; in looking
directly up to a roof, or to the stars in the zenith, because then all standards
disappear; on entering a round room, where there are no perpendicular lines
of light and  shade, as when the walls  and roof are covered with a  paper
which has  no regular arrangement of spot; on turning round, as in waltzing,
or if placed on a wheel; because the eye  is not then allowed  to  rest long
enough on any standard, &c.
   People when in the dark, and therefore blind people always, use standards
belonging  to the sense of touch;  and it  is because, on  board of a ship,
the standards both of sight and touch  are lost, that  the  effect is so very
remarkable.                                    *
   But sea  sickness also  partly depends on  the irregular pressure of  the
bowels among themselves and against the containing parts, when the  influ-
ence of their  inertia and weight varies with the  rising and falling of the ship.
   From the nature of sea sickness, as discovered in these facts, it is seen
why persons unaccustomed to the motion of a ship, often find relief by  keep-
ing their eyes directed  to the fixed  shore, where visible ; or  by lying  down
on their backs and shutting their eyes ; or by taking such a dose of exhilarat-
ing drink as shall diminish their sensibility to all objects of external sense.
   As no  condition or form of matter escapes from the great laws of nature,
we find the attitudes and general condition of vegetable as well as of animal
bodies, characterized by the necessity of having  the centre of gravity sup-
ported over the base.   With what admiration may  we contemplate the pine
and other trees in the forests of  nature, springing up to heaven  as perpen-
dicularly as if the plummet had been at work  to direct them ;  and no less
on the bows of precipitous hills than in the level plains.   On a smaller scale,
we see the grasses  and corn-stalks of our fields illustrating the same truth.
And whenever, in tree  or shrub, accident or peculiar nature causes a devia-
tion from perpendicularity, additional strength and support are provided.
  Beauty of form or position is often felt to exist in bodies, merely because
they possess  the shape and support required,  that the centre of gravity may
be stable.
   In architecture, how displeasing is a wall or pillar that is not quite upright;
or a column with too small  a  base; or a very tall narrow house  ; or a long
slender chimney.   On the other hand, how beautiful in a lofty edifice  is  the
suitable succession  of  columns, from  the massive  Doric of the basement,
supporting the whole superstructure, to the light Corinthian or kindred forms
seen above.  The Chinese pagoda is a fine example of the union of certain
requisites for stability, viz., perpendicularity and expanding  base,  with  the
other  qualities of perfect  symmetry, graceful proportion, and fanciful  orna-
ment.  When seen crowning a rising ground in a wooded island, or spring-
ing up from the centre of a rich garden, it forms, perhaps, one of the most
beautiful objects which fancy has ever designed.

  Beauty of attitude and grace of carriage in the human individual  are in
great part referable to the same principle. 
CENTRE OF GRAVITY.                       93
   The postures of opera dancers might pass as intentional illustrations of the
 number of ways in which the centre of gravity may be kept above a narrow
 base, by counteracting one disturbing motion or extension of a limb by some
 opposite and corresponding motion.  The common statue  of the god Mer-
 cury on tip-toe is a permanent familiar illustration  of such  a beautifully
 balanced attitude.
   Grace of carriage includes not only a perfect freedom of  motion, but also
 a firmness of step, or steady bearing of the centre of gravity over the base.
 It is usually possessed  by those who live in the country, taking much and
 varied exercise, or who  make gymnastics a part  of their discipline.  What
 a contrast is there between the gait  of the active mountaineer, enjoying the
 consciousness of perfect nature, and that of the  mechanic  or  shop-keeper,
 whose confinement to the cell of his trade soon produces in  his body a shape
 and air corresponding to it—and in  the softer sex what a difference is there,
 between that active and  graceful fair one who recalls to us  the fabled Diana
 of old, and that other sedentary being, who, having scarcely trodden  but on
 smooth  pavements and  carpets, under any new  circumstances, carries  her
 person as if  it were a load quite new and foreign  to her.
   The centre of gravity is also the  centre of inertia.   When a person lifts
 a uniform rod by its  middle, the inertia of both ends  being equal,  he over-
 comes it equally, and raises them evenly together.  When he lifts by a part
 nearer to one end, the shorter and lighter portion having less inertia, will rise
 the first, and there  will be a turning  motion of the rod  round the finger as a
 centre, proportioned to the excess of inertia in the greater side.
   The centre of gravity, or inertia, however, is not necessarily in the centre
 of the mass ;—for if a weight of three pounds, a,
 be affixed to one end of a rod, and  a weight of           ^S- 34-
 only one pound, b, be affixed  to  the other, the     J>
 two will still  be balanced, if supported  or lifted    C^/
 by a point of the rod,  c, three times nearer to the
 centre of the large weight than to the centre of
 the small one.  This fact is explained under the  head  of lever, a few pages
 hence.  For the sake of simplicity, in  describing such experiments, the weight
 of the connecting rod itself is neglected.
   The centre of gravity or inertia is also the centre of centrifugal force:—
 for if the balls a and  b of the last figure were made to spin round a common
 centre, as by making  the connecting  rod rest and  turn upon  a point or pivot
 at c;  unless  the point c were  the  centre of inertia of the two, the pivot
 would always be drawn in the direction of that end of the rod at which there
 was the greatest centrifugal force.  It is on this account that in the case of
 a mill-stone,  or great  fly-wheel, or of the balance-wheel of a watch, the axis
 must pass through the centre of inertia, to prevent its being more worn on
 one side than on the other.
   When we say, in astronomy, that  the earth revolves round the sun,  or that
 the moon revolves round the earth, we do not speak with absolute correct-
 ness, for in all such cases, both bodies are revolving round the common cen-
 tre of inertia of the two.   In the case of the sun and earth,  as the former is
 about a million times  larger than the latter, the Common centre of inertia of
the two is a million times nearer to its centre than to the centre  of the earth,
 and is therefore within its body or circumference.
   The centre of inertia in a body  moving evenly  is also its centre of  action
or percussion; because, if such centre come against an obstacle, the  whole
*Q 
94
MECHANICS.
momentum of the body acts there and is destroyed ; while, if any other part
than the centre hit, the body loses only a part of its momentum, and revolves
round the obstacle as a pivot or centre of  motion, to pass it on the side to-
wards which the greater inertia happened to be.
  In a hammer, or a  bar of iron used as a hammer, or in a pendulum, the
motion is not said to be even, because the velocity of the  different parts is
different, being greatest far from the hand  or centre of motion, and the centre
of all the motal Fnertia is nearer to the fast  moving end than to  the other.  Its
exact place, in  many cases, is easily  ascertained by calculation.   In a uni-
form rod moving as a pendulum, formstance, it is at the distance of one-third
from the lower  end.   In the pendulum it is called the centre of oscillation.
   If a man use a bar or rod of iron as a hammer, he must take care to make
it strike the object by its centre of action, or his own hand will receive a part
of the  shock.  A very heavy mass  thus carelessly used will seriously  strain
the wrist.   In a common hammer, as the chief part of the matter is  at the
end, the centre of percussion  is there too, and no precaution of the kind men-
tioned is required.
   If a  rod or small log of wood be suspended horizontally by  a string tied to
its middle, or be floating in water, and if  a forward blow be  given directly
across it near to one  end, the other end will be found, in the  first instant, to
have moved a little backward, or in a direction contrary to the blow, as if the
rod had  been fixed upon  an  axis.  The  inertia of the general mass, by re-
                                     sisting the motion becomes in effect a
               Fig. 35.                fixed axis.   This truth is  amusingly
                                     illustrated by laying the ends of a long
  <    a          a         ~~T^   stick on  two wine-glasses, and then
   ^7                        Y/     breaking  the stick by a smart down-
    ed                        o     ward blow of a poker on its centre.
                                     Instead of breaking the glasses also,
 as by such a blow might be expected, the ends of the stick rise at the instant
 of the stroke, to turn round certain centres of resistance in the fragments, as
 at a a and b, and then fall harmless on the table.
   In this section we have seen what admirable simplicity is given to many
 of our reasonings and operations, by considering bodies in reference only to
 their centre of inertia, under one or other of its names.

 "In a solid body moving about an  axis,  like a wheel or weighing-beam,
   the  different parts have different velocities, according to  their respective
   distances from the axis or centre."  (Read the Analysis.)
   The truth of this proposition is perceived at once on comparing the motion
 in the rim of a wheel, or  near the ends  of a  weighing-beam, with that  in
                          parts nearer the  centres.  Suppose  a d to be a
          Fig. 36.          line drawn across a wheel, or, along a weighing-
                          beam, the centre of motion  in either case being
                          at  c;  then the outer circular line or path, a c,
                          which a  point  at a  describes when  moving, is
                          longer  than the  corresponding inner line, b /,
                           which a point at b describes in the same time, as
                           a is farther from the centre than b.   This admits
                           of easy mathematical demonstration,  and  is in-
                           deed merely  an instance  of the  truth, that the
                           proportions existing between any  parts or lines in
                           one circle, hold with respect to the corresponding
 parts and lines in all circles.
 
SIMPLE MACHINES.
95
 "Hence forces with different speed may still be placed in continued connec-
   tion or opposition; and they will balance or be  equivalent, if the one be as
   much more intense than the other as it is slower."  (Read the Analysis.)

   This is the  important truth upon which the whole of  mechanics may be
 said to hinge.   It gives to man the simple machines or mechanical powers,
 as they have been called,—the Lever, Wedge, Pulley, &c, which enable him
 to adapt any species and speed of power which he' can  command to almost
 any work which he has to accomplish : and the discovery of it and of means
 to apply it may be said to have subjected external nature  to his control.  His
 works are of a thousand kinds, from the displacing of a rock to the spinning
 of a delicate thread ;  while  the natural powers or forces at his command are
 chiefly wind, waterfalls, fire and animal effort—and of which, in any particu-
 lar case, he may have only one kind at his service ;—still, being able to con-
 nect together his powerand resistance by solid media, of which different parts
 move  with any desired  difference of velocities, he can employ any force for
 a purpose of almost any kind.
   There is,  however, a false and most pernicious prejudice  very  generally
 existing with  respect  to the simple machines, which we  must  begin by
 removing,  viz., that they increase the quantity of power or force applied to
 them.  For  instance, when one pound, as a, at the end of a beam or lever,
 is seen balancing two pounds, at b, at half the distance on the other side of
 the axis, or four pounds, as  c, at a quarter of the distance, many  persons be-
 lieve that the lever itself gives or begets
 a force equal  to the difference of the               Fig. 37.
 weights so balanced.  But we shall now
 show, that levers and all  the other me-                   {)
 chanicalpowers (as from  the erroneous
 idea above  mentioned  they have been
 called,) merely enable us  to make such
 substitutions, so  that of a small weight
 descending  far, in place of a greater
 weight descending a little way, or of an                    ,--k-'."&"'
 inferior force working long, instead of a                    f".....'1
 superior force working  for a shorter                    l.._ ,.!
 time, — and  thus often to accomplish                      *
 ends to which the force possessed would
 be quite unsuited if applied directly.   In other words, the simple  machines
 enable us to  concentrate  or divide any kind or quantity of  force which we
 possess, so as to suit it  to our various  purposes, just as mill-pounds and
 branching channels enable us to accumulate or divide the force of a stream of
 water; but they no more  increase  the quantity of power than a mill-pound
 increases the quantity of water.  When any slender  force is caused through
 a machine, to produce  some  effect which  seems proportioned to an intense
 force, it  has  always to act longer, or  through more space than the other,
just in proportion as it  is more slender;  as a small stream of water acting
 for ten minutes, may  produce the same effect as a greater gush in one
 minute.  Twenty feet of the action of a small horse near the circumference
of a  great wheel, may be  rendered, by intervening machinery, equivalent to
ten feet of the action of a  heavy ox or elephant nearer the centre.   And one
horse in drawing through  six hundred feet, or a hundred  horses in drawing
through six feet, or the  piston of a great  steam-engine, in raising one from
the bottom to the top of its cylinder, &c, may all be made to do the  same
work. 
96
MECHANICS.
  To illustrate this subject farther;  we shall suppose a weighing-beam xy,
with a weitrht of one pound hanging at the end x; then if a spring issuing
from the fixed  box at E, with uniform force of one pound, be made to push
at the other end of the beam y, it will just balance the weight;  and if it be
                                     in the slightest degree stronger than
                                     the weight, it will push the end of
                                     the beam y down to B, and will raise
                                     the weight to  F.  If, instead of the
                                     single spring of one pound at the end
                                     of the beam,  two  such springs be
                                     applied at half-way from the centre
                                     to the end, so as to press at A, where
                                     there is just half the extent of  mo-
                                     tion, or room  to act, as at B, exactly
the same effect will follow.  Now because one string at the end of the beam
is seen here doing the  same work as two similar  springs, or a single spring
of double  strength at the middle, it might at first appear  that  there was a
saving of  power  by using the single  spring and  longer lever;  but let it be
observed,  that the two middle springs have each  issued from their box only
one inch,  while the single spring at the end has issued two inches: in both
cases, therefore, exactly two inches  of one-pound spring have been used.
   In the last experiment, pound weights or little buckets of water might be
used instead of the springs, and with exactly the same result—one pound or
pint at the end of the arm producing the same effect as two pounds or pints
at the middle of it: but it would be observed that the single  quantity fell two
inches,  while the double  quantity at half distance fell  only one inch; and to
replace  them after they had done their work, there would evidently be the
same labour, whether a person had  to lift the single quantity first one  inch,
and then another, or had to lift, first, one half of the double equipoise an inch,
and then the other half as much.—Each atom of matter may be considered
as held  to the  earth by its thread of attraction, and if one atom rise or fall ten
inches,  just as much of the supposed thread of attraction will be drawn out
or returned as if ten atoms rise or fall one inch.  And  so, where a weight of
one pound is made to do any work, instead of a weight of two pounds, there
is no more saving than in giving away two  yards of  single rope  instead of
one yard  of double  rope; and  in like manner for all other differences of
intensity.
   If a man were  to exert a force of one hundred pounds at A,  in the above
figure, to  lift the weight, a boy at B, with force of fifty pounds, might do
the same work;  but the man would only have  worked  or  pressed down
through one foot, while the boy would have worked through two;  and there-
fore, although the boy, with the assistance of the lever, seemed to  become as
strong  as the  man, the case  would merely be, again, that of the one-pound
spring unbending two  inches, to produce an effect equal to that of the two-
pound spring  unbending  one  inch.  The boy would be using two feet of his
smaller force, where the  man used  one foot of his greater force;  and  if the
work had to be long continued, the boy would  have  completely exhausted
himself, when the man remained yet fresh.
   A case of the lever, exhibited in this diagram, serves well  to explain the
nature of mechanical powers in general.   Suppose A  to be a weight of four
 pounds at the end of  a rod  or lever  A B, (p. 97) made to turn on c as an
 axis or fulcrum, and having the arm c B four times as long as the arm c A,
(but the two arms of the lever being equipoised so as not to conceal the action 
SIMPLE MACHINES.
97
of weights subsequently attached to them;) then one  pound at  the end B,
would balance the four pounds at the end of A, and with the slightest addi-
tional weight would preponderate.   Now let us suppose the arc B b to have
been fixed to the long arm of the lever with the four projections or shelves
here  shown, on  which  balls of one
pound might rest;  then if one of the                Fig. 39.
four balls from the plane d were to roll                        -g
upon  the first shelf, it would just ba-                  ^^S<?
lance A, and, with  one  grain more,              ^^^LzJlS^
would descend to the plane e, one inch     a,    ^^^       /\r=   ^
below; then a second ball of one pound   04^*"^........................>&=*&®-®&>--
would occupy  the  second shelf, and    w^  c              /-.• •
would descend in the same way, to be                        .•"-    c
followed by a third, and afterwards by                        /"■
a fourth; and when the whole four had
fallen from d to e, they would just  have lifted  the four-pound mass, at  the
other end of the lever, one inch.   So that, although one pound was seen
here  lifting  four pounds, it would  only have lifted them one-fourth part as
far as it fell itself, and the sum of the phenomena would be, that four pounds,
by falling one inch at the long end of the lever, had raised four pounds through
the same distance of  one inch at the short end.  No mechanical power or
machine generates force more than the lever does in this case.
   It appears, then, from all this, that as the quantity of motion in a body is
measured by its velocity and the number of atoms in  it conjointly, so  the
quantity of force exerted in  any case, is  measured by the intensity of the
force conjointly with the space through which it moves.   A clear mode,
therefore, of comparing forces, is to state  the lengths and the intensities—
for instance, to speak of ten feet of  one-pound force, as equal to  one foot of
ten-pound force, &c.
   A horse pulling with the force of fifty pounds goes generally at the rate of
six miles an hour; the steam-engine  piston is generally made  to move at the
rate of two hundred feet per  minute, bearing a pressure of steam  of about
twenty pounds  to each  square inch  of its surface;  a particular mill-stream
may have a  force of one hundred pounds, with a velocity of a hundred and
fifty feet per minute:—now it is easy, by  simple arithmetic, and the rule of
length and intensity above explained, to compare all these and other forces
as applicable to any given work.  We must warn the  reader, however, that
there  are many important considerations connected with the practical employ-
ment  of forces, according to their respective nature and  that of the resistance
to be  overcome, which cannot be entered upon in this elementary work.  In
very many cases there is a great waste or unavoidable loss of  force, because
the resistance, in yielding, runs away or escapes from the force;  as when a
ship runs away from the wind which is driving her, or the floats of a quick
moving water-wheel, from the stream which turns it.   Horses drawing boats
or carriages  at the rate of five miles an hour, might  exert great force, but to
have a speed exceeding twelve miles they might require their whole  effort
to move their own bodies.  As a general rule, although equal quantities of
force  balance each other when applied to parts of a lever or wheel altogether
or nearly at  rest, still when a force is made to act near its axis or fulcrum, to
produce considerable velocity in a more distant part of the machinery, much
of it is wasted in pressure against the fixed fulcrum.
   What an  infinity of vain schemes—yet some of them displaying great
ingenuity—for perpetual motion, and new mechanical engines  of power, &c, 
9S
MECHANICS.
 would have been checked at once, had the great truth been generally under-
 stood, that no form or combination of machinery ever did or ever can increase,
 in the slightest degree, the quantity of power applied.  Ignorance of this is
 the hinge on which most of the dreams of mechanical projectors have turned.
 No year passes, even now, in which many patents are not taken out for such
 supposed discoveries ; and the deluded individuals, after selling perhaps even
 their  household necessaries to obtain the means  of securing the expected
 advantages, often sink  into despair, when their attempts,  instead of bringing
 riches and happiness to their families, end in disappointment and ruin.  The
 frequency, and eagerness, and obstinacy, with which even talented individuals,
 owing to their imperfect knowledge of this part of natural philosophy, have
 engaged in such undertakings, is a remarkable phenomenon in human nature.
 Examples of such schemes will be noticed in different  parts of this work,
 where they may serve to illustrate points under consideration.

      " Lever, wheel and axle, fyc."  (Read the Analysis, at page 84.)
   These  are the simplest  of the  contrivances which  the circumstance of
 solidity  in masses has enabled man to adopt, for the purpose of connecting
 or opposing forces and resistances of different intensities.   We proceed to
 describe them, and to explain some of their useful applications.

                                " Lever."
   A beam or rod of any kind,  resting at one part on a prop or axis, which
 becomes its centre of motion, is a lever; and it has been so called, probably,
 because such a contrivance was first employed for lifting weights.
   This figure  represents a lever  employed to move a  block of stone: a is
 the end to which the power or force is applied,/is the prop or fulcrum, and
 the mass b is the weight or resistance.  According to the rule already given
                              and explained at page 96, the power may be
           Fig- 40«             as  much  less  intense than the  resistance as
                              it  is farther from the fulcrum, or moving
                              through a  greater  space.   A man at a, there-
                              fore, twice as far from the prop as the centre
                              of gravity of  the  stone b,  will be able to lift
                              a stone twice  as  heavy as himself; but he
 ...                      will lift it only one inch for every two inches
 that he descends :  and two men would be required, actino- at  half the dis-
 tance,  to do the same work.
   There is no limit to the difference, as to intensity, of forces which may be
 made to balance each other  by the lever,  except  the length and strength of
 the material of which levers have to be formed.  Archimedes said, « Give me
 a lever long e    ^   d      p strQng      ^ ^ ^                 i

 will lift the worId "   But he would have required to move with the velocity
 of a cannon-ball for millions of years, to  alter  the  position of  the earth by
 a smal  part of an inch.   As stated in a former part of the volume, this feat
 of Archimedes is, in mathematical truth, performed by every man who leaps
 from the ground: he  kicks the  world  away from him when he rises  and
 attracts it again when he falls back.                                  '
   To calculate  the effect of a lever, in practice,  we must always take  into
account the weight of the lever itself,  and the fact of its bendfng  n.or
less ; but in expounding the theory of the lever, it is usual to consider, first,
 
SIMPLE MACHINES.
99
 what would be the result, if the lever were a rod without weight and without
 flexibility.
   The rule for the lever, that the opposing forces, to balance each other, must
 be more or less  intense, exactly as they act  nearer  to or farther from the
 centre, holds in all cases, whether the forces be on different sides of the prop
 or both on the same side, and whether the force nearest to the propjfrave the
 office  of power or of resistance;  it holds, also, whether the lever be straight
 or crooked.

   The following are examples of levers with the prop between the forces.
   The handspike, represented in page 98, is a lever moving a block of stone.
 The same form, when made of iron, with the extremity formed into claws, is
 called a crow-bar.  Both kinds are used by gunners, in working cannon dur-
 ing battle: they  are also used generally for lifting and moving heavy  masses
 through small spaces, as the materials of the mason, the  ship-builder, the
 warehouse-man,  &c.   A short crow-bar is the instrument of house-breakers,
 for wrenching open locks or bolts, tearing off  hinges,  &c.
   The common claw-hammer, for drawing nails,  is another example.  A
 boy who  cannot  exert a direct force of fifty pounds,  may yet, by means of
 this kind of hammer, extract a nail to which half a ton might be quietly sus-
 pended,—because his hand moves through, perhaps,  eight inches, to make
 the  nail rise one-quarter of an inch.  The claw-hammer  also proves, that
 it is of no consequence  whether  the lever be  straight or crooked, provided
 it produces the required  difference of velocity between power and resistance.
 The part of the hammer resting on the plank is the fulcrum.
   A pincers or forceps  consists  of two  levers, of  which  the hinge  is the
 common prop or fulcrum.   In drawing a nail  with  steel forceps  or nippers,
 we have a good example of the advantages of using a tool: 1,  the nail is
 seized by the teeth of steel  instead of by the  soft fingers:  2, instead of the
 griping force of the  extreme fingers only, there is the force of the  whole
 hand conveyed through the handles of the nippers : 3, the force is rendered,
 perhaps, six times more  effective by the lever-length of the handles :  and 4,
 by making the nippers, in drawing the  nail, rest on one shoulder as a ful-
 crum,  it acquires all the advantages of the lever or claw-hammer for the same
 purpose.
   Common scissors are  also double levers, and those stronger shears with
 which, under the  power  of  a steam-engine, bars and plates of iron are now
 cut as  readily as  paper is cut by the force of the hand.
   The common fire-poker is a lever.   It rests on the bar of the  grate as its
 prop, and displaces or breaks the caked  coal behind as the  resistance.
   The mast of a ship, with sails set  upon it, may be regarded  as  a long
 lever, having the sails as  the power, turning upon the  centre of buoyancy of
 the vessel as the  fulcrum, and lifting the ballast  or  centre  of gravity as the
 resistance.  For  this reason lofty sails make a ship heel or lean over greatly,
 and  if  used in open boats,  are dangerous.  In  some of the  islands  in the
 Eastern and Pacific Oceans, for the sake of sailing swiftly, boats are  used
 so extremely narrow and sharp, that to  counteract the  overturning tendency
 of their large sails, they have an outrigger or projecting plank to wind-
 ward, on the extremity of which one or more  of the  crew may sit as a ba-
lance.
   Perhaps no  instance  of the lever,  with the prop  between the forces, is
 more interesting than the weighing-beam ; whether with equal arms, form-
ing the common scale-beam; or with unequal arms, forming the steel-yard. 
100
MECHANICS.
   We have seen why quantities of matter attached at equal distances from
 the  prop, must be equal to each other in order to balance.   A lever, there-
 fore, which enables us to place quantities thus exactly in  opposition to each
 other, and which turns easily on its axis, becomes a weighing-beam.  Of this
                             the annexed  figure shows a common form.
          Fig. 41.            The axis or pivot at  c  is sharpened below,
                             wedge-like, that the  beam may turn easily,
                             and that  its centre of  motion  may be  nicely
                             determined;—in a delicate  balance  for philo-
                             sophical purposes, the  axis is almost as sharp
                             as a knife edge, and rests on some hard smooth
                             surface  of support, so as to turn with the
                             weight of a small part of a grain.   The scales
 also of a weighing-beam are suspended  on sharp edges to facilitate motion,
 and to determine nicely the points of suspension.  If the two arms of a beam
 be not of perfectly equal length, a smaller weight at the end of the longer will
 balance a greater weight at the end of the shorter.   An  excess  of half  an
 inch in  the length of a beam-arm, to which  merchandize  is attached, where
 the arm should be eight inches long,  would cheat the buyer of exactly one
 ounce in every pound.  This case might be detected  instantly, by changing
 the  places of  the two things  balanced;  for so, the lightest would be at the
 short arm, and would then appear doubly too light.  Abeam intended for
 delicate purposes, and required, therefore, to turn easily, must have its centre
 of gravity very near the axis  on which the beam turns; for if otherwise, the
 beam will be in the predicament of a ship with the ballast too high or too low:
 in the former  case, when once inclined, it would fall over, and not to recover
 itself:  in the latter, it  would tend to remain horizontal, and therefore would
 be less free  to move.   The proper situation of the centre of gravity is a little
 below the axis or line of support, that the beam may return with sufficient
 readiness from any state of inclination, to its horizontal position of rest.
   There is  a  mode of arriving at very accurate results, even with a weigh-
 ing-beam  which is  not itself accurately made,  provided it has very free
 motion, viz.,  first, very nicely to balance in one scale  the  substance to be
 weighed, and  then to  remove it, and to put weights into the same scale, until
 a perfect balance is produced.   Such  weights must be the exact  equiva-
 lent or weight of the  substance, however unlike to each  other the arms of
 the balance  may be.  A projecting rod, or plank, or  branch  of a tree, may
 thus be made  to answer the  purpose of  a weighing-beam, by attaching any
 substance to its  extremity  and observing minutely how  far such substance
 bends it, and then trying what weights would bend it as much.
  The steel-yard is a lever with unequal arms, and any weight, as b, on the
                                long  arm, will balance  as  much more
            Fig. 42.               weight as a on the short arm, as  the for-
                                mer is supported farther from the fulcrum
           ±  2   3    -*  .5  g  than the latter.   Thus, if the  hook at the
            1   '   '  -^jjT'  '■   '  short  end be  one inch from the centre of
                     (3&       support, c, a pound weight b, on the long
                                arm  at four  inches,  will  balance four
                                pounds a,  at  the short arm.   This sup-
                                poses,  however, that the steel-yard when
                                bare,  hangs  horizontally, from  having a
greater mass of matter in  the short arm to counterbalance the  long  slender
 
SIMPLE MACHINES.
101
arm from which the shifting weight hangs.  When this is not the  case, a
corresponding allowance has to be made.
  The Chinese, who are so remarkable for the simplicity to which they have
reduced all their common implements, weigh any small objects by a delicate
pocket steel-yard.  It is a rod of wood or ivory, about six inches long, with
a silk cord passing  through it at a particular part,  to serve as a fulcrum, and
with a sliding weight on the long arm, and a small  scale attached to the short
one.

The following are examples of levers with both forces on the same side of the
  prop, and where the more distant force  acts as the power.

  A common wheel-barrow is a lever, in using which a man bears as much
less than the  whole weight of the load as the centre of gravity of the load is
nearer to the axle of the wheel than to his hands.
  When two porters  carry a load placed  midway between them, on a pole,
they share it equally, that is to say, each bears a  half, for  the pole becomes a
lever, of which  each porter is a fulcrum, as regards the other; but if the load
be nearer to one end than to the other, he to whom
it is nearestbears proportionably more of its weight.           Fig. 43.
A load at c is equally borne by a porter at a and    ff   ^    c______jfr
by one at b ;  but a load at  d gives  three-quarters
of its weight to the  man at a,  and only one-quarter
to him at b.
  Two horses drawing a plough, act from the ends
of a cross bar, of which the  middle usually is hooked to the plough.  The
horses must  thus  pull equally, to  keep the bar directly across.  When on.
heavy land, three horses are yoked, and two of them are  made to draw from
one end of the bar, it must be attached to the plough by a hook, not at its mid-
dle but half as far from one end of it as  from the other.
  The oar of a boat is a lever of this kind, where singularly the purpose of ful-
crum is served  by the unstable water.
  The common nut-crackers furnish another instance, by the lever-power of
which a person can break a shell many times stronger than he  could break
with the bare fingers.
  The consideration of this kind of lever explains why a finger caught near
the hinge of a shutting door is so much injured.   The momentum of the door
acts by a comparatively long lever, upon a resistance placed very near  the
fulcrum.   Children pinching their fingers near the hinge of a door, or of the
fire-tongs,  which furnishes a similar case—wonder why the bite is so keen.
  The phenomenon of the  branch of a  tree  giving  way, when in  autumn
overloaded with fruit, or in winter with snow, also exhibits the  action of this
kind of lever.   The resistance is the cohesion of the upper side of the branch
to the  tree, and the fulcrum is the part below which is last broken.

The following  are  examples of the lever, where  the two forces are  on the
  same side  of the  pivot, but  where that nearest to the  pivot acts as  the
  power.  In this kind, the power is more intense than  the resistance.
  The hand of a man who pushes open a gate while standing near the hinges,
moves through much  less space than the end of the gate,  and hence must act
with great force.
   When a man uses the common fire-tongs, the ends move much farther than
     8 
102
MECHANICS.
his fingers, and therefore with less strength.   No one fears a pinch with the
ends of the fire-tongs.
   The most beautiful and remarkable instances of this modification  of lever
are in the limbs of animals.   The object in these was to give to the extremi-
ties great range and freedom of motion, without  clumsiness of  form; and it
has been attained  most perfectly by the tendons or ropes which  move them,
being attached near to thejoints, which are the pivots or fulcra of the bone
levers.
   In  the human  arm, the deltoid muscle, which forms  the cushion of the
shoulder, by contracting its fibres less than an inch,  raises  the elbow twenty
inches, and of course, if it overcome a force of fifty pounds at  the elbow, it
must itself be acting with a force at least  twenty times as intense, or of one
thousand pounds.—What extraordinary  strength of muscle, then, is displayed
by a man who lifts another at the end  of his extended arm; yet this feat is
frequently accomplished, and even on both sides of the person at  once.
   How powerful  again must be  the wing-muscles of birds, which, by this
kind of action, sustain  themselves in the sky for many hours together.  The
great  albatross, with  wings extended fourteen  feet or more, is seen in the
stormy solitudes of the Southern Ocean, accompanying ships for whole days,
without ever resting  on the waves.
   A little contraction of the glutaei muscles  of the hips gives to  the human
step a length of four  feet.
   While the erroneous opinion prevailed, that  machines  increased power,
instead  of, as  they do, merely accommodating forces to purposes, this last
kind of lever, where a great force acting through a short distance is  made to
gain great extent  of  motion and other benefits, was  regretted by many as a
most  unprofitable contrivance, and was called the losing lever.
   It is almost unnecessary to say, that the same rule of comparative velocities
ascertains the  relations required between power and resistance,  where a com-
bination of levers  is used, as where there is onlv one.   If a lever  which makes
one balance/owr, be applied to work  a second lever which  does the same,
one pound at the  long  arm of the first will balance sixteen pounds  at the short
arm of  the second, and would balance  sixty-four at  the short arm of a third
such, &c.
   The  general rule for the lever, that a force may be less intense the farther
it is from the pivot,  supposes always  that the force  acts  at right angles, or
directly across the lever;  for if there be any obliquity, there is a correspond-
ing diminution of effect,  as explained under  the head of resolution of forces,
at page 57.   For instance, one pound at  b on the end of the long arm of the
                          bent lever b d a, because its  weight does not act
          Fig. 44.         directly across b  d, has influence only as if it were
   e     rl            .p    acting directly at the end  of a shorter horizontal
   ;.........-^p^—•..............:     arm df; and the two-pound weight at a acts only
   |   /   ^""^-^^^  \y    as if it were on a horizontal arm at e; now e being
   I  /          ^"XD    onty na^ as far  from  the centre as/, two pounds
  1/                      at a, in the  position of  the lever here shown,
 (^)                       would just balance the one pound at b.  In every
          c                case, the exact influence  of weights is known by
                           referring them to places  directly above or below
                           them, on a supposed horizontal lever ef.   What
 is called a bent-lever balance, is made on the principle  here explained.   It
 has on  one side a heavy weight as at a, and  on the other side a scale  attached
 at b; and the  weight of any thing put  into the scale  is indicated by the posi- 
SIMPLE MACHINES.                      103
Fie. 46.
 tion then assumed by the lever, marked by the point at which it  cuts an arc
 of divisions placed behind it.   In any common weigh-beam, the point of sus-
 pension of the scales being a little below the axis of motion of the beam, there
 is a degree of the property of the bent-lever balance, and enough to require
 notice in very nice experiments.

                         " The Wheel and Axle"
 is the next to be mentioned of the simple machines.  The letter d here marks
 a wheel, and e an  axle affixed to it; and we see that in turning together, the
 wheel would take up or throw off as much
 more rope than the axle,  as its circumference
 or diameter were greater than that of the axle.
 If  the proportions were as four to one,  one
 pound at b, hanging from the circumference of
 the  wheel,  would balance four pounds at a,
 hanging  from the opposite side of the axle.
 The proportions are equally indicated, and are
 usually expressed by  comparison of the diame-
 ters of the  wheel and the axle.
  This figure represents the same object as the
 last, viewed endways.  It explains why  the
 wheel with its axle has been called a perpetual
 lever; for the two weights hanging in opposition, on the wheel at a, and on
 the axle at b, are  always as if  they were  connected by a
 horizontal  lever at a c b, of which the arms are  respectively
 the diameters of  the wheel and the axle,  turning on  the
 centre c as the prop; and while a simple lever could only lift
 through a small space, it is evident that this  construction
 will lift as  long as there is rope to be wound up.
  A common crane for raising weights, consists of an axle
 to wind up or receive the rope which carries  the weight,
 and of a large wheel  at the circumference of which  the
 power is applied.   The power may be animal effort exerted
 on the rim or outside of the wheel, or the weight of a man
 or beast  walking within it, and moving it as a squirrel
 moves the cylinder of his cage.
  The capstan used on board of ships,  is merely a large upright axle or spin-
 dle b, which by turning, pulls the cable
 or rope a b c; and  it is  moved  by the
 men pushing at  the capstan-bars d, e f,
 &c, which for the time are stuck into
holes made  for them  in the broader part
or drum,  usually  appearing above the
deck, at the top of the spindle.  These
bars may be considered as the spokes of
a large wheel, and the effect produced by
a man working at one of  them is in pro-
portion to his distance from the centre.  The capstan is chiefly used on board
ships for lifting the anchor, and for doing any  other very heavy work; but it
is also applied to certain purposes on shore.
  The common  winch (represented as attached to  the wheel and  axle  at
the  letter  c,) with which  a grindstone is  turned, or  a crane worked, or a
Fie. 47.
 
104
MECHANICS.
watch wound up, is really in principle a wheel: for the hand of the worker
describes a circle, and there is no difference in the result whether an  entire
wheel be turning with the hand or only a single spoke of a wheel.
  That part of a common watch called  the fusee is  as  beautiful an illustra-
tion of the principle of the wheel and axle now under consideration, as it is
                                           a useful and ingenious contri-
                 Fig. 48.                    vance.  The spring of a watch,
                                           immediately after winding up,
                                           being more strained, is acting
                                           more  powerfully  than   after-
                                           wards  when slacker,  and if
                                           there were no means of equal-
                                           izing its action, it  would de-
                                           stroy the  wished-for uniform-
                                           ity in the  motion of the  time-
piece.  The fusee  is this means.   It  may be considered as  a barrel or
spindle, gradually diminishing from its large end b, to its small  end a, with
the surface cut into a spiral groove to receive the chain, by pulling at which
the spring in the box  c moves the watch.   Now when  the watch has been
wound up, by a  key applied  on the axis of the fusee, the fusee is covered
with the  chain  up  to the small end  a, and  the newly bent and  strong
spring begins to pull by  this  small  end  or short  lever; and  afterwards,
exactly as the spring becomes relaxed and  weaker, it is pulling  at a larger
and larger  part of the fusee-barrel, and  so  keeps  up an equal effect on the
general movement.
   A large fusee in place of a common cylindrical axle, is  often used with a
winch, for drawing water by bucket and rope from very deep wells. When the
bucket is near the bottom of the well, and the labourer  has to overcome the
weight of the long rope, in addition to that of the bucket and water, he does
so more easily by beginning to wind the rope on a small axle, that is to say,
on the small end of the fusee;  and in proportion as the length of rope dimi-
nishes, he lifts by a larger axle.
   By the double axle a b, very unequal intensities of force may be balanced.
                              We see that in  turning it, a rope unwinding
                              from the small end a is taken up by the large
                              end b, turn for  turn,  and that the  rope below
                              must be shortened at each turn by the differ-
                              ence between the circumference  of the ends
                              a and b. If the weight rise half an inch only,
                              while the handle  of the  winch  describes a
                              circle of fifty inches, one pound force at the
                              winch  would balance one hundred pounds
                              at d.
                                 By means of a wheel, which is very large
                              in proportion to its  axle, forces  of very dif-
 ferent intensities may be balanced, but the machine becomes of inconvenient
 proportions.  It is  found preferable, therefore,  when such an  end is desired,
 to use a  combination  of wheels of moderate size.   In  the adjoining figure,
 three wheels are seen  thus  connected.  Teeth on  the axle  d, of the first
 wheel c, acting on six  times the number  of  teeth  in  the circumference of
 the second wheel g, turn it only once for every six  times that c turns; and
 in the same manner the  second  wheel, by turning six  times, turns  the
Fie. 49.
 
SIMPLE MACHINES.
105
third wheel h once ; the first wheel, there-
fore, turns thirty-six times for one turn of
the last;  and as the diameter of the wheel
c, to which the power is applied, is three
times as great as that of the axle /, which
has the resistance, three times thirty-six,
or one hundred and eight, is the difference
of velocity, and therefore of intensity, be-
tween weights or forces that  will balance
here.—An  axle  with  teeth upon it, as  d
or e, is called  a pinion.
   On  the principle of combined wheels,
cranes are  made, by which one  man can
lift many tons.    It is even  possible to
make  an engine, by means of which a  little windmill, of a few inches in
diameter, should  tear  up the  strongest  oak by the roots;  but of course  it
would require a  very long time for its work.
   The most familiar instances of wheel-work are in our clocks and watches.
One turn of the  axle on which the watch-key is  fixed, is rendered equiva-
lent,  by the train  of wheels, to about four hundred turns or beats of the
balance-wheel; and thus the exertion during a  few seconds, of  the  hand
which winds up,  gives motion for twenty-four or  thirty hours.  By increas-
ing the number of wheels, time-pieces are made which go for a year:  if the
material would last, they might  easily be  made  to go for a  hundred or a
thousand years.
   Wheels may be connected  by bands  as              pj„ 51_
well as by teeth.   This is seen in the com-
mon spinning-wheel, turning-lathes, grind-
stones, &c. &c.   A spining-wheel, as a c,
of thirty inches in circumference, turns by
its band a pirn or spindle of half an inch, b,
sixty times for every turn of  itself.
Fig. 52.
                         " The Inclined Plane"
is the third means which we shall  describe, of balancing, by solid media,
forces of different intensities. A force push-
ing a weight  from c  to  d,  only raises it
through  the perpendicular  height e d, by
acting along the whole length of the plane
c d; and if the plane be twice as long as it
is high,  one  pound at b  acting  over the
pully  d, would balance two pounds at a, or
anywhere on the plain: and so  of all other
quantities and proportions, as already ex-
plained under the head of  "Resolution of forces," at page 86.
  A horse drawing on a road where there is  a rise of one foot in twenty,
is realy lifting one-twentieth of the load, as well as overcoming the friction
and other resistance of the carriage.  Hence the importance of making roads
as level as possible; and hence our forefathers often  erred in  carrying their
roads  directly over hills, for the sake of straightness considered vertically,
where by going  round the bases of the hills  they would scarcely have had
greater distance, and would have avoided all rising and falling.   Hence, also,
 
106
MECHANICS.
a road up a very steep hill must be made to wind or zig-zag all the way; for
to reach a  given height, the ease of the pull to the horses  is greater exactly
as the road is made longer.  This rule  of road-making is exhibited remark-
ably in  various parts of the world,  where hills  with  almost perpendicular
faces, have very safe and commodious roads upon them, leading to forts or
residences near their summits.  An intelligent driver, in ascending a steep
hill on which there is a  broad road,  winds from  side to side of the road all
the way to save his horses a little.
   The railways of modern times offer a beautiful illustration of this subject.
They are made generally quite  level, so that the drawing-horse or  steam-
engine has  only to overcome  the friction of the carriage;  or where heavy
loads are passing only  in  one direction, as  from mines, they are  made to
slope a very little, leaving to  the horse or other power only the  office of
regulating the movement.
   A hogshead of merchandize, which twenty men could not lift directly, is
often seen moved into or out of a wagon, by one or twro men, who have the
assistance of an inclined plane.   In some canals, or rather particular situa-
tions on canals, the loaded boats are drawn up by machinery or inclined
planes, instead of being raised by water in locks, as is  the usual mode.
   It is  supposed that the ancients (the Egyptians  particularly) must have
used the inclined plane, to assist in elevating and  placing those immense
masses  of stone, which still  remain from  their times, specimens of their
gigantic architecture.
   Our common  stairs are inclined  planes in principle; but being so steep,
are cut into horizontal and perpendicular surfaces, called steps, that they may
afford a firm footing.
   We may here recall, that  a body falling freely, in  obedience to gravity,
descends about sixteen feet in the first second, and that if made to descend
on an inclined  plane, it moves just  as  much less quickly  (besides the loss
from the friction  and the  turning  produced) as  the length  of the plane is
greater than the height.   On a plane sloping one foot in sixteen of its length,
a body would descend only one foot in the first second.
   The descent of a pendulum  in its  arc is investigated mathematically by the
laws of the  inclined plane.  And the laws of the inclined plane itself are
mathematically examined by the principle  of the resolution of forces, ex-
plained  at p. 57.

                             "The Wedge"
is  merely an inclined plane forced in between resistances to separate or over-
come them, instead of, as  in the last case, being stationary while the resistance
               is moved along its surface.   The  same  rule as to mechanical
               advantage has  been  applied to the wedge as to other  simple
               machines;  the force acting  on a  wedge being considered as
               moving through its length c d, while  the resistance yields to
               the extent of its breadth a b.   But this rule  is far from ex-
               plaining   the extraordinary  power of a  wedge.  During the
               tremor produced  by the blow  of the  driving-hammer? the
               wedge insinuates  itself,  and advances  much more quickly
               than the  above rule anticipates.
                 The wedge  is  used for many  purposes;  as for splitting
               blocks of stone and  wood: for squeezing strongly, as in the
               oil-press ; for lifting great weights, as when a ship of war, in
               dock, is  raised by wedges driven under the keel, &c.
                 An engineer in London, who had built a very lofty and
 
SIMPLE MACHINES.
107
 heavy chimney, common to all his  steam-engines and furnaces, found after
 a time that, owing to a defect in the foundation, it was beginning to incline.
 However, by driving wedges under one side of it, he succeeded in restoring
 it to perfect perpendicularity.
   Nails, awls, needles, &c, are examples of the wedge;  as are also all our
 cutting instruments, knives, razors, the  axe, &c.   These latter  are often
 used somewhat in the  manner of a saw—which is a  series of small wedges,
 —by pulling them lengthwise at the same time that they are pressed directly
 forward against the object.  They themselves, indeed, when viewed through
 a microscope, are seen to be but finer saws.  It appears that the^vibration of
 the particles produced  by the drawing of a saw, enables its edge to insinuate
 itself more easily.   The sharpest razor may be pressed directly against the
 hand with considerable  force, and will not enter, but if then  drawn along
 ever so little, it starts into the flesh.

                             " The Screw"
 is another of the simple machines.  It may be called a winding wedge, for
 it has the same  relation to  a  straight wedge  that
 a road winding up a hill  or  tower  has to a straight
 road of'the same length and acclivity.
  A screw may be described as a spindle a d, with
 a thread wound spirally round it,—turning or work-
 ing in a nut c,  which has a corresponding spiral fur-
 row fitted to receive the thread.  The  nut is some-
 times called  the female screw.  Every turn of the
 screw carries it forward  in  a fixed  nut, or draws a
 movable nut along upon it, by exactly the distance
 between two turns of its thread : this distance, there-
 fore, is the space  passed through by the resistance,
 while the  force moves  in the circumference of the
 circle described by the  handle of the  screw, as at f
 in the  figure.  The disparity between these lengths or spaces is often as a
 hundred or more to one; hence the prodigious  effects which a screw enables
 a small force to produce.
  Screws are much  used in presses of all kinds :  as in those for  squeezing
 oil  and juices from  such vegetable bodies, as linseed, rapeseed,  almonds,
 apples, grapes, sugar-cane, &c.: they  are used also—in  the cotton press,
 which reduces a greaty spongy bale, of which a few, comparatively, would
 fill a ship, to a compact  package, heavy enough to sink in water;—also, in
 the common printing-press,  which has  to force the paper strongly against the
 types:—a screw is the great agent in our coining machinery,—and in letter-
 copying machines :—it is a screw which draws together the iron  jaws of a
 smith's vice, &c.   The screw, although producing so much friction as to con-
 sume a considerable part of  the force used in working it, is an  exceedingly
useful contrivance.
  As a screw can easily he made with  a hundred turns of its thread in the
 space of an inch, at  perfectly equal distances from each other, it enables the
 mathematical instrument maker to mark divisions on his work, with a minute-
 ness and  accuracy quite extraordinary.   If we suppose such  a  screw to be
 pulling forward a plate  of metal, or  pulling round the edge of a circle, over
 which  a sharp-pointed steel marker can be let down perpendicularly, always
 in the same place, the marker, if let down once for every turn of the screw,
 will  make just  as many lines on the  plate as the screw makes turns;  but if
 
108
MECHANICS.
55.
 made to mark at every hundredth or a thousandth of a turn of the screw,
 which it will do with equal accuracy, it may draw a hundred thousand dis-
 tinct lines in one inch.
   The  instruments called micrometers, by which the sizes of the heavenly
 bodies  and  of microscopic objects  are ascertained, are worked  by fine
 screws.
   A perpetual screw is the name given where a screw acts on the teeth of a
 wheel, so as to produce a continued rotation of the  wheel.
   A common  cork-screw is the  thread of a screw without the spindle, and
 is used, noj to connect opposing  forces, but merely to enter  and fix itself
 in the cork".  Complicated cork-screws  are  now made,  which draw the
 cork by the  action of a second screw, or of a toothed  rod or rack and pinion.

                            " The Pulley"
 is another simple machine, by which forces of different  intensities  may be
                        balanced.  A simple pulley consists of a wheel as
                        a b, which rests with its  grooved circumference
                   -a   on the bend of a  rope, c a  b d, and to the axis of
                        which the weight or  resistance is  attached, as
                        at e.
                          In such a construction,  it is evident that the
                        weight (let it be supposed  ten  pounds)  is  equally
                        supported by each end of the rope, and that a man
                        holding up one end, only bears half of the  weight,
                        or five pounds ;  but to raise the weight one foot,
                        he must draw up two feet of rope; therefore, with
                        the pulley  he  is as if lifting five pounds two feet,
                        where, without the pulley, he  would have to lift
                        ten pounds one foot.
                          Many wheels  may be combined together, and
                        in many ways to form compound pulleys.   Wher-
 ever there is but  one rope running through  the whole, as shown here, the
relation  of power and resistance  is  known by the number of folds of the
rope which support  the weight.  Here there are four supporting  folds, and
a power of one hundred pounds would balance a resistance of four hundred.
                 As  persons using  pulleys generally find it more conveni-
                 ent to stand upon the ground than to go up  and apply
                 their force  directly to one of the  supporting  ropes, the
                 last of these is  commonly made  to pass over a wheel
                 above, and to come down apart from the others, as shown
                 here.  This portion not being directly connected with the
                 weight, adds convenience to the pulley, but is  not  to be
                 counted with the others, in estimating  the relation of the
                 power and resistance.
Fig. 56.
 Infixed pulleys, like those shown at a and c, p. 109, there
is no  mechanical  advantage, for the  weight just moves as
fast as the power; yet such pulleys are of great use in
changing the  direction of forces.  A sailor without mov-
ing from the deck of his ship, by means of such a pulley,
may hoist the sail  or  the signal-flag to the top of the
loftiest mast.   And in the building of lofty edifices, where
heavy loads of  material are  to  be sent  up every  few
minutes, a horse,  trotting away with the end of the rope 
SIMPLE MACHINES.
109
from d, in a level  courtyard, causes  the  charged basket b to ascend to the
summit of the building as effectually as if he had
the power of climbing, at the same rate,  the per-          Fig. 57.
pendicular wall.
  There is  a case,  however, in  which a fixed
pulley may seem a balancer of different intensi-      ( U J^
ties of force; viz., where  one end of a rope is
attached to a man's body, and the other is carried
over a pulley above, and brought down again to
his hands ;—for  safety this  end  also should be   'b\
attached to his body.  By using  the hands then
to pull with force equal to half  his weight, he
supports himself, and may  easily raise himself to          \f?)      cl
the pulley.   A man, by a pulley thus employed,        '•"</
may let himself down into a deep well, or from
the brow of a cliff, with assurance  of being able
easily to return, although no one  be near to help
him ; and cases have often occurred  where, by
such  means,  a fellow-creature's life  might have been saved, or other import-
ant objects attained.  How easily, for instance, might persons either reach
or escape from the elevated windows of a house  on fire, by such a pulley,
which might readily be found and used where ladders could not be obtained!
This  kind of pulley furnishes a convenient  means of  taking a bath from a
ship's stern windows, &c. €
   The chief use of the pulley is  on ship-board.  It is there called a block,
although, strictly speaking, the block is only the  wooden  mass which sur-
rounds the wheel or  wheels of the pulley.   It  aids so powerfully in over-
coming the heavy strains of placing the  anchor, hoisting the masts and sails,
&c, that, by means of it, a smaller number of sailors are rendered equal to
the duties of the ship.   Pulleys  are also used  on shore, instead of cranes
and capstans, for lifting weights, and overcoming other resistances.
   Surgeons,  in former days, when  they trusted rather to force than to the
address which better information gives, used pulleys much to help in the re-
ductions of luxations,—but often hurtfully, from not understanding the force
of the pulley.  A good  surgeon  now rarely needs a  pulley, and he  who
should ignorantly stretch his patient on  the rack, would be well requited by
similar treatment.
>   The cranks  of bell-wires,  seen  in the  corners of  our rooms, are bent
levers nearly equivalent to  fixed pulleys.

There is no reason, but old us^age, why  the appellation of mechanical power
   should  be confined to the six contrivances  now explained, for those of
   which the account is yet to follow equally  deserve it; and, as will be seen
   under hydrostatics and pneumatics, the most powerful mechanical engines
   do  not belong to solids at all.

   Engine of oblique action, is a title  which  may include  a considerable
variety of contrivances for connecting different velocities.
   Suppose c a and c b to represent two strong rods connected together, like
a carpenter's folding rule, by a hinge or joint at c.  If the distant ends be
made to bear against notches in two obstacles, at a and b, and by force then
applied to c,  either to push or to  pull, the joint c be straightened or carried
towards d, the joint c will move through  a much greater space than the simul- 
110
MECHANICS.
Fie. 58.
Fie. 59.
                       taneous increase of distance produced between a and
                       b; and, in proportion to the disparity, the power
                       applied at c will overcome a more intense resistance
                       at the extremities.   The  mechanical power of this
                       contrivance incieases rapidly, the nearer the jointed
                       rods approach to straightness.
                         If  we suppose the end  a to be steadied by a hinge
                       on frame-work, and the end b to bear upon  that part
                       of a printing press which carries the paper against
                       the types, we have imagined the simple press called,
                       from  its contriver, the Russell-press.  A man's force
                       at d,  at the moment when the rods are drawn nearly
                       to a straight line, becomes  equivalent to a  pressure
of many tons.
  For the  same reason, that by urging c towards d, in the last figure, the
extremities a and b are separated with great force, so by urging c in the con-
trary direction, the extremities would be drawn together with corresponding
force :  and if we suppose a c b to be part of a rope coming through pulleys
at a and b, to one end of which rope beyond a, great resistance is attached, one
man, by  pulling at c, may move a weight or resistance many times greater
than he could move by his direct  power.
  The following is another mode of connecting an oblique and a direct force
                        so as to balance them, although of  different inten-
                        sities.—If to turn  a wheel (represented  here by
                        the circle,) a weight be suspended from d, it is act-
                        ing directly, for it descends  just as fast as the cir-
                        cumference of the wheel moves, and would, there-
                        fore, be impelling with  its whole strength: but if
                        it were suspended from the  point e, it would then
                        be acting obliquely to the motion of that part of the
                        wheel, and from not descending so fast as if at d,
                        it would have as  much less effect on the wheel,
                        than if there, as the line e b is shorter than the line
                        d c.  The reason of this will be  understood by
                         referring  to  the  subjects of resolution of forces
                         and of  bent  levers,  in  former  parts of the work.
                         For the same reason,  if such  a wheel were used
                         in lifting weights, a man turning it  could lift as
much more attached at the point e  than  at the point d, as the line d c is
longer than e b.  A  man  turning  this wheel  in  the direction from e  to a,
with a weight  hanging at e, would  be lifting that weight exactly  as  if he
were  rolling  it up the  inclined plane  or  curve  e a.   This figure  is useful
                           in explaining  the varying intensity of the ac-
                           tion  of a crank  or winch~ in different parts of
                           its revolution, and of the combination  of levers
                           used  in the Stanhope printing press, in their
                           different positions:  it explains also the degrees
                           of strength  and  support afforded by oblique
                           stays in buildings and in ships'  rigging, and
                           many other kindred matters.
                             The arrangement  of cross-jointed wires, re-
                            presented here, connects different velocities, and
                            therefore, is really a mechanic power.  It has
Fig. 60.
 
SIMPLE MACHINES.
Ill
been applied to some curious purposes, but to none of much utility.   By
pressing the ends a and 6 towards each  other, the wires, from  being in the
position represented in the upper figure, immediately assume  the position
represented in the lower; so that the end c darts outwards much farther than
the ends a and b approximate.

Different intensities of force are balanced, although not simultaneously, by
   the following means;  which, therefore,  according to the  old idea, have
   some claim to the name of mechanic powers.
   A man  may have a purpose to effect, which a forcible downward push
would accomplish: but his body being too  weak to give that push directly,
he may employ a  certain  time in carrying a weight to such an elevation,
above his  work, that when let fall  its momentum may do  what is required.
Here the continued effort of the man in  lifting the weight, to a  height
of perhaps thirty feet, may be just sufficient to produce a blow which will
cause a stake or pile to sink into  the  earth one inch ;  and the contrivance
has therefore balanced forces, of which the relation as to intensity is marked
by the spaces thirty feet and an inch.
   So also hammers, clubs, battering-rams, slings, &c, are machines which
enable a continued moderate effort to overcome a great but short resistance.
   'The fly-wheel,  which, by persons  ignorant of natural  philosophy,  has
often been accounted a positive power, in common cases  merely equalizes
the effect  of an irregular force.
   In using  a winch to  turn a mill, for instance, a man does  not act with
equal force all around the circle: but a heavy wheel fixed  on  the axis mode-
rates acceleration, and receives or absorbs momentum, while  his action is
above par, and returns it again, giving  it to  the machine, while his action is
below par, thus equalizing the movement.  And in the common instances
of circular motion produced by a crank, as when, by the pressure of the foot
on a treadle, we turn a lathe or grind-stone, or spinning-wheel, the  force is
only applied during a small part of the revolution, or in the form of inter-
rupted pushes ; yet the motion goes on steadily, because the turning grind-
stone, or wheel, or lathe, becomes a fly and reservoir, equalizing the effect
of the force.  In a steam-engine  which moves machinery by a crank, the
upward and downward pushes of the  piston are converted, by means of a
heavy fly-wheel, into a very steady rotatory motion.
   A  heavy  wheel, however, has sometimes been used as  a concentrator of
force or a mechanic power.   By means of a winch, or a  weight, or other-
wise, motion or momentum  bejng gradually  accumulated  in the wheel, is
then  made  to expend itself in producing some sudden and  proportionally
great effect.   Thus, a man may lift a very heavy weight by first, in any way
giving motion  to a fly-wheel, and then suddenly hooking a rope from  the
weight  to the axle of the  wheel, which rope being wound upon the axle, lifts
the weight.
   A fly-wheel moved in the same  manner, and containing  the result of a
man's action during perhaps one hundred seconds, if made  to impel a screw-
press, will, with one blow or punch, stamp  a perfect medal, or from  a rough
flat plate of silver will form a finished spoon, or other utensil.
   A spring, in the same sense, may become a mechanical power.  A person
may expend some minutes in bending it, and may then let fly its accumulated
energy in  an instantaneous blow.  A gun-lock shows  this phenomenon on a
small scale.   The slow bending of a bow, which afterwards shoots its arrow
with such velocity, is another instance. 
112
MECHANICS.
  These, then, are the principal means which the solid state of bodies affords
us of balancing forces of different intensities.  We shall find other such means
or mechanic powers belonging to liquids and airs.   All of them are of inesti-
mable value to man, by enabling him to  accommodate  the forces which he
can command to any kind of work which he has to perform.  Thus he makes
his  millstone turn with the same velocity, whether  it be moved by the slow
exertion of a horse or bullock, walking in  a ring, or  by the quicker motion of
a river gliding  under the wheel, or by the rapid  gush of a water-fall, or by
the  invisible swiftness of the wind.  And  again, each of these forces he can
equally apply to turn the heavy millstone  or to twist a cotton thread.
  The wants of men seem first  to have led them to  use the simple machines
for  the purposes of raising great weights, or overcoming great resistances,
and hence the name long used  of mechanic powers,—particularly  for the
Lever, Wheel  and Axle, Plane, Wedge,  Screw, and Pulley: but the  term
conveys to the uninformed a false idea of  their real  nature, and  has begotten
the common prejudice with respect to them, that they generate force,  or have
a sort of innate power for saving labour.  Now so far is this from being true,
that in using them in any case, even  more labour  or bodily exertion is ex-
pended than would suffice to do the work without them.  This assertion is
intentionally rendered paradoxical to arrest attention, but its truth will appear
from the  following considerations.
  One man may be able, with a  tackle of pulleys having ten plies of the rope,
to raise a weight which it would require  ten men  to raise  at once without
pulleys.   But  if the weight is to be raised a yard,  the ten men will  raise it
by pulling at a single rope and walking one yard, while  the one man at his
tackle must walk until he has shortened all the ten  plies of rope of one yard
each ; that is, he must walk ten  yards, or  ten times  as far as the ten men did.
In both cases,  therefore, to accomplish the same end, we have just the same
quantity of man's work expended, in the  first, performed by ten men in one
minute, in the  second, by one man in ten minutes;  and if the work  were of
a nature to continue longer, let us say a whole day  for the ten men, it would
last ten days for  the single man, and there would  be ten days' wages of a
man to pay in  both cases, there is, therefore, no direct saving of human effort
from using pulleys; indeed, there is a loss, because of the great friction which
has to be overcome.  Now exactly the same is true of  all other simple  ma-
chines, or mechanic powers; none of them save labour, in a strict sense of
the phrase;  they only allow a small force to take  its time  to produce any
requisite  magnitude of effect, at  the expense of additionally overcoming a
certain amount of friction or other such resistance.
  The real advantages of these  machines  are such as the following:
  That one man's effort, or any small power, which  is  always at command,
by  working proportionally longer, will answer  the  purpose of the sudden
effort of many  men, even of hundreds or  thousands, whom it might  be most
inconvenient and expensive, or even impossible, to  bring together.
  A ship's company of a few individuals easily weighs  a heavy anchor by
means of the capstan.
  A solitary workman, with his screw or other engine, can  press a  sheet of
paper against types, so as to take off a clear impression; to do which  without
the press, the direct push of fifty men would scarcely be sufficient: and these
fifty men would be idle and superfluous except just at the instants of pressing,
which occur only now and  then. In this way the screw may be said to do
the work of fifty men, for it is as useful.
   A man with a  crow-bar  may move a  great  log  of wood  to a convenient 
SIMPLE MACHINES.
113
place, where twenty men would have been required to move it without  the
crow-bar; and although the  single man takes  twenty minutes, perhaps, to
do what the many men would have done in one minute, as the twenty might
not have been wanted  again for the rest of the day, the crow-bar may really
be as useful as the twenty men.
  It is  so important  to have correct notions on the subject of the simple
machines or mechanical powers, that more space has  been  here allotted to
the explanation of the  general principle, than  has been  usual in such works.
After the  examination which  it has now undergone, however, the author
hopes that none of his readers will have difficulty in conceiving clearly, that
"whatever, through a machine, is gained in power,  is lost in speed or in
time, and  vice versa"—or wi\\ have difficulty in detecting immediately any
common fallacy connected with  the  subject;—as that  of supposing,  for
instance, that a  lever, or jrreat pendulum, or spring, or heavy fly-wheel, &c,
can  never exert more force than  has  passed  into  it  from  some  source of
motion.

"By solid connecting parts, also, the direction of any xisting motion or
  force may be changed. Hence the endless variety of complex machines."
  (Read the Analysis  at p. 84.)

   It is this power of  changing the direction of motion, added to the power
of connecting and adjusting various intensities of force and resistance by the
simple machines last  described, which has enabled  man to make complex
machines, rivaling in  their performances the nicest work of human  hands.
It would  be endless to attempt the enumeration of the modes  in which the
directions of motions may thus be changed, for it would be to enumerate and
describe  the whole apparatus of the arts and  sciences; but we shall advert to
a few as specimens.
   Straight motion changed into rotatory.—The straight motion of wind or
water becomes rotatory in wind or water-wheels.—The straight-downward
pressure of the human foot, acting at intervals on a treadle  and crank, turns
round the  grindstone, and common lathe, and spinning-wheel.  The alter-
nate rising and falling of the piston of a steam-engine  is made, by means  of a
crank, to turn the great fly-wheel and any other wheels which a steam-engine
may move.
   Rotatory motion into straight.—An axle in  turning will  wind up a rope,
and lift a weight in a straight line.—A crank  on a turning axle, if connected
with a pump rod, will work the piston up and down ; or it will work a saw.
Pallets or teeth-on a turning-wheel act on the  handle of a great forge ham-
mer, so that every one in passing lifts the hammer and produces a blow.
   We need not multiply instances.  By a visit  to great manufacturing towns,
or, indeed, by simply directing the eyes to what is passing around, in  any
part of the civilized world, we discover miracles of mechanic art:—machines
driven by wind, water or steam  for grinding  corn ;—machines for  sawing
wood and giving it various forms;—machines in which rods of  metal are
seized between great  rollers, and  are flattened  at once into  thin plates, as if
they were of clay, and these  plates  again are slit into  bars or ribbons —
spinning machines, which perform their delicate office even more uniformly
than human hands, forming thousands of threads at once, in obedience, to die
impulse of a single steam-engine;—weaving machines, which accomplish their
difficult task with the  most admirable perfection;—paper-making cndnes,
which convert worn-out and apparently useless remnants of  our apparel, into 
114
MECHANICS.
 the uniform and beautiful texture of paper, a texture which, with the farther
 assistance of the pen, or types, or engraved plate, becomes a magic conserva-
 tory of mind, shutting up  among its folds  the brightest effusions of genius,
 and ready, at any instant, to disclose them again to the  delighted  student,
 nothing changed after revolving centuries ;—coining machinery, which from
 a bar or plate of metal cuts out and stamps thousands of beautiful medals in
 an hour, and keeps an exact record of  its  work ;—cranes,—pile-engines,—
 turning-lathes,—time-pieces,—all the implements  of agriculture, of  mining,
 of navigation, &c. &c.   If Aristotle deemed the title or definition of tool-using
 animal appropriate to man two  thousand  years  ago, what title should be
 given now?
  In many of the  complex  machines, several of the simple ones are found
 as elements ;  and in the same machine may be comprised many of the means
 of changing the direction of motion.

                "Friction."   (Read the Analysis, p. 84.)
In estimating the effects of mechanical contrivances, by the rule of compara-
  tive velocities of the power and resistance, there is an  important correction
  to be made, on account of the mutual  friction of the moving  parts.   In  the
  steam-engine, where the rubbing parts are numerous, the loss of power
  from friction often amounts to one-third of the whole.
  Impediment from friction seems to be owing  to  two causes: 1st, a degree
of cohesive attraction between the touching substances ;  2d, the roughness of
these surfaces, even where,  to the naked eye, they appear smooth.
  It is supposed to be, because the roughness, or little projections and cavi-
 ties, in pieces of the same or of homogeneous  substances  mutually  fit each
other,  as  the  teeth of similar saws would, so as  to allow the bodies, in a
degree to enter into each other, that the  friction  is greater between such than
between  pieces of different  or of heterogeneous substances with dissimilar
grain.
  The friction of one piece of iron, wood, brick, stone, &c, on another piece
of the  same substance, has been measured  by using the second piece as an
inclined plane, and then gradually lifting one end of it until the upper mass
began  to  slide,—the  inclination of the  plane, just before  the sliding com-
mences, is called the angle of repose.  This angle, different for different sub-
stances, is found to be, for metals, generally such as to mark  that the force
required to overcome  the friction between  small  pieces of them is equal to
about a fourth of the weight of the  moving piece, and for woods it is about
a half.   But for large pieces or great pressures,  the friction  is proportionably
much less.
  It is this angle in the substances concerned, which determines the  degrees
of acclivity which  can  exist in the sides of hills  composed of sand, gravel,
earth, &c, in  the banks of canals, rivers, &c.
  If the thread of a screw winds round  the spindle with an angle less than
this, the screw can never recoil or slide back  from force acting against its
point.
  But for friction,  men walking on the ground or pavement would always be
as if walking on ice;  and our rivers, that now flow so calmly, would all be
frightful torrents.   Friction is, therefore, in  these cases of great use to men.
  Friction is  useful, also, when it enables  men,  out of the comparatively
short fibres  of cotton, flax, or hemp,  to form their lengthened webs and
cordage,—for  it is  friction  alone, consequent  upon the  interweaving and 
FRICTION.
115
twisting of the fibres and threads, which keeps the material of these fabrics
together.

   The following  means are used to diminish friction between rubbing sur-
faces ; and they are used  singly or in combination, according  to  circum-
stances.
   1.  Making the rubbing surfaces  smooth;—but this must be done within
certain limits, for great smoothness allows the bodies to approach so near that
a degree of cohesion takes place.
   2.  Letting the substances which  are to  rub on each other be of different
kinds.  Axles are made of  steel, for instance, and  the parts on which  they
bear are made of brass : in  small machines, as time-keepers, the steel axles
often  play in agate or diamond.   The swiftness of a skater depends much
on the great  dissimilarity between steel  and ice.
   3.  Interposing some lubricating substance between the rubbing parts ;  as
oils for the metals, soap, grease, black-lead, &c, for the woods.   There is a
laughable illustration of this  in the holiday sport of soaping a lively pig's tail,
and then offering him as the prize of the  clever fellow who can catch and
hold him  fast by his slippery appendix.
   4.  Diminishing the extent of the touching surfaces ; as in making  the rub-
bing axis  of a wheel very small.
   5.  Using wheels, as in wheel-carriages,  instead of dragging a rubbing load
along  the ground.  Castors on household furniture  are miniature wheels.
   6.  Using what is called friction-wheels ;—which
still farther diminish the friction even of a  smooth          Fig. 61.
axis, by allowing it to rest on their circumferences,             a-
which turn with it.   Here a represents the end of
an axis, resting on  the exteriors of two  friction-
wheels, b and c.
   7.  Placing the thing  to be moved on rollers or
balls, as when a log of wood is drawn along the
ground upon  rounded pieces of  wood;  or when a
cannon, with  a  flat  circular base to its carriage, turns round by rollino- on
cannon-balls  laid on a hard level bed.  In these two cases, there is hardly
any friction, and the resistance  is merely from the obstacles which the rollers
or balls may  have to pass over.
   Of  all rubbing parts, the  joints of animals, considering the strength, fre-
quency and rapidity of  their movements, are those which  have the least
friction.   The rubbing  surfaces in these are  covered, first, with a layer  of
elastic cartilage, and then with an exceedingly smooth membrane, over which
there is constantly poured from the glands around, a fluid called  synovia,
more emollient and lubricating than any oil, and which is renewed constantly
as may be required.  We study and admire the  perfection of animal joints,
without being able very closely to imitate it.

   Wheel carriages merit notice  here, as illustrating many of the circumstances
connected with friction; and moreover as being among the most common  of
machines.
  Wheel  carriages have three  advantages  over the  sledges for which they
are the substitutes:
   1. The rubbing or friction, instead of being between an iron shoe  and the
stones  and irregularities of the road, is between the axle and its  bush,  of
which  the surfaces are smoothed and fitted to each other, and well lubricated.
 
116
MECHANICS.
Fie- 62.
   2.  While the carriage moves  forward, perhaps fifteen feet, by one revolu-
tion of its wheel, the°rubbing part, viz., the axle, passes over only  a  few
inches of the internal surface of its smooth greased bush.
   3.  The wheel surmounts  any abrupt obstacle  on the road  by the axle
                                           describing a gently rising slope
                                           or  curve,—as  shown in  this
                                           figure, where a represents an
                                           obstacle,  and where the curve
                                           from c, of which the beginning
                                           has the direction shown  by the
                                           line c e,  represents  the path
                                           of the axle in surmounting it.
                                           The wheel is as if rising on an
                                           inclined plane, and gives to the
                                           drawing animal the relief which
                                           such a plane would bring.  This
kind of advantage is greater in a large wheel, for evidently the smaller  wheel
here represented, in having to surmount the same size of obstacles, has to
rise in the steeper curve beginning at d,—but the difference of advantage, in
this respect, is not so great as the difference of size.  It is true  again,  that a
small wheel would sink to the bottom of a hole,  where a larger one  would
rest on the edges as a bridge, and would sink less.  The fore-wheels of car-
riages are usually made small, because  such construction, by allowing the
wheel to go under the body of the carriage, facilitates the turning of the car-
riage.   It is not true, however, according to the popular prejudice,  that the
large  hind-wheels of coaches, wagons, &c, help to push on the  little wheels
before them, as if the carriage were on an inclined plane resting on the wheels;
but there is the accidental advantage, that in ascending a  hill, when  the horses
have to put forth their strength, the load rests chiefly on  the hind-wheels, and
in descending, when an increased resistance is desirable, the load falls chiefly
on the fore-wheels.
   From the causes mentioned in the last paragraphs, the difference in per-
forming the same journey of a mile, by a sledge and by a wheel-carriage, is
that while the former has to rub over every roughness in the road and to be
jolted  by every irregularity,  the rubbing part of the latter, the axle,  glides
very slowly over about  thirty yards of a smooth oiled surface, in a gently
waving line.   Thus, by wheels, the resistance is reduced to  about the hun-
dredth part of what it is  for a sledge.
   On hilly roads, in descending, it is common to lock or fix one of the wheels
of a carriage,  and the horses have then to pull  nearly as much as  on a level
road with the  wheel free; showing the effect of a little increase of friction.
   The wheel  of a carriage, simple  as, from our extreme  familiarity with it,
it now appears to us, is  a thing of very nice workmanship, and which  has
exercised much  ingenuity.—It acquires astonishing  strength, indeed, that of
                        the arch, from what is  called its "dished form, seen
                        here in the wheel c, as contrasted with the flat wheel
                        a.  In a wheel of this form, the extremity of a spoke
                        cannot be displaced inwards,  or towards  the  car-
                        riage, unless the rim of the wheel be  enlarged, or
                        all the other spokes yield at the same  time,  and it
                        cannot be displaced outwards, or away from the car-
                        riage, unless  the  rim be diminished, or the other
                        spokes yield  in  the opposite  direction:—now the
 
FRICTION.
117
rim being strongly bound by a ring, or tire of iron, cannot suffer either increase
or diminution, and the strength of all the spokes is thus by it conferred on each
individually.   In a flat wheel a given degree of displacement outwards  or
inwards of the extremities of a spoke, would less affect the magnitude of the
circumference, and therefore the rim of such  a  wheel secures much less
firmly.   A watch-glass and  a round piece of egg-shell are stronger than  flat
pieces of like substances, for the same reason that a dished wheel is stronger
than a flat wheel.—The dished form of a wheel is farther useful by leaving
more room between the wheels for the body of the carriage, and is useful
also in this, that when the carriage  is on an inclined road, and more of the
weight  consequently falls upon the wheel of  the lower  side, the inferior
spokes  of that wheel become nearly perpendicular, and thereby support the
increased weight more safely.  The strongest  form of wheel is the doubly
dished,  that is, a wheel  having half of the spokes passing from within to the
rim, as from c to d, fig. 63, and the other half similarly from without.  This form
is adopted in the wheel recently constructed entirely of iron, in which there is
the  farther peculiarity that  the load is  supported  more by hanging by the
upper spokes  than by resting on the lower.—When wheels, instead of stand-
ing upright, like b and d shown, fig. 63, are made to incline outwards, as is
common, owing to the  ends of the axletree being  bent down a little to give
a security against the accident of the wheels falling off, the pull to the horses
in deep or sandy roads is  much increased; for  an  inclining wheel would
naturally describe  a curved and  outward path, as is  seen when  a hoop or
wheel-barrow inclines; and the  horses, therefore, in  drawing straight  for-
ward, have constantly to overcome the deviating tendency in all inclining
wheels.  This cause of resistance is still more remarkable when the wheels
have broad rims.  Such wheels must be conical, that is, of smaller diameter
at the outer than at  the inner edge, as the  end of a cask is  smaller than its
middle, and then, as the iron hoops or  tires which cover the different parts
cannot  all, by an equal number of turns, truly measure the same length of
road, there will be a constant rubbing or grinding forward of the lesser rings,
and a grinding backward of the larger, injuring  the road,  rapidly wearing
the iron, and exhausting the strength of the pulling animals.  Such wheels
rolling free would describe a circular path,  as is exemplified when a thimble,
or drinking glass, or sugar-loaf, which also are  conical, is pushed forward on
any plane surface.
   The  application of springs to carriages, which is an  improvement of com-
paratively recent date, not only renders  them soft  moving vehicles on rough
roads, but much lessens the pull to the horses.   When  there is no spring,
the whole load must rise with every rising  of the road, and if time be given,
must sink with every depression, and the  depression  costs  as much labour
as tb^e rising,  because the wheel must be drawn up again from the bottom of
it; but in a spring-carriage moving rapidly along, only the parts below  the
springs are moved in correspondence with  the road-surface,  while all above,
by the inertia of the matter, have a soft and even advance.  Hence  arises
the  superiority of these modern carriages, furnished  with  what are  called
under-springs, which insulate from the effect of shocks, all the parts, except-
ing the  wheels and axletrees themselves.   When  only the body  of the car-
riage is on springs, the horses have  still to rattle the heavy frame-work below
it over all irregularities, and  then the wheels as well as the structure generally
require to be  of much  greater strength and  weight  to bear the consequent
shocks.
      9 
118
MECHANICS.
   The subject of wheel carriages is interesting to medical men, from  their
having often to direct in transporting the sick or wounded.
   Is is perhaps difficult to conceive any thing more elegant and perfect than
the carriages of modern refinement;  and therefore a  man, who sees them
gliding swiftly along the prepared levels and slopes of our present landscapes,
and  thinks of the clumsy vehicles on the  bad roads  of former times, may
readily imagine that absolute perfection is at last attained.   Yet we are per-
haps now on the  eve of a farther change which, for many purposes, will be
of greater importance than all that has  yet been achieved—viz., the general
adoption of rail-roads, with new-fashioned carriages  to suit  them.   To all
who study such subjects, it is now known, that to drag a loaded wagon  up
one inconsiderable hill, costs more force than to send it thirty or forty miles
along a level rail-way; and the conclusion is obvious, that although the origi-
nal expense of forming the level line might considerably exceed that of mak-
ing an ordinary road, still, in situations of great traffic, the difference would
soon be paid for  by the savings, and  when once paid, the savings would be
as a profit for ever.   To  readers  conversant with political  economy, it
would be superfluous to speak here of  the advantages of  any greater facility
of intercourse, but to  those who are  not,  the  following  reflections may be
interesting.
   In reviewing the history of the human race,  we find that every remarkable
increase in civilization has taken place very much  in proportion to the facili-
ties of intercourse offered in the  particular situation.  First, therefore, civili-
zation grew along the banks of great rivers,  as the Nile, the  Euphrates and
the Ganges; or along the shores of inland seas and  archipelagos, as in the
Mediterranean  and the numerous islands of Greece ;  or  over fertile and ex-
tended plains, as  in many parts of India   When  the situation  thus bound a
great number  of individuals into one body, the useful new thought or action
of any  one unusually gifted, and which, in the insulated  state, would soon
have been forgotten and lost, extended its influence immediately to the whole
body, and became the thought or action of all who  could benefit by it, besides
that  it was recorded for ever, as  part of the growing science of art of the com-
munity.  And in a numerous society, such useful thoughts and acts would
naturally be more frequent, because persons feeling that they had the eyes of
a multitude upon them, and that the rewards of excellence would be propor-
tionally great, would be excited  to emulation  in  all the pursuits that could
contribute to the well-being of  the society.  Men soon learned to  estimate
aright these and many other advantages of easy intercourse, and after having
possessed themselves with avidity of the stations naturally fitted for their
purposes, they began to improve the  old and to make new stations.  They
created rivers and shores, and plains of their  own, that is, they constructed
canals,  and basins,  and  roads;  and so connected artificially regions which
nature  seemed to have separated for  ever.—In the British isles, whose fa-
voured children have taken so remarkable a lead  in showing what prodigies
a wise policy may effect, the advantages arising from certain lines  of canal
and  road first executed, soon led to numberless similar enterprizes, and within
half a century the empire has been thus bound  together in all directions : and
it seems as if the noble work was now to  be  crowned by the substitution of
level railways for many of  the  common roads and canals.*   Several  rail-

   * These observations were first published (the substance had been written Ion."- before,)
 soon after the Darlington rail-road, the first of any  note intended for passengers, was
 opened.   The Manchester and Liverpool rail-road has since then  admirably verified the
 anticipations. 
STRENGTH OF MATERIAL.
119
roads of short extent have already been established, and although they and
the carriages upon them are far from having the perfection which philosophy
says they will  admit, the results have been very satisfactory.  If we sup-
pose the progress to continue, and the price of transporting things and persons
to be thus reduced to a fourth of the present charge—and in many cases  it
may be less—and if we suppose the time of journeying with safety also to
be  reduced  in some  considerable degree,—of which there can be as little
doubt—the general adoption of such roads would operate an  extraordinary
revolution and improvement in the state of society.   Without in reality
changing the distances  of places, it would in effect bring all places nearer to
each other, and would give to every spot in the  kingdom the conveniences
of  the whole,—of town and country, of sea-coast and  of highland district.
A man, wherever residing, might consider himself virtually near to any other
part, when, at the expense of time and money now expended in travelling a
short way, he might travel very far  and he would thus  find remarkably ex-
tended, the sphere  both of his business  and of  his  pleasures.   The over-
crowded and  unhealthy parts of towns would scatter their inhabitants into the
country ; for  the man of business could be as conveniently at his post from a
distance of several miles, as he is now from an adjoining  street.   The present
heavy charges for bringing distant produce to market being nearly saved, the
buyer everywhere would purchase  cheaper, and the producer would be still
better remunerated.
   In a word, such a change would  be effected as if by magic, the whole of
Britain had been compressed into a circle of a few  miles in diameter, yet
without any part losing aught of its magnitude  or  beauties.—All this may
appear visionary ;  but  it is  less  so  than  seventy years ago it  would have
been to anticipate much of what, in respect to travelling, has really come  to
pass,—as, that the common time of passing  from London to Edinburgh
would be forty-six hours.   At the recent opening (in 1825) of the  rail-road
near Darlington, a train of loaded carriages was dragged by one little steam-
engine a distance of twenty-five  miles within two hours ; and in some parts
of the journey the speed was more than twenty miles an hour:  the load
was equal to  a regiment of soldiers, and the coal expended was not  of the
value of a crown.  An  island  with such roads would be an impregnable
fortress ; for in less time than  an  enemy  would require to disembark on
any part of the coast, the forces of the country might be concentrated to de-
fend it.

" Strength depends on the magnitude, form, and position of bodies, as well
   as on the degree of cohesion in the material."   (Read the Analysis, page
   84.)

   The minute details connected with this branch  of the subject belong to the
practical engineer, but there are some of the general truths which should be
familiar to every body.

       Of similar bodies the largest is proportionally  the iveakest.

   Suppose two blocks  of stone left  projecting from a hewn rock, of  which
blocks one, as d, p. 120, is twice as long, and deep, and broad as the other, b.
The larger one will by no means  support at its end  as  much more weight
than the smaller, as its mass is greater, and for two reasons.  1st.  In the
larger, each particle of the surface  of attachment  at c, in helping to bear 
120
MECHANICS.
                         the weight of the block itself,  has to support by
                         its cohesion twice as  many particles beyond it in
                         the double extent of projection, as a particle has
                         to support in  the  shorter block at a;  and 2dly,
                         both the additional substance,  and any thing ap-
                         pended  at its outer extremity, are acting  with a
                         double lever advantage  to  break it, that is, to de-
                         stroy the cohesion at c.   Hence, if any  such mass
                         be made to  project very far, it  will be broken off,
                         or will fall by its own weight alone.   And what
                         is  thus  true of a block supported  at one  end, is
                         equally true  of a block supported at both ends, and
                         indeed of all masses, however supported,  and of
                         whatever forms, if they have projecting parts.  It
                         is to be  observed also  that masses, like an abso-
                         lutely perpendicular cliff, which have no  projecting
or overhanging parts, are still limited  as  to size by the degree of cohesive
force among their  particles, for the upper part of  such a mass tends to
crush or break down  the lower.   A  lofty pillar cannot be formed  of soft
clay.
   That a large body, therefore, may have proportionate strength to a smaller,
it must  be still thicker  and more clumsy than it is longer: and beyond a cer-
tain limit no proportions whatever  will  keep it together, in opposition merely
to the force of its own weight.

This great truth limits the size and modifies the shape of most productions
   of nature and of  art;—of hills,  trees, animals, architectural or mechanical
   structures, &c.

   Hills.  Very strong or cohesive material may constitute hills of sublime
elevation, with very projecting cliffs and very lofty perpendicular precipices;
and  such accordingly are seen where the hard granite  protrudes from the
bowels of the earth, as in the Andes  of America, the Alps  of Europe, the
Himalayas of Asia, and the Mountains of the Moon in Central  Africa.—
But  material of inferior strength  exhibits more humble risings  and more
rounded surfaces.   The gradation is so striking and  cons tan t,°from granite
mountains, down to those of chalk, or gravel, or sand, that the geologist can
often tell the substance of which a hill is  composed  by observing the pecu-
liarities  of its shape.
   Even in granite itself,  which is  the  strongest of rocks, there is  a limit to
height and projection ;  and if an instance of either, much  more remarkable
than now remains on earth, were  by any chance to be produced  again, the
law which we are considering would prune the monstrosity.   The  grotesque
figures of rocks  and mountains  seen in  the  paintings of the Chinese,—or
actually formed in  miniature for their gardens,  to express their notions of
perfect sublimity and beauty,—are caricatures of nature for which originals
can never have existed.  Some of  the  smaller islands in the Eastern Ocean,
however,  and some of the  mountains of the chains seen  in the voyage
towards China, along the coasts of Borneo and Palawan, exhibit, perhaps,
the very limits of possibility in singular shapes.   In the moon, where the
weight or gravity of bodies  is less than on earth, on account of her smaller
size, mountains  of  a given  material might be  many times higher than on
earth—and observation proves that the  lunar mountains are in  fact very
high.
Fig. 64.
______\b
o
el
o 
STRENGTH OP MATERIAL.
121
  By the action of winds, rains, currents, and frost, upon the mineral masses
around us, there is unceasingly going on  an  undermining and wasting of
supports, so that every now  and then immense rocks, or almost hills, are
torn by gravity from the station which they have held since the earth received
its present form, and fall in obedience  to the law now explained.
  The size of vegetables, of course, is obedient to the same law.   We have
no trees reaching a height of three hundred feet,  even  when  perfectly per-
pendicular, and sheltered in forests that have been unmolested from the be-
ginning of time:  and  oblique or horizontal branches are kept within com-
paratively narrow limits by the great strength required to support them. The
truth, that to have proper  strength, the breadth or diameter of bodies must
increase  more quickly than the length, is  well illustrated by the contrast
existing between the delicate and slender proportions of a young oak or elm,
yet in the seedsman's  nursery, and the sturdy form of one which has braved
for  centuries all the winds of heaven, and  has become the monarch of the
park or forest.
  Animals furnish  other interesting illustrations of this law.
  How massive and  clumsy are  the  limbs of the elephant, the rhinoceros,
the heavy ox, compared with the slender  forms  of the stag,  antelope, and
grayhound!  And unless the bones were made  of stronger material than now,
an animal much larger than the elephant would fall  to  pieces  owing  to its
Weight alone.   The whale is  the largest  of animals, but feels not its enor-
mous weight,  because lying constantly in the liquid support of the ocean.  A
cat may fall with impunity from a greater height than would suffice  to dash
the bones of an elephant or ox to pieces.
   For the reason which we are now  considering, the giants of the  heathen
mythology could not have existed upon this earth; although, on our moon,
where, as already stated, weight is much less, such beings might be.   In the
planet Jupiter, again,  which is many times  larger than the earth, an ordinary
man from hence would be carrying in  the simple weight of his body, a load
sufficient to crush the  limbs which supported him.   The phrase a little com-
pact man, points to the fact that such  a person is stronger in  proportion to
his size than a taller man.
  The same law limits the height and breadth  of architectural structures.  In
the houses of fourteen  stories, which formerly stood for protection, close under
the Castle of Edinburgh, there was danger of the superincumbent wall crush-
ing the foundation.
  Roofs.   Westminster Hall approaches the limit of width that is possible
without either very inconvenient proportions  or  central  supports;  and the
dome of the church of St.  Peter in Rome is in  the same predicament.
  Arches of a bridge.  A stone arch,  much larger than those of the magnifi-
cent bridges in London, would be in  danger of crushing or  splintering its
material.
  Ships.   The ribs or timbers of a boat have  scarcely a hundredth part  of
the bulk of the timbers of a ship only ten times longer than the boat.  A ship's
yard of ninety .feet contains, perhaps, twenty times as much wood as a yard
of thirty feet, and even then is not so strong in proportion.  If ten men may
do the work of a three-hundred-ton ship,  many more than three times that
number will be required to manage a ship  three times as  large.  Very large
ships,  such as the two built in Canada, in the year 1825, which carried each
nearly ten thousand tons of timber, are weak from their size alone;  and the
loss of these first two specimens of gigantic magnitude will not encourage to
the building of others. 
122                           MECHANICS.
a
 The degree in which the strength of structures is dependent on the form and
   position of their parts, will be illustrated by considering the two cases  of
   longitudinal and transverse compression.   And the rule for giving strength
   to any structure will be found to be, to cause the force tending to destroy
   it, to act, as equally as possible, on the whole  resisting mass at once, and
   with as little mechanical advantage as possible.
   In longitudinal compression, as produced by a body a, on the atoms of the
 support b, the weight, while the support remains straight,  can only destroy
 the support, by crushing it in opposition to the repulsion and impenetrability
                             of all  its atoms.   Hence a very  small pillar, if
         Fig. 65.             kept  perfectly  straight,  supports a very great
                             weight; but a pillar originally crooked, or be-
                             ginning to bend, resists  with, only  part of its
                             strength; for, as seen in  c d , the whole weight
                             above is supported  chiefly on the atoms of the
                             concave side, which are  therefore in greater
                             danger of being oppressed and crushed, while
                             those on the  convex side,  separated from their
                             natural helpmates, are in the opposite danger of
                             being torn asunder.   The atoms near the centre
                             in such a case are almost neutral, and  might be
                             absent without the strength of the pillar being
                             much lessened.
                               Long pillars or supports are weaker than short
                             pillars of the same diameter, because they are
                             more easily bent; and they are more easily bent
                             because  a very inconsiderable,  and  therefore
                             easily  effected yielding between each adjoining
                             two of their many atoms, makes  a considerable
                           a very short pillar there  cannot be much bend-
                                 ing without a  great change in the relation
                                 of proximate  atoms, and  such as can be
                                 effected only by great force.  The weight
                                 resting on any pillar, and bending it,  may
                                 be  considered as  acting (with obliquity
                                 dependent  on  the  degree of bending) at
                                 the end of a long lever which reaches
                           \     from the extremity to the  centre of the
                                 pillar, against the strength resisting always
                                 directly  at a short lever reaching  from the
                                 side d to the centre:  the strength of the
                                 pillar, therefore, has relation to the differ-
                                 ence between these levers and to the degree
                                 of bending.  Shortness, then, or  any  stay
                                 or projection, as a e b, which, by making
                                 the resisting lever longer, opposes bending,
                    \| | |/        really increases the strength of a  pillar.
                     fy            A column with ridges projecting from
                                 it, is on this account stronger than one that
                                is perfectly  smooth.
  A hollow tube of metal is stronger  than the same  quantity of metal as a
solid rod, because its substance standing farther from the centre resists bend-
bend in the whole; while in

           Fig. 66.
ci
■ e-
cb~\ 
STRENGTH OF MATERIAL.
123
ing with a longer lever.   Hence pillars of cast-iron are generally made hollow,
that they may have strength with as little metal as possible.
  In the most perfect weighing-beams for delicate purposes, that  there may
be the  least possible weight with the required  strength, the arms, instead of
being of solid metal, are hollow cones, of which the substance  is not much
thicker than writing paper.
  Masts and yards for ships have been made hollow in  accordance with the
same principle.

In  Nature's works we have to admire  numerous illustrations  of the same
  kind.
  The stems of many vegetables, instead of being round externally, are ribbed
or angular  and fluted, that they may have  strength to resist bending.   Many
also are hollow, as corn-stalks, the elder, the  bamboo of tropical climates,
&c, thereby combining  lightness with their  strength.—A person who has
visited the  countries where the bamboo grows, cannot but admire the almost
endless uses to the inhabitants, which its straightness, lightness and hollow-
ness, fit it  to  serve.  Being found of all  sizes, it has merely to  be cut into
pieces  of the lengths required for any purpose, and nature has already been
the turner, and the polisher, and the borer, &c.  In many of the Eastern
Islands it is the chief material, both  of the  dwellings, and of the  furniture;
there are the bamboo huts and bungalows, and then the fanciful chairs, couches,
beds, &c; flutes and other  wind instruments there, are merely  pieces of the
reed with holes bored at the requisite distances : conduits for water are pipes
of bamboo ; bottles and casks for preserving  liquids are single joints of larger
bamboo with the natural partitions remaining;  and bamboo split into threads
is twisted  into rope, Sic.
   From the animal kingdom also we have illustrations of our present subject:
—as in the hollow stiffness of the quills of birds ; the hollow bones of birds ;
the bones  of animals, generally—strong and hard, and  often angular exter-
nally,  with light cellular texture within, &c.

                           Transverse Pressure.

  When a horizontal beam is support-
ed  at its extremities, as at a and b, its                Fig. 67.
weight bends its middle down more   «/-\^                  -^O^
or less, as  here shown,  the particles     \^^T_~——   c'_____^^^^
on  the upper side being compressed,       ^--C!___^--~-—^T~^ff^^^^
while  the parts below are distended;             ~~    ^
and the bending   and  tendency to
break are greater,  according as the beam is longer and its thickness or depth
is less.
  The danger of breaking,  in a beam so  situated, is judged of, by consider-
ing the destroying  force as acting by a long lever reaching from an  end of
the beam to the centre, and the resisting force or strength as acting only by
a short lever from the side  d to the centre:  while  only a little of the sub-
stance  of a beam on the under side is allowed  to resist at all.   This last cir-
cumstance  is so remarkable, that the scratch of a pin on the under side of  a
plank resting, as here supposed, will sometimes suffice to begin the fracture.
  Because the resisting lever is small in proportion as the beam is thinner,  a
plank  bends and breaks more readily than a beam, and a beam resting on its
side bears  less weight than if resting on its edge.  Where a single beam can- 
124
MECHANICS.
not be found deep or broad enough to have the strength required in any par-
ticular case, as for supporting the roof of a house, several beams are joined
together, and  in  a great variety of ways, as is  seen in  house-rafters, &c,
which, although consisting of three or more pieces, may be considered as one
very broad beam, with  those parts cut  out which would contribute least to
the strength.
   The arched form, resting  against immovable abutments, bears transverse
pressure so  admirably  because by means of it the force  that would destroy,
is made to  compress, not  one  side only, but all  the  atoms  or parts of both
                                    sides nearly in the same degree.  By
              Fig- 69.                comparing this figure with the last,
                 d                   we see  that the atoms on  the  under
                                    side of  an arch, must be compressed
               ~~c      """"^^"^ik    about as much as  those on the upper
  flX^'                   ^-v/0    s\c\e, and are therefore in no danger of
                                    being torn, or overcome separately.
The  whole substance  of the arch  therefore  resists, nearly like that of a
straight pillar  under a weight, and is nearly as strong.
   An error, which has  been frequently committed by bridge-builders, is the
neglecting to  consider  sufficiently the effect of the horizontal  thrust of the
arch on its piers.  Each arch is an  engine of oblique force (see page 56,)
pushing the pier away  from it.   In some instances, one arch of a bridge fall-
ing, has allowed  the adjoining piers to  be pushed down towards it, by the
thrust, no longer  balanced, of the arches beyond, and  the whole structure has
given way at once like  a child's house or bridge built of cards.
   It is not known at what time  the arch was invented, but it was in com-
paratively modern times.   The  hint  may have  been taken from nature, for
there are instances in Alpine countries of natural arches, where rocks have
fallen between rocks, and have there been arrested and suspended, or where
burrowing water has at last formed a wide passage under masses of rock, and
has left them balanced among themselves as an arch above the stream.  No-
thing can surpass the strength and beauty of some modern stone bridges ;—
those, for instance, which  span the Thames as it winds through London.
   Iron bridges have been made with arches twice as large as those of stone;
the material being more tenacious and easily moulded, is calculated to form
a lighter whole.  The bridge of three fine arches  lately built between the city
of London  and Southwark, is a noble specimen, and compared with those
erected in the preceding century, appears almost a fairy structure of lightness
and grace.
   The great domes of  churches, as those of  St. Peter's in Rome  and St.
Paul's in London, have strength on  the same  principle as simple arches.
They are in general strongly bound at the bottom with chains and iron-bars,
to aid the masonry in counteracting the horizontal thrust of the superstruc-
ture.
   The Gothic arch is a pointed arch, and  is calculated to bear the chief weight
on its summit  or key-stone.  Its use, therefore, is not properly to span rivers
as a bridge, but to enter into the composition of varied pieces of architecture.
With  what effect  it does this, is seen in the truly sublime Gothic structures
which still adorn  so many parts of Europe.
   The following are instances, in smaller bodies, of strength obtained by the
arched form.—A  thin  watch-glass bears a very hard push ;—a  dished or
arched wheel for a carriage is many times stronger to resist all kinds of shocks
than a perfectly flat wheel ;—a  full cask may fall with  impunity, where a 
STRENGTH OF MATERIAL.
125
strong square box would be dashed to pieces ;—a very thin globular flask or
glass, corked and sent down many fathoms into the sea, will resist the pres-
sure of water  around it, where  a  square  bottle, with sides of almost any
thickness, would be crushed to pieces.
  We have, from the animal frame, an illustration of the arched form  giving
strength, in the cranium  or skull, and particularly  in  the skull of  man,
which is the  largest in proportion to  its thickness:—the  brain  required the
most  perfect security, and in the arched form of the skull  has obtained  it
with  little  weight.—The  common  egg-shell  is  another  example of the
same class:  what hard blows  of the spoon or knife  are often required to
penetrate this wonderful defence of a  dormant life !  The  weakness of a
similar substance not arched, is seen in a scale from a piece  of  freestone so
readily crumbling between the fingers.
  To determine, for  particular cases, the best forms  and positrons of  beams
and joists,  and of arches,  domes, &c, is the  business of strict calculation,
and belongs therefore to mathematics, or the science of measures.
  It was a  beautiful problem of this kind,  which Mr. Smeaton, the English
engineer, solved so perfectly,  in the construction of  the  far-famed  Eddy-
stone light-house.   He had  to determine the form  and dimensions of a
building, which should  stand  firm on  a sunken rock, in the channel of a
swift ocean tide, and exposed  to the fury of tempests from every quarter.
Only the man who has  himself been driven before the irresistible storm in
the  darkness of night,  and  in the midst of dangers, and whose eyes have
watched the steady ray from the light-house which saved him, can appreciate
fully the importance of  the studies which bring such useful results;  or can
feel  how happy he is  to  have fellow men, whose talents, although exerted
usually  for individual good, are  yet, by God's providence,  made to accom-
plish the most philanthropic ends, and  to bind the whole of human kind
into one great society of helping brotherhood.
  [For Animal and Medical Mechanics, see Part V. Sec, 1,] 
126
HYDROSTATICS.
           PART   III.

THE  PHENOMENA OF FLUIDS *
SECTION  I.—HYDROSTATICS.
                      ANALYSIS OF  THE SECTION.

The particles of a fluid mass are freely movable among one another, so
  as to yield to the least disturbing force; and if bearing force at all, can
  be at rest only when equally forced in all directions.  Hence:
1. In a mass of fluid submitted to compression, the whole is equally affected,
  and equally in all  directions.   A given pressure, for instance,  made by
  a plug forced inwards upon a square inch of the surface of a fluid filling
  a vessel, is suddenly communicated to every square inch of the vessel's
  surface, however large,  and to every inch of the surface of any body
  immersed in  the fluid.
2. In any fluid, the  particles that are below bear the weight of those that
  are above, and there is, therefore, within the mass, a pressure increasing
  exactly with  the perpendicular depth, and not influenced by the size, or
  shape, or position of the containing vessel.
3. The open surface of a fluid is level; and if various pipes or vessels
  communicate with each other, any fluid admitted to them will rise  to the
  same level in all.
4. A body immersed in  a  fluid displaces exactly its own bulk of  it, which
  quantity having  been just supported by  the fluid  around, the body is
  pressed  upwards, or supported, with a force  exactly equal to the weight
  of the fluid  displaced, and must  sink or  swim according  as  its oivn
  weight is greater or less than this.  By comparing, therefore, the weight
  of a body with the force which holds it up in a fluid, the  comparative
  weights, or specific gravities, are found.

                               "Fluid."
  It was  explained  in Part I., that the  same- atoms  may exist in the form
of a solid  or of a fluid ;  and  as a fluid,  they may either constitute a  dense
liquid like water, or a light  elastic mass like air.  A  pound of  ice, or  a
pound of water, or a pound of steam,  differs  only in the  particles being more
or less distant  from  each  other, owing  to the  different  quantities of heat
among them.  In the ice, they are comparatively near, and are held together
by attraction, as if they were  spitted  or  glued to each other;  in  the  water,
the repulsion of heat seems nearly to  balance attraction,  and to leave the par-
* Read again the Synopsis, page 20. 
FLUIDS.
127
tides at liberty to glide about among each other almost without friction ; and
in the steam, the repulsion altogether overcomes the attraction, and the par-
ticles separate to  a  great distance, as if held apart  by some bulky elastic
medium.  The  few facts not evidently reconcilable with this simple and
satisfactory explanation of so many phenomena,—as that water in freezing,
and even in  cooling down from forty degrees to the freezing point, increases
in volume, instead of contracting, like things in  general, and like itself in
cooling at other temperatures,—and  that  baked clay, in proportion as it is
more heated, contracts  instead of dilating,—are treated of in other parts of
our work.
   Whether  matter be in the solid or fluid form, the properties of the indi-
vidual atoms remain unchanged, that is, the atoms always exist in accord-
ance with the " general truths;" but as,  in the  chapter on Mechanics, we
found  so  many  important modifications of  effect produced by the circum-
stance of  the attraction being in the  degree  which  produces  solid cohesion
among the  particles,  in this chapter on  fluids we shall  find  as many
important results springing  from the circumstance  of non-cohesion  or
fluidity.
   In a liquid the particles, although comparatively near to one another, seem
not to be  in actual contact;  for the mass  may be condensed  indefinitely  by
pressure.   The  force required, however, to change the volume of a liquid in
any sensible degree, is  so great, that until improved means of experiment,
recently contrived, liquids were  accounted  absolutely incompressible.   In
aeriform fluids, on the contrary,  each particle, under common  circumstances,
has about two thousand times as much space to itself as when forming part
of a liquid or solid ; and hence it is that these fluids are so extensively com-
pressible and dilatable—or elastic, as  they are called.   On account of this
elasticity, they exhibit so many important phenomena, in addition to  those
of mere fluidity, that the consideration of them  requires to  be gone  into
apart, and forms the branch of the subject called pneumatics, from a Greek
word, signifying "spirit" or "breath!"

" In a quantity  of fluid submitted to compression, the whole mass is equally
   affected, and  similarly in all directions.  A  given pressure, therefore,
   made upon an inch of the surface of a fluid confined in a vessel, as by a
  pfyg forced inwards, is suddenly  borne by every inch  of  the surface  of
   the vessel, however large, and by every inch  of the surface of any body
   immersed in the fluid."
   This truth  is of  great importance,  both from its explaining so many
remarkable phenomena  of nature, and from  the useful applications of it  in
the construction  of machinery.
   When a man compresses  in his hands a bladder full of air, he readily
conceives  that  the air  immediately  under  his fingers  is  not at  all  more
compressed  than that in every  other part  of  the  bladder;  and of course
that every part of the bladder's  surface must be pressing the air  as much
as those parts of it on which his fingers rest, and  must be bearing a reaction
or resistance of  the air in an equal degree ;  and  that every  single particle
of air must  be acted upon equally on every side, so that if a small opening
were  made  in  the  bladder  anywhere, the air would issue from it with
equal readiness.   This  is in accordance with  the characteristic  of fluidity,
" that the particles glide about among one another almost  without friction,
so that a  particle  can  never be  at rest unless when  equally  urged in  all
directions," 
128
HYDROSTATICS.
Fig. 69.
l&
~"Kcl
:b
J
  In like manner, if a close vessel B be filled with water, and into the top of
                       it a tube a  c be screwed, and if then, by means of
                       a cork or movable plug in the tube at c, the surface
                       of the water in the vessel be pressed upon with  a
                       force of one pound, the water throughout the whole
                       will be squeezed or condensed  in proportion to the
                       pressure, and every other  portion of the vessel B,
                       of equal surface with c, will be keeping up the con-
                       densation just as  much as c, and will be bearing the
                       resistance or elasticity of the water to the extent of
                       one pound.  And if there were  another similar tube
                       b, also with a plug, screwed into the top of the box B,
                       the force of one pound depressing the plug c would
be  pushing up the plug b, with  the same force.   And  if there  were many
other similar tubes and plugs, by acting on  one, all would be equally affected;
and a plug or piston of double size  would be  twice as much affected as  the
smaller one ; and a plug d, of ten  times  the  size, would  be lifted  with  a
force of ten pounds.  Hence it appears that, through the medium of confined
fluid, a force of one pound, acting upon an inch square of the fluid  surface
in a vessel, may become a bursting force of ten, or a hundred, or a  thousand
pounds, according to the size of the vessel, or may be used as a mechanical
power  to  overcome a force  much  more  intense  than  itself.  It  will be
explained  below that the well-known hydrostatic press is merely  a large
plug or piston as  here  described, forced up against  the substance to be
pressed by the  action of a smaller piston in another barrel.
  If, in the above figure, the tube a  were such as to contain just one pound of
water,  on the plug c being withdrawn from it, and water being poured in to
fill it, the  same pressure or  condensation  would take place in the box B as
when the  plug  was pressed with the force of one pound;  and of course ex-
actly the same  effects would follow on the sides of the vessel and  on the
other pistons ;  and if, in the other tubes also, water were substituted for the
pistons, it  is evident that, to effect a balance in  all, it  would require  to
stand as high in every one as in the  tube a c, producing the same level in all,
whatever their size.
                 The fact, that the weight of one pound of water, or any
               other force of one  pound similarly applied, may be made,
               through the medium of extended fluid surface, to produce a
               pressure of hundred  or of thousands of  pounds,  has been
               called the "hydrostatic paradox," yet there is nothing in
               reality more paradoxical in it than  that one pound at the long
               end of the lever should balance ten pounds at the short end:
               indeed it is but another means, like the  contrivances usually
               called mechanical powers, and described in the last chapter,
               of balancing different intensities of force, by applying them
               to parts  of an apparatus moving with different velocities.
               Here the  tube a being ten times smaller than the tube e, the
               piston in a must descend ten inches to raise the greater piston
               in e one inch.
                 This law of fluid  pressure is  rendered very striking in
               the experiment of  bursting a strong  cask by the weight or
               action of a few ounces of water.  Suppose a cask a already
               filled with water, and that a long small  tube b c is  screwed
               tightly into its top, which tube will contain only a few ounces
Fig. 70.
  17,
 
FLUIDS.                            12g
J
a,
Fig. 71.
of water; by pouring these few ounces into the tube, the cask will be burst.
In explanation, it is unnecessary to say more than that if the tube have an area
of a fortieth of  an  inch, and contain, when filled, half a pound of water, that
water would produce a pressure of half a pound  upon every fortieth  of  an
inch all over the interior of the cask; or nearly 2,000 lbs. on the square foot,
—a pressure greater than a common cask'can bear.
  A similar effect is seen in what is called the hydrostatic bellows.  This
consists of a long small tube a b, into which  water is poured to enter the body
of the apparatus at c, which resembles the common bellows, in having wooden
boards above and below, and strong leather connecting them.  If the tube a
b holds an ounce of  water, and has itself only one-thousandth of the area of
the top of the bellows, an ounce of water in it will balance weights of a thou-
sand ounces placed on the top of the bellows at d.  If mercury were substi-
tuted in this machine for water, the effect would  be fourteen times  greater,
because mercury is fourteen times heavier in  the same
bulk.  And if a man stand on a large bellows of  the
kind, he may raise himself by blowing  into  the tube
with his mouth.
  The annexed cut will give an idea of Mr. Bramah's
singularly powerful and useful hydrostatic or hydrau-
lic press;  which, if compared with the  bellows', exhi-
bits merely a strong  forcing-pump instead of the lofty
tube, and a barrel with its piston instead of the leather
and boards.  The  letter e points out the piston of the
forcing pump  worked  by the  handle  d, and driv-
ing water along the  horizontal tube into the space/
under  the  large solid piston c,  which last,  with its
spreading top is urged against the object to  be com-
pressed.  If the small pump have only one-thousandth
of the area of the large barrel, and if a man, by means
of its  lever-handle d,  press its piston  down with a force of five hundred
pounds, the piston of the great  barrel will rise
with a force of one thousand times five hun-
dred pounds, or more than two hundred tons.
Scarcely any resistance can withstand the pow-
er of such a press;  with it the hand of an infant
can break a strong iron bar; and it is used to
condense substances, as cotton or hay for sea
voyages, to raise great weights, to uproot trees,
to tear things asunder, &c.
   The Dilater  is a surgical instrument of extensive applicability, of which
the action  depends  on the  principle of  the communication of fluid pressure.
It was  proposed by  the author some years  ago,  and was brought to great
practical perfection by his brother, Dr.  James Arnott, (now superintendent
surgeon in the service of the Hon. East India  Company,) in whose  publica-
tion it is minutely treated of.  Many professional men in this country doubted
of its power, from  not being aware of the nature of fluid action; but it is  in
reality  a kind of hydraulic press, allowing the operator to act with the most
gentle or most  energetic force.  Farther remarks are  made upon it in the
medical section  which follows this chapter.
Fig.. 72.
cl
\
%
et
7
"In any fluid, the particles that are below, bear the weight of those that are
  above,  and there is therefore a pressure among them increasing in exact 
130
HYDROSTATICS.
   proportion to the perpendicular depth, and not influenced by the size, or
   shape, or position of the containing vessel."

   The atoms of matter having gravity, it is evident that the upper layer of
 any mass of fluid must be supported by the second, and this with its load by
 the third, and the third with its double load by the fourth, and so on.  This
 truth is experimentally proved by putting different heights of liquid into an
 upright tube, of which the bottom is closed by a flap having a spring or lever
 to support it, and to indicate the force acting  on it.  And what is true of the
 entire column of water in the tube, may be considered true  of any single line
 of atoms; just as it would be true of a line of bricks piled one above another.
   A tube of which the area is  an inch square, holds, in two feet of its length,
 nearly a pound of water; hence, the  general truth, well worth recollecting,
 that the  pressure of water, at  any depth, whether on the side of a  vessel or
 on its bottom, or on any  body immersed, is nearly one pound on the square
 inch for every two feet of depth.
   The striking effects from the increase of pressure in a fluid, at great depths,
 are of course most commonly exhibited at sea.  The following instances will
 illustrate them.
   If a strong square glass bottle, empty, and firmly corked, be sunk in water,
 its sides  are  generally crushed inwards by the pressure before it reaches a
 depth of ten fathoms.
   A chamber of air similarly let down with a man in  it, would soon allow
 him to be drowned by the water bursting in upon  him;—as really happened
 to an ignorant projector.
   When a ship founders  in shallow water, the wreck, on breaking to pieces,
 generally comes to the surface, and is cast upon  the beach;  but when the
 accident happens in deep water, the  great pressure  at the bottom forces water
 into the pores of the wood, and makes it so heavy that no part  can ever rise
 again to reveal her fate.
  A bubble of air or of steam, set at liberty far below the surface of water,
 is small at first, and gradually  enlarges as it rises.
   A man who dives deep, suffers much from the  compression of his chest,
 as the elastic air within yields under  the strong pressure.   This limits the
 depth to which divers  can safely go.
  It is not  known whether there is  a limit to the  pressure which fishes  can
bear with impunity, but they are chiefly found living in the  shallower waters
 on coasts, or  on banks in the midst of the ocean, such as the banks of New-
 foundland, the Dogger-bank, and other fishing stations out at sea.  In round-
ing the Cape of Good  Hope, at a considerable distance from  land, ships pass
over the  bank of Lagullas, where a hook let down with  a bit of red ra<r or
 almost any thing as a  bait, immediately secures its codfish.
  By sending a vessel prepared for the purpose,  down into the deep sea,
 we can readily prove  the  compressibility of water.  Suppose the vessel to
be made with only one entrance, and that a  small round opening, into which,
instead of a cork, a sliding rod has been closely fitted.   If, then, when filled
 with water, and  having the rod  inserted into  the- opening, it  be allowed to
 sink in the sea, the pressure around will push the rod inwards, in a degree
 proportioned  to  the yielding  or compression  of  the water within: and if
there  be on the rod a stiff sliding-ring, or other contrivance to indicate on the
 return of the  vessel how far the rod had been driven inwards,  the apparatus
 will show the degree  of compression  at the greatest depth to  which it has 
PRESSURE AS DEPTH.
131
 descended.  Water a  thousand fathoms below the surface is less bulky by
 about one-twentieth part than when at the surface.

 The following are  proofs of the pressure of weight in an open fluid, ope-
   rating in all directions, as any pressure does in the case of a confined fluid.

   A bottle-cork carried far under water, is not flattened as if  it were pressed
 unequally, but is reduced in all its dimensions so as to appear a phial-cork
 of the usual form.
   If a  corked empty bottle be sent down into the sea, the cork is generally
 forced inwards  at a given depth,  and equally so in whatever direction the
 mouth of the  bottle may happen to point.
   If a  vessel containing  water have an opening in the  side, covered by a
 valve or flap so contrived  as to tell the force required to keep  it shut, we find
 that the water tends to escape just as powerfully through such an opening
 as it would through one in the bottom, with the same  elevation  of water
 over its centre.   And different equal openings in the side of a vessel require
 to be closed with forces exactly proportioned to the heights  of liquid above
 their centres.
   In an open square-sided vessel  full of water, the whole pressure on any
 upright side is just half the pressure on an equal extent of horizontal bottom:
 for the  centre of the side being just half as deep as the bottom, the pres-
 sure  on any point there is only half as great as on a point at the bottom, and
 on points above the level of the centre is just as  much less than half, as,  at
 corresponding distances below,  it is more than half, and so it amounts to an
 exact half in the whole.  Considering that the pressure on  every point be-
 low the central level is greater than on every point above it,  we see the rea-
 son why, to support a sluice or flood-gate by a single stay  on the outside, the
 point at which the pressure has to be made is below the central level.  Cal-
 culation discovers that this point, called the centre of pressure, is at one-third
 from the bottom.  The knowledge of such facts furnishes rules for the con-
 struction of large vessels for liquids, canal embankments, Sic.
   The  pressure on a given extent of  the side of a narrow vessel is just as
 great as on  the same extent of the side of a wide vessel, having the same
 depth of fluid: because, as now explained, it depends  entirely on the extent
 of surface acted upon and the depth of liquid.
   Hence a flood-gate or sluice which shuts out the ocean, as in docks open-
 ing to the sea, bears no more pressure than if it stood  only against  an equal
 depth of lake or river;  or than if it  were one of two such flood-gates be-
 come the sides of a very narrow  vessel, made to contain only a few hogs-
 heads of water.
   Hence, again, the fear is unfounded which has  been  expressed with refer-
 ence  to the formation of a canal between the Red Sea and the Mediterra-
nean,—that because the former, owing to the  effect of easterly winds at its
mouth,  Sic, is twenty feet higher than the latter, it might burst through the
flood-gates, and  carry devastation along its course.
  A deep crevice in a  rock, when filled by a shower, is often the  cause of
the rock being torn  asunder, and of part being precipitated.
  Extensive walls  or faces of masonry, intended to confine banks of sand
or earth, if no  openings  were left for water to escape from behind  them,
would be burst after a rain unless they had the strength of flood-gates of the
same size.   Ignorance of this  danger has led to some extraordinary catas-
trophes. 
132
HYDROSTATICS.
   Other examples of the pressure in fluids being in all directions, and pro-
portioned to the depth, are;—the swelling and bursting of leaden pipes when
filled from a very elevated source :—the  tearing up of the coverings of sub-
terranean drains or water courses, when, during a flood, any accident chokes
them near their lower openings :—the violence with which water escapes by
an opening near the bottom of any deep vessel, or enters by an opening or
leak near the keel of a deep-floating ship:—the great strength required in
the lower hoops and securities of those enormous vessels of porter-brewers,
called vats, some of which contain many thousand barrels of liquid.
   In speaking of the pressure of a fluid in all directions, some  persons have
difficulty  in conceiving that there is an upward as well as a downward and a
lateral pressure.  But if, in a fluid mass, the particles below had  not a ten-
dency upwards equal to the weight or downward pressure of the fluid over
them, they could not support that  fluid, which entirely rests upon them.
Their tendency upward is owing to  the pressure around them from  which
they  are trying to escape.  Accordingly, if a long tube, open at both ends,
and with  a sliding plug or piston in it near one end, be partially plunged into
water by  the plugged end, the water  is found to press the plug upwards with
force proportioned to the depth to which it is carried, and  exactly equal to
the force  with which water presses upon an equal  extent of the  bottom or
side of any other  vessel having in it the same depth;  or, with which, in the
same vessel, it would press other plugs in  other branches of the  tube pro-
jecting in various directions.  On removing such a plug altogether, the up-
ward pressure is visibly proved and measured by the column of water pushed
into  the tube from below, and supported there to the level of the water around.

The pressure in a mass of fluid is proportioned to the perpendicular  depth,
   and is not at all influenced by the size, shape, or position of the containing
   vessel.
   A body immersed in the water of a lake, one foot under the surface, is
just as much pressed upon  as if it were one foot under  the surface of the sea,
and no more than if it  were one foot under the surface  of a small cistern.
  Suppose vessles differing from each other in form and capacity, as sketched
                            here at a, b, and c, but all having flat bottoms,
         Fig. 73.             of exactly the same area;  if  fluid be poured
                            into all to the  same level or  perpendicular
                            height, as represented here by the dotted lines,
                            although the quantity be very different in each,
                            the pressure on the bottom will be the same in
                            all. This truth is easily proved experimentally,
                            by having  the bottoms  movable, and held to
                            their places by  weights  or springs capable of
                            measuring the pressure :  or by letting the three
vessels all communicate with the same vessel of water below them°, and then
observing that  the water in all has still the same level.—These results are
other exemplifications  of the truths,  "pressure equal in  all directions;'
"pressure as depth," and "pressure as the extent  of surface."   For as a
column of the fluid, resting on the middle of each bottom, just presses with
its whole  weight, and therefore  according  to its altitude, this column could
not remain at rest if there were any greater or less pressure than its own
near it;  then as the  fluid really is at rest in all the cases, and in all  a cen-
tral column is of the same height, the pressure must be equal on all the bot-
toms.  The case of the largest vessel, a,  is in a degree illustrated by sup- 
FLUID LEVEL.
133
posing the water in it  to be suddenly converted into smooth upright small
columns  or rods of ice or glass;  then, evidently, only those  pieces  which
rested on the bottom, could press on it while the others would  be supported
by the oblique sides of the vessel, and by the lateral resistance of the  pieces
around them.

            "Level surface of a Fluid."   (Read the Analysis.)
  That the surface  of a fluid must be level, follows from the facts of  all the
particles  being equally attracted  towards the centre of the earth, and being
perfectly movable among themselves.   The  particles forming the surface
may be regarded as the tops  of so  many columns of particles, supported  at
any given level  below, by a uniform resistance or pressure;—for no particle
of an inferior level  can be at rest unless equally urged in all directions, and
therefore all the particles at such a level, and which, by equally urging one
another,  keep themselves at rest, must all be bearing the weight of equal
columns : thus a higher column must sink and a lower one must rise, until
just balanced by those around ;  that is, until all become alike.   Besides, just
as a ball  rolls down a slope  or inclined plane, so do the particles of  a fluid
slide  or  move from any higher situation  among themselves, to any lower
unoccupied situation near them.   The account now given explains why an
accidental  elevation or depression of a fluid surface, usually called a wave,
continues to rise and fall, or to oscillate, for some  time with gradually dimi-
nishing force ;—for when the mass is raised above the general  level, it is not
quite  supported, and therefore soon sinks, but in  sinking, like a falling pen-
dulum, it acquires momentum which carries it below the general level, until
opposed  and  arrested  by a  resistance greater than its weight,  it then rises
again, but by acquiring new momentum in its  rise, it has to fall  again, again
to rise, and this alternation  continues, until the lateral sliding of the parti-
cles, and the friction among them, gradually destroy it.
   A perfectly level surface  on  earth really means one in which every par-
ticle is equi-distant from the centre of the earth, and  it is  therefore truly a
spherical  surface;  but so large is the sphere, that  if a slice of it of two
miles in diameter were cut off, and laid on a perfect plane, the centre of the
slice would only be four inches higher than the edges.  Any small portion  of
it, therefore, for all  common purposes, may be accounted a perfect plane.
   So truly smooth  does a fluid surface become, that it forms a perfect mirror;
that is, it  reflects  or  throws back the  rays of light, which fall upon it  so
exactly in  the order which they had on leaving the object, that an eye which
receives  them may fancy the object to be placed in the direction of the mir-
ror.—It  was over  the glassy surface of the fountain or the lake, that the
shepherdesses of  the  young world  bent  themselves, to learn the charms
which nature had  bestowed on  them.   And a child  contemplates with
wonder and delight, through the  window of a still pool or gliding stream,
another sky below the  ground, with its clouds, and sun or stars; and another
landscape,  with  inverted woods and  mountains, the  supposed  dwelling  of
fairy beings.
   In the cutting of canals, the  making of railways,  and in many other ope-
                        rations of engineering, it is of essential importance
       Fig. 74.          to determine the level or horizontal direction at any
                        place;  and this is usually done by a tube  or glass,
 P'      fr,______<?     a c, filled with spirit  except one  bubble  of air  b,
 v----------------'    and called a spirit level.   When this tube is hori-
                        zontal, the bubble has no tendency  to  move to either
     10 
134
HYDROSTATICS.
end; but if the tube inclines ever so little, the bubble rises to the end which
is highest; or to speak  more correctly, the denser spirit  falls down  to the
lower end, and forces the light bubble away from it.   Such a tube properly
fixed in a frame, with a telescope attached to it, or simply with sight-holes
to look through, becomes the  engineer's guide in many of his most  import-
ant operations.
   A hoop surrounding the earth would bend away from a perfectly straight
line four inches in  a mile.  In cutting a'level canal, therefore,  which may
be considered as part of a hoop, there  must be everywhere a falling from
the straight line, called  by geometers a tangent, in  the proportion now de-
scribed.   All rivers also have the curvature of hoops applied to the surface of
the earth.
   Canals  leading from  sea-ports to the interior of countries have generally
to ascend ; but as water cannot become stagnant in any channel which is not
level, the canal is divided, by gates or sluices, into portions at different levels,
like steps of  a stair, the rising at the joinings being generally  from  six to
twelve feet.   The  boats are  raised or lowered  from one level to another
by the contrivance called a lock, which is merely a portion of the canal, of
sufficient capacity for the boat to lie in, furnished with high walls, and with
flood-gates at both  ends ; so  that when the gates below are shut, and water
is gradually admitted  from above, it becomes  part  of the high  level, ready
as such to  deliver a boat, or receive one : and  when the upper flood-gates
are shut, and  the  water is gradually allowed to escape from  the  lock, it
becomes a part of  the low level, and a boat may enter it, or leave it by its
lower gates.
   The cutting of canals  is one of the great items in the mass of modern
improvement, which  both mark and hasten  the progress of  civilization.
Adverting to the importance  of easy intercourse, as explained  in a  former
section, we need only say here, that a horse which  can draw only one ton
on our best roads, can draw thirty with the same speed in a canal-boat.
   And what  a glorious triumph to science and art it is, to be able to conduct
vessels of all  kinds, even those originally intended for the  ocean surge alone,
through  the  quiet valleys of an  interior country !   In Scotland, now, along
the Caledonian canal, a noble frigate  may be  seen, wandering as  it were
among the inland solitudes, and displaying her grace and majesty to the asto-
nished gaze of the mountain shepherd; and when she has traversed the king-
dom, and visited the  lonely lakes, whose waters until lately had borne only
the skiff of the hunter,  she descends again by the steps of her  liquid stair,
and safely resumes  her place among the waves.
   It was lately in contemplation  to lead a ship canal across the isthmus which
joins  North  and  South  America.  The elevation to  which the canal must
reach, to surmount the central  ridge, is considerable, and will increase the
difficulty;  but such important consequences would follow the  accomplish-
ment of  the  object, that, with the continuance of general peace, and  the in-
crease of political  wisdom, it will  probably be  attained.   If so, the  loaded
vessel, rising from the Atlantic, would soon be described among the mountain
heights, and, a few  hours after, would be safely lodged in a port of the oppo-
site sea; having performed, by a near cut,  a voyage which at present costs
months of delay and hazard, in a tedious navigation round the whole southern
continent.—And if  the Red Sea and Mediterranean were joined in the same
way, as has also been proposed, the operation  would, in  effect, bring India
nearer to Europe, and would more and more strengthen the bonds ofmutual
utility and brotherhood among  the nations of the earth. Then, indeed, might 
FLUID LEVEL.
135
it  be said with truth, that the world is  a vast garden, given to man for his
abode, of which every spot has its peculiar sweets and treasures ; but, because
the cultivator of each may exchange a share of its produce for shares in return,
the same general result follows  as if every field or farm contained within itself
the climates and soils and capabilities of the  whole.
   In a canal, the least deviation from the true level would immediately cause
any  water admitted  into it to flow towards  the lower end.  This flux to a
lower  situation is what is going on in the myriads of  streams, which render
the face of the earth a scene of such varied beauty and incessant change.
   As in the animal body, from every the minutest point, a little vein, endowed
with living power, takes  the blood which has just brought life and nutriment
to the part, and delivers it into a larger vein, whence it passes into a larger
still, until, at last,  in the great reservoir of the heart, it meets  the  blood re-
turned from every  part of the body, so, in this terraqueous  globe, where the
magic moving power is simply fluid seeking its level, does the rain, which
falls  to sustain vegetable and animal life, and to renovate nature, glide  from
every point of the surface into a lower bed, and  from thence into a lower still,
until the countless streams, so formed, after every variety of course, combine
to form the  swelling rivers, which return the  accumulated waters into the
common reservoir of the ocean.  In the  living body, the arteries carry back
the  blood with renewed vitality to every point whence  the veins had with-
drawn it, and so complete the circulation; and in what may be called the
living universe, the circulation is completed by the action of heat and of the
atmosphere, which, from  the extended face of  the ocean, raise a constant
exhalation of watery vapour of invisible  purity, which the  winds then carry
away and deposit as rain or dew on every spot of the earth.
   A very slight declivity suffices to give the running motion to water.  Three
inches per mile, in a smooth straight channel, gives a velocity of about three
miles  per hour.   The  Ganges, which gathers  the waters  of  the Himalaya
mountains, the  loftiest in the world, is,  at eighteen hundred miles from its
mouth, only eight  hundred feet above the  level of the  sea—that is, above
twice the height of St. Paul's Church  in London ; and  to fall these eight
hundred feet, in its long course, the water takes nearly a month. The greater
river Magdalena, in South America, whose channel, for a thousand miles, is
between two ridges of the Andes, falls only five hundred feet in all that dis-
tance. Above the commencement of the thousand miles, it is seen descending
in rapids and cataracts  from the  mountains.   The gigantic Rio de la  Plata
has so gentle a descent to the ocean, that, in  Paraguay, fifteen hundred miles
from its mouth, large ships arrive which have sailed against the current all the
way, by the forne of the  wind alone: that is  to say,  which on the beautifully
inclined plane of the stream, have been gradually lifted by the soft wind, and
even against the current,  to an elevation greater than that of our loftiest spires.
   A  small lake or extensive  mill-pond, with uneven  bottom, if  suddenly
emptied by a sluice or opening at its lowest part, would exhibit a number of
pits or pools of various size and shape left among the inequalities.  But sup-
posing rain to fall,  and frequently to recur, the water seeking its level would
soon effect a very remarkable change.  In consequence of each pool discharg-
ing over its lowest part, that is, sending out  a streamlet either  into another
lower  pool,  or into a channel leading directly to the sluice or opening, there
would be a constant wearing down of the part or side of the pool over which
the water was running,  that is  to  say, a  deepening of a breach or channel
there, and the surface of  water in the pool would be consequently becoming
lower, while, at the same time,  the bottom would be rising, owing to the de- 
136
HYDROSTATICS.
posit of sand or mud washed down by the  rain from the elevations  around;
and  these two operations continuing, the pool would at last altogether disap-
pear.  And by this change going on in every pool through the whole of the
emptied mill-pond, the general bottom  would at last exhibit only a varied or
undulated surface  of dry land, with a  beautiful  arrangement of  ramifying
water channels, all sloping with a precision unattainable by art, to the general
mouth or estuary.—The reason that, in the supposed case, and in every other,
a watercourse soon becomes so singularly uniform, both as to dimension and
descent, is, that any  pits or hollows in it are filled up by the  sand and mud
carried along in the stream, and deposited where the current is slack ; while
any  elevations are  worn away by the action of the more rapid current which
accompanies shallowness.
  The above paragraph describes, in miniature, what has been going on over
the general face of our earth ever since  that convulsion  of  nature which pro-
duced its  present form.   In many places  the phenomenon  is already com-
plete ; in others it is  only in progress.  The whole of what is now dry land,
has  at some  period  been under water, and much of it has evidently been a
gradual deposition  from water.  By some extraordinary convulsion, therefore,
our  present continents and  islands  must have been thrown up from  the bot-
tom  of an ocean, or  an ocean must have subsided away from them;  and  in
either case the land  must  have  merged as  checkered  and unsightly as the
bottom of the emptied lake  above supposed.   And it is  the gradual operation
of water seeking its  level which has at last converted the earth into the para-
dise  which we now behold.
  The marks of the former  state of the world, and of the progressive  change,
are everywhere most strikingly evident to the enlightened eye of philosophy.
The  present kingdom of Bohemia, for  instance, is the  bottom of one of the
great lakes formerly existing over Europe.  It is a basin or amphitheatre,
formed by a wall of  mountains, and  the only gate or opening to it, is  that
remarkable one by which the water now escapes from it, and which evidently
has been gradually cut or formed by the action of the  running stream.  As
the bottom became uncovered, owing to the sinking of the  water,  and the
formation of a regular sloping channel  from  every part, the former lake was
converted into a fine and fertile country, a  fit habitation for man;  and the
continued drain from it of the rains which fall over its surface, and either pass
rapidly away, or sink into  the earth, and ooze again  more gradually in the
form of springs,  is the  beautiful river which  we now call the Elbe.
  In Switzerland,  many of the valleys which were formerly lakes, have the
opening for the exit of water so narrow, that, as  happened in one of them a
few  years ago, a mass of snow or ice falling into it, converts the valley once
more into a lake.  On the  occasion alluded to, the accumulation of water
within was very rapid;  and although, from the danger  foreseen to the coun-
try below, if  the impediment should suddenly give way, every means  was
tried to remove the water gradually, the attempt had not succeeded when the
frightful burst took place, and involved  the inferior country in common ruin.
  The magnificent Danube  is the drain of  a chain of basins or lakes, which
must, at one  time, have discharged or run over one into  another; but
owing to the continued stream  cutting a passage at last low enough to empty
them all,  they  are now regions of fertility, occupied by civilized man, instead
of the fishes which held them formerly.  This operation is still going on in
all the lakes of the earth.  The Lake of Geneva, for instance, although con-
fined by hard rock,  is lowering its outlet,  and the surface  has consequently
fallen within the period of  accurate  observation and records ; and as, at the 
FLUID LEVEL.
137
same time, the wearings of the neighbouring mountains, brought down by the
winter torrents, are filling up its bed, if the town of Geneva last long enough,
its inhabitants may have to speak of the river in the neighbouring valley, in-
stead of the picturesque lake which  now fills it.   Already several towns and
villages, which were close upon  the lake a century ago, have fields and gar-
dens spreading between them and the shore.
  Illustrating this subject,  it is very  interesting to  observe the contrast
between the pure blue water of the  Rhone issuing from the lake of Geneva,
and the turbid streams which join its course a little farther down.   The tor-
rents which fall into  the lake all  around, are equally charged with the debris
or wearings of the mountains ; but,  having deposited all their load in the still
bosom of the lake, the pure water alone escapes to form  the river.   The
streams, however, coming to the  Rhone directly from the Alps, and bringing
with them their charge of broken-down earth,  even after they have joined it,
are long distinguishable by their muddy waters.   It is  the mud deposited as
here described, which is gradually filling up all lakes, and which has formed
the vast regions of flat country seen about the  mouths of great rivers.   The
greater part of Holland is deposition of this kind, the whole of lower Egypt,
a great part of Bengal, &c. Sic
   There are  some lakes on the face  of the earth which have no outlet towards
 the sea,—all the water which falls into them, being again carried off by evapo-
ration  alone—and such  lakes are never of fresh water, because every sub-
 stance, which, from  the beginning of time, rain could dissolve in the regions
around them, has necessarily been  carried towards them by their feeding
streams, and there has remained.  The great majority of lakes,  however,
being basins  with the water constantly running over at one part towards the
sea, although all originally salt, have, in the  course of time, become fresh,
because their only supply, being directly from the clouds, or from rivers and
springs fed  by the clouds, is fresh, while what runs away from  them  must
always be carrying  with it a proportion of any substance  that remains dis-
solved in  them.  We thus see how the face of the earth has been  gradually
washed to a  state of  purity and  freshness fitting it for  the uses of  man, and
why the great ocean necessarily  contains in solution all the  substances which
originally existed near the surface of the earth, soluble in water:—viz., all
the saline substances.  The city of Mexico stands in the centre of a vast and
 beautiful plain, 7,000 feet above the level of the sea, and surrounded by sub-
 lime ridges of mountains, many of them snow-capped.    One side of the plain
 is a little  lower than the other, and  forms the bed of a lake, which  is salt for
 the reasons stated above ;—but the  lake will not long be salt, for it now has
 an outlet.  About 150 years ago, owing to unusual rains, an extraordinary
increase of the water took place, and covered the pavements of the  city.  An
artificial  drain was  then cut from  the plain,  at  the distance of about sixty
miles from the city, to the lower external country.  This soon freed the city
 from  the  water, and  since then,  becoming every year deeper by the wearing
effects of the uninterrupted stream,  it is still lowering the surface of the lake,
 is daily rendering the water less salt, and is converting the vast salt marshes,
 which formerly surrounded the city, into fresh and fertile fields.
   The vast  continent of Australasia, or New  Holland, (as  large as Europe,)
 is supposed  by some to have been formed at a  different time from what is
 called the Old World, so different  and  peculiar are many of its animal and
 vegetable productions ; and the idea of a later formation receives countenance
 from the existence of immense  tracts of marshy  or imperfectly drained land 
138
HYDROSTATICS.
discovered in the interior, into which rivers flow, but seem not yet to have
worn down a sufficient outlet or discharging channel towards the ocean.
   Where the soil or bed of a country through which a water-track passes is
not of a soft consistence, so as to allow readily the  wearing down of higher
parts, and the filling up of hollows by deposited sand, lakes, rapids and great
irregularities of current remain.   We have, for instance, the line of the lakes
in North America, the rapids of the St. Lawrence,  and the stupendous falls
of Niagara, where at one leap the river gains a level lower by a hundred and
sixty feet.  A softer barrier than the rock over which the river pours, would
soon be cut through, and the line of lakes would be emptied.
   The contemplation of the fact, that water in seeking its level is constantly
wearing where  it  rubs, and carrying the  abraded portions down  to  lower
levels, and ultimately to the bed of  the ocean, brings irresistibly the  awful
idea,  that this  earthly abode of ours, owing  to  natural causes already in
operation, can have but a limited existence in its present stale.   No shower
falls that does not  send  portions  of mountains and plain into the depths of
the ocean, and thus cause a corresponding encroachment on the shores by the
rising water;  and  with revolving ages,  unless new convulsions of nature
disturb the progress, or art  succeed, as in Holland and elsewhere, in shutting
out the ocean from extensive low  tracks by means of sea dykes  or embank-
ments, the dry land must at last  disappear, and another gradual deluge em-
brace the globe.
   There is, perhaps,  nothing which  illustrates in a more striking manner
the exact resemblances among nature's phenomena,  or their accordance with
the few general expressions or laws which describe them  all, than the perfect
level  of the ocean  as a liquid surface.  The sea never rises or falls in any
place, even one inch, but in obedience to fixed laws,  and therefore its changes
may generally be foreseen and allowed for.  For instance,  the eastern trade-
winds and other causes force the water of the Indian Ocean  towards the Afri-
can coast, so as to keep the  Red Sea about twenty feet above  the general
ocean level; and the Mediterranean is  a little  below that level,  because the
evaporation from it is greater than  the supply of  its  rivers, causing it to
receive an additional supply by the Strait of Gibraltar ;—but in all such cases,
the effect is as constant as the disturbing  cause, and therefore can be calcu-
lated upon with confidence.
   Were  it not for  this perfect exactness, in what a precarious state  would
the inhabitants exist on the sea shores, and on the banks of low rivers !  Few
of the inhabitants of London, perhaps, reflect, when standing by the side of
their noble river, and gazing on the  rapid flood-tide pouring  inland through
the bridges, that although sixty miles from the  sea, the water there is, at the
moment, lower than the surface of the sea, which may at the time be heaving,
moreover, in lofty  waves, covered perhaps with wrecks and the drowning.
   The horrible  destruction that would follow any alteration in the level of
the ocean, may be judged of by the effects of occasional floods, produced by
rains  and melting  snow in the  interior of countries, or  by these combined
with winds and high tides on the coasts.   The flood at St. Petersburgh, in
1825, was dreadful, in which strong westerly winds had retarded the flow of
the Neva  so much, that the water rose forty feet (the height of an ordinary
house) above its  usual  mark, covered all the low  parts of  the town, and
drowned thousands of the people.
   In  Holland,  which  is a low flat, formed chiefly by  the  mud  and sand
brought down by the Rhine and neighbouring rivers, much of the country is
really below the level of the common spring-tides, and is only protected from 
FLUID LEVEL.
139
daily inundations by artificial dykes or ramparts,  made  strong enough to
resist the ocean.   On  one occasion  the water broke into such an enclosure,
and  drowned more than sixty  thousand people.   What awful uncertainty
then would hang over the existence of the Dutch,  if the level of the sea were
subject to change: for while we know that its waters, owing  to the centri-
fugal force  of the earth's rotation, are seventeen miles higher  at the equator
than at the poles, if the level, as now established,  were from any cause to
be suddenly changed but ten feet, millions of human beings  would be the
victims.
   Where inundation is  regularly periodical, as in the Nile and many other
rivers, the hurtful effects can be guarded against, and the occurrence may even
become useful, by fertilizing the soil.
   Tracts of land in contact with rivers, of which land, the  surface lies be-
tween the  levels of ebb and flood-tide, if surrounded with  dykes, may be
kept constantly covered with water, by opening the sluices only at high water;
or may be kept constantly drained, by opening the sluices only at low  water.
A vast extent of rice fields, near the mouths of rivers in India  and  China, is
managed in this way, the admission or exclusion of water being -regulated by
the age of the rice plant.   A great part also of the rich sugar plantations pf
Demerara,  Esequibo, &c., on the  coast of South America,  are in the  same
predicament;  and another advantage which these have over the plantations
on the West-India Islands, is the  saving  of the labour of transport effected
by the canals which  intersect all the fields.

" If various tubes and vessels communicate with  one another, fluid admitted
   to them will rise to the same level in all."  (Read the Analysis, p. 84.)
   The following sketch may represent  a variety  of tubes and vessels, fixed
upon and opening into  the cistern or box G.   Water poured into any one
would fill the box, and would then rise to the same  level in all.  The  dotted
lines from  a to f, may represent the surfaces of the  fluid  in the different
vessels.  In the  figure at p. 128, it
was seen why, in all  upright cylin-
drical vessels, as  a, b and c, the fluid
rises to the same level; and the figure
at p. 132, explained why shape of the
vessel cannot effect the level.   Al-
though in  the oblique  vessel c, re-
presented  here, there  is  more  water
than in  a,  still  there  is  the  same
pressure at the bottom  of both, be-
cause c  supports part of the weight
of its contained fluid on the principle of the inclined plane.
   If a tube twenty miles long, and rising and descending among the inequali-
ties  of a country, were filled with water, and could  have its ends brought
together for comparison, it would exhibit two liquid surfaces having precisely
the  same level;  and on either end being raised, the fluid would sink in it to
overflow from the other.
   An easy mode of determining a level line at any  spot is to  have an open
tube, bent  up at its ends a and b, and nearly
filled with liquid: by then  looking along the
two liquid  surfaces,  or through floating sights
resting  on them,  an observer looks in a line
which is quite horizontal at the middle point be-
tween them.
Fig. 76.
a^
J" 
140
HYDROSTATICS.
  If there were two lakes on adjoining hills of different heights, a pipe of
communication descending across the valley and connecting them, would
soon bring them to the same level; or if one were much higher than the other,
would empty that one into the other.
  A projector thought that the vessel of his contrivance, represented  here,
was to solve the renowned problem of the perpetual motion.   It was goblet-
shaped, lessening gradually towards the bottom  until it became a tube, turned
upwards at c, and pointing with an open extremity into the goblet again.  He
reasoned thus: A pint of water in  the goblet a must more than counterbalance
                    an ounce which  the tube b will contain, and must there-
      Fig. 77.        f0re be  constantly pushing the ounce forward into the
        ^—-^      vessel again, and keeping  up a stream or circulation,
     _^_/\\     which will cease only when the water dries up.   He
 Cll............-\  \W    Was confounded  when a trial showed him the same level
  I        ill     always in a  and  in b.
  V      J J J       A glass tube  inserted near the  bottom of a cask or
     \^  C^y       cistern of any sort, not air-tight above, which tube is
      £>—-^        then bent upwards, to appear  on  the outside like a
                    barometer tube, shows by  the elevation of a fluid  in it,
the height of the greater mass of  fluid within.
  In  like manner a tube  brought from a river  into a neighbouring cellar or
pit, will indicate the height of the water in the river.
  A knowledge of  the truth,  that  water in pipes will always rise again to
the height or level of its source, has enabled men in modern times to con-
struct those admirable systems of iron pipes, which distribute water in  great
towns.   The water brought to any elevated site, in or near the  town, may
be delivered from a reservoir there, by the effect of gravity alone, to every
cistern which is under the level of the reservoir; the result not being affected
by  the pipes having to  rise over  heights and to descend into  valleys  many
times in their course.
   On the hill north of  London, on which Pentonville  stands, there is such a
reservoir to which water is brought from Hertfordshire,  by a channel cut for
the purpose upwards of thirty miles  in length, and  called the New River.
Another reservoir has lately been  constructed, by the West Middlesex Water
 Company, at Primrose Hill, higher than any house in town.   It  is filled by
operation of steam-engines at the  Company's works,  near Hammersmith,
five miles off.  It will  supply water  to  the summits of all the houses con-
nected with it, and  is exceedingly useful in cases of fire.
   Many persons have  believed that the ancients were ignorant of the law,
that fluid in pipes rises to the level of  its source, because, in all the ruins of
their aqueducts, the channel is a  regular slope.   Some  of the aqueducts, as
works of magnitude, are not inferior to the great wall of  China, or the Egyp-
tian Pyramids ; yet, at  the present day, a single pipe  of cast-iron is  made
to  answer  the same purpose, and  even more  perfectly.   It is now  ascer-
 tained,  however, that it was  not  ignorance of the principle, but  want  of fit
 material for  making the pipes, which  cost our forefathers such enormous
labour.
   The supply and  distribution of water in a large city, particularly since the
steam-engine has been added to the apparatus, approach  closely to the  per-
 fection of nature's own work in the circulation  of blood through the animal
body.   From the great pumps or a high reservoir, main pipes issue to the
 chief divisions of the town ;  these then send suitable  branches to the streets,
 which branches again divide for the lanes and alleys ; and at last subdivide 
FLUID LEVEL.
141
until every house has its small leaden  conduit carrying its  precious freight,
if required, even into the separate apartments, and  yielding it anywhere to
the turning of a cock.   A corresponding arrangement of drains and  sewers,
most carefully constructed in obedience to the law of level, receives the water
again when it has answered its purposes, and sends it to be purified in the
great laboratory of the ocean.   And so admirably complete and  perfect is
this counter-system of sloping channels, that a heavy shower may fall, and
after washing and purifying every superficial spot of the city, and sweeping
out all the subterranean passages, may, within the space of an hour, form part
of the river passing by.  It is the recurrence of  this almost miracle, of ex-
tensive, sudden, and perfect purification, which  makes modern London the
most healthy,, while it is the largest city in the world.
   English citizens have now become so habituated to the blessing of a sup-
ply of pure water, more than sufficient for all their purposes, that it no more
surprises them than  the regularly returning light of day  or  warmth of sum-
mer.   But a retrospect into past times may still awaken them to a sense of
their obligation to advancing art.  How much of the anxiety and labour of
men in former times had  relation to  the supply of this precious element!
How often,  formerly, has periodical  pestilence arisen from  deficiency of
water; and how often has  fire devoured whole cities, which a timely supply
of water might have saved  !  Kings have received almost divine honours for
constructing  aqueducts, to lead the pure streams  from the mountains into the
peopled towns.   In the present day, it is he who has travelled on the sandy
plains of Asia or Africa, where a well is more prized than mines of gold, or
who has spent months on ship-board, where  the fresh  water  is often doled
out with more caution than the most  precious product of the still, or who,
in reading history, has vividly sympathized with the victims of siege or ship-
wreck, spreading out their garments to catch the rain from heaven, and then,
with mad eagerness, sucking the delicious  moisture—it is  he who can ap-
preciate fully the blessing of that abundant supply which most of us  now so
thoughtlessly enjoy.  The author of this  work  will long remember the
intense momentary regret with which, on once approaching a beautiful land
after months spent at sea, he saw a stream of fresh water gliding over a rock
into the salt waves—it appeared to him as if a most precious essence, by
some accident, were pouring out to  waste.

The subject  of fluid level leads to the consideration of springs or wells, and
                   of the  operation of boring for water.
   The water which falls  from the clouds, and which must  all ultimately
return to the sea, may find its  way to  the rivers, either by running  directly
along the surface of soils which refuse it admittance; or by first sinking into
porous earth, and again oozing out  at lower situations in the form of springs.
If  a spring be as low as the bottom of the porous earth from which it issues,
that is to  say,  as low as  the  surface of the  impermeable clay or rock on
which at some depth all such  earth rests, it  may drain the whole; but if not,
the water will stand at a certain level among the earth  as it would among
bullets in a water-tight vessel.   If a hole or pit be  then dug in such earth,
reaching below the level of the water lying in it, the pit will  soon be filled
with water up to the level, and will be called a well.  In  many places this
water-level is very far below the surface of the ground ; and in some places,
by reason  of the water having an  easy drainage towards the sea, or of the
superficial soil being altogether impermeable to it, there is  none to be found
within an accessible depth. 
142
HYDROSTATICS.
  A remarkable illustration of this subject occurred a few years ago, in Kent,
on the occasion of cutting between Rochester and  Gravesend the canal called
the Thames  and  Medway Canal.   This canal consists of but one cut or
level, seven miles long, of which two are in a tunnel through the hill—which
level  is that of high  water in the  connected rivers;  the  intention  having
been to let the canal  be filled always from the  rivers at high water:—but as
the level of the subterranean water in the surrounding land, and therefore of
all the inhabitants' wells there, is, as might be anticipated, half-way between
the levels of high and  low tides,  the  salt water  from the rivers was no
sooner admitted to the canal,  than  it spread into the  land on either  side,
where the resisting internal water-level  was lower, and  destroyed  all the
wells.   If the canal had been dug a few feet lower, the mischief would not
have  occurred, and  the  company would have  escaped paying  the heavy
damages, which rendered their undertaking a very ungainful speculation.
  All the  wells and springs in  the world are merely  the rain water which has
sunk into the earth, appearing again,  and gradually escaping at lower places:
nature thus admirably making the bowels of the earth an ever-stored reservoir
of the substance most indispensable to the comfort and existence of man, and
of all living creatures.  It is worthy  of remark here, that high cultivation or
agricultural improvement of a  country has a great  effect on the quantity of
spring-water  in it.   While the  face of a country is rough, the  rain-water
remains long among its inequalities, slowly sinking into  the earth  to feed the
springs, or slowly running away from the surface as from bogs and marshes
towards the rivers.  The rivers hence have a comparatively  uniform and
regular supply, even when rain has not fallen for a long time :—but in a well-
drained country,  the rain, by a thousand prepared channels, finds its way to
the brooks and  rivers almost immediately, producing often  dangerous floods
or inundations of  the neighbouring low grounds.   A friend of the author had
a waterfall and mill in Surrey, which he formerly let for a rent of £1,200 a
year; but  after agricultural improvements in the district from which the water
came, the supply of water was generally either superabundant or deficient,
and the value of the mill was reduced to one-half.
  The surface of our globe is formed of different strata or layers,  as of clay,
chalk, sand, gravel, &c. <fcc, which appear all to have been  at former periods
horizontal, formed under water, and  to have been afterwards thrown up, by
some convulsion or convulsions of nature, into every variety of position.  In
particular  situations, the upper surface is now concave  or  basin-shaped, the
different strata or layers,  when water-tight, being like cups or basins placed
one within another; and  as water poured  in, to fill the space  between two
basins so placed,  would spring out to the height of its upper or level surface,
through any hole  made in the side of either, so on boring for water, through
an innermost or superior water-tight stratum or basin of earth, the water often
springs  out and rises far above the surface of the ground.  London stands in
a hollow of which the first-met layer is a basin of clay, placed over chalk,
and on boring through the clay (sometimes of three hundred feet thickness,)
the water  issues,  and  in many places will  form a  jet considerably above the
surface of the ground ; showing that there is a higher  source or level some-
where—as among the hills of Surrey, or those north of London.
  When fluids of different kinds  and of different weights under the same bulk,
are made to oppose, or to balance each other in communicating vessels—as
water, for instance, in one leg  of the bent tube b d c,  and  oil in the other—
the surfaces will  not at all rest or settle at the same height or level, but that
of the lighter fluid will be just as much higher than that of the other as it is 
FLUID SUPPORT.
143
 lighter.   Thus a column of oil must be of a
 length as d o, to  balance a column of water
 d w: and alcohol, because lighter than oil,
 to balance the same water,  would have  to
 stand higher still, as at  a; while mercury,
 because  thirteen times weightier than water,
 would stand only about m.   The shape, size
 or position of the vessels in which the oppos-
 ing  fluids might stand,  would have  no in-
 fluence on the relative heights of the surfaces;
 for if we suppose a larger vessel, such as is
 represented here by the dotted lines between
 the letters efm, to be substituted for the leg
 c d of the tube, the various fluids to  balance
 the water in b d, would have  to stand just as
 high in it as in the smaller tube.

 " A body immersed in a fluid, displaces exactly Us own bulk of it, which
   quantity having been just supported by the fluid around, the body is held
   up with force exactly equal to the weight of the fluid displaced, and must
   sink or swim according as its own weight is greater or less than this.
   A bladder full of air, and maintaining the bulk of a pound of water, requires
 a force of  one pound (except a few grains, the weight of the air), to plunge
 it under  water.  The same bulk of gold is held up in water with exactly the
 same force; so that, if previously balanced at the end of a weighing beam, it
 appears  on immersion to have lost one pound of its weight.
   And a piece of wood, ivory or any other substance, having exactly the same
 bulk, is opposed on entering the  fluid by the same resistance.
   The reason of this is obvious, for the immersed body takes the place of water
 which weighed one  pound and yet was supported,  and whose pressure was
 necessary for the equilibrium of  the rest.   In a vessel of water represented
 here by  the figure a b, let us attend to any por-
 tion  of the water,  a single column of particles  for           Fig. 79.
 instance, represented  by  the  line cd: we know
 that  each  column is steadily supported  in  its
 place, because the particle of the liquid  immedi-
 ately under it is tending upwards to escape from
 the  surrounding  pressures, with force exactly
 equal to  the weight of the column; and what is
 true  of a  column of single particles, is true of any
 other portion, such as the larger column represented by the figure y/i g.   If
 such portion weighed exactly a pound, the surface under it would be tending
 upward with the force of  a  pound ; and  if the portion,  without changing its
 bulk  or form, were to become ice, it  would still be exactly supported by the
 surface below pressing upwards with  force of a pound ; and farther, if a simi-
 lar column of wood, or stone, or metal, were there, the surrounding pressures
 would still be the same.  Again, if we  suppose only half the  column to be
 solidified, the portion h g for instance, it  would still be pressed upwards with
a force of one pound at g; but its own weight of half a pound, and the weight
of the half pound of water above it, would produce an exact balance and main-
tain rest.
Fis-. 78.
IV
\e
If
d
1	j		r»
1.....		7i	&
	,9		&
 
144
HYDROSTATICS.
  It is very important to have clear notions on this subject; and as different
minds apprehend such matters with different degrees of facility, and in dif-
ferent ways, we shall state the same general truth  in other words.
  Let us consider a mass of fluid as consisting of a vast number of extremely
minute columns of single particles standing side by side, where every particle
supports those  above it by the tendency upwards  which it acquires through
the pressure of the fluid surrounding it.  Now if we suppose the particles of
a portion of a fluid  mass, of any shape, to stick together, or to become ice
without change of bulk or weight, that portion when solid would still be
between the same forces as when fluid, and therefore would be equally sup-
ported, and would remain at rest.   And if gold, or silver, or glass,  or wood,
having the same bulk, were substituted for the supposed ice, such  new sub-
stance  would still be sustained with the same force; so that a substance of
exactly the same weight as the ice or water displaced, would  have no ten-
dency either to rise or to fall more  than the water itself had; but a substance
heavier would  sink, and one  lighter  would swim,  and  in either  case with
force exactly proportioned to  the difference between its weight and that of
an equal bulk of water.
  Few persons, in now reading the statement of  this truth—in appearance
so simple and obvious—would imagine that it had remained so long unknown,
and that the discovery of it may be accounted one of  the most  important
which  human sagacity ever made,—but such is the case.   We owe the dis-
covery to  one of  the master-minds of antiquity—that of Archimedes.  He
caught the idea one day while his limbs were resting on the liquid  support of
a bath: and as  his god-like intellect darted into futurity, and perceived  many
of the  important uses to which  the knowledge was applicable, he is said to
have become so moved with admiration and delight, that  he leapt from the
water, and unconscious of his nakedness, pursued his  way homewards, call-
ing out "sv^xa, iv^xa," I  have found it.  He was thinking chiefly of the
ready means, thus obtained, of ascertaining in all  cases what has since  been
called the specific gravity of bodies, viz., the comparative weights of equal
bulks of different substances; as of gold, or silver, or copper,  or iron,  com-
pared with water;  and in the  case of mixtures,  as of gold with  silver for
instance, of declaring at once  the proportion present of each—important
problems, which, until then, could  not be correctly solved.
  The  hydrostatic  law now explained, has since led to great advances in
various arts.   It may be regarded as a chief foundation of chemistry, for by
it the chemist distinguishes one substance from another, distinguishes a pure
from an impure substance, and discovers the  nature  of many mixtures or
compounds.  The merchant often  judges by it of the  worth of his  merchan-
dize.   In any case  it enables an inquirer to ascertain  at once the  exact size
or solid bulk of a mass, however irregular—even of a bundle  of twigs.  It
has become the cause of improvements in navigation, in marine architecture,
and in  many other arts.
   We shall now discuss more particularly the subject of comparative weights
or specific gravity.

"'ITie force with which a body is held up in a fluid, being the exact weight
   of its bulk of that fluid, by ascertaining this force and comparing it ivith
   the weight of the  body itself, the comparative weights or specific gravi-
   ties are found."   (head the Analysis, p. 126.)
   If any body, c, a  mass of gold for instance, be suspended by a  thread or 
FLUID SUPPORT—SPECIFIC GRAVITY.
145
Fig. 80.
hair from the bottom of one  scale 6
of a weighing-beam, and be balanced
by weights put into the other scale a,
and if a vessel of water be then lifted
under it  so  that the water shall sur-
round it,  the body is pushed up or
supported by  the  water  with force
equal to the weight of the water which
it displaces;  the weights, therefore,
then required in the scale b to restore
the  balance,  show truly  the exact
weight of the water displaced; or. of
water equal in bulk to the body;  and
the weights  in the two opposite scales
show the comparative weights of the
body and of its bulk of water.  In the
supposed case, whatever weight the
gold had in   the air, it would  seem to lose, when the water surrounded it,
about a nineteenth part of such weight;  that is, the water would  support it
with this force; and gold would thus be proved to be about nineteen times as
heavy as water.
  In making a table of specific gravities, it was necessary to select a common
standard with which all other substances  should be compared, and this has
been done in choosing water; the reason of preference being, that water can
be  so easily  procured in a state of purity,  and  therefore of uniformity, in all
situations.   When we say, therefore, that gold is of the specific gravity  19,
and copper 9, and  cork \, we mean that  these substances are just so much
heavier or lighter than their bulk of pure water in its densest state, viz., at
the temperature of 40 degrees of Fahrenheit's  thermometer.
   As the substances in nature differ  as  to  form and  other  qualities, corre-
sponding differences  have to be made in the  manner of ascertaining their
specific gravities: the following cases are the most important.
   Solid bodies insoluble  in water and heavier than it—as the metals, &c,
are merely suspended by a thread or  hair, having nearly the specific gravity
of water, to one scale of the hydrostatic  balance (simply a  good  weighing-
beam with a water-vessel  below one of the scales;) and  the body being first
balanced or weighed  in the air, and then  in water, as already described,  the
 weight and  the loss, represented, if the operator chooses, by the weights in
the opposite scales, are the weights of equal  bulks of the two substances;
and by  finding, through the arithmetical operation of division, how often  the
weight of the water is contained in the  weight of the solid, we find the specific
gravity of the solid, or how much it is weightier than its bulk of water.—It
is almost superfluous to  remark,  that putting weights into the  scale b, or
taking them  out of the scale a, are equivalent operations.  We shall explain
afterwards, that for very delicate purposes, bodies must be weighed first in a
vacuum, instead of in air, or a suitable allowance must be made; for air itself
supports a little any body immersed in it.
   Solids lighter than  water, as  cork, are  weighed  in it  by attaching to
them a mass of metal or glass heavy enough  to sink them, and already ba-
lanced in water for the  purpose; or by making the line which connects them
with the weighing beams  pass under a small  pulley  fixed at  the bottom of
the vessel, so that the rising«of the end  of  the beam  to which they are at-
tached shall  draw them down. 
146
HYDROSTATICS.
   A solid  soluble in water, as  a  crystal of  any salt, may be protected
 during the operation of weighing in water, by previously dipping it in melted
 wax, so as to leave a thin  covering on  it; or it may  be weighed  in some
 liquid which doe? not dissolve  it, allowance  being afterwards made for the
 difference between the weight of such liquid and of water.
   Powders insoluble  in  water, such as gold dust, are weighed  in  a glass
 cup which has previously  been  balanced in water for the purpose.
   Powders soluble in  water, must be weighed in some other liquid.
   Mr. Leslie,  the highly  endowed  professor of natural philosophy in  the
 University of Edinburgh,  has lately suggested a novel and  ingenious mode
 of ascertaining the specific gravities of pulverized or porous bodies ; but as
 it can be understood only by persons acquainted with the doctrines of pneu-
 matics,  the consideration of it must cor.:e under that head.
   Other liquids maybe compared with water  in  several ways.   1st. If a
 phial be made to hold  exactly  one thousand  grains  of distilled  water, at
 the temperature of 40°, the weight of the same  measure of any other liquid
 is found, by simply filling  the phial, and weighing it.  Of sulphuric acid,
 for instance,  such a phial will contain nearly nineteen hundred grains, while
 of alcohol it will receive only about eight hundred.   2d.  A bulb of glass,
 which loses one thousand grains when  weighed in  water, (which  thousand
 grains is therefore the  weight of its bulk in water,)  may be weighed, in other
 liquids, and the difference of loss marks the  specific  gravity, as in the last
 case.  The bulb for  this purpose  may  be of any size, but  one which loses
 in water exactly one thousand grains, is preferable, from the  simplicity there-
 by given to  the calculations:—This remark  applies  also to the  phial last
 mentioned.  3d. A contrivance which renders  the beam and scales altogether
 unnecessary, is a hollow floating bulb of glass or metal a, with a slender stalk
 rising from it to support the little scale or dish b, and with another stalk de-
 scending to carry the  weight or weights at c, which serve as ballast to it.  The
 whole is so adjusted that when displacing one thousand grains, or other known
 quantity of pure water, it shall float with a certain mark upon the upper stalk
                 just at the surface of the water.  By then  immersing it in
                  other liquids and finding how much weight must be added
                  to, or taken from it  above  or below, to make it float in
                  them at the same elevation,  the  comparative weights of
                  these other liquids and of water are  found:—or the differ-
                  ence of weight which makes it float at different elevations
                  in water, having been previously ascertained, it  will only
                  be necessary,  in any other  case, to note exactly  its eleva-
                  tion : an inch of the slender stalk may be equivalent to a
                  difference.of  ten  grains.  This  instrument is called  an
                  hydrometer.   There  are generally printed  tables  and di-
                  rections accompanying  all forms of it, telling  the exact
                  import of the  several indications, and the  allowances to
                  be made for temperature, &c.  It may be used for weigh-
ing solids as well as liquids, for if any mass be put into  the saucer b, weiglits
exactly equal to the mass must be taken  out of the  saucer b, or from below
at c, to restore the equilibrium of  the instrument.  The  mass may be after-
wards placed at c, and weighed in water.  4th.  The shortest mode  of ascer-
taining the specific gravities of liquids, is to  have a set or  series of small
glass bubbles of diflerent specific gravities, so  that when they are  thrown
into any  liquid, those heavier than ft will sink, nnd those lighter will swim,
while that one which  marks its specific gravity will remain merely suspended.
 
FLUID SUPPORT--SPECIFIC GRAVITY.
147
The bubbles must, of course, be numbered, and the specific gravity of each be
previously known.
  A common use of  hydrometers is  to ascertain the quality of the distilled
spirits brought to market, as of rum, brandy, gin, Sic  All these  consist of
alcohol more or less diluted with water; and duty or tax is levied upon them
in proportion to their strength, or the quantity of alcohol which  they contain.
A delicate hydrometer discovers this at once.
  A shop-keeper in  China sold  to the purser of a  ship, a quantity of dis-
tilled spirit according to a sample shown; but not standing in  awe of con-
science, he  afterwards, in  the privacy of his  store-house, added a certain
quantity of  water to each cask.
  The spirit having been delivered on  board, and tried by the hydrometer,
was discovered to be wanting in strength.  When the  vender  was charged
with the intended  fraud, he at first  denied  it,  for he  knew of no human
means which could have made the discovery ;  but on the exact quantity of
water which had been mixed  being specified, a superstitious  dread  seized
him, and, having confessed his roguery,  he  made ample amends.  On the
instrument of his detection being afterwards shown to him, he offered any
price, for what he foresaw might be turned to great account in his  trade.
  The specific gravity of  aeriform  substances is ascertained  by means of
a glass flask of known  size, furnished with a stop cock.   It is  first weighed
when emptied  by the air-pump, and  afterwards  when  filled successively
with water and with different airs  or  gases.  Comparison of the weights
gives the specific gravities, as already described.
   The following table shows, in round  numbers, the comparative weights
or  specific  gravities  of some common substances.   Water is the standard
kept in view, and any equal bulk of another substance  is heavier  or  lighter
than water,  according to the numbers severally attached to them.
Platinum
Gold
Mercury
Copper
Steel and Iron
Diamond  .
Glass   .
Common stones .
22|	Common Salt .
19|	Brick .
13*	Alcohol
81	iEther .
8	Cork
U	Atmospheric Air
3	Hydrogen Gas .
n	
 80~6"
___I
12000
   Complete tables are found in systems of Dictionaries of Chemistry.
   A cubic foot of water happens to weigh very nearly one thousand ounces
avoirdupois, or 62^ pounds.  Hence, in the foregoing table, the  figures de-
noting the specific gravities tell how many times a thousand ounces  of the
different  substances  a cubic foot contains.   Of gold, for instance, a cubic
foot  contains more than nineteen  thousand  ounces, being worth  in money
about £63,000 sterling.   A cubic foot of common air contains only  a little
more than  one ounce;  and of hydrogen g;,s,  the lightest of ponderable
things, a  cubic foot contains less than a drachm.

The following facts also are illustrations of the truth, that a body  immersed
   in a fluid is held up, or has its entrance resisted, with  force equal  to the
   weight of the quantity of fluid which it displaces.
   A stone which on land requires the strength of two men to lift  it, may be
lifted and carried in water by one man.   There are cases, therefore,  where 
148
HYDROSTATICS.
the support of water thus rendered useful is equivalent to the assistance of an
additional hand.   A boy will often wonder why he can lift a certain stone to
the surface of water, but no farther.
   The invention of the diving-bell in modern times, having enabled men, in
the building of piers, bridges, Sic, to work under water almost as freely as
above, many have experience of this influence of water:  but  workmen are
generally surprised at  first, to find that  below, they can move much larger
and heavier stones than they can in  the  air.   Some had supposed the fact
accounted for by saying that the denser air of the diving-bell when  received
into the lungs gave greater strength.   In recovering property from a sunken
ship by the diving-bell, every thing is found to be lighter in the proportion
now stated.
   This law explains also  why stones, gravel, sand and mud, are so easily
moved  by waves  and  currents.  Many  people expressed astonishment, in
March 1825, to learn that at the  Plymouth  Breakwater, the  storm had dis-
placed blocks of stone,  of many tons weight; but we now see that the moving
water had only to overcome about half the weight of the stone.
   When a person lies  in  a bath, the limbs are so nearly supported by the
water as to require scarcely any exertion on the part of the individual.   When
this softest of all beds  has  been indulged in for  half an  hour  or more, the
person, on first lifting a limb out of the water, feels surprise at its great appa-
rent weight.  The workers about diving-bells always  experience the  sensa-
tion now spoken of, on returning to the air.
   The bodies of most  fishes are nearly of the specific gravity of water, and,
therefore, if lying in it without making exertion, they neither sink nor rise
very quickly.   When  this subject was less understood, many persons be-
lieved that fishes  had no weight in water; and it  is related as  a joke  at the
expense of philosophers, that a king  having once  proposed  to his men of sci-
ence, to explain this extraordinary fact,  many profound disquisitions came
forth, but not one of the competitors  thought of trying  what really was the
fact.  It was beneath the dignity of science in  those days to make an experi-
ment.   At last a  simple man balanced a vessel  of water  in scales, and on
putting a fish into the water, showed  its  scale preponderating just as much
as if the fish had been  weighed alone.
   In the sense now explained, water  is said to have no weight in water.  The
least force will raise a bucket of water from the bottom of a well to the surface;
but if the bucket be lifted at all farther, its weight is felt just in  proportion to
the part of it which is above the surface.


"A body lighter than its bulk of water will float, and with force propor-
          tioned to the difference."   (Read  the Analysis, p. 126.)

   The reason of this is clear.  If any body, the cylinder a b c d for instance,
                         be partially immersed in water, we know that the
                         upward pressure of the water on the bottom c d, is
                         exactly what served to support the water displaced
                         by the body, viz., water of the bulk, efcd.  The
                         body, therefore, that it may  remain out as far as
                         here represented,  must have exactly the weight of
                         the water which the immersed part of it displaces;
                         and if it be lighter than this,  it will rise farther; if
                         heavier, it  will sink farther until  the exact balance
                         be produced.
e		f........ d
c		
 
FLUID SUPPORT—SWIMMING.
149
   Hence of any body which floats in water, a pound weight displaces just a
 pound of water, whether the body be very light in proportion to its bulk, as
 cork, or heavier, as a piece of dense wood.  This is experimentally shown
 by putting such bodies to float in a vessel originally full of water.  The water
 displaced by each  must run over the sides of the vessel, and may be caught
 and  measured.
   Hence a porcelain basin  weighing four  ounces will sink in water only as
 far as a similar wooden basin or bowl of the same weight; and the weight of
 either basin may be in the substance of which it  is formed, or  in any thing
 else  put into it as a load.
   Hence a boat made of iron floats just as  high out of water as a boat of simi-
 lar form and size made of wood, provided "the iron be proportionately thinner
 than the wood, and therefore not heavier on the whole.   An  empty metallic
 pot or kettle is often seen floating with a great part of it above the surface of
 the water.—Prejudice for a long time prevented  iron boats from being used,
 although, for various purposes, they are superior to others ; and there are still
 people who would fear to go on board of a ship built of the strong and singu-
 larly durable Indian teak, because it is heavier than water, and in the form of
 a log, therefore sinks in water.  Many  fine  ships of the line, however, and
 East-Indiamen of fifteen hundred tons or more, are now built of teak.
   Hence a ship carrying a thousand tons weight will draw just as much water,
 or float to the same depth,  whether her  cargo be of cotton or of lead :—and
 the exact weight of any ship and her cargo may be determined by finding how
 much water she displaces.   In canal boats, which are generally of a simple
 form, this truth affords a ready rule for ascertaining the quantity of their load.


 The human body,  in an ordinary healthy state with the chest full of air, is
   lighter than water.

  If this truth were generally and familiarly understood, it would lead to the
 saving of more lives,  in cases of shipwreck and in other accidents, than all
 the mechanical life-preservers which man's ingenuity will ever contrive.
   The human body with the chest full of air naturally floats with a bulk of
 about half the head above the water,—having then no more tendency to sink
 than a log of fir.   That a person in water, therefore,  may live and breathe it
 is only necessary to keep the face uppermost.  The reason that in ordinary
 accidents so many people are drowned who might easily be saved, are chiefly
 the following:—
   1st. They believe that the body is heavier than water, and therefore that
 continued exertion  is necessary to keep  it  from sinking ; and hence, instead
 of lying quietly on the back, with the face upwards, and with the face only
 out of the water, they generally assume  the position of a swimmer, in which
 the face is downwards, and the whole head has to be kept out of the water
 to allow  of breathing.   Now, as a  man cannot  retain this position but by
 continued exertion, he is soon exhausted, even if a swimmer,  and  if he is
 not, the  unskilful attempt will scarcely  secure for him even  a  few  respira-
 tions.  The body raised for a moment  by exertion above  the natural level,
 sinks as far below it when the exertion ceases ; and the  plunge, by appearing
the commencement of  a permanent  sinking terrifies the unpractised  indivi-
dual, and renders him  an easier victim  to  his  fate.—To  convince a person
learning to swim  of the natural buoyancy of his body, it is a good plan to
throw an egg into water about five feet deep, and then desire  him to  bring it
up again.  He discovers that instead of his body with the chest full of air
     11 
150
HYDROSTATICS.
naturally sinking towards the egg, he has  to force his way downwards, and
is lifted again by the water as soon as he ceases his effort.
   2d.  They fear that water entering by the ears may drown, as if it entered
by the nose or mouth, and they make a wasteful exertion of strength to pre-
vent it; the truth  being, however, that  it can only fill the outer ear, as far
as the membrane  of the  drum, where its presence  is of no  consequence.
Every diver and swimmer has his ears thus filled  with water, and cares not.
   3d.  Persons unaccustomed to the water, and in danger of being drowned,
generally attempt in their struggle to keep  their hands above the surface,
from feeling as if their hands were imprisoned and useless while below; but
this act is most hurtful, because any part of the body held out of the water,
in addition to the face which  must he out, requires an effort to support it,
which the individual is supposed at the time ill able to afford.
   4th. They do not reflect, that when  a  log of wood or a human body is
floating upright,  with a small portion above the surface, in rough water, as at
sea, every wave  in passing must cover it completely for a little time, but will
again leave its top projecting in the interval.  The practised swimmer chooses
this interval for breathing.
   5th. They do not think of  the importance of keeping the chest as full of
air as possible;  the doing which has nearly the same effect as  trying a blad-
der of air to the  neck, and without  other effort, will  cause nearly  the whole
head to remain above the water.   If the chest be once emptied, while from
the face being under water the person cannot inhale again, the  body remains
specifically heavier than water, and will sink.
   When a man  dives far, the  pressure of deep water compresses, or dimi-
nishes the bulk of the air in his chest, so that, without losing any of that air,
he yet becomes  really heavier than water, and would  not again rise, but for
the exertion of swimming.  The author of this work once  saw a sailor (a
Une-bodied West India negro) fall into the calm sea from a  yard-arm eighty
feet high.  The  velocity  on his reaching the water was so great, that he shot
deep  into it, and, of course, his chest was compressed as  now explained:
probably also the shock  stunned him,  for although he was  an excellent
swimmer, he only moved his arms feebly  once or twice, and was  then seen
gradually sinking for a  long time afterwards, until  he appeared  only as a
black and  distant  speck, descending towards the unknown regions  of the
abyss.
   Every person  needs not learn to swim ; but every one who makes voyages
should have practised the easy  lesson of resting in the water with the  face
out.   The head, from the large quantity of bone in it, is a heavy part of the
body, yet,  owing to its proximity to the  chest, which is comparatively light,
a little action of  adjustment with the hands, easily keeps it uppermost; and
there is an accompanying motion of the feet, called treading  the water, not
difficult to learn, which suffices to sustain  the entire head above the surface.
Many of the seventy passengers who  were  swallowed  up  on the sudden
sinking of the Comet steam-boat near Greenock, in November, 1825, might
have  been saved by the  boats,  which so soon went  to their assistance, had
they known the  truth which we are now explaining.
   A man having to swim far, may occasionally rest  on his back for a time,
and resume his labour when he is  somewhat refreshed.
   So little is required to  keep a swimmer's head above water,  that many in-
dividuals, although unacquainted with what regards  swimming or floating,
have been saved after shipwreck, by catching hold of a few floating chips or
broken pieces  of wood.   An oar will suffice  as a support to half a dozen 
FLUID SUPPORT--STABILITY.
151
people, provided no one of the number attempts by it to keep more than his
head out of the water ; but often, in cases where it  might be thus serviceable,
from each person wishing to have as much of the security as  possible, the
number benefited is much less than it might be.
  The  most common  contrivances, called life-preservers, for preventing
drowning, are strings of cork put round the chest or neck, or air-tight bags
applied round the upper part of the body, and filled, when required, by those
who wear them blowing into them  through  valved pipes.
  On the great rivers of China, where thousands of people find it more  con-
venient to live in covered boats than in houses upon the  shore, the younger
children have a hollow ball  of some  light material attached  constantly to
their necks, so that, in their frequent falls overboard, they are not in danger.
  Life-boats have  a large quantity of cork mixed in their structure, or  of
air-tight vessels  of  thin copper or tin plate: so that, even when  the boats
are filled with water, a considerable part still floats above the general surface.
  Swimming is  much easier to quadrupeds than  to man, because the ordi-
nary motion of their legs in walking and running is that which best supports
them in swimming.  Man is at first the most helpless of creatures in  water.
A horse while swimming can  carry his rider with half the body out  of the
water.   Dogs commonly swim well on the first  trial.—Swans, geese, and
water-fowls in general, owing to the great thickness of feathers on the under
part of their bodies, and the great volume of their  lungs, and the hollowness
of their bones, are so bulky and  light, that they float upon the water  like
statelv ships, moving themselves about by their webbed feet as  oars.
  A water-fowl  floating  on plumage half as bulky as  its naked body, has
about half that body above  the surface of the water; and similarly a  man
reclining on a floating mattrass, as in the  hydrostatic bed afterwards to  be
described, has nearly as much of his  body above the level of the water-surface,
as he forces of the mattrass under it.  His position, therefore, depends on
the thickness of the mattrass.
  A man walking in deep water may tread  upon sharp flints or broken glass
with impunity, because his weight is nearly  supported by the water.
  But many men have  been drowned in attempting to wade across  the fords
of rivers, from forgetting that the  body is  so  supported  by the  water, and
does not press on the bottom sufficiently to  give a  sure footing against a  very
trifling current.  A man, therefore, carrying a weight on his head or in his
hands held over his head, as a soldier bearing his  arms and knapsack,  may
safely pass a river,   where,  without a load, he would be carried down the
stream.
  There is a mode practised in China of catching wild ducks, which requires
that the catcher be  well loaded or ballasted.   Light grain being first strewed
upon the surface  of the water to tempt them, a man hides himself in the midst
of it, under  what appears a gourd or basket  drifting  with the stream, and
when the flock  approaches and surrounds him, he  quickly obtains a rich
booty by snatching  the  creatures down one by one—adroitly making them
disappear as if they were diving, and then securing them below.  Each bird
becomes as a piece  of cork attached to his body.
  Fishes can change their specific  gravity,  by  diminishing or increasing the
size of a little air-bag contained to their body.   It is  because this bag is
situated towards  the under side of  the body, that a dead fish floats with the
belly uppermost.
  Animal  substances, in undergoing the process of putrefaction, give out
much aeriform matter  Hence the bodies of persons drowned and remaining 
152
HYDROSTATICS.
 in the water, generally swell, after a time, and rise to the  surface, again to
 sink when the still increasing quantity of air shall burst the containing parts.
   A floating body sinks to the same depth whether the mass of fluid support-
 ing it be great or small:—as is  seen  when a porcelain basin is placed first in
 a pond, and  then in  a second basin  only so much larger than itself that a
 spoonful or two of water suffices to  fill up the interval between them.  One
 ounce of water in the latter way may float a thing weighing  a pound or more,
 exhibiting another instance of the hydrostatic paradox:—And if the largest
 ship of war were received into  a dock, or case, so exactly fitting it that there
 were only half an inch of interval between it and the wall or side of the con-
 taining space, it  would float as completely, when the few hogsheads of water
 required  to fill this little interval up to its usual water-mark were  poured in,
 as if it were on the high sea.  In some canal locks, the boats just fit the place
 in which they have to rise and  fall, and thus the expense of water  at the lock
 is diminished.
  The preceding examples  of  floating are all illustrations  also of the truth
 that the pressure of a fluid on any immersed body is exactly proportioned to
 the depth and extent  of the surface pressed upon.  The lateral pressures just
 balanced one another, and the  upward pressure  has to be  balanced by the
 weight of the body.

 Similar reasoning to  that which proves that the whole weight of a body acts
  as if lodged in the point called its centre of gravity,  proves that the whole
  buoyancy of a body, or the upward push of the fluid in which a body is
  immersed, acts as  if lodged in the point which was the centre  of gravity
  of the fluid  displaced.   This  point consequently is called the " centre of
  buoyancy."

  A floating body, to be  stable  in its position, either must have its centre of
gravity below the  centre of buoyancy—in which case it resembles a pendu-
lum ; or it must have a very  broad bearing on the water, so that any inclina-
tion may cause the centre of gravity  to ascend,—in which case it  resembles
a cradle or rocking-horse.
  Hence arises, in the stowing of a ship's cargo, the necessity  of putting
the heavy  merchandize  underneath,  and generally of putting iron ballast
under all  the  merchandize.   Hence, also, the  danger of  having a cargo  or
ballast which is liable to shift its place.  A ship loaded entirely with stones,
is sometimes lost by  a wave  making her incline for a  moment so  much that
the load shifts to one side, which is then kept down.   For a similar reason,
a cargo of salt or sugar  has a peculiar danger attached to  it, for if the ship
leak, the  cargo may be dissolved, and then pumped out with  the bilge water,
leaving her with altered trim.   In a fleet coming home from India, in  1809,
four fine ships disappeared during a hurricane off the Isle of France, and from
what happened to the other ships that were saved, the cause of the destruc-
tion was supposed  to be,  that the saltpetre of the cargoes had been dissolved
and pumped out, and that the ships in consequence became  unmanageable.
  Bladders used by beginners in swimming are dangerous, unless secured so
 as  not to  shift towards the lower part of the body.
  A  great inventor (in  his own  estimation) published  to  the world, that he
had solved the important  problem of  walking safely upon the water: and he
invited a crowd to witness his first essay.   He stepped boldly upon the wave,
equipped in bulky cork boots,  which he  had previously tried in a butt of
water at home; but it soon appeared that he had not pondered sufficiently on 
FLUID SUPPORT AMONG FLUIDS.
153
the centres of gravity and of floatation, for in the next instant all that was to
be seen of him was a pair of legs sticking out of the water, the movements
of which showed that he was by no means at his ease.   He  was picked up
by help at hand, and, with his genius cooled and schooled by the event, was
conducted home.—Some soldiers once finding a few cork jackets among old
military stores, determined to try them;  but mistaking the shoulder straps
for lower fastenings, they put them on as drawers, and on then plunging in,
with the hope of being able to sit pleasantly on the water, their heavy heads
went down, and they were nearly drowned.
   When, on the  return of summer, the ice  breaks up in the polar regions,
immense  islands of it are set  afloat, rising high into the air and sinking deep
into the sea.  The melting process, in most cases, does not go on equally  in
the water and in the  air, and from the  mass, consequently, changing form, its
stability is often lost, and one of the grandest phenomena in nature follows—
the overturning of a mountain—the sudden subversion of an island—produc-
ing a tumult in the ocean around, felt often at the  distance of many leagues.

The phenomena of pressure, floating, &c, in fluids, vary in proportion to the
   weight  or specific gravity of the fluid.
  A ship  draws less  water, or swims lighter, by one thirty-fifth, in the heavy
salt-water of the sea than in  the fresh water of a river:  and for the same
reason, a man swimming supports himself more easily in the sea than in a river.
   Many kinds of wood that float in water will sink in oil.
   A man  floats on mercury  as the lightest  cork floats on water, and with
practice he might be able to walk upon mercury.
   Had the water of our ocean  been but a little heavier than it is, men after ship-
wreck might have died of famine and cold, but would not have been drowned.
   Oil floats on water, but sinks in  alcohol or aether.   The term proof spirit
means spirit light enough for  oil to sink in it.   The strength of spirit is pro-
portioned  to its lightness.
  Cream  rises in milk, and forms a covering to it.
  Blood,  allowed to rest after flowing from the  living body, separates  into
parts or layers, which arrange themselves according to their specific gravities.
The buffy coat of inflammation (where this exists)  is uppermost, forming
the surface of the general coagulnm : towards the lower part of the coagulum
there is an accumulation of  red globules; and the whole of the solid part
floats in the serum,  which  is therefore lowest of all.  When the  red  glo-
bules escape from the coagulum, they  fall to the bottom even  of the  serurn.
  Wine, if slowly and carefully poured on water,  will float  upon it.  In a
vessel  shaped like a common  sand-glass, only with a larger
opening between the chambers at c, if  wine be put into the     Fig. 83.
under  chamber,  and  water into the upper, the two liquids
will  gradually change places: and if the lower half of the
glass be covered, so as to leave the upper  half with the ap-
pearance  of a simple goblet, the  water will seem to have
been changed into wine.  The liquids are less mixed, and
change places sooner, when there  is a tube b to carry the
water down to the bottom without touching the wine, and a
tube  a  to carry the wine directly to the top.
  Mercury, water, oil, air, and some other fluids may all be
shaken together in the same vessel, and  on  standing  will
separate again and arrange  themselves in  the order of their
specific gravities.
 
154
HYDROSTATICS.
  When, in a mass of water, part of it is heated more than  the rest, that
part, by its expansion, becomes specifically lighter than the rest, and rises
to the surface.   Hence, when heat is applied  to  the bottom of a vessel con-
taining  water, a circulation  is established, which goes on from  the first
moment until the operation of heating finishes:—water is always rising from
the hotter parts of the vessel, and descending over the colder parts.
  In like  manner, when a tall glass  containing hot water is dipped into cold
water, a downward  current takes place within ihe glass near the sides all
round, and there is an upward current in the middle.   This motion may be
rendered very obvious  by small portions of amber thrown into the water, for
these being nearly of the specific gravity of water, rise and descend with it.
On account of the current established  in such cases,  heat applied to the bot-
tom of a vessel of liquid is soon equally diffused  over it; but heat applied at
the top is there confined, because the  heated and lighter fluid does not descend.
Water may be made to boil at its surface, while a piece of. ice lies at  the
bottom.   The converse is impossible.
  The  current  in a fluid, produced by local change of temperature, is an
important part of the following process, which  the author deems applicable
to  various useful  purposes.—Heat  may  be transferred from  one liquid to
another, without mixing  them, by making the hot liquid  descend in a very
thin metallic  tube, through the cold liquid rising around it in  a larger tube.
Boiling water  from the  vessel e, for instance, may descend slowly by the
                                          small tube e a bf,  which is sur-
                 Fig. 84.                   rounded from  a to b  by  cold
                                          water ascending  through  the
                                          tube  c g.   Then, as the tem-
                                          perature of two liquids, brought
                                          so nearly into contact with each
                                          other, will  not, after a very short
                                          time, differ, in any one place
                                          more than a  few degrees, it fol-
                                          lows that the water lately cold,
                                          will, on  leaving the part of the
                                          tube g, which is in contact with
                                          the boiling water descending di-
                                          rectly from e, be r\early boiling,
                                          while the water lajely  hot will,
                                          on leaving the  tube at b, which
                                          is in  contact with cold  water
                                          just  arrived  from  h,  be  itself
                                          nearly cold:   and  thus  equal
quantities  of hot and cold water will have exchanged  temperatures.  The flux
of  the hot water is to be regulated by a cock b, and that of the cold  water by
a cock h.  The water in the part of the tube c g d rises, because it is hotter and
therefore specifically lighter than that in the  part h c.—The author believes
that an  apparatus  made on this principle,  with an arrangement of many thin
flat tubes instead of a single large tube, for the descending fluid, and a spacious
box c g to contain these and the  rising fluid, would be  an excellent refrigera-
tor in  a distilling apparatus, and  for cooling the  wort of brewers;  or would
serve as a means of diminishing the expense of  warm  baths, by transferring
the heat from the water lately used to  pure water.  In distilling, the wash  or
low wines, about to enter the still, might be used as the cold condensing fluid
 to  surround the warm  or vapour  tubes, and thus, without expense, would  be
 
FLUID SUPPORT AMONG FLUIDS.
155
heated in its progress to the still.  Half the original expense of a great por-
ter brewery is in the construction of the numerous water-tight floors on which
the hot wort is thinly spread  to cool.  The practice of warm bathing, so
conductive to health, is less common in this country, because the  present
expense is so great.
   It is a general truth in nature, that substances contract in size as they cool.
There is,  however, in  water, a  curious exception to this  rule, which,  ope-
rating through the  principle  of specific gravities,  effects most important
purposes in the economy of nature.   Water contracts only  down to the  tem-
perature of 40 deg., below which, towards 32  deg. or the  freezing point, it
goes on dilating again, and as ice is much lighter than as a  fluid.  Ice, there-
fore, floats on the surface of water, and being a very slow conductor of heat,
defends the water underneath from the cold  air, and  preserves it liquid, and
a fit dwelling for the finny tribe, until  the return  of the  mild season.   And
not only is the extreme of cold below thus prevented, but because very cold
water remains floating on the surface of a wintry lake, as cream floats on milk,
it preserves underneath that warmth which is agreeable to the fishes, just as
very  hot  water in  summer remains uppermost,  preserving underneath an
agreeable  coolness.  By the dilation of  very cold water,  then, and  the for-
mation of  ice, nature has prepared a winter garb  for the inhabited lakes and
rivers, as  complete and effectual as for terrestrial  animals, by the periodical
thickening of their wool or fur.  Had ice become heavier than water, so that
it must have fallen to the bottom, and have left the surface without protection,
a deep lake, in European  winters, would  have been frozen  into a solid life-
less mass, which summer suns would no more have melted  than they now do
the glaciers of Switzerland.  But for this important exception, therefore, to a
general law of  nature,  many of the now most  fertile and lovely portions of
the earth's surface would  have remained for ever barren and  uninhabited
wastes. 
156
PNEUMATICS.
PART   III.
THE  PHENOMENA OF FLUIDS.
SECTION II.—PNEUMATICS.
                       ANALYSIS OF THE SECTION.

 In aeriform fluids, that is, in such as have their particles held far apart by
   mutual repulsion, which yields, hoivever, to any force applied, so that the
   mass suffers great change of volume under different degrees of compres-
   sion,—the phenomena are modified by the great  lightness and elasti-
   cityof the fluids, but are still in strict accordance with the general pro-
   perties of fluids already explained, viz., pressure equal in all direc-
   tions--PRESSURE AS THE DEPTH--LEVEL SURFACE, and  FLUID SUPPORT.
   The pressure of air, in all directions, and as the depth, may be studied in
   the effects of our atmosphere—on solids—on liquids :—or when it concurs
   with heat, in producing the phenomena of boiling,  evaporation, clouds,
   rain, dew, &c.; or when, by varying in degree,  it  allows certain sub-
   stances to exist sometimes in  the liquid  and sometimes  in the aeriform
   states.   The fluid support in air is exemplified by balloons, the ascent of
   flame,  and smoke, winds, &c.

   What a change  has taken  place in the degree of man's knowledge of
nature, since philosophers thought that air was one of four primary elements,
viz., air, fire, water and earth, of which all things  were  composed, and each
of which was for ever distinct from the others.  We now  know that air or
gas is merely an accidental state, in which any body  may exist, according to
the quantity of  heat pervading it:  the body being solid  when the absence of
heat allows its atoms to obey freely their mutual attraction, and to cohere-
as in ice, for instance;  being liquid, when so much heat is present as nearly
to balance the attraction, and to let them slide freely among each other—as
they do in water; and being aeriform when still more heat is added, causing
the atoms mutually to repel and dart asunder to a great distance__as they do
in steam.   But  in any one of these three  states, the  various substances are
as much themselves as  in  the  others, and  at  the command of the chemist
will assume any of the forms  which he desires.  As  most substances in
nature have a different relation to heat, there are some which, at the medium
temperature of our earth are solid, some which are liquid, and some aeriform.
The solids, in general, are the heaviest under a given volume, and therefore
sink down and form the great mass or centre of the earth  ; the liquids follow
next m order, and float upon this solid centre, filling  up  its inequalities with 
AERIFORM FLUIDS.
157
'  a level surface, so as to constitute the ocean ;  while the airs are  the  lightest
  of all, and as a second ocean, rest above the sea and above the highest moun-
  tains, to an elevation of about fifty miles.  Among the substances whose
  relation to heat causes them, when not restrained in  certain combinations, to
  assume the form  of air at very low temperatures, there are two in particular,
  viz., oxygen andnitrogen, which are very abundant in nature in suchuncom-
  bined state,  and  of these,  therefore, the atmosphere chiefly  consists ;  but
  smaller portions  of almost every other substance are found in  it.   Water,
  among  the supplementary matters, is much more abundant than any of the
  others,  and in its  various states of cloud, mist, rain, dew and snow, it answers
  a thousand useful purposes, and serves beautifully to vary the scenes of na-
  ture.   The atmosphere is about fifty miles high or deep, and therefore, in
  relation to the bulk of the earth, is as a covering of one-tenth of an inch in
  thickness to a common library globe of a foot in diameter.
     The  atmospheric ocean is the great laboratory in which most of the actions
  of  life  go  on, and on  the  composition  of which they depend.  A human
  being requires for breathing a gallon of  fresh  air every minute, dying equally
  if deprived of air, or if confined to the same.   All other animals  also require
  fresh air, but in various proportions.   And  in  the  vegetable creation, the
  beautiful green leaf and delicate flower are merely broad and tender expan-
  sions of surface for the contact of the vivifying air.   Animals give out to the
  atmosphere  a substance which vegetables absorb, and  vegetables, by the
  absorption,  fit the air again for the use of animals; so that, upon the whole,
  in  the various changes of nature, there is a perfect balancing of  actions, which
  preserves the atmospheric mass in a uniform  state, constantly fit  for its admi-
  rable purposes.
     While the ancients had that notion of air,  which made them apply to it
  vaguely, and almost indifferently, the names of air, ether, spirit, breath, life,
  Sic,  they never  dreamed of  making experiments  upon  it, with a  view to
  prove its relation to common matter:—and  one of the most beautiful portions
  of the modern history of man's progress in knowledge, is that which exhibits
  the light gradually breaking in upon this most interesting subject.   Galileo
  was led to conclude that air made  a  definite pressure upon things at the sur-
  face of the earth; Torricelli and Pascal proved that  this was  occasioned by
  its weight, and hence, moreover, they deduced the height of the aerial ocean;
  Priestley, Black, Lavoisier, and others, discovered that  air might be united
  with  a metal, so as to  increase its  weight, and to  produce a compound of
  totally  new qualities, for  they showed that many of the ores of our mines
  are merely metals concealed,  by being  thus united  with  a substance which,
  when set free, ascends  as one of the  ingredients of the atmosphere.   They at
  last analyzed the atmosphere itself, and exhibited its two great ingredients
  as  distinct substances.   And within  a few years the nature of  air or gas  has
  been  so thoroughly  investigated, that we  can now take a little of many a
  light, invisible, impalpable  fluid such as we breathe, and squeezing the heat
  out of  it by strong pressure, can make  its particles collapse from their  aeri-
  form distances to assume the state of a tranquil fluid; which may then be
  retained as such  for ever, or may be decomposed and made solid in combi-
  nation with other bodies, or may be again set at liberty.
     The suspicion once excited, that air was as much  a material fluid as water,
  only much less dense, by reason of a greater separation and repulsion of the
  particles, it was easy to follow out the parallel, and to confirm the supposition
  by reference to the commonest facts.   Thus, a leathern sack or  pouch, opened
  and dipped  into water so far as to  become full, if its mouth be then carefully 
158                          PNEUMATICS.
closed, retains the water, and its sides cannot afterwards be pressed together:
a similar sack or bladder, opened out, and then closed in air, is found to
remain, in a corresponding way, bulky and resisting, and forms what is called
an air-pillow.  The  motion of a flat board is  resisted in water: the motion
of a fan is resisted in the air.   Masses of wood, sand, and pebbles, are rolled
along or floated by currents of water:  chaff, feathers, and  even rooted trees
are swept away  by currents of air.  There are  mills driven by water; and
there are mills driven by the wind.   Oil set free  under the surface of water,
or placed  there in a  bladder, rises to the surface: hot  air or hydrogen gas
placed in a balloon, rises in the air.  A fish moves itself by its fins in water:
a bird moves itself by its  wings in the air; and as on taking the water from
a vessel in  which a fish  swims, the creature falls to the bottom, gasps a few
moments, and dies, so,  on exhausting  the  air from a vessel  in which  birds
or butterflies are enclosed, their useless  wings  may flap; but they sink to the
bottom, and if the cruel experiment be  continued, they soon become  motion-
less  and for ever.
  We proceed now to prove that air or gas, as a fluid, differs from the other
fluids, which we call liquids, only in the two circumstances of great  light-
ness or rarity, and of being very extensively elastic,  that is to say, the  parti-
cles being so related,  that pressure brings them  much more nearly into  contact,
and on ceasing, allows them to regain their former distance.

                           Lightness of Air.
  The lightness or rarity of  atmospheric air, as it is found on the  general
surface of  the earth, is  such, that if, by the action  of a pump, a bag of it
holding a cubic  foot  be  emptied into the copper ball of an air-gun,  the ball
weighs about an ounce  and a quarter more than before.  The same volume
of water weighs  nearly a thousand ounces ;  so  that common air is about eight
hundred times lighter than water.  Other gases, or  substances in the aeri-
form  state, have their various specific gravities,  just as the same substances'
have when liquid or solid.  Thus water in the  form of air, that is to say,
when existing as steam, and of the common density, is little more than half
as heavy as the  same bulk of common  air: hydrogen is only one-fourteenth
part as  heavy: and carbonic acid gas, which is the air that rises out  of soda-
water, brisk ale, champagne wine, &c, is  so much  heavier, that even  in the
atmosphere, it may be poured out  of one open vessel into another, as a liquid
might, or,  more exactly, as water  might be poured out under oil.

                           Elasticity of Air.
  A small bladder full of  air may be pressed or  squeezed between the hands
  Fig. 85.    so as t0 De much reduced in size, but on being relieved from the
            pressure, it will immediately resume its  former bulk.
              If a metallic tube  or barrel  of  perfectly uniform bore a, b, be
            fitted with a movable  plug or  piston  c,  which is  covered with
            leather and oiled, so as  to slide up and down without  allowing
      «•    the air to pass by its sides, the air  between  the piston and the
            close bottom b may be compressed to a  hundredth or less of its
            usual bulk ; but when allowed, will push the  piston back again
            with the same force as it opposed to  the condensation,  and will
            recover the volume which  it had before the experiment.
              Again, if the plug  at the commencement of the experiment
            were only an inch from the bottom, enclosing  air of the  usual
            density, on drawing it up to the top, the inch of air beneath it
|Cl 
ELASTICITY OF AIR.
159
would expand so  as to occupy the  whole tube, having  become, of course,
proportionally less dense.
  To  the question why the air, which  admits of such  various density, is
found to have that certain degree of it met with at the surface of the earth,
we  answer,  that as the water, in any place near the  bottom of the ocean, is
pressed with force exactly proportioned to the quantity of water above it, so
the  air at the surface of the  earth bears the pressure of the superincumbent
mass of air,  and on account of its extensive elasticity, suffers, like the lower-
most bags of cotton  or wool in a great heap, that  degree of compression
which the superincumbent mass is calculated  to produce.   We shall see be-
low, that the density of the air near the earth  is changing with every circum-
stance which affects the weight of the atmosphere above, as winds, clouds,
rain, &c, and that it bears  relation  to the altitude of  the  place of  observa-
tion above the level of  the sea.
The tube with its  piston, described  in the last page, becomes, according to
   the position of its valves, either a  syringe for injecting and condensing air,
   or a pump for exhausting or removing it from any vessel; both operations
   depending on the elasticity of air.

   A barrel and piston is a condensing syringe, when, in a passage of com-
munication between the bottom of the syringe and a receiving vessel, there
is a flap or valve allowing air  to pass towards the  receiver but not to return.
The piston, therefore, at each stroke, forces what the barrel contains of air
into the receiver.   When the  piston  is lifted again after the stroke, air re-
enters the barrel from the atmosphere, either through a valve in the piston,
or through a small hole near the top  of the barrel.—That useful contrivance,
a valve, for whatever purpose used,  and in whatever way formed, is in prin-
ciple merely a movable flap, placed on an opening, against which it is  held
by its weight, or by some other gentle and yielding force.   Such  a flap, it
is evident, will allow fluid to pass only in one direction, viz., outwards from
the opening, for any fluid tending inwards must  shut the flap, and press it
the closer, the greater the tendency.
   To convert a forcing syringe or pump into an exhausting syringe or pump,
commonly called an air-pump, it is only necessary to reverse the position of
the valves; then, on the descent of the piston, all  the air between it and the
bottom, instead of  entering the vessel or receiver, as in the last  case, escapes
by a valve in the piston itself towards
the atmosphere, and on the rising of
the piston, a perfect vacuum would be
left under it, but that the valve  below,
then opened by the elasticity of the air
in the receiver, allows a  part  of that
air to follow it. Thus, at each stroke,
a quantity of the air, proportioned to
the size of the pump, is  removed from
the receiver.    In  a  good  air-pump,
there are two similar pumping barrels,
as a and b, to quicken  the operation
of exhausting; and  both  are  worked
at the same time by the reciprocating
winch  or handle f,  with its pinion e,
acting on the teeth of the piston rods d
Fisr. 86.
 
160
PNEUMATICS.
and c.    This double construction has the farther  advantage, that the atmo-
spheric pressure, if fifteen pounds per square inch on the upper surface of
either piston, and  which for a single piston would have to be overcome by
the worker in  lifting  it,  as here  balanced always  by  the corresponding
pressure  on the other piston.   Both pumps communicate with a tube g A,
which at h  rises  tightly through the round plate of the machine to i. This
flat plate is so smooth,  that  a glass  bell  or  receiver k,  with a  smooth
ground lip, when  placed  upon  it,  forms an  air-tight joining.   On work
ing the pump, such a bell is exhausted of its air, and fitted for showing the
many interesting phenomena which the air-pump  can display,—and which
will pass under review as we proceed.  The supporting  frame-work of the
pump is not shown here.

The law of the elasticity of air is, that its spring,  or resistance to com-
  pression, increases exactly with its density or the quantity of  it collected
  in a given space.  Hence, by finding in any case either the density of the
  air, or the spring, or the compressing force, we know all the three.

  It has been ascertained by experiments described a few pages  farther on,
that in the atmospheric ocean surrounding the earth, there are nearly fifteen
pounds of air above every square inch  of the surface of the earth ; and that
the air nearest the earth, and bearing this superincumbent weight or pressure,
has the density of an ounce and  a quarter of weight to a cubic foot of volume.
We further find that such air is reduced to half its bulk, or  becomes of what is
called double atmospheric density, by an additional pressure of fifteen pounds
on the inch, and of triple density by triple pressure, and so forth ;  and on
the other hand, that it dilates to  double bulk if the pressure be diminished to
half, and to any greater bulk, even  beyond  a thousand-fold, if the pressure
be diminished  in  a corresponding  degree; and  any air bearing  a given
force or  pressure,  is always acting as  a spring of that force on whatever it
touches.
  It is very important to be  familiar  with this truth or law, for it  holds very
nearly with respect to all aeriform fluids as well as common air, and  throws
light, therefore, on the action of steam-engines, air-guns, pneumatic machines
generally.   It also explains the condition of our atmosphere as to density at
various elevations ; telling us, for instance, that when  a  balloon has risen
through half of the atmospherical mass, the air around  it  will  be  of only
half the density which exists at the surface of the earth.
   We know not exactly to what extent the rarefaction of air may go on the
removal  of pressure ; in other  words,  at what distance  the gravity of the
particles  becomes just a balance to their mutual repulsion  ; and therefore we
know not exactly what the degree of rarity  is at the top of our atmosphere;
but we know that it must be exceedingly great, from the fact that the air left
in the receiver of an air-pump has still spring or elasticity enough to  lift the
valve of  the pump, when less remains than  the thousandth part of the origi-
nal quantity.  In the most perfect air-pumps, that the exhaustion may be as
complete as possible,  the machine itself is made to raise the valve.
  The expansion of air is well  illustrated by a bladder, having a very little
air in  it, placed under the receiver of an air-pump.  On exhausting the
receiver, the  bladder gradually swells,  with  force sufficient to lift a moderate
weight laid  upon it, and at last  appears quite full, and may even be burst.
A shriveled apple  treated in the same way becomes plump.   The explana-
tion of such phenomena is, that at first the air in the bladder or apple is in  a 
ELASTICITY OF AIR.
161
Fig:. 87.
state of condensation, like all air at the surface of the earth under the pressure
of the superincumbent atmosphere ;  but  that its volume increases  as  that
pressure is diminished  by the air-pump:—it is rarefied  in the same  propor-
tion as the air which remains in the receiver surrounding it.
  The curious instrument called the  air-gun has  a strong globular vessel of
copper attached under  the lock, into which air  is usually forced to be thirty
or forty times as dense as the air in the atmosphere around : hence the pres-
sure or elasticity tending outwards is  thirty or forty times fifteen pounds on
the inch, and when the valve is opened for an instant by the action of the lock,
a portion of the  air issues  and propels  the charge with this force.  The
effect of air thus condensed nearly equals that of gunpowder, and one charge
of the ball suffices  for many shots, the force, however, becoming less  for
every successive discharge.
  If a bottle or vessel  a b, partly filled with water, have a
tube c d passed tightly through the cork to near the bottom
of the water; and if more air be then forced through this
tube in any way, so as  to accumulate in the upper part of the
vessel above  the water surface a b ; on turning the cock c,
which opens  the tube, the elasticity  of the  condensed  air
will  press the water  out as a beautiful jet, to a height pro-
portioned to the condensation, and  gradually diminishing as
the condensation diminishes.  Or if such a vessel, with air
of common density, be placed under a tall air-pump receiver,
on working the pump  so  as to diminish the density of  the
air in the receiver, the jet of water will equally arise.—A
table-lamp, by the force  of condensed air, may be supplied
with oil from a reservoir far  below the wick :  and lately an
enema syringe and a shower-bath have been constructed on
the same principle.
  The elasticity of air is rendered very serviceable in con-
nection with great water-pumps, such as those  used for the
supply of cities.   A pump  throws its  water by a distinct
gush at  each stroke,  while  the current through the pipe
towards  the  city should be  uniform.  Now uniformity is
attained by causing the gushes from the pump a to enter by
the  passage b at one side of a large vessel c, of which  the
upper part is full of condensed air, and from the other side of which at d, the
water issues  on its way.  The air in this vessel
(called the air-vessel) is condensed, as a spring, by
the entering water, and its resisting elasticity, both
immediately, and  afterwards during the interval of
the strokes, forces  the water along the pipe d.  Each
entering gush has only  the  effect of compressing
the air a little more  for the time, while the flow in
the  great  pipe continues nearly  uniform.   The
pump itself is made to take in a little air at each
stroke, so  that not only is the vessel always sup-
plied, but some air  is constantly passing on with
the water, and effecting  the  highly useful purpose
of giving an elasticity to  the  whole contents of the
pipe and its ramifications.
  The same object is  attained by the same means in  the fire-engine used to
check conflagration.   In it there are generally several water-pumps working
Fig. 88.
 
162
PNEUMATICS.
together, which throw their interrupted supply into an air-vessel whence it
passes in a nearly uniform jet to the point desired.
   The compressibility and corresponding spring of air are remarkably exhi-
bited in that singular contrivance of modern times, the diving-bell, in which
men now descend with safety to considerable depths in the ocean, there to
reside and labour, attaining many objects of high importance to them :—they
recover sunken treasures,—they are  enabled to pursue works of submarine
architecture, as in constructing light-houses and noble  harbours, where for-
merly no foundations could have been laid, &c.   The diving-bell, in point of
utility, has proved a remarkable contrast to its sister invention,  the balloon,
which, although so wondrously bearing man aloft to the regions of the clouds,
has brought him as yet little advantage to compensate for the many fatal acci-
dents which its use has occasioned.
   The diving-bell is a large  heavy open-mouthed vessel, with accommoda-
tion in it for one or more  persons.   It is let down  into the water with its
mouth undermost, from a crane to which it is suspended, and which rests on
a suitable carriage either on the shore, or on the deck of a ship, or barge
fitted for its service.   On first  entering the water it appears full of air; but
air being compressible, according to the law now explained, and  the pressure
of the water around the descending bell increasing  with the depth, the vo-
lume of the air gradually diminishes, and at thirty-four feet is reduced to half.
The  bell, then, unless more air be supplied, will of course be half full of
water, and a person breathing in it, at each inspiration will receive twice as
much air into the lungs as when breathing at the surface.  A constant supply
of fresh air is sent down  to the bell by a forcing-pump above ; and the  heated
and contaminated air, which has served for respiration, and which rises to
the top of  the bell, is  allowed to escape by a cock  placed there for the pur-
pose.  The men who work at a distance from the  bell have tubes of com-
munication with  it, by which they inhale the air required;  and they allow
the used air to rise through  the water above them.   A man  cannot breathe
comfortably by such a tube if he be either much above or much below the
level of the water in the bell: for if above, the air  in the bell is more com-
pressed than his chest, and is forced  towards him so as to require an effort
to control its admission ; and if below, his chest is  bearing greater pressure
than  the air in the bell, and he must therefore act strongly with the muscles
of the ribs to  draw the air  down  to  him.  A phenomenon  similar to this
takes place when two  bladders of air are connected by a long tube, and im-
mersed  in  water to unequal depths, the  air being  always  strongly forced
from the lower into the upper one, because the lower one is more pressed.
The difficulty of pumping air down to the diving-bell  increases, of course,
with the depth to which it has descended: for if the bell be so  low that the
water is  pressing on  the air in  it with  a force  of fifteen  pounds per inch,
(which would happen at thirty-four feet,) it is evident that a syringe or pump
cannot inject more air unless it act with a force  greater  than this.  Divers
might often, if not always, more  conveniently receive  their supply of air
through tubes  from  an  air-vessel kept charged to  the necessary  density in a
boat over them, or on  the shore, than from a bell  below.   If  they would,
moreover, dress in India-rubber cloth, and use a hood of metal with windows
for the head, they might work under water without wetting any part but their
hands.
  It is remarkable, when the use of the  diving-bell has  become  so  familiar,
that a kindred and still more simple  contrivance of  the same class has  not
been introduced for certain purposes, particularly of sudden emergency, such 
ELASTICITY OF AIR.
163
as to aid in the recovery of  the bodies of drowning persons.   A  ten-gallon
cask, or vessel of any kind, filled with air, and made heavy enough just  to
sink in water, with a breathing tube from it like that of a diving-bell, would
be a provision of air for a man below water for ten minutes; and a man with
it under his arm, might instantly descend from a boat or walk from  the shore,
into water of any depth, to recover the body of a fellow-creature lately sunk,
and in time probably to save the life, which a few minutes wasted in waiting
or in unsuccessful dragging  would suffer to be lost.   The author would  pro-
pose  this as an addition to the apparatus of the Humane Society for the
recovery  of  persons apparently drowned.—It shows the remoteness from
common  trains of  thinking of the truths connected with the constitution of
our atmosphere and sea, when a means so simple and easily procured should
never have been thought of or tried in any way by pearl-fishers, or by per-
sons who gain their bread by diving to recover things dropped overboard  in
harbours or anchoring stations; all of whom  have hitherto been  limited  to
the single gulp of air taken on descending.   In the case of a  man working
under water, cask after cask of air might be sent down, to enable him to remain
as long as necessary.
   There is an exceedingly beautiful  philosophical toy, of which the action
depends chiefly on the  elasticity of air; and as it moreover
illustrates most of the laws of fluidity, it is deemed worthy of     Fig. 89.
description here.  It is a small balloon or thin globe of glass        ri
c, having an opening at the bottom, and its little car or basket
hanging to it.  If put to float in water  while  the globe con-
tains  air only, it is so light that half the globe remains above
the surface ; but water may be introduced to adjust the specific
gravity of the whole, until it becomes  only a  little  less than
that of water.  If the balloon be  then placed in a  tall jar of
water a b, the mouth of which is closely covered by bladder-
skin or India-rubber tied upon it, on pressing such covering
with the hand, the  balloon  will  immediately descend in the
water, to rise again when the pressure ceases, and will float
about, rising, or falling, or standing still, according to the      """"-y—""
pressure made.  The reason of this is, that pressure  on the
top of the jar first condenses the air between the cover and the
water surface; this condensation then presses upon the water below, and by
influencing it through its whole extent, compresses also  the air in the balloon
globe, forcing as much more water into this as to render the balloon heavier
than water, and therefore heavy enough to sink.  As  soon as the pressure
ceases, the elasticity of the  air in the balloon  repels the lately entered water,
and the machine, becoming  as before, lighter than water, ascends to the top.
If the balloon be adjusted to have a specific gravity  too  nearly that of water,
it will not rise of itself  after once reaching the bottom, because the pressure
of the  water then above it will perpetuate the condensation of the  air which
caused it to descend.  It may even then, however, be made to rise again by
inclining the  water-jar to one side,  so that the perpendicular  height  of water
over it shall be diminished.
  This toy proves  many things—the materiality of air, by the pressure  of
the hand on the top beino. communicated to the water  below through the air
in  the upper part of the jar—the compressibility of air, by what happens  in
the globe just before it descends—the clasticforce of air shown in expansion,
when,  on the perssure ceasing,  the water is again expelled from  the globe—
the lightness of air, in the buoyancy of the globe :—it shows also that in a 
164
PNEUMATICS.
fluid the pressure is in all directions, because the effects happen in whatever
position the jar be held—it shows  that pressure is as the depth, because less
pressure of the hand is required the farther that the globe has descended in
the water—and it exemplifies many circumstances of fluid support.  A young
person, therefore, familiar  with this toy,  has  learned  the  leading  truths of
hydrostatics and pneumatics, and has had much amusement as well as instruc-
tion.
                        On the same principle as the balloon now described,
      Fig. 90.          three  or four little  figures of men may be formed of
                      glass, hollow within, and having each a minute open-
                      ing at the heel, by  which water may pass in or  out.
                      If these be  placed in ajar as the balloon was, and be
                      adjusted by the quantity of water admitted into them,
                      so that, in specific gravity, they shall differ a little from
                      each other, and if then, a gradually increased  pressure
                      be made on  the cover of the jar, the heaviest figure will
                      descend first, and the others will follow in succession;
                      and they will stop  or return  to the surface in reverse
                      order when  the pressure ceases.  A person while exhi-
                      biting these figures to spectators who do not understand
                      them, may  appear  only  carelessly to rest his hand on
                      the cover of the jar while he is making the required
                      pressure, and he will seem to have the power of order-
                      ing their movements by his will.  If the jar containing
                      the figures be inverted, and the cover be placed over a
                      hole in the  table, through which, unobserved, the ex-
                      hibitor can  act by a rod  rising through the hole and
                      obeying his foot, he may produce the most  amusing
      Fig. 91.          and surprising evolutions among the little men, in per-
        -             feet obedience to his word of command.
       (til               The beautiful fountain, called the fountain of Hiero,
                      by which water is made to spout far abpve its source,
                      depends for its action upon the  resisting elasticity of
                      compressed air.   The vessel d is first filled with wa-
                      ter,  while b and a contain air only.  OnMhen pour-
                      ing water into a, the water of d darts upwards through
                      the jet-pipe e, to  an  elevation nearly equal to the length
                      of the tube  from a  to b.   The reason  is, that the wa-
                      ter from a descends by the tube to b, and compresses
                      the air at c; which compression conveyed along the
                      other tube from  c to d, acts on the water  in the vessel
                      (/, and causes it  to jet.   As the  pressure  is produced
                      by the  column of water a b, the jet is proportioned
                      to the length of  that column.'—This kind of fountain
                      may have its parts concealed under a variety of forms
                      as here exemplified, and may thus become a beautiful
                      ornament among flowers in a summer drafting-room.
                      It may be made  of size to play for an Jtour'Or more,
                      and  it will always  recommence  on the ivater being
                      shifted from the  low to the high reservoir.—The use-
                      ful table-lamp, consisting of a simple column or pillar
                      with the oil  rising to the flame from  far below, is a
                      Hiero's  fountain,  only  the oil,  instead  of being al-
                      lowed to  jet out, rises in a tube to the flame.  The
 
PRESSURE AS THE DEPTH.                   165
 contrivance  for maintaining the two'columns always  of the  same length,
 notwithstanding the expenditure of oil has to  be explained some pages
 hence.
   Having now explained the two  peculiarities which  distinguish  aeriform
 from other fluids, viz., their lightness and  extensive elasticity, we proceed
 to show that they have the four other properties already described under hy-
 drostatics, as belonging to fluids generally:  and first,


   "Prcssurein all directions."  (Read the Analysis at pages 140 and 172.)

   A quantity of air or gas shut up  in any vessel  and compressed, is equally
 affected throughout, and its tendency to escape from the pressure is equal in
 all directions, as is proved by the force necessary to keep similar valves close
 wherever placed.   Hence the hydrostatic press and hydrostatic bellows de-
 sribed in the last section, which depend  for their action on this law, may be
 worked by air or gas as by a liquid.
   Owing to this law, air, when allowed, will always rush from where there
 is more pressure to  where there is  less.  The actions of the common  fire-
 bellows, and of the  animal chest in breathing, blowing,  sucking, Sic, are so
 many instances.
   The suddenness with  which any compression made on part of a confined
 aeriform fluid is communicated through  the  whole, is strikingly seen in the
 simultaneous increase or burst of all the gas-lights over an extensive building
 or even in a long street,  at any instant when the  force  supplying the gas is
 augmented.
   Many very interesting illustrations of the fluid  pressure  of air  being in
 all directions, will occur  under the  next head, joined with proofs of the at-
 mospheric pressure  being as the depth.

                        "Pressure as the depth."
   On first approaching this  subject, a person is naturally surprised to hear
 the depth or height of the atmosphere spoken of as something  perfectly as-
 certained, although no body can ever have approached the surface to  measure
 it; but science  often furnishes means of reaching precise  truth,  in cases
 where ignorance would not  even dream of the possibility of making an ap-
 proximation.  It may facilitate the apprehension of this point as regards air,
 to describe first some parallel cases  in which  water is concerned.
   The  bottom of a lake evidently supports  all the water in the lake, and
 each portion  bears just  die weight of the water directly over it:  a means
 then of ascertaining the weight or pressure of water on any portion of the
 bottom, would tell how  much' water stood  over that portion, and  by the
 known  relation  of the weight and bulk of water would tell also the depth
 at that part.   In like manner the ocean of air which surrounds the globe
 rests with its whole  weight  upon the surface  of the globe, and each portion
 of the  surface bears its  share: if  we ascertain,  then, the pressure of  the
 atmosphere on a given extent of the surface, we find how much air is stand-
 ing directly over it;  in other words, the  weight of  a  column of air resting
 on such surface as  its  base, and reaching  to  the  top  of the  atmosphere.
 Having then the weight  of  the whole column, and  finding  the weight of a
 given bulk of  it  at the  bottom (ascertained as  described at page 158,)  and
knowing the law of aerial elasticity (explained at page 158,) we  determine
the depth or height  of  the column by a simple calculation.   Now accurate
     12 
166
PNEUMATICS.
experiments show that  there  are  nearly  fifteen  pounds  of air over every
square inch of the earth's surface; producing the same pressure as would
be made by a depth of water of thirty-four feet, or by a depth of quicksilver
of thirty inches ;  and from this fact and the ascertained lightness and elasti-
city  of air, we know  that its depth on  earth must be nearly fifty miles,
which, as already stated, is about as much in relation to the size of the earth
as a  tenth of an  inch is to a globe of one  foot in  diameter.  The remaining
part  of this section has chiefly to  trace  the  effects of this  mass of matter
resting upon the earth's  surface, and as a fluid embracing and compressing
every object placed there.
   Water is a substance  much more obvious to the  human senses than air,
and which is constantly under observation; yet many of its  most important
agencies escape the notice of common observers.  Few persons, for instance,
of themselves discover the law explained in the last  section, of the pressure
in water being proportioned to the  depth: but when made to  observe that a
piece of cork plunged deep into it  is compressed  to  much  smaller bulk, and
that strong empty vessels of glass, or even of metal  under the same circum-
stances, are crushed or  broken inwards, and that pieces of sunken wood
are,  at great depths, filled with water  through all their  pores, so as to be-
come nearly as heavy as stone, Sic, their  minds are  roused to a sense of
the important fact that a fluid presses, and in proportion to its  depth.  If the
truths of hydrostatics thus escape notice,  we need not wonder that those of
pneumatics escape still longer.
   If a piece of bladder-skin or a pane  of glass  be  laid at the bottom of a
vessel, holding water, the bladder or glass  exhibits no sign of being pressed
upon, although it  bears on its upper side the whole  weight of the water
directly  above  it: the reason being that  water beneath the bladder resists
just  as strongly as the water above presses, in the same way that  one  stone
in a pillar resists those above  it:  but if the bladder be  tied closely over the
mouth of a common drinking glass or tumbler filled with water, and placed
at the bottom  of the  vessel, and if then, by means of a syringe or  pump,
the water be extracted from within  the glass, the bladder  itself has to bear
the  whole pressure of the water  above it, (independently of a pressure of
air to be explained afterwards,) and will probably be torn or burst.  The de-
gree of pressure, and consequently the depth of the water, in such a case,
might be ascertained by placing some support, of which  the action could be
measured, under the bladder to sustain it after the removal  of the interior
water.—Now this  case may be closely copied in  our  atmosphere or sea of
air.   A glass held in the hand is immersed in the fluid air, and is  full of it
as the other glass was supposed full of water: its  mouth may be covered over
with bladder, and no  external  pressure will be apparent, because there is a
resistance  of the air within, just equal to  the pressure of  the  air on the out-
side:—but if the air be extracted  from under the covering by means of an
air-pump,  the bladder is first seen sinking down  and becoming hollow from
the weight of the air over it, and at last bursting inwards  with a great noise
or crack.  By placing a circular piece of wood  under the bladder-skin, for
it to rest on, and a spring of known force to support the wood, we may as-
certain very nearly the weight and pressure of the air over it.  This mode,
however, of ascertaining the weight of the atmosphere, is not that commonly
used, but is described here as a good illustration  of  the present subject; the
problem being solved much more  elegantly and  accurately by means of the
barometer described farther on.   The  phenomenon  of  atmospheric pressure 
ATMOSPHERIC PRESSURE  ON SOLIDS.
167
is often exhibited by placing the hand on the mouth of a glass so as to cover
it closely, and then extracting the air from underneath the hand : the weight
of the  atmosphere holds the hand down upon the mouth of the glass with a
force of fifteen pounds to the inch.
  As should follow, from the pressure of fifteen pounds per inch thus detected
at  the  surface of the earth, being the weight of our superincumbent atmo-
sphere, we find that exactly as we rise from the earth, and leave part of the
atmosphere beneath  us, the pressure  diminishes.  This fact now furnishes
the readiest means of ascertaining the  height of mountains  and  of balloon
ascents, as will be explained in considering the barometer.
  After the many explanations here given of fluid pressure being equal in all
directions, it is almost superfluous to  remark, that the downward  weight of
the atmosphere becomes a pressure in all directions.  This is  seen in the fact
of the  bladder described  above, being as readily burst if turned sideways as
if turned directly upwards.  Every body or substance, therefore, on the surface
of the  earth, dead or living,  solid or fluid, is compressed with  this force.
In general the pressure on one side of a body is just balanced by the equal
pressure  on the other, so that no sensible effect  follows ; and it is on this
account that philosophers were so long in discovering it at all, and  that half-
informed persons are still  disposed  to doubt its  existence; but the proofs
offered on all sides to the now awakened attention are irresistible.   We shall
speak  first of

                    " Atmospheric pressure on solids."
   The atmosphere, then, presses on the two sides of a plate of glass or metal,
with force of fifteen pounds on the inch.   Under ordinary circumstances, no
sensible effect follows, because the opposite pressures counterbalance; but if
two plates of smooth glass or metal be laid  against each other, and the air be
prevented from entering between them, they cannot be separated by less force
than fifteen pounds per inch of their surface.
   In like manner, to draw down the piston pf a syringe from the bottom of
its  barrel, while no air is allowed to enter  between  them, requires force of
fifteen  pounds to  the square inch of surface of the piston.  But if the experi-
ment be made in  the exhausted receiver of an air-pump, the  piston falls by
its  own weight.  It  is pushed back  immediately on  re-admitting the air.
Wherever a vacuum  is produced at the surface of the earth, there is an exter-
nal pressure, of the force stated, seeking admittance all round.
   An  air-pump receiver of five  inches diameter  has  nearly twenty square
inches of surface in  its upper part or roof, and bears a weight or pressure of
atmosphere, of twenty times fifteen, or three hundred pounds.  While it has
air within it, this pressure is exactly balanced, and is not sensible ; but when
exhaused on the plate of the air-pump, it is  pressed  against the plate with
this force.  As the atmospheric pressure is in all directions,  the pump-plate
of course is equally pressed upwards against the receiver, and  the sides of the
receiver are pressed  towards each other.   This explains why air-pump re-
ceivers must be made arched or of dome-shape to withstand the great pressure.
A  flat piece of glass  of great thickness, laid upon the upper  mouth of a re-
ceiver, so as to form  an air-tight cover to it, is broken instantly by exhausting
the air beneath;  and a bottle  or receiver with flat sides,  when  exhausted,
suffers in the same manner.
   Illustrative of this  pressure on solids there is the experiment of the Magde-
burgh  hemispheres,  as it is called.   Two hollow half globes of metal a and 
168
PNEUMATICS.
              b, are fitted  to  each  other, so that their lips when touching
              may be air-tight.  While there is air between them or within,
              resisting the pressure of the outward air, they can be separated
              from each  other without difficulty; but when the air is ex-
              hausted from within  by the  air-pump, a force is  required to
              separate them of as  many times fifteen pounds as there are
              square inches in the  area of the mouth. The air is extracted
              by unscrewing  one of the handles at  b, and  then  connecting
              the  remaining  stalk  (which  is hollow, and  has a stop-cock)
              with the air-pump.—This experiment merits recollection, be-
              cause it was one of the  first which drew attention  to the ma-
              terial nature and  properties of the air; and it astonished the
world.   Otto Guericke,  Burgomaster of  Magdeburgh, the  inventor, had
hemispheres  made of three feet in diameter, and  once when he exhausted
them, on  the  occasion of  a  public exhibition, twenty coach-horses  of the
emperor were unable to pull them asunder.   There being no air-pump when
Guericke beo-an his experiments, although he himself invented it afterwards,
he originally°emptied the  balls of their air  by  first  filling  them with water,
and then extracting the water  by a  common  pump or syringe applied at the
bottom.             '
  It is a phenomenon of the  same kind as the last  described, when  a boy
with his foot presses  a  circular piece of wet leather as a, against a flat-faced
stone as b, and then lifts  the stone by pulling at a cord c, rising from the
centre of the leather.   If  the  leather be so close in its texture that air cannot
pass through  it, and stiff enough not  to be  puckered or  drawn  together, he
must exert  a force  before  detaching  it, of as  many times  fifteen pounds as
                     there are square inches  of surface covered by  it, for
      Fig. 93.         such is  the weight or  pressure of the air over it, while
                     there is  no counterbalancing  pressure underneath nearer
                     than on the  other side of the stone.   The weight of
                     the  stone that may be lifted is thus determined by the
                     size of  the leather.   The contrivance has been called
                     a sucker, or  pneumatic tractor.  A very  large sucker
                     applied  upon  a  rock  or  wall, would resist the pull of
                     horses like the Magdeburgh hemispheres.
                       This contrivance  seems  suited  to some purposes of
                     surgery.  It  might assist, for  instance, in raising de-
                     pressed portions of a fractured skull, and might thus
                     sometimes save  the operation  of trepanning:—for such
                     a purpose it  would be preferable to the small cupping-
                     glass sometimes used, from  its being perfectly inactive,
                     except during the instants when pulled at; whereas the
                     cupping-glass, by keeping up a continued flow of blood
 to the part, might do injury.  There is another surgical application  spoken
 of  in the last section of the present part, which the professional reader may
 consult immediately.
   It is  from having feet  that act  on  the  principle  of the  tractor, that the
 common fly and other insects ran move along ceilings, and even  polished
 surfaces of glass or metal with their bodies  hanging downwards;  and there
 are many marine animals which attach themselves to rocks, or other  objects
 by a similar action.
   If two pneumatic tractors be applied to each other, men  pullino- opposite
 ways, to separate them, must act with a force  of fifteen pounds  to the square
 
ATMOSPHERIC PRESSURE ON LIQUIDS.
169
inch of the surface of contact, as if they were separating the  Magdeburgh
hemispheres.
  The case of the pneumatic  tractor may be well illustrated  by an experi-
ment made in a vessel containing a liquid. * If a body with a flat surface be
applied to the bottom of the vessel so as perfectly to exclude the  liquid, the
body bears the whole weight of liquid directly over it,  and cannot be detached
without force equal to this.  The case is striking when a flat piece of cork is
pushed against the smooth bottom or side of a vessel containing mercury, and is
found not to rise again when the hand is  withdrawn from it, but to be firmly
held down by  the weight of the mercury.   We have to remark that in such
experiments made in vessels open to the air, the weight of the atmosphere
on the liquids adds a pressure of fifteen pounds on every inch of  the surface
of a body immersed in it.

                   "Atmospheric  pressure on liquids."
  The  pressure  of  the  atmosphere on  liquids produces many important
effects, and  now  that we comprehend  them, we wonder  that they should
have been so long misunderstood.   We have familiar examples  of it in the
working of pumps  and syphons.   All  such phenomena, in former times,
were referred to what was called nature's horror of a vacuum, or to an ob-
scurely imagined principle of suction.   It was not until the time of Galileo
that their true nature began to be detected.   The discovery has led to many
very important results in the arts.
   Persons may at first have a difficulty in conceiving that a fluid  so rare and
subtle as air should affect or resist a dense liquid  like water:  but the action
or resistance of air  in  contact with water is familiarly shown in the facts,
that a glass does not become full of water when plunged, with its open mouth
downwards, from the air into water; and that when a tube, open  at  both
ends, has been  partially immersed in water, and, therefore, partially filled, the
water can be forced out of it by blowing air in at the upper end, to  return only
when  the blowing  ceases.  Then it may be recollected that  a  hundred
pounds of feathers are as great a load as a hundred pounds of  lead.
   That  there  are fifteen  pounds  of  air  above every square inch of the
earth's surface, is confirmed by the effects above described of the atmospheric
pressure on solids;  and we  now proceed to  show that many  of  the pheno-
mena among  liquids, which long  appeared  so mysterious, are  merely the
necessary consequences of the same pressure upon them.   It will  facilitate
the  comprehension of these  effects, if we first view  them  as they may be
produced by more visible agents, viz., by one liquid  pressing upon  another;
and for this purpose the author has contrived  the apparatus represented in the
next page in which alayer of oil rests upon alayer of water, or upon a layer of
mercury.
   It has already been shown, that  an  ocean of oU, spread over the earth, to
have the same  weight as our atmosphere, requires to be  about thirty-seven
feet deep. A vessel, then, a b c, with water in it up to the level W, and with
thirty-seven  feet of oil above this, up to the level O, is fitted to  illustrate
many of  the phenomena of atmospheric pressure on liquids.  The follow-
ing are the seven principal cases.
   1st. The weight of the oil pressing with a force of fifteen lbs.  per inch on
the  water at W,  would not at all  disturb the level surface  of the  water.—
Neither  does  the weight  of the atmosphere of fifteen  lbs.  per inch disturb
any liquid surface. 
170
PNEUMATICS.
                                 2d. If the oil were gradually poured into
           Fig. 94.              the vessel a b  c, over the water, the  water
                               would  rise in  the  tube i w, as already ex-
                               plained by the  figure at page 143; so that
                               when there were thirty-seven feet in height,
                               or fifteen pounds  in weight of oil on  the
                         O    inch, the water in i w would stand thirty-
                               four feet above its level in the large vessel.
                               If these thirty-four  feet of water were then
                               lifted out of the  tube by a plug or piston
                               drawn up from the bottom of it at i, a second
                               equal quantity  would  be  pressed up by the
                               oil, to be removed, if desired, in the same
                               way  as  the first and the tube  and piston
                               would constitute a  pump.—Now when the
                               atmosphere, instead  of  the  oil, is  allowed
                               to press upon  the water surface in such a
                               vessel,  but is  excluded from  the  tube,  the
                           f    water rises in  the  tube  thirty-four feet, as
                               in the last case; and if this quantity be lifted
                         M    ou' °^ tne ttme ^y a piston> a second  equal
                               quantity  is pressed  up,  and the tube and
                               piston  become a complete  example of  the
                               common lifting or sucking-pump.  We have
to describe it more particularly hereafter.
   3d.  If there were a quantity of mercury or  of quicksilver at  the bottom
of the vessel a b c, filling it up to the level' M,  and if a tube i m issued from
under this level,  the  mercury  pressed  upon  by  thirty-seven  feet  of  oil,
would rise in this short tube as  the water did  in the  larger; but by reason
of its greater specific gravity, it would only reach a height of thirty inches
above its level, the  water  having stood at  thirty-four  feet. — Now  thirty
inches of mercury is the height of column which the atmospheric pressure,
acting in the same way, really produces, as is seen in a similar apparatus made
expressly for measuring that pressure, and called a barometer or measure of
iveight.
   4th. If a tube d, of an inch square, and open at both ends, were  plunged
into the oil, it would  of course  always be full  up to the level of the oil on
the outside of it; and if  it  were  pushed low enough  to touch the water at
W, it would just contain fifteen pounds of oil resting on  an  inch square of
the water-surface at its mouth ; which surface  would  therefore be bearing a
weight of fifteen pounds like  every inch of the surface around, but would
not yield, owing  to the force with which it tended upwards  to escape from .
the pressure corresponding to its depth in the  oil.   Then if the tube were
pushed a little farther down, and if, by a piston or  plug in it, the fifteen
pounds of oil were lifted out of it, water would rise into it until enough  had
entered to reproduce the pressure of fifteen pounds on the surface below as
before ; that is to say, the water would rise thirty-four feet, as in the exter-
nal tube w i.    This internal tube and piston again would form  a pump.—In
like manner, when a tube open  at both  ends  is plunged from  the  air into
water, the air  presses on the surface of the water within the tube, as on the
surface around it, with a force of fifteen pounds to the inch, and the two sur-
faces are not affected by  the equal pressures; but if, by a piston, we lift the
 
ATMOSPHERIC PRESSURE ON LIQUIDS.
171
air out of the tube, as we suppose the oil lifted in the last experiment, the
water will then rise, following the piston to  the  altitude of thirty-four feet.
This arrangement of parts is the most  usual for the lifting or household
pump.
   5th. If a common bottle or vessel of any other shape, as the bent  tube e,
were filled with water, and placed under the  oil  with its mouth or mouths
reaching  below the  water surface at the level W, it would remain full of
water, owing to the  pressure of the oil surrounding it.—For a similar rea-
son, any such vessel or tube, surrounded  only by the air, when filled with
water, and placed with its mouth or mouths under the surface of water, re-
mains full; and if such a bent  tube has one of its  ends in  another  vessel
lower than the first, a current is established  in it;—the contrivance  being
then called a syphon.
   6th. A fish in the water below the level W, would be bearing the pressure
of the oil from O to W, as well as the pressure of the water.—So a fish in
water open to the air, is bearing the atmospheric  pressure of fifteen pounds
per inch,  in addition to that of the water  itself.   This is  proved by extract-
ing the air from over water in which a fish is swimming : for then the air-bag
of the fish, situated  near its  under side, as already described, immediately
dilates and turns the fish upon its back.
   7th. To separate the Magdeburgh hemispheres, or to produce a vacuum
in any way, under the water level W, would require force proportionate to the
weight of oil above, in addition to that required  on account of the water;—
and to separate the Magdeburgh hemispheres under any water-surface pressed
upon by the atmosphere, a force is  required of fifteen pounds per inch be-
yond what would balance the effect of the water itself.
Fig. 95.
The following remarks illustrate more minutely some of the objects which
   we have just been explaining.
   The common  lifting-pump (or sucking-pump as it used to be called,) is
then merely a barrel a  b, with a close-fitting movable plug or piston in it c.
When the lower end b, is plunged into water, and the piston is drawn up
from the bottom, the atmosphere being prevented from press-
ing on the surface of the water within  the tube, the pressure
on the surface external to the tube, drives the water up after
the piston.   That  the water  which thus  rises may not fall
again, there is a valve or flap at the lower part of the pump-
barrel b, which opens only to water passing upwards;  and
that the piston may be allowed to pass downwards through
the water in the  barrel, to repeat its stroke, there is in  it a
similar  valve.   The  piston, in  rising during a second or
succeeding stroke,  causes all the water above it to  run over
at the spout d.—Formerly a lifting-pump was said  to act by
sucking the water  up  from the well beneath it;  the  true
meaning of which phrase we now  perceive to  be, that the
piston merely lifts  or holds off the air which was  pressing
on the water within the  barrel, and allows the  water to rise
there in obedience to the external pressure of the air around.
The reason is apparent, then, why, in the lifting-pump, the
water will only follow the piston to a certain elevation, viz.,
until its weight balances  the external pressure  of the atmo-
sphere.
 
172
PNEUMATICS.
Fig. 96.
                        When the piston of a pump is solid, or without a
                      valve, as at c, the machine is called a forcing-pump.
                      The water rises  beneath the  piston, as  already ex-
                      plained for the  lifting-pump, but then,  as it cannot
                      pass through the descending piston, as in the lifting-
                      pump, it is forced into any other desired direction, as
                      to  d.  A  forcing-pump  can bring water from  only
               nb      thirty-four feet below the piston, but can send it to
            J -^      any elevation.   In forcing-pumps, it is usual to make
                      the  water enter  an air-vessel d a (already explained
                      at page  161,) from which  it is  again urged by the
                      elastic air, through the pipe b,  in a  nearly uniform
                      stream.
                        The animal action of sucking is an approximation
                      to what  we have described in the lifting-pump.  The
                      difference is, that the chest or  mouth can  make only
                      a partial vacuum, and therefore cannot raise a liquid
                      very far.
                        A syphon remains  full of liquid, although partially
raised above the general surface of the liquid, as explained above.   For com-
mon purposes, a syphon  is made of the  form here represented, viz., a bent
tube c  b a, with one end longer  than the other.   To use it, the end c is first
immersed in liquid, and the end a being  then stopped for the  time by the
finger or a cock, the air is extracted by the mouth or otherwise,  through the
small tube a d, and the atmosphere immediately fills the whole tube  with
liquid from c.   If the instrument be then left to act, the liquid will run from
the longer leg, because a long column of liquid overbalances a  short one,
until the shorter has drunk up all within  its reach.   Whether  the  external
                                extremity  be in the air only, or immersed
                                in liquid, makes no difference, except that
                                the immersion shortens so much  the de-
                                scending column.  If both extremities  be
                                immersed in liquid, and in different vessels,
                                by alternately lifting  one  vessel  or  the
                                other, the  liquid will be made  to pass and
                                repass, and will come to rest in the syphon
                                only when the surfaces in the  two vessels
                                are at the same level.   Thus the same leg
                                becomes alternately the long and the short
                                leg,  according to  the height of the liquid
                                in which  it  is immersed.  A  syphon is
                                sometimes made with  both  legs equal and
                                turned up, as here represented, so that it
                                remains  full of liquid although lifted away
                                from  the vessel, and  therefore is  always
                                ready for action.   As it is the same cause
which lifts the  water in a pump and in a syphon, the top of  a syphon must
evidently be within thirty-two feet of the water-surface below.  In the syphon,
as the cases of balancing liquids, described at page  131 (which see.) the
comparative diameters of  the legs  are of no importance, nor their oblique
length, provided the perpendicular heights of the two columns have the neces-
sary relation:—even  an inverted tea-pot may be used as a  syphon.  This
truth is well exemplified in what may be called the syphon-paradox, an exact
 
PRESSURE ON LIQUIDS—SYPHON.
173
counterpart of  the  paradox of  the  " Hydrostatic Bellows,"
already explained.   If the apparatus of the  bellows  be filled    Fig. 98.
with water in the ordinary way (see page 130,) and be then re-      ^—>.
versed or turned so that the tube becomes like the long leg of a      !(\
syphon, the little stream  of water issuing from it at a will lift
as great a weight suspended/rora  the board d, as the same slen-
der column in the standing position can lift upon the board.  As
farther illustrative of the atmospherical pressure exerted in pro-
ducing this effect, and in rendering a  syphon active,  we may
advert to the  striking fact, that  a  long small  tube of  water
screwed  into the side or bottom  of a close cask of water so
as to communicate with  it, and then allowed to discharge like   V~/   V~9
the long leg of a syphon, will  cause  the cask to  be  crushed
inwards, just as the same tube screwed into the top of the cask, as represented
at page 131, causes the cask to burst outwards.
   The syphon is very useful for  drawing off liquids,  where there is a sedi-
ment that should not be  disturbed, or  where  it is desirable not to make an
opening in the lower part of the vessel.  A large syphon would empty a lake
or mill-pond over its bank without injuring the bank.  To fill a large syphon
that it may act the most convenient way is, instead of pumping out the air
from it, to close the two  ends for  the time, and to pour in water through  a
cock at the top.
   There is a  pretty syphon-toy,  called a Tantalus-cup, having in  it  a stand-
ing human figure which conceals  a syphon. The short branch of the syphon
rises in  one  leg of the  figure to  reach the level of the  chin, and the long
branch descends in the other leg  to pierce the bottom of the cup towards  a
reservoir below.  On pouring water into the  cup, the syphon  begins  to act
as  soon  as the water reaches the chin of the figure, and  the cup is then
emptied as if by magic.
   Among the  infinitely varied water-drains or courses in the bowels of the
earth, some are syphons, and produce  what are called intermitting wells or
fountains.  These may alternately run and cease for longer or shorter  periods,
according to  the comparative magnitudes of the collecting reservoir and the
drain.   The reservoir may be  an internal cave of a  mountain, receiving  a
regular supply of water by a slow filtering  of moisture from above,  and the
drain is a syphon-formed  channel,  which, like that of the Tantalus-cup, begins
to act only when the water in the reservoir has reached  the level of  the  top
of the syphon,  and then carries off the water faster than it  is supplied.
There are some fountains that flow  constantly, but at  regular intervals have
a remarkable increase.  In them a common spring is joined with a syphon-
spring.
  The author has suggested an application of the syphon, which obviates  a
strong objection to  the high  operation for  stone, as explained  in the next
medical section.

The following  facts have close relation to  those  now explained, as farther
  illustrative of atmospheric pressure on liquids.

  A long glass of jelly, if inverted and placed with its mouth just  under the
surface of warm water, will soon  be found to have lost the jelly, but to  be
full of water in  its stead.   The jelly' is heavier than water, and when melted
by the heat sinks down, and is replaced by water from below, sent up by the
atmospheric pressure. 
174
PNEUMATICS.
  The slaves in the West Indies steal rum, by inserting the long neck of a
bottle full of water through the  top aperture of the rum-cask.  The water
falls out of the  bottle into the cask, while the lighter rum ascends in its stead.
  The common water-glass for bird-cages has its only opening near the bot-
                       torn through the neck  b;  yet no water can escape
       Fig. 99.          from it  but when the level of the water at c, in the
           o           open part, becomes low enough for some air to pass
                       into the body of the glass  by the channel b.  When
                       a bubble of air does pass in, an equal bulk of water
                       comes out, and by raising the water level in c, pre-
                       vents the passage of more.—An ink-glass made on
                       this principle preserves the  ink well, because there
                       is so small a surface exposed to the air ; if made too
               ___      large, however, the accidental  expansion of the air
                ~JjC    in it by heat may cause it to overflow.
                          In the  common Argand or fountain-lamp, the
provision of oil is in a vessel like an inverted bottle, higher than the flame,
and with its mouth  immersed in  a small reservoir of oil, nearly on a level
with the  flame,  then no oil can escape from above, but as the flame consumes
the  free oil from the small reservoir, which supply is  thus maintained always
at the same elevation.—In the Hiero's fountain-lamp, mentioned atpage 164,
that the two balancing columns of oil may be always of the same height, the
oil is supplied to  them from high reservoirs, with  the mouths dipping into
them  as  above  described, and keeping their tops, therefore, always at the
same level;  and that the descending column may not be  shortened by the
rising of  the oil in the low  reservoir c, the tube containing  it is turned up at
the  bottom like an end of the " ever ready syphon,"  and discharges near the
top of c.

We have hitherto been contemplating only the  direct weight  or downward
  pressure of the atmosphere on liquids: in the following instances we have
  proof of the same pressure acting upon them in all directions.

  If a bottle or cask be filled with  liquid, and  closely corked, and if a small1
hole be then drilled in the  bottom or side, the liquid will not escape by it,
because  of the  resisting pressure of the atmosphere, and of their not being
room  in the opening for a current of air to  enter while  the current  of water
escapes:  but if a second hole  be drilled  in the  top, a jet from the lower
opening will follow  immediately, because then the air will press on the upper
surface of the liquid as well  as  on the lower,  and the  weight of the liquid
will be free to  act:—thus, a cask  of beer  or  wine cannot be emptied by a
cock near the bottom, unless what  is called a vent-hole be  made at  the top.
If the lower opening, however,  in any case be so large,  that the  air may
enter  by  one side of it, while the liquid is escaping by the other, the vessel
may be emptied by it, as is seen in decanting a bottle of  wine.  In such a
case, it is the interrupted entrance of the air which causes that gurgling sound
so delightful to  the ear of the drunkard, instead of allowing the smootlfstream
which falls from a funnel.
  Even  a large opening at  the bottom of a vessel which is close above, may
be  prevented,  by the pressure  of the air,  from discharging liquid,  if any
mutual passing of the two currents of air and liquid be rendered difficult.  An
inverted  bottle of water will not  discharge, if a piece of paper be simply ap-
plied against its mouth.   Even a wineglass filled with water may be inverted, 
ATMOSPHERIC PRESSURE--BAROMETERS.
175
and yet will spill none, if the piece of paper, laid loosely upon its mouth,
be held to it during the turning,—the pressure of the atmosphere against the
paper keeping it in its place, and supporting the water above it.   Any vessel
or tube of water of less height than thirty-four feet may be kept closed at the
bottom in  the same way.

The animal body is made up of  solids and  fluids, and is affected  by the
                     atmospheric pressure accordingly.

   There  is a difficulty at first in believing that a  man's body should  be
bearing a pressure of fifteen pounds on every square inch of its surface, while
he remains altogether insensible of it; but such is the fact, and the reason of
his not feeling the fluid pressure  is its being  perfectly uniform  all  around.
If a pressure of the same kind be even many times greater, such, for instance,
as fishes bear in deep water, or as a man supports in the diving-bell, it equally
passes unnoticed.  Fishes are at  their ease in a depth of water where the
pressure  around will instantly break or burst  inwards almost the strongest
empty vessel that  can be sent down; and  men  walk on  earth without dis-
covering a heavy atmosphere  about them, which, however, instantly  crushes
together the sides of  a square glass bottle emptied by the air-pump, or even
of a thick iron boiler, left for a moment by any accident, without the coun-
teracting internal support of steam or air.
   The fluid pressure on animal bodies, thus unperceived under ordinary
circumstances,  may be rendered instantly sensible by a little artificial arrange-
ment.  In water, an  open tube partially immersed becomes full to the level
of the water around it, and the water contained in it is supported, as already
explained, by that  which  is immediately below its  mouth: now a flat fish
resting closely against the mouth of the tube, would evidently be bearing  on
its back the  whole of this weight, perhaps one hundred pounds; but the fish
would not thereby be pushed away, nor would it even feel its burden, because
the upward  pressure  of the water immediately under it would just counter-
balance the weight,  while the lateral  pressure around  would  prevent any
crushing effect of the upward and downward forces.  But  if, while the fish
continued  in the situation supposed, the  hundred pounds of water were sud-
denly lifted  from off its back  by a piston in the tube, the opposite  upward
pressure of one hundred pounds would at once crush its body into the tube.
At a less depth, or  with a smaller tube, the effect might not be fatal, but there
would  be a bulging or swelling of the substance of the fish  into the mouth of
the tube.—In  air  and on the  human  body  a perfectly analogous  case is
exhibited.  A man without pain or any peculiar sensation, applies his  hand
closely to  the mouth or  opening of a tube, or of any vessel containing air,
but the instant that the air is withdrawn from within the tube  or vessel, the
then unresisted  pressure of the external air  fixes the hand upon the opening,
causes the flesh to swell or bulge into it, and makes the blood  ooze from any
crack or puncture in  the skin.
  These  last lines describe closely the surgical operation  of cupping; the
essential circumstances of which are, the application of a cup or glass, with a
smooth blunt lip, to the skin of any part of the body, and the extraction by a
6yringe or other means,  of a portion of the air from  within  the cup.  To
some minds  the exact comprehension of this phenomenon may be facilitated,
by considering the case of a small bladder or bag of India-rubber full of any
fluid and pressed between the hands on every part of its surface except one :—
at that one part it would  swell, and even burst if the  pressure  were strong 
176
PNEUMATICS.
enough.   So in cupping, the whole body, except the surface under the cup,
is squeezed by the atmosphere, with a force of fifteen pounds to  the square
inch, while in that one situation  the pressure is diminished according to the
degree of exhaustion in the cup, and the blood consequently accumulates
there.   The application of a cup with exhaustion only, constitutes the opera-
tion called dry-cupping.   To obtain blood, the cup is removed and the tumid
part  is cut into by the simultaneous stroke of a number of united lancets: and
the cup is then applied again as before and exhausted, so that the  blood may
rush forth under the diminished pressure.
  The partial vacuum in the cup may be produced  either by the action of a
syringe, or by burning a little spirit  in the cup  and applying it while the
momentary dilatation effected by the heat has driven out the greater part of the
air.   The  human mouth  applied upon any part becomes a small cupping
apparatus, and formerly, in  cases of  poisoned  wounds,  was used  as such.
Our  present perfect cupping-glasses, of stronger and more permanent opera-
tion, are not yet  always used, as they might  be, to assist in removing the
poison after the bites of rabid or venomous animals.
  The author has suggested an extension and modification of the operation
of the dry-cupping, which he believes will prove an  important remedy in the
hands of the medical practitioner.  It is intended  as a substitute for bleeding
in certain cases where blood can ill be  spared, and as a more sudden  and
effectual  check than  even bleeding itself, in  certain cases of inflammatory
disease.  It is explained in the next medical section of this work.
  The atmospheric pressure on living bodies produces an effect which is rarely
thought of, although of much importance, viz., keeping  all the parts about
the joints, firmly  together, by an action similar to that exerted on the Magde-
burgh  hemispheres.   The broad surfaces of bone forming the knee-joint, for
instance, even if not held together by  ligaments, could not, while the capsule
surrounding the joint remained air-tight, be separated by  a force of less than
about a hundred pounds; but on  air being admitted  to the articular cavity, the
bones at once fall to a certain distance apart.  In the loose joint of the shoulder,
this support is of  great consequence.   When the shoulder or other joint is dis-
located, there  is no empty space  left, as might be supposed, but the soft parts
around are pressed in, to fill  up the natural place of the bone.   When a thigh
bone is dislocated, the deep socket called the  acetabulum instantly becomes
like a cupping-glass,  and is  filled partly with  fluid  and  partly with the soft
solids.   In all joints, it is  the atmospheric pressure which keeps the bones in
such steady contact,  that they work smoothly  and without noise.
   The barometer, we have seen  at page 170, is a column of fluid  supported
in a  lube by the  pressure of the atmosphere, and  therefore  indicating most
exactly the degree of that  pressure.   It is an instrument now of such import-
ance, both in a scientific point of view and in  the business of common life,
that  for the sake of minds which conceive such  subjects with difficulty we
shall add here the two following farther illustrations of its nature.
  If mercury be  poured into a bent tube open at  both ends, it  will stand at
the same level in the two  legs, as at a and b, and the air  will be pressing on
the two surfaces at a and b with equal force of  15 lbs. per square inch.  If the
air be then removed from one leg a, by a piston or otherwise, while  it continues
to press in the other leg b, the mercury will be pushed down in  b, until the
growing height of the column in a produces  a weight so much greater than
that in b, as just  to  counteract the  pressure:  now  this balance takes place,
in fact, when the  mercury in a stands about thirty inches higher than in b ; that
being the height  of a column of mercury weighing 15 lbs. on the square inch. 
ATMOSPHERIC PRESSURE—BAROMETERS.
177
Fi«j. 101.
If the top of the tube a were then closed permanently, the mer-
cury would for ever remain elevated in it, marking most perfectly    Fig. 100.
the atmospheric pressure ; now this construction, only with the
empty and useless part of the tube above d  cut  off or wanting,
forms a common barometer.  The exact altitude of the mercury
in it is  known by observing how  much the surface near c  is
higher than that near d.  Often, in such a barometer, a little mass
of metal is placed to float on the meicurial surface at d, and as
it  rises  and falls, is caused, by a thread passing from it over a
wheel or pulley, to move an index like  the  hand of a clock
connected with the wheel, and this index tells the degree of ele-        \\cl
vation.   This modification is called the wheel barometer
   Again, as water at a, in the bottom of a closed pump-barrel, if
pressed  upon by the piston b c,  of which the rod d were hollow
or tubular, would rise in the rod to a height proportioned to the pressure made
by the piston, so, in a straight exhausted barometer-tube, which is as this hol-
low  piston-rod, the mercury or water rises, because the atmospheric  pres-
sure around it is as the  piston  forcing the fluid up.   To make a barometer
of this kind it is Only necessary  to procure a glass tube more than thirty inches
long, and close at one end, and then having filled it with
mercury, to  plunge  its mouth  (stopped  by the finger
while turning) into a small cup  or basin of mercury ;—
the  fluid falls away a little from the  top of the tube,
leaving  a vacuum there, and  stands  at the  elevation
which  the atmospheric pressure is fitted to maintain.
We know from the law of hydrostatics already explained,
that it is of no importance, in such a case, what the shape,
or inclination, or  size of  the tube may be, as only the
perpendicular height can measure or be measured by the
pressure.  This fact enables us to construct barometers
with the upper part of the tube bent obliquely, so that
for one inch rise of mercury in a perpendicular tube, there
shall be an advance of several inches in the oblique top, rendering any change
of elevation so much more apparent.
   Galileo had found that water would rise under the piston of a  pump to a
height only of about  thirty-four feet.  His  pupil Torricelli, conceiving the
happy thought, that the weight of the atmosphere might be the cause of the
ascent,  concluded that mercury, which is  about thirteen  times heavier than
water, should only rise  under the same influence to a thirteenth of the eleva-
tion ;—he tried and found that this was ?o, and the mercurial  barometer was
invented.  Pascal then, to afford farther evidence that the weight of the atmo-
sphere was the cause of the phenomenon,  carried the tube of mercury to the
tops of  buildings and of mountains, and found that it fell always in exact pro-
portion  to the portion of the  atmosphere left below  it;—and he found that
water-pumps in  different situations varied as to sucking power, according to
the same law.
   It was soon afterwards  discovered, by careful observation of the mercurial
barometer, that even when remaining in  the same  place, it did not always
stand at the same elevation ; in other words, that the weight of the atmosphere
over any particular part of the earth was constantly fluctuating; a truth which,
without the barometer, could never have been suspected.  The observation
of the instrument being carried still farther, it was found, that in  serene dry
weather the mercury  generally  stood high, and that before and during storms
		ci
I)		
	CL	
 
178
PNEUMATICS.
and rain it fell:—the instrument, therefore, might serve as a prophet of the
weather, becoming a precious monitor to the husbandman or the sailor.
  The reasons why the barometer falls before  wind and rain will  be better
understood a few pages  hence; but we  may remark here, that when  water
which has been suspended in the atmosphere, and has formed  a part of it,
separates as rain, the weight and bulk of the mass are diminished:  and that
wind must occur when a sudden condensation of aeriform matter, in any situ-
ation, disturbs  the equilibrium of the air; for the air around will rush towards
the situation of diminished pressure.
  To the husbandman the barometer is of considerable use, by aiding and cor-
recting the prognostics of the weather which he draws from local signs familiar
to him ;  but its great use as  a weather-glass seems to be to the mariner who
roams over the whole ocean, and is often under skies and climates altogether
new to him.   The watchful captain of the present day, trusting to this extra-
ordinary monitor, is frequently  enabled to take in sail and to make  ready for
the storm, where, in former times, the dreadful visitation would have  fallen
upon him unprepared.—The marine barometer has  not yet been in general
use for many years, and the  author of this work was one of a numerous crew
who probably owed their preservation to its almost miraculous warning.   It
was in a southern latitude ;  the sun had just set with placid appearance, closing
a beautiful afternoon, and  the usual mirth of the evening watch was proceed-
ing, when the captain's order came to prepare with all haste for a storm.  The
barometer had begun to fall with appalling rapidity.  As yet, the oldest sailors
had not perceived even a threatening in  the sky, and were surprised at the
extent and hurry of the preparations: but the required measures  were not
completed, when a more awful hurricance burst upon them  than  the most
experienced had ever braved.   Nothing could  withstand it; the sails already
furled and closely bound to the  yards, were riven away in tatters:  even the
yards and masts themselves  were in great part disabled; and at one time the
whole had nearly fallen by the board.  Such, for a few hours, was the min-
gled roar of the hurricane  among the  rigging, of the waves around, and of the
incessant peals of thunder, that no human voice could be heard, and amidst
the general consternation, even the trumpet sounded in vain.  In that  awful
night, but for the little tube of mercury which had given the warning, neither
the strength of the noble ship, nor the skill and  energies of the commander,
could have availed any  thing, and not a man would  have escaped to tell the
tale.  On the following morning the  wind was again at rest, but the ship lay
upon the yet heaving waves, an unsightly wreck.
  The  marine barometer differs from that used on shore, in having its tube
contracted in  one place to a very narrow bore,  so as to prevent that sudden
rising and falling of the mercury which every  motion of the ship would else
occasion.
  Civilized Europe  is  now familiar with the barometer  and its  uses, and
therefore, that Europeans may conceive the first feelings  connected with it,
they almost require  to  witness the astonishment or incredulity with which
people of other countries  still regard it.  A Chinese once conversing on the
subject with  the  author, could  only imagine of the  barometer, that It was a
gift of miraculous nature,  which the  God of Christians gave them in pity, to
direct them in the long and perilous voyages which they undertook to un-
known seas.
  A  barometer is of great  use to persons  employed about those mines in
which hydrogen gas, or fire-damp, is  generated and exists in the  crevices.
When the atmosphere becomes unusually light, the hydrogen beino- relieved 
ATMOSPHERIC PRESSURE—BAROMETERS.
179
from a part of the pressure which ordinarily confines it to its holes and lurk-
ing-places,  expands or issues  forth to where it  may meet the lamp of the
miner, and explode to his destruction.  In heavy states of the  atmosphere,
on the contrary, it is pressed back to its hiding-places,  and the miner advances
with safety.
  We  see from this  that  any  reservoir or vessel containing air would itself
answer as a barometer if the only opening to  it were through  a  long tubular
neck, containing a close-sliding plug; for then,  according to the weight and
pressure of the  external  air, the density of that in the vessel would vary,  and
all changes would be marked by the position of the movable plug.  A baro-
meter has really been made on this principle,  by using a vessel of glass, with
a long slender neck,  in which a globule of mercury is the movable plug.
  The state of the atmosphere,  as to  weight, differs at different times  in the
same situation, so as to produce a range of about three inches  in the height
of the mercurial barometer;  that is to say, from twenty-eight  to  thirty-one
inches.  On the occasion of the  great  Lisbon  earthquake, however, the mer-
cury fell so far  in the barometers, even in  Britain, as to disappear from  that
portion at  the  top usually left uncovered  for  observation.—The uncovered
part of a barometer is commonly of five or  six inches in length, with a divided
scale attached to it, on which the figures 28, 29, &c, indicate the number of
inches from the surface of the mercury at the bottom to the respective divi-
sions :—on the lower part of the scale the  words wind and rain are generally
written, meaning that when the mercury sinks to them, wind and rain are to
be expected ; and on the upper part, dry and fine appear, for a corresponding
reason : but we have to  recollect, that  it is  not the absolute height of the mer-
cury which indicates the existing or coming weather, but the recent change
in its height: a falling barometer usually telling of  wind and rain ; a rising
one  of serene and dry weather.

The barometer answers  another  important purpose, besides that of a weather-
   glass—in enabling us  to ascertain  readily the height of mountains, or of
   any situation to  which it can be carried.

   As the mercurial column in the barometer is always an exact indication of
the weight  or pressure of air above its  level, being, indeed, as explained in the
foregoing paragraphs, of the same weight as a column of the air  of equal base
with itself, and reaching from  it to the top of the atmosphere —the mercury
must fall when the instrument is carried from any lower to any higher situa-
tion, and  the degree of falling must always tell exactly how much air has been
left below.  For instance, if thirty inches barometrical height mark the whole
atmospheric pressure at the surface of the ocean, and if the instrument be
found, when carried to some other situation, to stand  at only twenty inches,
it proves that one-third of the  atmosphere  exists below the level of the  new
situation.   If our atmospheric ocean were  of as uniform density all the  way
up as our watery oceans, a certain weight of air thus left behind in ascending
would mark everywhere a change of  level nearly equal, and the ascertaining
any  height by the barometer would become one  of the most simple of calcu-
lations : the air at  the surface of the earth being between eleven and twelve
thousand  times lighter  than  its bulk of mercury, an inch rise or fall of the
barometer  would mark everywhere a  rise  or fall in the atmosphere of  nearly
twelve thousand inches  or one thousand feet.   But  owing to the elasticity of
air,  which causes it  to  increase in volume as it escapes from  pressure, the
atmosphere is rarer in proportion as we ascend, so that to leave a given weight 
180                          PNEUMATICS.
 of it  behind, the ascent must be  greater,  the higher the situation where the
 experiment is made:  the rule, therefore, of one inch of mercury for a thou-
 sand feet, holds only for rough estimates near the surface of the earth.  The
 precise calculation, however, for  any case,  is still very easy ; and  a good
 barometer, with a thermometer attached, and with tables, or an  algebraical
 formula expressing all the influencing  circumstances, enables us to ascertain
 elevations much more  easily, and  in many cases more correctly, than by tri-
 gonometrical survey.
   The weight of the whole atmospherical ocean surrounding the earth being
 equal to  that of a watery ocean of thirty-four  feet deep, or of a covering of
 mercury of thirty inches, and the air found at the surface of the earth being
 828 times lighter than water, if the same density existed all the way  up, the
 atmosphere would  be 34 times 828 feet high, equal to about  five miles and
 a half.  On account of the  greater rarity, however, in  the superior regions,
 it really extends to a height of nearly fifty miles.   From the known  laws of
 aerial elasticity, explained at page  158, we can deduce what is found  to hold
 in fact, that one-half of all the air  constituting our atmosphere exists within
 three  miles and a half from the earth's surface ; that is to say, under the level
 of the summit of Mont Blanc.  A  petson unaccustomed to calculation, would
 suppose the air to  be more equally distributed through the fifty miles than
 this rule indicates, as  he might at first also suppose  a tube of two feet dia-
 meter to be  only twice as capacious as a tube of one foot, although in reality
 it is four times  as capacious.
   In carrying a barometer from the level of the Thames to  the  top of St.
 Paul's Church, in  London, or  of  Hampstead  Hill, the mercury  falls about
 half an inch, making an ascent  of  about  five hundred feet.    On  Mont
 Blanc it  falls to  half of the entire barometric height,  making  an elevation
'of fifteen  thousand feet; and in Du Lucts' famous balloon ascent it fell to
 below  twelve inches, indicating an  elevation of  twenty-one thousand  feet,
 the greatest to which man has ever ascended from the surface of his  earthly
 habitation.
   The extreme rarity of the air on high mountains must of course affect ani-
 mals.   A person breathing on the  summit of Mont Blanc, although expand-
 ing his chest as much as usual,  really takes in at each  inspiration only half
 as much air as he does below—exhibiting a contrast to a man in the  diving-
 bell, who, at thirty-four feet under water, is breathing air of double density, at
 sixty-eight feet  of triple, and so on.   It  is known  that travellers, and even
 their practised  guides, often  fall down suddenly as if struck  by lightning,
 when  approaching  lofty summits,  on account chiefly of the thinness  of the
 air which they are breathing, and some minutes elapse before they recover.
 In the elevated  plains of South  America, the inhabitants  have larger chests
 than the inhabitants of lower regions—furnishing another admirable instance
 of the animal frame adapting itself  to the circumstances in which it is placed.
 It appears from all  this, that although our atmosphere be fifty miles high, it
 is so thin beyond  three miles and a half, that mountain ridges of greater ele-
 vation are  nearly as effectual barriers  between nations of men, as islands or
 rocky  ridges in  the sea are between  the finny tribes inhabiting the opposite
 coasts.   The intense  cold  which  appertains to high situations, and forms
 another obstacle to human approach, remains to be considered in our next
 division.
   A barometer  connected with  an  air-pump, indicates exactly the progress
 and degree of exhaustion in the receiver.   When the mercury'falls to  half its
 height, it shows that half of the air  is extracted ;  and so for all other propor- 
ATMOSPHERIC PRESSURE--BAROMETERS.
181
102.

Cl
tions.   A barometer then is a necessary appendage to a complete air-pump ;
but as its chief purpose is to mark when the exhaustion is carried nearly  to
completion, a very short tube, corresponding to the bottom of a common
barometer, is all that is generally provided, and it is usually made of syphon
form.
   The ingenious method, mentioned at page 145 of ascertaining the specific
gravity of the solid material  forming any porous mass or powder, includes
the agency of a barometer.   It  proceeds  upon  this  reasoning.   The inter-
stices of a porous or pulverized mass are filled with air of the density of the
surrounding atmosphere, and if the atmospheric pressure on which that dens-
ity depends be diminished upon the mass in any given degree, an exactly
corresponding proportion of the  air will issue from  the  pores, and if mea-
sured, will declare  the whole quantity, and, therefore, the amount of inter-
stices or pores in the solid mass.  Now if the substance were inclosed at the
end or bottom of a syringe, the pressure  of the atmosphere  might be held
off from it in any degree by drawing at the piston, and the air would issue
from  the pores  as  described, and would  follow the  piston ; but as, owing
to the friction of a solid piston, it would be difficult  to measure the precise
action, the liquid piston of a  mercurial column has been substituted, of which
the force is  always proportioned to  the length.   The  operator
takes an open glass tube, a e, of  known dimensions, and pre-    Fk
pares a part of its top, a b, as a receptacle for the substance
under trial, by affixing a  partition at b, which shall support the
substance, but allow passage to air.   Having then filled a b
with the substance, he gradually immerses the tube in a vessel
of mercury df, until the  mercury stand both  inside and outside
of the tube at the level of b, the air from the tube  having passed
out through the substance in  a b.   It  is  evident that on then
closing the tube at a in an air-tight manner, and lifting  the tube,
a column of mercury will remain standing in  it, above the level
of the external mercury at d, and will be acting  as  a piston    -----tf,
pulling down from b with force proportioned to its height.  If
the tube  be  lifted until such  mercurial column c d he just of
half the  length of the  column in a common barometer, the air
in the pores of the substance will be relieved from half of the
atmospheric pressure, and will dilate to double  bulk : so that
while half of the air will remain in the  pores,  the other half
will have issued forth  to occupy a space, as b c, between the
surface of the mercury and the partition at b.   This spaced c,
therefore, will be exactly equal to the amount of the pores or
interstices ; and as it may be  measured and compared  with the
whole space a b, its ascertained  magnitude willxsolve  the pro-
blem.  It has been found in  this way that charcoal,  which is
usually said to be only half  as heavy as its  bulk of water, is
really formed of matter nearly four  times as heavy ; proving,
in a new way, the identity of charcoal and diamond, and that
light pumice-stone consists of matter heavier than  granite or marble.  This
very ingenious application of the barometer may lead ultimately to many use-
ful results : and the contrivance merits consideration here, as exhibiting-, under
a new and interesting aspect, the rationale of barometric action and the elasti-
city of air.
13 
182
PNEUMATICS.
 Atmospheric pressure determining the liquid or aeriform state of certain
                substances.  (See the Analysis, page 156.)
   It has already been stated that the permanent gases—or substances usually
in the aeriform state—may  be reduced to the liquid, or even solid form, by
simple pressure, and abstraction of the heat which is combined with them
in the aeriform state.  Carbonic acid, the common coal gas, &c, have been
treated in  this way.   Now it becomes an interesting question whether many
of the substances known  as liquids on the face of the earth, where they are
bearing the pressure  of the atmosphere, would not appear as airs if that pres-
sure did not exist.
   On investigating this subject by experiment,  we accordingly find, that
nether, alcohol or ardent spirits, volatile oils, Sj-c, and  even ivater itself, are
known to  us here as  liquids, only because their  particles are kept  together
by the weight and pressure of a superincumbent atmosphere.   Any of these
substances, relieved  by  art from  such  pressure, quickly becomes an  air or
gas,  just  as a common  gas, which has  been kept in  the state of liquid by
any  great  pressure, becomes air again on being relieved.
   In our first chapter we explained the dependence of the three forms which
any body  may assume, viz., of solid, liquid, or air, on the quantity of heat
diffused among the  panicles; we now see, however, that to  understand the
subject completely, we  must consider also the effect of  accidental pressure;
for, while  heat is the power separating  the atoms in the changes mentioned,
it  has to overcome both  the mutual attraction of the atoms and the additional
force of the atmosphere  pressing them together.   The combined influence of
these forces  is fully displayed  in the  two phenomena  called  boiling and
evaporation,  which exhibit  the progress of the change of a liquid into an
aeriform fluid.   We  now proceed to examine these phenomena.
   Boiling.—If  water be placed in a suitable  vessel over a common fire, or
over the flame of a lamp, it is gradually  heated to  a certain degree; and then
small bubbles of aeriform  matter, viz.,  water, in the state called  steam, are
seen forming at the bottom of the vessel;  and successively rising to the sur-
face, where they disappear by mixing with the atmosphere; and  the opera-
tion  being continued, the quantity of water diminishes  with every bubble,
until the whole vanishes under the new  form of air.
   This change takes place in  water, under common circumstances, at the
degree  of  heat marked 212° on Fahrenheit's thermometer, and called, on that
account, the  boiling point of water;  at which, therefore, the repulsive power
among the particles is just sufficient to overcome both their natural attraction,
and the compressing force of the atmosphere of fifteen  pounds on the inch.
But a less  degree of heat suffices if the pressure of the atmosphere be lessened
or removed ;  and a greater  degree is required if pressure be increased.  Water
on the top of Mont Blanc  boils at 180°, because  relieved from the  pressure
of the air  that is below the level of the mountain's summit; and at all inter-
mediate heights in  descending to  the level of the sea, and beyond that into
mines, there is  a corresponding  increase of the boiling temperature.  So
exactly is  this the case, that  we  now find it to be a good method of ascertain-
ing the  heights of places, merely to  observe the heat of boiling  water at
them.  To many persons the information here given that boiling water is not
equally hot in all places, will appear extraordinary : and they will not under-
stand, that even in the same place, at different times, when the barometer is
high or low, there will be corresponding differences.—Again, near the bottom
of a boiler, the water is hotter than above, because it is bearing an additional 
ATMOSPHERIC PRESSURE —BOILING.
183
pressure proportioned to the depth, and does not, therefore, give out the steam
which it would part with if a little higher up.   In very large and deep boilers,
therefore, such as are used  in great porter breweries, the liquor is much
more heated than it can be in smaller vessels;—a circumstance which proba-
bly has an influence on its ultimate quality.
   While water, under common atmospheric pressure, or when the barometer
stands at thirty inches, boils at 212°, other substances, with other relations
to heat, have  their boiling points  higher or lower:—aether, for instance, at
98°; spirit  or  alcohol  at 174°; fish-oil  and  tallow at about 600°; mercury
at 650°.
   It is in consequence  of the different temperatures at which the  particles of
different substances acquire repulsion enough to rise against the atmospheric
resistance, that we are enabled to perform the operation  called distilling.   If
a  mixture of spirits and water, for instance, be heated up  to  180 deg., the
spirit will pass off in  the aeriform state, leaving the water behind, and
may be caught apart and cooled to condensation in any  fit receiver.   Distil-
lation is the best means we possess of separating many substances from each
other, as spirit from wine or  other fermented liquor; various  acids from
water ;  water itself from its  common impurities ;—and even the separation of
mercury from  silver or gold which it has been  used to dissolve from among
the rubbish of a mine or river-bottom, is merely a distillation.
   We must call to mind here what was mentioned in a former  part of the
work, that a large quantity of heat combines with every  substance during the
change of form from solid to liquid, or from liquid to air;  a quantity which,
from not remaining sensible to the  thermometer, has received the name  of
latent or concealed heat.   The whole of this is given out again in the contrary
change.   In the conversion of water into steam, the  heat which  thus disap-
pears is about 1,000 degrees, or six times as much as is  required to raise the
cold water to the boiling point: this is proved by the time and fuel expended
in boiling any quantity to dryness, and  by the fact  that  a  pint of water in
the form of steam will combine instantly with six pints of cold water, raising
the whole to boiling heat.
   But for the fact of latent heat, the conversion of a  liquid  into air would not
be the gradual process  of boiling which we now see, but a sudden and terri-
ble explosion:  for  when any  quantity of water were raised to the boiling
heat, one decree more would be sufficient to convert the  whole into steam.
And but for  the same reason, the  thawing of winter snow would  always  be
a sudden  and  frightful  inundation; the  whole  load of  a  mountain or plain
becoming at once as a lake bursting from its enclosing  barriers.  On the
other hand,  if water in freezing had not to give  out again its  latent heat, after
any  quantity were once cooled down  to the freezing point, the abstraction of
one degree more would instantly convert the whole into  a solid mass.  Thus,
then, by an  arrangement effecting most important purposes in nature and art,
all changes from solid  to  liquid and from liquid to  air, and  the  converse
changes, are very gradual.
   If a little heat.be  abstracted  from steam, a part of the steam  proportioned
to  the abstraction is immediately condensed into  water.  What is called steam,
in  common language—as the vapour which becomes visible at a little distance
from the spout  of a boiling  kettle or the chimney of a tea-urn—is not truly
steam, but small  globules of  water already condensed by the  cold air and
mixed with  it.   Steam is as dry and invisible as air  itself; but the instant
that it comes in contact with air or other bodies colder than itself,  it becomes
water. 
184
PNEUMATICS.
   By means of the exhausting air-pump on one hand, and of the condensing
syringe on  the other,  all the above-mentioned  phenomena, depending  on
the atmospheric  pressure, and its increase or diminution, may be strikingly
shown.
   Thus, to  exhibit the effect of diminished  pressure, water not heated  by
several  degrees  to  the  boiling point  of  ordinary low situations, but which
would be boiling at the top  of  Mont Blanc, is caused to boil instantly  by
placing it under the receiver of an air-pump, and  making a few strokes of the
piston; if the  exhaustion be  rendered  nearly complete, the  water will  boil,
even when colder by 20 degrees than the blood  of animals; and at degrees
of temperature still much lower, it will rapidly assume the form of air, although
not with force sufficient to produce the violent agitation of  boiling.  Other
liquids, as spirits, aether, Sic, from requiring inferior degrees of heat to sepa-
rate their particles  to aeriform distances, boil under  the receiver of an air-
pump at very low temperatures; aether, for instance,  when as cold as freez-
ing water.
   On the other hand, to exhibit the effect of increased pressure, if  we con-
fine the particles of a liquid  still more than  by a common  atmospheric  or
equivalent pressure, degrees of heat higher than the common boiling point
will be required  to separate them.  In a diving-bell, the boiling point of water
is  higher than  212 deg.  in proportion to the depth which the bell has reached:
and if, at the surface of the earth, we heat water in a close vessel into which
air is forced so as to press thirty pounds on the inch instead of fifteen, as the
atmosphere  does;  or from which we  prevent the  steam's escaping  until it
has acquired the force  of a double atmosphere,—^before making the liquid
boil, we shall have to raise the heat, in a corresponding proportion beyond
212 deg.  Under a very strong pressure, water may be rendered almost red-
hot, but the force with  which its particles are  then  tending to separate is
almost that of inflamed  gunpowder.  Even then, however, if a gradual issue
were allowed,  only a certain quantity of the water would absorb and render
latent the existing excess of heat above 212 deg.,  and would become common
steam, leaving behind a considerable portion as boiling water of the ordinary
temperature.

The fact that liquids are driven off, or made to boil at lower  degrees of heat
   when the atmospheric pressure is lessened or removed, has recently  been
   applied to some very useful purposes.

   The process for  refining sugar is to dissolve  impure sugar in water, and
after clarifying the solution, to boil off or evaporate the water again, that the
dry crystallized mass may remain.
   Formerly this  evaporation was performed under the atmospheric pressure,
and a heat of 218° or 220° was required  to make the  syrup boil; by which
degree of heat, however, a portion of the  sugar was discoloured and spoiled,
and the whole  product was deteriorated.  The valuable  thought occurred  to
Mr. Howard, that the water might be dissipated by  boiling tbe syrup in a
vacuum, or atleast a place from which air  was nearly excluded, and" therefore
at  a low temperature.   This was done accordingly ; and the saving of sugar
and the  improvement  of quality were such,  as to  make the  patent-right,
which secured the emoluments of the process  to him and other parties, worth
many thousand pounds a year.   The syrup, during this process, is not more
heated than if  in a vessel merely exposed to a summer sun.
   In the preparation of  many medicinal substances, the process  of boiling in 
ATMOSPHERIC PRESSURE--BOILING.
185
vacuo is equally important.   Many extracts from vegetables have their vir-
tues impaired, or even  destroyed, by a heat of 212°; but when the water
used in making the  extract is driven off in vacuo,  the  temperature need
never be higher than blood-heat, and all the activity of the fresh plant remains
in the extract.
  In the  same  manner, in the process of distillation,—which is merely the
receiving and  condensing again in appropriate vessels the aeriform  matter
raised by heat from any mass,—substances which  are changed  and injured
by an elevated  temperature, may be obtained of admirable quality by carry-
ing on the operation  in a vacuum.  The essential oils of lavender, pepper-
mint, &c, never had the natural flavour and virtues of the plants until  within
the last few years,  since this plan has been adopted.
  The influence on the human system of vegetable  medicines obtained  in
the old or in the new  way, is so different,  that the prescriber should carefully
advert to the circumstance.
  The apparatus for  evaporating and  distilling in vacuo consists of vessels
strong enough to bear, when quite empty,  the external atmospheric pressure,
and which are therefore generally of arched form.   The vacuum is produced
and maintained by air-pumps driven by a steam-engine or otherwise;  or by
first admitting steam to expel the  air, and then condensing the steam  into
water.
   The author has suggested a very simple contrivance to answer, in certain
cases, the purpose of such air-pumps and steam-engines or
apparatus.   It  is  merely  to establish a  communication       Fig. 103.
between a close boiler,  as a, and the vacuum at the top of
a water barometer, as b.  To produce that vacuum,  the
strong vessel  b forming the top  of the  barometer,  and
thirty-six feet of tube  below, reaching to d, are first filled
with water through a cock  c at the top;  this cock being
then shut, and  another  cock d at  the bottom, which was
shut, being opened, the water will sink down out of  the
vessel b, until the  column in the  tube be  only thirty-four
feet high, as aty, that being the height which the atmosphere
will support.   On then  opening a communication between
the boiler a and the vacuum  in b, the operation will go on
as desired, and the steam rising from a may be condensed
in b by a little stream of cold water allowed constantly to
run through from above.  This water, it is evident, would
always pass downwards to form part of the column below,
without filling up or impairing the vacuum.  If air should
find admittance in  any way, the original degree of vacuum
could easily be reproduced as at first; and  to prevent inter-    r.^...::;
ruptions, it might  be  convenient to  have  two vessels  like    ;  Jj]cl\
b, of which one could always be in action while the other    '"-......'
was being emptied of air.   The author planned this  as a simple apparatus
for the preparation of medicinal extracts;  and it appears well suited also for
the  manufacture of sugar in  the  colonies, where  air-pumps and  nice ma-
chinery can with difficulty be either obtained or managed.   On many sugar
estates there  is a fall  of water, which  would supply the barometer without
the trouble of pumping.  The tube d c need  not be perpendicular, provided
it be longer in  proportion to its obliquity; and it may be very  small: some
yards of common  lead-pipe would answer.
   When it was understood  that, at common temperatures, water  and many
 
186
PNEUMATICS.
other liquids would be existing in the form of air, but for an  atmospheric
pressure opposing the separation of the particles, it became of great import-
ance  in  many  of the  arts,  and for comprehending certain  phenomena of
nature, to ascertain, very exactly, with respect to some of these  liquids, the
degrees of expansive force belonging to them at different degrees of tempera-
ture.   The subject, as  water is concerned, has been investigated with great
care, and the following  table shows part of the results.  The left-hand column
marks temperatures from 32 deg. of Fahrenheit's thermometer, or the freez-
ing point of water, to 290 deg.; and the  right-hand column  marks die cor-
responding  degrees of force  with  which the  water lends to expand into the
state of steam,Cand therefore also the force and density  of the steam existing
in any vessel  above the water which it  contains.   One  ounce and  a half
per square inch, is the force exerted on the sides of any containing vessel by
steam rising from freezing water,  that is  to say, the force  with which freez-
ing water seeks to dilate  into steam or air; and sixty pounds  per inch is the
force  of  water heated to 290 deg.   To many readers the  idea will  be quite
new and surprising, that  if some freezing water, or even ice,  be placed in a
bladder containing nothing else, and the  bladder  be then  placed in the ex-
hausted receiver of an  air-pump or other  vacuum, the  bladder  will quickly
be distended with steam strong enough to support one ounce and a half on
every square inch of its upper surface.

                At 32p force of steam is   l£ oz. per inch.
                  50.....     2f oz.
                  100.....13  oz.
                  150.....    4   lbs.
                  180.....    7i lbs.
                  212.....15  lbs.
                  250.....30  lbs.
                  272.....45  lbs.
                  290.....60   lbs.

   In this table  we have to remark how much more rapidly the tendency to
become  steam  increases  than the temperature  of  the water: for a rise of
eighteen degrees, viz., from 32° to 50°,  at the beginning of the scale, only
increases the dilating  force one ounce  and a quarter on the  inch, while an
equal rise at the top of the scale, viz., from 272 deg. to 290 deg., increases it
fifteen pounds.  It is most  important  to distinguish, however, between the
tendency to  form  steam at any  temperature, and  the bulk  or quantify of
steam formed by a given quantity of heat; for the matter imperfectly under-
stood has led to many  vain schemes for improving the steam-engine.  The
truth  is,  that high-pressure steam  is nearly condensed steam, as high-pres-
sure air is condensed air ; in other words, the  density of steam is greater,
or there  must  be more of it, exactly as its force  is  greater according to the
rule explained at page  160 ; and the heat  absorbed in its formation being pro-
portioned to the quantity of steam in a given space, or the density, the force
and the cost in fuel have always nearly the same  relation to each other.  In
one pint of  steam, at 290 deg., having an elastic force of sixty pounds on the
inch, there are  very nearly four times as much water and four times as much
latent heat as in one pint of steam at 212 deg., which  has a  force of fifteen
pounds on the inch;—indeed, the  one pint, at 290 deg., may be changed into
the four  pints at 212 deg., or the  contrary, by merely  changing the degrees
of pressure.  It does not accord with the plan of the present work to enter 
STEAM ENGINE.
187
farther into  the details of this subject, but they may be found  in various
modern treatises.
  Seeing the rapid increase of the expansive force in the preceding table, we
have the explanation of the terrible effects occasionally produced by confined
water when  overheated.  A boiler of any kind completely closed, and hav-
ing no safety valve, if heated  to a certain degree, will explode as  if charged
with gunpowder.   Unhappily the  instances are too numerous where the in-
cautious or ignorant use of steam has produced explosions, which have shat-
tered buildings and destroyed  whole neighbourhoods.
  The prodigious force  generated by heating water would at first only sur-
prise and terrify men, but in the  course of time would lead  inventive minds
to inquire whether  it  might not  be turned to  use ;  in other words, whether
some mechanism,  to be called a steam-engine, might not  be contrived to en-
able men to make it aid them  in their various labours.  To this  inquiry,  after
numerous less successful attempts,  a glorious  answer has  been  given in our
own day by the illustrious Watt ;—and to this part of our work it belongs
to consider what he has accomplished, viz., to describe

                          The  Steam-Engine,

which, in the few  years  since the genius of Watt carried it to  its present
state of perfection, has changed  the  direction  of human industry, and  may
almost be said to have elevated man in the scale of existence.
   The name of steam-engine, to  most  persons, brings  the
idea of a machine  of the most complex  nature, and  hence to     Fig. 104.
be understood only by those  who will  devote much  time to       ^p)
the study of it; but he who can understand a common pump,
may understand a  steam-engine.  It is  in fact only a pump
in  which the fluid  passing through  it is  made to impel  the
piston instead of being impelled by it, that is to say, in which
the fluid acts as the power instead of  being  the  resistance.
It may be described simply as a strong barrel  or cylinder c d,
with a closely fitting piston in it, here shown at b, which is
driven up and down by steam admitted alternately above and    ^ ^
below it from a suitable boiler; while to the  end of the pis-      "'^rtf*
ton rod a, at which the whole force may be considered as
 concentrated, there is attached  in  any convenient  way the
 work  which is to be performed.   The power of the engine  is of course pro-
 portioned to the size or area of the piston, on which the steam  acts with a
force,  according to its density, of from   15 to  100 or more pounds to  each
square inch.  In some of the Cornish mines  there are cylinders and pistons
of more than ninety inches in diameter, on which the pressure of the steam
equals the efforts of six  hundred horses.
   In one place this wonderful piston-rod may be seen acting upon the end
of a great vibrating beam, to  the other end of which capacious water-pumps
are attached, whose motion causes almost a river to gush up from the bowels
of the earth.  In another place,  it is seen working a crank, and urging  com-
plicated machinery.  One steam-engine four miles from London  is at the
 same instant filling all the water reservoirs, and baths, and fountains of the
 finest quarter of the town.   One engine stretching long arms over  a  great
barrack or manufactory, keeps  in one quarter, thousands of spinning-wheels
in  motion, while in another it is carding the  material of the thread, and  in
another weaving  the cloth.  In like manner, one  steam-engine, in  a  great 
188
PNEUMATICS.
metropolitan brewery, may be seen at the same time grinding the malt, pull-
ing up supplies of all kinds from  wagons around the building, pumping cold
water into some of the coppers, sending the boiling wort from others up to
lofty cooling-pans, over  which it  is turning the fans,  perhaps also  working
the mash-tub, drawing water from the deep wells  under ground, and loading
the drays—in a word, performing the offices of a hundred hands.  Again,
there are manufactories where this resistless power is seen with its mechanic
claws seizing masses of iron, and  in a few minutes delivering them out again
pressed into thin sheets, or cut into bars and ribbons, as if the  iron had  be-
come to it like soft clay in the hands of the potter.   And now for some years,
over nearly the whole world, has this wonderful  piston-rod,  working  at  its
crank, been turning the  paddle-wheels of innumerable steam-boats, thereby
setting at defiance  the violence of the winds and waves, and  the  currents of
the fleetest rivers, while it  carries  men  and civilization into the remote
recesses  of all the great continents.  To wherever a river leads, the region,
although concealed perhaps since the beginning of the world, is now  by  the
steam-engine called as  it were from its  solitude, to form a part of the great
garden which civilized man is beautifying.—Such are a few of  the prodigies
which this machine is already performing, and every day is witnessing new
applications of  its  utility.
  The following account of the parts of the steam-engine is intended, with-
out entering into minute practical details, still fully to explain the  principle
or general nature of the machine.  It should serve to render very interesting
to an  attentive  reader, a visit to  any place where a steam-engine  is in use:
and it should make  evident the  folly of many of the modern  schemes  for
improving the engine.   To avoid complexity in the figure, the parts  which
the reader can easily conceive are not here sketched.

                                Fig. 105.
  1st.  The part which  first claims attention is the great barrel c d, already
spoken of as the centre or main portion of the machine, in which the piston
P is moved up and down by the action of steam entering  alternately above
and  below it, through the pipes e c  and e d.  The barrel or  cylinder is
bored with extreme accuracy, and the  piston is  padded round its edge with
hemp or other soft material so as to be  perfectly air or steam-tight.  Lately 
STEAM ENGINE.
189
pistons have been made altogether of metal, and, in some cases, from work-
ing with less friction, these answer even  better than the others.—2d. The
next part to be mentioned is the boiler B, which is made of suitable size and
strength.—3d. The steam passes from the boiler along the pipe to e, and there
by any suitable cock or valves, worked by the engine itself,  is directed alter-
nately to the upper and under part of the barrel;  and while it is entering to
press on one side of  the piston, the waste  steam is allowed to escape  from
the other side, either to the atmosphere, for high-pressure engines, or into—
4th, the condenser at C, for those of  low-pressure; the  condenser being
always kept at a low temperature by cold water running into it and pumped
out again by the piston k.—5th.  The supply of steam from the boiler to the
cylinder is regulated by a valve placed somewhere in  the pipe B e, and made
obedient to what is called—6th, the governor, a contrivance not represented
here, but already described at page 52,  to illustrate  centrifugal force.   We
may recall it by saying, that it consists of two balls hanging by jointed rods
like the legs of a tongs, from opposite sides of an upright spindle, which is
made to revolve by connection with some turning part of the machinery :—
when the spindle turns at all faster than  with the desired speed, the balls fly
more apart and are made to affect the steam valve so as to narrow the passage;
and, on the contrary, when it turns more slowly than is desired, they collapse,
and by  so  doing open the valve  wider.—7th.  The supply  of water to the
boiler is regulated  by afloat on the surface of the water in the boiler; which
float, on descending to a certain point, by reason of the consumption of wa-
ter, opens  the valve  to admit  more.—8th. There is a safely valve in the
boiler, viz., a well fitted flap or stopper,#held against an opening by a weight,
but loaded so as to open before danger can arise from the over-heating of the
water.—9th. The rapidity of the combustion, or  force of the fire, is exactly
regulated  by the state of the boiler and the  wants of the machine, thus:—
there is a  large open tube (not represented here) rising from near the bottom
of  the boiler,  through its top, to the  height of several feet, and when the
water in the boiler is too hot, and the steam  therefore too strong, part of the
water is pressed up into this tube, and by the agency of a  float which rests
on  its  surface, it  shuts the chimney valve or damper: the draught is then
diminished and the fuel saved, until a brisker  fire is again  required.—10th.
In this figure a i g marks the place of  the  great  beam, turning  on an axis
at i, and  transmitting the force of the piston  to the remote machinery.
When the object is to raise water, the pump rods are  simply connected with
the end g of the beam ; but when any rotatory motion is wanted, the end g
is made to  turn.—11th. A crank Inhy the  rod g 1;  and uniformity of mo-
tion is obtained by the influence  of—12th, the greatly wheel m, fixed to the
axis of  the crank.
  The  smallest  and simplest steam engine, and  therefore the cheapest, is
that called  the high-pressure engine.   In it steam is used  of  great density,
and consequently of  great force,  as of 50 lbs. or more to the inch ; and while
the fresh steam is admitted to press on one side of the piston,  the  steam
which  has already worked, is allowed to escape, or is driven  out to the air,
from the other side.   The atmospheric  resistance to the  issue of  the  steam
diminishes the working force of  the piston just 15 lbs. per  inch.   The sim-
plicity  of this form  of engine  recommends it, but  the danger of a  large
boiler of overheated  water, always, like  inflamed gunpowder,  seeking to
escape,  has, by numberless fatal  accidents, been proved to  be  so  great, that
the use  of  such an engine is limited to certain situations.   Notwithstanding
all the ingenious securities recently contrived  against the danger,  and  which 
190
PNEUMATICS.
will suffice  for small engines, such as are  required for steam  carriages, the
high-pressure engine is not employed in a single English  passage-vessel.*
   In the  low-pressure engine, the  steam is  used generally of force  not ex-
ceeding 20  lbs. on  the inch, which force is  only 5 lbs. more  than  the atmo-
spheric pressure, and is insufficient to burst a common boiler, or to do serious
mischief:! but as the interior of the low-pressure engine is kept in a state of

  * In this country, also, what is called the low pressure engine, is at present employed in
all the steam-boats on  our  eastern waters, but we have reason to  believe that few if any of
them are worked with a less pressure than 25 lbs.  on the square inch, and we have seen
many of them worked with a much higher pressure.  Tt is stated, that the steam guage on
board the Pulaski, just before the  fatal explosion, indicated a pressure of 26 lbs.  whicli was
considered by the engineer a safe working force.  The British Ocean Steamers work with a
pressure of only 3h to 4i lbs. to the square inch.  On our western waters, the low pressure
engine has been discarded in favour of the high pressure.                      Am,  Ed.

  + Our author must be understood as saying, only, that explosions do not take place in low-
pressure engines whilst working under a  pressure of only 20 lbs. to  the square inch.  But
from various  circumstances the elastic force of the steam in a low pressure engine may be
greatly  increased, and instead of its ordinary power of 20 lbs., it may acquire one of 100 or
200 lbs., and if the boiler, as is often the case, is unable to bear this pressure, it will burst.
In this wav explosions have repeatedly taken place  in  low-pressure engines.   Several of
these will be  found related  in an interesting memoir by M. Arago, originally published in
the "Annuaire du Bureau, des Longitudes,'" and which has been translated and published in
that useful periodical, "The Journal of the Franklin Institute of Pennsylvania."  M. Arago,
in this memoir, remarks,  " I ought not to  conclude so  long a paper on the subject of the
explosions of steam boilers, without explaining why 1 have  not separated the examples of
the explosions of high-pressure boilers from those of the low-pressure ; it is because 1 think
there is no reason to make such distinction.   Every one must in fact  admit, that at the time
of an explosion, all boilers contain high pressure steam."
  The belief that low pressure boilers are  not liable to burst, or do mischief, has led, as
has been already observed, to their exclusive use in passage vessels ; this belief, however, is
founded in error.   M. Arago, in the memoir already quoted, observes, "it does not ap-
pear established by any means, that explosions take place more frequently in high than in
low pressure  boilers ;  the contrary has been maintained by different engineers, among whom
may be  classed Messrs. Perkins,  Oliver Evans, &e." Indeed-, a little  reflection will show
that high-pressure boilers ought not to be more liable to explosions  than the low.  Boilers
may he  made of either iron  or copper of sufficient strength to resist a much greater force,
thap that of the steam ever employed in high-pressure engines.   Now boilers are always
constructed of a strength proportionate to the pressure they are to sustain.  Thus, in a low-
pressure engine working with a force of 20 lbs. the boiler is made of a strength calculated
to support from 3 to 5 times that pressure; in a high-pressure engine destined to work with
a pressure of 150 lbs., the boiler  is constructed so as to resist from  3 to 5 times that pres-
sure. It will  immediately be asked why cannot the boiler of a low-pressure engine be made
of the same strength as if it were for a high pressure-engine?  Tn answer to this it may be
remarked, that independent of many difficulties to be overcome before we can  exceed cer-
tain limits  in the thickness of boilers, the weight and cost of the low-pressure engine are
already so great that it would he impossible to persuade owners of steam-boats to incur any
addition in these particulars—and even would they do so, perfect safety would still not thus
he obtained.   We have already observed that at the time of an explosion, all boilers contain
high-pressure steam, and as we know no  limits to the force of this steam, however strong
the boiler may be, it may burst, unless this be prevented hv other means.
  It was long ago known that if a  vessel,  however strong it  might be, containing water, be
placed over a fire, it will burst, unless an opening is provided for the escape of the steam
as fast as produced. The temperature which will cause the rending of a vessel must depend
upon its form and  dimensions, and upon the tenacity and thickness of the material of which
it is made.  If we could keep the  heat of our furnaces below a certain limit, no other pre-
caution  would he required to prevent explosions.  But it is evident that this cannot be done;
we must therefore resort to some other  expedient, and the safetv valve  invented by Papin
would seem to answer this purpose.   We must be allowed to anticipate the subject a little
in order to explain the nature of this valve.
  The safety valve consists of a hole, say of an inch  square, made in  the upper part of the
boiler, upon which is placed a metal plate loaded  with a certain weight.  It is evident that
the hole will remain closed as long as the pressure of the steam within the boiler is less than
the weight of the valve, together with that of the atmosphere, upon the square inch, and that
as soon as the pressure within shall exceed this, the valve will be  raised and give a free vent
to the steam.
  It would lead us too far to explain  how  it has happened that  so  simple  and apparently 
STEAM ENGINE.                          191
 vacuum, except where the  steam is acting, the whole pressure of 20 lbs. is
 made available, and the engine has  the same power, if of equal size, as  a
 high-pressure engine working with steam of 35 lbs. on  the  inch.  The re-
 quired vacuum is  preserved by means of a separate vessel or box, represented
 at C, called  the condenser, into which cold  water is constantly running  to
 condense the  steam, and  is  afterwards pumped  out with  the  condensed
 steam, and with any little air that may have entered : the pump is represented
 at k  in  the  figure.   Steam, on coming into contact with a cold body,  is
 condensed almost with the rapidity of an explosion ; and therefore  the instant
 that the  opened valves  make a communication between the  cold  condenser
 and any  part  of the engine containing  steam, this  rushes to  the condenser,
 and becomes water, leaving a vacuum behind.   The great merit of Mr. Watt
 was  in the contrivance of this  separate  condenser, for, until his time, cold
 water had always been thrown  directly into the working cylinder, cooling it
 so much, that twice or thrice  its fill of steam was destroyed at each stroke to
 warm it  again before it could  work.   This single change saved three-fourths
 of the quantity of fuel formerly expended.
   Before Watt's day, the only steam-engine in use was  a rude  single-stroke
 engine, as it  was called, in which steam,  admitted  under the  piston, allowed
 the weight of the pump-rods at the far end of the beam to lift the piston, and
 the steam being then condensed so as to leave a vacuum  in  the  cylinder, the
 pressure of the atmosphere pushed  the piston down to do its work:  on this
 last account the engine was also called an atmospheric engine.   It was used
 almost solely for pumping water; but it wasted so much fuel, from causes of
 which the chief is mentioned  in the last paragraph,  that the expense was not
 much less than that of employing horses.
   In  the atmospheric engine, the steam which  lifted  the piston against the
 atmospheric pressure, required to be at least as strong as that pressure, to the
 very end of the stroke.  Another of Watt's great improvements was,  his ex-

 efficient means, has not always proved efficacious ; since these causes are various and many
 of them as yet not perfectly understood.
   Those who wish to investigate the subject, we refer to the memoir of  M.  Arago, and
 to the report of a committee of the Franklin Institute, who have collected an account of
 all the explosions in this country, and who have  instituted a  very interesting series of ex-
 periments, in order to examine into the causes of the explosion of steam boilers, and devise
 means for  its prevention.  This report has been  published in the Journal of the Institute.
   We must not omit, however, to mention that when a low-pressure boiler does explode,
 it has been found to produce greater destruction than  a high-pressure one,  in consequence
 of the greater  size, and, therefore, larger quantity of water contained in the  former. It
 may, perhaps, be supposed that the steam  from a high-pressure engine would  scald more
 severely than that from  a low-pressure one. This, however, is not the fact : on the contrary,
 whilst the steam issuing from a low-pressure engine scalds at all moderate  distances  from
 the boiler,  that from a high-pressure one scalds only at certain distances.  Thus the hand
 may be placed an inch from an aperture in a high pressure engine without any inconvenience
 beingfelt; at greater distances, however, it will scald most severely.  A friend has informed
 us that he  has placed his hand within an inch of the aperture in a boiler from which the
 steam  was  issuing at a time when the force of the steam within the  boiler was equal to 300
 lbs. on the square inch, without feeling any inconvenience.  Some  interesting experiments
 on this subject have been instituted by Peter Ewart, Esq., and an account of which will be
 found in the fifth volume of the Journal of the Franklin Institute.
  It must not be supposed from  any thing that we have said in this note, lhatexplosions of
 steam-boilers cannot be  prevented.  But we may be allowed to quote on this  subject the fol-
 lowing  remarks  of VT.  Arago.  " No cause of explosion exists which cannot  be avoided,
 by means at once simple and within the reach of every one.   As we should  not trust fire-
 arms in the hands of children, so, I think, we should not trust the direction of a steam-
 engine to a man either unskilful, without experience, or wanting  in intelligence.  It is a
mistaken idea, that because  steam-engines usually move without attention to  them, sm-h
attention is not required ; Watt contended strongly against this error."        Am. Ed. 
192
PNEUMATICS.
eluding altogether the  air from  his machine, by doing which he not only
avoided the cooling effect of the air, but was at liberty to shut off the steam,
as it is expressed, or to stop the  supply for each stroke, before the cylinder
was full, and then to make the  farther expansion of the quantity admitted
impel the piston to the end of the stroke.  This principle of causing the mere
expansion of steam to do work was afterwards carried to  a great  extent by
Messrs. Hornblower, Woolfe, and others, who constructed engines with two
barrels, in the first and smaller of which, the  steam  was made to act  in its
dense or strong state, as it issued from  the boiler, and when it had finished
a stroke there, instead of being at once sent useless to the condenser, it was
admitted to a larger piston, which it moved by its continued expansion alone:
—the same steam thus doing double work or more.  All the advantages of
the two cylinders, however, are obtainable from the single cylinder with its
condenser, as now used in most of the Cornish mines.  Steam of about 60 lbs,
pressure on  the inch is admitted  to the  cylinder, until  the piston is driven
nearly one-third of its way, and the valve being then shut, the  same steam is
left to finish the stroke by its expansion.  The pressure of the expanding
steam gradually diminishes, it is  true, in proportion as the volume  increases;
but in pumping water there is a  great saving  of time, from having  the power
more intense at the beginning of the stroke, when the vast mass of water and
machinery has first to be put into motion.  Steam, while doubling  its volume
by mere expansion,  will do about two-thirds  as much work as  while origin-
ally rising from the boiler,  and by every subsequent doubling it might  do as
much as by the first: the  increasing size of the cylinder, however, and in-
creased friction, confine this mode of using it to narrow limits.              (
   It might be supposed that high-pressure engines without condensers would
be comparatively wasteful, because in them the steam which has  acted must
be driven out of the cylinder against the powerful resistance of the atmosphere,
while in the low-pressure  engine it has instant access to the condenser, and
leaves effective the whole pressure of the fresh steam on the opposite side of
the piston.   But as in the low-pressure engine, nearly half the power of the
steam is expended in overcoming the friction and other impediments of the
numerous parts, while in that of  high-pressure, the parts are so much fewer,
and the piston is so much smaller in proportion to the  force acting upon it,
that the loss from friction  is often less than a  fourth or even a sixth of the
steam-power, although the resistance  of the air is to be overcome by the
high  pressure engine, still there  is often  a saving on the whole.   The saving
becomes very considerable if the steam  be allowed to  act by its expansion
 also, as described in the last paragraph.
   From misapprehension of the law of increase of force by increase of heat
in water, explained by the table  at page 186, some exceedingly false conclu-
sions have been drawn, and acted upon at great expense  (as  lately by Mr.
Perkins,) in attempts to make engines  work  with  an excessively  high pres-
sure.  Besides making the error now alluded to and others, Mr. Perkins over-
looked the fact, that we possess  no material for cylinders and pistons, strong
 enough to bear the contemplated pressure and friction  even for  a moderate
 time.   Perhaps more striking examples could not be adduced  of the absurdi-
 ties into which  even highly ingenious  men  may  fall, when not sufficiently
 acquainted with the  general truths of nature on which the arts which occupy
 them are founded, than in the history of supposed  inventions and improve-
 ments connected with the steam-engine.
   The fertile genius of James Watt did not stop at the accomplishment of the
 two or three important particulars described above, but throughout the  whole 
EXPLOSION.
193
detail of the component parts, and of the various applications of the engine,
he contrived miracles of simplicity and usefulness.   We should exceed  the
prescribed bounds of this  work by entering more minutely into the subject;
but we may remark that, in the present perfect state of the engine, it appears
a thing almost endowed with intelligence.   It regulates with perfect accuracy
and uniformity the number of its strokes in a given time, counting ox record-
ing them, moreover, to tell how much work it has  done, as a clock records
the beats of its pendulum;—it regulates the quantity of steam admitted to
work ;— the briskness of  the fire;—the supply of water to the boiler;—the
supply of coals to the fire  ;—it opens and shuts its  valves with absolute  pre-
cision as to time and manner;—it oils its joints ;—it takes out any air which
may accidentally  enter into parts which should be vacuous;—and when any
thing goes wrong which it cannot of itself rectify, it warns its attendants by
ringing a bell:—yet with all these talents and qualities, and even when exert-
ing the force of hundreds of horses, it  is obedient to the hand of a child ;—its
aliment is coal, wood, charcoal, or other  combustible;—it consumes none
while idle ;—it never tires, and wants no sleep ;—it is not subject to malady
when originally well made;—and only refuses to work when worn out with
age;—it is equally active in all climates, and will do work  of any kind;—it
is a water-pumper, a miner, a sailor, a cotton-spinner, a weaver, a blacksmith,
a miller, Sic,  Sic ; and a small engine in the character of a steam pony', may
be seen dragging  after  it on a rail-road a hundred tons  of merchandize, or a
regiment of soldiers, with thrice the speed of our fleetest horse coaches.  It is
the king of machines, and a permanent realization of the  Genii of Eastern
fable, submitting  supernatural powers  to the command  of man.
  We need  not wonder that the inventor of an engine having such qualities,
should be deemed deserving of the highest honours from his fellow-men.  In
November, 1825, a public  meeting was called, to vote a monument to Watt,
then not long deceased; and the most  distinguished men of the empire, of all
parties,  philosophers and  statesmen, met to vie with each other in  speaking
his  praise.  Perhaps  a series of  such eloquent discourses has rarely been
pronounced at one time; but perhaps in the progress of the  arts of civiliza-
tion there can rarely be offered such  motive and  occasion.   The  common
voice of that  assembly scarcely  exaggerated,  when attributing to  Watt's
genius and perseverance that increase of our national commerce and  riches,
which had enabled  free Britain, single-handed, at an extraordinary crisis of
human affairs, to  contend  with Europe combined against her, and  at last to
triumph, so as to secure her own happy destinies, and  probably much to in-
fluence those of the human race.
  As science and  the twin  sister art are making constant advances, who shall
say that  even  the steam-engine, perfect as we have described  it, forms  the
limit to human  discovery of mighty yet obedient force?  It is true that the
nature of steam, and the laws of  its formation  and action, are now so well
understood, that the intelligent engineer no more hopes for great improvement
in steam-engines, than he hopes for it in the mode of using a waterfall to turn
a mill; but still there are kindred  regions of nature left almost unexplored.
We shall have occasion to  make a remark on this subject in our chapter on
the nature of heat.
The explosion of gunpowder and of all fulminating mixtures bears so strong
  an analogy to the phenomenon of the formation of  steam, that  the mind
  may advantageously  contemplate the subject in this place.
  The ingredients of  which gunpowder  is formed are chiefly substances 
194
PNEUMATICS.
 which, when separate, exist, at any common temperature, in the form of air;
 and the combustion sets them loose,  with a production of intense heat, caus-
 ing an increase of volume which is instantaneous, and almost irresistible.  By
 experiment and mathematical deduction, it appears that the exploding particles
 begin to separate  from each  other with a velocity as if ten thousand volumes
 of air had been condensed into one: and this explains  the corresponding force
 and swiftness with which a  bullet is  propelled.
    All the fulminating metals are chiefly combinations of the like substances
 with the metals ;  and the ingredients  are held together by so slight a tie, that
 a little friction or  elevation of temperature disunites them so as to produce the
 explosion.
    The escape of  condensed  air from the chamber of an air-gun, is a species
 of explosion ; but is very gentle compared with the shock of discharged gun-
 powder.
    It has lately been shown  that a gun-barrel may be  connected with a hio-h-
 pressure steam-boiler, in the same manner as with  a  chamber of condensed
 air; and as the steam may be supplied as long as water remains in the boiler,
 if  bullets be allowed to fall into the barrel fast enough, a hundred  or more
 may be  thrown out  every minute, with  the  same force  and precision as if
 each issued from  a common  fire-arm.  The rapid succession resembles the
 issue of water from a jet pipe; and if such an engine were used in a field of
 battle, its barrel of death, made to point gradually along a line of men,  would
 mow them down like corn-stalks before the scythe—none could escape.  The
 horrible idea and  proposal have  been excused by saying, that to prove the
 possibility of such carnage  must have  the effect of  putting an end to war
 altogether.
   The invention ofgunpowder,"with the consequent change of military tactics,
 because it gave to a handful of men possessing it the mastery over thousands
 who had it not,  was hailed by the philosophers of the day as a certain secu-
 rity against the relapse of civilized mankind into such  a state of barbarism as
 followed the irruption into Europe of the  Goths and Vandals :—none but well-
 instructed and disciplined armies could then enter a European kino-dom. This
 consideration, however, has lost its interest, since the invention of printing,
 and other  changes in society, have  afforded still better and more humane
 securities.

 Besides the interesting instances above cited of the pressure of the atmosphere
   determining whether certain substances shall or shall not have the form of
   air,  there are others that deserve  mention, where  the effect is modified  by
   the mutual attraction of substances.

   The pressure of the atmosphere at the surface of the earth  keeps a certain
 quantity of air in combination with water, so as to form part of  the  liquid
 mass.  This air re-appears at once on taking off the pressure.  If we place a
 glass of water under the receiver of an air-pump and  then exhaust  this, the
 water is soon crowded with bubbles of air, seen adhering to the glass all round,
 or rising through the water.  This  admixture of air in water is necessary to
 the life of fishes.  It is  driven off by boiling, and  hence the vapid taste of
 water that has recently been boiled.
  In the making of beer, wine, and other fermented liquors, there is formed,
 during the fermentation, a large quantity of the substance called carbonic acid.
 Much of it flies off in  its usual form of gas, but, because of  the  pressure of
the atmosphere, much still remains in union with the liquid.    On removing 
PRESSURE AFFECTING TEMPERATURE.
195
 this pressure suddenly, the liquid appears almost to boil, as when a glass of
 warm beer is placed in the air-pump vacuum.
   A degree of  pressure still greater than that of the atmosphere keeps a pro-
 portionally larger quantity of this carbonic acid in liquid  combination ; as in
 bottled porter or sparkling champagne before the cork is drawn ; but as soon
 as the compression maintained by the cork is removed, the gas escapes, caus-
 ing the  thin champagne to sparkle, and the more viscid  beer, which retains
 the little bubbles as they rise, to be covered with froth.   After the  sparkling
 or frothing has ceased  under  the  atmospheric pressure,  the  phenomenon
 may be renewed by placing the glass in the air-pump receiver.
   Carbonic acid so readily becomes liquid when its attraction for water assists
 the compression, that enough of it may be united with water to make a pint
 become a pint and a half.  The  soda water, or aerated  water, now so gene-
 rally used as drink in warm weather, is water with several times its bulk of
 carbonic acid forced into it by pressure; and  a part of this is seen escaping
 always  at the instant of the confining cork being drawn.
   Carbonic acid forms nearly half of the substance of marble or lime-stone.
 When an acid with stronger attraction, as vinegar or sulphuric acid, is poured
 upon marble, it dispossesses the carbonic acid, and unites itself with the pure
 lime.  The carbonic acid  in rising, constitutes the effervescence which then
 appears.   Carbonic acid, for the  manufacture of the common soda water and
 other aerated drinks, is  obtained in this  way.
   Many mineral waters contain carbonic  acid, which  remains in tranquil
 combination while the water is bearing a certain pressure under ground, but
 which in part escapes as soon as the  water issues  to the air and  only the
 atmospheric pressure remains:  such waters are called sparkling waters.
   The reason that champagne and the  aerated waters are so cool when  first
 decanted is, that the carbonic acid, in assuming its gaseous form, absorbes  as
 latent heat, a large proportion of the heat which was previously existing in the
 liquid.

 The  atmospheric  pressure, by making  the density of the  air in any place
  dependent upon the height of the place above the level of  the sea, causes
   corresponding differences of temperature.
   The explanation of this  is simple.  If a gallon  of air,  at the surface of the
 earth, contain a certain quantity of heat, this must  be diffused equally through
 the space of the gallon;  but if  the air be then compressed  into  one-tenth  of
 the bulk, there  will be ten times as much heat  in that  tenth as there was be-
 fore;  the increase affecting the thermometer to an extent modified by circum-
 stances explained in a future part of this work.  In like manner, if by taking
 oft" pressure, the gallon be made to dilate to ten gallons, the heat will be  in
 the same degree diffused, and any one part will be colder than before.   It is
 known that air  may be so much compressed under the piston of a syringe,
 that the  heat in it, similarly concentrated, becomes intense enough to inflame
 tinder attached to the bottom of the piston:—this  means, under  the name  of
 the match-syringe, being in common use for obtaining an  instantaneous light.
  Now, for the reason here explained, the air  near the surface of the  earth,
 forming the bottom  of the atmosphere, because condensed  by the weight  of
 the air above it, is  much  warmer  than if it were suddenly carried higher up,
to where, from the pressure being less, it would be more expanded or thin.
In many cases the height of mountains may be  estimated  by the  difference  of
temperature observed at the bottom  and at  the top.  While a thermometer 
196
PNEUMATICS.
stands at 60° at the bottom of St. Paul's Cathedral, in London, another marks
only 58° at the top of the dome;  and in the lofty ascent of a  balloon, the
thermometer soon  falls  to the freezing  point  and below  it, the cold  to the
aeronaut becoming almost insupportable.
  In every part of the earth, at a certain elevation in the atmosphere,  differ-
ent according to the  latitude or proximity to  the equator, the  thermometer
never rises  above  the freezing  point,—and this  limit in  the atmosphere  is
called the-line or level of perpetual  congelation.   In Norway it is at five
thousand feet above the level of the sea; in Switzerland at six thousand five
hundred; in Spain and  Italy at  seven thousand;  farther south,  at Teneriffe,
at nine thousand ; directly under the sun, as in central Africa, and among the
Andes in America, it  is  about fourteen thousand.  We see therefore why the
snow-capt mountains  are not the tenants  only  of high northern  and southern
latitudes.   It is this effect of elevation  which renders many  of the tropical
regions  of the earth not  only tolerable abodes for man, but as suitable  as any
others,  contrary to the opinion of the ancient philosophers of Europe, who
accounted them, by reason of the great heat, an everlasting barrier, as regard-
ed  man, between  the northern and  southern hemispheres.  Much  of the
tropical land of America is so raised, that it rivals,  as to agreeable  tempera-
ture, even a European climate ; while the lightness and  purity of the air, and
the brightness of the  sun, add delightfully to its charms.  The vast expanse
of table-land forming  the empire of Mexico is of this  kind, enjoying the im-
mediate proximity  of the  sun, and yet, by its elevation of seven  thousand
feet above the  level of the  ocean, possessing  the most  healthful freshness.
   The  land in many parts has the fertility of a cultivated garden, and can
produce naturally most of the riches which vegetation offers over the diversi-
fied face of the globe.   The plains of Columbia, in South America, and
indeed  all along the ridge  of the Andes, are similarly circumstanced.  The
contrast is very striking, after  sailing a thousand miles up  the level river
Magdelena, in a heat scarcely  equaled  on the plains  of India, at once  to
climb to the table-land above, where Santa Fe de Bogota, the capital of the
republic, is  seen smiling over interminable plains, that bear the livery of the
fairest fields of Europe !
  Persons   not understanding the  law  which we are now illustrating, will
express surprise that wind  or  air blowing down  upon them from a snow-
clad mountain, should still be warm and  temperate.  The truth is, that there
is just as much heat combined with an ounce of the air  on the mountain top
as in  the valley: but above, the heat is diffused through a space perhaps
twice as great as when below, and, therefore, is less sensible.  It maybe the
same air which sweeps along as  a warm gale on a plain at the foot of a moun-
tain,—which then  rises  and freezes water on the  summit—and  which in  an
hour after,  or less, is  playing among the flowers of another valley, as warm
and genial as before.
  As the temperature in different parts of the atmosphere depends thus upon
the rarity of the air, and therefore upon  the height, the vegetable productions
of each  fiistinct region or elevation are of a distinct character; and many other
peculiarities of place and climate acknowledge  the  same cause.

Because the atmospheric pressure  determines  the  temperature  of the air  in
  different situations,  as now explained,  it has  also a corresponding influence
  upon  the  state of aerial humidity, which is modified by  the temperature.
  It was explained atpage 184, that water and other liquids under a vacuum, 
PRESSURE AFFECTING TEMPERATURE.
197
rise in the form of air or vapour with force, and in quantity having a strict
relation to the temperature—heat being in fact the cause of their rising ; and the
table  at  page 186 exhibits the force, and therefore the  density of watery
vapour corresponding to some certain temperatures.   Now it is a remarkable
circumstance, that vapour  in  the  same  quantity and  of equal tension rises
from any liquid, whether placed under the pressure of air, or under a vacuum;
only through a space containing air it diffuses itself more slowly than  if the
air were not present.  As regards  the former  case, it was  for a long time
supposed that the air dissolved the  liquid as a liquid dissolves a salt: but it
now appears that there is  merely a  mechanical mixture of the two.  If the
vapour, while rising from  a liquid, has not a tension or elastic force equal  to
the pressure of the atmosphere the process is tranquil, and is called evapora-
tion,  and it goes  on  only as the vapour can diffuse itself among the particles
of the air, and therefore slowly in air perfectly  quiescent, but quicker  as the
air is  moving more, or as the density of the air is less.  But when the vapour,
owing to greater heat, is strong enough to overcome the atmospheric pressure
of fifteen pounds  per inch,  and the weight of a certain quantity of liquid over
it, the phenomena of boiling arises as already described.
   For the reason now explained, the air of our atmosphere contains diffused
through  it a large quantity of  invisible aeriform water; and if there were no
intestine motions, and no changes  of temperature  in the atmosphere, the
quantity of  water would soon everywhere reach  a  maximum, or  would  be
the greatest that the  temperature of the place could  support: instead of this,
however, from a  variety of causes to be explained below, the air  is moving
about constantly as winds, and the local temperatures are ever fluctuating, and
when the temperature is lowered, in situations where a maximum of watery
vapour is present, part of this is instantly reduced to the state of water again,
and appears, according to circumstances, in the form of mist, rain, snow  or
hail; while to supply material for these phenomena, evaporation is going on
wherever,  over water, there is not  a maximum of vapour in the air.  These
opposing operations of evaporation and condensation keep up that constant
circulation of moisture which is the life of nature.
   When a given quantity  of water  assumes the aeriform state, it'contains the
same quantity of latent heat in all  cases, whether  rising, for instance, from
a boiling caldron, or from the surface of  a lake.   Hence  we see why evap-
oration is so cooling a process  to any liquid or moistened solid from which it
is arising:   and as we have already  shown that a  rapid passing of dry air,  or
the substance being placed in  a vacuum, quickens evaporation, we now see
why  both of these conditions accelerate the cooling.   Wet linen placed in a
strong wind, which  does  not contain a maximum of moisture, becomes dry
almost immediately ; a bottle of wine covered with a wet cloth and suspended
in a current of air, as is practised in warm climates to prepare wine  for the
table, is quickly  cooled ; mats  hung around the walls of houses in India, and
frequently  wetted through the day, preserve  a  delightful  freshness in the
apartments.   Sprinkling  water or  vinegar over  a  hot sick-room  cools and
refreshes it; and watering the streets of a city moderates  in them  the  inten-
sity of summer  heat.  In  warm climates water is cooled ,for drinking by
being put into vessels so porous that the external surface is always moist, the
vessels being then suspended in a current of  air, or during a calm being made
to vibrate in the manner of a pendulum.  Again, the rapidity of evaporation
from  water under the exhausted receiver of an  air-pump,  and particularly
when some other substance which  powerfully absorbs watery vapour is in-
cluded in the receiver, is  so rapid, and carries off the heat so quickly, that
      14 
198
PNEUMATICS.
the mass of water freezes before much of it has been carried away.  This
process is used for making ice in India.
  It is partly because air saturated with moisture, that is to say, having as
much water diffused in it as can be  supported in  the invisible or aeriform
state of the existing  temperature,—lets  fall a part on any reduction of the
temperature, that air which, as a portion of the  atmosphere, has been heated
by the sun during the day, and has received much, moisture, lets it fall again
during the night, and exhibits the night  fogs of certain seasons, which float
upon the surface  of the earth, until  again  acted upon by  the beams of the
next morning's sun.  Fog, when farther condensed,  by groups of the minute
particles uniting, forms rain;  and rain when cooled becomes snow or hail.
  The quantity of dew which falls at night is influenced by the quantity of
moisture taken up by the atmosphere during the heat of  the day; and the
immediate cause of the  dew is, as was ingeniously proved  by Dr. Wells,
some years ago,  that the temperature of the objects on which it settles has
become lower during  the night than that of the air around,  and than is
required to maintain in the invisible  state, the  moisture in the  surrounding
atmosphere.   There is a tendency in  heat to diffuse itself uniformly among
bodies, by a  constant radiation from  one to  another, rapid in proportion to
the differences of temperature, and which, if  continued, would reduce all to
the same degree.   The  earth,  therefore, during the  day, receives radiated
heat from the  sun, and becomes comparatively  hot, and during the night it
gives out heat again by radiation towards the sky, from which  there is little
or no return.  When there  are clouds in the atmosphere at  night,  they
receive the heat  darted upwards from the bodies on the earth's  surface, and
they radiate heat back, becoming thus, as it were, a clothing to maintain the
warmth of the earth beneath them,—and on cloudy nights there  is no dew,'—
but with a clear  sky, the heat radiated upwards, darts into boundless space,
and is lost altogether to the objects which  emitted it.   These objects, there-
fore,  which during the day had  the same, or even a higher temperature than
the atmosphere  around, now become  colder,  and the aeriform  water which
comes in contact with them is condensed, and forms what we call dew.  This
beautiful  provision  of nature supplies the necessary moisture to vegetables
during seasons when rain is  deficient.  Dew on very cold objects freezes as
it  settles, and is then  called hoar frost.   A  phenomenon  which may be
classed with dew,  is the perspiration,  as it  is  vulgarly called, of massive
walls and furniture, occuring on the sudden setting  in of warm weather, or
on the occasion  of a warm  moist air of higher temperature than the walls
being suddenly introduced, as when a crowd assembles in a cold  church :—
the wall or other object then, from not  having  yet acquired the temperature
of the  surrounding air,  condenses  upon itself  a copious deposition of the
atmospheric moisture.   For a similar reason a bottle of wine  brought from a
cold cellar or  from  an ice-pail, into  a room with company, is  soon covered
with  thick moisture or dew;  as are  the  glasses  also into which the wine is
poured.   It is another phenomenon of the same  kind  when we see the
moisture of warm breath condensed on any cold polished surface, as on a
mirror's face, or on the glasses  of a carriage shut up, or on the windows of
a room in winter, when the surface is very cold, the moisture being frozen
with the appearance of beautiful aborescence.
   Many instruments  have been contrived, with the name of hygrometers,
for indicating the quantity of water in the atmosphere.  A prepared human
hair  is the  essential  part of one of the best  of those formerly used; the
lengthening or shortening of the hair, according to the quantity of moisture 
PRESSURE AFFECTING MOISTURE.
199
around it, being caused to move an index like that of a wheel-barometer, to
mark the degrees.  This, however, and other common hygrometers, are only
philosophical toys; but Mr. Daniel (see his excellent work, entitled Meteoro-
logical Essays) has lately given  to the  philosophical world a correct  and
simple instrument for the purpose, depending on the principle explained
above,—that whenever the temperature of a body in the atmosphere is reduced
below that at which the quantity of watery vapour in the air around it can be
maintained in the aeriform or invisible state, dew forms an  the body.   His
apparatus consists  of a bulb of glass, which can be cooled to any desired
degree from being connected  with another bulb enveloped in an evaporating
liquid ; and when moisture begins visibly to settle upon the first, its tempera-
ture is exhibited on a thermometer enclosed within it;  and the proportion of
water mixed with the air around is then, as  indicated by the table, partially
copied here, at page 186.
   A  great fall of the  barometer marks a diminished pressure in  the atmo-
sphere around, with a consequent dilatation of the air and fall of temperature,
as explained a few pages back ; and if the  air at such a time hold a maxi-
mum of moisture, a part of this must become visible as fog  or rain.  Thus
a fall of the barometer, a fall of temperature, and a fall  of rain, often occur
as associated phenomena.
   Illustrating this by experiment, we find, that on the extraction of air f om
the receiver of an air-pump, a cloud  or mist generally appears in it with the
first strokes of the piston:—the reason being that the still remaining air, because
cooled by the rarefaction, absorbs heat from the vapour in combination with
it, and renders  the water visible.   The mist is then removed by  the subse-
quent action of the machine,  or is re-dissolved when the usual quantity of air
is re-admitted.
   We understand from  this why rain happens  so  much more  frequently
among mountains than on extended plains.   When air saturated with mois-
ture approaches a mountain ridge to rise over it, for every  foot that it rises,
it escapes from a degree of the pressure which it bore while lower down, and
in then dilating, it becomes" colder, and lets fall part of its moisture. It is
the rain copiously thus produced in  mountainous regions  which  constitutes
the chief supply of their  many  rivers, and which, with periodical changes of
wind bringing more moisture, causes the extraordinary annual overflowing
of such rivers as the Nile, the Ganges, Sic
   Those who have visited the Cape  of Good Hope, will recollect a striking
phenomenon illustrative of our present subject, observed there when the wind
blows from  the south-east.   Beyond the city, as viewed from the bay, there
is a mountain of great elevation, called, from its  extended flat summit,  the
Table Mountain.  In general its rugged steeps are seen rising in a clear sky;
but when  the south-east wind blows, the whole summit becomes  enveloped
in a cloud of singular density and beauty.   The inhabitants call the pheno-
menon the spreading of the table-cloth.  The cloud does not appear to be at
rest on the hill, but to be constantly rolling onward ; yet, to the  surprise of
the beholder, it never descends, for the snowy wreaths seen falling over the
precipice towards  the  town below, vanish  completely before  they reach it,
while others are formed on the other side to replace them.   The reason of
the phenomonen is this.   The air constituting the wind from  the  south-east
having passed over the vast southern ocean, comes  charged  with  as much
invisible moisture as its temperature can sustain.  In rising up the side of
the mountain it is rising in the atmosphere, and is therefore gradually escap-
ing from a part of the pressure lately borne ;  and on attaining the  summit it 
200
PNEUMATICS.
has dilated so much, and has consequently become so much colder, that it
lets go part of its moisture.   This then appears as the cloud just described;
but it no sooner falls over the edge of the mountain, and again  descends in
the atmosphere to where it is pressed, and condensed, and  heated as before,
than it is re-dissolved and disappears :—the magnificent apparition dwelling
only on the mountain top.
   When the  elevation to which moisture  is suddenly carried is very great,
the fall  of temperature  is proportioned, and the separating water becomes
snow instead of rain.   This phenomenon is remarkably illustrated by a great
Hieroi's fountain, used in one of the mines of  Hungary; during the play of
which, the air in one place  is so compressed, that on being suddenly released,
it expands and cools enough to cause the moisture driven out with it to appear,
even in  summer, as a shower of snow.
   The foregoing reasoning explains why,  along the sides of mountain ridges,
clouds are generally  seen floating at  a certain  height only, and  therefore in
horizontal strata.  The water is separated from the air at a certain tempera-
ture, which is dependent on the height, and above that height the  air is at
the time too  dry and rare to have clouds. Very lofty summits are always
seen much above the clouds, and  the admirer of nature who climbs towards
them, may often contemplate  the grand  phenomena of the thunder-storm
far beneath his  feet.   Teiienffe soars so sublimely,  that the  distant  sailor
not unfrequently mistakes  the  line of clouds  hanging around  its sides for
the white streak which  elsewhere indicates the cliffs  and  waves of the  sea-
shore.

    Fluid support or floating, in air.  (Read the Analysis, page 156.)
  When it was explained under " Hydrostatics," that any body immersed in
a fluid has its downward tendency or weight resisted  with exactly the force
which supported the quantity  of the fluid previously occupying the same
space, and therefore that the body will sink or swim, according as it is heavier
or lighter than its bulk of the fluid, the reasoning, was as applicable to the
case of a body immersed in an  air or gas as in a liquid.
   We hence see why a body weighed in air  appears lighter, by the exact
weight of its  bulk of the  air, than  when weighed  in  an empty space or
vacuum ;—and why, for the same reason, the jocular question, whether a
pound of lead or a pound of  cork be the heavier, is not  truly answered by
saying that they are of equal weight; the cork being  really the  heavier, for
when balanced  in air, bulky cork is  more supported than dense lead.  A
small weighing-beam having attached to its opposite ends pieces of cork and,
lead which equipoise in the air, if placed under the exhausted receiver of an
air-pump, quickly exhibits  the cork preponderatino-.
   As any liquid lighter than  water, such  as oil or spirits, on  being  set at
liberty under the surface of water, will  rise, while any heavier liquid, such
as brine, syrup, or sulphuric acid, will  sink;  and in both cases with force
proportioned  to the difference of specific gravities:—so we find, that in com-
mon air, a mass of  hydrogen, or hotter  air  descends,  because specifically
lighter; while  oxygen, carbonic acid  gas, or colder  air, descends, because
specifically heavier.   This truth is well exemplified in

                              The Balloon,
which is a thin light bag of varnished silk, generally shaped like a globe or
egg, and filled with a fluid lighter than common air.   It is made sufficiently 
FLOATING--BALLOONS.
201
large that the difference between its weight when filled and  that of an equal
bulk of common air, may enable it to carry aloft the material of which it is
constructed, with the aeronauts, and their apparatus.  It is  in principle like
a bladder of oil immersed in water.  A globe of thirty-five feet diameter has
a capacity of nearly twenty-two thousand cubic feet.  This quantity of com-
mon air weighs about sixteen  hundred pounds, and  the same quantity of
hydrogen gas, of easily obtained purity, weighs only one-eighth as much, or
two  hundred  pounds.   Such a globe, therefore, being buoyed up, or sup-
ported in common air, with a force of sixteen hundred pounds, while, if filled
with hydrogen, it only weighs two hundred, will carry  up into the sky four-
teen hundred pounds of material and load.
   The first balloon was exhibited by a  man ignorant of what he was really
effecting.  Seeing the  clouds float  high in the  atmosphere, lie thought that
if he could make a cloud and enclose it in  a bag, it might  rise and  carry him
with it.  Then, erroneously deeming smoke and a cloud the same, he made
a fire of green wood, wool, Sic, and placed a great bag over with  the mouth
downwards to receive the smoke.   He soon had the joy to see the bag full,
and, when set free, ascending; but he understood not that the cause was the
hot and dilated air within, which, being  lighter  than  the  surrounding  air,
was buoyed up; while the visible part of the smoke, which chiefly engaged
his attention, was really heavier  than the air, and was an impediment to his
wishes.
   This modification, called the hot air or fire balloon, was  afterwards better
understood, and was used by aeronauts, until the more commodious* and less
dangerous modification, called  the  inflammable air  balloon, or  balloon of
hydrogen gas, was substituted.
   Since the modern  introduction of gas lights,  the carburetted hydrogen
prepared for them  is  generally employed  for filling  balloons.   It is consi-
derably heavier than pure hydrogen, but is so much more readily obtained,
that aeronauts like better to make a larger balloon  to suit it, than a smaller
one which obliges them to prepare the  other.—A  thin paper bag,  filled with
the hot air rising from a large lamp, is'a miniature hot air or fire balloon;
and a common soap bubble, filled with hydrogen, is a little  inflammable air
balloon, which mounts with great rapidity.
   There are, perhaps, few  occasions on  which a youth is more surprised
and delighted than when he first beholds a balloon  sailing high in  the bosom
of the air and bearing a human being to regions far beyond what the soaring
eagle has ever reached: while to the intrepid aeronaut  himself, the scene of
a world displayed  beneath him is unquestionably the grandest, except that of
the  starry  heavens, which mortal  eye  has ever compassed.  To him even
 wide spread London, the queen of the  cities of the earth, and a little world
 within'itself, when viewed from a great elevation  in the sky, appears but as
a dusky patch  upon a  map, with the far-famed Thames winding  there as a
silvery line, and the magnificent temples  and palaces  scattered around, ap-
pearing but as darker points rising  out of the general  mist of buildings, in
which a million and a half of human beings reside.
   The first aeronautic expeditions astonished the world, and endless reveries
passed through men's minds of  important uses to  which  the new discovery
mio-ht be applied:  but more mature reflection, and now  frequent trials have
shown that the balloon, while furnishing  philosophers with the opportunity
of making some  observations in  elevated  regions  of the  atmosphere, is  still
interesting chiefly as a philosophical toy.  The French, under the Directory
in 1796, attempted to  use it as a  military station, from which the position 
202
PNEUMATICS.
and  motions of an enemy might be descried: but  the plan was  eventually
abandoned.   It has since been thought of as a means by which travellers
might obtain information while penetrating into unknown countries, like the
almost interminable plains of Australasia.   Although aeronauts, while aloft,
have the power of making the balloon rise farther by throwing out part of
the  sand-ballast which  they carry  with  them, or of making  it descend by
opening  a  valve  at the top through which the hydrogen may escape, still
they have  no power of producing a lateral motion.  The idea  which yet
stongly excites the minds of some projectors, that by wings or other means,
a balloon may be directed in the sky nearly as a ship is directed on the sea,
is not much more reasonable than to suppose that an insect, suspended to a
huge block of wood, driven along at the  rate of eight or ten miles an hour
by river  torrent, should  have power to stop or  sail against  the stream.  A
man in a balloon  would  generally have to resist or change a motion exceed-
ing fifty  miles in  an hour.
  A  balloon which  is  only half full at the surface of  the  earth, becomes
quite full when it has risen three miles and a half, because, at  that altitude,
air from  below doubles  its  volume  on account of the diminished pressure.
A balloon, therefore, if quite distended on first  rising, must let  air escape as
it ascends, or  it will burst: this is true  also of the drum of the  human ear
under the same  circumstances, and in  a  contrary  way under  the opposite
circumstances of  a person descending in a diving-bell.
  The downy seeds  of plants seen floating about  upon the winds  of autumn
are not lighter than air,  but have so much bulk and surface in  proportion to
their weight, that the friction upon  them of the moving  air is greater than
their weight, and carries them along.
  A  sheet  of paper,  made in some degree  to resemble a balloon, by its
having a little weight,  representing  the  hanging car, attached by threads
from its angles, is often  seen rising at a street corner, to the delight of the
boy  who watches it.   Its rise  depends upon eddy winds or currents which
the corner produces.

                     The ascent of flame and smoke
in the atmosphere, affords  other examples  of  a lighter fluid rising  in  a
heavier;  for both these are merely hotter air rising in the midst of colder.
  The phenomenon of flame is produced when a  burning substance contains
some ingredient capable, on being heated, of assuming the form of air or gas,
which ingredient, on ascending, burns or  combines with the oxygen of the
atmosphere, with intensity of action sufficient to produce a white heat.  It
is because  charcoal  and coke have nothing in them thus volatile, that they
burn without flame, appearing like red-hot stones.   The flame  of a lamp or
candle is merely the oil, wax, or  tallow  converted  into gas, and  allowed to
burn as it is disengaged  and rises.   The same  gas obtained by heating the
oil, &c,  in vessels which exclude  the atmosphere, so as to prevent imme-
diate combustion, and from  which  tubes lead to  suitable receptacles, is the
common  oil-gas used for illumination.
  Smoke consists of all the dust  and visible particles which  are  separated
from the fuel without being  burned, and are,  moreover, light  or  minute
enough to be carried  aloft by the rising current of heated  air;° but all that is
visible of smoke is really heavier  than air, and soon falls  again as powdered
chalk falls  in water.   In the  receiver of  an air-pump, where a candle has
been extinguished by  exhausting the air, the stream of smoke that conlinues 
FLOATING--FLAME AND SMOKE.
203
to pour from the wick after the exhaustion, is seen to fall on the pump-plate,
because there is no air to support it.
   Chimnies quicken the ascent  of hot air  by keeping a long column of it
together.  A column of two feet  high  rises, or is pressed up with twice as
much force as  a column of one foot, and so in proportion for all other lengths ;
just as two or  more corks strung together and  immersed in water, tend up-
wards with proportionally more force than a single cork; or as a long spear
of light wood,  allowed to ascend perpendicularly from a great depth in water,
acquires a velocity which4makes it dart above the surface, while a short piece
under the same circumstances rises very slowly.   In a chimney where  one
foot in height  of the column of hot air is one ounce lighter than the same bulk
of the external cold air, if  the chimney be  one hundred feet high, the air or
smoke in it is  propelled upwards with a force of one hundred ounces.  In all
cases, therefore, the draught, as  it is called, of a chimney, is proportioned to
its length.  The following facts are consequences of this truth.
   In  low cottages, and in the upper floors of houses, the annoyance of smoky
rooms is much more frequent than where chimneys are longer.
   If there are  two fires in the same room, or in any rooms open to each other,
 which have chimnies  of different lengths, and of which the doors and win-
 dows are very close, so that air to supply the draughts  cannot enter by them,
 the taller chimney will overpower the shorter,  and cause it to smoke into the
 room; just as  the long leg of a syphon  overcomes the short  one, or as a long
 log of wood, held down in water by a cord  passing from it round a pulley at
 the bottom to  a  shorter log also floating, will rise, and  pull down the shorter
 l°g-
   A  long chimney, for the reasons above explained, causes a current of air
 to pass through the fire very rapidly, and it has the advantage also of acting
 more uniformly than any bellows .or blowing machine.   On these accounts,
 or fires of steam-engines, and many others, it is the means of blowing gene-
 rally preferred.  The importance of length in a chimney explains the remarka-
 ble appearance  of some mining districts and modern English towns, where
 steam-engines abound.
   When we heap dying embers together, so  that the  hot air rising  among
 them may become a mass or column of considerable altitude, this column has
 the effect of blowing them  gently, and helps to light them up again.  A piece
 of burning paper thrown upon the top of a half-extinguished fire, often makes
 it blaze afresh, by causing  a more rapid current of air to pass through it from
 below.

 The  action or draught of a chimney, influenced  as we have seen, by its length,
   depends also  on the degree in which the air  in it is heated, because this de-
   termines the dilatation, or comparative lightness, which makes the air ascend.

   In what are called  open fire-places, such as  those in  the sitting-rooms of
 Britain, a large  quantity of air directly from the apartment enters  the chimney
 above the fire, and mixes  with the hot air from the fire itself.   This mixture
 ascends more slowly than if hot air alone  entered, and in  a proportion de-
 pendent on the  degree  of  mixture.  The effect of excluding a part  of this
 colder air, is  seen when a board or plate  of  metal is suspended across the
 opening  of the chimney,  so as to narrow  the entrance:—almost instantly a
 quicker action is produced, and  the fire begins to roar as if blown by a bel-
 lows   This  means is often used  to blow  the fire instead of bellows, or to
 cure a smoky chimney, by increasing the draught.  What is called a register 
204
PNEUMATICS.
stove is a kindred contrivance.   It has a flap placed in the throat of the chim-
ney, which serves to widen or contract the passage  at pleasure.  Because the
flap is generally opened only enough  to allow that air to pass which  rises
directly from the fire, the chimney receives only very hot air, and therefore
acts well.   The register stove often cures smoky chimnies:  and by prevent-
ing the too ready escape of the moderately warmed air of the room, of which
so much is wasted by a common fire-place, it also  saves fuel.  In what are
called close fire-places, as those of steam-engines, or brewers' coppers, when
the furnace door  is shut, no air can enter  the chimney but directly through
the fire; hence the action of such chimnies is very  powerful.
   In a room with two fires, or in drawing-rooms  communicating with each
other, although the chimnies be of equal length, that one  over the best fire
will act the most strongly ; and if the  doors and windows  of the apartment be
so close as to prevent a sufficiency of air  from  entering by them  to  supply
both fires, cold air will enter by that chimney which has the weakest fire, and
the smoke from it will spread into the room.   How often is an  assembling
dinner party annoyed by the smoke of a second drawing-room fire just lighted
before their arrival, and which had therefore to  contend  with the  antagonist
fire already in powerful action all the day.  While only one fire was lighted,
the cold chimney was admitting the air to feed it, just as an open pane in the
window would have done.   A room  may be so close, that  no air can find
entrance, and in such a case the smoke of its fire must all spread into the room.
   When all the windows and doors of a house fit  so closely as not to admit
air for the acting chimnies, the supply comes down the chimnies that are not
in use.  Inattention to this  fact causes many a good chimney to incur the
imputation of being  smoky, because on the attempt being made to  light a fire
at it, the smoke at first is always thrown back.   The truth is, that  at the time
when the  servant begins to light the fire, there is a downward  current in the
chimney,  repelling,  of course, any heated air and smoke that approaches it,
and spreading them  over the  whole house; but were the room door to  be shut
for a few minutes, so as to cut off communication  with the other drawing
chimnies in the house, while at the same time the windows were opened, the
chimney would act at once ; and when sufficiently  heated, would continue to
act in spite of the others, and as well  as they.
   There are some cases of smoky rooms not to be so easily corrected  as what we
have now mentioned. When a low house adjoins a lofty house, the wind blow-
ing towards the latter, is obstructed and becomes a gathering or condensation of
air against the  wall; and if the top of a low chimney be there, the compressed
air enters it, and  pours  downwards.   The same happens occasionally from
the proximity of trees or rocks.   In  such cases, to avoid the influence, the
chimnies of the low  house are often made very lofty.  Again, whenever, from
the nature of buildings, eddies  of wind  occur, or unequal pressures, as at
street corners, &c, the chimnies around do not act regularly.  It is proverbial,
that corner houses, or those at the ends of a row, are smoky houses;  and we
see the uniformity of architecture in a street often destroyed  by the necessity
of lengthening the chimnies of the houses  at the extremities.
   When smoke is found  descending  into  a room where  there is  no fire, the
empty chimney is serving as an inlet for air to the house,-while  the  smoke
of a neighbouring chimney is passing closely over the top of it.
   In summer,  when fires are not in use, there  is often a strong smell of soot
perceived  in the  apartments  during the  whole of the day, but which ceases
at night.   The reason is, that during the day the chimney is colder than the
external air, and by condensing the  air which enters it, causes a  downward 
FLOATING--FLAME AND SMOKE.
205
current- through the soot.   During the night, again, when the external air
becomes colder, owing to the absence of the sun, the chimney, by retaining
the heat absorbed during the day, is hot enough to warm the air in it, and to
cause an upward current.   These currents, in  chimnies  left open  during the
days and nights of summer, are almost as regular as the land and sea breezes
of tropical countries.
  All these remarks prove how important it is to be able to conceive clearly
of the motions going on, according to  the simple laws of matter, in the in-
visible air around us.   Were such subjects better and more generally under-
stood, many prevalent errors in the arts of life,  influencing much the comforts
and health of the community, would soon be corrected.
   If we are filled with admiration on discovering how  perfectly the simple
law of a lighter fluid rising in a heavier, provides a constantly renewed  sup-
ply of fresh air to our fires, which supply we should  else have to furnish by
the unremitted action of some expensive blowing apparatus, still more must
we admire that the operation of this law should  effect the more  important
purpose  of furnishing the ever-renewed  supply of  the same vital fluid  to
breathing creatures.  The air which a man has once respired becomes poison
to him; but because the temperature  of his body is generally higher than
that of the atmosphere around  him, as soon as  he has discharged any air
from the lungs, it ascends completely away from him into the great purify-
ing laboratory of the atmosphere, and  new  air takes its place.   No art or
labour of his, as by the use of fans or punkas, could have done half so well
what this simple law unceasingly and  invisibly accomplishes, and accom-
plishes without effort or even attention  on his part, and in his sleeping as in
his waking hours.  Truly in this, may he be  said to be watched over  by a
kind Providence.

                 The warming and ventilating of houses
is an important art, founded chiefly on  the foregoing  considerations, and  at
present too little understood, not only by the  public at large, but even by
medical practitioners, whose management of  disease, though judicious in
other respects, is often rendered vain by error or  omission in this.
   Excellent fuel is so cheap in Britain, owing to the profusion with which
beds of rich coal are scattered in it, that a careless  domestic expenditure has
arisen;  which, however,  instead of securing  the comfort and health  that
might be expected, has led to plans of  warming  which often  prove  destruc-
tive to both.  The mischief lies chiefly in the unsteadiness or fluctuations of
our domestic temperature;  for in still colder countries, and where fuel is
more  expensive, as in the north of continental  Europe, the  necessity for
economy has led to contrivances which give steady temperature and impunity.
   In cold countries, to retain and  preserve the heat once obtained, the houses
are made with  thick  walls, double windows,  and nice  fittings; and more-
over with close  stoves or fire-places, which draw  their supply of air, not from
the apartments  where they are placed, wasting the  temperate air  of these,
but directly from without.   Thus fuel is saved to a great extent, and a uni-
formity of temperature is produced, both as regards the different parts of the
room, so that the occupiers may sit with comfort where they please, and as
regards the different times of the day, for the stove being once heated in the
morning, often suffices to maintain a steady warmth until nitrht.   The  tem-
perature can be carried to any  required degree,  and  sufficient ventilation is
easily effected.
  In England,  again, the apartments,  with their open  chimnies,  may be 
206
PNEUMATICS.
compared to great  air-funnels, constantly pouring  out their  warm contents
through  a large opening, and consantly requiring to be replenished.  They
thus waste fuel exceedingly, because the chimney being large enough to allow
a whole room-full of air to pass away in two or three minutes, the air of the
room has to be  warmed, not once in the course of the day,  but very many
times.  The temperature in them is made to fluctuate  by the slightest causes,
as the opening a door, the omitting to  stir the  fire, Sic  The  heat  is very
unequal in different parts of the room, rendering it necessary in general for
the company to sit near the fire ; where they must often submit to be almost
scorched on  one side, while they are chilled on the other.   There is generally ■
a warm  stratum of air above the level of the  chimney-piece, surrounding,
therefore, the  upper part of the bodies of persons in  the room, while a cold
stratum below  envelops  the  sensitive feet and legs.  As a very rapid current
is constantly ascending in the chimney, a corresponding supply must be en-
tering some  where; and  it can  only  enter by the crevices  and defects in the
doors, windows, floors, Sic:—now there is nothing more dangerous to health
than to sit near such inlets,  as is proved  by the rheumatisms, stiff necks and
catarrhs, not to mention  more serious diseases, which so frequently follow the
exposure.  There is an old Spanish proverb, thus translated,

                  "If cold wind reach you through a hole,
                   Go make your will and mind your soul,"

which is scarcely an exaggeration.
  Consumption  is the disease which carries off a fifth  or more of the persons
born in Britain;  owing in part, no doubt, to the changeableness of the exter-
nal climate, but much more  to the faulty modes  of  warming  and ventilating
the houses.  To judge of the influence of temperature in producing this disease,
we may consider,—that miners who  live  under ground,  and are always,
therefore, in the same temperature, are strangers to it, while  their  brothers
and relatives, exposed to the vicissitudes above, fall victims,—that butchers
and others, who live almost  constantly  in the open  air, so as to be  hardened
by the exposure, enjoy nearly equal immunity,—that consumption is scarcely
known in Russia, where close stoves  and  houses preserve a  uniform tempera-
ture within doors, while fit  clothing gives safety on going out,—and that in
all countries and situations, whether tropical, temperate   or  polar, the fre-
quency of the  disease bears  relation  to the degree  and manner of change.
We may here  remark, also, that it is not consumption alone  which  springs
from  changes  of temperature, but  a great proportion  of acute diseases, and
particularly of the common winter  diseases of England.   There  are a few
cases of these  in which the invalid  has  not to remark,  that if he had avoided
cold or wet on some certain occasion, he  might yet have been well.
  While temperature is thus so frequently an original cause of disease, it is
also a circumstance of the very highest importance  in the  treatment,—as is
proved by every fact bearing upon  the  question.  We may, therefore, at first
wonder  that it should be so negligently and unskilfully controlled as we often
see it; disease and death being thence allowed to lurk,  as it were, undisturbed
in  the sanctuaries of our homes: but when we reflect on the subtile and in-
visible nature of air and heat, and  that the science  which detects their agen-
cies has been hitherto so little an object  of general study, and is, indeed, of
modern  discovery, the fact is accounted for.
  In England, the open fire-place is  so generally in use for common dwell-
ings, and the  cheerful blaze is accounted so essential  to the comforts of the
winter days  and long evenings, that it would be difficult to  persuade persons 
              FLOATING—WARMING AND VENTILATING.          207

to abandon it: let us  hope, then, that when the subjects which we are now
discussing  come to be better and more  generally understood, the open fire,
with close flooring, better  for double windows, doors that fit well, register
stoves, and good  general  management, may be rendered  almost  as efficient
for warming, and as safe to health, as any other contrivance.
  The following considerations present  themselves  in  this place.—Small
rooms in winter are more dangerous  to health than  large ones, because the
cold air, entering towards the fire by the doors or windows, reaches the per-
sons in  the room before it can be tempered by mixing with the  warmer air
already  around them.—Stoves in halls  and stair cases are useful, because
they warm the air before  it enters the rooms; and they  prevent the hurtful
chills often felt on passing through a cold stair-case from one warm room  to
another.—It is important to admit no more cold air into the house than is
just required for the fires and for ventilation;  hence  there is a great error in
the common practice of leaving all the chimnies that are not in use quite open,
each admitting air as much as a hole in the wall, or an open pane in the window
would do.—Perhaps the best mode of admitting air to  feed the fires is through
tubes, leading directly from the outer air to the fire-place, and provided with
what are called throttle-valves, for the regulation of  the quantity ; the fresh
air  admitted  by them  being made  to spread in the  room either at once,  or
after having been warmed during its passage  inwards,  by coming near the
fire.—In a very close apartment, ventilation  must be  expressly provided  for
by  an opening near the ceiling, through which the impure air, rising from the
respiration of the company, may pass away.   With  an open fire, the pur-
pose is  effected, although less perfectly, by the frequent change of the whole
air of the room which that construction occasions.
   With a view to have, in  rooms intended for invalids, the  most perfect
security against cold blasts and fluctuation of temperature, and still to retain
the so much valued appearance of the open fire, a glazed frame or window
may be placed  at the entrance to the chimney or stove, so as completely to
prevent the passage of air from the room to the  fire.  The room will then  be
warmed by the fire through the glass, nearly as a green-house is warmed by
the rays of the sun.   It is true, that the heat of combustion does not pass
through glass so readily as  the  heat of the  sun; but the difference for the
case supposed  is not  important.   The glass of such a window must, of
course,  be divided into small panes, and supported by a metallic frame-work
to resist the heat; and there must be a flap or door in  the frame-work, for
the purpose of admitting the fuel and stirring the fire.  Air must be supplied
to the fire, as described above, by a tube leading directly from  the external
atmosphere to the ash-pit.   The ventilation of the room  may be effected  by
an opening into the chimney near the ceiling;  and the temperature may  be
regulated  with  great precision by a valve placed in  this  opening, and made
to obey the dilatation and contraction of a piece of wire affixed to it, the
length of  which will  always depend on  the temperature of the room.—The
author contrived the arrangements here described, for the winter residence of
a person threatened with consumption, and the happy issue of that particular
case, and of others treated on similar principles, has  led him to doubt, whe-
ther many of the patients with incipient consumption who are  usually sent
to warmer climates, and who die there after suffering hardships on the jour-
ney, and distress from the banishment sufficient to shake even strong health,
mio-ht not be saved by judicious treatment in properly warmed and ventilated
apartments, under their own roofs,  and in the midst of affectionate kindred.
And if a boy be almost certainly secured from consumption by being made 
208
FNEUMATICS.
a miner or a butcher, may we not hope that, when all the influencing circum-
stances come to be better understood, something of the same immunity may
be obtained  for  persons  in  all the  professions and conditions of civilized
society ?
  It must not be supposed that  the remarks made in this section exhaust
even nearly the very important subject of temperature as  affecting  health.
The questions of clothing, of  hot and cold bathing, of exercise, and  others,
equally belong  to it, but  the consideration of them falls under other  depart-
ments of study.

                   Winds or currents in the atmosphere

are also phenomena, in a great measure dependent on the law, that lighter
fluids rise  in heavier.  As  oil let loose under water  is pressed up to the
surface and swims,  so air near the surface  of the earth, when heated by the
sun, rises to the top of the atmosphere, and spreads there,  forced up by the
heavier air around  ; this heavier air rushing inwards, constitutes  the wind
felt at the surface of the earth.   The cross currents in the atmosphere arising
as now described,  are often rendered  evident  by the motion  of clouds or
balloons.
   If our globe were at rest,  and the sun were always beaming over the same
part, the earth  and  air directly under the sun would become exceedingly
heated, and the air  there would be constantly rising like oil in  water, or
like the smoke  from a great fire : while currents or winds below would be
pouring towards the central  spot,  from  all directions.  But  the  earth is
constantly turning round under the sun, so  that the whole middle  region or
equatorial belt may  be called the sun's place: and therefore, according to the
principle just laid down, there should be over it a constant rising of  air, and
constant currents from the two sides of  it,  or the north and south, to supply
the ascent.   Now this phenomenon is really going on, and has been going on
ever since the beginning  of the  world, producing the  steady winds of the
northern and southern hemispheres, called trade winds, on  which  in most
places within thirty degrees of the equator, mariners reckon  almost  as con-
fidently as on the rising and setting  of the sun himself.
   The  trade winds, however, although thus moving from  the poles to the
equator, do not appear on the er.rth to be  directly north and  south, for the
eastward whirling,  or  diurnal  rotation of the earth, causes a wind from the
north to appear as if coming from the north-east, and a wind from the south
as if coming from  the south-east.  This fact is illustrated by  the case of a
man on a galloping horse, to whom  a calm  appears to be a strong wind in his
face ; and if he be riding eastward, while the wind is directly north or south,
such wind will appear to him  to come from the north-east, or south-east:—
or again, is illustrated  by the case of a small globe made to turn upon a per-
pendicular axis, while a ball or some water is allowed to run from the top of
it downwards;—the ball or  water will  not  immediately acquire the whirling
motion of the globe, but will fall almost directly downwards, in a track which,
if marked upon the  globe, will appear not as a direct line from the axis to the
equator, that is from north to  south, but as a line  falling obliquely.   Thus,
then, the whirling of the earth is the cause  of the oblique and westward direc-
tion of the trade winds, and  not, as has often been said, the sun drawing them
after him.
   The reason why  the trade winds at their external confines, which are about
 30 degrees from the  sun's place,  appear almost directly  east, and  become 
FLOATING—WINDS.
209
more nearly north and south as they approach the central line, is, that at the
confine they are like fluid coming from the axis of a turning wheel, and which
has approached the circumference, but has not yet  acquired the velocity of
the circumference; while, nearer the line,  they are  like the fluid after it has
for a considerable  time been turning on the circumference, and has acquired
the rotary motion there, consequently appearing at rest as regards that motion,
but still leaving sensible any motion in a cross direction.
   While, in the lower regions of the atmosphere, air is thus constantly flow-
ing towards  the  equator  and  forming the steady trade winds between the
tropics, in the upper regions there must of course be a counter-current distri-
buting the healed air again over the globe:  accordingly, since reasoning led
men to expect this, many striking proofs have been detected.   At the summit
of the Peak of Teneriffe, observations now show that there is always a strong
wind blowing in a direction contrary to that of the trade wind on the face of
the ocean below.  Again, the  trade winds among the West India Islands are
constant, yet volcanic dust thrown aloft from the Island of St. Vincent, in the
year 1812, was found, to  the.  astonishment of the inhabitants of Barbadoes,
hovering over them in thick clouds, and falling, after coming more than 100
miles directly against the  strong .trade wind, which  ships must take a circui-
tous course  to avoid.   Persons sailing from the Cape of Good Hope to St.
Helena, have often to remark  that the sun is hidden  for days together, by a
stratum of  dense clouds passing southward high in the atmosphere;  which
clouds consist of the moisture raised near the equator with the  heated air,
and  becoming condensed again as  it approaches the  colder regions of the
south.
   Beyond  the tropics, where  the heating influence of the sun is  less, the
winds occasionally obey other causes than those we have now been consi-
dering, which causes have not  yet  been  fully investigated.  The winds of
temperate climates are in consequence  much less  regular, and are called
variable; but still, as a general rule, whenever air is moving towards the
equator, from the north or south poles where it was at rest, it must have the
appearance of an east wind, or a wind moving in the contrary direction of the
earth itself, until it has gradually acquired  the whirling motion of that part of
the surface of the earth on which it is found; and again, when air is  moving
from the equator, where it had at last acquired nearly the same motion as that
part of  the earth,  on  reaching parts nearer the poles, and which  have less
eastward motion, it continues to run faster than they, and becomes a westerly
wind.   In many situations beyond the tropics, the westerly winds, which are
merely the upper equatorial currents of air falling down, are almost as regular
as the easterly winds within the tropics, and might also  be called trade winds:
—witness the usual shortness  of the voyages from  New York to Liverpool,
and the len<nh of those made in the contrary direction.  North of the equator,
then, on earth, true north  winds  appear to be north-east, and true south wind
appear to be south-west:—which are the two winds that blow in England for
three hundred days of every year.  In southern climates the converse is true.
   While the sun is beaming directly over  a  tropical island, he warms very
much the surface of  the soil, and therefore also the air over it; but the rays
which fall upon the ocean around penetrate deep into the mass, and produce
little increase of superficial temperature.   As a consequence of this, there is
a rapid  ascent of hot air  over the island during the day, and a cooler wind
blowino- towards  its  centre from all directions.  This wind constitutes the
refreshino- sea-breeze of tropical islands and coasts.   A person  must have
been among these, to conceive the  delight which the sea-breeze brings after 
210
PNEUMATICS.
the sultry stagnation which precedes it.   The welcome ripple shorewards is
first perceived on the surface of the lately smooth or glassy sea ; and soon the
whole face of the sea is white with little curling waves,  among which the
graceful canoe, lately asleep on the water, now shoots swiftly along.
   During the night a phenomenon of opposite nature takes place.  The sur-
face of the earth, then no longer receiving the sun's rays, is soon cooled by
radiation, while the sea whicli absorbed heat during the day, not on  the sur-
face only, but through its  mass, continues to  give out heat all night.   The
consequence is, that the air over the earth becoming colder than that over the
sea, sinks down, and spreads  out on all sides, producing the  land-breeze of
tropical climates.  This wind is  often charged  with  unhealthy exhalations
from the marshes and forests, while the sea-breeze is all purity and freshness.
Many islands and coasts would be absolutely uninhabitable but  for the sea-
breeze.
   The peculiar distribution of land in the Asiatic part of the globe, produces
the curious effect there of  a sea-breeze  of six months, and a  land-breeze of
six months.  The great continent of Asia lies  chiefly north of the line, and
during its summer, the air  over it is so much heated, that there is  a constant
steady influx from the south—appearing south-west, for the reason given in
a preceding page ; and during its  winter  months, while the sun is over the
southern ocean, there is a constant land-breeze from the north—appearing,
for a  like reason, north-east.   These  winds  are  called monsoons; and if
their utility to commerce were to be a  reason for a name, they also deserve
the name of trade  winds.  In early periods of navigation, they served  to
the mariner the purpose of compass, as well as of  moving power ; and one
voyage outward, and another  homeward with the changing monsoons,  filled
up his year.—On the western shores of Africa and America also, the trade
winds are interfered with by the heating of the land ; but  much less so than
in Asia, and always in accordance with  the laws now explained.
   The frightful tornadoes, or  whirlwinds, which occasionally devastate cer-
tain tropical regions, making  victims  of every ship  or bark  caught on the
waters,  and the shore gusts  or squalls  met  with everywhere, are owing
to some  sudden chemical changes  in  the atmosphere, not yet fully under-
stood.

                 The Pneumatic Trough and  Gasometer
of the chemist are contrivances constantly displaying the truth now under
consideration, "that a lighter fluid is pushed up and floats  on a heavier."
They are important parts of the apparatus for operating on substances while
in the form of air.
   The trough a may be made of metallic plate, or of wood lined with metal,
                          and of any convenient size.   It is  nearly  filled
        Fig. 106.           with water, and has at one end  about  an inch
                          under the surface of the water, a shelf, on which
                          jars  or vessels, as  b  and c,  may  rest.   Any
                          particular air or  gas  is preserved  separate  from
                          the atmosphere,  by being placed in one of these
                          jars  with  the mouth downwards.  The gas is
                          passed into the jar by  the operator first immers-
                          ing the jar in the trough, so as  to fill it  with
                          water and to expel the common air from it; and
                          then holding its  mouth over the gas while rising
                          under the water  from another  vessel or  pipe:—
 
FLOATING—GAS APPARATUS.                 211
0'
a.
III M; \\e f\\
a
d represents a long-necked vessel, used to contain the ingredients for the pro-
duction of gases by chemical action.  The gas of course rises to the top of
the jar b, and gradually displaces the water.   During the operation of filling,
the jar may be  supported by the hand or by resting upon the shelf;—in the
latter case the gas is allowed to rise into it through a hole in the  shelf,  pro-
vided with  a small funnel gaping downwards to catch  the  air more readily.
The shelf may have room on it for many  jars, and it  may have more  holes
than one;  and if the gas under  operation  be such that water  absorbs  or
changes it, some other liquid, as mercury, may be used instead of water.
   A gasometer or gas-holder, is merely a larger jar or vessel as a, dipping
into  water, with its mouth downwards, in a
trough of its own  shape, b c, and so sup-            Fig. 107.
ported or counterpoised  by a  weight at  d,
over pullies, that  very little force  suffices  to
move it  up or down.   Air forced  into it
through a pipe f opening under it, causes it
to rise or float  higher in  proportion  to the
quantity.   The air  is made to pass from it
again when wanted, either through  the same
tube or through another as e.
   The huge gasometers, exceeding in size    -^
an ordinary house, and containing the supply
of gas for the lamps of a town, are vessels
suspended as above represented, in great pits
or troughs, filled with water.  The gas issues
with force  proportioned to the  downward
pressure of the  containing vessel, which may    —
be nicely regulated  in a variety of ways, and
is generally made to equal the action of a  column of water of two inches in
heiffht; that is to say, such, that a pipe issuing from the gas holder, and dip-
ping into water as its other end, shall allow gas to escape, if immersed less
than two inches perpendicularly.
   It would be encroaching on the province of the chemist to treat here par-
ticularly of  the substances which most generally exist in the aeriform state ;
but to give an increased interest to the description of the gas apparatus, a few
leading facts may be mentioned.
   Of about fifty distinct substances known as the  materials of our globe, five,
when uncombined,  and under  common circumstances of heat and pressure,
exist as airs or gases.  The water used to fill  the  apparatus above described
is a compound of two of the  substances,  viz., oxygen and  hydrogen.   By
directing an electrical current through water, it is gradually decomposed, and
from one side, a stream of aeriform oxygen may be received, and from the
other a stream of hydrogen.   The two gases  may be again  united  to form
water, by mixing  them in a proper  vessel, and  passing  an electric  spark
through them.   They combine with explosion.
   This oxygen, so called from its relation to  acids, (the name consisting of
two Greek words, signifying acid and to form,) has  been accounted, for
many  reasons, the  most important substance in nature.   It forms  eight-
ninths, by  weight,  of the ocean ;  one-fourth of  the  atmosphere; and per-
haps,  one-fourth of the solid  matter of the globe : possibly,  therefore,
although most  persons think of it only as an  air  or gas, there is  not  a mil-
lionth part of the  quantity of oxygen in  the world,  existing as  air.   It
unites readily with most other substances, and generally with  such intense
V- 
212
PNEUMATICS.
action  as to produce the phenomena of fire or combustion ; the word  com-
bustible chiefly applies to substances that quickly combine with oxygen.
  Oxyo-en  assumes a sino-ular  variety of character in  its different  combina-
tions.  Thus  with  hydrogen, it forms water; with lead, it forms the sub-
stance  called red-lead;  with nitrogen, m one proportion, it forms atmospheric
air, in  another proportion, the nitrous oxide, or what is called the laughing
gas, in a  third, the acid  called aqua forth;  with sulphur,  it forms the
sulphuric acid or oil of vitriol;  with iron,  and all metals, it forms  their ores
called oxides : and so forth.   But the most important character in which we
know it, is as that ingredient of our atmosphere,  without which animals and
vegetables cannot live,  and fire cannot burn.  Oxygen, from this part of its
history, was long named vital or pure air.
  Pure oxygen, in the  state of air is a little heavier than common air; but
when holding a quantity of charcoal in solution, it forms aeriform  carbonic
acid, which is nearly twice as  heavy as common air, and may be poured out
of one  vessel into another  like  water.  Carbonic acid is what issues  from
soda-water,  brisk ale, champagne, Sic, while  they sparkle.  If drawn into
the  lungs in breathing,  it is fatal  to life.  A charcoal fire left in a close room
with sleeping  persons, has often  been fatal to them,  because carbonic acid
gas is  the product of the combustion.  So likewise, houseless wretches in
winter lying down in a brick-maker's field  to  leeward  of a burning heap of
bricks, often fall asleep for ever.  The famous  Grotto del Cane, in Italy, is
a cavern always full of carbonic acid, which  springs  into it from below, as
water springs into a well, and runs over like water from a well:—it received
its name from  the circumstance of dogs dying instantly when thrown into it.
Carbonic acid rising in  fermentation has often  proved fatal to persons leaning
over the edge of fermenting vats.  It is common  to see a rat die instantly, in
the  attempt to run a plank  laid across the mouth of a fermentino- tub.
  Hydrogen, the other ingredient of water, and so called from its relation to
wafer (the name consists of the Greek words for water and  to form,) when
in the state of air, is sixteen times as  light  as oxygen.   With it balloons are
filled.  When it holds  in solution a  certain  quantity of carbon or charcoal, it
becomes the common gas used for illumination, and is the fire-damp  of mines,
of which the burning and explosion are so terrible.   It forms one-ninth of the
ocean,  and  much of animal and vegetable bodies.
  Nitrogen, so  called  from its relation to  nitric acid, is the third and last
substance which  we shall mention.   It is  what remains of the  atmosphere
when the oxygen is removed.   It forms about four-fifths of the atmosphere,
one-fourth of the animal flesh, and is found in small quantities in  the other
combinations.   It will  not support life by itself,  and therefore formerly was
called azote: with a larger  proportion of oxygen, it forms nitric acid or the
aquafortis of old.
 _ The last few paragraphs may serve to show how many of the manipula-
tions of chemistry are directed by the  principles  of physics or mechanical
philosophy; and therefore, how essential  to  the chemist  the preliminary
study of physics becomes. 
DISCHARGE FROM APERTURES.                213
           PAET  III.

                   OR

THE  PHENOMENA OF  FLUIDS.

              (continued.)
SECTION  III.—HYDRAULICS—PHENOMENA OF  FLUIDS  IN
                              MOTION.
                      ANALYSIS  OF THE SECTION.

Whether the particles of matter exist in  the form of solid or fluid, the cir-
  cumstance does not affect their properties of inertia, and gravity.—
  Hence liquids and airs, in proportion to their quantity, resist,  receive,
  and impart motion, and have weight and friction, as is true of solids.
  This is seen in the phenomena of
1. Fluids issuing from vessels, or moving in pipes and channels.
2.  Waves.
3. Fluids resisting the motion of bodies immersed in them; or themselves
  moving against other bodies.
4. Fluids lifted, or moved in opposition  to gravity.
         " Fluids issuing from vessels, or moving in channels."

  Water admitted to a tube ascending from near the bottom of a reservoir,
will rise in it, as already explained, to the level of the liquid surface in the
reservoir.  If such a tube  be afterwards cut off, except a small part at the
bottom, then prepared as a jet-pipe, the water will spout from this still to the
same height, with a certain deduction for the resistance of the air and friction.
Now as a  body shot upwards to  any height has that velocity  in departing,
which it again acquires by falling back to the same place or level, (with a
certain deduction  for the resistance of the air,) as explained at page 60, it
follows that fluid issues from any orifice in a reservoir with velocity equal to
what a body acquires in falling as far as from the level of the fluid surface in
the reservoir to  the orifice.  By referring them to the law of falling bodies, as
explained at page 59, we may learn the velocity of the issue  of water in any
case, and therefore  the quantity delivered by an opening of a given magni-
tude.
    15 
214
HYDRAULICS.
  Thus, a body by gravity falls sixteen  feet in  the first second, with speed
gradually increasing, and at the end of the second has a velocity of thirty-two
feet per second ;  therefore a reservoir with an  opening of an inch square at
sixteen feet below the  water's surface, will deliver, in one second of time,  with
a certain deduction for resistance  of  air, friction, Sic, thirty-two feet of a jet
of water of an inch square; and according to the  same rule, an  opening at
four times the depth should deliver a double quantity ; at nine times the depth,
a trible quantity ; and so on, as really happens.  An inquirer is  at first sur-
prised that the  quantity should not  be  quadruple, and where the height of
column  or pressure forcing it out  is quadruple, ninefold when the pressure is
ninefold, Sic;  but on reflection, he may perceive that the real effects, as
stated above, are  still  exactly proportioned to  the  causes ; for,  when  only
twice as much  water  is forced out in the  same time,  there  is still an effect
four times as powerful, because each particle of the  double quantity issues
with twice the force or velocity, and increase of velocity  costs just as much
force  as increase of quantity.  Similar reasoning holds with respect to the
triple or other quantities.   Because a body shot upward with a double velo-
city gains a quadruple height, (see page 60,)  the jet issuing with only double
velocity from four times the depth, still reaches the level of the surface of the
reservoir.
  The knowledge of this rule for discharging orifices is of the greatest import-
ance in the construction  of water-works, because, when joined  with other
rules assigning  the effects of friction, bending unequal width, Sic, in pipes, it
ascertains the quantity of water which a conduit of any magnitude, length and
slope, will deliver.
  It is a curious  fact, that more  water issues from a vessel through a short
pipe, than through a simple aperture of the same diameter  as the pipe ; and
still more if the pipe be funnel-shaped, or wider towards its inner extremity.
The explanation is, that the issuing particles  coming from all sides to escape,
cross and impede each  other in  rushing through a  simple opening, as is
proved by the narrow  neck which the jet exhibits a little beyond the opening;
but in a tube,  this  narrowing of the jet cannot happen without leaving a
vacuum  around the part, and the pressure of the atmosphere, resisting the
vacuum, causes a quicker flow.  The funnel-shape again leads the water by a
more gradual inclination to the point of exit, and thus considerably prevents
the crossing  among the particles ; besides that,  because its mouth surrounds
the narrow neck of the jet, it allows that part to be deemed the commence-
ment of the jet.
  Another remarkable effect of atmospheric pressure  on running liquids is,
that in  a tube of considerable length, descending from a  reservoir, it much
quickens the discharge.  Water naturally  falls like any other body with  acce-
lerating velocity, but  if it so fall  in a tube which it fills like a piston, either
portions of it below  must  outstrip  portions above, leaving vacuous spaces
between, or  water from above must be  pressed into the tube by some  other
force than its weight.   Now the atmospheric  pressure becomes this force,
and it prevents a vacuum, partly by impelling water more rapidly  into the  top
of  the tube, and partly by resisting  the discharge from below.  The forcing
in of the water at the top of the tube causes that depression of the water-
surface in the  reservoir over it,  which  becomes more conspicuous as  the
depth in the reservoir diminishes, and  at last  is a deep hole  in  the  water
extending far into the tube, and sometimes even as  in a common funnel ex-
tending quite through.
   The friction or resistance which  fluids suffer  in passing  along  pipes is 
DISCHARGE FROM APERTURES.
215
much greater than might be expected.  It depends on the cohesion of the
particles to the  surface of the pipe and among one another, and on the parti-
cles  near  the outside being constantly driven from their  straight course by
the irregularities in the surface  of the  pipe.  An inch tube of two hundred
feet in lenglh, placed horizontally, is found to discharge only a fourth part of
the water which escapes by a simple aperture  of the same diameter.   Air
passing along tubes  is still more retarded.  A  person who  erected a great
bellows at a waterfall, to blow a furnace two miles off, found that his appa-
ratus was totally useless.  When  gas-lights were first proposed, some engi-
neers feared that the resistance by friction to the passing air would be fatal
to the enterprise.
   Higher temperature in a liquid increases remarkably the quantity discharged
by an orifice or pipe,—apparently by  diminishing that cohesion of the par-
ticles which exists in certain degrees in all liquids, and affects so much their
internal movements.  The addition of 100 degrees of heat will, in certain cases,
nearly double the discharge.
   The flux of water through orifices under uniform circumstances is so steady,
that  before the  invention of clocks and watches, it was employed as a means
of measuring time.  The vessels were called clepsydrae.  That of  Ctesibius
is famous, in which the issuing water took the form  of tears from the eyes of
a figure, deploring the rapid passing of precious time ;  and these tears being
received  into a fit vessel, gradually filled it up and raised  another  floating
 figure, who pointed to the hours marked on an upright scale.  This vessel
 was daily emptied by a syphon, when charged to a certain height, and its dis-
 charge  worked machinery which told the  month and the day.—The common
 hour-glass of running sand is another modification of the same principle, with
 this remarkable difference, however, that  depth of the sand does not quicken
 the  flux.
   The progress of water in an  open conduit, such as the channel of a river or
 aqueduct, is influenced by friction, &c, in the same  manner as in close pipes.
 But for this, a  river like the Rhone, drawing its waters from the elevation of
 1,000 feet above the level of its mouth, would  pour them out, with the velo-
 city of water issuing from the bottom of  a reservoir 1,000 feet deep; that is
 to say, at the rate of about 170 miles per  hour.   The ordinary flow of rivers
 is about three  miles per hour, and their channels slope three or four inches
 per mile.
   The velocity of a water current is easily ascertained by immersing in it an
 an upright tube, of which the bottom bent at right angles  becomes  an open
 mouth turned  towards the  stream.  The water in  the tube
 will stand above the surface of the stream, as much as would     Fig. 108.
 be  necessary  in  a  reservoir,  according  to the explanation
 given above, to cause a velocity of jet equal to the velocity
 of the stream.   A modification  of this contrivance may be
 made to measure the velocity of the wind.—A common mode
 of telling the velocity of an open stream, is  to observe with
 a stop-watch the progress of a body floating in  some part of
 it from which its medium speed may be known ; and know-
 ing that  speed and the  depth  and width  of the channel, the     \\_X\.
 quantity delivered in  a given time becomes a matter of sun-      ---\J>
 pie calculation.   The  speed of the wind may be ascer-
 tained  by observing how long the shadow of  a cloud takes
 to pass across  a field of known dimensions.
   The friction of water moving in water is such, that a small stream directed 
216
HYDRAULICS.
 through a pool, with speed enough to rise over the opposite bank, will soon
 empty the pool.  Extensive fens have been drained on this principle. The fric-
 tion between air and water is  also singularly strong, as is proved on a great
 scale by the magnitude of the ocean-waves, which  is a consequence of it;
 and on a small scale, by the amusing experiment of making a light round body
 dance or play upon the summit of a water-jet,—a chief cause of its remain-
 ing there  being, that the current of air which  rises around the jet by reason
 of  the friction, presses it  inwards again, whenever it  inclines to fall  over.
 Oil thrown upon  the surface of  water, soon spreads as  a film over it, and
 defends it from farther contact and friction of the air.   If oil be thus spread
 at the windward side of a pond where the waves begin, the whole surface of
 a pond soon becomes  as  smooth as glass;  and even  out at sea, where the
 commencement of the  waves cannot be reached,  oil  thrown  upon  them
 smoothes  their surface to leeward of the place, and prevents their  curling
 over or breaking.   It is said that boats having to reach the  shore through a
 raging surf, have been preserved by the crews first spilling a cask of oil in
 the offing.
   The most magnificent examples that ever existed, or probably ever will
 exist, of artificial  water-courses,  were the aqueducts of ancient Rome, about
 twenty in number.   Several of them exceeded forty miles in length, passing
 through hills in their way, and resting on tiers of splendid arches across the
 valleys.  They were constructed of such durable materials, and so skilfully,
 that the principal  of them remain perfect to this day.  Considered  as one
 object,  they  rank, in point of magnitude, with  any other work of  human
 labour, not excepting the pyramids of Egypt.
   While the aqueducts are cited as specimens  of grandeur, we may mention
 the  fountains in the gardens of France  and Italy as  specimens  of beauty.
 Those at Versailles are  well known.   In them the most magical effects are
 produced by varying the ways in which water is made to spout from orifices.
 In one place it is seen darting into the air as a single upright pillar: in others
 many such pillars rise  together,  like  giant stalks of corn: sometimes an in-
 clination given to the jets makes them bend so as to form  beautiful arches,
 of which a portion appear  as  the roofs of apartments built  Of water, while
 others mingle together with endless variety:  here and  there water-throwing
 wheels send out spiral streams, and hollow spheres with a thousand openings
 are the centres of immense bushes or trees of silvery boughs.  Such effects,
 amidst cascades, smooth lakes, and scenes of  lovely landscape, constitute a
 whole as enchanting, perhaps, as art by moulding nature has ever produced.

                               " Waves."

   The form, magnitude and velocity of waves, are subjects admitting of deep
 mathematical research; and are  rendered the more  interesting, because cer-
 tain phenomena of sound and light are of kindred nature.   Here, however,
 they must be treated with great brevity.
   A stone thrown into a smooth pond, causes a succession of circular waves
 to spread from the spot where  it falls as a common centre.   They become of
 less elevation as they expand,  and  each  new one is less raised than the pre-
 ceding, until  gradually the liquid  mirror becomes  again  perfect as  before.
 Several stones falling at the  same time  in different places, cause crossing
 circles, which, however, do  not disturb the  progress of one another,—a
phenomenon seen in beautiful miniature at each leap of the little insects which
cover the surface of our pools in the calm hours of summer.—The rationale 
WAVES.
217
of the formation of waves in such cases is as follows.   When the stone falls
into the water, because the liquid is incompressible, a part of it is displaced
laterally, and becomes an elevation or circular wave around the stone.  This
wave then spreads outwards in obedience to the laws of fluidity, already ex-
plained, and  the circle is seen to widen.   In the mean time, where the stone
descended, a hollow is left for a moment in the water,  but owing to the sur-
rounding pressure, is soon filled up, chiefly by a sudden rush  from below.
The rising water does not stop, however, at the  exact  level  of  that around,
but like a pendulum sweeping past the centre of its arc, it rises  almost as far
above the level as the depression was deep.  This central elevation now acts
as the stone did originally, and  causes  a second wave,  which pursues the
first;  and when the centre subsides, like  the pendulum still, it sinks again
almost as much below the level as it had mounted above: hence it has to rise
again, again to fall, and so on for many times, sending forth a new  wave at
each alternation.  Owing to the friction among the particles  of the water,
each new wave is less raised than  the preceding, and at last the appearance
dies away.
   A wave passing through any gap or  opening, spreads from  it as a new
centre;  and a wave coming against a perpendicular surface of wall  or rock,
is completely reflected from this, and acquires the appearance of  coming from
a point  as far beyond the reflecting surface, as its real origin  or centre is
distant on the side where it is moving.
   So absolutely level is a liquid surface, and so sensitive or mobile, that the
effect of any disturbing cause is perceived at great distances.  A boat rowed
across a still lake, ruffles its surface to  a great extent;  and although the
widening waves become at last such gentle risings as not to be perceptible
to the eye, they still produce  a rippling noise where they fall among the
pebbles on shore.  In seas liable to sudden but partial hurricanes, the roar of
breakers on distant coasts often  tells of the storm which does not otherwise
reach them.   The author once, in the eastern ocean, had an  opportunity of
contemplating  waves  of extraordinary  magnitude  rolling along  during a
gloomy calm, and therefore with unbroken surface, appearing like billows of
molten lead.   At that very time, about a hundred and fifty miles to the north-
east, four  of the  finest  ships of the India  Company  were  perishing in a
storm.—In the polar seas, which are comparatively tranquil, because partially
defended from the wind  by the floating islands  of ice, a few sudden waves
are occasionally observed, and quickly all is calm again. Such a  phenomenon
announces, that the occurrence described at page 153 has happened somewhere,
of an island of ice turning over,  when the place of its centre  of gravity is
changed by partial melting.
   The common cause of waves is the friction  of the wind upon the surface
of the water.   Little ridges or elevations first appear, which by continuance
of the force, gradually become loftier  and broader, until they are the rolling
mountains seen where the winds sweep  over a great extent of water.   The
heaving of the Bay of Biscay,  or still more  remarkably,  of the open ocean
beyond the southern capes of America and Africa, exhibits one extreme, and
the stillness of the tropical seas, which are sheltered by near encircling lands,
exhibits the other.  In  the vast archipelago of the east, where Borneo and
Java and Sumatra lie, and the Molucca islands  and the Philippines, the sea
is often  fanned only by the land and sea breezes  and is  like a smooth bed, in
which these islands seem to repose in bliss—islands in which the spice and
perfume gardens of the world are embowered, and where the bird  of para-
dise has its home, and  the  golden pheasant, and a hundred other  birds of 
218
HYDRAULICS.
brilliant plumage, among thickets so luxuriant, and scenery so picturesque,
that European strangers find there the fairy land of their youthful dreams.—
One who has visited°these islands in his early days, may perhaps be pardoned
for thus adverting to their beauties.
   In rounding the Cape of Good Hope, waves are met with, or rather a swell,
so vast, that a few  ridges and a few depressions occupy the extent of  a mile.
But these are not so dangerous to ships as what is termed a shorter sea, with
more perpendicular waves.  The slope in the former is comparatively gentle,
and the rising and falling are much less felt; while among the latter, the sud-
den tossing of the vessel is often destructive.  When a ship is sailing directly
before  the wind, over the long  swell now  described, she advances as if by
leaps;  for as each  wave passes, she is first descending headlong on its front,
acquiring a velocity so wild that she can scarcely be steered ; and soon after,
when it has glided under her, she appears climbing on its back, and her motion
is slackened almost to rest, before the following wave arrives.  To a passen-
ger perched at such a time on the extremity of the bowsprit, and looking back
on the  enormous body of the ship, with perhaps its thousand of a crew, a hun-
dred feet behind him, heaved by these billows as a cork is on a ruffled lake,
the scene is truly sublime.   When a coming wave lifts the stern and in the
same degree depresses the bow, he is deep in the hollow or valley between
the waves,  and sees only the ship rushing  headlong down  towards him  as if
to be  engulfed; but soon after,  when the stern is  down, and the bow is
raised, he looks from his station in the sky upon an awful scene beneath him
and around.
   The velocity of  waves has relation to their magnitude.   The large waves
just spoken of, proceed at the  rate of from thirty to  forty miles an hour.—
It is a  vulgar belief that the water itself advances with the speed of the wave,
but in  fact theybrm only advances, while the substance, except a little spray
above, remains rising and  falling in the same  place,  with  the regularity of a
pendulum.   A wave of water, in this respect, is exactly imitated by the wave
running along a stretched rope  when one  end is shaken;  or by the mimic
waves  of our theatres, which are generally undulations of long pieces of car-
pet, moved by attendants.  But when a wave reaches a shallow bank or beach,
the water becomes really progressive, for then, as  it cannot sink directly
downwards, it falls over and forwards, seeking the level.
   So awful is  the spectacle of a storm at sea, that it generally biases  the
judgment;  and, lofty as waves really are,  imagination  pictures them loftier
still.  Now no wave rises much more than ten feet above the ordinary sea-level,
which, with the ten feet that the surface afterwards descends below this, give
twenty feet for the whole  height, from the bottom  of any water-valley to an
adjoining summit.  This is easily verified by a person who tries at what height
on a ship's mast the horizon remains always in  sight over the top of the
waves—allowance being made for accidental inclinations of the vessel, and
for her sinking in the water to considerably below her water line, at the time,
when she reaches the bottom of the hollow between the two  waves.  The
spray of the sea, driven along by the violence of the wind, is of course much
higher than the summit of the liquid wave; and a wave coming against an
obstacle, or entering a narrow  inlet, may dash to an elevation  much  greater
still.   At the Eddyslone light-house, which is about ninety feet high, placed
on a solitary rock ten miles from land, when a surge breaks which has been
growing under a storm all the  way across the Atlantic, it often dashes to 100
feet above the lantern at the summit.
   The magnitude of waves is well judged  of when they are seen breaking 
WAVES.
219
on an extended shore or beach.  In the deep sea the wave is only an eleva-
tion of the water, sloping on either side; but as it rolls towards the shore, its
front becomes more and more perpendicular, until at last it curls over and falls
with its whole weight, and when several miles of it break at the same instant,
its force and noise may shake the country abroad.
  Along the east, or  Coromandel Coast of India, at certain seasons, vast
waves are constantly breaking; and as there are no good harbours there, com-
munication between the sea and land is rendered impossible to ordinary boats.
The natives of the coast, at Madras, for instance, have hence become almost
amphibious.   They reach ships beyond the breakers by the help of what are
Called catamarans, consisting of three small logs of wood tied together.  On
these they secure themselves,  and boldly advance up to  the  coming wall of
water, which they shoot into,  and rise to the smooth surface beyond it, like
water-fowls after diving.  Boats unsuited to the breakers often perish in them.
The author of  this work had  gone on shore with  a  watering party on the
coast of Sumatra, and during the hours spent there, a swell had risen in the sea,
which on their return was already bursting along the beach and across the
river's mouth in lofty breakers.  The boat in which he happened to be, re-
gained the high sea in  safety, but a larger boat which  followed at a short dis-
tance was overwhelmed, and an officer and part of the crew perished.
   There is a phenomenon observed at the mouths of many great rivers, called
the Boar, which has resemblance to a wave.   When the tide returning from
the sea meets the outward current of the  river, and both have the force which
in certain situations belongs to them, the stronger mass from the ocean assumes
the form of an almost perpendicular wall,  moving inland with resistless sweep.
 This is called the boar.  It is in fact the great sea-wave of the tide, produced
twice a day by the attraction of the moon, rolling in upon the land and inlets,
 where contracting channels concentrate its mass.   In the  different branches
 of the Ganges the boar is seen in a remarkable degree.   Its roaring is heard
long before it arrives.   Smaller boats and skiffs cannot live where it comes;
 and as it  passes the city of Calcutta, even the large ships at anchor there are
 thrown into such commotion,  as sometimes to be torn away from their moor-
 ings.—The nature and effects of this boar are  strikingly illustrated upon cer-
 tain coasts where extensive tracts of sand  are left  uncovered at low water.
 In such situations, of which there are many on the  western shores of Britain,
 the returning tide is seen advancing with steep front, and  with such rapidity,
 that the  speed of a galloping horse can scarcely save a person who  has in-
 cautiously approached too near. Many, every year, are the victims of temerity
 or ignorance on these treacherous plains.
   In the end of the year 1831, on the low flat coast of the Indian peninsula,
 north of Madras, one great wave of the kind now described was produced
 during a very high spring-tide of midnight, by  an extraordinary wind, and
 spread ten  miles in upon the  inhabited land.   It had retired with the ebbing
 tide before morning, but the next day's sun disclosed a scene of devastation
 rarely matched.   Amidst the  total wreck of the villages and fields, there lay
 the drowned carcases  of more than ten thousand human beings, mixed with
 those of  elephants, horses, bullocks, wild tigers and the other inhabitants of
 the land.                                         .
    It has been proposed lately to construct sub-marine boats, or vessels cal-
 culated to swim so deep in the water as to be below the superficial  motion of
 the waves, and  therefore  beyond the influence of storms at surface.   Such
 a boat has been tried  with considerable  success; and men's increasing fami-
 liarity with sub-marine matters since the invention of the diving-bell, may 
 220                          HYDRAULICS.

 ultimately lead to improvements rendering the sub-marine  vessel, for certain
 purposes, commodious and safe.

 "Fluids resisting the motion of bodies immersed  in  them, or themselves
        moving forcibly against other bodies."   (See the  Analysis.)
   The  same force is required to give or to take away, or to bend motion, in
 a fluid,  as in an equal quantity of solid matter.  A pound  of water enclosed
 in a bladder is not more easily thrown to a given height than a pound of ice
 or of lead ;  nor, if falling into  the scale of a weighing beam, does it require
 less as a counterpoise; nor, if  made to revolve  at  the end  of a sling, does it
 render the cord less tight.
   A convenient measure of  the force of moving  water  on  an obstacle, or of
 the resistance of still-water  to a moving body, exists  in the facts already ex-
 plained, that the pressure of a  known height of fluid column produces from
 an orifice a  certain velocity of jet, while  conversely, that jet, or a  current of
 equal speed, directed against the orifice, supports the column.  The impulse
 given or received, therefore, by a flat surface in water, such as the vane of
 a  water-wheel, whether  that of a steam-boat pressing against  the water, or
 that of  a corn-mill pressed by  it, is  measured by  the weight of the column
 alluded  to, the  height of  which  is, according to the  velocity and the breadth
 or diameter, according to the breadth or extent of the solid surface concerned.
 This estimate supposes that the pressure of or upon the surface is direct; if
 it be oblique, there is a  diminution according  to the rule  given  under the
 head of " resolution of forces."
   Many persons, looking carelessly at the  subject of fluid  resistance, would
 expect  that  if a body, as a boat, moving  through a fluid at  a given rate,
 meets a given resistance, it should just meet double resistance when moving
 twice as fast.  Now the resistance is four times greater with a double rate.
   This  fact is  but  another  example of a principle  already explained, and
 when more  closely examined, is easily understood.  A boat" which moves
 one mile per hour, displaces or throws aside a certain quantity  of water, and
 with a certain velocity;—if it move twice as fast,  it of course displaces twice
 as many particles in the  same  time, and requires  to be moved by  twice  the
 force  on  that  account;  but  it also  displaces every particle with a double
 velocity, and requires another doubling of the power  on  this  account;  the
 power then  being doubled on two accounts becomes a power of four.   In the
 same manner with a speed of three, three times as many particles are moved
 and each particle with three  times  the velocity;  therefore, to  overcome the
resistance, a force of nine is  wanted; for a speed of four, a power of sixteen;
 for a speed of five, a power of twenty-five, and so forth: the relations being that
which mathematicians indicate by saying that the  resistance increases as the
square of the speed.  The  corresponding numbers, up to a speed of ten,
are as here shown.
Corresponding ^ resistance . .. \	1	2	3	4	5	6	7	8	9	10
	1	4	9	16	25	36	49	64	81	100
  Thus, even if the resistance at the bow of a vessel were all that had to
be considered, the force of one hundred horses would  only drag  the vessel
ten  times as fast as the force of one  horse.   But there is another  important
element in the calculation, viz., the lessening, as the vessel's speed quick- 
WAVES.
221
ens, of the usual water pressure on the stern,—which pressure, while she is
at rest, is equal to, the pressure on the bow,  and the force therefore required
to produce an increased velocity is still  considerably greater than as noted in
the table.
   There is not a more important truth in  physics  than the law of fluid re-
sistance to moving bodies here treated of;  it exlains so many phenomena of
nature, and becomes a  guide in so many matters of art. We will now set forth
some interesting  examples.
   It explains at what a heavy expense  of coal high velocities  are obtained
in steam-boats.   If an engine of about 50 horse power would drive a boat
7 miles an hour, two  engines  of  50, or one of 100 would be required to
drive it 10 miles, and three  such to drive it 12 miles; even  supposing the
increased resistance at  the bow, as  already stated,  to be the measure of the
whole work  done, which it is  not, and that  engines  worked to the same
advantage with a high velocity  as with a low, which they do not.—For the
same reasons, if  all the  coal which a ship could  conveniently carry were
just sufficient to  drive her  1,000 miles, at a rate of 12 miles per hour, it
would drive  her more than 3,000 at a rate of 7 miles per hour; and more
than 6,000 at a rate of  five miles per hour.  This is a very important consi-
deration for persons concerned in steam navigation  to distant parts.
   The same  law  shows the  folly of putting very large sails on a ship; the
trifling advantage in point of  speed by no means  compensating  for the addi-
tional expense of making and working the sails, and the risk of accidents in
bad weather.  The ships of the prudent Chinese have not, for the same ton-
nage, one-third so much sail as  those of the  Europeans, and yet they move
but little slower on that account.  A European ship under jury-masts  does
not lose so much of her usual speed as most people would expect.
   This law explains also  why a ship glides  through the water one  or two
miles an hour when there is very little wind, although with a strong  breeze
she would only sail at the rate of eight or ten  miles.  Less than the 100th
part of that force  of wind which drives her ten miles an hour, will drive her
one mile per hour, and less  than the 400th  part will drive  her half  a mile.
Thus, also, during  a calm, a few men  pulling in a boat  can move  a large
ship at a sensible  rate.
   These considerations show strikingly of what importance to  navigation it
might be to have, as a part of a ship's ordinary equipment, one or two water-
wheels, (or ready means  of forming them) to be affixed upon the ship's side
when required, like the paddle-wheels of a steam-boat, and by turning which,
the crew might easily deliver themselves from the tedium, or even disastrous
consequences of a long calm at sea.—This idea occurred to  the author while
in a ship completely becalmed for weeks on the Line :  during which  weari-
some period, the breezes were often seen roughening the water a mile or
two farther on; and any  means that could have enabled the ship's company
to advance her that little distance might have saved the delay.   The wheels
might be driven by connection with the capstan, at which, under such circum-
stances, the crew  would most willingly work.  Delay in a large vessel often
costs hundreds of pounds per day, and may retard the execution of important
projects.—But the propelling of the ship in a calm  seems, by no means, the
most important purpose which  such wheels  might serve.   If from disease,
fatigue, or other cause,  the crew were inadequate to existing necessities, two
wheels affixed to the extremities of an axis crossing the ship might be equiva-
lent in  many cases to additional  hands, or to a steam-engine of great power; 
222
HYDRAULICS.
for when acted upon by the water as the  ship sailed, they would turn with
the force of water-wheels on shore, and might be made to move the  pumps,
to hoist the sails, and to do any work which  a steam-engine could perform.
Many a gallant vessel has perished because  the  exhausted crew could no
longer labour at the pumps, where such water-wheels as now contemplated,
or a windmill-wheel  in the rigging would have  performed the duty most
perfectly.

The law that resistance to a body moving in a  fluid increases  in a greater
   proportion than the speed of the body, applies where the fluid is aeriform,
   as well as where it is liquid.
   A bullet shot through the air with a double  velocity, for the reason assigned
above, experiences four times as much resistance in front,  as with  a single
velocity:  the  motion is retarded also by the  diminution of the usual  atmo-
spheric  pressure of 15 lbs. per inch on the posterior surface, which  diminu-
tion  is proportioned to the speed.  It is farther true, that when the velocities
of bodies moving in the air are very great, the resistance increases in  a still
quicker ratio than in liquids,—probably because  the compressibility  of air
allows it to be much condensed or heaped up before the quick moving  body.
It is  useless to discharge  a cannon-ball with a velocity exceeding 1,200 feet
in a  second, because the powerful resistance of the air to any velocity beyond
that, soon reduces it to that at least.

The rule of reciprocal action  between a solid and fluid,  now explained, holds
   equally when the fluid is in motion  against the solid, as when the solid
   moves through the fluid.
   If a  ship be anchored in a  tide's way, where the current is four miles an
hour, the  strain on her cable is not one-fourth part so great as if the current
were eight miles.
   A wind moving three miles an hour is scarcely  felt;  if moving six miles,
it is  a pleasant breeze; if twenty or thirty  miles, it is a brisk gale; if  sixty,
it is  a storm; and beyond eighty, it  is a frightful  hurricane, tearing  up trees
and destroying every thing.
   Supposing the wind to move one hundred  miles  per hour, there  are one
hundred times as many particles of  matter striking any body exposed to it,
as when it moves only one mile per hour, and  each particle strikes, more-
over, with one hundred times the velocity or force, so that the whole increase
of force is a hundred  times a  hundred, or ten  thousand.   This explains how
the soft invisible air may by motion acquire force sufficient to unroof houses,
to level  oaks which have  been stretching their  roots around for a century,
and in some West India hurricanes, absolutely to brush every projecting thing
from the surface of the earth.

The law of rapidly increasing resistance assigns  a limit to many velocities,
   both natural and artificial.

   It limits the velocity of bodies  falling  through the air.  By the law of
gravity, a body would fall with a constantly accelerating speed, but as the
resistance of the air increases still more quickly than the speed, at a certain
point, this resistance  and the gravity balance  each other, and the motion be-
comes uniform. 
ACTION BETWEEN FLUIDS AND SOLIDS.
223
  The parachute, by means of which a person may safely descend to the
earth from a balloon" at any  elevation, furnishes a good example.  The con-
trivance resembles a  large  flat umbrella.  The aeronaut  attaches himself
underneath it, and when it is let loose from the balloon, he is partly supported
by the resistance which its broad expanse experiences in falling through the
air, and falls, therefore, in a corresponding degree more slowly.   After the
first second or  two, for the  reason  stated above, it descends with a uniform
motion;  and its  breadth is  generally  made  such, as to allow a velocity of
about eleven  feet in a second, or that which a man acquires in jumping from
a chair two feet high.
  No  ship sails faster than  fifteen miles  in an hour.—And it is because the
resistance to  be overcome in steam-carriages on rail-ways, viz.,  their friction,
does not increase with their velocity like the fluid resistance to steam-boats,
that the speed of the former may  so much exceed that of the latter.
   No  fish swims with  a velocity exceeding  twenty miles  an hour; not the
dolphin, when  shooting a head of our swiftest frigates, nor the salmon, when
darting forward with  a speed which lifts  him over a water fall.
   And the flight of birds through the thin air has  a limited celerity.  The
crow,  when flying homewards against the storm, cannot face the wind in the
Open sky, but  skTms along the  surface of the earth  in the deep valleys, or
 wherever the swiftness  of  the wind  is retarded  by terrestrial  obstructions.
The great albatross, stemming upon the wing the current of a gale so as  to
keep company with a driving ship where the air  is passing at the rate of a
hundred miles  an hour, often takes shelter momentarily under the lee-side of
 the lofty billows.  The bird called the  stormy petrel  abides chiefly in  the
 midst of the Atlantic Ocean, but the irresistible violence  of the wind occa-
 sionally sweeps it from the  waves, and causes its appearance on the western
 shores of Europe.   Vessels  from the high sea, approaching  a  coast from
 which the wind blows, generally become resting-places  to exhausted land
 birds  driven off the shore by wind which they have not had strength of wing
 to stem ;—sad evidences of the myriads which are constantly perishing where
 no resting-place  is found, and where no  human eye notes their fate.

 The action or  resistance between a meeting fluid and  solid, is  influenced by
   the shape of the solid.
   This follows from  what has already been said of direct and oblique impulse.
 If a flat surface  directly opposed to 'the  fluid experience a certain resistance,
 a projecting surface like that of a sphere or short wedge is resisted in a less
 decree,  and  a  concave surface in a greater.   The explanation is, that a flat
 or "plane surface  throws the particles of fluid  almost directly outwards from
 its centre to its circumference, and therefore with greater velocity, while the
 convex or wedge-like surface, although displacing them just as far, still does
 so more slowly, and therefore with less expenditure of force, in  proportion
 to the obliquity of surface, or as its  point is in  advance of its shoulder  or
 broades-t part;  and a concave surface must  give to some of the particles a
 forward as well  as lateral  motion.   The shape of the hinder part of a solid
 moving through a fluid is of importance for corresponding reasons.
    The following are instances of projecting  or wedge-like surfaces, intended
 to diminish  the resistance.  Fishes  are wedge-like both before  and  behind,
 their  form bein<r modified, however,  in relation to other objects  than  mere
 speed of motion.  Birds are so  also; and they stretch out their necks while 
224
HYDRAULICS.
flying, so as to make their form perfect for dividing the air.  In the form of
the under part of boats and ships, men have, in a degree, imitated the shape
of fishes.  The light wherries  which shoot about upon the surface of the
Thames, appear the very essence of what imagination can picture of form
combining  utility and  grace.  There are boats used in China called  snake-
boats, which  are only a foot or two broad, but perhaps  a  hundred feet  in
length, and when moved, as  they often  are, by nearly a hundred  rowers,
their swiftness is extreme.  The problem of which it is the object to assign
for a ship's hull or  bottom the  best possible form that  she  may have speed
of sailing, is not  yet completely  solved ; so that a kind of empiricism prevails
in the matter, and very unexpected results often arise.  Yet the subject merits
much attention,  for  when vessels  have to chase and to flee, speed becomes
of the greatest importance; and at all times  the sailor's heart swells  with
delight to find his well-beloved vessel performing well.
   The following instances exhibit the mutual influence of meeting solids and
fluids, where  the surface of the solid is plane or concave.—In a water-wheel,
whether  the water be moving against the wheel, as is the case where a stream
acts  to drive  machinery, or the wheel be moving against the still water, as
in the case of the paddle-wheels of a steam-boat, the extended faces of the
vanes or  float-boards  give or receive a  powerful impulse.   When a wheel
with float-boards has  its lower part merely dipping into a  stream of water,
to be driven  by the momentum, it is  called an undershot-wheel; when the
water reaches the wheel near the middle of its height, and turns it by falling
on the float-hoards of  one side as they sweep downwards in a curved trough
fitting them, the modification is called a breast-wheel;  and when the float-
boards are  shut  in by flat sides, so as to become the bottoms  of  a circle of
cavities or  buckets surrounding the wheel, into which  the  water is  allowed
to fall at  the top of the wheel, and to act by its weight instead of its momen-
tum, the  modification  is called the overshot-wheel.   To have a maximum of
effect from  wheels moved  by the momentum of water, they are generally
made to  turn with  a velocity about one-third as  great as that of the water:
and wheels moved by the simple weight of  water usually have their circum-
ference turning with a velocity of about three feet per second.  The subject
of water-wheels is  one of the  most important in  practical mechanics; for
moving water performs a great deal of labour for man.
   Oars for  boats are  made flat, and often a little concave, that the mutual
action between them  and water may be as great as possible.   The webbed
feet of water-fowl are  oars: in advancing, they collapse like a  shutting um-
brella, but  open outwards in  the thrust backwards, so  as  to offer a broad
concave surface  to the water.  The expanded wings of  birds are in like man-
ner a little concave towards the  air which they strike.  The sails of ships,
when they  are receiving a fair wind, are left slack so as to swell and become
hollow.

The resistance between a meeting solid and  fluid being nearly proportioned
   to the  breadth of  the solid, it follows that large bodies, because  containing
   more matter in proportion to their breadth or surface than smaller bodies
   of similar form, are  less resisted, in proportion to their weights, than smaller
   bodies.

   The science of measures tells us that a bullet or other solid of two inches
diameter, has eight  times as much matter in it as a similar solid of one inch 
ACTION BETWEEN FLUIDS AND SOLIDS.
225
diameter, while  it has only four times the breadth or surface.  Thus, by
putting eight dice or little cubes together, as here represented, we have a
larger cube, of  which compared with a single dice,  the  edge is evidently
twice as  long, the surface four times as  great,  and the quantity of matter
eight times  as great;—again,  twenty-seven dice simi-
larly put together form a cube with sides three  times        FlS- 109-
as long, and the  surface nine times as great; and sixty-
four dice form a cube with  sides four times as  long,
and a surface sixteen times as great.  All solids similar
have to each other this kind of relation, whicli, in the
language of the  science of quantity, is called the rela-
tion of cubes: they are said to be to each other as the
cubes of any of their corresponding lines.  Hence, if a
bullet of eight pounds, and a bullet of one pound  be shot off with equal velo-
city, because that of  eight pounds has only half  as much  surface in propor-
tion to its weight, and therefore to its motal inertia or force, as  the other, it
will go  much farther  than the  other.
   This important rule explains why shells and  large shot may be thrown
four or five miles,  while  smaller cannon-balls, musket-bullets, pistol and
swan shot,  and  the  common  small-shot of the  sportsman, all of which are
generally discharged  from their respective pieces  with the  same commencing
velocity, have a shorter range, as the size of the projectile is less.  Even
water is sometimes  thrown from a gun or powerful  syringe to stun birds,
that they may be obtained with uninjured  plumage; but it  soon divides in the
air so minutely  that it reaches only to a short distance.
   Water falling through the  air from  a great  height, goes on suffering  a
gradual division into  smaller and smaller portions, which at last may be said
to be nearly all  surface; and then the resistance of the air lets them fall very
slowlv indeed.   The relation  of the size and resistance is  well shown by the
difference  of  celerity in the descent of a minute fog, a drizzling mist, and
common rain.    The toy called the water-hammer, is  merely a little  water
enclosed in a tube  exhausted or empty  of  air; and when, by turning the
 tube, the water  is made to fall from one end to  the other, as there is  no air
to impede or divide it in its descent, it falls as one mass,  and makes a sharp
 noise like the blow of a hammer.
   The  same law explains why a spider's thread or a single filament of silk
 floats so long in the air before it falls;—why there is  almost constantly sus-
 pended  in  the  air, wherever  active man resides, that  immense quantity of
 very minute solid particles, which, when rendered visible  by the sun's light
 passing  directly through them, are  called motes in the sunbeam—particles
 which'are constantly settling on household furniture, and rendering necessary
 the daily operation  of dusting or  cleaning;—why the fine dust sent aloft
 during the eruption of volcanoes is often carried  by the wind to a distance of
 hundreds of miles;—why  in the deserts of Africa  the  strong winds  often
 transport fine sand from place to place, overwhelming caravans, and forming
 new mountains, which succeeding blasts  are again to  lift;—why in the bot-
 tom of a river, or in a tides-way, fine mud is found where the current is slow;
 sand where it is quicker;  pebbles, or large stones, where it is quicker still;
 while in  rapids and  water-falls, only massy  rocks can  resist the fluid  force.
 Now rock, pebble, sand and  mud, may all be the same material in portions
 of diflerent magnitude.
    This law explains the operation of levigating, by which substances inso-
 luble in water  are obtained in the state of a very fine powder. Any such
 
226
HYDRAULICS.
substance is first ground or powdered in  the ordinary way, and mixed with
water.  The grosser parts then  soon fall to the bottom, while the fine dust
remains longer suspended.  This is afterwards obtained separately by pouring
the liquid which bears  it into another vessel, and allowing more time for the
slow subsidence.   The fine  powder of flint used in the manufacture of por-
celain is obtained by levigation;  as is also that of calamine stone, and other
powders used in medicine and various arts.
   This law farther explains  how, by means of air or water, bodies of  differ-
ent specific  gravities,  although  mixed ever so intimately,  may  be  easily
separated.   If pieces of cork and lead be let fall together through the air, the
lead will reach the ground  first, and may be swept away before the cork
arrives ; but in a  vacuum the whole would  reach  the ground at the same
time,  as is proved  by the common experiment of the guinea and  feather fall-
ing in the exhausted receiver  of an air-pump.  Again, when a mixture of
corn and chaff, as  it  comes from any threshing  machine, is showered down
from a sieve  in a current of air,  the chaff being longer in falling, is carried
farther  by  the  wind, while the heavier  corn falls almost perpendicularly.
The farmer, therefore,  by winnowing in  either a natural or artificial current
of air, readily separates the  grain from the chaff; and, if he desire it, may
even divide the grain itself into portions of different quality.  Similar to the
operation of separating chaff from corn by wind, is  that of separating sand
or mud from gold-dust  by water:—the soil containing gold-dust is first spread
on a flat surface, over which a current of water is then made to pass;  which
current carries  away the lighter rubbish, and leaves  the gold.  If a mass of
metal be affixed on the end  of a rod of wood, the rod then, whether simply
falling through the air,  or advancing as an arrow, will follow the heavier metal
as its point. The cork of a shuttlecock is always foremost for the same reason.
   The instances  enumerated under this head serve  to show  how many and
varied the results may  be which  flow  from a single principle.

When a fluid and a solid meet each other obliquely, the impulse or effect is
   still perpendicular to the surface of the  solid, as if they met directly, but
   is less forcible as the obliquity of the approach is greater.

   Suppose a b to represent  the  upper edge of a smooth board or of any flat
polished surface standing in a current, the fluid approaching this surface,  in
whatever direction, must act upon  it as if  approaching  perpendicularly,
                    because, on account of its smoothness, the fluid can take
      Fig. 110.       no hold of  it to push it endways, either towards  a or b.
                    But the impulse of  a stream acting on the  surface will
                    be less forcible if the surface be oblique to the  stream,
                    both because less fluid will touch, and because the velo-
                    city of the  effective approach will be less.   The line c
                    d marks the breadth, and therefore  force, of the part of
                    a stream reaching the board  directly; and the  shorter
                    line/c marks the smaller breadth that can touch it, of a
                    stream  coming obliquely  in the direction  c  b:  in  the
                    oblique stream, moreover, if the line c b mark the whole
                    velocity, the shorter line c a marks the slower  rate of
                    the direct approach  of  any one  particle to the board.
                    (This subject was treated of at  page 57, under the head
 of Resolution of Forces.)
    Hence the wind  blowing upon the sail of  the ship, however obliquely,
a
/ 
OBLIQUE FLUID ACTION.
227
always presses it directly forward or perpendicularly
to its surface, but acts less forcibly as the obliquity is
greater.  If the wind be represented, as to direction
and strength, by the line e d approaching the sail a b,
it will  act on the sail as  if it came  from/, but with
the smaller force / d, instead of the whole force  e d.
The effect, therefore, is the same as if  the sail were
pulled  by the rope d c.   We see in  this, how a ship
can be made to sail in a certain  degree against the
wind:—for all the sails being adjusted so as to receive
the wind in the  direction here shown, they all act to
produce the same result as if ropes were pulling from
each in the direction d c; and a force like/rf, or a rope like d c, urging side-
ways as well as forwards—as instanced in the tow-rope of a canal boat—
makes the vessel advance rapidly forward, but scarcely at all sideways, be-
cause the form of vessels causes them to pass forward at least twenty times
more easily with their sharp bow than sideways with their long  keel; and
therefore a force urging equally sideways and forwards makes a ship advance
twenty miles in the direction of her keel, that is forwards, for one mile which
she deviates  sideways.—The deviation  sideways, which, in sailing vessels,
must take place to a certain extent whenever the wind is
at all oblique, is called the lee-way.
   A vessel  having to sail from  b to a, while the wind
blows  directly against her course, or from a to b, is ob-
liged to sail close to the wind, as represented in fig.  Ill,
first perhaps to e, as represented by this figure, with the
left or larboard side to the wind, then to tack, as it is call-
ed, or  turn around, at e, and  to sail to d, with the right
or starboard  side in the wind: then  to go on the larboard
tack again to c, and thence to the  port at a.
   In making way against a contrary wind, the sails of a
ship are pointed so  nearly edgeways to the wind, that
unless  very flat, a great portion of their surface becomes useless.  The Chi-
nese manner of rigging is, in this respect at least, superior to the European;
for in it bamboo reeds attached across the sails, render them as flat as boards.
When a Chinese ship has her sails pointed edgeways to a spectator, he only
sees the masts which support them.
   The reason why a ship with several masts generally sails faster when the
wind is more or less from a side, than when directly astern, is, that  in  the?
former case  all the sails are acting, although individually not to the best
advantage, while, in the latter, the sails in front are becalmed by those behind
them.   A ship with a side-wind may move faster than the  wind itself, as is
often true of the outer extremities of a wind mill's vanes.   A corresponding
relation of motions is observed when a slippery  wedge is forced out two  or
three inches  laterally from  its place, by a weight which descends  only one
inch perpendicularly.
   The law now under consideration explains the action of the  rudder of
ships,—that  contrivance, by which  a single steersman can  direct  the course
of an enormous vessel through rocks and  shoals more steadily and  safely
that an adroit charioteer  can guide his tiny vehicle on a common road.  The
helm or rudder is a flat projection from the stern-post of the ship, turning on
strong hinges, in the manner of a door or gate, and moved by a beam or lever
Fig. 111.
Fig.  112.
     a
cl,--'
,>« 
228
HYDRAULICS.
   Fig. 113.    called the tiller, which proceeds from it forward to where the
       I       steersman stands.   In small vessels the tiller is above the deck,
              and the steersman applies his hand directly to it; but in large
     /">.     ships it is below, and is moved by ropes, rising from it to the
     I    \    wheel on the deck, where the steersman stands,  with the com-
         |    pass before him.   While the rudder  points directly astern, as
     \    /    to a, like a continuation of the keel and stern post, it does not
     \    /     affect the vessel's course; but if it be inclined ever so little to
   c \ . /     one side, as to b on the left or  larboard side,  the water imme-
    ./•-vr      diately acts on it  in the direction  c b, perpendicular to its sur-
     "'  \c      face, and pushes  the stern  to  the right or starboard side,—an
              action equivalent to pulling  the bow to the left or larboard,
   It is possible to make a  ship or boat steer itself, by placing a powerful
 vane on the mast-head, and connecting it with the tiller-ropes by two  pro-
 jecting arms from its axis.  If it were desired to make the ship sail directly
 before  the wind, the tiller-ropes would be fixed  to  the arms of  the vane so
 that the helm should be in the middle position, when the  vane  was pointing
 directly forward:  should  the vessel then  from  any cause deviate from her
 course, the vane by  its changed  position with  respect  to her,  would  have
 produced a corresponding change on the position of her helm, just such as
 to bring her back to her course.   Again, it is evident that, by adjusting such
 a vane  and rudder to each other in different ways, any other desired course
 might be obtained, and which would alter only  with the  wind.   The vane,
 to have the necessary power, would require to be of large size;  it would be
 a wide hoop, for instance,  with  canvas stretched upon  it;  and the rudder,
 to turn with little force, might  be hung on an axis passed nearly through its
 middle, instead  of,  as usual, by binges at  one edge.  Cases have occurred
 where shipwrecked persons might have sent intelligence of  their disaster to
 a distant coast, by a small vessel, or  even  a  block of wood fitted up  in  this
 way.   The  method admits  also of other applications, particularly in  war.
   As fluids act  on  surfaces, in  a direction perpendicular to  them, the water
 on the right  side of  a ship's bow is always pressing it towards the left  side,
 but owing to the equivalent and  contrary pressure there, the ship holds her
 course evenly between the two, or straight forward.   When a ship, however,
 owing to a side wind, lies over, or heels, as it is called, that  side of the bow
 which sinks  most is more pressed than the other ; and were there not a coun-
 teracting  inclination of the rudder then  made, constituting what is called
 weather-helm, the ship's head would  come round to the wind.   Now ships
 so rarely have the wind exactly astern, that to diminish the almost constant
 necessity for weather-helm, the mast or masts, and consequently the mass of
 the sails, are placed more towards the bow than the  stern.
   Again, because the bow of a ship is oblique downwards  as well  as side-
 ways, the water, when she moves,  is  constantly tending to lift the bow;
 hence when the vessel is dragged by a low horizontal rope, as in the  case of
 a boat attached to a sailing ship's stern, or is moved by paddle-wheels,  like
 steam-boats, the bow rises much out of the water, and the stern sinks in the
 hollow or furrow of the track : but when she is driven by sails, as these are
 high on the mast, and are acting therefore on a long lever to depress the bow,
 the two  opposite tendencies just balance each  other,  and  the vessel  sails
evenly along.
   The form of the  fore part of  a ship has less influence upon her speed of
sailing than the form of the hind part, called the run, from the middle to the
stern.   When a ship is at rest, there is of course as much forward pressure 
OBLIQUE FLUID ACTION.
229
 of water about the stern as of backward pressure on the bow ; but when she
 sails, she is running away from the propelling  pressure, and  is increasing
 the resisting pressure.   A gradual  tapering of the hind part, therefore, or  a
fine run, as it  is called, which allows the water to apply itself readily to it,
 as it passes along, must influence much the rate of sailing.  The fore part
 of any mass drawn  through the water, however bliint or square, becomes in
 effect sharp or rounded  by  a quantity of water which it pushes on before it.
 A tree, or the tapering mast of a ship, can be drawn through the water more
 easily with  the large end foremost than in a contrary way.
   The common windmill furnishes another illustration of the action of fluids
 on oblique  surfaces.   The face of the  windmill
 is turned directly to the wind, but the four flat          Fig. 114.
 vanes or sails,  of which the great wheel consists,
 are individually oblique.  Thus the edge a of the         C
 vane a  e, is more forward as  regards the coming
 wind or a spectator  in front, than  the edge e, and      ,
 the action  of the wind, therefore,  being  perpen-     i
 dicular to the oblique surface  a e,  pushes it in a     \^
 degree  towards a.   The same remark applies to
 each of the other vanes where the  edges b c  and
 d are in front, and  those marked  by the fainter
 lines  are behind; so that each vane  produces an
 equal effect in turning the wheel.  The law of the " decomposition of forces,"
 explained in page 57, tells in what proportion the force of the wind  is exerted
 to push the  wheel backwards against its supports, and to turn it round.
   Windmills were first used in Europe  in the fourteenth century, and they
 are still of  great importance  in countries where there are no waterfalls, and
 little fuel for steam-engines.   In some of the richest European landscapes,
 every  height is crowned by  its  busy windmill, grinding  corn, or sawing
 wood, or pressing oil-seeds; and over the  plains, similar wheels are pump-
 ing water for domestic use, or incessantly draining the land.
   The smoke-jack of our chimnies is a small windmill, driven by the ascend-
 ing current of air in the chimney.
   The feathering of an arrow acts in part on the principle of the windmill.
 The feathery projection from the shaft is  not quite straight, but winds  round
 it a little, like  the thread of a screw; and the  arrow, therefore, constantly
 turns as it flies, and goes straight to its object although the shaft itself be bent,
 because any deviation is constantly correcting itself.
   The rifle-barrel in  fire-arms has spiral furrows or threads along its interior
 surface, so that the  bullet in passing out receives  a turning motion  corre-
 sponding to  that of an arrow, and producing similar results.   A bullet which
 receives any other turning motion  than round the line of  its  course—and
 most bullets from  a common barrel do acquire such, owing to the irregularity
 of their form, or unequal friction at the mouth of the piece—is sure to deviate
from its course, because unequally pressed or  resisted by  the  atmosphere.
A good rifle  fixed to its place will  send  a succession of shots through  the
hole made in the target by the first shot, at the distance of 200 yards.  Duels
have been fought with rifles, and the parties having fired at the same moment,
have been corpses the moment after.
   It might be supposed that a wheel which the wind turned by direct action
on the rim,  as water  turns common water-wheels, would be preferable to the
windmill-wheel above  described, which is turned by oblique action on the
face: accordingly, a  wheel like a water-wheel only with broader vanes, has
    16
I 
230
HYDRAULICS.
been placed in a house or cover, so that only one side at a time was exposed
to the wind;—but it is a powerless machine.  The oblique vane wheel may
apply to use only half or less of the  force of the air which  reaches it, but
its wide expanse receives a stream of air of thirty feet in diameter, while an
ordinary window would admit that required for a  wheel of equal size of the
other construction.
  There are some situations where it would be an advantage to have water-
wheels like the common windmill-wheel, viz., where the stream is sluggish,
and is deep enough to allow a large wheel to be wholly immersed.
  A small  wheel of this sort,  with broad oblique vanes, has been used as a
means of ascertaining the rate of a ship's sailing.   It is allowed to drag astern
in the water; and the number of revolutions made in a given time marks the
ship's speed.
  A windmill wheel made to turn during a calm by force applied to its axle,
would  be pressed  endways, or in the direction of its axle, just as if  wind
were blowing  upon it, owing  to the  reaction of the still air, through which
its oblique  vanes were made to sweep.  Such a form of wheel fitted to  work
in water, and called a vvater screw, has been applied at the  bow or stern of
steam-boats, to  propel  them  in canals where  there was no room for side
wheels.  But as from the obliquity of the surfaces only a part of the applied
power becomes  propulsive—the remainder being wasted in the lateral strain
or twisting of the water—the method  is not applicable to general purposes.
  Two small windmill-wheels placed horizontally one above the other, on
the same axis, and  made to turn in opposite ways by springs or otherwise,
would rise  in the air, carrying a certain load with them, and would consti-
tute, therefore, a flying  machine.
  The effect of  a single oar projecting from the stern, used to propel a boat
or vessel, in the manner called sculling, is referable to the law now under
consideration.    The oar or scull rests on a round-headed prop or nail at the
stern, and is made  to vibrate from side to side.  In all its positions it has the
surface which presses the water turned obliquely backwards; hence the reac-
tion of the water drives the boat forward.—In China, large vessels are moved
by a single sculling oar, which half of the ship's company may be urging at
the same time.   A sculling  oar may be regarded as a single vane of such a
propelling  wheel or water-screw as above described, made to sweep across,
behind the  vessel,  alternately to the right and to the left.
   The action of a  fish's tail and of the bending of an eel or snake in water,
partly resembles that of the sculling oar.   Many  people believe that the tail
of the  fish  is only  the rudder of the body, and that the fins give it forward
motion—as is  true of a bird's tail and wing's,—but the  fish's tail is in fact
                       the  great instrument of motion, while the  fins serve
                       chiefly to steady and direct the motion.
                        A paper kite  rising  in the air is another example
                       belonging to this place.   Its cord d is attached to it
                       above  the  middle of its loop, and therefore so as to
                       make it present always an  oblique surface to the wind;
                       and  by the action  of the wind, perpendicular  to its
                       surface, it  rises as  if pushed up in the direction c a,
                       or as if drawn up  in the  direction of a b.  A kite
                       might be made large  enough to  lift  a man.  Cats
                       have been sent up at  kite's tails, and  have  fallen
                       down  safely under parachutes from the greatest  eleva-
                       tions.   It might be  safer  for a man to rise at a kite's
 
LIFTING OF WATER.                      231
 tail to reconnoitre an enemy's position, or to survey an unknown country,
 than under a balloon, as was practised by the French during the revolutionary
 wars.   He might have the security of a parachute, and the power of regula-
 ting the obliquity of attachment of the rope, so as to command his ascent or
 descent  at  pleasure.  An exhibition was  made in October, 1827, between
 Bath and London, of a car  drawn along the  highway by kites.   That they
 might  ascend to a great elevation, where the wind is generally stronger than
 below, they were attached to each other in  a row, so that the second kite
 mounted as if its cord were held by a hand at the first, the third as if rising
 from the second, and so forth.  The projector of  this novelty hoped that he
 had pointed out a most valuable means of travelling across extensive plains,
 sandy  deserts, tracks of snow, Sic, and in all cases, nearly with  the speed of
 the wind.

       "Fluids lifted in opposition to gravity."    (See the Analysis.)
   Water, as we have seen in  former parts of this  work, is to the living uni-
 verse,  in some degree, what the blood is to the animal  body, and a constant
 supply and circulation  are required.  This supply has  been provided for to
 an extraordinary extent, by the operation of  natural  causes;  but for many
 purposes of human society, water is still required where none naturally exists.
 A great variety of means have been employed for raising it, some of which,
 sufficient to illustrate the  whole, are now to be considered.
   Water may be raised in a bucket which is  attached to a rope to be pulled
 up by  the  hand.—The rope carrying the  bucket may be drawn up  more
 easily  by being  wound round a barrel or axle turned by a winch.—There
 may be a succession of  buckets on a rope, rising one after the other, and
 when emptied, descending again on the opposite  side of the wheel or axle
 which  lifts them :  the rope to  which they are  attached being a circle or end-
 less rope, and constituting with them what is called the bucket-machine.—
 Instead of  buckets on such an endless rope or chain, there may be a succes-
 sion of flat  pieces of wood, which, on being  drawn up through  a large tube
 or barrel, like loose-fitting pistons, will raise a copious stream of water: this
 is the contrivance called the chain-pump.—Or simply an endless rope of hair,
 very rough, passing round one wheel above, another below,  may be whirled
 quickly by turning the upper wheel,  so that a mass  of water adhering  by
 friction to its rising half, shall  be thrown into a reservoir at the top where it
 passes  over the upper wheel:  several such ropes may be joined side by side
 to increase  the effect.—But the most  important of all water-raising engines
 are the lifting and forcing pumps, already described at pages  171 and  172.
 They are used to draw from  wells, to  drain mines, to send a supply over
 eities, to  pump  ships, to  throw water for extin-
 guishing  fires, and for many other purposes.              FiS- 116-
   A stream of water passing  through a garden,
 or in the midst  of fields, may have beauty with
 little utility, unless it can be employed to irrigate
 the vegetable creation around.   In the fields and
 gardens of  Persia, where  the  heat of the sun  is
very intense, the streams are caused, by their own
 action,  to lift a part  of their water into elevated
reservoirs, from  which it  again flows in sloping
 channels  to  wherever  it  is required.  A  large
 water-wheel is placed so that the stream may turn
 
232
HYDRAULICS.
Fig. 117.
Fisr. 118.
it, and around its  circumference buckets  are  attached, to be  filled as they
sweep along below, and to be emptied into a reservoir as they pass above—
or instead of buckets, the spokes of the wheels are themselves made hollow,
and curved as here represented, so that as their extremities dip into the water
at each evolution, they receive a quantity of it, which runs along them as they
rise, and is discharged  into  a reservoir  at  the centre.   These  are usually
called Persian wheels, but they are as commonly employed on the banks of
the Nile and elsewhere  as in Persia.
  A pipe wound like a screw upon a sloping barrel, and made to dip its lower
mouth into water at each revolution of the barrel, will also raise water:  the
lower portions of the turning pipe will always be  full of it, and  it will be
                                        rising in them to  the top, as if on
                                        an  inclined plane.  Archimedes
                                        was the inventor  of this beautiful
                                        water-screw, and his name  has re-
                                        mained to it.  It may be turned by
                                        hand, or by a passing stream which
                                        acts on the  vanes of a water-wheel
                                        affixed upon it.
                                          Water may be raised by produc-
ing centrifugal force at the upper end of  a bent pipe which dips into a reser-
              voir.   Supposing the pipe to be bent as here represented, and
              the horizontal.arm a to turn like the spoke of a wheel, round
              the upright portion as  the axis,—if the pipe be  once filled
              with water, and be turned with sufficient speed, it will con-
              tinue to throw out a constant stream  from  the end a.   To
              increase the  discharge there may be several horizontal arms
              from one larger upright pipe, all emptying themselves into a
              circular trough or reservoir; and to prevent  the  necessity of
              refilling the apparatus  after every interruption of its motion,
              a valve opening upwards must be placed at the bottom.   This
              contrivance has been called the centrifugal pump, because the
water is raised at b as in a pump, by the pressure of the atmosphere, to supply
the place of that which is thrown out  from  a  by the centrifugal force.  The
velocity of rotation  must bear proportion to  the height of  the discharging
aperture a, above the surface of the water in the reservoir.
   It had long been  observed in  household experience and elsewhere, that
while water is running through a pipe, if a cock at the extremity be suddenly
shut, a  shock and noise are produced there.   The reason is, that the forward
motion of the whole water contained  in the pipe being instantly arrested, and
the momentum of a liquid being  as great as of a solid, the water strikes the
cock with as much force as  if it were a long bar of  metal or a rod of wood
having  the same weight and velocity as  the water.   Then as a fluid  presses
equally in all directions, a leaden  pipe of great length may be widened, or
even burst in this experiment.—Lately  this forward  pressure of an arrested
stream  has been  used as a force for raising water, and the arrangement of
parts contrived to render it available has been called, on account of the shocks
produced, the water-ram.  The ram may be  described as a sloping  pipe in
which  the stream runs, having a valve at its  lower end, to be shut at inter-
vals to  arrest the stream, and having a small  tube rising from near that end
towards a reservoir above, to receive a portion of the water at each interrup-
tion.   Now water allowed to run for one second, in a pipe ten yards  long, 
LIFTING OF WATER.
233
two inches wide and sloping six feet, acquires
momentum enough to drive about half a pint,
on the shutting of  the cock, into a tube lead-
ing to a reservoir  forty feet high.  Such an
apparatus, therefore, with the  valve  shutting
every second,  raises about sixty half-pints or
four  gallons in  a  minute.  The valve  is so
contrived that the stream works it as desired.__
In this figure which represents the lower end
of the water-ram, a is the opening by which
the stream escapes from it, and  the valve or
flap seen below  the opening is that which by suddenly shutting arrests the
stream.   The  valve is made so heavy, that the stream must run for a certain
time to acquire force enough to shut it;  and in the instant of its shutting, a
little of the advancing water passes upwards  through the valve b towards°the
reservoir.   The water in the  main pipe then becoming stagnant again, no
longer has power,  by its weight alone, to keep the valve a shut: this, there-
fore, falls open and the stream begins again, again to  be arrested as before ;
and as long as  the supply of water lasts, the action of the apparatus continues.
The action of  a  water-ram has been compared to the beating of an animal's
pulse.   The upright tube has usually a reservoir at the bottom, where it
first receives the water, constituting there an air-vessel b, (described at pa«-e
161) which, by the air's elasticity, converts the interrupted jets first received,
into a nearly uniform current  towards the reservoir.  The supply of air to
this vessel is maintained by the contrivance called a snifting-valve.
  In the preceding examination of the doctrines of fluidity, we have had to
touch on many of those phenomena of nature and art  which  are the  most
important  to man ; yet we have  seen how beautifully  simple and intelligible
they are all rendered  when referred, by  a methodical  arrangement, to a few
heads dependent on the  " fundamental truths."   Each one of the many par-
ticulars belonging to this department, and which when now explained appears
so simple and obvious, has yet been a distinct step in the slow progress of
discovery or invention,  and  probably when  first made has filled some inge-
nious mind with intense and purest delight.
Fie. 119.
HI
a   __j| 
234
ACOUSTICS.
PART   III.

    (continued.)
          SECTION IV.—ACOUSTICS,
OR PHENOMENA OF SOUND AND HEARING.
                      ANALYSIS OF THE SECTION.

1. Sound is heard when any sudden shock or impulse is given to the air,
  or to any other body which is in contact directly or indirectly with the ear.
2. If such impulses be repeated at very short intervals, the ear cannot attend
  to them individually, but hears  them as a continued sound, which is
  uniform, or what is called a tone, if the impulse be similar and at equal
  intervals, and is grave or sharp, according as they are few or many in
  a given time; and all continued sound is but a repetition of impulses.
3. When the number of impulses in a given time producing some uniform
  continued sound has a simple relation, as of half, third, fourth, fyc, to
  the number producing some other such sound which is heard either simul-
  taneously with it, or a little before or after, the ear is generally much and
  pleasingly affected by the circumstance ;  and the  sounds are said to have
  musical relation to each other, or to be accordant, while  all others are
  termed discordant.
4. The shock which causes the sensation of sound spreads or is propagated
  in all  bodies, somewhat  as a wave spreads  in water, with decreasing
  strength as the distance increases, but with a velocity nearly uniform, and
  which in air is about 1,112 feet per second.
5. Sound is REFLECTED/rom smooth surfaces, and hence arise many curious
  and pleasing effects called echoes, 8rc.
6. The structure of the ear illustrates the laws of sound.
  Early inquirers into nature had remarked that in most instances of noise
or sound there was present a  shock or  trembling of the sounding body,
often visible, but sometimes only sensible to the touch, or discoverable by
other means ; it was noted, for  instance, in the  string of a harp, in the reed
of a hautboy, in the prongs of a tuning-fork, in the lip of a bell: but it was
reserved for the moderns to understand fully, that the animal organ called the
ear, is  merely  a structure  of parts admirably adapted to be  affected by the
concussions or tremblings of things  around ;  and that  sounds in all their
varieties are merely such  motions, affecting the ear through the medium of 
CONTINUED SOUND.
235
the air which surrounds it, or of some other body, or series of bodies, reach-
ing from the trembling thing to the ear.
  The delicacy and complexity of an organ  destined to feel and to distin-
guish such slight and varying influences, and the vast  importance  of it to
man, as that which makes him capable of using  language, besides being his
ever-watchful monitor of surrounding occurrences, and the channel by which
the fascination of music  enters,  render this subject, to all who love to read
in nature the attributes of its author, a most favourite study.
   Because all the bodies around us are immersed, in common with ourselves,
in the ocean of air which covers the earth, we  are  much more frequently
warned of the shocks and tremblings of which we  have been speaking, by
their effect on the air, than in any other way ; hence the early prejudice that
air was necessary to sound, and hence, in part, the reason why the doctrines
of sound have generally been accounted a part of pneumatics.  We shall now
find, however, that all bodies are more or less  fitted to convey these trem-
blings, and that air in many cases is  neither the quickest nor the best conduc-
tor.   Although our notions on the subject are thus corrected, it  is still con-
venient to study the theory of sound as a  part of Pneumatics.

 I. " Sound is  heard when any sudden  shock or impulse occurs in a body
   having  communication, through the  air  or otherwise, with the  ear."
   (Read the Analysis.)
   Common instances of a single impulse are—the blow of a hammer—the
 clap of  hands—the crack of  a whip—a pistol-shot—any  explosion—the
 thunder-clap.
   The loudness of sound conveyed  by air depends on the air's density.   A
 bell enclosed in  the receiver of an air-pump is heard  less and less distinctly
 as the air is exhausted, and in a vacuum is not heard  at all.—Even the blow
 of a  hammer, in  a  vacuum,  is not heard if care is  taken to  prevent the
 shock from being communicated through  neighbouring solid  bodies.—In the
 thin air surrounding a lofty mountain-top  the report of a pistol is much less
 loud, and human voices  are weaker.—In  the condensed atmosphere of a div-
 ing-bell a whisper is loud.— When  volcanoes and various other resemblances
 to the constitution of our earth were first discovered in the moon, some per-
 sons fancied  that  during the stillness of  night we should hear the  thunder
 there :—but supposing the thunder to happen, and to be ever so loud, it could
 not be heard on earth, because there is no medium to bear thither the pulses
 of sound—there is a vacuum between.

 2. Impulses quickly repeated cannot be individually  attended to by the ear,
   and hence they appear as one continued sound,  of which the pitch  or
   tone depends on the number occurring in a given lime; and all continued
   sound is but a repetition of impulses.   (Read the Analysis.)
   If a wheel with teeth be made to turn and to strike any elastic  plate, as
 a piece of quill, with every tooth, it will, when moved slowly, allow  every
 tooth to be seen and every blow to  be  separately heard ; but with increasing
 velocity the eye will lose sight of the individual teeth, and  the ear, ceasing
 to perceive the  separate blows,  will at last hear only  a smooth continued
 sound, called a tone, of  which the character will change with the velocity ot
 the  wheel.                     .        ,       .      , .,  •, •  T,„„. dipt
   In like manner the vibrations of  a long harp-string, while it is very  slacK,
 are separatTly visible, and the pulses produced by it in the air are separately 
236
ACOUSTICS.
audible;  but as it is gradually tightened, its  vibrations quicken, so that,
where it  is moving, the eye soon sees only a broad shadowy bellying line;
and the distinct sounds which the ear lately perceived, seeming now to run
together on account of the shortness of  the intervals, are felt as one uniform
continued tone, which constitutes the note or sound  then belonging to the
string.
  Again, if a current of air passing through a tube or  opening,  be in any
way interrupted at regular and very short intervals, as by a  little  stop-cock
placed in the opening, of which cock the plug, instead of being only partially
turned by a cross handle, as in a common beer-cock, has a wheel fixed upon
it, so that any desired rapidity of rotation may be given to it,—then at every
time when the  passage  for air becomes open, there will be a  certain shock
given to the air around, and the repetition of such shocks will constitute a
musical tone.   This apparatus can produce all tones, and it enables us with
great precision  to ascertain the number  of pulses required to constitute any-
given tone.
  It is the elasticity of any string used  to produce a tone which  causes  the
repetition of the percussions, and therefore  the  continuance of the sound,
thus:—the string having been pulled at  its middle to one side, and then let
go,  is, owing to its elasticity, carried back quickly to the straight position;
but by the time that it has reached this, it has acquired a momentum which,
like the momentum of a vibrating pendulum, carries it nearly as far beyond
the   middle  station  as  the distance whence it  came:—it  has  to  return,
therefore, by its elasticity, from this  second deviation, in the same way;  but
still passing the middle as before, it has  again to  return ; and thus continues
vibrating uniformly as a  pendulum does, until  the resistance of the air and
friction gradually bring it to rest.   A large vibration of any string, like a large
oscillation of a pendulum, occupies  very nearly the same time as a  smaller,
because the  farther that the string is displaced or bent, the more forcibly, and
therefore quickly, is  it  pulled  back again by its elasticity:  hence  the uni-
formity  of the tone produced by a musical string is  not injured by the differ-
ent  force with which the finger of the player may touch the string.   Accord-
ing, however, as the  vibrations  of a string  are more extensive or  quicker,
the  impulses given to the air are more sharp or forcible, and hence the sound
becomes louder.   And  this  explains why sharp sounds are generally also
loud.  Vibrations  which are  comparatively  few and slow,  strike the  ear
very gently, as in the  flapping  of  a pigeon's wing, or in  the  play of a
switch.
  The most familiar instance of sounding vibration is that of  an elastic cord
extended between  two fixed points,  as in stringed instruments of music:  but
because elastic bodies generally, when by any force their natural form is for
a time altered, recover  it when allowed, not by a  first  effort, but  like  the
string of a pendulum, after a series of oscillations, almost all such bodies
repeat many  times an impulse once given to them, and thus may become the
means of producing a continued sound.—If a solid rod of steel, glass, or any
other elastic substance, be fixed firmly at one end and left free at  the other,
and  if that other be then pulled a little to one side of its station of rest,  and
suddenly let go, it will immediately seek its station again, but by the momen-
tum acquired in the approach, will go beyond it:  it will then  return as be-
fore, but again to pass, and so will continue to vibrate with diminishing force
for  a considerable  time.—A boy at school, thus, sticks the point of his pen-
knife into the bench, and by one touch makes it produce a continued uniform
sound of considerable duration.—The prongs of a tuning-fork, or of the com- 
CONTINUED SOUND.
237
mon sugar tongs, vibrate and sound in the same way.—In the musical snuff-
boxes and chimney-clocks, the sounds are produced by the vibration of little
rods of steel, fixed by one end, in  a row,  like the teeth of a comb,  and
touched by small pins or points projecting from a turning barrel.—Any elastic
flap, as of metal or of tough wood, placed  over an opening, so  as  to stand
away from it a little when not pressed by passing air, but  to close the open-
ing if so pressed, becomes a sounding reed  when air is gently forced through
the opening: thus, the air first pressing on the flap to  close it causes a mo-
mentary interruption of the  current, but the flap immediately recoiling from
the blow,  as well as by  reason of its own elasticity, again opens the  passage,
and the continued rapid alternation of the shutting and opening produces the
tone.—The reed of a clarinet is a thin plate of elastic wood, made to vibrate
in this way.—The drone of the bag-pipe and the common straw-pipe, are
reeds of nearly the same kind.—The Chinese organ, and  the  sweet instru-
ment lately introduced under the name of iEolina, have reeds which  differ
from these, by beating through the opening instead of merely  on its face.—
Elastic rods simply resting on supports  at both ends, or suspended  by their
middle, will also vibrate; a  musical instrument  is thus made of pieces of
glass laid upon two strings,  and struck  by a cork hammer: in the Island of
Java, a rude instrument of the same kind is made of  blocks of hard elastic
wood.—A portion of a hollow sphere of elastic metal very readily takes on
a vibration, during which its  form is constantly changing from  the perfect
round to the oval, and conversely ; there are consequently repeated percussion
of the air, and  a continued  sound, and  the thing is  called a bell.  A bell
admits of  great variety of shape,  and may be made of any elastic substance,
as metal, glass, earthenware, (buyers ring earthenware to ascertain its sound-
ness,)  and even  of hard wood.—The  Chinese  gong is  a metallic vessel
shaped like a common sieve, having a manner of vibration  very peculiar, and
producing sounds  that are rousing and  sublime.—The drum has a tense
elastic membrane on which  the  blows  of the drum-stick  are  received : its
tone ceases quickly, because the  motion of so broad a  surface is much re-
sisted by the air.—In the flute, flagelet, common organ-pipes, &c, the air is
forced through narrow  passages, and is divided by sharp  edges, in such a
way as  to suffer repeated but perfectly regular condensations or interruptions
sufficient to affect the ear; and hence the endless variety of sweet continued
sounds which these contrivances are known to produce.
  To the production of a  tone, it is of no consequence  in  what  way the
pulses of the air are caused, provided they  follow with sufficient regularity;
witness, in addition to  some of the instances given above, the  pure sound
produced by the motion of a fly's wing—supposed by many to be the voice
of the insect.   The clacking of a corn-mill, and  the noise  of a stick pulled
along a grating, are not tones, only because the pulses follow too slowly.
  Where  a  continued sound  is produced  by impulses which do not, like
those of an elastic body, follow in regular succession, the effect  ceases to be a
clear uniform sound or tone, and is called a noise.—Such is the  sound of a
saw or grind-stone—the roar of the waves breaking on a rocky  shore, or of a
violent  wind  in a forest—the roar and crackling of houses or  of a  wood in
flames—the mixed voices of a talking multitude—the diversified sounds of a
great city,  including the rattling of wheels, the  clanking of hammers, the
voices  of  street-criers, the  noises of manufactories, Sic;  which rough ele-
ments, however, at last mingle so completely that the combined result  has
often been called  "the hum  of men," from analogy to the smooth  mingling
miniature sounds which  constitute the hum  of a  bee-hive. 
238
ACOUSTICS.
           " Grave and sharp sounds."  (Read the Analysis.)
  The difference of sounds, which depends on the different number of vibra-
tions of the sounding body in a given time, divides them into those called bass,
low, or  grave notes, for  comparatively few and slow vibrations;  and those
called high, shrill, or sharp, for vibrations more numerous and quick.
  The frequency  of vibrations in strings increases with their shortness, light-
ness and tension—for if a string be long or heavy, there is a greater mass of
matter to be moved, and hence a slower motion ; and if a string be slack, the
force  of elasticity which pulls it  from  any deviation back to the straight
line is so much the less.  It is found that a string  taken of half the  length,
or of one-fourth the weight,  or of quadruple  the  tension of  another string,
vibrates twice as fast on any one of these accounts.
  These truths are  familiarly illustrated  in the violin.   The low or bass
strino- is thick and  very heavy from being covered with metallic wire, and the
others gradually diminish in  magnitude  and weight, up  to the smallest or
treble.  The  strings are tuned to  each other by being attached by one  end
to  movable pins,  which, when  tuned, increase or diminish  their tension;
and the sound produced by each may be afterwards varied to a certain extent,
by  the  performer  pressing different parts of it with the  finger against the
board, so as to shorten the vibrating portion.
  An analogous law, as to  the influence upon tone, of weight and dimen-
sions, holds with  respect to  bells, glasses, reeds, &c, and enables us to use
these also in the construction of musical instruments.

3.  " When the number  of impulses producing some continued  sound has
  a simple relation, as of half, third, fourth, 8,-c, to the  number producing
   some other sound which is heard  either simultaneously, or a little before
  or after  it, the  ear is much and pleasingly affected ; and the sounds are
  said to have musical relation to each other, or to be accordant, while all
   others are termed discordant."  (Read the Analysis.)
   Understanding now  that all continued uniform  sounds  are  produced by a
repetition of  similar beats or vibrations, we perceive that  in the series from
grave to sharp, there must be such as, with respect to the number of beats in
a given time, are  related to  each  other, as the numbers  1, 2, 3,  4, Sic, or,
which is the  same thing, as  10, 20, 30, Sic   Now as between two  sounds,
one of which has 20 beats while another has 10, there will be a coincidence
at every second beat of the quicker, and between  sounds whose beats are  to
each other as 30 to 20, there must be a coincidence at every third  beat of the
quicker, and  so forth,  we should  naturally expect the ear to be  differently
affected by such  correspondence   than  when the coincidence is  either less
frequent, or is irregular.   Accordingly wre find that all sounds which have
such simple  relations  to each other, are remarkably agreeable to the  ear,
either when heard together, or in close succession ; while those in which the
coincident beats are farther apart, are heard with indifference, or  are felt  to
be  positively harsh and disagreeable.   It is in  fact offering itself to be noticed
here, that  the coincident or double pulses of any  two concordant  sounds be-
come  the  cause or elements  of a  third sound, perfectly  distinct  from them,
but which is always heard with them, and is called their grave harmonic or
resultant: it is the same as  a simple sound having  as many vibrations in a
given time as there are coinciding  beats between the two other sounds.
   If a long musical string be made to sound, and the number  of its vibrations
in  a given time be ascertained, we find that if only half of it be  allowed to 
MUSIC.
239
vibrate at a time, as when a finger  presses its  middle against a board, that
half will vibrate twice as fast; and similarly, a third part three times as fast;
a fourth part four times as fast; and so on, producing the sounds or tones most
nearly related to each other.  A fine illustration of this is afforded by the string
of a violoncello, when made to vibrate by a bow moved very gently across it,
near the bridge ; for it often divides itself spontaneously into two, three or four,
&c, equally vibrating parts or bellies, with points of rest between them called
knots : when this happens, there are heard not only the sound or note belong-
ing to the whole length of the string, but, also, more  feebly, the subordinate
notes belonging to its half, third, or fourth, Sic, according to circumstances,
beautifully mingling with the first sound,  and forming with it a rich harmony.
Often in such a case tbe subordinate sounds swell with such force as to over-
power for a time the fundamental note ; but any one such sound is rarely of long
duration.  The same harmonic sounds  may be produced still more certainly,
while drawing the  bow across the string, by touching the string lightly with
the finger, at one of the points where we wish it to divide.   Even a tune may
be so played.
  The sounds thus belonging  to a single  cord or string, and produced by its
spontaneous division into different numbers of equal  parts, constitute, when
heard together or in succession, what may be called the simple music of na-
ture herself.  It is  produced pleasingly, as just described, by the single string
of a violoncello; but in the most perfect manner by the instrument called the
iEolian harp.
   The iEolian harp is a long box or case of light wood, with harp or violin
strings extended on its face.  These are generally tuned in perfect unison
with each other, or to the same pitch, as  it is expressed, except one serving
as a bass, which is thicker than the others, and vibrates only half as fast; but
when the  harp is suspended among trees, or in any situation where the fluc-
tuating breeeze  may reach it, each string, according to the manner in which
it receives the blast, sounds either entire, or breaks into some of the simple
divisions above described ; the result of which is the production of the most
pleasing combination and succession of sounds that ear has ever listened to,
or fancy perhaps conceived.  After a pause this fairy harp may be heard
beginning with a low and solemn note, like the bass of distant music in the
sky:  the sound then swells as if approaching, and other  tones break forth,
mingling with the  first, and with each  other; in the combined and  varying
strain, sometimes one clear  note predominates  and sometimes another, as if
single musicians alternately led the hand: and the concert often seems to
approach and again to recede, until with the unequal breeze it dies away, and
all is  hushed again.—It is no wonder that the ancients,  who understood not the
nature of air, nor consequently even of simple  sound, should have  deemed
the music of the iEolian  harp supernatural, and, in their warm imaginations,
should have supposed that it was  the  strain  of invisible beings from above,
come down in the  stillness of evening  or night to  commune with men  in a
heavenly language of soul intelligible to both.   But, even now that we under-
stand it well, there are few persons so insensible to what is delicate and beau-
tiful in nature, as to listen to this  wild  music without emotion ;  while the
informed ear finds  it additionally delightful, as affording  an admirable illustra-
tion of those laws of sound which human ingenuity at last has traced.
  As the simple scale of sound, called a  chord,  which nature thus gives by
the  spontaneous dividing of a single string, has  considerable vacancies in it,
human taste or feeling, long before there was any theory of music, had joined
to it the notes of two additional strings, one sharper or more  acute  than it, 
240
ACOUSTICS.
and the other more grave; of which additional notes, while part agreed, or
were in unison with certain  notes of the principal chord, the remainder just
served to fill up its larger intervals, and to complete a scale of nearly uniform in-
terval—as three ladders havingunequal intervals between their steps, might still,
if placed together, complete a stair of easy ascent.   The relation between these
strings or chords is such, that the principal beats thrice for twice of the low
chord,  and  the high  chord beats thrice for twice of the principal:—and in the
complete scale of notes, the  principal is five notes above the lower  and five
notes below the higher.  So truly natural is the scale thus formed, that it has
arisen in all nations, however remote or unconnected; and an untutored indi-
vidual, in attempting to raise his voice by regular steps, falls into it almost as
readily as the learned professor.   The scale has eight steps or notes between
any tone, and the tone above it vibrating twice as fast, or the tone below it vibrat-
ing half as  fast; these two tones or notes being hence called the octaves above
and below the note with which they are  compared,  and the intermediate notes
which  fill up either  octave from  the fundamental note are distinguished by
the names  of second, third, fourth, Sic, in ascending or descending.  The
numbers which express the  relations of beats among the notes of an  octave
are easily found, from our knowing the relative number of beats  in the notes
of any one simple chord, and the  relation  as above described of the three
chords forming the compound scale.  The following table exhibits these num-
bers or the  arithmetical expression for the notes of an octave, as well  as the
corresponding lengths of a  given string required to produce  them, and the
English designation of the notes by letters, and  the continental designation
by names,  these names being the  first  syllables of certain  verses  sung by
learners.
Number of vibra-?	1	2. 8	s 4	4 3	3 2	5 3	1 s 8	2
Length of string . . English characters .	1 C	8 0 D	4 5 E	3 F	2 3 G	3 5 A	8 J 5 B	i C
Continental names .	ut	re	mi	fa	sol	la	si	ut
  The musical scale, however far extended, is a repetition of similar octaves,
so that any note in it vibrates just twice as often as the corresponding  note
in the octave below, and half as often as that in the octave above.   The
lowest note which is perceptible  to the human ear has about thirty beats in a
second, and the highest about thirty thousand ;  and there is  included between
these two,  a range  of nearly ten octaves.  To certain ears the extremes  of
this range are totally inaudible, as if their power did not reach so  far.  Some
persons do not  hear at all the sharp note of the grasshopper, while some are
equally insensible to the lowest  tones of an organ or piano;  and yet to all
the perception of intermediate sounds may be equally perfect.  Few musical
instruments comprehend  more  than  six  octaves, and the human voice  in
general has only from one to three, the  female voice being in  pitch an octave
higher than the male.
   If the intervals in  the  musical  scale were all equal, a  performer  mi^ht
choose indifferently any note as a fundamental or  key-note, and  would only
have to attend to  the number of intervals above and below it; but, in  fact,
the relation of the three constituent chords is such that the  third and seventh
intervals, in ascending from a key-note, are only about half  as large as the
others.   It is owing to this circumstance  that in changing the  key on any
instrument, certain  notes belonging to other keys are half  a note too low or 
MUSIC.                             241
too high, that  is, too fiat or too sharp, and must be changed accordingly.
And hence, when an instrument is to be used to play in all  keys, its laro-er
intervals must be divided into two parts. The fact of these unequal intervals,
ill understood, is what gives an appearance of great complexity and difficulty
to musical science.
   Melody,  in  music, is  when notes, having the simple numerical relations
of beat which  we have been describing, are played in succession; harmony
is when two or more such notes are sounded together.   The effect of both
is delightfully increased by  what is called measure, viz., making the dura-
tion of the  notes or strain correspond with  certain regular divisions of lime.
This gives  to  the ear a prescience, to a certain  degree, of what is coming,
with the pleasure of having  expectation realized, as happens similarly from
the metre and  rhyme of poetry: it moreover  enables the memory to retain
musical combinations of sound—for the airs of the iEolian harp, which observe
no time, cannot be  learned or  repeated.  The  music  of a single  drum is
that of time only.
   Melody,  harmony, time and varying intensity of sound, are the four con-
stituents of music, and it seems that almost every state of mind has, in some
combination of these, an  appropriate  expression, intelligible to the general
feeling of the human race.   The exact relation between the  movements of
the animal spirits, as it  has been  expressed, or the fluctuating stream of
feeling, and the varying flow of sound in a musical composition, is not clearly
understood, but the  fact of their correspondence  and its consequences are
most remarkable.  Under many circumstances, the association between the
feeling and  expression is so strong,  that the latter is often spontaneously be-
traying itself;—witness the almost constant humming, or low song of some
contented beings—the singing and  whistling of careless childhood, or of the
light-hearted rustic living  among the beauties of nature—the heart-rousing
strain of the hunter or warrior—and the  tender  expression  of many of the
modifications  of anxiety and sorrow.  The musical  sensibilities are  by  no
means limited  to the human race, for there  is no expression more exquisite
than in the song of the nightingale  during  the evenings  of spring, or of the
thrush and blackbird, in the same season, amid the quiet retreats  of our
woodlands,—the music of which untutored  songsters  is made up of the same
elements as our own.
   The accompaniment of an air afforded to a singer by one or more instru-
ments, and  which is so pleasing, is  chiefly the  sounding,  simultaneously, in
a subdued manner, some other notes of the chords to which  the several vocal
notes belong.   Duetts and more complicated concert-pieces  have their origin
from the same  source: and  highly cultivated musical  sense can even follow
and enjoy several melodies played together.
   Musical notes, by whatever instrument produced, have to each other the
same numerical relations  in  the  beats or vibrations which constitute  them.
The different  qualities of tone,  therefore,  from  different instruments, can
only depend on the peculiarities of  the single  beat, as to whether they are
sharp  or soft, strong or weak, &c.   Such is the extraordinary nicety of per-
ception which  the human ear possesses in this respect, that it can  not only
distinguish different kinds of instruments, as a flute and clarinet, playing the
same note, but different instruments of the same  kind, even  to the extent,
for instance, of recognizing each  one of a hundred voices singing the same
air.  One of thexgreatest charms of concert music is, that the  voice and the
different instruments may take  up  separately, parts of the  strain suited to
their individual expression—the flute and clarinet, for instance, breathe soft- 
242
ACOUSTICS.
 ness;  the trumpet and drum arouse; the harp rolls forth its brilliant chords;
 the violin leads the flowing sound through rapid and endless variety ;  and
 so of the rest.
   That there might be correspondence in instruments when  played together
 and a known pitch when played apart, it became necessary to fix  on some
 tone or number of vibrations as a point of comparison.  Hence, tuning-forks
 have been made of steel, with length of prongs calculated to produce a certain
 note.  This note is usually the fourth, A or la  from the bass of the piano-
 forte, and vibrates about 430 times in the second ;—and when the note of the
 same name on any instrument is tuned in unison with  this, the other notes
 can be easily adjusted according to the harmonic relations above explained.
   Almost every substance or contrivance that can produce  a uniform con-
 tinued sound may enter into the composition of a musical instrument: hence
 the almost endless variety which the world has seen. ,  The  chief classes of
 instruments are stringed instruments,  wind instruments and bells or rods.
   Of the stringed instruments, we may mention the harp, the lyre or lute,
 the guitar, the viol of all sizes, and piano-forte.  The  harp, lyre  and lute
 were the inventions of antiquity, and  have  brought down with them to the
 present times a thousand delightful associations.   They awakened to inspi-
 ration  the bards and  poets of the young world, and  they were the beloved
 companions  of many of  the noblest spirits  of succeeding times.  Their
 great charm  appears to have been in their power to heighten the emotions
 produced by music's twin sister, poetry;  and the combined  effects  seem to
 have been magical.—The  other instruments mentioned are of comparatively
 modern  invention, particularly the piano-forte;  and their  perfection  has
 assisted in carrying  the combinations of musical sound to degrees of com-
 plexity and difficulty of which  antiquity dreamt not.  It is a question, how-
 ever, whether the style of much of the music now in vogue  does not prove
 rather a degeneracy, than a desirable refinement of musical taste.   Music is
 a language of nature, intelligible at once to all susceptible minds, and, in a
 degree even  to inferior animals; but modern art is  attempting to  make of it
 an artificial and conventional language, in which  there  may  be fashion and
 change.  The ornaments and accompaniments  are now often so overwhelm-
 ing,  that the melody,  in  which  the  idea  and  sentiment really  reside, is
masked and almost lost;  and an unpractised ear, particularly if listening to
an organ, often discovers  only an  unmeaning succession  of chords.  And
when a singer, abandoning the  natural simplicity of melody, strains to exe-
cute with the voice  the complicated movements which  belong properly to
instrumental accompaniments, the attempt destroys the poetry, by either ren-
dering  the words inaudible, or by sacrificing their natural expression to some
supposed appropriate expression of the ornamental music.  These considera-
tions may account  in part for the insensibility of so many highly-endowed per-
sons to ivhat is now called excellent music.   Some of  the tricks on the voice
and on instruments, at present  so common,  are, to natural or  graceful music,
what tumbling and rope dancing are to natural orgraceful gesture. And when
we hear noted professors avow their inability to sing a simple ballad, or to play
an unadorned melody, must we  not conclude that the  natural  sense of music
has left them, as the relish for simple but the most invigorating fare  has  left
the morbid epicure ?
  The guitar, as  affording an accompaniment to vocal music, has  many ad-
vantages.  It is not too loud, yet the  strains are very distinct; it admits of
most touching expression; it is very easily  learned by any one who  should
attempt to learn music ; it is portable and" cheap.  The great facility of ac- 
MUSIC.
243
 companiment on it  depends  on this, that  the player is able by one position
 of the hand to touch the strings so that the sounds of all the six shall belong
 to the same chord :—three positions of the hand, therefore, for one key, pro-
 duce all the notes and chords which a simple accompaniment requires; and
 the hand soon falls  into these so readily, that the player is hardly sensible of
 exerting volition.
   Among wind instruments  are the flute, the flagelet, the organ, the  clari-
 net, the hautboy, the horn, the  trumpet, Sic   The pitch or tone of a tubular
 wind instrument, just as of a musical string, has relation to its length  ; and
 the vibrations causing the  sound seem  to be waves or condensations of air
 passing from the mouth to the  extremity of the tube ; being more frequent,
 therefore, as  the tube is  shorter;—when the bottom  of the tube is  closed,
 the wave  has to come back again, and thus renders the note twice as grave.
 It appears, also, that on blowing more  strongly, the  air in the tube divides
 into separate vibrating portions, as  a  siring may divide to produce its  har-
 monic sounds, and produces thus all the harmonic sounds  belonging to the
 fundamental  note of the tube.   By blowing into  a common German  flute,
 for instance,  it is possible to  produce five ascending harmonics without  mov-
 ing the fingers at all.   The music of a trumpet is  limited  to these five  notes
 of the same  chord; but in the flute, and  other instruments with holes, the
 effective length of  the tube is calculated from the upper  end to the nearest
 hole left open; and each length has its  harmonics.—If a tuning fork, Jew's-
 harp, or any such sounding body, be held at the open end  of a°tube or other
 empty space of dimensions calculated to produce a frequency of undulation,
 in its contained air, according with the pulses of the sounding body, then the
 tube  or space will immediately give  out its own beautiful tone;  and if the
 space be enlarged or diminished in a double, trible, or any  other simple pro-
 portion—as a tube may be, by a piston moved up or down  in it—then will its
 note become  the fifth, octave, twelfth, &c, above or below the original  tone,
 although that tone continues  unchanged.  The tones of the Jew's-harp are
 well  known to depend altogether on the varying dimensions of the player's
 mouth; but to obtain perfect  music from it, three harps at  least, to be substi-
 tuted one  for the other during  the performance, are required to produce the
 notes of the three constituent  chords of the common musical scale.—In wind-
 instruments with reeds, the tone depends on the stiffness, weight, length,  &c,
 of the vibrating plate or tongue of the reed,  as well as on  the dimensions of
 the tube or space with  which it may be connected.  This  truth  is well illus-
 trated in that instrument, the iEolina, already mentioned, which, in improved
 and varied forms, promises to become common, and one of the most express-
 ive of wind instruments.—The sounds of  the human  voice are the sweetest
 of all, and are produced by the  vibrations of two delicate membranes situated
 at the top  of  the windpipe, with a slit or opening,  called the glottis, left be-
 tween them, for the passage of  the air.   The tones of the voice  are grave or
 acute, according to  the varying  tension of  these membranes, and to the size
 of the opening.—In the organ there is a pipe for each note, and wind is ad-
 mitted from the bellows to the pipes, by the action of keys, like the keys of a
 piano-forte.  The organ maybe played  also very perfectly  by a barrel, made
 to turn slowly under the keys, and to lift them in passing, by pins  projecting
 from it at the required situations.  Very complicated pieces of music are  thus
 set on barrels, but at great cost of study and labour,  and, therefore, of money ;
 now a plain barrel, made to turn near the keys of an organ during perform-
ance on  it by the hands, might be made to record, with mathematical accuracy,
every touch of the most  finished player, by receiving marks of some  kind 
244
ACOUSTICS.
Fie. 120.
 from the keys as they were lifted ; and to repeat, with absolute accuracy, there-
 fore, any performance, however delicate and exquisite, it would only be far-
 ther  necessary to  drive pins into the barrel where  the marks  remained, and
 afterwards to make these pins lift the keys.
   Bells are often conjoined in sets, having  the musical relations, and  to some
 persons their music is very agreeable.   There are, in the tolling of  a single
 bell,  a  loudness and solemnity rendering it a  fit accompaniment of funeral
 rites.
   The Chinese gong partakes of the nature both of  the  bell  and of a great
 drum, and has  something in its sound which is singularly affecting.   In its
 own country it bears a part in one of the most imposing ceremonies which
 man  has ever imagined.   On  certain festivals, as  the sun is sinking in the
 west, the  whole population of China, a host of more than a hundred millions,
 issues forth under the single canopy of heaven, to  testify, amid the thunder
 of gongs and the continued discharge of  fire works, that adoration and grati-
 tude towards the Deity which  human nature, in all ages and climes,  has felt
 to be due, and has  eagerly sought to express, however  blind as  to the sublime
 simplicity of religious truth.
   Bells or goblets of glass sound still more perfectly than  those of metal,
 and  by gentle  friction on their edges with a bow or the wetted finger, their
 tones may be continued  for any length of  time, and  may be made  to swell
                                and diminish  like the human voice or the
                                notes of a violin.  A set of glasses, there-
                                fore, attuned  to each other, according to
                                the harmonic scale, becomes, for  certain
                                species of music, the most  perfect of all
                                instruments.  It is in fact an iEolian harp
                                at  command.   Dr.  Franklin,  who first
                                constructed a set, double the long line of
                                glasses upon  itself,  and placed the  half-
                                notes as outside rows.  The  author of this
                                work, however, during some experiments
                                on  sound, found the zig-zag arrangement
                                here represented to  possess certain advan-
                                tages.  The small open circles represent
                                the mouths of the glasses standing in a box
                                a b c, and the relation of the  glasses to the
                                written  musical  notes  is  shown  by the
 common music lines  and spaces which  connect them.   The learner  dis-
 covers immediately that  one row of  the  glasses produces the  notes written
 upon  the lines, and the other row the notes written between the lines;  and
 he is mentally  master of the  instrument by simple  inspection.  This ar-
 rangement  also renders the performance easy, for the  notes most commonly
 sounded in succession are contiguous : and the relations of the notes forming
 a tune are so obvious to the eye, that the  theory of musical combination  and
 accompaniment is learned at the same time.   The set  of glasses here repre-
 sented has two  octaves, and with the additional flat  seventh and fourteenth,
 seen at a and c, which, when required, may be substituted  for the correspond-
 ing glasses  in the rows, it is capable of playing the greater part  of our simple
melodies.   All  the half-notes, if desired, may  be placed  in  outside rows.
The player stands at the side of the box between a and b, and  has the notes
ascending towards  the right hand, as in a piano-forte.
 
MUSICAL EAR.
245
                              Musical ear.
  Philosophers have not yet been able to account for a remarkable difference
among individuals, as  regards-their perception  of the musical relations of
sounds.  Many persons,  without understanding any thing of acoustics, or
having studied  music as a science,  can  tell instantly whether various notes
heard together  or in succession, have the mutual relations which we  call
musical—and which we now know to depend on the comparative numbers
ot beats in a given time ; and  they quickly recognize  and learn to repeat
tunes, and to sing a fit second or bass  to the performance of another ;—while
there  are persons, again, with an equally  perfect sense of hearing, who can
neither know if an air be played in tune, nor what air it is, nor can they ever
sing alone  or accompany.  The former class of persons are said to have a
musical  ear, and  the latter to want it; and although  cultivation will raise
mediocrity to considerable  expertness,  it  cannot bestow  the  faculty where
originally deficient.  On this subject there is a very common misconception,
which becomes a source of great mortification on one side, and of arrogance
on the other, viz., that the possession of a musical ear, or the power of  dis-
tinguishing  notes, is the  indication of  all the finer qualities of the mind,
while the want of it proves an opposite deficiency ; and Shakspeare's opinion
of him " that hath no music in  himself," is often  triumphantly cited as ap-
plicable to all who want the distinguishing ear.   The truth, however, is,  that
many who possess this characteristic in  a  remarkable degree, are deficient in
almost all else  that humanity reveres,—witness the weak minds and disor-
derly  lives of so many professed  musicians,—while many, again, who have
it not, are otherwise examples of excellence, and  exquisitely sensible toother
beauties and harmonies of nature.  They may not be  deaf, for instance, to
the general music of spring, when all nature bursts forth in voice of rejoicing,
nor to the awful  music of the storm—they may feel as touching music  the
silence of a  lone wood,  contrasted with  the unceasing din of multitudes—or
even the stillness of night in a great city,  where  the astronomer, contemplat-
ing the wondrous spheres above, hears only the  tongues  of  passing time in
the church towers, or the call of watchmen, faintly sounding in the distance.
In fine, many distinguished poets and  philosophers have had no musical ear.
—That the charm of music  is often as much from early associations as from
peculiar aptitude  in the individuals, is proved by the effects so well  known
of the Swiss airs, when heard  by native Swiss in foreign lands ; and, indeed,
of the national  melodies of  all countries, whose  people are happy, and mix
song with their usual occupations,—it not  being in  nature, that at any period
of life, or in any clime,  a  man  should cease  to  deem  those modulations
lovely, which recall the ecstatic emotions of his infancy and  childhood;
modulations learned in general from a parent's voice, perhaps an  excellent
mother's, whose affection  was so long around him as a shield, whose tears
fell  to chide  his  errors, and to reward where there was promise of virtue ;
whose steady judgment was his guide,  whose faultless life was his exam-
ple, and who in all things to him was  a personification of God's goodness on
earth.
  It is the prejudice of  which we are  now speaking with respect to musical
ear and  musical taste, that in  the present  day, condemns many young wo-
men, possessed of every species of loveliness and talent except that of note-
distinguishing, to waste years of precious time in an attempt to acquire this
talent  in spite of nature;  but when they have succeeded  as far as they can,
they have only the merit of being  machines, upon which  tunes are set as
    17 
246
ACOUSTICS.
 upon a barrel-organ, and of which the performance is  often far from being
 pleasing to good judges.  Such persons, when  liberty  comes to them with
 age or marriage,  generally abandon the offensive occupation;  but tyrant
 fashion will force their daughters to run the same course. The waste of time
 now spoken of, is  only one of many evil consequences which arise from  the
 prevailing false  notions with  respect to music:  a subject which, however
 interesting, cannot be farther pursued in this place.

 "The trembling which causes the sensation of sounds spreads in all bodies,
                    solid or fluid."  (Read the Analysis.)

   As air consists of material  particles held far apart from each other by  the
 repulsion of heat among them, we can conceive  how  an.impulse given to a
 certain portion of the particles is transmitted to those beyond, by the increase
 of repulsion as they approximate ; and from the second layer  in the same
 manner to a third, and so on.  And as  in fluids the particles all mutually rest
 against, or repel each other, we can conceive why a motion produced in any
 part of a mass should be felt in  every direction.  The explosion of gun-
 powder,  in which  there is a sudden  formation of a quantity of  air, gives a
 shock all round which spreads a spherical wave  to a great distance.
   Although  material particles in the  form of liquid or solid are much nearer
 to each other than in the form  of air, we still  have many proofs, as stated at
 page 30, that  they are not  in absolute  contact,  and  we therefore see the
reason  why the impulses producing  sound should be  transmitted through a
liquid or solid  in the same manner as through air, and  even, by reason of the
 greater proximity of the particles, more quickly and forcibly than in air.
   Instances  of air  carrying sound were given at page 235.—As farther ex-
 amples, we may cite the cases  of what are called sympathetic sounds.  Every
 elastic body being sonorous, that is to  say, being fitted to tremble when struck,
with a certain  frequency of oscillation,  depending on  its weight  and shape,
 &c, if the air around it be made to tremble by any cause, with  the velocity
 which it  is fitted to take on or  produce, it  immediately begins to tremble in
unison with  the air; and its motion or sound may continue after the original
 cause has ceased.—Thus almost any sound produced near a piano-forte whose
 dampers  are raised, finds a responsive string, and if bits of paper are strewed
 upon the strings generally, those falling on the strings which return unisons
 or octaves to the sounding body are soon shaken off, while the others remain.
A harp or guitar in a room with talking company, is often  mingling a note
 with their conversation.—A wine-glass or goblet  may be made  to tremble,
and if on a table at all inclined, even  to fall, by a  person  sounding on a vio-
loncello near it, the note accordant to its own.
   Sounding bodies vibrate much  more quickly, or have sharper tones, if
 placed in light hydrogen, than  in common air; and more quickly in common
 air than in any of  the heavier  gases :—because the lighter the  surrounding -
fluid, the less is the resistance  to  a body moving in it.  Thus also a bell will
 ring under water, but with a much graver sound than in  the air.
   That water  is a vehicle of sound, is proved  by the fact last mentioned,__
by the distinctness  with which the blows of workers around a diving-bell 'are
heard above,—by the fact that  fishes hear very acutely, Sic.
   And the following are instances of  sound conveyed  by solids.__A scratch
of a pin at one end of a wooden log is distinctly heard by a person  applying
his ear at the other end, although through the air it is  not at all audible even
 to the person who makes it.—Savages often discover  the proximity of ene- 
SPREADING OF SOUND.
247
mies, or of prey, by applying an ear to the ground and hearing the tread.—
The approach of horsemen at night is easily discovered in the same way.—
The report of a cannon placed on the ice is carried much farther by the ice,
than by the air around.—In the military operation of mining, or cutting away
under ground for the purpose of entering a citadel, or blowing up fortifications,
the approach of the enemy is often  discovered by the subterranean sound of
the pioneer's tools.—The awful muttering of earthquakes is  merely the sound
of subterranean explosions, conveyed from amazing distances, by the solid
earth.
  A superstitious man sleeping in the upper story of a lofty house had for
some time heard, during the stillness of the nights, a singular beating  noise
near the head of his bed.   There was no adjoining house  beyond the wall,
nor was there any thing going  on near him in his own house to account for
it, and  he at last deemed it supernatural.   Accident at last discovered that in
a hovel built at the bottom and outside of the wall against which his bed stood,
there was a wooden clock hanging, of which the sound, while all  else was
still, became  audible aloft..
  It is  easy to ascertain whether a kettle boils, by putting one end of a stick
or poker on the lid, and the other end to the ear; the  bubbling of the  water
then appears  as loud as the rattling of a carriage in the street.—A slight blow
given to a steel poker or common triangle, of which an end  is held to the ear,
produces a sound which is even painfully strong.
  The readiness with which  solids receive and transmit sound is farther per-
ceived  in the  fact, that a small musical box, while held in the hand, is scarcely
audible, but when pressed against a table, or a door, will rival a little  harp.
The vibration communicated from the box pervades the whole of the wood,
and the extended surface  then acting on  the  air increases  the effect.   The
construction of violins, harps, guitars, Sic, and of sounding-boards generally,
is governed by the  same law.  In the dancing-master's kit or small fiddle,
which  he carries in his pocket, there may be the same strings and  the same
bow as for a  violin, but it has very little sound, because the extent of its sur-
face is  so small.  A heavy piece of metal called a sourdine, when fixed upon
the btidge of a violin,  damps the sound, because it is  a dead mass  resisting
the motion of the elastic wood.
   The fact of solids conveying sound so much more perfectly than air has
lately been applied  to useful purposes in medicine.  Dr. Laennec, of Paris,
proposed some years ago to listen to what was going on in  the interior  of the
body, and of the chest particularly, by applying one end of a wooden cylinder
which  he called a stethoscope or chest inspector, to the surface, and resting
the ear against the other end.   The results of this happy thought have been
important.
   The actions going on in the chest are, the  entrance  and exit of the air in
respiration, the voice, and the motion of the  blood in the  heart  and blood-
vessels ;—and so perfectly do all these declare themselves to a person listening
through the stethoscope, that an ear once familiar with the natural and healthy
sounds, instantly detects certain deviations from  them.  Hence this  instru-
ment becomes a means of ascertaining certain diseases in the  chest almost as
effectually as  if there were  convenient windows for visual inspection: and
when it is  considered that a fourth or fifth part of the inhabitants of Europe
die of  disease of the chest, such as inflammations, abscesses, consumptions,
dropsical collections, aneurisms, and various affections of the heart and blood-
vessels, each of which requires an appropriate treatment, the importance of
such a means may be judged of.  By many medical men this instrument was 
248
ACOUSTICS.
at first ridiculed as quackery and nonsense, and  many have yet to learn the
use of it.   May not both of these facts be attributed to  the. error which has
existed in  medical education, of leaving  so many practitioners without that
knowledge of the general laws of nature,  which should enable them to appre-
ciate at o°nce any means likely to be useful in their art, from whatever quarter
offered ?

                " Velocity of sound."  (See the Analysis.)
  The velocity of light is  such, that for any distance on earth its  passage
may be regarded as instantaneous.   The  velocity of sound is very much less.
__If a woodman be observed at his occupation on the hill, his axe is seen to
fall a considerable  time before the sound  of his  blow reaches the spectator's
ear.—The flash of a gun  fired at a distance is seen long before the report is
heard.
  Most accurate experiments have  been made to ascertain the velocity with
which sound travels in the atmosphere;  and  it  is found  to be 1,142 feet per
second, or a mile  in about four seconds and a-half; varying little either with
the density or temperature of the air.
  By  noting then  how long the flash of a gun is seen before the report reaches
the ear, we learn the  distance of the ship  or battery from which the gun is
fired.   A chasing ship may thus  often discover whether she be nearing or
not the object of her pursuit.   In the same manner the distance of thunder
may  be ascertained:  and  the reason of the  long-continued roll of thunder
is, that although  the lightning darts instantly through  the chain of clouds,
perhaps of miles  in length, the claps or  explosion at each interruption of
the chain are only heard successively, as the sound arrives  at the ear.  The
pulse  at the wrist  of a healthy man is a convenient measure of time for ascer-
taining distances by the motion of sound,—each beat making nearly a second,
and therefore indicating a distance of nearly a quarter of a mile.
   A line of muskets fired  at the same instant  cannot appear a single report
to any person who is not  in the centre of a circle, of which the line forms a
part.
   An extended orchestra of musicians cannot be heard  equally well from all
situations near them.
   Wind affects the velocity of sound just as  a current in water affects the
motion of a sailing ship.
   Sound decreases in intensity from the  centre where it originates,  according
to the same law as gravitation or light; that is to say, at double distance it is
only one-fourth part as strong, at triple,  a  ninth, and  so on.
   By confining it, however, in tubes, which prevent its spreading, its force
 diminishes much  less rapidly, and  it will, therefore, extend  to  much  greater
 distances.—In many manufactories,  and even private  dwellings now, there
 are pipes for the conveyance of sound leading to all parts; so that on ringing
 a bell to attract attention, verbal orders may  be given through them to great
 distances.
   Sound travels in water four times quicker, and in solids from ten to twenty
 times quicker, than  in air.  The blow of a  hammer given to a wall by a
 person at one end, may be heard twice by a person at the other, viz., almost
 immediately by an ear applied to the wall, and a little after through the air.

               " Reflection of sound."  (Read the Analysis.)
   As a wave of water turns back at a smooth wall or obstacle, so that at any 
REFLECTION OF SOUND.
249
distance after the reflection, it appears what it would  have been at the same
distance beyond the wall,  only  moving  in  an opposite  direction; so the
pulses  or  waves of  sound are regularly reflected from  flat  surfaces,  and
produce what is called an Echo.   Such flat  surfaces  of nature's works are
found only among the rocks and hills ; and hence arose the beautiful fiction of
the ancient poets, that Echo was  a nymph who dwelt concealed among the
rocks.  Science has now disclosed the secret of the viewless Echo ;  but who
does not vividly recollect the wonder and delight with which he has listened,
in the morning of his days, to his shrill call returned to him from some bold
precipice, across  the plain or river, or, perhaps, sent down to him again from
the vaulted roof of ocean's caves!
   The  quickness with which  an echo is returned to  the spot where the
sound originates, depends of course upon the  distance  of the  reflecting sur-
face ; and, as sound travels 1,142 feet in a second, a rock at half that distance
returns  a sound exactly in one second.  The number of syllables that can be
pronounced in a second,  will, in such a case, be repeated distinctly, while the
end of  a longer  phrase  would mix with the commencement of the echo.
The breadth of a river may easily be ascertained where there is an echoing
rock on the farther shore.  A perpendicular mountain's side, or sublime cliffs,
such as  in many parts  skirt the British  coasts, return an audible echo of
artillery, or of thunder, to a distance of many miles.
   If two bold faces of rock or wall be parallel  to each other, a sound  pro-
duced between them  is repeated often, playing like a  shuttlecock  between
them, but becoming more faint each time until it is heard no more.   In some
situations, particularly when the sound plays thus above the smooth surface
of water, a pistol-shot may be counted forty times.
   The resonance of enclosed spaces  depends  on this continual  reverberation.
It often  increases the effect of music by converting a simple melody, which
is a succession of notes,  into a harmonized piece, where each note is accom-
panied  by  some accordant tones;  and a young flute-player  is  often  first
charmed with his own music when he finds himself performing a duett with
Echo in a cave or under a spacious arch :—but resonance injures the distinct-
ness of speech, so as even in some ill-contrived halls of assembly or theatres,
to render the articulation unintelligible.  Small  rooms or near  surfaces  give
no perceptible echo, because the interval of time between  the original sound
and its repetition is too short for the ear to appreciate.
   It is worthy of remark that every apartment or confined space has a certain
musical note proper to it, the pitch of which depends upon the number of
pulses or repetitions of a sound produced there in a given time by the returns
from  its walls.   The velocity  of sound being  uniform, this  number  must
depend on the size of the apartment.
   There is a curious effect of echo which both illustrates  the  nature of the
phenomenon, and proves that a tone or musical  sound is merely a repetition
of pulses following each  other very  quickly.   Iron  railings  are generally
formed of square bars, of which any side is a plane surface, and may produce
an echo. Now a sound, such as the sharp blow of a hammer, occurring near
the end  of such a railing, is echoed to a corresponding place on the other side
by every bar in it; and as the echoes do not return all at once, but in regular suc-
cession, according to the increasing distances of the bars,  the consequent regu-
lar succession of slight pulses, with uniform and small intervals, affects the ear,
not as the echo of a single blow, but as a continued musical tone, the  pitch
of which depends on the distance of the bars from  each other.—The writer
of this had observed,  in passing on horseback along a particular portion of 
250
ACOUSTICS.
road, where there was first a piece of wall  and then two  pieces of paling
with rails or bars of different width,—that there was from  the wall a clear
echo of the horse's  cantering feet, and  afterwards, opposite  the palings, a
ringing sound for every step of the horse.   He  at first concluded that  the
road there was singularly hard, although it did not appear so, and he slackened
the horse's pace to save  his  feet, until, observing one day  that the ringing
sound  was of different pitch opposite the two pieces of paling, and so as
to correspond with  the  different width  of the  bars,  the true  explanation
occurred to him that the sound was  an echo of the nature above described.
   That an echo  may be  perfect, the surface  producing it must be smooth,
and of some regular form ;  for the wave of sound rebounds according to  the
same  law as  a wave of  water, or a ray  of light, or an elastic ball, Sic, as
explained at page 65, viz., perpendicularly to the surface, if  it fall perpen-
dicularly, but if it fall obliquely on  one side, departing with  an equal degree
of obliquity on the other.  To express this very important  law shortly,  we
                             say  that "the  angle of  reflection is  equal to
                             the angle of incidence."—According to  this
                             law, any irregular surface must break an echo;
                             and  if the irregularity be  very considerable,
                             there can be no distinct  or audible  reflection
                             at all.   A regular concave surface, on the con-
                             trary,  as  e  g, may concentrate  sound,  and
                             bring all which falls upon it,  as from abed,
                             to the  same centre or focus,  as at f, so as to
                             produce there  a very powerful effect.
                               We thus see the reason  why echo is much
less perfect from the front of a house which has windows  and doors, than
from the plane end, or  any plane wall  of  the  same magnitude,—and why
the resonance of a room is so irregular and indistinct when the  room contains
curtains, carpets  and other  furniture, or a crowded assembly.   Halls  for
music  have generally plane bare walls.   Theatres for the drama, again, have
boundaries broken in all  ways by rows of boxes,  and various ornaments.
   The concentration of  sound by concave surfaces produces  many curious
effects both in nature and in art.
   There are remarkable  situations  where the  sound from a cascade is con-
centrated by the surface of  a  neighbouring cave so completely, that a person
accidentally bringing his  ear into the focus, is suddenly astounded, as if the
universe  were  crushing around him.  A chair placed  in the cave, so that a
person sitting down in it must bring his ear  into the focus, insures the success
of the sometimes  amusing experiment.
                 The centre of a circle  is  the focus in which  sound issuing
               from it is again collected after reflection : hence the powerful
               echo near the centre of a  round apartment.  An oval has two
               centres or foci—one towards each end, as a and b—and the
               nature of the curve is such, that sound, or light, or heat,
               issuing around from either of the foci, as  a, by obeying the
               law of reflection above stated, is all directed from the various
              points, as at c d e, Sic, to the other focus, as  at 6.  Hence
              a  person  uttering a  whisper in one focus of  an oval room is
              very  audible at the  other, although he may not be heard by
persons placed between.   Such a room  may be called a whispering gallery.
Concave surfaces  facing  each other, as two alcoves in a garden, or covered
recesses on opposite sides of  a street or bridge, will enable persons seated in
Fig. 121.
d    t   j.   d
 
REFLECTION OF SOUND.
251
 their foci to converse by whispers across louder noises in the space between,
 ano  without themselves being overheard in that space.
    1  he reason why a tube conveys sound so far, is, that its sides confine or
 repress, by a  continued reflection, the advancing sound which, in the open
 air, would  quickly spread laterally and be dissipated.   And the reason  that
 the plane surface of a smooth wall, or of water, &c, also conveys  sound so
 lar, is, that it similarly prevents the lateral spreading and dissipation, although
 only on one side.—Persons far  apart may converse along  a smooth wall.—
 1 he  barking of dogs or the clear voice of a street-crier, in  a town situated
 on the  border  of a lake, may be  heard across the water in a calm evening at
 a distance of more than five miles—the sound of bells, of  course,  is aud,ble
 much farther—And in  the  stillness of night, even  the splashing  oars of a
 boat  will announce  its approach to persons waiting at a great distance.
   If  a sound-reflecting surface be curved inwards, that is,  be concave, it not
 only  prevents  the spreading of any sound which passes along it, but is con-
 stantly condensing the sound by  driving the external part inwards.   Hence,
 in a circular space, such as a gallery under a dome, persons close to the wall
 may  whisper to each other at all  distances.
   An ear-trumpet is a tube wide at one end where the sound enters,  and
 narrow at the  other where the ear is applied: its sides are so curved that,
 according to the law of  reflection, all the sound which enters is brought to
 a focus in the narrow end.   It thus increases manyfold the intensity of a
 sound which reaches the ear through it, and enables a person who has become
 deaf  to common conversation, to mix again  with pleasure in society.   The
 concave hand held behind  the ear answers in some degree the purpose of an
 ear-trumpet, and in  a very  large theatre is sometimes useful even to persons
 of quick hearing.   A notorious  instance of a sound-collecting  surface was
 the ear  of Dionysius, in the dungeons of Syracuse: the roof of the  prison
 was so  formed as to collect the  words and  even whispers of the unhappy
 prisoners, and  to direct them along a hidden conduit to where the tyrant sat
 listening.  The wide-spread sail of  a ship, rendered concave by  a  gentle
 breeze, is also  a good collector of sound.  It happened one day on board a
 ship sailing along the coast of Brazil, far out of sight of land,  that the per-
 sons  walking on deck, when passing a particular spot,  heard very distinctly,
 during an hour or two, the sound of bells, varying  as in human rejoicings.
 All on board came to listen, and were convinced, but  the phenomenon was
 most  mysterious.   Months afterwards it was ascertained, that at the time of
 observation the bells of the city of St. Salvador, on the Brazilian coast, had
 been  ringing on the occasion of  a festival:  their sound, therefore, favoured
 by a gentle wind, had  travelled over perhaps  100 miles of smooth water, and
 had been brought to a focus by the concave sail in the particular situation on
 the deck where it was  listened to.   It appears from this that a machine might
 be constructed having the same relation to sound that a telescope has to light.—
 A  friend of the  author, on the 18th of June, 1814, while sitting near the wall
 of his garden, situated near Dover, heard  distinctly the firing of the cannon
 at the battle of Waterloo.
  The speaking-trumpet is made  according to the same law of reflected
 sound, with the view of directing the strength of the voice  to a particular
 point.   The  sea captain  uses it  to hail ships at a distance, or  to  send his
orders aloft, where the unaided voice would be lost in the noise of the wind
and waves.  A similar form of mouth is used for the bugle horn and common
trumpet, and fits them  to  sound the note of command  amid the uproar of con-
tending armies. 
252
ACOUSTICS.
  Some  amusing effects have been produced  by operating on sounds  with
tubes and concave surfaces.   What was termed the invisible girl, was  a
contrivance where the questions of visitors were caught  by a concealed con-
cave, and carried to the director who sat at a distance; and his replies, as in
the whispering gallery, became audible to the inquirers alone.
  The concave, undulating, and perfectly polished surface of many sea-shells,
fits them to  catch, mix, and return  the  pulses of sounds that happen to be
trembling about them, so as to produce that curious resonance from within
which closely resembles the sound of the distant ocean—so closely, that the
spirited boy, after studying the interesting stories of voyagers which paint
dangers  to be nobly braved, and charms of  nature to be seen in distant lands,
often feeds  his imagination with this  voice of a shell, and fancies himself
already riding among the billows.

                            The animal ear,

so admirably adapted to perceive the evanescent tremblings of the air, has of
course a  structure in nice  relation  to their nature as now  explained.   The
parts of the ear, and  the progress of sound to  the sentient  nerve, may be
simply described as follows:
  1st.  There is external to the head, a wide-mouthed tube or ear-trumpet
                             a, for catching and concentrating the waves of
                             sound.  It is movable in many  animals, so
                             that they can direct it to the place from which
                             the sound comes.
                               2d. The sound concentrated at the bottom
                             of the ear-tube, falls upon a membrane stretched
                             across the channel, like the parchment of an
                             ordinary drum,  over the space  called the tym-
                             panum or drum of the ear b,  and causes the
                             membrane to vibrate.  That its motion may
                             be free, the air contained within the drum has
free communication  with the external air, by the open passage f, called the
Eustachian  tube, leading to  the back of the mouth.  A degree  of deafness
ensues when this tube is obstructed, as by wax;  and a crack or sudden noise,
with immediate return of natural hearing, is generally experienced when, in
the effort of sneezing or otherwise,  the obstruction is removed.
   3d.  The vibrations of the membrane  of the  drum are  conveyed farther
inwards, through the cavity of the drum, by chain of four bones (not here
represented on account of their minuteness,) reaching from the centre of the
membrane to the oval door or window leading into the labyrinth e.
   4th. The labyrinth, or complex inner compartment of the ear, over which
the nerve of hearing is spread as alining, is full of  water; and therefore by
the law  of fluid pressure (see  page 128,)  when   the  force of the moving
membrane of the drum, acting through the chain of bones, is made to com-
press the water, the pressure is felt instantly over the whole cavity, as in a
hydrostatic press.—The labyrinth consists of the vestibule e, the three semi-
 circular canals c, imbedded in the hard bone, and  a windinor cavity, called
 the cochlea d, like  that of  a snail-shell, in which fibres, stretched across like
 harp-strings,  constitute  the lyra.—The separate uses  of these various parts
 are not  yet perfectly known.  The  membrane of the tympanum may be
 pierced, and the chain of bones may be broken without entire  loss of hearing.
 Considerable diversity of  form and dimension is found in different animals.
Fig. 123.
 
THE EAR.
253
The bone containing the cavities of the ear is the hardest in the body, and is
the first formed.
  The ear has  the power of judging of the direction in which sound comes.—
A person in a thicket, listening to the song of various birds, although  they
be concealed from his eye by the luxuriance of the vernal foliage, still judges
correctly by the ear  in what  tree every little songster  is concealed.—The
same truth is strikingly exemplified in the fact that,  when horses or mules
march in company at night, those in front direct their ears forwards;  those
in the rear, backwards; and  those  in  the  centre, latterly or across;—the
whole  troop seeming to be actuated by one feeling, which watches the com-
mon safety.
   The intensity of sound  is to the ear a measure of distance.—In a windy
night,  the sound of a distant bell may be brought so  quickly, that it has not
yet had time to spread and be weakened ; and a person is often roused from a
reverie by its  unusual loudness  and apparent nearness.—When  a stormy
wind  blows directly upon a coast, and rolls the great waves in upon the
sandy  beach or among the rocks, the countryman living far inland  hears the
uproar, as if the ocean had burst  its barriers, and were  pouring in upon the
land.—The scene-contrivers at our  theatres heighten the illusion of an ap-
proaching procession, by letting the accompanying music be first heard  from
a closed chamber or in a feeble tone, and afterwards with gradually increasing
loudness.  To the imagination, already excited perhaps  to the highest pitch
by the drama of some divine mind, the  advancing host is thus most vividly
portrayed; and when at last, with the thunder of drums  and trumpets  from
the front of the stage, the troop also appears, the effect is complete.   It is the
varying loudness of  the iEolian harp which produces  the feeling  that the
heavenly choir is sometimes approaching and sometimes receding.
   \_For an account of the Doctrines of Fluidity in relation to animals, see
Part V. Sec.  II.] 
254
IMPONDERABLE SUBSTANCE.
PART   IV.
        DOCTRINES OF IMPONDERABLE SUBSTANCE.


  To minds beginning this study, it may facilitate the conception  of a sub-
stance which is without  weight, or at least is  imponderable by human art,
to consider the nature of air.  Until lately men were so imperfe ctly acquainted
with the constitution of the universe around them, that a person placed in an
apartment which offered to view nothing but the naked walls, would have
said  that it was empty, meaning literally what he said; and even when adver-
tised that there was air in the room, he would  still have been  far from pos-
sessing a clear notion that it  was  full  of aerial fluid  just as an open vessel
immersed in the sea is full of water, and that if air were not allowed to escape
from it, even so small a body as an apple could not be  pressed into  it addi-
tionally by less force than fifty or sixty pounds.  This truth, however, is now
clearly understood, and  daily exemplified in easy pneumatic experiments, and
in no way more strikingly than by the recent adoption of the substance of air
in place of feathers, as stuffing for beds and pillows.   An air-tight bag or sack
suspended by its lip in the air, and held quite open by a hoop near its mouth,
would appear empty, but if then firmly closed above the hoop, it would have
imprisoned its fill of air, just as a bag similarly managed under water would
imprison its fill of water ; and while in some respects the air would be softer
and locally more yielding than feathers, its entire mass  would  be  much less
compressible.  Now this air, when weighed by means which modern science
has furnished, is found  in a  cubic foot to  contain somewhat  more than an
ounce, and by strongly pressing it, or by causing it to combine chemically
with some other substance, we can reduce it to very small bulk,  either with the
form of a liquid or of a solid:  proving how small a  quantity  of ponderable
matter, under certain circumstances, will occupy great space.  And common
air is by no means the lightest known substance, which as powerfully resists
the intrusion of other bodies where it exists.    Hydrogen gas,  for instance, of
the same space-occupying force, weighs only a fourteenth part as much, and
therefore a few drachms of it confined in a bag or bed as broad as  the  foun-
dation of a house, would support  a house or a  cask as large as a house filled
with water to a height of thirty  feet, the gas itself being then eighty thousand
times lighter than its bulk of gold ;—and if the pressure on it  were diminished,
it would readily expand  to a  volume  a thousand  times as  great, and would
still be exerting a considerable outward elasticity.  Again, a mixture of oxygen
and  hydrogen gases, while uniting with explosive force to form water, dilates
for the time, even under the  great pressure of the atmosphere,  to a bulk about
twenty times greater than the gases have while separate.
   The mind, pursuing the idea of such expansion or occupancy of space by
a small quantity of matter, and reflecting on the wonderful divisibility of mat-
 ter or minuteness of the ultimate  atoms, as explained in Part I. of this work,
 might almost admit as  a possible reality Newton's hypothetical illustration, 
IMPONDERABLE SUBSTANCE.
255
of that divisibility, viz., that even one ounce of substance  uniformly distri-
buted over the vast space in which our solar system exists, might leave  no
quarter of an inch without its particle.  Now a fluid in any degree approach-
ing in rarity to this, although it might press, resist, communicate motion, and
have other influences in common with more ponderable matter, would  have
neither weight nor inertia discoverable by means at  present known to man.
While we are contemplating, then, or modifying the agencies of what causes
the phenomena of heat and  cold, of light and darkness, of electricity in  its
forms of thunder and lightning, of galvanism, or of magnetism, in a word, the
most striking phenomena of  nature, we may be  dealing with matter of the
subtile constitution now spoken of.  And as in the terrestrial atmosphere there
are at least two fluids present, viz., oxygen and  nitrogen, of distinct nature,
so in a more  subtile ether, filling all space, there may be various ingredients.
   A majority of philosophers now incline to the opinion here sketched, that
there is at least one such subtile fluid or ether occupying completely the space
of the universe, and tending to uniform diffusion by reason of a strong mutual
repulsion of  its  particles, which fluid pervades denser material substances
somewhat as  water pervades  a sponge or a mass of sand, being attracted in a
peculiar way  by each substance, and which fluid may or may not have weight
and inertia.   They believe farther that the phenomena above alluded to, and
which human art can exhibit with highest beauty, or with  awful intensity,
are produced  by  the motion of other affections of that fluid, as the sensation
of sound- in all its varieties is produced in the delicate structure of the ear by
a certain  motion  in the air, or in any other body, having communication with
the ear; or as the sensation of jar is perceived by a  hand held 1o one  end  of
a log of wood when  a blow  is given to the other end.   Some philosophers
again suppose that the causes of the  phenomena are material particles  pro-
jected through space, somewhat as sand might be scattered by an explosion,
and which particles are present only when the effects are apparent.  Some
combine  these two hypotheses.  And some hold all the  phenomena of heat
to be mere motions in the common matter of the bodies in which the  heat
exists.
   We mention these hypotheses, not with the view of entering upon a minute
examination of their respective merits, or even of asserting  that any one  of
them is true,  but merely to make the  reader aware  of the directions  which
inquirers' minds  have taken in  pursuing  the investigation.   To understand
the subjects as far as  men yet usefully understand them, and sufficiently for
a vast number of most useful purposes, it is only necessary, as in other de-
partments of  science, to classify important phenomena, so that  their  nature
and resemblances may be clearly perceived.  When, in treating of the human
mind, we speak of its retaining an idea,  or being depressed, or being heated
ivith passion, Sic, we speak of subjects sufficiently definite, although we
may have no  hypothesis as to the intimate nature of the phenomena:—and
in the same manner may we  speak  of the accumulation, radiation, or other
affections  of heat and light.    We know nothing of the cause even of gravity,
the grandest influence in nature, but we can calculate its effects with admira-
ble precision. 
256
HEAT.
PART   IV.
SECTION I.—ON HEAT.
                      ANALYSIS OF  THE SECTION.

Heat (by some called Caloric) may be strikingly referred to as  that which
  causes the difference between winter and summer, between tropical gar-
  dens and polar wastes.   Its inferior degrees are denoted by the term cold.
  It cannot be exhibited apart, nor proved to have weight or inertia, and
  the change of its quantity in bodies is most conveniently estimated by the
  concomitant change of their bulk ; any substance so circumstanced as to
  allow this to be accurately measured constituting a thermometer.
Heat diffuses itself among neighbouring bodies until all have the same tem-
  perature,  that is, until all similarly affect  a thermometer.   It spreads
  partly through their structure, or by conduction, as  it is called, with a
  slow progress, different for each substance, and in fluids modified by the
  motion of their particles; and  it spreads partly also by  being shot or
  radiated like light from one body to another, through transparent media
  or space,  with readiness affected by the material and  state of the giving
  and receiving surfaces.
Heat, by entering  bodies, expands them, and through a range which in-
  cludes, as three successive stages, the forms of solid, liquid  and air or
  gas ;  becoming thus in nature the grand antagonist and modifier of that
  attraction which holds corporeal particles together, and which, if acting
  alone, would reduce the whole material universe to one solid lifeless mass.
  Each particular  substance, according  to  the nature, proximity, fyc, of
  its ultimate particles, takes a certain quantity of heat (said to mark  its
  capacity), to produce in  it a given change of temperature  or calorific
  tension;  undergoing expansion then in  a  degree  proper  to itself, and
  changing its form to liquid and air at points of temperature proper to
  itself; the expansion in  bodies  generally increasing more rapidly than
  the temperature, because the cohesion of their particles lessens with in-
  crease of  distance ; being remarkably greater therefore in liquids than in
  solids ; and in air than in liquids ; and the  rate of expansion, moreover,
  being much quickened as the bodies approach their points of changing
  form to liquid or air, to produce which changes, a large quantity of heat
  enters them, but in the  new  arrangement of particles  and increased
  volume of the mass,  it becomes hidden from the thermometer, and is
  therefore  called latent heat.   For any given substance  the changes of
  form happen so constantly at the same  temperature, that they mark fixed
  points in  the general scale of temperature, and enable us to regulate and 
CAUSE OF SEASONS AND CLIMATES.               257
  compare thermometers.—Heat, by expanding different substances une-
  qually, influences much their chemical combination.
Heat influences also the functions of vegetable and animal life.   The great
  source of heat is the sun; but electricity, combustion and other chemical
  actions, condensation, friction, and the actions of life, are also excitants*
" Heat may be strikingly referred to as  that which causes the difference
  between  winter and  summer, between  the gardens of the equator  and
  polar loastes."  (See the Analysis, page 256.)
  In the winter of climates, where  the temperature is for a time below the
freezing point of water, the  earth with its waters is  bound up in snow  and
ice, the" trees and shrubs are leafless, appearing everywhere like withered
skeletons, countless multitudes of living creatures, owing either to the bitter
cold or deficiency of food, are perishing in the  snows—nature seems dying
or dead ; but what a change when spring returns, that is, when heat returns !
The earth is again uncovered and soft, the rivers flow, the lakes  are again
liquid  mirrors, the warm showers come  to  foster  vegetation, which soon
covers  the ground with beauty and plenty.   Man, lately inactive, is recalled
to many duties;  his  water-wheels are everywhere  at work, his boats are
again on the canals  and streams, his busy fleets of  industry are along the
shores; winged life in new multitudes  fill the sky,  finny life similarly  fills
the waters, and every spot of earth teems  with vitality and joy.   Many  per-
sons regard these changes  of season as if they came  like  the  successive
position of a turning-wheel, of which  one necessarily brings the next; not
adverting that it is the single  circumstance of change of temperature  which
does all.   But  if the colds of winter arrive too early, they unfailingly  pro-
duce the wintery scene, and if warmth come before its time  in spring, it
expands the bud and the blossom, which a return of frost will  surely destroy.
A seed sown in an ice-house never awakens to life.
   Again, as regards climates, the earthly matters forming the  exterior of our
globe, and  therefore entering into the composition of soils,  are not  different
for different latitudes,—at the equator, for instance, and near the poles. That
the aspect of nature then in the two situations exhibits a contrast more strik-
ing still  than between  summer and winter, is  merely to an inequality of
temperature, which is  permanent.   Were it not for  this, in both situations
the same  vegetables  might grow, and  the same  animals  might  find their
befiting support.  But now,  in  the  one, namely, where the heat abounds,
we see  the magnificent scene of tropical fertility ;  the earth covered  with
luxuriant vegetation in endless,  lovely variety, and even the hard rocks fes-
tooned with green, perhaps with the  vine,  rich in its  purple clusters.  In the
midst of this scene, animal existence is equally abundant, and  many of the
species are of surpassing beauty—the plumage of  the birds is as brilliant as
the gayest  flowers.  The warm  air is perfume from  the spice-beds, the sky
and clouds are often dyed in tints as bright as freshest rainbow, and  happy

  * It is to be remarked here, that many phenomena in which heat plays an important part,
have been already described in preceding chapters of this work;—for instance, 'he action
of the steam-engine, the phenomena of winds, many facts in meteorology,  &c, under-the
head of Pneumatics.  In a  separate treatise on heat, these could not with P^I»'ety *™
been omitted ; but in a comprehensive system of science like the present, they tiiul tueir
fit place, where, being surrounded by subjects resembling them in more intricate particu-
lars they can be more concisely and clearly explained. 
258
HEAT.
human  inhabitants call the scene a paradise.   Again, where heat is  absent,
we have the dreary spectacle of  polar barrenness, namely, bare rock  or
mountain, instead of fertile  field;  water everywhere hardened to solidity;
no rain, nor cloud, nor dew; few motions but drifting snow;  vegetable life
scarcely existing, and then only in sheltered places turned to the sun—and
instead  of  the  palms and other  trees of  India, whose single leaf is almost
broad enough to cover a hut, there are bushes and trees, as the furze  and fir,
having what may be called hairs  or bristles, in the room of leaves.  In the
winter time, during which the sun is not  seen for  nearly six months, new
horrors are added, viz., the darkness and dreadful silence, the cold benumb-
ing all life, and even  freezing mercury—a scene into  which man may pene-
trate from  happier climes, but  where he can only leave his protecting ship
and fires for short periods, as he might issue from a diving-bell at the  bottom
of the ocean.   That in these now desolate regions,-heat only is wanted  to
make them like the most favoured countries  of the earth, is proved by the
recent discoveries  under  ground  of the remnant of animals and vegetables
formerly inhabiting them, which now  can live only near the equator.  While
winter,  then, or the  temporary absence of heat, may  be  called the sleep of
nature,  the more permanent torpor about the poles appears like its death ;
and when we farther  reflect, that heat is the great  agent in numberless im-
portant processes of chemisty and domestic economy, and is the  actuating
principle of the mighty steam-engine  which now perforins half the work of
society, how truly  may heat, the subject of our present chapter, be considered
as the life or soul of the universe !

"Heat cannot be exhibited in a separate state, nor proved to have weight or
                inertia."   (Kead the Analysis, page 256.)

  Although heat is known  to be  abundant  in  the sunbeam, and to  radiate
around from a blazing fire, we cannot  otherwise arrest or detect it in its pro-
gress than by allowing it to enter, and remain in some ponderable substance.
We know hot iron, or hot water,  or hot air, but nature no where presents to
us, nor has art succeeded in  showing us heat alone.
  If we balance a quantity of ice in a delicate weigh-beam, and then leave
it to melt, the equilibrium will not be  in the slightest  degree disturbed.  Or
if we substitute  for the ice,  boiling water  or red-hot iron, and  leave  this  to
cool, there  will be no  difference in the result.  If we place a pound of mer-
cury in  one scale of the weigh-beam and a pound-of water in the other, and
then either heat or cool both through  the same number of thermometric de-
grees, although about thirty times more heat,  (as will be explained  below)
enters  or leaves the bulky  water than the  dense  mercury,  they will still
remain equivalent weights.
  Again, a  sun-beam, with its intense light and heat, after being concentrated
by a powerful lens or mirror, may be  made to fall upon  the scale of  a most
delicate balance, but will produce no depressing effect on the scale, as would
follow if what constitutes the beam  had the  least forward motal inertia or
momentum.                         -f
  Such  are the  facts which have led certain inquirers to deny the material or
separate existence  of  heat, and to hold that it is merely motion of one kind
among the material  particles of  bodies generally, as  sound  is  motion  of
another  kind  among  the  same particles.  The following facts they consider
to have the same bearing  in the argument.  Heat can be produced without
limit by friction, as—when savages light their  fires by rubbing together two 
CAUSE OF SEASONS AND CLIMATE.
259
 pieces of wood—when Count Rumford made great quantities of water boil
 by causing a blunt borer to rub against a mass of metal immersed in water—
 when Sir Humphrey Davy quickly melted pieces ofice by rubbing them against
 each other in a room cooled below the beezing point, &c.   Intense heat is
 produced  by the explosion of gunpowder  or other fulminating mixture, yet
 it cannot be conceived to  have existed in the small bulk of the powder before
 the explosion.   Other inquirers, on the contrary, have deemed to be proofs
 of the separate materiality of heat such facts as now follow ;—that it is radi-
 ated through the  most perfect vacuum which we  can produce, and even more
 readily than through air ;   that it radiates in the  same place in  all directions,
 without impediment from  the crossing rays;—that it becomes  instantly sen-
 sible on the condensation  of any material mass, as if then squeezed out from
 the mass;  as when, by compressing air suddenly, we inflame a match im-
 mersed in it; or when, on reducing the bulk of iron by hammering we ren-
 der it very hot, the warming being  greater at the first blow (which most
 changes the bulk) than afterwards,—that when, on mixing bodies which com-
 bine so intimately as to occupy less space than when separate, there is a dis-
 engagement of heat proportioned to the  diminution of  the volume:—that the
 laws of the spreading of heat in bodies do not resemble those of the spreading
 of sound, or of any  other motion known  to us :—and that, as  to the great and
 sudden  extrication of heat by friction or explosion, it may be as truly a rush
 of the fluid to  the part, as in the  case of an  electrical accumulation or dis-
 charge.   These facts, moreover, they think, square well with their assump-
 tion that the phenomena  of heat  are produced  by an exceedingly subtile
 fluid, or ether,  pervading  the whole  universe, and  softening, or melting,  or
 gasifying bodies,  according to the  quantity present in each ; its own parts
 being strongly repulsive of each other, and seeking,  therefore, widest  and
 most equable diffusion.

 "The change of its quantity  in bodies is most conveniently estimated by the
   concomitant change of  their bulk,  any substance so circumstanced, as  to
   allow this to be accurately measured, constituting a thermometer."  (Read
   the Analysis, page 256.)

   If we heat a wire, it is lengthened; if we  heat water in a full vessel, a  part
 runs over:  if we heat air  in a bladder, the bladder is distended: in a word,
 if we heat any substance, its volume increases in some proportion to the
 increase  of  temperature,—and  we may measure the increase of volume.
 The reasons why, in such investigations, a contrivance in which the expan-
 sion of mercury may be observed,  viz.,  the mercurial  thermometer, is com-
 monly preferred to  others, can only be fully understood by the mind which
 has considered  the  whole  subject  of heat; and we touch upon the matter
 here, only for the  purpose  of stating that a mercurial thermometer is a small
 bulb, or  bottle of glass  filled with  mercury, and  having a long very narrow
 stalk or  neck, in which the mercury rises when  expanded by heat, or falls
 when heat is withdrawn;  the stalk  between the points  at which the mercury
stands in freezing  and boiling water, being divided into an arbitrary number
of degrees,  which division appearing  on a scale applied to the stalk, is con-
tinued similarly above and  below these points.

"Heat diffuses itself among neighbouring bodies until all have acquired the
  same temperature; that is  to say, until all will similarly effect a thermo-
  meter."  (See the Analysis.)
  An  iron  bolt thrust in among burning coals soon becomes  red hot like 
260
HEAT.
them.   If it be the heater of a tea-urn, it will, when afterwards placed amidst
the water, part with its lately acquired heat to the water, until both are of the
same temperature.  Boiling water, again, soon imparts heat to an egg placed
in it, and a feverish head yields its heat to a bladder of cold water or ice.  A
hundred objects enclosed  in the same apartment, if tested, after  a time, by
the thermometer, will all indicate the same temperature.

       "The inferior degrees of heat are denoted by the term cold."
   When the hand  touches  a body of  a  higher temperature than itself, it
receives heat according to the law now explained, and it experiences a pecu-
liar sensation; when it touches a body of lower temperature than  itself, it
gives out heat for a like reason,  and experiences another  and very different
sensation.   The  two are called  the sensations of heat and of  cold.  Now
heat and cold, considered as existing in the bodies themselves, although thus
appearing opposites, are really degrees of the same object, temperature, con-
trasted by name, for convenience sake,  in reference to the particular tempera-
ture of  the individuals speaking of them—just as any two nearest mile-stones
on a road, although merely marking degrees  of  the same object, distance,
might receive from  persons living between them  the opposite names of east
and  west, or of north  and south.  It  is to be remarked, moreover, that the
sensation of heat is  producible also by a body colder than the hand, provided
it be less  cold than a  body touched immediately before, or  than the usual
temperature; and the sensation of cold is producible under the  opposite cir-
cumstances of touching a comparatively warm body, but which  is less warm
than something touched just before.  This explains the remarkable fact that
the same  body may appear  at the same time, and to the same  person, both
hot and cold.  If a person transfer one  hand to  common spring-water from
touching ice, that hand will deem  the water very warm: while, the other
hand, transferred to it from a warm bath, would deem  it very cold.  For a
like  reason, a person  from India, arriving in England  in the spring, deems
the air cold, while the  inhabitants of the country are diminishing their cloth-
ing, because the heat to them is becoming oppressive.   Such facts show how
necessary it was  for men  to discover  more correct thermometers  than their
bodily  sensations.

" Spreading partly through their structure, or by conduction, as it is called,
   with a  progress  proper to each  substance."   (Read the  Analysis, page
   256.)
   If one end of a rod of iron be held in the fire, a  hand grasping the other
end soon feels the heat coming through it.  Through a similar rod  of glass
the transmission is much slower, and  through one of wood it is slower still.
The hand would  be burned  by the  iron, before  it felt warmth in the wood,
although the inner end were blazing.
   On the fact that different  substances are permeable  to  heat, or have the
property of conducting it, in different degrees, depend many  interesting pheno-
mena in nature and  in the  arts: hence it  was important to ascertain  the  de-
grees exactly, and  to  classify the  substances.   Various  methods  for this
purpose have been  adopted.  For solids—similar rods of the different sub-
stances, after being  thinly coated with wax, have been placed with their infe-
rior  extremities  in hot oil, and then the comparative distances  to which, in
a given  time, the wax  was  melted, furnished one set of indications of the com-
parative conducting powers:—or, equal lengths of the different bare rods 
SPREADING BY CONDUCTION.
261
 being left above the oil, and a small quantity of explosive powder being placed
 on the top of each, the comparative intervals of time elapsing  before the
 explosions gave another kind of measure :-or, equal balls of different sub-
 stances, with a central cavity in each to receive a thermometer, being heated to
 the same  degree, and then suspended in the air to cool, until the thermometer
 fell to a given point, gave still another list.   A modification of the last method
 was  adopted  by Count Rumford to ascertain the relative  degrees in which
 furs, feathers  and other materials used for clothing, conduct heat, or, which
 is the same thing,  resists its passage.  He covered  the ball and stem of a
 thermometer  with a certain thickness of the substance to be tried, by placing
 the thermometer in a larger bulb and stem of glass, and then filling the inter-
 val between them with the  substance; and, after  heating this apparatus to a
 certain degree, by  dipping it in liquid of the desired temperature, he sur-
 rounded it by ice, and  marked the comparative times required to cool  the ther-
 mometer a certain number  of degrees.   The figures following the names of
 some of the substances in the subjoined list, mark the number of seconds re-
 quired respectively for cooling it 60°.
   These experiments have shown,  as a general rule, that density in a body
 favours the passage of heat through it.   The best conductors are the metals,
 and then follow in succession diamond, glass, stones, earths, woods, &c, as
 here noted :

                 Metals—silver, copper, gold, iron, lead.
                 Diamond.
                 Glass.
                 Hard stones.
                 Porous earths.
                 Woods.
                 Fats or thick oils.
                 Snow.
                 Air.....576
                 Sewing silk  ....   917
                 Wood ashes -    -     -         927
                 Charcoal    ....   937
                 Fine lint    ....  1,032
                 Cotton       ....  1,046
                 Lampblack  ....  1,117
                 Wool       -    -     -    -  1,118
                 Raw silk    .... 1,284
                 Beavers' fur  -    -     -    - 1,296
                 Eider down  .... 1,305
                 Hares' fur   -     -     -    - 1,315
  Air appears near  the  middle  of the preceding list, but if its particles are
not allowed to move about among themselves, so as to carry heat from one
part to another, it conducts  (in the  manner of solids) so slowly that Count
Rumford doubted whether it conducted at all.  It is probably the worst con-
ductor known, that is, the substance which when at rest impedes the  passage
of heat the most.   To  this fact seem to be owing, in a considerable  degree,
the remarkable non-conducting quality of  porous  or spongy substances, as
feathers, loose filamentous matter, powders, &c, which have much air in
their structure, often adherent with a force  of attraction which immersion in
JVater, or even being placed in the vacuum of an air-pump, is insufficient to
overcome.
     18 
262
HEAT.
  While contemplating the facts recorded in the above table, one cannot but
reflect how admirably adapted to their purposes the substances  are  which
nature has  provided as clothing for the inferior animals;—and which  man
afterwards  accommodates with such curious art to his peculiar wants.   Ani-
mals required to be  protected against the chills of night and the biting blasts
of winter,  and some of them which dwell among eternal ice, could not have
lived  at all, but for a garment which might shut up within  it nearly all the
heat which their vital functions producer!.  Now any covering of a metallic,
or earthy, or woody nature, would have been far from sufficing; but out of a
wondrous  chemical union of carbon with the soft  ingredients of the atmo-
sphere, those beautiful textures are produced called fur and feather,  so greatly
adorning while they  completely protect  the  wearers ;—textures, moreover,
which grow from the bodies of the animals, in the exact quantity that suits the
climate and season, and  which are  reproduced  when by any accident  they
are partially destroyed.   In warm climates the  hairy coat of quadrupeds is
comparatively short and thin; as in the elephant, the monkey, the tropical
sheep, &c.  It is seen to thicken with  increasing latitude, furnishing the soft
and abundant fleeces of the temperate zonesf and towards the poles it is ex-
ternally shaggy and coarse, as in the  arctic bear.   In amphibious animals,
which have to resist the cold of water as well as  of air, the furs grow particu-
larly  defensive, as  in  the otter and  beaver.   Birds, from having very warm
blood, require  plenteous clothing, but  require  also  to  have  a  smooth sur-
face,  that they may pass  easily through the  air ;—both objects are secured
by the beautiful structure of feathers, so beautiful and wonderful that  writers
on natural  theology have often particularized it is as  one of the most striking
exemplifications of creative wisdom.   Feathers, like fur, appear in kind and
quantity suited to particular climates and seasons.   The birds of cold regions
have  covering almost as bulky as their bodies, and  if it be warm in those of
them  which live only in air, in the water-fowl it  is warmer still.  These
last have the interstices of the ordinary plumage filled  up by the  still more
delicate structure  called  down, particularly on  the  breast,  which  in swim-
ming first  meets and  divides the cold wave.  There are animals with warm
blood which yet live  very constantly immersed  in water, as  the whale, seal,
walrus,  Sic   Now neither  hair nor  feathers,  however  oiled, would  have
been a fit covering  for them: hut kind nature has prepared an equal protec-
tion in the vast mass of fat or thick oil which  surrounds their bodies—sub-
stances  which are  scarcely less useful to man than the  furs and  feathers of
land  animals.
   While speaking  of clothing we may remark, that the bark of trees is also
a structure very slowly permeable to heat, and  securing, therefore, the  tem-
perature necessary to vegetable life.
   And while  we admire what nature has thus done for animals and vegetables,
let us not  overlook her scarcely less remarkable  provision of ice and snow, as
 winter clothing for the lakes and rivers,  for our fields  and gardens.  Ice, as
 a protection to water and its inhabitants, was  considered in Sec. I. in the
 explanation of why,  although solid, it swims on water.   We have  now to
 remark that snow, which becomes  as  a pure white  fleece to the  earth, is a
 structure which resists the  passage of heat nearly  as much as feathers.  It,
 of course, can defend only from colds below 32° or the freezing point;  but it
 does so most effectually, preserving the roots  and  seed3 and tender plants
 during the severity of winter.   When the green blade of wheat and the  beau-
 tiful  snow-drop flower appear in spring rising through the melting  snow, they
 have recently owed an important shelter to their wintry mantle.  Under deep 
SPREADING BY CONDUCTION.
263
 snow, while the thermometer in the air may be far below zero, the tempera-
 ture of the ground rarely below the freezing point.  Now this temperature,
 to persons some time accustomed to it, is mild and  even  agreeable.  It is
 much higher than what often prevails for long periods in the atmosphere of
 the centre and north of Europe.  The Laplander, who during his long winter
 lives under ground, is glad to have additionally over head a thick covering of
 snow.  Among the hills of the west  and north of Britain, during the storms
 of winter, a house or covering of snow frequently preserves the lives of travel-
 lers, and even of whole flocks of sheep,  when "the keen north wind catching
 them unprotected, would soon stretch them lifeless along the earth.
   It is because earth conducts heat slowly, that the most intense frosts pene-
 trate but a few inches into it, and that the  temperature of the ground a few
 feet below its surface is nearly the same all the world over.  In many mines,
 even  although open to the air, the thermometer does not vary one degree in
 a twelvemonth.   Thus also water in  pipes two or three feet under  ground
 does not freeze,  although it may be frozen in all the smaller branches exposed
 above.  Hence, again, springs  never  freeze, and  therefore become remarka-
 ble features  in a snow-covered country. The living water is seen issuing from
 the bowels of the earth, and running often  a considerable way through fringes
 of green,  before the gripe  of the frost arrests it; while around it, as is well
 known to  the sportsman, the snipes and wild duck  and other birds are wont
 to congregate.   A spring in a frozen pond or lake may cause the ice to be so
 thin over the part where it issues, that a skater arriving there will break through
 and be destroyed.  The same spring water which appears warm in winter, is
 deemed cold in summer, because, although always of the same heat, it is  in
 summer surrounded by warmer atmosphere and objects.  In proportion  as
 buildings  are massive, they acquire more  of those qualities which have now
 been noticed of our mother earth.  Many of the Gothic halls and cathedrals are
 cool in summer and  warm  in  winter—as are also old-fashioned  houses  or
 castles with thick walls and deep cellars.   Natural caves in the mountains or
 sea-shores furnish other examples of a similar kind.
   When in  the arts it is desired to prevent the passage of heat  out of, or into
 any body  or situation, a screen  or covering of a slow conducting" substance is
 employed.  Thus, to prevent the heat of a smelting or other  furnace from
 being wrasted, it  is lined with fire  bricks, or is covered  with clay and sand, or
 sometimes with  powdered charcoal.   A furnace so guarded  may be  touched
 by the hand, even while containing within it melted gold.  To prevent the
 freezing of water in pipes during the winter, by which occurrence the pipes
 would be burst, it is common to cover them with straw ropes, or coarse flan-
 nel, or to enclose them in a larger Outer pipe, with dry charcoal, or saw dust,
 or chaff, filling up the interval between.  If a  pipe, on the  contrary, be for
 the conveyance of steam or other  warm fluid, the heat  is retained, and, there  ■
 fore, saved by the very same means.   Ice-houses  are generally made  with
 double walls, between which, dry straw placed, or  saw dust,  or air, prevents
 the passage  of heat.   Pails for  carrying ice in summer,  or intended to serve
 as wine coolers,  are  made on the  same principle—viz., double vessels, with
 air or charcoal, filling the interval between them.  A flannel covering keeps
 a man warm in winter—it is also the best means of keeping ice from melting
 in summer.   Urns for hot  water, tea-pots, coffee pots, &c, are made with
 wooden or ivory handles, because, if metal were used, it would conduct the
 heat so readily that the hand could not bear to  touch them.
  It is because glass and earthenware  are brittle, and do not allow ready pas-
sage to heat, that vessels made of them are so  frequently broken by sudden 
264
HEAT.
change of temperature.   On  pouring boiling water  into such  a  vessel, the
internal part is much heated and expanded (as will be explained more fully
in a subsequent page) before  the external part has felt the influence, and this
is hence riven or cracked by  its  connection  with the internal.  A chimney
mirror is often broken by a lamp or candle placed on the marble shelf too near
it.  The glass cylinder of an electrical machine will sometimes be broken by
placing it near the fire, so that one side is heated while the other side receives
a cold current of air approaching the  fire from a door or window.  A red hot
rod of iron drawn along a pane of  glass will divide it almost like a diamond
knife.   Even cast iron, or backs of grates, iron pots,  &c, although conduct-
ing readily, is often, owing to its brittleness, cracked by unequal heating or
cooling, as from pouring water on  it  when hot.   Pouring cold  water into a
heated glass will produce a similar effect.   Hence glass vessels intended to
be exposed to strong heals and  sudden changes, as retorts, for distillation,
flasks for boiling liquids, &c, are made very thin, that the heat  may pervade
them almost instantly and with impunity.
   There is a toy called a Prince Rupert's Drop, which well illustrates our
present subject.   It is a lump of glass  let fall  while fused into  water, and
thereby suddenly cooled and  solidified on the outside before the internal part
is changed ; then as this at last hardens and would contract, it is kept extended
by the arch of external crust, to  which it coheres.   Now if a portion of the
neck of the lump be broken off, or if other violence be done, which jars its sub-
stance, the cohesion is destroyed, and the  whole crumbles to dust with a kind
of explosion.  Any glass cooled suddenly when first made, remains very brit-
tle, for the reason now stated. What is called the Bologna jar is a very thick
small bottle,  thus prepared, which bursts by a grain of sand falling into it.
The process  of annealing, to render glass ware more tough and durable, is
merely the allowing it to cool very slowly by placing it in an oven, where the
temperature is caused to fall gradually.  The tempering of  metals by sudden
cooling seems to be a process having some relation to that of rendering glass
hard and brittle.
   It is the difference of conducting power in bodies  which is the cause of a
very common error made by persons in estimating the temperature of bodies
by the touch.  In a room without a fire all the articles of furniture soon
acquire the same temperature; but if in winter, a person with bare feet were
to step from the carpet to the wooden floor, from this to the hearth-stone, and
from the stone to the steel fender, his sensation would deem each of these in
succession colder than the preceding.  Now the truth being that all  had the
same temperature, only a temperature inferior to that of the living body, the
best conductor, when in contact with the body, would carry off heat the fastest,
and would, therefore, be deemed the coldest.   Were  a similar experiment
made in a hot-house, or in India, while the temperature of every thing around
were 98°, viz., that of the living body, then not the slightest difference would
be felt in any of the  substances: or lastly, were the experiment made in a room
where by any means the general temperature were raised considerably above
blood heat, then the carpet would be deemed considerably the coolest instead
of the warmest, and the other things would appear hotter in the same order
in which they appeared colder in the winter room.   Were a bunch of wool
and a piece of iron exposed to the severest cold of Siberia, or of an artificial
frigorific mixture, a man might touch the first with impunity, (it would merely
be felt as rather cold;) but if he grasped the second, his hand would be frost-
bitten and possibly destroyed;  were the two substances, on  the contrary,
transferred to an oven, and heated as far as the wool would bear, he  might 
SPREADING BY CONDUCTION.
265
 T. L     ^ wool with impunity (it would then be felt as a little hot,) but
 the iron would burn his flesh.   The author has entered a room where there
 was no lire, but where the temperature from hot air admitted was sufficiently
 high to boil the fish, &c., of which he afterwards partook at dinner; and he
 breathed the air with very little uneasiness.  He could bear to touch woollen
 cloth in this room, but no body more solid.
   The foregoing  considerations  make manifest the  error of supposing that
 there is  a positive warmth in  the  materials of clothing.  The thick cloak
 which guards  a Spaniard  against the cold of winter, is also in summer used
 by him as protection against the direct rays of the sun:—and while in Eng-
 land, flannel is our warmest  article of dress, yet we cannot more effectually
 preserve ice than by wrapping the vessel containing it in many folds of soft-
 est flannel.
   In every case where a substance of different temperature  from the living
 body touches  it, a thin surface of the substance  immediately shares the heat
 of the  bodily part  touched—the hand  generally; and while in a good conduc-
 tor, the heat so received quickly passes inwards, or away from°the surface,
 leaving this in a state  to absorb more, in the  tardy conductor the  heat first
 received tarries at the surface, which  consequently soon  acquires nearly the
 same temperature as the hand, and therefore, however cold the interior of the
 substance may be, it does not cause the sensation of cold.  The hand on a
 good conductor has to  warm it deeply, a slow conductor it warms Only super-
 ficially.   The following cases farther  illustrate  the same principle.   If  the
 ends of an iron poker,  and of a piece of wood of the same size, be wrapped in
 paper and then thrust  into a fire, the paper on the wood will begin to burn
 immediately, while that on the metal  will long resist:—or if pieces of paper
 be laid on a wooden plank and on a plate of steel, and then a burning coal be
 placed  on each, the paper on the wood will begin to burn long before that on
 the plate.   The explanation is, that the paper in contact with  the good con-
 ductor  loses to this so rapidly the heat received from the coal, that it remains
 at too low a temperature to inflame, and will even cool to blackness the touch-
 ing part of the coal; while on the tardy conductor the paper becomes almost
 immediately as hot as the coal.  It is because water exposed to the air cannot
 be heated beyond 212°, that it may be made to boil in an egg-shell or a vessel
 made of paper, held over a lamp, without the containing substance being  de-
 stroyed; but as soon as it is dried up, the paper will burn and the shell will
 be calcined,  as the solder of a common tinned kettle melts under the  same
 circumstances. The reason why the hand judges a cold liquid to be so much
 colder  than a solid of the same  temperature is, that from the mobility of the
 liquid particles among themselves, those in contact with the hand are constant-
 ly changing.   The impression  produced on the hand by very cold mercury
 is almost insufferable, because mercury is both a ready conductor and a liquid.
 Again,  if a finger held motionless in water feel cold, it will feel colder still
 when moved about; and a man in the air of a calm frosty morning does not
 experience a sensation  nearly so sharp  as if with the same temperature there
 be wind.  A finger held up in the wind discovers the direction in which the
 wind blows by the greater cold felt on one side, the effect being still more
 remarkable, if  the finger is wetted. If a person in a room with a thermometer,
 were with a fan or bellows to blow the air against it, he would not  thereby
lower it, because it had already the same temperature as the air, yet the  air
blown against  his own  body would appear colder than when at rest, because,
beino- colder than his body, the motion would supply heat-absorbing particles
more quickly.  In like manner,  if a fan or bellows were used against a ther- 
266
HEAT.
mometer hanging in a furnace or hot-house, the thermometer would suffer no
change, but the air moved by them against  a person would be distressingly
hot, like the blasting sirocco of the sandy deserts of Africa.  If two similar
pieces of ice be placed in a room  somewhat warmer than ice, one of them
may be made to melt much sooner than the other, by blowing on it with  a
bellows.  The reason may here be readily comprehended why a person suf-
fering what is called  a  cold in the head, or catarrh from the eyes and nose,
experiences so much more relief on  applying to the face a handkerchief of
linen or cambric than one of cotton:—it is, that the former, by conducting,
readily absorbs the heat and diminishes the inflammation, while the latter,
by refusing to give passage to the heat, increases the temperature and the dis-
tress.  Popular prejudice has held  that there was a poison in cotton.

    " Heat  spreading in fluids chiefly by the motion of their particles."
                     (Read the Analysis, page 256.)

   Owing to the mobility among themselves of fluid particles, heat entering a
fluid anywhere below the surface, by dilating and rendering specially lighter
the portion heated, allows the denser  fluid around to sink down and force up
the rarer; and the  continued currents  so established, diffuse the heat through
the mass much more quickly than  heat spreads by conduction in any solid.
   Count Rumford's experiments led him at first to conclude that liquids, but
for this carrying process, by the particles changing their place, were  abso-
lutely impassable  to heat.   A piece of ice will lie very long at the bottom of
water which is made to  boil at  the top by the contact of any hot body; and
when it at last melts, Count R. believed that it did so entirely from the heat
which passed downwards  through the sides of the vessels containing  the
water.  But an ingenious experiment by Dr. Murray decided the  question
differently.  He made a vessel of ice, which of course could not carry down-
wards any  heat greater than 32°, as ice melts at that degree; and having put
into the vessel a quantity of oil at 32°, with the bulb of a thermometer being
a quarter of an inch under the surface of the oil, he placed  a cup of boiling
water in contact with the surface of  the  oil:—in a minute and a  half the
thermometer  rose  nearly a  degree,  and in seven minutes  it rose five degrees,
beyond which it did not go.   The  heat then  must have passed downwards
through the liquid, proving  a conducting power;—unless, indeed, it passed
by radiation, as explained in a subsequent page.
   The internal currents  or circulation produced by heat in fluid masses, and
of which there are so many important instances  in  nature, were more fitly
explained in the chapter on Hydrostatics and Pneumatics; we shall  here,
therefore, allude to them very shortly.
   Perhaps  the best experimental illustration of the subject is obtained  by
placing a tall glass jar, filled with water in which small pieces of amber are
diffused to show its movements, first in a warm bath, and then in a cold bath.
In the first  case, the water and amber near the outside of  the jar where they
are heated,  will exhibit a rapid upward current, while in the centre of the jar
they will form an  opposite  and downward  current.   In  the second case, or
when the jar is placed in a cold bath, the direction of the  currents will be
reversed.
   Consideration of these currents led the author  of this work, some years
ago, to propose what he  deemed a great improvement on the construction and
management of boilers and  evaporating pans  generally;  namely, to convert
the upward and downward current in  the mass of boiling  liquid into a lateral 
SPREADING BY CONDUCTION.
267
current below, constantly and rapidly sweeping the bottom of the vessel.
In ordinary boilers, when a portion of liquid is converted into steam in contact
with the horizontal bottom, it does not separate from the bottom immediately,
but remains until a steam-bubble of considerable size be formed,  and in the
mean time the part of the boiler defended by it from the contact of the liquid,
becomes overheated, and the following evil consequences ensue:—1.  Rapid
destruction of the boiler, and on that account a rapid expense ; 2.  Necessity
for having  originally much thicker, and therefore dearer boilers ;  3. The
thickness being an impediment to the passage of heat, there is a proportion-
ate waste of fuel; 4, and last, When the liquid  is a vegetable juice or extract,
as sugar-cane  juice, of a nature to be carbonized and blackened when over-
heated, the quality of the product  is often exceedingly deteriorated.  The
patentees of an apparatus, described at page 184, for boiling sugar in vacuo,
and therefore at a low and steady temperature, gained, it was said, more than
£40,000 a year by  preventing the  injury now spoken of.   And when the
liquid is a saline  solution, like the sea-water used in steam ships, the salt
soon encrusts the bottom of  the boiler; and powerfully both prevents the
passage of heat and destroys the boiler.   Now a current sweeping the bottom
prevents all these consequences, and may be  easily  obtained.   The most
obvious method is, to place  in the liquid  some upright  tubes with open tops
at a certain distance under the surface of the liquid, and with the bottoms also
open, but laterally, and all in one direction: the consequence will be, that as
soon as the liquid begins to  boil, the general mass, consisting of liquid mixed
with bubbles of steam, becomes of considerably  less specific gravity than the
liquid in the  tubes, remaining unmixed, because  steam will not enter the
lateral mouths of the tubes, and the columns of heavier liquid will  therefore
descend rapidly, and issuing by the  lateral openings of the  tubes all in one
direction along the bottom of  the boiler,  will powerfully and uninterruptedly
sweep it.   In a long wagon-shaped boiler the tubes, instead  of being round,
should  be  made flat and broad enough to reach from  side to side;  and if  a
very rapid current be desired, they must be made larger  than  the spaces
between them in which the  steam has to rise, for thus  the steam  bubbles,
being driven closer together, will make the rising column so much the lighter,
and its ascent consequently  the more rapid ; and there will be a  correspond-
ing rapidity of issue of the sweeping current.   In a moderate-sized pan or
boiler of the usual basin or  half-globe shape, the simplest method of produc-
ing the current is to have a  smaller vessel of similar shape made of thin
metal, and placed within the  other  so as  to leave about an inch  space all
round between them, and having one large opening at its bottom,—then all
the steam  mixed with fluid  will rise between  the  outer and inner  vessel,
while the unmixed liquid will descend through the open bottom of  the inner
vessel, and  spread in every  direction,  sweeping the bottom of the outer.
The sweeping of the bottom  of a boiler might also be effected  by a wheel
kept turning, to cause  the  liquid to resolve horizontally (as was done with
another view  in the large Scotch whisky stills,) or  by a frame made some-
what like a rake or gridiron, kept moving backwards and forwards  upon  the
bottom.
   As stated in a previous section, it is the heating and dilatation  of the fluid
air over a tropical island while acted upon during the middle of the day by
the powerful  rays of the sun, which allow the colder and  heavier air from
the face of the ocean around to press inwards upon it and  force it upwards
in the  atmosphere—the  cold  current forming the delightful sea-breeze of the
climate.  And it is the general heating of the  air over the whole equatorial 
268
HEAT.
belt  of the earth, which, rendering it specifically lighter than the air nearer
the poles,  allows  this to assume  the form of  cool  trade-winds, constantly
blowing towards the sun's path,  and pressing upwards the hot air, which
then spreads away on the top of the atmosphere towards the poles, to miti-
gate  the severity of the northern  and southern  cold.  In  the watery ocean
also  there is a circulatory motion of  the same kind, although less in degree,
tending to distribute heat and equalize temperature, and contributing to pro-
duce some of the great sea currents known to mariners.
  The vertical currents produced by heat, in the ocean and in  great masses
of water generally, preserve in and over them a comparatively uniform tem-
perate freshness, while the rocks and soil on the shores around may  be either
parched under a burning  sun, or bound up in frost.   A keen frost chills, and
soon hardens in its icy grasp the surface of the ground; but of water simi-
larly exposed, the  part first  cooled  descends to the bottom by its increased
density, and forces up a war mer water to take its place; this in its turn is
cooled and descends, and a continued circulation is  established, so  that  the
surface cannot become ice until the whole mass, of whatever depth,  has been
cooled down to its greatest density.  Hence the very deep sea is not frozen
even in the coldest climates, and in  temperate  climates, the  severest winter
does not freeze even  the ordinary lakes.   During this intestine movement in
the water, that  which ascends to  the surface  to be  cooled, by losing one
degree of its heat, warms more  than 500 times its  bulk of air one degree,
and thus tempers remarkably the air passing over it.  Hence places in  the
vicinity of the sea and  of lakes are warmer  in winter than places farther
inland, although nearer to the equator.   England is  much warmer in winter
than central Germany, which lies south of England; and the coast of  Scot-
land and of the north of Ireland  are warmer than London :—snow never lies
long upon these coasts.   As continental or inland  countries have thus in
winter an extreme of cold, so they have in summer an  extreme  of  heat.
Water admits the rays of the sun, and absorbs the heat into the whole thick-
ness of its mass, and therefore  is warmed very slowly; but the dry  earth
retains all the heat near its surface, and is therefore soon heated to excess.
   The ventilation of our dwellings and halls of assembly (as explained pre-
viously) is owing to  the  motion produced by the changed  specific gravity of
air when heated.   The air which is within the house becomes warmer than
the external air, and  the latter then presses in at every opening or crevice to
displace the other.  The ventilation of  the person  by the slow passage of
air through the texture of our clothing  is a phenomenon of the  same kind;
and  thicker clothing  acts  chiefly by diminishing the rapidity of this  passage.
Hence an oiled-silk or other air-tight covering laid on a bed, has greater influ-
ence in preserving warmth than one or  two additional blankets, and is not
generally used, only  because it prevents ventilation,  and, by shutting in the
insensible perspiration,  soon  produces  dampness.   From the  part of bed-
clothes immediately over  the person there is a constant outward oozing of
warm air, and there is an  oozing inward of cold air in  lower situations around.
In many persons the circulation  of the blood is so feeble that in winter, they
have great difficulty in keeping their feet warm, even in bed, unless  with the
assistance of a bottle of hot water or some such means, and in consequence they
often pass sleepless nights, and suffer in their general health.  In such cases,
at the suggestion of  the  author, a  long  flexible tube  has been used,—as of
spiral wire, covered with leather or varnished cloth, by which a person can
send down to his feet his hot breath, and thus  supply to them effectually a 
SPREADING BY RADIATION.
269
 natural animal warmth, as in a cold day he does to his hands by blowing
 upon them through his gloves.                                         °
    The power of fluids to diffuse heat being due to their power of carrying,
 and  not of conducting it, the consequence should  follow,  that any circum-
 stance whicli impedes the internal motion of the fluid particles, should dimi-
 nish the diffusing power.  Accordingly we find, that fluids in general transfer
 heat less readily in proportion as they are more viscid.  Water, for instance,
 transfers less  quickly than spirits; oil  than water;  molasses  or syrup than
 oil:  and water thickened  by  starch dissolved in it,  or which has its internal
 motion impeded by feathers or thread immersed in it, less quickly than where
 it is  pure and at liberty.   Cooling being merely a motion the reverse of heat-
 ing,  it is influenced by the same law.   Hence the  reason why thick  soups,
 pies, puddings, and all semifluid masses, retain their heat so long—so much
 longer than equal bulks of mere fluid.   The same law affords  explanation of
 the facts,  that very porous masses and  powders, as charcoal, metal  filings,
 saw  dust,  sand, &c, conduct  heat more slowly than  denser masses,—their
 interstices being filled with air, which scarcely conducts heat, and which, by
 the structure of the substance, has no freedom of  motion  or  circulation by
 which it might carry the heat.

 "Heat spreads, also, partly by being radiated or shot light from one body
   to  another, through transparent media or space with readiness affected by
   the material and the slate of the giving and receiving surfaces"   (Read
   the Analysis, page 256.)

   If a heated  ball  of metal be suspended in the air, a hand brought in any
 direction near to it  will experience the sensation of  heat; and  beneath it the
 sensation will  be as strong as  on the sides, although  the heat  has to shoot
 down through an opposing current of air approaching the heated ball, to rise
 from it, as explained in a  preceding section.   A delicate thermometer substi-
 tuted for the hand  will equally detect  the  spreading heat, and  if held  at  dif-
 ferent distances, will prove it to diminish in the same ratio as light diminishes
 in  spreading  from  any luminous  centre,  viz., to be only  a fourth part as
 intense at a double distance, in  a corresponding proportion for other distances.
 If  the heated  body be  enclosed in a vacuum, a  thermometer placed near it
 will still be affected in the same manner.   If a screen be interposed between
 the body and  the thermometer, the latter will not be affected at all, proving
 the heat to spread in straight lines.  Heat, when diffusing itself in this way,
 to distinguish it from heat passing by contact or communication, as described
 in the last  section,  is called radiant heat; that is to say, spreading in radii
 or rays all  around its source as light spreads.
   Radiant  heat resembles  light yet in other respects.   It as rapidly perme-
 ates certain transparent substances, and  its course suffers in them a degree
 of the bending, termed refraction by opticians.   It  is reflected from  many
 kinds of polished surfaces, just as  light is  reflected from a common mirror;
 and many such surfaces directed to  one point or centre (as when Archimedes
 made the sun  his assistant to  burn the  Roman  ships) or a single concave
 surface, having its own centre or focus,  will concentrate  heat  just as light.
 Its motion  in the sun-beam is so rapid, as for any distance at which men can
 try the  experiment, to appear  instantaneous; and  the rays of heat from hot
 iron or burning charcoal concentrated at  great distances by suitable mirrors,
affect a thermometer as quickly as the heat of the  sun similarly reflected.
Although light and heat are united  in the sun's  ray,  they  are still separable 
270
HEAT.
by our glass prisms or lenses; and the focus of heat behind a  burning glass
is  not precisely the focus of light.   Heat, in radiating through air, does not
warm the air, and its passage is not sensibly affected by winds or any other
motion of the air.—These resemblances in the phenomena of light and heat
have by some  inquirers been held to  prove  that the two classes of appear-
ances are only different modifications of action in the same subtile substance
or ether.
   The diffusion of heat by radiation,  as it takes place in an instant to any
distance, and begins whenever there is any inequality of temperature between
bodies exposed to each other, would produce  instant balance of temperature
throughout nature,  but that heat leaves and  enters bodies with readiness
depending on the condition of their surfaces, and on  their internal conducting
powers.   A black stone-ware tea-pot, for instance, will radiate away 100
degrees of its heat in the same time that a pot of polished metal will radiate
only 12 degrees.
   Professor Leslie was the first  to see the importance of investigating this
subject, and he had the merit of contriving well-adapted means,  and of detect-
ing many of the important facts.  As common thermometers  are not suffi-
ciently delicate to determine very sudden changes of temperature, where the
influence is so light as in many  cases of radiant heat, he used the beautiful
differential thermometer contrived by himself, in conjunction  with  concave
mirrors, (as represented on next page,) to concentrate the heat and accumu-
late its energy.  Then taking as his heated body a cubical tin vessel filled
with boiling water, and covering it successively with plates or layers of dif-
ferent substances and  with different colours, and  exposing the thermometer
to  it for a given time under all the changes, he noted the  number of degrees
which the thermometer rose (as seen in the  table which  here follows) and
thus ascertained the radiating power of each sort of covering.
       Lamp black........100°
Writing paper
Crown glass
Ice .
Isinglass
Tarnished lead
Clean lead   .
Iron polished
Tin plate
98
90
87
75
45
19
15
12
        Gold, silver and copper    ......   12
   He next  reversed the experiments by using his  hot-water vessel always
 in the same state, and covering the thermometer bulb with the  different sub-
 stances and colours, and thus he  ascertained that the comparative absorbing
 powers of the substances and colours were very nearly proportioned to their
 radiating powers:  lamp-black, for instance, absorbed or was  heated 100°,
 while the polished metals absorbed or were heated only 12°, and so for the
 others.  And, lastly, the absorbing powers being  an indication of the oppo-
 site or reflecting powers (for  a body absorbing only a given proportion of
 the  heat which falls  on it, must reflect the remainder,)  he, by the same expe-
 riments, ascertained the radiating, absorbing, and reflective or mirror powers
 of the bodies, and therefore all the important points respecting radiant heat
 in its  relation to them.
   It seems paradoxical that the putting  a clothing  of a  thin cotton  or woollen
.fabric upon the  polished tin vessel, should cause  the heat to be received by 
SPREADING BY RADIATION.
271
it or dissipated from it much sooner than if the vessel were naked, but such
is the fact.   And  metal with a scratched  or  roughened surface radiates or
receives much more rapidly than polished metal.
  The property  of absorbing heat depends  much upon  the colour  of the
substance, and, as a general rule, the dark colours, viz., those which absorb
most light,  absorb  also most heat.  Dr. Franklin proved  this by  laying
pieces of cloth of different colours  on snow, and exposing  them during a
given period to the sun's rays :  while he noted the different depths to which,
by the melting of the snow under them, the pieces sank.   Hence  comes
the ^importance of having a  white  dress  in  summer,  that by it, with  the
sun s light, the heat also may be repelled.  And a white dress in winter is
good because  it radiates little.  Polar animals have generally  white furs.
White horses are both less heated in the sun, and less chilled  in winter, than
those of darker hues.
  The rate of cooling in heated bodies must be  influenced by all the parti-
culars noted  above,  viz., substance, surface, colour, and  by  the excess of
heat in the cooling body as compared with those around it.
  The concentrating apparatus  used  for experiments  on the radiation of
heat consists  of two  concave tin mirrors, here represented  at a and b, so
formed and placed in  relation to each other that all  the rays of light or  heat
issuing from  the  focus 'of one, as  at c,  shall, after a  double reflection, be
collected in the focus  of the other, d.   A stand under one focus c  is intended

                               Fig. 124.

'^
 to support the body giving out or receiving  heat, and  a stand  under the
 other d is  meant to support the thermometer.   For farther explanation of
 the action  of such  mirrors, we may refer to what was  said of the concen-
 tration of sound in  the section on Acoustics, or to what  follows in the sec-
 tion on Optics, on the concentration of light.  The general rationale of such
 facts is, that heat, light, sound, an elastic ball, Sic, when reflected from any
 point of a surface,  returns, if it fall perpendicularly  to  that point, in  the
 same line by which it approached;  but if it  fall obliquely, or from one side
 of the perpendicular, it returns in a line deviating as much on the other side.
 Now the surfaces of concave  mirrors  are so formed, that every ray issuing
 from the focus shall, when reflected, become parallel to every other ray—as
 represented by the  dotted lines in  the  figure; and it is the property of a
 similar mirror receiving parallel rays to make them all meet in  its focus:—
 thus, any influence  radiating from  c towards the mirror d, will again, after
 two reflections, be collected at d.  The purpose  and effect of such  mirrors
 in experiments on heat, are merely to concentrate feeble influences, so that
 they may  be more accurately  estimated.   To  show  the  effect  and  mode
 of action of such mirrors, they may be  placed exactly  facing each other at
any convenient distance, and then a hot body of any kind, as a  metallic ball
or a canister of boiling water, being placed in one focus while a thermometer 
272
HEAT.
stands in the  other, the thermometer will instantly rise; although  if left in
any intermediate  situation nearer to the hot  body, and therefore not in  the
focus, it will not be affected.  If burning charcoal be placed in one focus,
and a readily combustible substance in the other, the latter may be set fire to,
at the distance of thirty feet or more.
  If, in one focus of the mirror apparatus described above, there be placed,
instead of the canister of hot water, a piece of ice, the thermometer in  the
other focus immediately falls.  This has  been called  the radiation of cold,
and  persons were at one time disposed  to think that it proved cold to have
a positive existence distinct from  heat.   The case, however, is merely that
the  thermometer  happens  then  to be  the hotter body in one focus of  the
mirrors, placed in close relation with a  colder  body, the ice, in  the other,
and consequently by the law of equable diffusion, it must share its heat with
the ice, and will fall.   The mirrors in any case have  merely the  effect, by
preventing the spreading and dissipation of the radiant heat from either focus
except towards the other of  making two distant bodies act upon each other
as if they were very near.   All the heat that seeks to radiate from  the ther-
mometer d in the direction of the surface of  the  mirror b, if not met by an
equal tension or force of temperature in the other  mirror or focus, to which
they are directed  at a and c, will radiate away  to c, and become deficient at
d.  Some inquirers have believed that  heat was constantly radiating in  ex-
change from substance  to substance (as light radiates between opposed bodies)
only more copiously from  the  side  where  the  temperature was  highest:
others have held that motion took place  only where there was excess of heat;
that  is, when the balance  of temperature was  destroyed;  and this  is  the
simplest view.
  There is a remarkable difference in one respect between the heat of  the
sun and that radiated from  any other source, namely, that  the first passes
through air, glass,  water, and  transparent bodies  generally, very readily,
while the latter, although not obstructed by air, is almost totally intercepted
or absorbed, in passing  through any of the other substances named.  In our
drawing-rooms it is common to have plate-glass  fire-screens, which, while
they allow the light to pass, defend the  face  from the  heat:  but all persons
know that the heat  of the sunbeams, as  well  as their light,  enters our green-
houses through the  glass which covers them.  A glass screen interposed  be-
tween the concave mirrors in the apparatus above described, destroys almost
entirely the effect of the heated body placed in one focus, on the thermometer
in the other, and the trifling effect really produced has appeared to some  to
be owing to the heat that is absorbed by the screen on one of its sides, and
then after passing through it by conduction, is radiated from the other.  This
conclusion seemed  to be supported by  the fact that screens  of metal or of
glass, covered  with lamp  black, paper, &c, allow transmission nearly in
proportion to their several absorbent and radiant  powers.   More careful  ex-
periments, however, have seemed to prove that,  even  at a  low temperature,
a certain portion of the heat is suddenly radiated through the glass, and at a
high temperature, a much larger portion.  A  glass mirror reflects the light of
a fire, but at first retains nearly all the heat, and only radiates it afterwards as
a hot body.
  The doctrines of radiant heat make us aware of the  importance of having
vessels of polished  metal for containing  liquids or other things which we  de-
sire to keep warm; hence, tea and coffee-pots, dishes  for soup, &c, should
be polished.   As a black  earthen  tea-pot loses  heat by radiation nearly in
proportion to the number  100, while one of silver or other polished metal 
SPREADING BY RADIATION.
273
loses  only as 12, there will be a corresponding difference in  their aptitude
for extracting the virtues of any substance infused in them.  Pipes for the
conveyance of steam or hot air, if left naked, should be  of polished metal;
but after arriving at a place where they have to give out their heat, their sur-
face should be blackened and rough.  A coat of polished mail is not a cold
covering.   A mirror intended to reflect heat  should be  of highly polished
metal, and such, for an obvious reason,  the interior of a screen behind roast-
ing meat  is attempted to be made.  A fireman's mark  is usually covered
externally  with smooth tin foil.   It is of advantage that the bottom of a tea-
kettle or other cooking vessel be externally black, because the bottom  has to
absorb heat, but the top should  be polished because it has to confine.
  The interesting phenomenon of dew was not at all  understood until lately,
since  the laws of radiant heat have been investigated.  At sun-rise,  in parti-
cular  states of the sky, every blade of grass and leaflet is found not wetted,
as if by a shower, but studded with a row of distinct globules most transpa-
rent and beautiful, bending it down by their weight, and falling  like  pearls
when the blade is shaken.  These are formed in the course of the night by
a gradual deposition on bodies rendered by radiation colder than the air around
them, of part of the moisture which rises invisibly from  water surfaces into
the air during the heat of the day.   In a clear night the  objects on the sur-
face of the earth radiate heat to the sky through the  air which impedes not,
while there is nothing nearer than the stars to return  the radiation ; they con-
sequently  soon  become  colder, and if the  air around has its  usual load of
moisture, part of this will be deposited on them, in the form of dew, exactly
as the invisible moisture  in the  air of  a room is deposited on a cold bottle of
wine when first brought  from the cellar.  Clouds, by obstructing the  radia-
tion spoken of, obstruct  the formation of dew.  Air itself seems not to lose
heat by radiation.   A thermometer placed upon the earth any time between
sunset and sunrise, generally  stands  considerably lower  than another sus-
pended in  the air a few feet above it; owing to the radiation of heat upwards
from it  and from the earth, while the  surrounding air remains nearly in the
same  state.  During the day, while the sun shines, the earth is warmer than
the air.  The reason why the dew falls, or is formed so much  more copi-
ously  upon the soft spongy surface of leaves and flowers,  where it is wanted,
than on the hard surface of stones and sand, where it would be of no use, is
the difference of their radiating powers.   There is no state of the atmosphere
in which artificial dew may not be made to form on a body,  by sufficiently
cooling it and the degree  of heat at which the dew begins to appear is  called
the dew-point, being an important particular in the meteorological  report of
the day. In cloudy nights heat is radiated back  from the clouds, and the earth
below not being so much  cooled, the dew is scanty or deficient.   And it is,
when  uninformed persons would least  expect the dew, viz : in very  warm
clear nights, and  perhaps when the beautiful moon invites to  walking, as in
some of the evenings of autumn with  the harvest moon and harvest occupa-
tions—that the dew  is more abundant, and the  danger greater to delicate per-
sons of  taking harm by  walking among the grass. 
274
HEAT.
"Heat by entering bodies expands them, and through a range which includes
  as three successive stages the forms of solid, liquid, and air or gas ;  be-
  coming thus, in nature, the grand antagonist and modifier of the effects
  of that attraction which holds corporeal atoms together, and which, If act-
  ing alone, would reduce the whole  material universe to one solid lifeless
  mass."   (Read Analysis, page 256.)
  If an experimenter take a body which is as free from heat as human  art
can obtain  it—a  bar of solid mercury, for  instance,  as it exists  in a  polar
winter—and  if he then gradually heat such  body, it will acquire an increase
of bulk with every increase  of temperature; first, for a time, there will be
simple enlargement or expansion in every  direction ;  then the mass will in
addition be softened ;  then  it will be  melted or fused; that  is to say, in  the
case supposed, the solid  bar will be reduced to the state of liquid mercury,
with the cohesive attraction of the atoms nearly overcome : if the mass be still
farther heated, it  will gain  bulk until  at a  certain point, the atoms will be
repelled from one another to much greater distances, constituting then a very
elastic fluid called an air or gas, many hundred times  more bulky than the
same matter in the solid or liquid state, and capable of forcibly distending an
appropriate vessel as common air distends a bladder;  susceptible, moreover,
of dilating indefinitely farther, by farther additions of heat,  or by diminution
of the atmospheric, or other  pressure, against which it had to rise during its
formation.   A subsequent removal  of the  heat from  the  gaseous  mercury,
will cause a progress of contraction corresponding to the previous progress of
expansion, and the various conditions or forms of the substance above enu-
merated, will be re-produced  in a reverse order, until the solid mass re-appear,
as at first.
   What is thus true of mercury is proved, by modern  chemical art, to be true
also of all the ponderable elements of our globe, and of many of the combi-
nations  of these  elements,—as water, for  instance, familiarly known in its
three forms of ice, water and steam; although compound substances generally,
by great changes  of temperature, are decomposed into their elements.
   A student might at first have difficulty in  believing that the beautiful variety
of solid, liquid, and air, found among natural bodies could depend  upon the
quantities  of heat in them, because these forms are all seen  existing at  the
same common temperature;  but he afterwards learns that each substance has
its peculiar relation or affinity to heat, and  that hence, while at the medium
temperature of the earth, some bodies contain so little as to be solids—like
the metals, stones, earths, Sic ; others have enough to be liquids—as mercury,
water, oils, Sic ;  and others have enough  to be airs—as oxygen,  nitrogen,
hydrogen, &c.  Men, until  better informed, are prone to deem the tastes in
which bodies are  most frequently observed by them, the natural or essential
states of such bodies; and  the Indian king reasoned but in the  usual way
when he held the Dutch navigators, newly  arrived on his shores, to be gross
impostors, because they said that in  their  country, at one time of the year,
water became so  hard that they could walk upon it, and drive their carriages
upon it, and shape it into solid blocks.  All persons err like this king, who
in thinking of the different substances familiarly known to them, regard their
accidental state of solid liquid or gas, which state is really dependent on the
temperature of the bodies, and therefore on the particular climate or situation
on earth where they are  found, to be in them an essential natural  character.
As well might a person who had  never seen silk, but  as  a delicate gauze or 
CAPACITY FOR HEAT.
275
satin enveloping some lovely human form, refuse to  recognize it in  the un-
sightly coil of the worm which produces it.
   The degrees in a general scale of temperature at which the substances most
important to man change their states from solid to liquid, or from liquid to
air, will be  noted  in  a  future page.   Here we have only to remark,  that the
differences  are very  great.   Mercury melts at about 80 degrees  below the
melting point of ice,  and  porcelain at about 30,000  degrees  above.   There
are some substances which  require so high a  temperature for their fusion  or
for their conversion into gas, that human  art has difficulty, or even  finds it
impossible, to produce the changes by simple  concentration of heat; but  all
such substances are quickly reducible to the liquid form when placed in  con-
tact with others for which they have  a chemical affinity, and which  possess
already the  form of liquid or air;  as when gold and platinum are dissolved in
nitro-muriatic acid—flint in the fluoric acid—carbon  in hydrogen gas.  Now
many  persons may not have reflected that the dissolving a  solid in any fluid
menstruum is merely  another mode of melting it by heat; yet this is the truth,
for the menstruum is  itself fluid, only because  of the much heat which it con-
tains, and in dissolving the more obdurate substances, it does so merely be-
cause its attraction for the substance  brings the particles into union with the
heat which  already exists in itself.  Heat, then, is the one universal solvent
or cause of fluidity.   Its influence in this view is interestingly  seen in the
fact, that a fluid when heated  can dissolve much more of a solid  than when
cold.   Water while  hot keeps dissolved  twice as much of many salts  as it
can when its temperature has fallen, as is proved by the crystals of salt formed
in any saturated solution as it cools.—There are again in nature many  sub-
stances having such  an affinity  for  heat, that until lately, they have been
known only as  airs ;  and  even in the  present advanced state of art, they  can-
not by any degree of mere cooling be  reduced to the liquid or solid form ; yet
all such, when  pressure is added  to the cooling, or when the chemical attrac-
tion for them of some other substances which  already exist in the liquid  or
solid state, is made to co-operate, may be  reduced.  An instance is afforded
when oxygen is made part of a liquid acid, or  of a solid ore.
   Of solids, some on receiving heat become very soft before they are liquefied,
as pitch, glue, iron, Sic ; others change completely at once, as ice in becoming
water: and some pass at once to the state of air, without  therefore having
assumed at all  the intermediate  state of liquid—they are sublimed, as  it is
called, and on cooling again  may be caught in a powdery state, as seen in that
form of sulphur, or of benzoin, termed the flower of the substance.  Of the
latter class also are camphor, arsenic,  corrosive sublimate, and the  substance
called iodine, which last, from the state of rich ruby crystals, becomes at once,
on  being heated, a dense transparent gas of  the  same hue, and in cooling
resumes its  crystalline form.
  The reader having arrived at this place, may peruse again  with advantage
the five pages near the  beginning of  this  work, which treat of the influence
of heat on the constitution of masses.

" Each particular substance, according to the nature, proximity, &j-c, of its
  ultimate particles, takes a certain quantity of heat (said to  mark its capa-
  city) to produce in it a given change of temperature or  caloric  tension."
  (Read the Analysis, page 256.)

  A pound  of  water, for  instance, that its temperature may be raised  one
decree, takes thirty times as much heat as a pound of mercury.   This may 
276
HEAT.
be proved in various ways.  First, if the heat be derived from any uniform
source, the water must remain exposed to it thirty times as long as the mer-
cury.  Second, if both substances, after being equally heated, be placed in
ice until cooled to the freezing point, the heat which escapes from the water
will  melt thirty times as much ice as that which escapes from the  mercury.
Third, when a pound of  hot water is mixed with a pound of  cold  mercury,
instead of the two becoming of a middle  temperature, as in the  case when
equal quantities of hot and cold water are mixed, and  every  degree of heat
lost by the one quantity becomes j ust a degree gained by the other—the pound
of hot water, by giving up one degree to  the  pound of cold mercury, raises
the temperature of the latter thirty degrees ; and in the same  proportion for
other differences:—or on reversing the experiment, a pound of hot mercury
will be cooled thirty degrees by warming a pound of water one degree.
  Now each particular substance in nature, just as water or mercury, has
its peculiar capacity for heat;  and experiments made by the modes of mix-
ture  and of melting ice above described have led to the construction  of tables
which exhibit the relations.  The following short table is an abstract, show-
ing the comparative capacities of equal weights of some common substances.
Water, for  reasons of convenience, has been chosen as the standard of com-
parison.  It appears  that  a pound of  hydrogen gas takes about twenty times
more heat to produce it in a given change of temperature, than a pound  of
water, while a pound of  gold takes about twenty times less,  and therefore
four  hundred times  less than  the hydrogen.  The figures  in the  table, by
marking the comparative capacities for heat of various substances necessarily
indicate also the comparative quantities of ice melted by equal  weights of the
substances  in cooling through an equal number of degrees.—A pound of
water, the standard, must  cool 140 degrees, that is, must give up 140 degrees
of its heat, to melt one pound of ice.

                                Gases.
         Hydrogen
         Atmospheric air
         Carbonic acid gas
         Common steam
                                Liquids.
         Solution of carbonate of ammonia
         Alcohol      ...
         Water       ...
         Milk
         Olive oil
         Linseed oil   -
         Sulphuric acid
         Quicksilver  -
                                Solids.
         Ice  ....
         Wheat
         Charcoal    ...
         Chalk
         Glass       ...
         Iron         ...
         Zinc        ...
         Gold
21|
 U
 n
 n
2
ItV
1
1
 1
30
TO 
CAPACITY FOR HEAT.
277
   We may remark here that some late researches, by another mode of trial
 make the capacity of air to  be only a quarter that of water, although in the
 preceding table it appears to be one  and three-quarters.  Now as  the other
 aeriform fluids have been compared with water through the  medium of atmo-
 spheric air, if there be an error with respect  to this, it must run through all
 the figures noting the capacity of other aeriform substances.
   If we seek a reason or reasons why there should be among bodies the dif-
 ferences of capacity here stated, the circumstances chiefly attracting attention
 are the following.   1st. Equal weights of  the various substances have very
 different bulks or volumes,  and  therefore have  different room in which the
 heat may lodge ;—as a pound of mercury, for instance, is only one-fourteenth
 part as bulky as a pound of water.  That the bulk, however, is not the only
 influencing circumstance appears in the fact, that  mercury  only  has one-
 thirtieth of the  capacity of water.   2d. In equal, bulks  of  different sub-
 stances, the space may be more completely occupied by the particles of one
 than of  another—as is probably true of the particles of  mercury compared
 with those of water.   3d.  But as the facts are  not fully accounted for by
 these two circumstances, we must infer that there is some  difference in the
 ultimate particles of bodies affecting their relations to heat.   We shall now
 review more particularly the various circumstances mentioned.
   First. The influence  of bulk or volume, in determining  the capacity for
 heat, is proved by the facts stated in the preceding table, and by many others.
 In the table, for instance, it  is seen that  hydrogen and the  gases generally,
 with their great comparative bulk, have also great capacity;  that liquids
 have less capacity than gases;  that solids  have less than  liquids—but the
 capacity, as already stated, is not in strict proportion to bulk; for hydrogen,
 which is  many thousand times more bulky than an equal  weight of water,
 has  only twenty-one times  the  capacity.   Again, if any body whatever be
 suddenly compressed  into less bulk, heat will issue from it as  if squeezed
 out.  Thus iron or other metal, suddenly condensed by the  heavy blow of
 a hammer, is thereby rendered hotter, that is, expelled heat gradually spreads
 from it.   Because water and  spirit, on being mixed, occupy less space than
 when separate, there is from the  mixture  a corresponding discharge of heat.
 But the truth is most remarkably exemplified in airs or gases, owing to their
 great range of elasticity.  They may be condensed or dilated a hundred-fold
 or more, and there will be a  simultaneous concentration or diffusion of their
 heat, that is to say, the production, in the  space occupied by  them, of intense
 heat or cold.   The heat of air just condensed, or the cold of that which has
 just  expanded, is much greater than even  the most delicate thermometer can
 indicate, for there is  so little heat altogether even in a considerable volume of
 air, that  the mass of a mercurial thermometer, although  absorbing  a great
 part of it, would be  little affected.  The  extent, however, of  the change of
 temperature is seen  in the facts, that, by the sudden condensation of air we
 may inflame tinder immersed in it, and by allowing air suddenly to expand,
 we may convert  any watery vapour diffused through  it into  ice or snow.
 Nay, air, containing  carbon in perfect solution, as  is true of the common coal
 gas,  if first condensed  to expel heat, and  then allowed suddenly to expand,
 will  be so cooled  that the carbon will be separated like a black cloud, as snow
 is separated in the  case before described.   The cold  which  separates or
 freezes carbon from  a  gas holding it in solution, must be very intense.  It
 mio-htbe expected that air suddenly compressed into half its previous volume,
should become  just twice as  hot as before, or if suddenly dilated to  double
volume, should be only half as hot, thus  enabling us to  ascertain the whole
     19 
278
HEAT.
quantity of heat contained in it;  but the facts  are not so; the temperature
changes, near the middle  degrees of the scale at least, much less than the
density.   Air in doubling its volume from a common density, becomes colder
only by about 50° of Fahrenheit's thermometer.
  The different capacity for heat of air in different states  of  dilatation, pro-
duces effects of great importance in nature as well as  in the arts.  Thus,
  On the  surface of the earth near the sea-shore, the air of the atmosphere
has a certain density (a cubic foot weighs about one ounce and a quarter) de-
pendent on the  weight and pressure of the  superincumbent mass; but on  a
mountain  top 15,000 feet high, as half the mass of the atmosphere is below
that level  (see "Pneumatics,") the air is  bearing but half the pressure, and
consequently any quantity of it has twice the volume of an equal quantity at the
sea-side, with  a temperature consequently many degrees  inferior.  The air
which is at any time on a mountain-top, may, a little while before, have been on
an adjoining plain or shore, and in gradually climbing the mountain side as a
wind, it must have been gradually expanding and becoming cooler in propor-
tion to the diminishing pressure.   It is found that air, on rising from the sea-
shore, becomes one degree colder nearly, for the first 200 feet of perpendicu-
lar ascent, and that air becomes altogether about 50°  colder in rising  15,000
feet;  so that at this latter elevation, water exposed to the air  is frozen  even
near the equator, where the temperature of low plains is at least 80°.  It thus
appears that if a man  could travel with the wind so as to remain always sur-
rounded by the same air, he might begin his journey with it from the sum-
mer vineyards of the Rhine, might soon after find it the piercing blast of the
Alpine summits ; and  again, a little after, without any  change having occurred
in the absolute  quantity of its heat, might feel it as the  warm breath of the
flowers on the plains of Italy.
  The explanation is  thus given of why very elevated mountains  in all parts
of the earth are hooded in perpetual snows.  We have just said that  even at
the equator, where the average temperature near the sea is 84°, water will be
frozen wdien carried to an elevation of 15,000 feet.   A line, therefore, traced
round a mountain at this level would divide the portion of it destined to sleep
under lasting ice and snow from  the portion below covered with  green her-
bage.  This line, wherever found, is called the snow  line, or line of perpetual
congelation.   At the equator it is high in the  atmosphere, because there is
a difference of about 50° between the average temperature of the country and
the freezing point of water, viz., the difference between 84° and 32°, and an
elevation of 15,000 feet corresponds to this difference; but in a progress to-
wards the poles, the line  is met  with gradually nearer to the  earth, as the
difference between the average  temperature and the freezing point  is less.
In Switzerland, the snow  line is at 6,500 feet above the sea;  in Norway,  it
is below 5,000.  With respect to the line of congelation, it is  farther to he,
remarked, that in tropical countries, because the temperature of the air is nearly
uniform during the whole year, the line or limit of frost and snow is  distinct
and unvarying, that is to  say, is narrow, particularly where  the  acclivity  is
considerable ; but in countries to the north and south, which have  strong con-
trast of summer and winter, the line rises in summer and falls in winter, and
thus becomes broad and less evident; in the hot season much  snow is melted
or half melted above the middle of the line or belt, while in winter much snow
and ice is accumulated below that, to be melted  again when summer returns.
  In  the breadth of the line of congelation for changeable climates, we have
the reason of the formation of what are called glaciers around  snow-capped
mountains situated in such climates, and around such only.   The snow near 
CAPACITY FOR HEAT.
279
the upper part of the broad line having been only softened or half thawed in
the preceding summer, becomes in winter almost as solid as ice, and in the
succeeding summer vast masses of it, detached by the action of the sun  and
of the internal heat of the earth, and loaded with more recently deposited snow,
are constantly falling down into the  neighbouring valleys within the broad
line of congelation; where, being accumulated, and the crevices filled up with
snow or with water  which hardens to ice, they form at last the huge glaciers
or  seas of ice,  mers de glace, which render certain regions so remarkable.
The falling of the masses above described (called in Switzerland avalanches),
is what renders the ascent to  snow-clad mountains so terrific and dangerous!
Around  Mount Blanc, in the. awful solitudes of the elevated valleys, the
avalanches are thundering down almost without interruption durino- the whole
summer,—in which  season only the attempt to ascend the mountain can be
made; and a  pistol shot, or any considerable agitation of the air, may suffice
to set loose masses that would sweep away a whole convoy.   Beneath  gla-
ciers there is always going on a melting of that part of the ice  which is in
contact with the earth, and hence a stream of water constantly issues from
the bed of every glacier.    These streams in Switzerland  are the beginnings
of the magnificent rivers the Rhine and Rhone.—Like the avalanches break-
ing loose in summer  among the mountains, there are in polar seas vast masses
of ice detached  from  the shores, and which afterwards drift into warmer seas
to be melted.   These often become as rafts to the arctic bear, and to his sur-
prise, carry him to new  latitudes, and leave him at last to perish in the midst
of the wide ocean, when his support has vanished from beneath him.
  Although the proofs are not so immediately apparent, the line of congela-
tion exists as truly  everywhere in the  open  sky,  over  sea  and  plains, as
where  there are mountain heights to wear its livery; and considerably below
the line, the cold, aided by electrical agency, is sufficient to  produce in  the
form of mist or  clouds, a deposition from the  air of the  watery vapour con-
tained in it.  There  is thus in  nature an admirable  provision to  shade  the
earth at proper  times from the too powerful rays of the sun, or to supply rain
as wanted, without the transparency of the inferior regions of the atmosphere
being much affected.   As the watery vapour rising from sea or lake, and invi-
sibly diffused in the atmosphere, can only reach to the height where the cold
is great enough to  condense it, the clouds may in general be regarded  as  the
top of  that atmosphere of watery vapour or aeriform water, which is always
mixed  more or less with the atmosphere of mere air; and as the quantity of
watery vapour which can exist invisibly  in a given space,  depends altogether
on the  intensity of heat present, the clouds in a cold or a  humid atmosphere
will be low, and in a warm or a dry atmosphere will be high, or there may
be none.  An aeronaut mounting in his balloon through  a clear sky, often
enters  a dense  cloudy stratum, and for  a time is  surrounded by the gloom
almost of night, the face of earth being  hidden from him below,  while  the
heavenly bodies are equally veiled from him above; but rising still higher, he
again emerges to brightness, and looks down upon the fleecy ocean rolling
beneath him,  as the climber to a lofty peak  looks down from  the ever pure
atmosphere around it on the inferior region of clouds and storms.
  The diminished temperature of air in the higher regions of the atmosphere,
often enables  the natives of temperate climates, when forced to reside in  hot
tropical countries, inimical to their health, to find  near  at hand, on some
mountain  height,  the congenial temperature of  their early  homes.  The
author  of this work,  during a visit to the then not long inhabited Island of
Penano- in the strait of Malacca, examined this fact with pleasure not readily 
280
HEAT.
 forgotten.   The centre of the island is occupied by a lofty mountain ridge
thickly wooded, on the northern  summit  of  which a few residences visible
from  the sea-shore like eagles' nests  on a cliff, had just been constructed.
Towards these, one  morning at  sunrise, on an active little horse  of  the
country, and along a tolerable  road, he began to climb from the hot plain
below.  At first there  were around him purely tropical objects, inspiring
tropical feelings,—the latter modified indeed by the reflection that his track
lay through a forest, in  which until lately  the foot of man never penetrated,
and where  the trees nursed  through ages to  their greatest growth, and  the
stupendous  precipices and the sublime waterfall had so recently been  ex-
posed to human observation ; but as he gradually ascended, he perceived  the
character of the vegetation to be changing, and the air to  be becoming so light
and cool  as strongly to awaken in him  thoughts of distant England—nay,
almost the  illusion that  he was  there.  When he had reached the summit,
however, and a clear space opened to view  the whole  country around,  his
attention was soon recalled to the fervid land  of the sun.  At first, from  the
elevation being so  great, the eye took account only of the grander features of
the scene, and which  were such nearly as might be met with on a Grecian
or Italian  shore: the expanse of  sunny  water in that  beautiful strait, the
opposite continent with its river winding seaward across the plain, the town
and the roadstead near it crowded  with ships, which appeared only as specks
on a wide-spread map; but, on  closer inspection, and particularly with  the
aid of the telescope, were described the rich groves of cocoa-nut and banana,
the plantations of spice, and cotton, and sugar-cane,  the  tawny labourers,  the
bamboo dwellings, the fanciful canoes or prows, and other objects of the like
character.   And such was the scene, which even under the equator, a person
could place  under  his  eye, while  the  thermometer near him stood as in  an
English month of  May.
   The interiors of the islands of Jamaica and Hayti have  many situations
of great extent, which combine, as above described, the  advantages of tropi-
cal situation and temperate climate, and which might well be inhabited  by
English labouring colonists.  The vast plain of Mexico, and much of  the
central land of South America, are similarly circumstanced : and it is not un-
common, where the ascent to the  gigantic Andes is gradual, to  find at  the
bottom of the ridge a town, whose markets are stored only with the  produc-
tions  of the equator, while in a  town higher up will be seen  only what
belongs to the temperate skies of  Europe:—climates of the  earth naturally
distant, thus meeting, as it  were, in amicable vicinity, on the same rising
plain.                     ►
   The facts detailed  in the preceding paragraphs are  intended to illustrate
the subject of the relation of volume in a body to the capacity for heat. We
now proceed to speak of density in the same respect.
   Second.   It might be anticipated that a dense body,  or one in which  the
constituent particles may be supposed to fill more completely the space occu-
pied  by it  than the particles do in a rarer  body, would have smaller capacity
for heat, in  proportion to the smaller space left vacant in its mass : and in a
general comparison of the capacities of equal bulks of  different substances,
such  anticipation is partly verified,—as when  a pint of dense mercury is found
to have only about half the capacity which a pint of lighter water has.  The
relation, however,  is by no means universal, nor  at all  in proportion to  the
differences of density.   Water, which is  denser than  oil, and according to
the hypothesis should have less capacity, yet has nearly  double the capacity;
and  mercury, which  being  nearly fourteen times denser than water, might 
EXPANSION OF BODIES.                    281
be expected to have only a fourteenth of the capacity, has really for equal
volumes a half, or, as formerly stated, for equal weights, a thirtieth.
  Third.   We are at last,  therefore, compelled to admit,  that the relation
between various substances and heat, which we call capacity  for heat, depends
much more on the nature of the ultimate particles of the substances than
either on the absolute bulk or comparative  density of the masses. Throwing
much light  on this subject, it has  been ascertained in late times,  that all
material substances are composed of extremely minute unchangeable  atoms,
of which, in different  substances, the comparative weights have been deter-
mined, although not the absolute weights; that is to say, for example,  the
atom  of gold  is known  to weigh four times  as  much as the atom of iron,
although we do not know how many thousands  or millions  of atoms  are
required to  form  a grain of either.   Now very recent researches seem to
prove that for  each  ultimate atom, no matter of what substance, nearly  the
same  quantity of heat is  required to produce in a  mass of the atoms a given
change of temperature.  Thus an ounce  of iron, which has four times  as
many atoms as an ounce  of gold, has four times the capacity for heat.  This
law seems to hold for all simple substances; but for compounds there seems
to be  another law not yet ascertained.
  Instead of the  term capacity for heat used  in  the preceding pages, with
respect  to particular substances, that  of specific heat has  by  some  authors
been  preferred; but as the latter  gives to a commencing student the idea
rather of kinds of  heat than of quantities, the term capacity has been he re
retained.

"Each substance in nature, for  a given change of temperature, undergoes
  expansion in a degree proper to itself, the expansion generally increasing
  more rapidly than the temperature, as the cohesion of the particles  be-
  comes weaker from increased distance ; being  remarkably greater, there-
  fore,  in liquid  than  in  solids, and in airs than in liquids;  the rate
  being quickened, moreover,  near the points of change.   (See the  Analy-
  sis, page 256.)
  The following table, containing  the names  of some common substances,
solid, liquid, and aeriform, shows,  by the  figures  following each name, how
much the  substance increases in  bulk, by having its temperature raised from
that of freezing to that  of boiling  water.   A  lump of glass, for instance,
would gain one cubic inch for  every 416 cubic inches contained in it; while
a mass of water would gain one inch for  twenty-three, dilating thus  for  the
same  range of temperature eighteen times  more than the glass.
                                Solids.
              Glass gains one part in                  416
              Deal......416
              Steel......283
              Iron......271
              Brass......177
              Silver......173
Lead
117
                  Liquids.
Mercury gains one part in  -     -    -   55
Water......23
Fixed oils.....12
Alcohol..... 
282                             HEAT.
                                 Airs.

              Common air,")
              all gases and y gain one part in             3
              vapours     J

  We have to warn readers here  not to  confound the  increase by heat of
the general bulk of  a solid body with the increase of its length.   The latter
is only one-third as great as the former.   This will be understood by consi-
dering that the increase of bulk is divided between the length, breadth, and
depth (or thickness.)  If the substance of a metallic square  rod or wire, be
dilated by heat a one-hundredth part of its bulk, it does not gain  all  that
hundredth at its end, becoming 101 inches (or other measure) long, instead of
100 ;  but every part becomes deeper and  broader in the same  proportion as
it becomes longer (we may suppose it divided into a row of equal little cubes,)
and the rod gains in length  only the third part  of an inch.  A fluid  enclosed
in  a tube unchangeable by heat (if such  tube there were)  would  show its
whole dilatation in an increase of length, because there  could be no swelling
laterally, and  its  extremity, therefore,  from  any variation  of temperature,
would have a triple extent  of motion.   A degree of this consequence is ob-
tained in our common thermometers, because the containing glass, although
dilatable by heat, is so much less dilatable than the fluid within.   As regards
solids, we have to inspire so much more frequently respecting the dilatation
in  length, breadth, &c,  that is to say, the linear dilatation, in  one direction,
than the increase of general bulk, that tables are frequently made stating only
the linear dilatation.  It may be found at once  from the above table, by recol-
lecting that it is one-third of the increase of bulk :—thus, as glass, in passing
from the freezing to boiling heat of water, dilates one part in  416 of its bulk,
it will dilate only one-third  of a part in length, or a  whole part in an extent
of three times 416 or 1,248.
  The expansion of solids by heat has been ascertained by  bringing micro-
scope instruments to bear on rods of the different substances heated to various
degrees, in troughs of oil or water.   The expansion of fluids, again, is found
by filling a glass vessel  with a known  weight  of any fluid,  and then ascer-
taining how much is made to run over or escape by a given increase of heat;
or how much  the fluid rises into a long tubular neck like the stock of a ther-
mometer.   This quantity, added to what is required  to fill the increased di-
mensions of the heated glass vessel, (which from the ascertained expansion
of glass is known,)  forms the whole of the  increase.

The general and comparative expansion of solids by heat is exemplified in
   the following cases:

  An iron bullet, when heated, cannot be made to enter an opening, through
which when cold it passes readily.
  A glass stopper sticking  in the neck of the  bottle, often may be released
by surrounding the  neck with a cloth  taken out of warm water; or by im-
mersing the bottle  in the water up to the neck; or by making strong friction
on the neck by a tape or any soft rope  put around it, and  then pulled back-
wards and forwards.  By any one  of these means the binding ring of  the
neck is heated and expanded sooner than the stopper, and so  becomes for a
short time slack or loose upon it.
  Pipes of cast-iron for conveying  hot water,  steam, hot air, &c, if of con-
siderable length, must have joinings which allow a degree of shortening and 
EXPANSION OF BODIES.
283
lengthening, otherwise a change of temperature may destroy them.   An in-
competent person undertook to warm a large manufactory by steam from one
boiler.   He  laid a rigid main pipe along a passage, with lateral branches
passing through holes into the several apartments : but on his first admitting
steam,  the expansion of the main pipe tore it away from all its branches.
  In an iron  railing, a gate which during a cold day may be loose and easily
shut or opened, in a  warm day may stick, owing to  their being  greater ex-
pansion of it and of the neighbouring railing, than of the earth on which they
are placed.   Thus also the centre of the  arch of an iron bridge  is higher in
warm than in cold weather; while on the contrary, in a suspension or chain
bridge, the centre is lowered.
  The iron pillars now so commonly used to support the front walls of those
houses of which  the ground stories meant to serve  as shops have spacious
windows, in warm weather really lift up the  wall whicli rests upon  them,
and in  cold weather allow  it again to sink or subside considerably more than
if the wall were of brick from top to bottom.
  In some situations (as  lately was  seen in the  beautiful  steeple of Bow-
church, in London,)  where the  stones of a building  are  held  together by
clamps or bars of iron driven into  the stones, the expansion  in  summer  of
these clamps will force the stones  apart sufficiently for dust or sandy parti-
cles to lodge between them :  and then, on  the return of winter, the  stones
not being at liberty to close  as before,  will cause  the ends of  the shortened
clamps to be drawn out, and the effect increasing with  each revolving year,
the structure will at last be loosened and may fall.
  The pitch of a piano-forte or harp is lowered in a  warm day or in a warm
room, owing to the expansion of the strings being greater than of the wooden
frame-work;  and in cold the reverse will  happen.   A harp or piano  which
is well tuned in  a morning drawing-room cannot be perfectly  in tune when
the crowded evening parly has heated the room.
  Bell-wires, too, slack in summer, may be of the proper length in winter.
  One admirable contrivance for keeping  the pendulum of a clock always
of the same length, by making the greater expansion  by heat of a  middle
bar of brass counteract the  smaller expansion of  two side-rods of steel, was
explained  under the head  of "  Pendulum," as  was also the construction
of a balance-wheel  having a corresponding property.  A^difference of a
100th of an inch in the  length of a  common pendulum, causes  a  clock to
err  ten seconds in twenty-four hours, and a rise or fall of 25° of Fahren-
heit's thermometer produces this difference.  Another kind of  compensation
pendulum, not less admirable,  distinguished by  the  name of  its  inventor
Graham, is obtained by substituting for the solid  bob or ball at the bottom,
a glass vessel containing  mercury.  The mercury, on expanding by heat,
has its centre of gravity raised just enough to compensate for the  lengthening
of the  rod of the pendulum.
  Crystals, when heated, do not expand  quite  equally in breadth and in
length.  The same  is true of fibrous substances, as wood, which  expands
and contracts more  in breadth  than in length.  This is  instanced in the
leaking, during cold weather, of  a  ship's deck, which, in warm weather, is
tight; — an occurrence  which the author, in rounding  the Cape of Good
Hope,  had to regret as the cause  of destruction to  some valuable  specimens
of natural history which he had collected among the Eastern Islands.
  Bodies expanded  by heat, unless  when  their intimate  composition  is
changed by it, regain exactly their former dimensions on being cooled. 
284
HEAT.
 As is seen in the preceding  table, the expansion of liquids by heat is much
   greater than of solids.
   A cask quite filled with liquid in  the  winter, must, in  summer, force its
 plug  or  burst; and  a vessel  which has been  filled  to the lip with warm
 liquid, will not be full when the liquid has cooled.  Hence a cunning dealer
 in liquids has tried to make his chief purchases in very cold weather, and his
 chief sales in warm weather.
   There  exists  in  the  case of water, an extraordinary exception, already
 mentioned to the law of expansion by  heat and  contraction  by cold,  pro-
 ducing unspeakable benefits  in  nature.  Water contracts only  down to  the
 temperature  of 40°, while, from that to 32°, which is its freezing point, it
 again dilates.   One  curious consequence  of this  peculiarity  is  exhibited
 when a pool or well happens to be formed on the upper surface of a mass of
 ice, as on one of the glaciers  of Switzerland  and elsewhere, namely, that
 the  well goes on quickly deepening itself,  until  it penetrates to the earth
 beneath.   Supposing the surface of the  water originally to have nearly the
 temperature of the melting ice, or 32°, but to  be  afterwards heated by the
 air and sun, instead  of the water being  thereby dilated or  rendered speci-
 fically lighter, and  detained  at the  surface, it  becomes heavier  the more
 nearly it  is heated to 40°, and therefore sinks down to the  bottom of the pit
 or well;  but there, by dissolving some  of the  ice, and being consequently
 cooled, it is again rendered lighter, and rises to be  heated as before, again to
 descend; and this circulation and digging cease only when the  water  has
 bored its  way quite through.

             Airs are expanded by heat still  more than liquids.
   The expansion of aeriform bodies by heat  produces many important effects
 in nature. Some of  these have already  been considered in the preceding
 parts  of  this work, as the rising of  heated  air in the atmosphere, causing
 the winds all over the earth;  the same in  our fires and  chimnies supporting
 combustion and ventilating and purifying our houses;  the same again from
 around animal bodies, removing the poisonous  or contaminated air which
 issues from the  lungs, and insuring  a constant  supply of fresh air for the
 support of life,»&c.
   It is remarkable with  respect to aeriform  bodies, that, unlike solids  and
 liquids, they are all  equally dilated by  the same change of temperature,
 receiving an increase  of about a third part of their  bulk  (37g  parts  in 100)
 on being  heated from the freezing to the boiling point of water, viz., 180°,—
 their bulk being therefore doubled from  the  same  standard  point  by about
 500°.   This  general truth holds, not only with respect to the more perma-
 nent airs or gases, but also with respect to all steams or vapours in the  dry
 state, that is, when  not in contact with  the liquid producing  them.   The
 probable reason of this uniformity is, that cohesive  attraction which varies
so much in different  solids and liquids, modifying the effects of heat upon
 them,  in aeriform fluids does  not exist at all.
   The extent of this  dilatation  for airs is  so  much greater than for liquids
or solids,  that it forces itself much more  strikingly upon the  common atten-
tion.   Thus,  a bladder containing  considerably less than its  fill of air, be-
comes tense immediately on being held to the fire.  The air in a balloon just
escaping from a cloud, has been so suddenly  expanded by the direct rays of
the sun, as to injure  the  texture of the balloon: and  probably  some of  the
fatal accidents among aeronauts have been owing to this  occurrence.  Burn-
ing fuel conveyed into a vessel  or case which can be suddenly and strongly 
EXPANSION OF AIR.
285
 closed, will produce  an expansion  of the air confined with it, capable  of
 bursting any vessel of ordinary strength—in short, will produce an explosion.
   Now, if not before, at any rate soon after steam-engines began to be used,
 and had so  strikingly shown  to what important purposes  the  force  of an
 expanding aeriform  fluid  might be  applied, the thought  would naturally
 occur that the force of common air dilating  by heat might also  be rendered
 useful.   Accordingly a variety of air-expansion engines have been proposed,
 but as yet no one has been reduced to profitable practice.   Had the truth been
 generally known, which very recent investigations have proved, that a given
 quantity of heat, when used to  dilate air, produces  several times as  much
 expansive  power  as  when used to form steam, the  attempts to bring such
 an application of heat under control would probably  have been  more nume-
 rous, and possibly, by this time, more successful. The subject is so interest-
 ing that  we shall subjoin a few remarks upon it.
   To  produce a cubic foot of common steam  from water originally cold,
 about 1,150 degrees of heat are required, as  will be  explained  a few  pages
 hence.   The same quantity of heat would  double the volume of about five
 cubic feet of atmospheric- air,—as  is known from the  comparative capacities
 for heat of the two substances, and the rate of dilatation of air when heated.
 Now the value for work of the  foot of steam passing from the  boiler  into a
 working cylinder would  be, to press  up  the piston of the steam-engine
 through a foot, as from  c d  to a b, with a  force  all  the  way of 15 lbs. per
 inch of the piston surface; while the working valve of the five feet of air in
 dilating to double  bulk, would be  to lift up the piston five  times as  far as
 the steam, viz., from  g h  to e f, but with a  force
 gradually diminishing  (represented here by the
 shaded part of the figure) as the expansion  went
 on, from 15 lbs per inch at the beginning until the
 air had dilated  to its  destined volume,  when the
 force would altogether cease;  its  whole effect,
 therefore, would be five feet impulsion of the pis-
 ton with a pressure averaging between 15 lbs. and
 nothing,  viz., Ik lbs.  per inch;  and  the friction in
 the two cases and the varying intensity of the lat-
 ter pressure being neglected, the force  of  the  air
 would be 2\ times as great as that  of the  steam.
 But it is  farther to be considered,  that only about
 half the heat of a fire is applied to use in a steam-     fy^
 engine, viz.,  that  part winch  enters the boiler,
 while the remainder passes up the  chimney; and
 in an air-engine probably  the  whole might  be applied.  In an air-engine,
 moreover, there might be a great increase of  power from the combustion, or
 semi-explosion of the inflammable gas evolved from the fuel.  We see from
 this of  what importance  the  discovery would be of a means  enabling us
 effectually to apply the  force of expanding air.
   If we suppose a fire a to be placed on a grate  near the bottom of  a close
 cylinder, d a, and the cylinder to be full of  fresh air  recently admitted, and
 if we farther suppose the loose piston g d to be pulled upwards, it is evi-
 dent that all the air  in the cylinder above d will be made to  pass by the
 tube e through the fire, and will receive an increased elasticity tending to the
expansion or increase of  volume,  which the fire is capaple  of giving it.  If
 there were only the single close vessel d a, the expansion  might be so strong
 as to burst it; but if another vessel 6 c of equal size were provided, com-
 
286
HEAT.
Fie. 126.
                          municating with the first through the passage b,
                          and containing a c/ose-fitting piston cf, like that of
                          a  steam-engine, the expansion of the air  in the
                          first cylinder would act to lift the said piston, and
                          so might  work  water-pumps, or do any other
                          service which a steam-engine can perform.   At
                          the end of the lifting  stroke of the  piston f c, it
                          might be made to  open an escape-valve  for the
                          hot air, placed in  any convenient part of the ap-
                          paratus, and to cause the descent of the blowing
                          piston d to expel  that air, while a  new  supply
                          of fresh air would enter by another valve into the
                          cylinder above d.  The engine would then  be
                          ready to repeat  its  stroke  as  before, and the
                          working would be continued as in a steam-engine.
  The preceding simple conception of an air-engine occurred to the author's
thoughts while considering the application of a condensed air-furnace to some
ehemical purposes.  It appeared to him that, in  applying  any such engine
to use, the chief difficulties to be surmounted would be,  to prevent the very
heated air and dust from injuring the valves  and other working parts  of the
engine,  and to obviate the inconvenience of the inequality of power at differ-
ent parts of the stroke.  Various expedients occurred to him.   The over-
                  heating might be prevented by surrounding  the cylinder,
                  &c, with water;  and both cylinder and  piston  would
                  suffer less from dust, if, instead of the common piston c,
                  represented  above, a great hollow plunger a were used,
                  (such  as  is  here  represented, and is now  common in
                  water-pumps for mines) embraced by an air-tight neck or
                  collar  at b c, which neck would be the only part of the
                  cylinder requiring to be made with nicety.   But a more
                  complete security would be obtained by interposing water
                  between the hot air and the piston, as represented in this
                  other sketch, where the working cylinder d has a water-
                  vessel b connected with it, and the heated air is admitted
                  to b to press upon a float  on the water-surface, to lift the
                  working  piston d e.   This construction, too, if desired,
                  would  allow the fire-chamber a to be made larger than
                  the cylinder, and to be kept constantly filled with highly
expansive air, each discharge of which into the space b would be replaced by
                                            cold air, either from the space
Fie. 127.
d
Fig. 128.
•'.■_;__,."__.........		e\		
'': cP {		\		
V				
\\				
1 b		Wi^		cl
^_	—			
above the piston d, driven in
through  a tube as the piston
ascended, or from  a distinct
blowing  cylinder worked by
the beam.  And  if  it were
wished  to  apply the  same
principle to an engine work-
ing with double strokes, that
is,  forcing  the  piston alter-
nately up and down, as in the
double stroke  steam-engine,
the object might be  attained,
by having a  second water-ves-
sel/ communicating with the 
/pf*
EXPANSION OF AIR.
287
Fig. 129.
 part of the working cylinder above the piston d; and the air would pass alter-
 nately to the one or the other vessel b or/, by the operation of the cock c, as
 steam passes in a  steam-engine;  the supply of fresh  air to  the chamber a
 would be given by a blowing cylinder worked through  a connection with the
 engme, as the air-pump of a steam-engine is worked.
    The sketch of an air-engine, as here given, was included in the specifica-
 tion ol a patent for another object, engaged in some years ago by a friend of
 the author s; but that friend being almost  immediately called  to other busi-
 ness, and the author's professional engagements forbidding his attention to the
 subject, it was not  prosecuted.  In the specification, drawn up by an  engi-
 neer in the town, some minor adaptations were described.  One  experiment
 has lately been made by a Swedish engineer with the simple form of dry
 apparatus described at page 285,  for the purpose of ascertaining its  power,
 and  the effect was found  to be several fold greater than  of steam from the
 same quantity of fuel;  but the apparatus was rude, and only calculated to
 I prove in a short trial, the  existence  of the power, but not the fitness of the
 Imachine to endure  uninjured, or to be rendered easily obedient to control ;  a
 ■complete experiment, therefore, remains still to be  made.  Could an obedi-
 lent and durable engine be contrived, at all approaching in simplicity to the
 Iplan given  above, its  advantages over the  steam-engine would be very con-
 siderable.   First, its original cost would be  much less,  by reason of its small
 [comparative size, its  simplicity,  and the little nicety of workmanship re-
 ( quired.   Secondly, it would  occupy much less room, and  would be  very
 light; hence  its  peculiar fitness for  purposes of propelling ships and wheel
 carriages.   Thirdly, the quantity of fuel required being
 so much less, would not load the ship or carriage, leav-
 ing little room  for any thing else.   Fourthly, the ex-
 pense of fuel and of repairing would be little.  Fifthly,
 the engine could be set to work in a few minutes, where
 a steam-engine might require hours.   Sixthly, little or
 no water would be  required for it.
   Another modification of air-engine, called  a gas va-
 cuum engine,  has  lately  been  proposed, and many
 expensive trials have been made of it; but it is in its
 nature a most wasteful machine,  evidently throwing
 away at least  nine-tenths of the  power which  the
 combustion  generates.  It was of this nature in an ex-
 periment which the author witnessed.  A little of the
 common coal-gas was admitted  by the cock b at the
 bottom of the cylinder a, and was  there inflamed, the
 lid c being at the time raised.   The  combustion rari-
 fied the lower stratum  of air, so that the air above was
 expelled, and about  one-fifth of the  original contents
 of the cylinder was  caused to occupy the whole.  The
 lid  was shut down, as nearly as could be judged, at
 the moment of greatest expansion, so that  when the
 small portion of air and vapour remaining within was
 again cooled, the interior of the cylinder approached
 nearly to  the  state  of vacuum.  Tt,  in fact, retained
 only a fifth  of the air.  A communication being then
 opened from the vacuous cylinder  by the tube e to  a
water reservoir ten feet below, the water was  driven
up by the atmospheric pressure, until it filled more
e. 
*r
*»
288

'% i
HEAT.
Fig. 129.
than half of the cylinder.   The  water so raised was then made  to turn a
common water-wheel, and  to do  work.  A much larger quantity of water,
however, could be raised  to  the  same height at less expense by a steam-
engine.   The proposer also hoped that he would be able to make  the atmo-
sphere pressing into his imperfect vacuum, act directly upon a  piston as
steam does, and with power cheaper than that of steam ; but in this anticipa-
tion too he  was completely in error.   To produce his imperfect vacuum  cost
him very nearly at the same rate  as it costs to. produce the perfect vacuum in
                 a steam-engine, and his vacuum for  equal bulks was worth,
                 as a working power, only about one-fourth as much as the
                 steam vacuum.  This may be understood by considering,
                 that in a perfect vacuum a piston rises all the way with the
                 same force, which,  if common  steam be used, is 15 lbs.
                 per inch, (the piston may be sdpposed to rise from c  d to
                 a b,) but if the vacuum were only three-fourths towards
                 being complete, the pressure on the piston would be onlyj
                 three-fourths of 15 lbs., at the commencement of the stroke,!
                 and then rapidly diminishing, would have ceased altogether!
                 when the piston had made three-quarters of its journey, orj
                 to/.  The force in  the first  case would be represented byl
the whole line c dand the whole space c dba, and in the second by the short-]
ening lines and the triangular space c ef.
   On considering the foregoing diagrams, we may perceive that in the vacuum-
engine,  by far the greater part of the force produced by the combustion of the
gas is absolutely wasted, or put to no use, namely, the whole expansive force
during the sudden combustion or explosion.  It is evident that if a tenth  part
of the aeriform contents of  a cylinder acquire elasticity enough (a fourteenth
part in a nice experiment does so) to be able afterwards to occupy  the whole
cylinder, that tenth must begin its expansion with the  force  of a tenfold atmo-
spheric  condensation, or pressure, of 150 lbs. on the square inch of a piston
withstanding it, which pressure will then  gradually diminish as  the  piston
rises, but will amount to an average of five times the  atmospheric pressure,
or 75 lbs. per inch all the  way; being therefore quadruple or more, that of
steam against a perfect vacuum, and, therefore, again, by  our former calcula-
tion, more  than twelve times greater than the force obtained from the imper-
fect vacuum of the engine under consideration.
   It is a question which the author thinks will one  day be answered  in the
affirmative, whether nearly the whole force of exploding gas may not be  con-
                   verted  into a calmly working power,  producing from a
                   given expenditure, ten times or more the effect obtained
                   in the vacuum engine described above, and, therefore, an
                   effect more than equal to  that of a steam-engine incur-
                   ring the same expense.  There are probably various ways
                   in which the object may  be  attained.  The  following
                   sketch is offered  merely to give  the reader  an  idea of
                   a machine for such a purpose.
                     Suppose b to be a very heavy close-fitting piston sliding
                   in the cylinder containing  it;  and suppose the space d
                   open to the cylinder, to be filled with atmospheric air of
                   double or greater density ; then if a mixture of explosive
                   gases admitted by a cock to the chamber a (formed be-
                   tween the piston and end  of the  cylinder) be inflamed,
                   the heavy piston will be shot forward, like a cannon-ball,
Fig. 130
	a.	
c	i jj	
¥ 
EXPANSION OF AIR.
289
  against the condensed air in d; and owing to the momentum acquired in the
  first instants, it will advance much beyond the point where the exploded gas
  and air in d would balance each other at rest.  The quantity of gases admitted
  would be just such as to carry it to the end of the cylinder.  The piston rod
  e would then by a catch or racket, be connected with the  work to be done,
  and after the condensation  of the exploded gases in  a cylinder,  would be
  pressed back again, with the greater than  atmospheric force in d, as if urged
  by  high pressure steam.   Figure 127 at page 286 represents a form of cylin-
  der which might also answer for this purpose, the heavy plunger being thrown
  up, to work  by its  weight in descending.
    It is to be  remarked that the first  modification of air-engine described at
  page 285, is  partly an explosive engine such as contemplated  above, for the
i  gas separated from the coal during the moment of slackened combustion while
  (the lately used air is  passing out, becomes an explosive accumulation for the
  fresh air about to enter.  The trial alluded to above proved this to be the  fact.

  "The  expansion of bodies by heat increases more rapidly than the tempera-
    ture, and particularly near the melting and boiling points, that is, their
   points of changing into liquid or air being, however, exactly proportioned
   to the temperature  after the change into air.   (See Analysis, page 256.)

f   If a given  quantity of heat, that, for instance, contained in some measure of
* boiling water or of common steam, be added to a mass of cool water, it  will
  produce in this a certain increment of bulk; and if'other equal quantities of
  heat be  afterwards successively added, under the nice management which
  such an experiment requires, each new addition will produce a greater incre-
  ment of bulk than the preceding, particularly when the water approaches to
  boiling;  but after the water  is converted into steam, any farther increase of
  bulk will be exactly proportioned to the increase of temperature.   The same
  truths  may be proved by the converse experiment of abstracting successively
  equal  quantities of heat  from steam or water (as  by making  it melt equal
  quantities of  ice,)  and noting the rate of contraction.  What is thus true of
  water  in relation^ to heat, is  true also of bodies  generally, each, however,
  having a rate of expansion and temperature for melting and boiling proper
  to itself.  The quickened rate of expansion  in solids and liquids might have
  been anticipated from reflecting, that each successive quantity of heat added
  to a mass, meets with less resistance to its  expanding power than  the  pre-
  ceding quantity, owing to the diminishing force of cohesion of the particles
  as the  mass enlarges; while in an air or gas, again, as cohesion  has  alto-
  gether  ceased, each addition of heat is at liberty to produce its full and equal
  effect.—If the capacity of substances for heat did not increase with their bulk,
  the terms "increase of heat" and "increase of temperature" would have the
  same meaning-, and the subject would be more simple.
   The reflection will naturally occur here,  that as in the common thermo-
  meter,  the mercury must rise  or expand more for a given quantity of heat
  added  at a high than at a low temperature, the  scale should  be  divided to
  correspond with the inequality.  Now this  reasoning is good, but the diffi-
  culty of complying with it in practice is such, that the inconvenience of the
  slio-ht error arising from an equal division is commonly submitted to.   An
  air  thermometer with equal divisions is very correct, but from wanting many
 of  the advantages  of the mercurial  thermometer,  is little employed;  and
 fortunately in the  mercurial thermometer there is such a counterbalancing
 relation between the expansion of the mercury and of the containing glass, 
290                             HEAT.
as to render the error alluded to, at least for any middle range of temperature,
very trifling.  The subject of unequal thermometric dilatation in the same
liquid, and of the differences in  that respect in different liquids, depending
on the proximity to their boiling points, &c, is well illustrated by Du Luc's
experiment of filling various thermometer-glasses with different liquids, and
while they are being heated through the same range of temperature, noting
their comparative indications.  He marked on each tube the points at which
the liquid in it stood when  the bulb was  placed, first in freezing and after-
wards in boiling water, and he then  divided the intervening space into eighty
parts or degrees.   The discordance of the dilatations in  the  different tubes
when the instruments were afterwards placed together, and heated from the
freezing to the boiling degrees of water, was as here detailed.
Mercury.	Olive Oil.		Alcohol.			Water.
0	0	.	0	-	-	0
10	9.5	-	7.9	-	-	0.2
20	19.3	-	16.5	-	-	4.1
30	29.3	-	25.6	-	-	11.2
40	39.2	-	35.1	-	-	20.5
50	49.2	-	45.3	-	-	32
60	59.3	-	56.2	-	-	45.8
70	69.4	-	67.8	-	-	62
80	80	-	80	-	-	80
  The singular discrepancy in the case of water is owing to the peculiarity
described  in former pages, of its contracting by cold only down to 40° of
Fahrenheit, and then dilating again until it freezes.
  Laborious investigations  have been made by the  French chemists to dis-
cover a comprehensive law determining the  rate of  expansion in  all bodies,
but the object is not yet satisfactorily accomplished.

"To melt  a solid body, or  to vapourize  a liquid, a large quantity of heat
  enters it,  but in the new arrangement of the particles  and generally
  increased volume of the mass, the heat becomes hidden from the thermo-
  meter and is  called latent heat.   It reappears during the contrary
  changes, after ivhatever interval."   (See the Analysis, page 256.)
  The expansion  of  bodies by heat, instead of proceeding throughout in
some nearly uniform or gradual manner, exhibits in its  course two  singular
transformations of the body: the first, when the solid  breaks down into a
liquid; the second, when the liquid swells  out into an  air or gas; so that
there are,  in all, three very distinct  modifications or states of existence for
the body  dependent on the agency of heat.  The substance of water, for
instance, when at a low temperature, exists in the solid  form called ice; but
at 32° of Fahrenheit,  on  receiving more heat, it gradually becomes liquid or
water, and on receiving more at 212°, even under the resisting pressure of
the atmosphere, it acquires a bulk nearly 2,000 times greater than it had as
a liquid, (gradually as regards the whole, but suddenly as regards each sepa-
rate portion,) being then  called steam or  aeriform water.  And other bodies
under analogous  circumstances,  undergo  similar  changes.   It  is farther
remarkable, that although during the changes a large quantity of heat enters
the mass,  producing in one case liquidity, in the other  the  form  of air, the
temperature is the very  same, immediately after, as  immediately before  the 
                             LATENT HEAT.                       29l


 w^S^na fLTiV^ h6at beC°ming Mdden  °r latent in the  ™ss:-thus
 "en ove TotZZ  "§ "* ^ Uw lhermometer b»< - the ice does, and
 „,ii TZt      g 7  [eJ apPears no holter tha» the vvater. The glory of oriei-
 SSshe frls'to rra11 Which' the terms  lat™1 tUlrcZc
 Ltlt fo'n of the r f66" US6d' bel°n^S t0 the il,ustrious Dr- Black-   The
 h—m^ ^tzt^'zran early result of kindred—

 We select the following instances as serving to display the subject of latent
                       neat in its various bearings.

   A mass of ice brought into a warm room, and there  receiving  heat from
 every object around it, will  soon reach the temperature of mefiina or 32°
 but afterwards both the ice and  the water formed from it will continue  a
  hat  emperature  until all be  melted ;-the  heat which continues to enter
 effecting a change only in the form of the mass.  And in the case supposed
 whatever time was required  for heating the mass of ice one degree, just one
 hundred and forty times as  much will  be required for melting it; proving
 that 140° is the latent heat of water.                             i     8
  , l^P Similarj flalks' one  filled with *ce at 32°,  and the other with water
 at &Z  , be placed in the same oven or over the same  flame, the water will <rain
 140 degrees  of heat while the ice is nearly being  melted into  water at&32-
 and in the course of the experiment, a correspondence will always exist be-
 tween the phenomena;  for instance, when the water has gained 14° of heat
 it will be  found that just a tenth part of the ice is melted.
   If equal quantities of hot and cold water  be  mixed together, the whole ac-
 quires a middle temperature, each  degree lost by the hot water becoming a
 degree gained by the  cold ;  but if a pound of ice  at 32°, and  a  pound of
 water 140° hotter be mixed together, the 140° of heat will go merely to melt
 the ice, for there will result two pounds of water at 32°.
   If a flask of water at 32°, or its freezing point, and a similar flask of strong
 brine (which  does not freeze until cooled to  near zero) also at 32°, be exposed
 together in the same cold place, it will be found that when the brine has lost
 10° of its  heat the water flask will still exhibit an undiminished temperature,
 but a fourteenth part of  its contents will be converted into ice.  Now as in
 such a case the  water flask must continue to radiate  away heat just as much
 as the other, it can maintain its temperature  only by absorbing into its general
 mass the heat which was latent in the portion of water froze°n.
   It is possible, by slowly cooling water which is kept in perfect repose, to
 lower its temperature,  while yet liquid, ten degrees below its ordinary freez-
 ing point; but then, on  the slighest agitation, ice will be formed.  It might
 be expected, in such a case, that the whole  water would instantly freeze, be-
 cause all is colder than common ice; but no, only a fourteenth part freezes;
 and singularly, both that fourteenth and the remaining liquid are  rendered in
 the moment ten degrees warmer—rising to 32°.  Here the 140° of latent
 heat escaping from  the  fourteenth part of the water which freezes, become
 10° of sensible heat for the whole mass, so  that the remaining water has the
 temperature at which it only begins to freeze.
  Strong solutions  in  hot water of various neutral salts, if allowed to cool
 while exposed to atmospheric  pressure, soon deposit crystals of the salts;
 but in a close  vessel, which protects them from such pressure, they will re-
main liquid even when cold.   Now at the moment of opening such a vessel
to admit the  pressure, the salt  immediately crystallizes, and the  latent heat 
292
HEAT.
 given out by the solidifying particles warms very  sensibly the remaining
 liquid and the vessel.
   From  the preceding facts it may be perceived, that the quantity of ice
 formed or melted in any case, becomes a correct measure of the quantity of
 heat transferred.  From this  consideration, the illustrious Lavoisier con-
 structed his calorimeter, or heat measure.  It is a case  or vessel lined with
 ice, and the quantity of heat given out by any body  placed in it is indicated
 by the quantity of water collected from the melted ice.
   Had the latent heat of water been only 1° or 2° instead of 140°, the earth,
 except in its tropical regions, would have been scarcely habitable.  The cold
 of a single night might have frozen an ocean, and the  heat of a single day
 might have converted the accumulated snows  of a winter into one sudden and
 frightful inundation.   As the fact is, however, both changes are  beautifully
 gradual, and easily controlled or prepared for.
   The fact of latent heat in other liquids than water is familiarly exhibited in
 the  slow melting of various substances, as—of the metals ; lead or pig-iron
 for instance—of butter or oils— of glass, &c; and on the other hand, in the
 slow solidification of any melted masses when heat is again abstracted.
   The substances  below  enumerated,  while passing from the  solid to the
 liquid state, absorb and render latent the quantities of heat here noted; which
 quantities are therefore called the latent heats of the liquids.

              Ice                               -     140°
              Mercury      -        -       -          142
              Bees'-wax       -        -         -     170
              Tin     ....          442
              Zinc     ...         .     492

   If a piece of frozen  mercury (the temperature of which  is at least 40° be-
 low zero) be thrown into  a little water at 32°, the latent heat of the water
 immediately passes into the mercury and melts it; but, singularly, the water
 in the act of melting the mercury, is itself frozen.

                    " Latent Heat of aeriform fluids."

   Water in a vessel placed over a fire gradually attains the boiling tempera-
 ture or 212°, but afterwards its temperature  rises no more, for  the  farther
 addition of heat becomes latent in the steam  escaping during the ebullition.
 One way of determining the quantity  of heat which becomes latent in steam
 is to note how much more time is required for boiling a quantity  of water to
 dryness, than for merely heating it to  the boiling point, or  through any cer-
 tain  number of degrees.  The experiment indicates about 1,000°; that is to
 say, that 1,000 times as much heat is  latent in any quantity of water formed
into steam, as would raise the temperature of the liquid water one  degree.
 Watt had found that water in a  vessel placed over a lamp was about six times
 as long in being completely evaporated, as in being originally heated from an
 ordinary temperature to that of boiling.
   If we place in the same oven, or over similar flames, two like vessels con-
 taining water, one of which is open at  top and the other is strongly closed,
the two  will gain heat equally up  to the boiling  point, but afterwards the
open vessel from giving out steam will remain at the same temperature, while
the other, by confining the  heat which enters, will show the temperature
continuing to rise as before, until  the increasing  tendency of the water  to 
LATENT HEAT.
293
dilate forces the vessel open.  Supposing the water in the latter vessel, be-
fore vent is given, to have become 100° hotter than common boiling water,
instead of the  whole, when  at liberty, being immediately converted into
steam, as might be  expected, only a tenth part will be so changed (the same
quantity as will be found to have already escaped  from the other vessel,) for
the tenth part requiring in the form of steam 1,000° of latent heat, will take
the excess of 100° from the other nine parts, and will leave them as common
boiling water.   If, however,  water heated  considerably beyond  the boiling
point be allowed  to expand  very suddenly, the whole  is blown out of the
vessel  as a mist, by the steam formed at the same instant through every part
of the mass; but the whole mass  in such a case is no more converted into
true steam than the whole of very brisk soda  water is converted into  gas
when  similarly  thrown out by the sudden extrication of the carbonic acid
gas, on uncorking the bottle.  Misconception of this matter  has led to most
wasteful experiments on steam-engines of very high pressure.   Mr. Perkins,
for instance, thought he truly described what was accomplished, by saying of
the water that it  had "flashed into steam."
   The same indication of the latent heat of steam is obtained by the converse
experiment of first converting a quantity of water into steam, and then admitting
it to cold water or to ice.   A pound of steam will raise the  temperature of ten
pounds of cold  water 100 degrees, or will melt about 85 pounds of ice.
   In the great quantity of heat which becomes latent  in steam, we perceive
the reason why water projected upon a raging fire so powerfully represses it:
and hence again why fire and water are so  often adduced proverbially as ex-
emplifying a fierce antagonism.
   It was when Watt had discovered how  much heat was lost when  steam
was lost, that he contrived the separate condenser for his steam-engine, by
which he at once saved three-fourths of the fuel formerly used.
   Substances differ among  themselves in  regard  to the latent heat of their
vapours as much as in their other relations to heat.  Thus the latent heat of the
vapour or steam of:

            Water       -        -       -       -  is 1,000°
            Vinegar          -       -      -       -   900
            Alcohol      -..-       442
            Ether    -              -      -       -   300
            Oil of turpentine      ...       177

   From the less latent heat in these last-mentioned  vapours than in that of
water, we might at first suppose that there would  be great advantage from
using them in steam-engines.   Accordingly numerous experiments have been
made,  and patents secured under this idea;  but the fact is, that  in the same
proportion as the heat is less, the volume of the vapour is less,  and therefore
no mechanical advantage  is obtainable.

The influence of external pressure in keeping the particles of liquids together,
  in opposition  to the repulsion of heat seeking to render their mass aeriform,
  was considered in the chapter on " Pneumatics :" but to make the present
  section complete, the subject must be here shortly resumed.
  Because water or any liquid, under the pressure  of the atmosphere,  while
receiving heat, remains tranquil, and apparently unchanged, until it reaches
what is called its boiling point, at which a bubbling  or conversion into vapour
takes place, we might suppose its  ordinary boiling temperature necessary to
     20 
294
HEAT.
Fie. 131.
enable it, under any circumstances, to assume or to maintain the form of air.
But  this.is no more true than that a common spring compressed against any
obstacle or force, has no tendency to expand or recover itself until the moment
when at last it overcomes  the  obstacle.   Liquid water with its  heat is really
a spring compressed by the powerful weight of the atmosphere, and seeking
to expand itself into steam with force proportioned to its temperature.  Even
at 32°, or its  freezing point, as is found  by placing it  in  a vacuum, it seeks
to assume the form of air, with a force of pressure 1| ounce on each square
inch of its surface, and can be restrained only by a counter-pressure of that
amount;  and at  any  higher  temperature, to correspond with the greater
dilating-tendency, the restraining force  must also be  greater: at 100°, for
instance, it must be 13 ounces; at 150°, 4 lbs.;  at 212°, 15 lbs.; at 250°, 30
lbs., and so on as stated in the preceding part of this work :—and whenever
the restraining force is much weaker than the expansive tendency, the forma-
tion of steam takes place rapidly and far below the surface of the liquid, so
as to produce the bubbling and agitation called boiling.   Now it is because
the atmosphere or ocean of air which surrounds the earth happens to have in
it 15 lbs. weight of  air over  every square inch of the earth's surface, and
presses on all  things  there accordingly, that 212° happens to be called the
                        boiling point of water.   An atmosphere less heavy
                        would have allowed liquid to burst into vapour at
                        lower  temperatures, and one more heavy  would
                        have had a contrary effect.—The exact degree of
                        expansive force for every degree of  temperature in
                        water and other liquids, has been  ascertained by
                        heating them in vessels furnished either with prop-
                        erly  loaded valves, as at/in this figure, or with a
                        tall upright tube, as d b,  into  which the liquid c may
                        force a column of mercury to an elevation marking
                        the  expansive tendency; the valve and  mercury
                        being of course  protected from the  external atmo-
                        spheric pressure, or the  necessary allowance being
                        made for that pressure.   Boiling at  the bottom of a
                        deep vessel is  resisted by the weight  of the liquid
                        in addition to that of the atmosphere, as already ex-
                        plained, and consequently the temperature at which
                 d     it occurs there, is higher than near the surface of the
                        vessel.   Boiling heat is greater also—in  a deep
                        mine, where of course there is additional depth and
                        weight of atmosphere over any exposed liquid,—
at times when the barometer is unusually high, that is to say,  when the atmo-
sphere is unusually heavy—in cases where air or steam is confined over the
boiling surface so as to press  more upon it as when brewers for a time shut
the lid or  valve of their great  boilers, &c.  Water placed on  the fire in a
strong vessel, from which steam  cannot at all escape, may be rendered even
red-hot, without a bubble forming or one particle being  dissipated ;  but the
tendency to expand  into  steam is  then great enough °to burst any known
material  of moderate thickness.   The  Marquis  of Worcester exploded  a
cannon by shutting up water in it, and then surrounding it with fire.—Boiling
temperature is lower again when the experiment is made on mountains or in
other situations above the level of the sea, where there is less height of air
resting over the boiler.  In the city of Mexico, which is 7,000 feet above the
sea, water boils before  it  reaches  the heat of 200°, instead of, as in  places
 
BOILING.
295
near the sea-level, at 212°.   Wollaston's thermometer, beautifully adapted
for  determining the height of mountains, balloon ascents, &c, by merely
indicating the heat of boiling water in any situation, is  a  fine illustration of
this truth.  If in any place we  take off the atmospheric pressure from a liquid,
as by placing it in the receiver of an air-pump, it will boil  at very low tempe-
ratures indeed.  Water thus treated boils at a temperature many degrees below
the  heat of English summer days ; and ether boils  when colder than common
ice.—Generally, in a vacuum, substances boil at  a temperature 124° lower
than while restrained by the atmospheric pressure.

  Consequences of these truths respecting the boiling temperature, are the
                                following.
  As water at any temperature  is tending to dilate  itself  into steam,  with
force proportioned to the  temperature,  the steam rising  from any mass of
water presses on the surface of the vessel containing it with that force;  and
in a steam-engine, therefore, the temperature of the water in  the boiler tells
the degree of force with which the steam is acting on the piston.
  Because in  the case of steam : the same law holds as  for aeriform fluids
generally, viz.,  that the outward elasticity or spring increases  in proportion
as the fluid is  more  condensed—high-pressure steam  is merely condensed
steam, just as high-pressure  air is condensed air ; and to obtain a double or
triple pressure, we must have twice or thrice the quantity  of steam under the
same volume.
  The reason that high-pressure steam issuing from a boiler heated to  300°
or more, is not hotter than low-pressure  steam from a boiler at 212°, is, that
in the instant when the  high-pressure or condensed steam  escapes into the
air, it expands until balanced by the pressure of the atmosphere, that is,  until
it becomes low-pressure steam, and  it is cooled by the expansion, as air is
cooled on escaping from any condensation.
   The vessel called  Papin's Digester, is merely a  metallic pot or boiler,
 which  can be kept closed in spite of the force of the  steam formed within
it;  and in such  a vessel, water can be heated far beyond the ordinary boiling
point,—sufficiently, for instance, to dissolve and extract all the gluten or jelly
of bones, and to form from them a rich soup where common boiling would
procure nothing;—or even to  melt lead  lying in the water.
   The person  who increases the fire under a boiling pot with the hope of
 making the water hotter, is foolishly  wasting the fuel, for the water can only
 boil, and  it does boil at 212° of the thermometer.
   As different substances,  under any  given pressure, become aeriform at dif-
 ferent  temperatures,  mixtures of such may be decomposed  by heat.   If a
 mixture of spirit and water, for instance, be placed over a fire, the spirit will
boil off long before the water.  If the spirituous vapour be caught apart and
condensed, Ihe  operation is called Distillation.   All distillations are of the
 same nature.
   The instrument here represented consists of a glass tube blown into bulbs
 at the  two ends a and b, and  hermetically sealed
 after receiving into  it  some water,  but no air.           Fig. 132.
 There will always be  in  the apparently empty
 part a  stream  or aeriform water of density pro-      ^_s?_          <*
 portioned to the temperature.  If one  of the bulbs    (^ <~
be  heated  more than the other, the steam or va-    {^_J
 pour in that one will, for the reasons stated above,
be  denser and stronger than in the other, and will
 
296
HEAT.
therefore be forcing its way into the other;  where, owing to the lower tem-
perature, a part of it will  be relapsing into the state of  water, making room
for more.   Hence, if the difference of temperature between the bulbs be long
maintained, the whole water will, by a sort of distillation, gradually pass into
the colder bulb.   If the difference of temperature become at any  time con-
siderable, the liquid will boil in  the warmer bulb, even  although the  source
of heat be only the living  hand grasping it.
   To the author of this work it appears that by a larger apparatus made on
this principle, fresh water might be conveniently obtained from salt-water on
board ship, or on islands  deficient in  fresh springs.  Suppose  any two air-
tight vessels like a and  b, of large size, communicating by a tube  furnished
with a stop-cock near 6 : then  if the vessel a  were filled with salt-water, and
were heated by being exposed to the sun, (its surface being blackened to ab-
sorb heat, and protected by glass  from  the cooling effect of the air,) and if the
other vessel b were  made a vacuum  by pumping  out  from its bottom the
water with which it had been  previously filled, and were  then  kept as cold
as possible by wetted coverings and a current of air,—on opening the cock
at b, vapour would pass over from the warmer vessel to be condensed in the
colder, and there would be a distillation from sea-water by  the natural action
of the sun alone, of  a  water perfectly fresh and  pure.   Cases have occurred
where  a knowledge of this  fact  would have  saved shipwrecked crews from
perishing by thirst; and there are rocky islands in  the ocean where  there is
no  supply of fresh water but from precarious rains or importation from abroad,
but which might be rendered pleasantly habitable by the adoption  of such a
means.
   When a substance has reached the  temperature at which it boils, that is to
say, at which its power of emitting vapour becomes rather more than a balance
to the atmospheric pressure, its dilating force  is  strong indeed. Persons may
not reflect  that 15 lbs. on a square inch is  about a ton on a square foot,—
and such is  the power with  which the vapour of all boiling substances rises
from them—sufficient in a single Cornish  steam-engine to urge the piston
with the force of 600 horses !    But even at temperatures much below boil-
ing, the tendency to expand, as already stated, is still very great, and although
not attracting common attention, is  silently working many  beautiful and im-
portant ends in the economy of nature.—As  into  a  perfect vacuum, freezing
water gives out a steam or vapour that would lift  an opposing weight with
force of I3  ounce per inch, or 16 lbs. on a square  foot; and even solid ice
gives out its vapour of nearly equal strength,—so also do many other liquids
and solids give out their vapours.  Thus in the apparently empty space called
the Torricellian  vacuum,  over the mercury in a barometer tube, there  is
always an  aeriform mercury, dense in proportion to the  temperature ; and
around camphor, and  the  essential or  volatile oils, &c,  there is similarly an
atmosphere of the substance in the form of air.
   It had  for a considerable time been known  that into  a perfect vacuum
many bodies emitted almost instantly in the  form of air, a quantity of their
substance proportioned to their  temperature; but  it was  reserved for Mr.
Dalton to make the admirable  discovery, that even into  any spacefilled with
air, these vapours arise in quantity and density the same as if air were not pre-
sent—the two fluids seeming to  be independent of each other, with the ex-
ception that in a vacuum the equal diffusion of a vapour takes place at once,
while in a situation  already occupied by air,  it proceeds more slowly as the
vapour can force its way through the particles of the air, and in general takes
place by a tranquil  evaporation  from  the surface instead of the agitation  of 
EVAPORATION.
297
ebullition.   In an apartment with an open vessel of water in it, there is soon,
although invisible, a steam of watery vapour mingled with the air,  as  dense
as if the room were a vacuum at the same temperature.

          Consequences of this important truth are the following.

  That it is only an atmosphere of the  substance  of  each body, which, by
pressing on the body, can prevent its farther dissipation by heat.   Thus we
can save camphor, musk, smelling oils,  spirits, water, &c, only by placing
them in closed bottles or vessels, in which, additionally to the air present, an
atmosphere of their own substance is formed, involving the remaining masses
with pressure proportioned to their temperature and its density.
  The important process of drying things is merely the placing them under
an elevated  temperature if attainable, and in an atmosphere not containing so
much of the liquid as to be saturated at the temperature.  The effect of wind or
motion of the  air in quickening evaporation, is owing to its removing air satu-
rated with the moisture, and  substituting  air which is not—thus producing
nearly the case of the substance placed in a  vacuum.
  If air at a certain temperature, contain mixed with it as much water as can
be sustained in the form of invisible vapour at that  temperature, and if then,
by any cause, as by rising in  the atmosphere, the air be cooled, it will abstract
heat from the  vapour and cause a portion to be precipitated or visibly con-
densed into a fog or rain.   Water rising as invisible vapour from the surface
of a lake or river, often,  when it has reached a certain height, is condensed
into the stratum of  clouds which there appears, and which for  a time may
remain usefully protecting the fields from  the intense  meridian sun, or may
fall again as refreshing showers over the country.
  It is the tranquil and invisible evaporation of which we are now speaking,
which lifts from the surface of the wide ocean all  the vvater which, after con-
densation, returns to the ocean in the form of the myriads of river streams
which give life and beauty to the face of nature.
  In warm climates there are inlets of the sea, occasionally shut off from the
parent ocean, and where, after the sun's rays have drunk up all the water, the
deposited salt  remains to be carried away in loads for the uses of man, as
sand is carried from any ordinary shore.  There are  in the bowels  of the
earth prodigious accumulations of salt, some of which may have been formed
in this way, during  the  revolutions  of the  world in  remote past time, and
which are now turned to man's account as salt-mines.   When the Nile over-
flows its banks with water holding in solution, although in almost impercepti-
ble proportion, mineral substances brought from the interior  of  Africa, some
of that water admitted into reservoirs, and afterwards dried up  by the  sun's
heat, leaves a rich store chiefly of crystallized natron or soda.
  The following are other instmces of vapour which is invisible while at a
higher temperature, but  is thickly precipitated when  air, with  which it is
mfxed, is cooled, or when it touches a colder solid body  :—the steam  observed
at night and morning hovering over brooks  and marshes heated by the sun
during the day:—the frost-smoke, as it  is called, which  lies on the whole face
of the Greenland seas in the beginning of winter, where  the water  warmed,
by the lono- day of the polar summer, continues to emit its vapour for a con-
siderable time  after summer is past, into an atmosphere, become too cold to
preserve it invisible:—the breath or perspiration of animals, of horses in par-
ticular, after strong exertion, becoming so strikingly visible in cold and damp
weather, or even°in warm weather, when  the air is  already charged with 
298
HEAT.
moisture:—in cities where there are deep drains communicating with kitchens,
manufactories, Sic, and constantly filled with moist and warm air, the vapour-
loaded air, although clear or transparent in the drain immediately on escaping
into a  frosty  atmosphere lets go  its  moisture, with the appearance of steam
issuing from  a great subterranean  caldron.   Steam over  water in any boiler
is transparent or perfectly aeriform—as may be seen when water is made to
boil in a vessel of glass, but as soon  as it is  cooled  by contact  or admixture
of colder air, it ceases  to be true steam, and  is condensed into small particles
of water suspended in the air.  Many persons, while thinking of steam, figure
it only in this latter state, as particles of water mixed with air nearly as a
subtile powder might be mixed, and  its substance occupying really no more
space than the original  water did.   Now until steam is cooled and condensed,
it is of a nature to fill alone any appropriate vessel and  powerfully distend it,
just as air fills and distends a bladder.   Steam issuingfrom the spout of a kettle
is hardly seen near the mouth, but as its distance from  the spout increases, it
is cooled into a thick cloud or vapour.
  In a vessel from which air and atmospheric pressure are excluded, even the
temperature of freezing water being sufficient to  maintain permanently in the
state of gas or air, many substances  which exist as  liquid under  the atmo-
spheric pressure,—and the whole mass of such a substance, when placed in a
vacuum, not  being instantly converted into  gas because the portion which
first rises becomes an atmosphere weighing upon the remaining  mass, and
because, moreover, that portion, by absorbing from the mass much heat  into
the latent state, cools the mass much  below the freezing point;—we see why
the liquids now spoken of are so rapidly cooled to at least the freezing point if
placed where a vacuum can be maintained, that is to say, where, after com-
mon air has been removed, the aeriform matter rising from them, and absorb-
ing their heat, is  also promptly and in a continued  manner abstracted.  It is
thus that water placed in the exhausted receiver  of an air-pump is so rapidly
cooled, and that when there is beside it  a vessel of concentrated  sulphuric
acid, or other substance capable of absorbing the watery vapour as formed, it
is soon reduced to the  state of ice;  or again, that water, or even mercury,
surrounded by ether evaporating in  a vacuum,  is so  quickly frozen.  It is
thus also that if one bulb of  the instrument described atpage 296, be immersed
in a freezing  mixture, the water in the other and distant bulb will soon  be-
come ice;  for the vapour rising from that water into the vacuum maintained
throughout the apparatus by the freezing mixture, is immediately condensed
again in the immersed bulb, and leaves the vacuum still free for the ascent of
more vapour, to carry away more heat from  the  water  as latent heat, and to
make it freeze.
  As we have explained, also,, that in  a liquid there is the same tendency to
evaporate,  whether it be or be not exposed to the air, we see  the reason why
all  evaporation is a very cooling  process.  The effect, however, in air, is
neither so rapid nor so great as in a vacuum ;  first, because the presence of
the air impedes  the spreading of the newly-formed vapour  from the liquid
surface, and keeps it where its  pressure resists the formation of more vapour;
and, secondly, because  the  air in contact with  the  liquid, shares  its higher
temperature  with the  liquid.   Still in India flat dishes  of vvater, placed
during the night on beds of twigs and straw kept wet and in a current of air,
soon exhibit  thin  cakes of ice—and thus, ice  is procured in India for pur-
poses of luxury.
  The absorption of latent heat in the evaporation which goes on from  the
sea and earth in all warm climates, greatly tempers  the  heat of these climates, 
EVAPORATION.
299
and the vapour afterwards spreading to the poles, as explained in "Pneuma-
tics,  under the  head of Winds, carries warmth  thither to be given  out
when it is recondensed into the form of rain, or is solidified as snow.   The
formation anywhere of mist or rain  warms the air most sensibly, by the
liberation of the latent heat  from the precipitated vapour.  Again, the liquid
water  which,  during winter, is  converted into  snow or ice  had been a
reservoir of latent heat  stored  to temper  the  frosty air of the commencing
cold season; and in the following spring, such ice and snow serve  as empty
receptacles, in which the first violence of the returning sun hides or expends
itself;  allowing  the  temperature  to change  more gradually,  and for many
living  beings,  therefore,  more  safely.   The  vast  stores  of  ice and  snow
among high mountains, as among the Alps  and  Pyrenees, are often, during
the summer, stores of mild temperature to regions around: for besides cool-
ing the air near them, they are  the never-failing sources  of the rivers which
run from them during the whole of summer, carrying freshness through dis-
tant lands:—from the Alps,  for instance, proceed  the Rhine and the Rhone
—most romantic and beautiful of European streams; and from the Pyrenees,
the rapid Gave,  &c, which,  while channels around from lower regions are
almost dried up by the summer heat, flows only the more freshly as the heat
is greater, and the feeding snows are more abundantly dissolved.

Men in artificially raising temperature, are generally causing the liberation of
   heat which  had been previously latent; and in  lowering  temperature or
   producing cold, they effect  their purpose almost solely by  rendering a
   quantity of  heat latent.
   Lavoisier thought that the heat of all combustion was merely  the latent
heat of the  oxygen  gas  concerned in the combustion, given out during its
combination with the burning body.   It is so in part, but we now know that
it depends more on the intensity of the chemical action between the combin-
ing substances.   The water thrown  upon quick lime to slake it,  becomes
solid in combination with the lime, and gives out  its latent heat so remark-
ably as often to set fire to a wooden vessel or ship containing  it.
   When dwelling-houses, green-houses, manufactories, &c, are warmed, as
is now common, by the admission of steam  into  systems of  pipes which
branch over them, the heat is chiefly that previously latent in  the steam, and
which spreads around  as soon as the steam, by  touching  pipes of lower
temperature, is condensed to a state of water.  The modes of most  profitably
effecting these purposes have to be considered in a future chapter.
   For producing artificial cold, our processes  generally involve the circum-
stance either of  a solid  changing into a liquid, during which  it absorbs, and
hides in  its  new constitution  much  of the heat previously sensible in it and
in the liquid dissolving it; or of a liquid changing into vapour, during which
heat equally becomes latent.  Thus  by dissolving a salt, nitre,  for instance,
in water, we obtain a solution which is very cold.
   In India, the common  mode of cooling wine for  table is to surround the
bottles with nitre thus melting; and  the water of the solution  being evapo-
rated again  before next day, the salt is left ready for use as before.   Such is
the  mutual  attraction of water and many salts, that they readily combine,
assuming the liquid  form, even when the water is  used  in the solid state of
ice; and as, in that  case, both  the water and  the salt render  heat latent, the
fall  of temperature  is very  great.   Thus common salt and snow (or ice)
when mixed,  dissolve into liquid brine 37° colder than freezing water, or 5°
below the zero  of Fahrenheit.—The last-mentioned fact  explains the  com- 
300
HEAT.
mon practice of sprinkling  salt on an ice-covered pavement before a street
door to  clear away the ice.   The salt and ice  quickly combine and form
liquid brine, which either of itself runs off  into the gutter and disappears, or
is easily swept off, or its water evaporates, leaving only the salt behind.  It
is true that the brine is at first a refrigerating mixture, which cools still more
the pavement and the  neighbouring  ice, but all which is touched by the salt
is melted.  Servants usually err in  using a pickaxe or spade immediately
after the sprinkling, instead of waiting, and with  a broom spreading  the
melting  salt completely over the place.
  The following is a short table of easily procured freezing mixtures:

                         Frigorific Mixtures.
        Substances mixed.                       Thermometer sinks.
Common salt  ...  -  1 part )  From  any  temperature  to 5°  below
Snow or pounded ice -  -  2 — ^             zero.
Common salt  ----5 — ~)t_i                         nc0  u i
c                        to     I  rrom any  temperature to  25°  below
Snow or ice   -   -  -  - 12 — v         J     r
Nitrate of ammonia  -  -  5 — J

tv, .  ,   , ,      ".,"     .,     >  From 32° above to 23° below zero
Diluted  sulphuric acid   -  2 — ^

c  h  P                  0     >  From 32° above to 51° below zero.
Snow......3 — ^
Nitrate of ammonia  -  -  1 — ?  y     ™o .  ia i
■yy  .                      ,     >  r rom 50 to 4° above zero.
Sulphate of soda  -  -  -  8 — 5  ,-,     _no t  _0
Muriatic acid  -  -  -  -  5 - \  Fr0m 50 t0 °  or zer0"
  We have already  described  under  other  heads the frigorific effect of
evaporating in a vacuum or in  the  air, and of the operation of condensing
a gas to  squeeze  the  heat out of it before letting it expand  again  to a  great
volume.
For any given substance, the changes of state from solid to liquid, and from
  liquid to  air, happen under  similar circumstances, so precisely at the
  same temperature, that they mark fixed points in a general scale of tem-
  perature,  and enable us to regulate and compare our various thermome-
  ters.  (See Analysis, page 256.)
  As  we can neither weigh heat, nor  measure its bulk, nor see it, and as,
even if our sense of touch  were a  correct judge in  the  matter, which it is
not, we dare not touch  things which are very hot or cold, some other means
were wanted for estimating  the  presence in  bodies  of this  very subtile prin-
ciple;—and  a mean has been found in the measurement of its most  obvious
and constant effect, namely that dilatation  or expansion of bodies, which
again  ceases when the  heat is withdrawn.   Any substance so circumstanced
as to allow this expansion to be accurately measured, becomes to us a thermo-
meter or measure of heat.
  In solid substances, the direct expansion by  heat is so small as to be seen
or measured with difficulty.  In airs, again, the expansion is very extensive:
but  there  is  the  objection that  in any  apparatus yet contrived, which  will
allow  their expansion completely to appear, they cannot be protected from the
varying pressure of the atmosphere—an influence which affects their volume
even more than common changes of temperature.   But liquids are free from
both disadvantages, and when placed in  a  glass bulb, as  a,  having  a long 
TABLE  OF TEMPERATURE.
301
i o	L 6
 neck or stalk ab, into which the liquid may rise when expanded by heat, to
 be measured, they form the most generally convenient
 ol thermometers.   Then,  among liquids, mercurv is,        Fig. 133.
 on several accounts, singularly pre-eminent.  In mer-
 cury, the range of  temperature  between  freezing and
 boiling reaches a higher point than in any other liquid,
 and a lower point than in all others except alcohol; its
 little capacity for heat and ready  conducting power,
 cause it to be very quickly affected by change of tem-
 perature; its expansion is singularly  equable for equal
 increase of heat through the important middle  part of
 the scale, which includes the common temperatures on
 earth, namely, from freezing to the boiling heat of wa-
 ter; and it is easy to proportion  the bulb  and the stalk
 to each other, so that a small difference of temperature
 shall cause the mercurial column in the stalk to rise or
 fall very conspicuously.
   Now, when the important fact was ascertained that solid water or ice melts
 in every case at precisely the same temperature, and that pure  liquid water
 in a metallic vessel, and under a  given atmospheric pressure, boils always at
 the  same temperature, it followed that by placing  such a thermometer  as
 above described, first in melting  ice and then in boiling  water, and marking
 upon the stalk the  two points at which  the mercury stood, as represented
 here by  F and B, two  fixed or  invariable points would be obtained, and the
 interval between them might be  divided on the  glass, or  on a suitable scale
 to be attached to the glass, into  any convenient number of parts to be called
 degrees: it  followed farther, that by  continuing the divisions to any extent
 both above and below the fixed points, a general scale of temperature would
 be obtained,  with respect to which all thermometers  made on the same prin-
 ciple would  perfectly  agree, although the size  of the divisions on the stalks
 would vary according to the comparative  capacities of the bulb  and stalk in
 the different  instruments.   Our Newton had the honour  first to  propose the
 regulating  points  of freezing  and  boiling, and they are now universally
 adopted, but  the interval between them has been variously subdivided;—that
 is to say, there has not been agreement among  philosophers as to what they
 would call a  degree  of heat.  In the  Centigrade thermometer, which is the
 most simple, the division  is into 100 equal parts;  in  Reaumur's, which  is
 commonly used in  France, it is  into 80 parts;  and in Fahrenheit's, which is
 used  in  England, it is into  180°.  In  Fahrenheit's, moreover, the freezing
 point, instead of being called zero, as in  the others, is  called  32°, because
 the maker chose to begin counting from  the lowest heat which he met in
 Iceland,  and which was  32° below freezing  of his  scale.—To turn the
 degrees of any  one  of these thermometers into degrees of any other, we
 have only to  recollect  that 9° of Fahrenheit are equal to 5° of the  Centi-
 grade, and to 4° of Reaumur.   Therefore, multiplying by 9 and  dividing by
 5 or 4, or the  reverse, and adding  or subtracting the 32° of  Fahrenheit,
 gives, as the  result, the degree desired.
   The bulb of a mercurial  thermometer  is formed by heating to  fusion in
 the flame of  a lamp, the end of a glass  tube, which has  a very small  and
 equable bore, and then blowing into  the tube  until the softened end swells
 like a soap-bubble, to the size desired.  The mercury is forced  into such a
 bulb through  its long stalk by the pressure of the atmosphere,—thus:  First,
 a portion of  the air  originally in the bulb  being expelled by warming the
bulb, the open end of the stalk is immersed in  mercury, and  when the air 
302
HEAT.
remaining in the bulb cools and contracts, a little mercury enters.  Secondly,
this admitted mercury having been made to boil, so as to fill with its vapour
the whole capacity of the bulb and tube, and to expel the air, on the open
end being again immersed in mercury, and the mercurial vapour  within being
condensed, the  atmosphere  presses  in  fresh  mercury to fill the  whole
vacuum.  To  complete the making  of the  thermometer, the bulb  is again
heated to expel so much of the mercury as  that when  cold,  the tube shall be
about one-third full of  it,  and then,  before  the heated mercury begins to
recede, the end or  opening is hermetically closed  by directing upon  it  the
point of a blow-pipe flame, which fuses the glass.
  Although  the  direct  expansion of any solid  body by a moderate change
of temperature is so inconsiderable  as to  be with difficulty measured, M.
Breguet, of Paris,  lately  with  much  ingenuity contrived  a  thermometer
which makes it very evident.   Having soldered  side by side two very small
flattened wires of silver and platinum, or of any other metals having  different
expansibility by  heat, he found that  all changes of temperature made such
compound wires bend to a great extent, as a sheet of damp paper  curls on
being held before the fire.  The metal most shortened or  least lengthened
acting like a bow-string  to pull the  other into the arched form; and he then
found, on giving to the  compound wire  a spiral  or  cork-screw form,  and
securing the upper end of it to a fixed stand, while the lower was left free to
move, that an index like the hand of  a  watch attached  to the lower  end  was
turned completely round by a certain  change of temperature, and  afforded
on  a circle of  degrees placed like  a  watch  face  below it, indications  which
perfectly agreed with those of good mercurial thermometers.  Other modifi-
cations of the same principle have since been successfully tried,  so simplified
and reduced in bulk as to be introduced into the structure of a pocket watch.
  Air is a substance on  several accounts admirably adapted to the formation
of a thermometer;   for it has great extent of dilatation from small increase of
heat; it  quickly receives  impressions, and its dilatation is equal for equal
increments of  heat at all  temperatures:—but, as  already stated, there  is  the
strong objection that the pressure of  the atmosphere cannot be excluded,
without at the same time confining the air, and effecting its expansion.   Mr.
Leslie, however, has used for particular purposes an air thermometer,  which
he calls the differential thermometer.    It consists of two bulbs a and b, filled
                  with  air  and connected  by  a bent tube d c,  containing
                  liquid, the instrument being  hermetically sealed, so  that
                  the atmosphere cannot affect the air within.  The greater
                  heat in the bulb  b  than in the other, as when that bulb is
                  touched by the warm hand or is exposed to the sun's ray,
                  is marked  by the descending  of the liquid  in the tube d,
                  which has a scale attached to it.—We may observe  that
                  equal divisions or degrees marked  on  the scale of  this
                  thermometer, do not mark equal  changes of temperature,
                  for the increasing condensation and resistance of the air in
                  the cold bulb  require the force overcoming it progressively
                  to increase.  If the resistance, on the contrary, were un-
                  varying, as in an air-thermometer open to a steady atmo-
                  sphere, equal extent of  motion in  the fluid would mark
                  equal increments of heat.  An air-thermometer made  of a
                  simple bulb and long stalk of semi-transparent porcelain,
                  with the mouth downwards, and containing in  its neck
                  melted lead or other fusible metal instead of mercury, is
 well adapted for measuring very high temperatures.
Fie. 134.
?   jQ*
 
TABLE OF TEMPERATURE.
303
 alcoho7wh ch s^h!     v."' °f freeZ,"S mercur* ™ usually measured by
 £of hS,.'ch Substance has not yet been frozen;  and temperatures  higher
 1,  ahnvP nTg muTUy are  meas^ by the expansion of air or of metals,
  \ Tt,  7   ,' ,°r b^ the ^nfaction of pieces of baked clay, which,
 when highly heated, lose water and become semi-vitrified.  The use of baked
 clay  was proposed  by Wedgewood, and the apparatus has been  called
 Wedgewood s Pyrometer, or fire-measure.   All contrivances for measurine
 heat may be graduated so as to correspond with the  scale adopted for the
 mercurial thermometer.
   It is  most interesting while considering the vast number and importance
 of  the phenomena produced by heat, to observe the  degrees in the general
 scale of temperature  at which these severally take place.  In the following
 table, a selection of the facts is classified, the temperatures beino- all referred
 to the scale of Fahrenheit's thermometer.

 Table of facts connected with the influence of heat corresponding to certain
                              temperatures.

                                                Wedgewood.  Fahrenheit.
 Highest temperature measured   ...      240°       32,277°
 Chinese porcelain softened  ...           156        21,357
 Cast iron thoroughly  melted   ...      150        20,577
 Greatest heat of a common smith's forge    -         125        17,327
 Flint glass furnace       -       -             -      114        15,897
 Stoneware baked in   -     -       ...      102        14,337
 Welding heat of iron     ...          90 to 95        13,427
 Delft ware baked in   -            -       -           41         6,407
 Fine gold melts        ....         32         5,237
 Settling heat of flint glass   -            -       -       29         4,847
 Fine silver melts    -       -      -       -      -    28         4,717
 Brass melts   -        -       -     -       -         21         3,807
 Full red heat (the beginning of Wedgewood's
       Pyrometer) -----     0         1,077
 Heat of a common fire         ....        .       790
 Iron red in the dark     -          ....           750
 Quicksilver boils   -       -         ...        .       660
 Linseed oil boils       ...        .       .            600
 Lead  melts   ...       -          -      -        -       594
 Sulphur melts  -      -      -        -       -       -            226
 Water boils       ...         .       .        -       212
 A compound of three parts of tin, five of lead, and eight of bismuth
       melts      .....          -       210
 Alcohol boils         -       ...        .             174
 Bees' wax melts          -       -       -       -                 142
 Ether boils     -              ....               98
 The present medium temperature of the globe      -                   50
 Water freezes  -             ....               32
 Milk freezes      .....          -        30
 Vinegar freezes      .....               28
 Strong wine freezes      ....                   20
 Weak brine  freezes -         ....    zero       0
Quicksilver freezes       ...        .     below zero    40
The air  sometimes at Hudson's Bay  -       -       -               50
Greatest artificial cold  yet measured     -         -                  91 
304
HEAT.
   There is reason for thinking that  the higher temperatures noted  in this
table are considerably too high, owing to the insufficiency of the thermometer
or pyrometer (Wedgewood's) by which they were estimated.
   It is a curious inquiry, suggested by contemplating the preceding table,
how much heat may yet remain in bodies at the lowest temperature which
we know?  No conjecture was hazarded on the subject until Dr. Irving thought
it  might be elucidated by comparing the quantity  of heat which becomes
latent in a body on  changing form, with the capacity of the body before and
after the change.  For instance, with respect to water, he said: as it requires
one-tenth more heat to make a certain change in the temperature of water
than of an equal quantity of ice, probably ice-cold water contains  altogether
just one-tenth  more heat than of an equal quantity  of ice at the melting point:
then as we know the water to contain exactly 140° more heat than the ice, viz.,
its latent heat, the whole or absolute quantity of  heat in it will be ten times
140°, or 1,400°.  By applying this reasoning, however, to  other substances
than water, it evidently is fallacious ;  and the conclusion follows that we have
as yet no means of  solving the question ;—the  thermometer no more telling
us the absolute quantity of heat in any body than the rising and falling of the
tide between any  two rocks tells us the total depth of the rocky chasm.
   From  what  is said in the last and  in preceding paragraphs, it  is evident
that the thermometer gives very limited information with respect to heat: it
merely indicates,  in fact, what  may be called the  tension of heat  in bodies,
or the tendency of the heat to spread from them.  Thus it does not discover
that, a pound of water takes thirty times as much heat to raise its temperature
one degree as a pound of mercury ; nor does it discover the caloric of fluidity
absorbed when bodies change  their form, and which is called "latent heat"
only because hidden from the thermometer; nor does it tell that there is more
heat in  a gallon of  water than in a  pint;  and if an observer did  not make
allowance for  the increasing rate of expansion with increasing temperature,
in the substance used as a thermometer, he would believe the increase of heat
to be greater than it is;  and lastly, when a fluid is  used as a thermometer,
the expansion  observed is only the excess of the expansion in  a  fluid over
that in the containing solid, and subject to the irregularities of expansion  in
both substances;—all proving that the indications  of the thermometer, unless
interpreted by  other circumstances and our knowledge of the general laws of
heat, no  more  disclose the true relations of  heat to  bodies, than the money
accidentally in a man's pocket  tells his rank and riches.

"Heat, by its different relations to different substances, has a powerful influ-
     ence on their chemical combinations."  (See Analysis, page 256.)

  By observations made and recorded through past ages, man has  now come
to know that the substances constituting  the  world around  him, although
appearing to differ in their nature, almost to infinity, are yet all made up of
a few simple elements variously combined;  and he  has  discovered that the
peculiar relations of these elements to heat, particularly their being unequally
expanded by it, and their undergoing fusion and vapourization °at diflerent
temperatures,  furnish him with ready means of separating, combining, and
new-modifying them to serve to him most useful purposes.  Where the primi-
tive savage, looking around on  rocks and soils, saw in their diversified aspect
almost as little  meaning as did the inferior  animals which participated with
him  the  shelter of the wood or cave,  his descendant, with penetration sharp-
ened by science,  descries  at once the  treasures of  the mine, and aided by 
ARSOLUTE QUANTITY OF HEAT.
305
heat, whose wonderful energies he has learned to control, pursues through
all the Protean disguises of ores, and salts, and solutions, each of the wished-
for substances, until he secures  it apart.   For instance, in what to his fore-
fathers for thousands of years appeared but a red dross, he knows that there
lies concealed the precious iron—king of metals!  and soon  forcing this in
his ardent furnace  to assume  its metallic form, he afterwards, with imple-
ments  made of it, moulds all other bodies  to  his will:  the  trees  from the
forest,  and  the rocks  from the quarry, in obedience to these, are  fashioned
by him as if they were of soft clay, and at his command rise into magnificent
structures of  palaces and ships, with which  the earth and the ocean are now
so thickly covered.—The minute detail of the  relations to heat of  particular
substances forms a great portion of the department of science called chemistry
(a name taken from an Arabic word signifying fire;) but a general  review of
the subject belongs to this work.
   The  most common ores of metals are combinations of the metals with
oxygen, carbonic acid, or sulphur, substances  all of which are volatilized at
much lower  temperatures than  the metals.  Now simple roasting, as it is
called, or strongly heating the ores, suffices often to  drive away great  part
of these adjuncts;  and where additional assistance is required, it is obtained
by mixing with the ore something which when heated  attracts the  substance
to be expelled more strongly than  the metal does.   Charcoal, for instance,
heated with oxide-ore, takes the oxygen, and  flying off  with it as carbonic
acid, leaves at the bottom of the furnace or crucible the vivified or pure metal.
   Mercury mixed with the dross of a mine, dissolves any particles of gold
or of silver existing in the dross, and the ingredients of  the solution may
afterwards be obtained separate by mere heating—the mercury passing away
as vapour to where it is cooled and again condensed for subsequent use, and
the more fixed gold or silver remaining in the vessel—and just as in all other
distillations, like that  of spirit from  wine, or  of essential oils from water, &c,
there is the separation by heat of a more volatile from a less volatile substance.
The only difference between what  is called  drying by heat and distilling, is,
that in the one case the substance vapourized, being of no use, is allowed to
escape or become dissipated in the atmosphere; while  in the other, being
the  precious part,  it  is caught  and condensed  into the liquid form.  The
vapour from drying linen, if caught, would be distilled water.   The abundant
vapour from wheaten bread while being baked, is chiefly a spirit like what is
obtained from malt, and, by an ingenious  apparatus lately contrived, may be
caught and preserved.
   A piece  of cold  charcoal lies in the  air for any length of time without
change:  but if heated to a certain degree, the mutual cohesion of its particles
is so weakened,  in  other words, the particles are so repelled and  separated
from each  other, that their attraction  for the  oxygen in  the air  around is
allowed to operate, and they combine with that oxygen, so as to produce
the phenomenon  of combustion.  The same  is true, under similar  circum-
stances, of almost any dry vegetable or animal substance, and of several of
the metals.                                                          .  .
   Nitre, sulphur and charcoal, while cold, may be mixed together most inti-
matelv without any change taking place; but if the mixture, or any part of
it, be heated to a certain degree, the whole explodes with  extreme violence,
for it is gunpowder.   By the change of  temperature, and  the consequently
altered relative attractions of the different substances, a new chemical arrange-
ment of them then  takes place with the intense combustion  and expansion.
which  constitute  the explosion. 
306
HEAT.
  Sea sand and soda very intimately mixed, and even ground  together, if
remaining cold, remain  also merely an opaque and useless  powder:  but if
the mixture be heated, to diminish the cohesion of the particles of each sub-
stance to those of its own kind, so that the mutual attraction of the two sub-
stances may come into  play, the two substances melt together, and unite
chemically into the beautiful  compound called glass; a product  than  which
art has formed none more admirable—which, in domestic  use  alone, is fash-
ioned into the brilliant chandelier and lustre, into the sparkling  furniture of
the side-board, into the  magnificent mirror plate, and  when extended  across
the window opening, admits the light while it repels  the storm.
  Perhaps the influence of temperature on chemical  union is nowhere more
remarkably exhibited, than in  retarding or hastening the  decompositions of
dead  vegetable and animal substances.   The functions  of life bring  into
combination, to form the various textures of these organic  or living bodies,
chiefly four substances, viz.,  carbon,  or  coal;  the ingredients of water, or
oxygen and hydrogen;  and lastly, nitrogen—which substances,  when in the
proportions found in such bodies, have but  slight attraction for each other,
and all of which, except the carbon, usually exist as airs.  Their connection,
therefore, is easily subverted ; and particularly by a  slight change of tempe-
rature, which either so  weakens their mutual hold as to allow new arrange-
ments to be formed, or altogether disengages the  more volatile of them.—At a
certain temperature, a solution  of sugar (which consists of the three substances
first mentioned, carbon, oxygen and  hydrogen)  undergoes a  change  into a
spirituous  wash, from which spirit or alcohol  may then be  obtained by dis-
tillation:  but if the heat be continued under certain circumstances the liquid
undergoes a second change, or new arrangement of constituent particles, and
becomes vinegar: under still other circumstances, it undergoes  a third change,
which is a destructive decomposition, or rotting, as we call it, and the oxygen
and hydrogen ascend away as airs.  But  sugar, and  many  similar vegetable
compounds, preserved at a low temperature, remain unchanged for ages.
   Again, as regards dead animal substances, we find that although at a certain,
not very elevated,  temperature, they  undergo that change in the relations of
their elements which we call putrefaction, during which  nearly their whole
substance  rises again to form part of the atmosphere, still at or below the
temperature of freezing water, they remain unaltered for any length of time.
 In the middle of summer, recently caught  salmon, or other  fish, packed in
boxes with ice, may be  conveyed fresh from the most remote parts of Britain
 to  the capital.  In our warmest weather,  any  meat or game may be  long
 preserved in an ice-house.  In Russia, Canada, and other  northern countries,
 on the setting in of the hard frosts, when food for the cattle and poultry is
 with difficulty procured, the inhabitants kill their winter supply, and store up
 their provender of frozen flesh or fowl,  as in other countries men store that
 which is  salted or pickled.—The most  striking illustration which we can
 adduce of this kind is the fact, that on the shore of Siberia, in  1801, in a vast
 block or island of ice, then accidentally broken  and partially melted, the car-
 cass of what has been  called the antediluvian elephant was found, perfectly
 preserved—an elephant differing materially from those now existing on earth,
 and  having a skeleton exactly similar to  the  fossil specimens found  deep
 buried in various countries.   The carcass was soon discovered by the hungry
 bears of the district, which were seen eagerly feeding on its flesh, as if it had
 died but yesterday, although it must have been of an  era long anterior to that
 of any existing monument on earth, of human art, or even of human being.
 After it had fallen  from the ice to the sandy beach, and  its tusks  had been 
INFLUENCE OF ANIMATED BEINGS.
307
 heen'lvrZ/01" "le K* Tungusian  fisherman, and much of its flesh had
 Xe feet  and'it" I1"'31181 fr°m *?' Petersb»rgh wh° visited it found an ear
 still perlect  and its long mane, and part of its upper lip, and an eye with the
 pupil, which had opened on the glories of a former or younger world ! About
 301 lb*, weight of its hair, which had been trodden into the sand by the bears
 while eating the carcass, and  part of the skin,  were preserved, and are now
 distributed in different  museums of natural curiosities.   A  piece of the skin
 with the hair upon it is to  be seen in the museum of the London College of
 Surgeons.                                                         6


 " Heat has powerful influence also on animated nature, both vegetable and
                 animal."  (Read the Analysis, page 256.)

   As the detail of the  relations of heat to particular inanimate substances
 belongs to the  province of chemistry, so does the detail of its  relations to
 particular living vegetables and animals belong  to the department of Phy-
 siology; but a general  review of the subject is required in  a treatise  on Na-
 tural Philosophy.
   The influence which heat exerts on  inanimate nature, is, by the common
 mind, more immediately and completely perceived than its influence on beings
 which have life.  Thus, to all  it is obvious, that the contrast between a winter
 and summer landscape, is owing chiefly to the effect of heat on the water of
 the landscape;—that during the absence'of heat, there is the dry barren deform-
 ity of accumulated ice and snow, covering every thing, the roads impassable,
 the rivers bound up, perhaps hidden, the air deprived of moisture, and loaded
 often with powdery drift:—but that  on heat  returning,  the gliding streams
 again appear, the cascades pour, the rills murmur, the canals once more offer
 their bosoms to the boats of commerce,  the lake and pool again show their
 level face, reflecting the glories of  the heavens, and the genial shower falls
 upon the bosom of the  softened earth, become ready to receive the spade  or
 the ploughshare.   But this change is not at all  greater  than what happens
 to a winter tree  acted upon  by the warmth of spring.—Again, it may  be said
 with truth, that heat applied to the cold boiler of a steam-engine, is the cause
 of all  its succeeding motions;  of  the  heaving of its beam and pumps, the
 opening and  shutting of its valves, the turning of its wheels, and its ultimate
 performance  of any work, as of spinning, or weaving, or grinding, or propell-
 ing vehicles by land and water; but as truly may  it be said, that heal coming
 to a seed which  has lain cold for ages, is the cause of its immediate germina-
 tion and growth ; or coming to  a lately frozen  tree is the cause of the rising
 of its sap, the new budding and unfolding of  its leaves  and blossoms, the
 ripening of its  fruits.   And what  is  true of one seed or tree, is true  of the
 whole of the  vegetable creation.   When the warm gales of spring have once
 breathed on the earth, it soon becomes covered, in field and in forest, with its
 thick garb of green, and soon  opening flowers  or blossoms  are everywhere
 breathing back again a  fragrance to heaven.—Among these  the heliotrope is
 seen always turning its  beautiful disc  to the sun, and many delicate flowers
 which open their leaves only to catch the direct solar ray, closing them often
 even when a  cloud intervenes, and certainly when the chills of night approach.
 On the sunny side of a hill, or in the sheltered crevice of a rock, or on a garden
 wall, with warm exposure, there maybe produced grapes, peaches, and other
delicious fruits, which will  not grow in situations of  an opposite character—
all acknowledging heat as the immediate cause, or indispensable condition, of
vegetable life. 
308
HEAT.
  And among animals, too, the effects of heat are equally remarkable.  The
dread silence of winter, for instance, is succeeded  in  spring by one  general
cry  of joy.   Aloft in the air the lark is everywhere caroling; and in the
shrubberies and woods, a thousand little  throats are similarly pouring forth
their sono-s of gladness—during the day, the thrush and blackbird  are heard
above the  rest, and  in the evening  the  sweet nightingale: for all  birds  it
is the season of love and of exquisite enjoyment.   And it  is equally so for
animals of other kinds: in favoured England, for instance, in April and May,
the whole face of the country resounds with lowings,  and bleating, and bark-
ings of joy.  And  even man, the master of the whole, whose mind embraces
alf times and places, is far from being insensible  to  the change  of  season.
His far-seeing reason of course draws delight from the anticipation of autumn,
with its fruits°; and his benevolence rejoices in the happiness observed among
all inferior creatures;  but independently of these considerations, on his own
frame the returning warmth exerts a direct influence.  In his early life, when
the natural sensibilities are yet fresh and unaltered  by the habits of-artificial
society, spring to man is always a season of delight.   The eyes brighten, the
whole countenance is animated, and the heart feels as  if new life were come,
and has longings for fresh objects of endearment.   Of those who have passed
their early years in the country, there are  few who, in their morning walks
in spring, have not experienced, without very definite cause, a kind of tumul-
tuous joy of  which the natural expression would have been, how good the
God of nature is to us !   Spring, thus, is a time when sleeping sensibility  is
roused to feel that there lies in nature more than the grosser sense perceives.
The heart is  then  thrilled  with sudden ecstacy, and wakes to aspirations  of
sweet acknowledgment.
  Besides the effects of heat now mentioned,  and  which are comparatively
transient as being connected with the seasons, there are other effects  on  ani-
mated nature of a  more permanent character.   Certain species of vegetables
and animals,  by their relation to heat, are confined  to certain latitudes or cli-
mates ; as the orange tree and bird of paradise, to warm regions ; the  fir tree,
and arctic bear, to  those that are colder;—and when individuals of either class
can support diversity of climate, they acquire a certain character  according
to the climate—as seen in the sheep and dogs of the  various regions of the
earth.   In this latter respect there is no instance more interesting than that
furnished by the varieties  of the human race.   If  we assume that the whole
sprung from  one  stock, what a contrast is there between the native of equa-
torial Africa,  of temperate Europe, and  of  the Polar zone :  between the
Negro, the Greek, and the Esquimaux: or again,  between  the  dark  slender
children  of Hindostan, the strongly-knit fairer Roman or Spaniard,  and the
taller,  ruddy, powerful Briton.   And in  the  female  sex of the  last-named
countries,  we may remark the  gentleness and singular devotedness of the
Indian woman, the more commanding dark eye and gesture of the  graceful
nymph of Italy or Spain, and the happily  attempered  mixture of qualities  in
the fair and much-favoured daughters of Britain.
  The very important influence of heat  upon the  temporary bodily  state  of
animals, becomes  an object of much study to the physician.  It explains,
among many other facts, the connection of temperature with the rise of fevers
and other pestilences, the powerful remedial efficacyof hot and cold bathing, of
changes  of climate, of regulating the  temperature of air breathed by invalids,
the protection from clothes, houses, &c. 
THE SUN THE  GREAT SOURCE.
309
  » The great natural source of heat is the sun." (See Analysis, page 256.)

   To admit this, it is only necessary to  think of the comparative tempera-
 tures of night and day, of seasons and of climates, and to reflect that  the sun
 is  the sole cause of the differences.   We need not wonder, then,  that, to
 many savage nations, seeking the source  of their life and happiness,  the sun
 has been the object, not only of admiration, but of worship.
   The heat comes from the sun with his light.  If a sunbeam enter by a
 small opening an apartment otherwise closed and dark, it illuminates intensely
 the spot or object on which it first falls, and its light  being then scattered
 around, all the objects  in the room become feebly visible.  Again, a cold
 thermometer, held  to receive the direct ray, rises much, while, in any other
 situation, it is less affected ; proving the heat to be like the light, at first con-
 centrated,  and then  widely diffused, losing proportionately of  its intensity.
 Light passes from the sun to the  earth in  about eight minutes of time, as will
 be fully explained in  a future chapter; and  there is every reason to conclude
 that heat travels at  the same rate.
   Human  art can gather the sunbeams together, and  the  intense heat exist-
 ing in the focus of  their meeting, is another proof that the  sun is the great
 source of heat.  A pane of glass in a window, or a small mirror, will reflect
 the sun's ray so as strongly to  affect the eye at a distance of miles—and the
 heat accompanies  the ray, for  by many such mirrors directed  towards  one
 point, a combustible object placed there  may be inflamed.  Archimedes set
 fire to the Roman ships by sunbeams, returned from many  points  to one,
 his god-like genius thus rivaling by natural  means, the supposed feats of
 fabled Jupiter with his thunderbolts.   Again, when the light of a broad sun-
 beam is made by a convex glass or lens to converge  to one point or focus,
 the concentrated heat is also  there—for a piece of metal held  in the focus
 will drop like melting wax: and if the glass be purposely advanced, its focus
 will similarly advance, and will pierce through the most obdurate substances,
 as red-hot wire pierces through paper or  wood.   A  hunter on  the hill, and
 travelling hordes on the plains, often conveniently light their fires at the sun
 himself, by directing  his energies through a burning glass.
   The direct ray of the sun, simply received into a box which is covered with
 glass to exclude the cold air, and is lined with  charcoal or burned  cork to
 absorb heat, and to prevent the escape of heat once received, will raise a ther-
 mometer in the box to the temperature of 230° of Fahrenheit, a temperature
 considerably above that of boiling water.   And the experiment succeeds in
 any part of the earth  where there is a clear atmosphere, and where the  sun
 attains considerable apparent altitude.  We see, therefore, that a solar oven
 might in some cases be used.   In operating with the apparatus suggested by
 the author, and described at page 295 for distilling vvater by the-heat of the
 sun, the vessel intended to absorb the heat, and to act as  the still, should be
 enclosed in a case covered and lined as above described.
   Reflecting on such  facts as now recorded, and  on the globular form and the
 motions of our earth,  we have a reason and the measure of the differences of
 climate and of season  found upon the earth.   It is evident that the part of the
 globe turned  directly  to the sun, receives his rays as abundantly as if  it were
 a perfect planr, directly facing  him, while on parts, which, as viewed from
 the sun, would be called  the sides of the globe, with the increasing obliquity
of aspect, an equal breadth or quantity of rays is spread over a larger and a larger
surface; and at the very edge the light passes level with the surface, and altogether
without touching,   the sunny side of many a steep hill in England receives
    21 
310
HEAT.
  the sun's rays in summer as perpendicularly as the plains about the equator;
  and such hill-sides are not heated like these plains, only because the air over
  them is colder—just as mountain tops, even at the equator, owing to the rari-
  fied  and, therefore, cold  air around them, remain for ever hooded  in  snow.
  In England, at the time of the equinoxes, a level plain receives only about
  half as much of the sun's light and heat as an equal extent  of level  surface
  near the equator : and in  the short days of winter it receives considerably less
  than a third of  its summer allowance.
    There are few contrasts in nature more striking than between the conse-
  quences of different intensity of  the sun's influence:  for instance, the inha-
  bitants  of India, at midday, with the thermometer  at 120°, are running to
  the shade of their bungalows, darkening  their windows, hanging wetted mats
  upon  the walls  and roofs, sprinkling water on the floors, fanning themselves
  with ever-moving punkas, and feeling the slightest  covering or exertion too
 much—while, on the other hand, the  dwellers in Greenland, with  the ther-
 mometer below zero, are loaded  with  furs, and are  seeking  the direct sun-
 shine  or heat from a fire, as their life and comfort.  Again, there is  the con-
 trast observed on passing, as the author  once did, very rapidly, from  such a
 paradise as Rio  de Janerio, with all its vegetable riches, to Tristan da Cunha,
 and the Isle of  Desolation in the Southern Ocean, which exhibit only cold
 and naked rocks;  but yet where the scene swarms with its  appropriate inha-
 bitants—the sea with seals, and the air with clouds of  sea-fowl, playing over
 the never-resting waves like flakes of eddying snow.   Were a person for a
 moment to doubt whether the sun be the real cause of such differences, and
 of the fact that certain creatures are found only in certain zones of the earth,
 let him reflect on the extraordinary migration  of animals, which have their
 home  not in any fixed region, but wherever the sun has for a time particular
 degree of influence, and which accordingly follow the  sun in the changes  of
 season.   We have the swallow in vast numbers coming to visit the British
 isles in the spring, to  play  over our woods  and waters, in  pursuit  of the
 insects which the heat then breeds to fill  the air,—welcome harbingers of the
 coming summer and its  riches;  and in  autumn, the same creatures are seen
 congregating on our shores, to wing their flight back in united  multitudes  to
 more southern countries, where, in turn,  there is a temperate influence of the
 sun.   The same season  brings to England the nightingale,  and makes our
 woodlands resound with the note of the cuckoo.  In the waters of  our bays
 and coasts, again, there appear with the seasons the vast shoals  of fish, as  of
 the herring and  mackerel, which  prove  such abundant food  for  millions  of
 human  beings;  and  we have salmon,  at  stated times, penetrating from the
 ocean far up the mountain streams, to  deposit its spawn for  future  supply:
 all by their movements contributing to the harmonious and beneficent system
 of the  universe.
   With respect to the sun as  a source of  heat, there have been two opinions
 among philosophers; one  class  believing that the sun is an intensely heated
 mass   which radiates  its heat  and light around, like  a mass  of intensely
 heated  iron; and another class holding  that heat is merely an  affection or
 state of an ethereal fluid, which occupies all space, as  sound  is an affection
 or motion of air, and that  the sun may  produce  the phenomena of light and
heat without waste of its  temperature  or substance,  as a bell may  produce
the phenomenon of sound: holding farther that the sun, below its luminous
atmosphere may be  habitable even by such animals as live  on this  earth.
 Those  who take  the  first view, are awakened to  the dread contemplation  of
a universe carrying  in itself the seeds of  its own decay, or at least  of great 
COMBUSTION.
311
 periodical revolutions: the others may view the universe  as  destined to last

 de"ro  "it        ' "    E "^ 3Ct °r WiU °f itS Creat°r Sha11 aSain altero*
   Of one fact there can  be no  doubt, namely, that the  present temperature
 of the  surface of the  earth is much lower  than the temperature in remote
 past time.   1 he rocks called primitive, as granite and gneiss, constituting
 the interiors of our great mountain masses  and the substrata of our plains,
 bear evident  marks of having been at  one  period  in a  molten state, from
 which  they have been solidified by a very gradual cooling: and  even the
 whole mass of the earth  at some time must have been so fluid or soft, as, in
 obedience to gravity, to have  assumed its rounded  form, and in obedience to
 the centrifugal force of its  whirling,  to have bulged out, at its great circum-
 ference or equator, the thirty-four miles which its equatorial diameter exceeds
 the polar; the same, by the  by, in degrees corresponding  to the  various
 speed of  rotation, being  true  of all the  other planets belonging to  the solar
 system.  Again, while in excavating below the surface  of the globe, or in
 examining its structure as exposed to view by volcanic or other  convulsions,
 men encounter, in very many situations, a thickness of more  than a mile, of
 the  wreck and remains of former states of the world— as, on digging eighty
 feet under vineyards near Mount Vesuvius, they encounter the more recently
 buried  cities of Herculaneum and Pompeii—they farther discover that the
 animal and vegetable  remains buried, without number,  in the  present cold
 climates  of the  earth, and evidently near  where  the  creatures lived, are
 all of kinds now inhabiting only the warmer or tropical regions.  Lastly, in
 the operations of mining, the  deeper men  go, the higher they find the tem-
 perature to be, at the rate of a degree for about 200 feet of descent; which
 fact, as heat tends to equable diffusion, proves both that the central  heat of
 our earth must have had another source than a radiation  from the sun of the
 present intensity; and  that the surface of  the earth is now radiating away
 more heat than it receives from the sun.   The conclusion then follows, that
 the temperature of the world is still  falling, although perhaps so slowly that
 a  change may not be  detected  even within  centuries.   Possibly, in  very
 remote  antiquity, that  may have been true which  the early Greeks errone-
 ously thought true in their day, viz.,  that the equator of the earth, by reason
 of its great heat, the  sun's influence there being joined to the heat  from
 within,  was a barrier, impassable by man,  between  the northern and south-
 ern  hemispheres.

           "Electricity a source of heat."   (See the Analysis.)
   This subject can only be satisfactorily  entered upon in the chapter devoted
 exclusively to  electricity,  and is, therefore,  deferred.  Suffice  it here to say,
 that  while an  electrical discharge of current passes from one  situation to
 another, the substance serving as a conductor is often heated, melted or dissi-
 pated, in such a manner as to make it doubtful whether  human art has any
 more powerful means  of producing  these  effects.  We may remark, too,
 that in certain cases of the electrical current, the heat is  accompanied by as
 intense a light as art can exhibit.

 " Combustion and other chemical actions as sources of heat."   (See Analy-
                             sis,  page 256.)
   Of the  phenomena of  nature  there is perhaps none which, to the  unin-
structed, appears so inexplicable and  so wonderful as that of fire or combus- 
312
HEAT.
tion—whether contemplated in its beauty or in its terrors.   Fire is seen in
its beauty when used  by man  for his domestic purposes, as when  it  blazes
cheerfully over his parlour hearth, or beams a steady light around from his
lamps and chandeliers.   It is  seen again in its terrors, when spreading by
accident from some focus, it envelops in sudden flame and quickly consumes
the surrounding objects, perhaps the draperies and other furniture of .a single
apartment; or wider spread, the valuable contents of a spacious mansion; or
still wider spread, and  with deafening uproar, a whole town or a forest:—and it
is fire which, labouring within  the bowels of the earth, first prepares and then
urges up to heaven the volcanic eruption  of flame and red-hot rocks,  during
which the region  around often  quakes and is uptorn, sothat the  cities are de-
molished into the  sudden tombs of the inhabitants, the course of the rivers is
changed, the  plains are converted into  lakes, or the lake-beds into dry land.
Fire is awfully seen also in some meteors, and when, intentionally lighted by
human hands, it bursts from the cannon to produce the carnage of battle.  Fire
among many nations of antiquity was regarded with awe and holy reverence,
the sun himself being honoured chiefly as its concentration or supposed abode.
There were sacred fires in many of the temples, and fire was used to complete
the splendour of the most august ceremonies.  Nay, even Moses, a worshipper
of the one  true God,  has given records of the Burning Bush and of burnl-
offerings made to that God: and  at the present day, in many Christian churches,
there are ever-burning lamps and  frequent magnificent illuminations.  Now
this principle of fire, which, when the savage man first saw it spreading after
the thunder-clap or the rubbing of forest branches in a storm, so as to threaten
universal destruction,  he so naturally  accounted the demon, if not the God of
nature;  this principle  man's art has now tamed to be a most obedient  and by
far the most useful of his servants.  Fire being in truth, but a concentration of
the element ofheat, which in its tranquil  and invisible diffusion we have al-
ready contemplated as the beneficent life or soul of the universe—the cause of
seasons and climates,  and of all the changes or activity which distinguish a
living world from a dead and frozen mass; man, by acquiring command  over
it, commands heat when and where  he wills, and  thus truly becomes  in a
second  degree the ruler of nature.   Fire in  man's service  may be figured
as a legion of serving spirits  to whom no labour is difficult, who, in any
particular case, have  power or magnitude exactly proportioned to the quan-
tity of food or fuel afforded; of whom, moreover, man can,  at any moment,
conjure up one or many by the magic stroke of his flint and steel.  In every
private  dwelling  he has of these fiery spirits as domestic slaves—in the
kitchen and in the parlour.  In his manufactories they are  melting  glass for
him, and reducing ores, and boiling and evaporating for a hundred purposes.
But it is chiefly  when chained to the steam-engine, that  they show  their
prodigious powers:—as when, putting forth a giant's strength, they heave a
river from the bottom of a  mine, or  urge a vast  ship through the winter
storm;  or when equaling, if not  surpassing, in nice dexterity,  the work
of human  hands, they twist the silken or cotton  threads, and weave them
into most delicate fabrics.   Men, grown  familiar with such prodigies,  have
almost ceased to be moved by them ; but even now few persons can resist a
feeling of wonder and admiration when chemistry  is calling forth the hidden
spirit of combustion  in some  new or less familiar guise:—as, for instance,
when a piece of iron wire, lighted as a taper in oxygen gas, burns with such
resplendent brilliancy;—or when phosphorus similarly placed, throws around
its overpowering flood of  flame; or when small portions of the metal called
potassium, cast upon  the surface of water, become as beads of most intense 
COMBUSTION.
313
light running about there, and  crossing as in a merry dance;—or, lastly,
when names produced from particular substances are seen rising deep-tinged
with most vivid and beautiful colours.
  Singularly  interesting, then,  to  philosophers, as  in such particulars the
phenomenon  of combustion  must  always  have appeared, one may wonder
that its true nature could remain to them  so long  a  mystery; but until the
admirable researches of  Davy, made only a few years ago, their conjectures
had scarcely approached the  truth.  An opinion long prevailed, that in every
combustible substance there  was present  a certain quantity of a  something
donommated phlogiston, which, on being disengaged or separated, became
obvious to human sense as  light and heat.   The white oxide of zinc, for
instance, named the flowers of zinc, and into  which the metal is changed by
burning, was  supposed to be the metal deprived of its phlogiston;  and when,
on this oxide being heated with charcoal, the metal again appeared,  it was
supposed simply to have recovered phlogiston from the°charcoal.  The illus-
trious Lavoisier had the merit of most clearly  disproving this hypothesis, by
showing that the flowers  or powders  obtained from a metal by  Burning it,
were heavier  than the piece of metal from which they were produced, by the
exact weight of the oxygen gas, which disappeared in the combustion, &c.;
and he shewed farther, that in  this and many other cases,  combustion was
merely the  act of two substances  combining chemically; but he fell into an
error almost as great as that which he overthrew, by supposing that for com-
bustion, oxygen  had always to be one of the combining  substances, and
that the heat and light given  out in every case had been previously latent  in
the oxygen.
   When Sir Humphrey Davy began his  labours on the subject, than  which
labours there is not, perhaps on record  a more perfect specimen of scientific
research, it was already known, first,—that bodies when compressed  or  by
any means reduced in bulk, generally give out a part of their heat, as—when
air condensed in  the match-syringe lights tinder,—or when water  and sul-
phuric  aeid, uniting into a compound  of smaller volume  than  the  separate
ingredients, become very hot,—or when  water, poured upon  quick-lime to
slake it, and becoming solid with it, produces heat sufficient to inflame wood,
as has been fatally proved  by the  burning of  many lime loaded ships;—and
that in  such  cases, the heat  produced  during the  chemical  union depended
more upon the energy of the action which united the substances than upon
the change of volume produced.
   Farther, it  was known that any substance  having its temperature raised,
by whatever means, to 800° or more of Fahrenheit's thermometer, became
incandescent  or  luminous—as  when iron, or stone, or any substance not
dissipated by heat, is placed  in a common fire;—in the first  degree the sub-
stance being said to be red-hot,  and at higher temperatures to be white-hot.
   Out of these two truths Davy constructed his explanation.   He  asserted—
that in  any case, combustion is  merely the  appearance produced  when sub-
stances, having a still stronger  attraction for each  other than quick-lime and
water,  are, with intense energy,  combining  chemically,  so as to  become
heated  atleast to the degree of incandescence; and that during the pheno-
menon  there  is not,  as  was formerly supposed, something altogether con-
sumed  or destroyed, or  something called phlogiston escaping, but that the
substances concerned are only assuming a new form or arrangement.   1 hus,
if a piece of charcoal be enclosed  in a  glass vessel filled with air, of which
vessel the mouth dips into  a liquid  to confine  the air, and if  the charcoal be
then heated to a  certain degree, by means of a burning-glass or otherwise, 
314
HEAT.
the cohesion of its particles  yields to their attraction for the  oxygen of the
air around them, and they immediately begin to combine with the oxygen so
energetically  as to produce  a heat still much greater, accompanied by the
light or incandescence of combustion.  The charcoal, under these circum-
stances, soon entirely disappears, or is dissolved in the air, as sugar may be
dissolved in water: but if the air be afterwards weighed, it is found to have
gained in weight the exact weight of the  charcoal which has disappeared;
and a chemist can  again separate the charcoal from the air, and use either
for any purpose as before.   In like manner, if a piece of iron wire be heated
at one  end, which is then plunged  into a jar of oxygen  gas, it will burn as a
most brilliant taper, and will gradually fall in the form of oxidized drops, or
scales of iron, to  the bottom of the vessel.   Now during this  process, the
quantity of oxygen will be diminished, but if the scales  mentioned be col-
lected, they will be found to weigh just as much more than the original  wire
expended, as there is of oxygen lost or combined with  them.   A chemist
can separate this iron and oxygen,  and exhibit them apart as before, without
loss.   Again, if iron and sulphur, in  certain proportions, be heated together,
they unite with vivid combustion,  but the product weighs exactly as much
as the original ingredients.
   While every instance of combustion is thus only a case of chemical union,
going on with  such intensity of action as to produce incandescence,  still,
according to the nature of the substances  combining, the appearance varies
much.   It may be, for instance, with  flame or without flame.   The  great
combining substance in nature, that is to say, the most  universally distributed,
is oxygen, of which the name is now become familiar even to  the ears of the
unlearned.   It forms four-fifths of the substance of water and one-fifth of our
atmosphere, being  on the  latter account present wherever man can be, and
ready to unite itself with any matter exposed to it at the necessary tempera-
ture.   Now of substances burning in air, those which are originally aeriform,
as coal gas, or which, on being heated, are rendered aeriform, or vapourized
before the union takes place,  as oil or wax,  assume the appearance of flame—
which  means that  the aeriform particles  usually invisible are raised to  the
incandescent  temperature;  but  when the substance  combining with  the
oxygen remains solid, while its particles  are gradually lifted away by the
oxygen acting only at the  surface of the mass, it appears, during the whole
time, only as a red-hot stone.   The latter is the case of charcoal, coke, Welch
stone-coal, &c, while in the case of wood, common  coal, Sic,  a greater  or
less portion of the  inflammable matter is by the heat of the combustion  con-
verted  into vapour, and produces  the beautiful appearance  of flame.
   Of the substances called combustible, and thus called because they combine
with oxygen so energetically as to become  incandescent, there  are only a
few, as the metals called potassium, sodium, Sic, which  will  begin to  unite
with oxygen, or to  burn at the common temperature of  our globe, the olhers
requiring to be at some higher temperature.   Thus  phosphorus  begins  to
burn at 150°, sulphur at 550°, charcoal at 750°, hydrogen at 800°, &c; it
appearing that up to these temperatures the attraction  of the atoms of the
substances among themselves is sufficient to resist the other attraction or that
of oxygen.   But when  the  combustion once begins, the temperature, from
the effect of the combustion  itself, rises instantly much beyond the degree
necessary for the  commencement of the  process.  Oxygen and hydrogen,
which begin to burn or combine at  800°, produce a flame of as intense heat
as human art can excite.
   On the circumstance that bodies require to have certain preparatory tem- 
COMBUSTION.
315
perature  before  beginning thus to combine with oxygen,  depend  many
important facts  in nature and art.  Hence the safety with which  most com-
bustibles may be exposed at ordinary temperatures to the contact of atmo-
spheric air:  otherwise coal, wood, Sic, in the moment of being exposed to
the air, would catch  fire, as really happens to phosphorated hydrogen gas ;
or to  the  metal  called potassium, even when  thrown into cold  water, the
metal  attracting the oxygen from the water instantly, and with  intense com-
bustion.  If a fire or flame be so small thai, from the rapid absorption of
heat  by bodies around, it  does not  produce heat enough to maintain the
inflaming temperature of_the substance, the  combustion  will soon be extin-
guished.   Thus a common coal fire, if not watched, and the  remaining fuel
occasionally gathered together,  to reduce the surface of wasteful radiation,
will be  extinguished long before the whole fuel is expended:—but  not so
with a fire of wood or of paper, which substances  burn more readily than
coal.   The Welch stone-coal can only be made to burn in very large masses,
or when mixed with a more inflammable coal or other fuel, or  when fed by
air already heated.   Some of our manufactures  have lately been improved
by causing the air  which feeds the working fire to pass through metal tubes
heated by  lying in another fire.   In common fires much  coal is burned  at
temperatures so low as to be  nearly useless.   A substance  placed in pure
oxygen gas burns  with much greater intensity, and will begin burning at a
lower temperature than if placed in  atmospheric air, which contains only
one-fifth of oxygen and four-fifths of  another  substance, nitrogen,  which
does  not aid the combustion—because, in the latter case, the nitrogen, by
absorbing much of the heat of the combustion, lowers the temperature. Iron
wire  will burn as a taper in oxygen, but not in common  air;  and a common
taper or flaming piece of wood just extinguished by blowing on it,  will imme-
diately be rekindled  if placed in oxygen.   Again, a lamp with a very small
wick, as of one thread, and producing, therefore, very  little heat,  will not
burn  in cold weather, or in an ice-house, and at any time will be extinguished
by a foreign body brought near it so as to cool it—a piece of ice, for instance,
or a small metallic nob, presented to  it on the end  of a wire,  or a metallic
ring let down over it; but if the ball or ring be hot, the effect will  not follow.
By more powerful refrigerating  processes, even a considerable lamp or candle
may  be  put out.  These discoveries led Davy to  the  construction  of his
miner's safety-lamp, which is merely a lamp surrounded by a wire gauze, of
which the  meshes are of such size that a flame of hydrogen gas attempting
to pass through is so cooled by the heat-absorbing and heat-conducting power
of the metal, as to be extinguished.  A wire gauze gradually let  down upon
any common flame,  annihilates  the part of it which should appear above the
gauze; but the combustible vapour passing invisibly through the gauze, may
be lighted  afresh on  its upper side.  Oxygen and hydrogen, which  are the
constituents of water, wben uniting, produce such intense heat that the mo-
mentary expansion of the newly-formed water, then in the state of steam, is
such  as to constitute a violent  explosion; and when a jet of the two gases
mixed gives a continued flame,  the most refractory substances melt in it like
wax in a common taper—yet these gases may be kept mixed together in the
cold reservoir of a blow-pipe without combining, and when they are set on
fire  while issuing as  a jet, the flame does not travel inwards through  the
opening as might be feared, because it is cooled by the  metal  of the orifice.
A mixture of oxvgen and hydrogen passing through the small channels left
in a tube filled with  wire, may  be lighted at the end of the tube without dan-
ger of explosion. 
316
HEAT.
   While solid bodies become very visible or incandescent at about 1,000° of
Fahrenheit, airs,  owing to their tenuity of condition, require  to be  heated
much farther before they take on the vivid appearance of flame ; and  airs  of
light atoms, like  hydrogen, require  to be heated still more than heavier airs.
Thus the flame of pure hydrogen is pale and blue, but a wire held in it be-
comes much more luminous  than the flame  itself; and the flame  of  mixed
oxygen and hydrogen  escaping from a very minute orifice  in a glass tube,
may itself  be scarcely  visible, while the extremity of the tube heated by it
becomes like a brilliant star.  Hence the light of many flames may be in-
creased by  placing a wire gauze or other solid body in the flame ; as is seen
when a piece of lime is placed in a flame of mixed oxygen and hydrogen.
Consideration of  this subject enables us  to explain why common  coal gas,
which consists of hydrogen holding a quantity of  carbon in  solution, gives a
stronger light than pure hydrogen;  and why oil  gas, which contains  about
twice as much carbon  as the coal gas, gives also about twice as much light:
—or it appears that  the atmospheric air, which first mixes  with these gases
as they issue to burn, is sufficient to combine with all their hydrogen (which
it most strongly  attracts)  but not at the same time with all  their carbon:
the particles of the  carbon, therefore, are first separated or precipitated  in
the flame,  and  become so many solid  particles  most intensely heated and
luminous ;  and afterwards when they have ascended a little higher, they meet
with new oxygen and  burn in their turn, giving a second quantity  of light.
That this decomposition of the gas really occurs is proved by placing  a wire
gauze  in the flame, for then we find that if it be held near the middle  of the
flame,  it becomes immediately  loaded  with  particles of charcoal separated
there, and  cooled by it so as  to cohere ; while if held at the  bottom  of the
flame where the carbon is not yet separated, it retains none, and if held  at
the top  of  the  flame,  where  they are  already burned, it retains none.   A
candle or lamp is said to smoke when the heat produced by it, or the quantity
of oxygen  allowed to approach the  flame, is not sufficient to effect the total
combustion of the carbon which rises in its  flame.  The hollow or circular
wick of the common Argand lamp, and the similar form given to gas flames,
is useful, by admitting  air to the inside as well as  to the outside of the flame,
and the lofty glass chimney is to quicken the currents of air.
   When oxygen  mixed with  certain of the inflammable gases or vapours  is
heated, although only to a temperature considerably below  that of  common
burning or  explosion, a union still takes  place, but very slowly, so that the
temperature never rises to what is necessary to exhibit flame.   This pheno-
menon has been called invisible combustion.   It is remarkably exemplified
on plunging  platinum or gold wire  moderately heated into such a mixture:
the combination then goes on  in the  immediate vicinity of the hot wire ; and
although without flame, still with sufficient disengagement of heat to  maintain
the wire in an incandescent or luminous state, as long as  there are ^ases left
to combine.  Thus the  vapour always arising at a  common temperature from
the mouth  of a phial of ether, (ether consists chiefly of hydrogen  and car-
bon), if made to pass through a coil of heated platinum wire, while combining,
by this slow combustion, with the oxygen of the  air around  it, give out heat
enough to keep the wire so luminous as to serve as a little lamp by which  to
read from the dial-plate of a watch through the night.   A beautiful modifica-
tion of this  principle has been  adopted in the miner's safety-lamp ; and when
the air of the mine is too impure to maintain the flame, it still suffices thus
to produce  a continued  light from the incandescent metal. 
FUEL.
317
                                 "Fuel."
   Heat being, in the sense already explained, the life of the universe, and
 man having command over nature chiefly by his power of controlling heat,
 which power again comes to him with the ability to produce combustion, it
 is o  great interest to inquire what substances he can most easily procure as
 food for combustion, or fuel, and how these may be most advantageouslv em-
 Pr°/r    l.?^     °n this SubJect at al1 fully in reference to the various arts
 pf lite would be to compose an extensive work, but an interesting sketch may
 be comprised within narrow limits.
   Although there are a great number of substances, which, in the act of their
 chemical union, occasion the heat and  light which constitute the combustion,
 still by far the greater part of these, in an uncombined state, are so sparingly
 distributed in nature, and are therefore procurable with  such difficulty, that
 heat obtained by  sacrificing them would be much too expensive to be within
 common means.   Providence, however, has willed that  the elementary sub-
 stance in nature which has the most energetic attraction for almost all other
 substances, and which therefore produces in uniting with them the most in-
 tense heat, is  also of all the most universally distributed.   This substance is
 oxygen.  It forms part of our atmosphere, and therefore penetrates, and is
 present wherever man can  breathe, offering itself at once  at  his  service.
 Then for the  purpope of combining with the oxygen, there are  chiefly two
 other substances  also very widely scattered,  and therefore easily procurable
 and  cheap.   These  are carbon  and hydrogen, the great materials of all vege-
 table bodies,  and therefore of our forest  trees, and of coal beds,  many of
 which are evidently the remains of antediluvian forests.  Carbon is found
 nearly alone in hard coal, but it is united with a large proportion of hydrogen
 in caking coal, and  in wood, wax, resins, tallow and oils.  The  gases ob-
 tained from  these last-mentioned substances and now used for illumination are
 merely hydrogen, holding in solution certain  proportions of carbon ; and all
 bodies which burn with flame give out such gases in the act of combustion.
 In the mass of the  earth,  as far as known to man, the stones, earths and
 water, forming its surface, are already combinations of oxygen with  other
 substances, and are therefore not in a state to produce fresh combustion; but
 carbon and hydrogen, by various processes of vegetable and animal life, are
 in numberless situations becoming accumulated, so as to  be fit for  fuel;—as
 by other processes the atmosphere  is always preserved with its due propor-
 tion  of oxygen.
  The name fuel  is given only to the substances which combine with oxygen,
 and not to the oxygen itself, probably because the former being solid orliquid,
 and therefore more obvious to sense, had attracted human  notice as producers
 of combustion long before the existence of the  aeriform  agent, oxygen, was
 even suspected.
  Oils, fat, wax, &c, from becoming  aeriform in their combustion, exhibit
 the appearance of flame, as  already explained, and hence these substances are
 chiefly used for the  purpose of giving  light; while wood and coal are more
 frequently used for mere  heating.  But the chemist's  lamp, by which he
 distils and evaporates, and his common blow-pipe for directing the point of
 any flame upon a substance to melt it, and  his blow-pipe fed with mixed
oxygen and hydrogen,  whose flame is capable of melting the most refractory
substances—prove that it is  chiefly the expense of the former kinds of fuel
which has limited  them  so  much  to  the  office of light-giving.   Lately an 
318
HEAT.
important application of coal-gas and of oil or fat as heat-giving fuel has been
made in a general cooking apparatus, which promises to effect a considerable
diminution of house-keeping expense.
  Wood was the common fuel of  the early world  when coal-mines were not
yet known, and still in many countries it is so abundant as to be the cheapest
fuel.  Charcoal is the name given to what remains of wood after it has been
heated in a close place; during which operation  the hydrogen  and  other
minor ingredients are driven away in the form of vapour.  Charcoal is nearly
pure carbon.  Coke,  again, is  the charcoal obtained by a similar process from
coal.  The wood and coal, if much heated in the air, would burn  or combine
with the oxygen of the air ; but  heated in ,a vessel or place which excludes
air, they merely give out their more volatile parts.
  Good  coal, where it abounds, is now for ordinary purposes by much the
cheapest kind of fuel; and since  within a few years men have learned to
separate from it, and to use instead of oil and wax, its illuminating gas, namely,
its hydrogen holding in solution a little carbon, it has become doubly precious
to them.  A person  reflecting that heat is the magic power which vivifies
nature, and that coal is what best gives heat for the endless  purposes of human
society, cannot without admiration think of the rich stores  of coal which exist
treasured up in the bowels of the  earth for man's  use.  And Britain, in this
respect, is singularly favoured.. Her extensive coal mines are in effect  mines
of latent labour or power vastly  more precious than the  mines of  gold  and
silver in Peru,—for a hundred men in England, of whom  a part dig coal, and
the remainder apply  it in manufactures, can get in  exchange for the merchan-
dize they produce more of either gold  or silver than a hundred men working
in the American mines are able to extract.  These coal-mines, therefore, may
be said  to produce abundantly every thing which labour and  ingenuity can
produce, or which our money can buy, and they have essentially contributed
to render Britain the mistress of the  industry and commerce of the earth.
Britain  has become  to the civilized world around, nearly what an ordinary
town is to the rural district in which it stands,  and of this  vast and glorious
city the  mines  in  question are the coal-cellars, stored at the present rate of
consumption for above 1,000  years; a supply which, as coming  improve-
ments in the arts of life will naturally bring  economy of  fuel, or substitution
of other means to effect similar purposes or changes in the channels of industry,
—may be regarded as exhaustless.
  The coal in  many mines is evidently the remains of forests,  which have
existed  in  very  remote  time,  swept together  either during  convulsions of
nature or by the more gradual operation of rivers,  (as may be seen at present
where the great  Mississippi is carrying an uninterrupted  stream of floating
timber to the  sea, and  depositing it at the bottom of the Mexican gulf)  the
accumulated wood being afterwards  compressed  and solidified  by superin-
cumbent deposits of  earth matters, aided probably by the action of heat.  In
many coal-beds the trees yet  retain their form, so that their species can be
easily distinguished,  and there are buried among  them other vegetable and
animal  remains of cotemporaneous inhabitants of the earth.  Coal is found
of different qualities.  In some places it is almost unmixed carbon, and  ex-
ceedingly solid,  as if it had been  coked by subterranean  heat.   Such is the
stone-coal of Wales,  wbich  in 100 parts contains 97 of pure carbon, with only
three of hydrogen and earthy matter.   In other places the coal contains hy-
drogen  in nearly as  large proportion as  wood  does, so combined with part
of the carbon as to form the  oily or  pitchy  substances  existing  in the coal, 
FUEL.
319
which, when burning, produce  flame, and when rising unburned, constitute
smoke.
  The comparative values, as fuel, of different kinds of carbonaceous matter,
have been found to be as in the  following tables.

     One pound of                                   Melts of ice
     Good coal  -                                     90 \^s^
     Coke   -       -       -        .       .       -    84
     Charcoal of Wood   -       -      -       .       95
     Wood  -       -       -        -       .       -    32
     Peat       ---..       19

  Lavoisier, in making experiments on  combustibles generally, to  ascertain
 the quantities of oxygen expended, and of heat given out during the combus-
 tion of a given quantity of each, obtained the followino- results :
One pound of	Melts of ice	Takes of oxygen
Hydrogen gas	370 lbs.	7^ lbs.
Carburetted hydrogen -	85	4
Olive oil	120	3
Wax -	110	3
Tallow	' 105	3
Charcoal	95	2-|
Phosphorus -	100	ii
Sulphur	25	i
The following remarks with respect to fuel, and the modes of using it, seem
                        to demand a place here.

  A pound of coke produces  nearly as much heat as a pound  of coal; but
a pound of coke is the produce of a pound and a third of  coal, although the
coke is more  bulky than the coal.
  It is wasteful to wet fuel, because the moisture in being evaporated carries
off with it as latent, and therefore useless heat, a considerable proportion of
what the combustion  produces.  It is a  very common  prejudice, that the
wetting of coal, by making it last longer, is effecting a great saving; but it
does so merely  by restraining the combustion or producing a smaller fire, and
with the bad fire, there is also much waste of heat.   To  illustrate the influ-
ence of watery vapour upon combustion, we may mention the fact, that a
manufacturer who tried to blow his fire by forrting  steam into  the  furnace
with the air, extinguished his fire;  and the analogous fact, that ordinary fires
burn better in winter  than  in summer, although the temperature be lower,
because cold air is generally much drier than that which is warm.
  Coal which  contains  much hydrogen, as  all  flaming  coal does,  is used
wastefully when anv  of the hydrogen is allowed to escape unburned;  for,
first, the great heat vvhich the combustion  of such hydrogen would produce
is not obtained; and, secondly, the  hydrogen while becoming gas,  absorbs
into the latent state still  more  heat  than  an equal  weight of water  would.
Now the smoke of a fire is the hydrogen of the coal rising in  combination
with a portion of carbon.    We see, therefore, that  by causing a  fire to
destroy or burn its smoke, we not only prevent a nuisance, but effect  a great
savin*.   The reason  that common fires give  out so much smoke is, either
that the smoke or vapourized  pitch is not sufficiently heated to burn,  or  that 
320
HEAT.
the air mixed with it as it  ascends  in  the  chimney, has already, by passing
through the fire, been deprived of its  free oxygen.   If the pitch  be very
much heated, its ingredients assume a new arrangement, becoming  transpa-
rent, and constituting the common coal-gas of our lamps; but at lower tem-
peratures, it is seen jetting, from cracks or openings in the  coal, as a dense
smoke—a smoke,  however, which immediately becomes a brilliant  flame if
lighted by a piece of burning paper or the approximation of the combustion.
The alternate bursting out  and extinction  of these  burning jets of pitchy
vapour, contribute to render a common  fire the  lively and  agreeable object
which it is in the winter evenings.   When coal is first thrown upon a fire,
a great quantity of vapourized  pitch escapes as a dense smoke, too cold to
burn, and  for a time the  flame  is smothered, or there is  none; but as the
fresh coal  becomes heated, its  vapour  reproduces the flame as  before.   In
close fire-places, as those of steam-engines, brewing  and dyeing apparatus,
&c, all the air which enters after the furnace-door is shut, must  pass through
the grate and the burning  fuel, so that  its  oxygen is  consumed by the red-
hot coal before  ascending to where  the smoke  is.  The smoke, therefore,
however hot, passes away unburnt; unless, sometimes, as overfoundery fur-
naces, where the heat is very great indeed, and  it burns as  a flame  or great
gas lamp at the chimney-top on reaching the oxygen of the open atmosphere.
   There have  been many modes proposed of  destroying smoke:  one has
been to admit to the space above the  fire, by a suitable  opening, a certain
quantity of fresh air, the oxygen of which may inflame the smoke.  This
plan is  efficient at  a certain point of time  after the  addition of fresh  fuel,
and then for a time effects the saving of fuel; but the difficulty  of admitting
just the quantity of air required to suit the varying demand for it,  has not
been overcome, and hence, from  there  being no saving on the whole, the
plan has been abandoned.   When just enough air entered, the flame pro-
duced gave so intense a heat as in several cases to have burned  or destroyed
the parts of valuable boilers exposed to it; and when, on the contrary, too
much air entered, it injuriously cooled the  boiler.  A  contrivance at present
commonly adopted  for burning smoke, is  that of Mr. Brunton, namely, a
circular fire-grate,  kept  turning like  a  horizontal wheel, on which, by ma-
chinery, coal is made to fall  in a  gradual manner,  so as  to be uniformly
spread over it.  The coal falls  so gradually, that although there is generally
a little smoke from it, there is  never much,—the oxygen  which finds en-
trance through  and around  the grate,  being always  in  quanlity the same,
and nearly sufficient.  A smoke-consuming fire  would be perfect, in which
the fuel were made to  burn only at  the upper  surface  of its mass, so that
the pitch and gas disengaged from it, as  the heat spread downwards, might
have to pass through the burning coals  where  fresh  air were  mixing  with
them;  and thus the gas and  smoke,  being the  most inflammable parts,
would burn first and  be all consumed. This  was  the principle proposed
in a fire-place suggested by  the author for  the great  brewery of Mr. Meux
in his neighbourhood,  and tried at the time  when attempts  were extensively
made to abate  the  nuisance of smoke  in towns.  The experiment proved
the theoretical perfection of the method, and that it would produce a saving
of 15 or 20  per cent, on the expenditure of coal; but before a durable grate
of the kind was completed, the Welch stone-coal  was introduced, which
has 97 per cent, of pure carbon, and therefore  has  no pitch to evaporate,
and no smoke,—and it was at once adopted there and  in many other places.
A little  common coal  is added to it to make  it  more easily  combustible. 
FUEL.
321
Coal in a  deep  narrow trough, as abed, if
lighted at its surface a b, burns  with a  lofty           Fig. 135
flame, as if u  were the wick of a large oil-
lamp; for all the  gas given out  from  the coal      ^r^^^m ,
below  as  that  is  gradually heated,  passes   *
through the burning fuel and becomes a flame.
Now,  if  we  suppose  several  such  troughs
placed  together with intervals between  them,   £
like the fire bars of a common grate or furnace,'
,u        u.
cl
 there  would  be a perfect  non-smoking fire-       eU        KJJ
 place.  Such was that  made  on the occasion
 mentioned; and although, as a  mere experimental apparatus, it was flimsy
 and imperfect, it  put beyond a doubt the possibility 0f accomplishing its
 object.  1 he reason of the vast saving of fuel by such a grate is, thai the
 smoke instead  of stealing away latent  heat-being  itself  also  the  most
 combustible  and precious part  of the fuel, here  gives  all its powers and
 worth to the purpose  of the  combustion.   The coal  rested on  movable
 bottoms in the  troughs,  (here  shown by dotted  lines) and  was moved up
 like the wick of a lamp by its screw,   the bottoms may be lifted  in many
 ways.  Or, without bottoms,  the coal  may be raised in each trough success-
 ively by an iron lever or bar introduced by the fire-man under the coal,  fresh
 coal being at the same time put  into the trough to fill the space under the
 lever.   The author believes that  this  principle of construction will still be
 extensively adopted  for the Newcastle or  flaming coal,—the  consequences
 would  be  so  important.   The  principle  has  been already  introduced for
 common parlour fires by  Mr. Cutler in his stove, which is  merely a  com-
 mon grate, having, instead of bottom-bars,  a deep box to  hold  the coal  for a
 whole day,  with a movable bottom,   which lifts the coal up  as  wanted.
 From  such a fire there  is  always ascending a long  beautiful flame; and much
 more heat is given out than from  the  same quantity of coal burned in the
 common way; the chimney never requires sweeping, for there is absolutely
 no smoke and therefore no soot.
   Smoke is effectually  consumed also  in a fire-place in which the air feeding
 the combustion is caused to pass downwards through the burning fuel, so as
 to carry the smoke  with  it,  instead of upwards  as usual.  This  result is
 attained by having the chimney to communicate with the ash-pit.  The  chief
 objection is, that the fire-bars are quickly destroyed by the intense heat to
 which they are exposed ;  and  to obviate this instead of solid bars, tubes  have
 been used  with  water passing through  them, and  admitting the feeding air
 only above.
   It is evident  that if a house or apartment  with the air in it, were  once
 warmed to a certain degree,  it  would for ever retain its temperature, but for
 the escape of heat through the walls  and windows, or with the air  from
 within, whether passing away as necessary ventilation  or as waste.   A per-
 fect system of heating,  therefore, would consist in diminishing as  much as
 possible these  causes of loss, with reference, however,  both to the expense
 of the means and the salubrity of the dwelling, and in  producing and distri-
 buting the heat judiciously.  It may be asserted that a fourth part of the fuel
generally expended in English houses, if more skilfully used, would better
secure  comfort and health than  all that is  now expended.   But it does not
accord  with the character of this general work to enter into minute detail on
the subject.   Remarks were made upon it in the chapter on "Pneumatics," 
322                             HEAT.
under the head of " warming and ventilating," and more minute information
may be obtained from Mr. Tredgold's work, expressly devoted to it.
  The detailed consideration of furnaces, blow-pipes, Sic, may appear to
some so  closely connected with  our present  subject as  to demand a place
here, but by so treating of them we  should be encroaching on the  province
of the chemist, &c.   We may state generally, that furnaces are merely struc-
tures by which coal or other  fuel heated to the degree at  which it combines
rapidly with the oxygen of the atmospheric  air, is placed in circumstances
favourable  to the  rapid renewal  of  the air,—and a  common blow-pipe is
merely a jet of air thrown from  a minute opening into any flame, so as  with
great decision to  direct the point of  the flame upon the body to *be heated.
The sand-bath and water-bath of  the chemist are  merely means  of insuring
a more uniform or steady temperature:—a vessel imbedded  in sand, so that
heat can reach  it only through the sand, cannot be very suddenly heated or
cooled, because sand is a slow conductor;  and a vessel immersed in  boiling
water,  can  never  have greater heat than 212°, or the boiling  heat of water.
For certain purposes, hotter baths, as of high-pressure steam, or  of solutions
of certain salts, or of vapour of oil of  turpentine, or of boiling whale oil,  have
been used.  On such subjects, readers  may consult works  on " chemistry
applied to the arts."

" Condensation and friction as causes of heat." (Read Analysis, page 256.)

  A soft iron-nail laid upon an anvil,  and receiving in rapid succession three
or four powerful blows of a hammer becomes hot enough to light a match,
and if longer hammered, will  become incandescent or red-hot,—partly  from
the diminished volume or condensation of  the  iron, on the principle already
explained, and partly from the  percussion or friction, in a way not  yet well
understood, but probably electrical.
  In the familiar case of the  mutual  percussion of  flint and steel, small por-
tions of one or both are struck off' by the violence of the collision,  in a state
of white heat, and the particles of the iron burn in passing through the air: in
a vacuum, the heated particles are equally produced, but are scarcely visible
from this combustion not occurring.   In both cases, they suffice to inflame
gunpowder, or to light  tinder.   When the materials are good the shower or
burst of sparks from  the sudden blow is most brilliant.
  The heat produced by friction alone  without perceivable  condensation of
the bodies concerned, is exemplified in many facts.  Two dry branches of a tree
kept strongly rubbing against each other by the wind, have  sometimes  set a
wood on  fire.  Savages light their fires  by analogous means.   Men warm
their cold  hands in   winter, by  rubbing them  against each other, or  against
their coat  sleeves.   Again,  the axeletree of a heavily laden wagon or  other
carriage,  if left without oil, is often heated  so as  to "inflame wood near it.
The line attached to a whale-harpoon as  it  runs over the side of the boat
when the whale dives after the harpoon has entered his  flesh, requires that
water  be constantly thrown  upon it to prevent its setting fire to  the  boat.
The cable of a ship drawn very rapidly through the hawse-hole by the falling
anchor, produces there  intense heat and smoke.   When a  great  ship is
launched from the builder's yard into the deep, and glides along the sloping
beams, a dense smoke usually rises from the points of rubbing contact. 
ANIMAL FUNCTIONS A SOURCE.
323
  " The functions of animal life a source of heat."  (Read the Analysis
                                p. 256.)
  It is one of the  remarkable facts in nature, that living animal bodies, and
 o a certain degree living vegetables also, have the property of maintaining in
 themselves a peculiar temperature, whether surrounded by bodies that  are
 hotter or colder than they.  Captain Parry's sailors, during the polar  winter,
 where they were  breathing  air that froze murcury, still  had in them  their
 natura warmth of 98° of Fahrenheit: and the inhabitants of India, where the
 same thermometer stands sometimes at 115° in the shade, have  their blood
 at no higher temperature.
  It was at one time  the favourite explanation  of this, that animal heat was
 produced  in the lungs, during  respiration, from the oxygen then admitted.
 I his oxygen combines with  carbon from  the blood, and  becomes carbonic
 acid as in combustion, and it was supposed fo give  out a portion of its latent
 heat, as in actual combustion ;  which heat, being then spread over the  body
 by the circulating blood,  maintained the temperature.  We now know that if
 such  a process assist, which it probably does,—for the animal heat has gene-
 rally  a relation to  the quantity of oxygen required in  any particular  case,
 and when an animal being already much heated needs no increase, very little
 oxygen disappears,—still much of the effect is dependent on the  influence of
 the nerves either directly, or  indirectly, through the vital functions governed
 by them.   Mr. Brodie, in his important experiments upon the subject, found
 that, although in animals apparently dead from injury done to the  nervous
 system,"he could artificially continue the action  of respiration, with the visual
 formation of carbonic acid, still the  temperature fell very quickly.   Again.
 the maintainance of low temperature in an animal immersed in air hotter than
 itself, is partly attributable to  the copious perspiration and evaporation which
 then take  place, and which absorb into  the  latent  form the  excess  of heat
 then existing.  Perspiration, both from  the skin  and internal surface of the
 lungs, occurs  generally in proportion to the excess of heat.   Dogs and other
 animals, when much heated, as they cannot throw off or diminish their natu-
 ral covering, increase the evaporating surface  by protruding a long, humid
 tongue.
  The power in  animals of preserving their peculiar temperature  has  its
 limits.  Intense cold coming suddenly upon a  man who  has not sufficient
 protection, first causes a sensation of pain, and  then brings on an  almost
 irresistible sleepiness, which  if indulged proves  fatal.   Sir Joseph Banks
 having gone on shore one day near  the cold Cape Horn, and  being fatigued,
 was so overcome by the feeling mentioned, that he intreated his companions
 to let him  sleep but for a little while.  His prayer, if granted, might  have
 allowed to come  upon him  that sleep  which ends not—as, under  similar
 circumstances, it came upon  so many thousands of the army which Buo-
 naparte led into Russia, and lost there during the  disastrous retreat through
 the snows.  The celebrated bulletin which allowed  that, in one night, when
 the thermometer of Reaumur stood at  19° below zero, 30,000 horses died,
 declared not  the number of  human  victims—tenderly  loved husbands and
 brothers, and children of thousands anxiously waiting their return, but doomed
never to see them more.  Cold in inferior degrees, and  longer continued,
acting on  persons imperfectly protected by clothing, &c,  induces a  variety
of diseases, which destroy more slowly;  as many of the winter diseases of
England.  A great excess of heat, again, may at once excite a fatal apoplexy, 
324                             HEAT.
and heat in inferior degrees, but long continued, may cause those fevers, Sic,
which prevail in warm climates, and  which are so destructive to strangers.
  Each species of animal has a temperature natural to it, and in the diversity
are found creatures fitted to live in all parts of the earth, what is wanting in
internal bodily constitution  being found in the admirable covering whicli has
been  provided to  protect them—covering  which grows  from  their bodies,
with form of fur or feather, in the exact degree required, and even so  as in
the same animal to vary with climate and season.  Such covering, however,
has been denied  to man; but the denial is  not one of unkindness,—it is  an
indication of his superior nature and destinies.  God-like reason  was be-
stowed on him, by which he subjects all nature to his use, and  he was left
to clothe himself.
  The human race is naturally inhabitant of  a warm  climate, and the  para-
dise described  as  Adam's first abode, may be  said  still to exist over vast
regions about the equator.   There the sun's influence is strong and uniform,
producing a rich and warm  garden, in which  human  beings, however  igno-
rant of the world which they had come to inhabit, would have their necessi-
ties at once  supplied.   The  ripe fruit is there always hanging from the
branches; of clothing there is required only what moral feelings may dictate,
or what may be supposed to add grace to the  form; and as shelter from the
weather, a few  broad leaves spread on  connected reeds, complete the Indian
hut.  The human  family, in multiplying and  spreading in all directions from
such a centre, would find to the east and west, only the lengthened paradise,
with  slightly varying  features of beauty;  but  to the north and  south, the
changes of season, which make  the  bee of high latitudes  lay up its winter
store of honey, and send migrating birds from country to country in search
of warmth and food, would also  rouse  man's energies to protect  himself.
His faculties  of foresight and contrivance  would come into play, awakening
industry : and as their fruits, he would soon possess  the knowledge and the
arts which secure  to him a happy existence in all climates, from the equator
almost to the pole.  And it is chiefly because man has learned to produce at will,
and to control, the  wonder-working principle of heat,  that in the  rude winter,
which seems the death of nature, he, and other  tropical animals and  plants
which he protects, do  not in reality  perish—as exemplified  when a Canary
bird  escapes  from  its cage, or an infant is exposed  among  the snow-hills.
By producing heat from his  fire,  he then obtains a novel but most pleasura-
ble sort of existence:  and in the night, while the  dark  and freezing  winds
are howling  over his roof, he basks  in the presence of his mimic sun, sur-
rounded by his  friends  and all the delights of society; while,  in his store-
rooms, or in those  of merchants at his command, he has the treasured delica-
cies  of every season and clime.   He soon becomes aware, too, that the
dreary winter, instead of being a curse, is really in  many respects a blessing,
by arousing from the apathy to which  the  eternal serenity of a  tropical sky
so much disposes.   He sees  that in  climates where labour and ingenuity
must precede enjoyment, every faculty of mind and body is invigorated ; and
that hence the sterner  climates produce  the perfect man ;—that in them the
arts and sciences have  reached their  present  advancement, and  the brightest
examples have arisen of intellectual and moral excellence:  while from them,
as centres, knowledge and  example  are spreading  over all the  earth, and
promising soon  to  render the whole of human kind but one large and happy
family. 
LIGHT.
325
             PART  IV.

                (continued.)


SECTION  II.—ON LIGHT,  OR OPTICS.
ANALYSIS OF THE SECTION.
Light is an emanation from the sun and other self-luminous bodies, becom-
  ing less intense as it spreads, and which, by falling on other bodies, and
  being reflected from them to the eye, renders them visible.  Its absence is
  called darkness.  It moves  with great velocity, and in straight  lines
  where there  is  no obstacle—leaving shadows  where it cannot fall.  It
  passes readily through some bodies—which are therefore  called transpa-
  rent—but when it enters or leaves their surfaces obliquely,  it suffers at
  them a  degree  of bending or refraction proportioned to the obliquity.
  And a beam  of white light thus reflected or bent, does not all bend equally,
  but is divided or resolved into beams of what are  called the elementary
  colours, which colours, on being again blended, become the white light as
  before.
Transparent bodies, as glass, may be made of such form as, by the powers
  of refraction thence received, to cause  all  the  rays which pass through
  them from any given point to bend and meet again in another point
  beyond them;—the body, then, because  usually inform somewhat resem-
  bling aflat bean or lentil, being called a lens.  And when the light thus
  proceeding from every point of an object placed before a lens is collected
  at corresponding points behind it, a perfect image of the object is  there
  produced, which may be seen from any situation on a white screen placed
  to receive it,  or in tlie air, if viewed from behind.  Now the most import-
  ant optical instruments and even the living eye, are merely various arrange-
  ments of parts for producing and examining such images as now described.
  When this image is received upon a suitable white surface or screen in a
  dark room, the arrangement is called, according to minor  circumstances,
  a CAMERA OBSCURA, a MAGIC LANTERN, 01' a SOLAR MICROSCOPE.  And the
  eye itself is, in fact, but a small camera obscura, enabling the mind to
 judae of external objects, by the size, brightness, colour, <y-c, of the very
  minute but most perfect, images or pictures formed at its back part, on the
  smooth screen of nerve called the retina.   The art of painting  aims at
  producing on a larger scale such a picture as is formed on the retina, and
  which, when  afterwards held before the eye, and reproducing itself in mini-
  ature upon the retina, may excite the same impression as  t^jmgiruU
  objects—When the image beyond a lens, formed as above  described, is
  viewed in the air, by looking at it from behind, that is, from a situation
    22 
326
LIGHT.
  where the light continued from it passes, then there is exhibited the arrange-
  ment of parts constituting the telescope or common microscope.
Light falling on very smooth or polished surfaces, is reflected so nearly in
  the order in which it falls, as to appear to the eye receiving it as if coming
  directly from the objects originally emitting it—and such surfaces are
  called'mirrors.  Mirrors may be plane, convex, or concave; and certain
  forms will concentrate  light, to produce images  by reflection, just  as
  lenses produce them by refraction ; so that there are reflecting telescopes,
  fyc,  as there are refracting instruments of the same name.   Light, again,
  falling on bodies of rougher or irregular surface, or which have other
  peculiarities, is so modified as to produce all those phenomena of colour
  and  varied brightness seen  among natural bodies, and giving them their
  distinctive characters and beauty.
                     "Light."   (See the Analysis.)

  The phenomena of light and vision have always been held to constitute a
most interesting branch of natural science; whether in regard to the beauty
of lio-ht, or its utility.   The beauty is seen spread over a varied landscape—
amoncr the beds of the flower-gardens, on the spangled meads, in the plumage
of birds, in the clouds around the rising and setting sun, in the circles of the
rainbow.  And the utility  may be judged of by the  reflection,  that if man
had been compelled to supply his wants by groping in utter and unchange-
able darkness,  even if originally created with all the knowledge now existing
in the world, he could scarcely have secured his existence for one day.  In-
deed,  without light, the earth would have been an unfit abode even for grubs,
generated and living always amidst their food.  Eternal night  would have
been universal death.  Light, then, while the beauteous garb of nature, cloth-
ing the garden and the meadow—glowing  in  the ruby—sparkling  in the
diamond, is also the absolutely necessary medium of communication between
living creatures and the universe around them.   The rising sun is what con-
verts  the  wilderness of darkness which night  covered, and which,  to the
voung mind not yet aware of the regularity of nature's changes, is so full of
horror, into a visible and lovely paradise;—no wonder,  then, if in early ages
of the world, man has  often been seen bending  the knee before  the glorious
luminary, and  worshipping it as the God of Nature.  When a mariner, who
has been toiling in midnight  gloom and tempest, at last perceives  the dawn
of day, or even the rising of the moon, the waves seem to him less lofty,
the wind is only half as fierce, and hope and gladness beam on him with the
light of heaven.   A man, wherever placed in light, receives by the eye from
every object around, nay, from every point in every object and at every mo-
ment  of time, a messenger of light to tell him what is there, and in what
condition.  Were he omnipresent, or had he the power of flitting from place
to place with the speed of the wind, he could scarcely be more  promptly
informed.  Then, in many cases  where distance  intervenes not, light can
impart at once knowledge which, by any other conceivable means, could
come only tediously, or not at all.  For example, when  the illuminated coun-
tenance is revealing the secret workings of the heart,  the tongue would  in
vain try to  speak, even in long phrases,  what one smile of friendship  or
affection can in an instant convey;—and had there been  no light, man never
could have suspected the existence of the miniature worlds of life  and activity, 
EMANATION FROM THE SUN, ETC.
327
 which, even in a drop of water, the microscope discovers to him;  nor could
 he have formed any idea of the admirable structure of many minute objects.
 It is light, again, which gives the telegraph, by which men readily converse
 Irom hill to  hill, or across an extent of raging sea—and it is light which, pour-
 ing upon the eye through the optic tube, brings intelligence of events passing
 in the remotest regions of space.                                       °

     "Emanation from the sun," <y-c.   (See the Analysis, page 325.)
  The relation of the sun  to light is most strikingly marked in the contrast
 between night and day.  In tropical countries, where the sun rises and sets
 almost  perpendicularly,  and allows not the long  dawn and twilight of tem-
 perate latitudes,  the   change from  perfect darkness to  the  overpowering
 effulgence of day, and the  contrary change are so sudden as to be most im-
 pressive.   An  eye turned  in the morning to the east has scarcely noted a
 commencing brightness  there, when  that brightness has already become a
 glow; and the clouds  floating near so as  to meet the upward rays, appear like
 masses  of golden fleece  suspended in the sky: a little after the whole atmo-
 sphere is  bright, and as  the stream of light reaches the  lofty mountain-tops,
 it makes them shine  like burnished pinnacles ; then as that stream descends
 to lower and lower levels,  the inhabitants in succession see the radiant orb
 first rising above the  horizon like a tip  of flame, and soon displaying all its
 breadth and glory, too bright for the  eve to  dwell upon.   With evening the
 same appearances recur  in a reversed order,  ending, as in the  morning thev
 began, by complete darkness.
   Light emanates also from the stars, but they are so distant as  in that respect
 to be of little  importance to this earth.   And  all bodies  in combustion are
 self-luminous, as exemplified in our common fires and lamps.  And there are
 still other transient sources in animal and vegetable nature, and among  solar
 phosphori.
  There have been two  opinions respecting the nature of light; one, that it
 consists of extremely minute particles darting all around from the luminous
 body ; the other,  that the phenomenon is altogether dependent  on an undula-
 tion among  the particles  of a very subtile  elastic fluid diffused  through space
 —as sound  is dependent on an undulation among  air-particles.  To admit the
 first opinion, the  particles of light must be held to be most wonderfully mi-
 nute, for a common  taper  can  fill with them during  hours a space of four
 miles in diameter ; and with the extreme velocity of light, if its particles pos-
 sessed at all the property of matter called  inertia,  their momentum should be
 very remarkable ;—yet, even a large sunbeam collected by a burning-glass,
 and with the precautious necessary in the case, thrown upon the scale of a
 most delicate balance, has not the slightest  effect upon the equilibrium. Such,
 and many other facts  to be  treated of in subsequent parts of this work, lead
 to the opinion that there  is  an undulation of an elastic fluid concerned in pro-
 ducing the   phenomenon of light,—although the fact of light spreading so
 nearly in straight lines, as  if only the crown of the wave had  existence, in-
 stead of beingDdiffused like  sound, is  an  important difference.

  " Becoming less intense as it spreads" (See the Analysis, page 325.)
  Any emanation from a central point, in spreading through water space,
becomes  proportionally thinner or less  intense.   Thus, if  a taper be placed
in the centre of a box, each side of which is a foot square, the  ight falling
on the sides of the box will have a certain intensity there:—if the taper be 
 328                             LIGHT.

 then placed in a box with sides  of two feet  square, there will be only the
 same quantity of light, but it will be spread over four times the surface, (a
 square of two feet is made up of four squares of one foot,) and will therefore,
 on any part of that surface, be only one-fourth part as strong or intense as in
 the first box:—and so for any other size of box or space, the intensity will
 diminish as the square of the distance increases.
  Hence four times as much light and heat fall upon a foot of this earth's
 surface as  upon a foot of the surface of the planet  Mars,  which is twice as
 distant from the  sun:—as four times as much light and heat fall on  a man
 who is at one yard from the fire, as on another who is distant two yards.

   " Falling on other bodies makes them visible."   (Read the Analysis,
                              page 325.)
  If the window-shutter of an apartment be  perfectly closed,  an eye  there
 turns upon an absolute blank: it perceives nothing.
  If a ray of the sun be then admitted, and made to  fall upon any object, that
object becomes bright, and affects the eye as if it were itself luminous.   It
returns a part of the light which falls upon it, and it is visible in all directions,
proving that it scatters the received light all around.  This scattered  light,
again falling on other objects, and reflected  from and among them until ab-
sorbed, like echo repeated many times and lost between perpendicular rocks,
makes all of them visible, although in a less degree, and the whole apartment
is said to be lighted.  If the sun's ray be made to  fall  upon a thing which,
from its nature, reflects much of the light, as a sheet of white paper, the apart-
ment will be well lighted :—if, on the contrary, it be received on black velvet,
which returns hardly any light, the apartment will remain dark ;—and, again,
if received  on a polished  surface or mirror, which returns nearly the whole
light, but in  one  direction only,  and therefore throws  it upon some  other
single object, the effect will  be according to the nature of that object, and
 nearly as if the ray had fallen directly upon it.
  Now all bodies on earth, and among these the constituent particles of the
mass of atmosphere surrounding the earth, retain and diffuse among them-
selves, for  a time, the light received directly from the sun, and by so doing,
maintain everywhere that milder radiance  so agreeable to the sight,  which
renders  objects visible when the sun's direct ray does not  fall upon them.
But  for this fact,  indeed,  all bodies  shadowed from the sun, whether by
intervening clouds or by  any other more opaque  masses on earth, would be
perfectly black or dark ; that is, totally invisible. And without an atmosphere,
the sun  would appear a round luminous mass in a perfectly black sky.  On
lofty mountain summits,  where half the atmosphere  is below the level, the
direct rays  of the sun are painfully intense, and the sky is of darkest blue.
  A shadow is the  name  given  to the comparative darkness  of  places or
objects which are prevented by intervening  things from receiving the direct
rays of some  luminous  body  shining on the things around.  The appa-
rent darkness of a shadow, however, is not proportioned to its  real darkness,
but to the  intensity of the surrounding lights.  A  landscape  may be very
bright, even when the sun  is veiled by a cloud, and then little or no shadow
is perceived; but as soon as the cloud  passes  away, deep shadows are cast
behind or beyond every projecting object.   Yet  the objects and places then
appearing  so dark, are in reality  more  illuminated  than before the shadow
existed,  for they are receiving, and again  scattering new light from all the
more intensely illuminated objects around them.  A finger held between a
candle and the wall casts  a shadow of a certain intensity; if another candle 
LIGHT MOVES WITH GREAT VELOCITY.            329
be then placedI in the same line from the shadow, the shadow will appear doubly
dark, although in fact more light will be reaching it and reaching the eye from
it than before;  it will be more dark only by comparison.   If the candles be
separated laterally, so as to produce two shadows of the  finger, but which
coincide or overlap in one part, that part will be of double darkness, as
compared with the remainders.  The most accurate mode of comparing lights
is to place them at such distances from  a screen or wall, as to make them at
the same time  throw equally dark  shadows  of the same  object; and then
according to the law of decreasing intensity explained above, to calculate  the
intensities of the sources of light  by the  difference of their distances from  the
wall.  The eye judges very easily of the equal intensity of compared sha-
dows of the same object.
   The real darkness of a shadow, then, depends  on the number and nature
of the light-reflecting objects around it.   Thus,  shadows are less  remarkable
opposite to any white surface, as that of  a recently painted wall, than in other
situations.  The reason why the moon, when eclipsed, that is, as will be
afterwards explained, when passing- behind the  earth, or through the shadow
cast by the earth in a direction away from the sun, becomes almost, if  not
quite invisible, is that there are  no other moons or bodies bearing laterally on
the moon to share their light with it.   And the  reason why our nights on
earth are darker than the shadows behind a house or rock in the sunshine of
day, is merely that there are not other earths near us  to reflect light into  the
great night-shadow of the earth, as  there are other houses and rocks to illu-
minate the day-shadow of these.  The moon is  the only light-reflecting body
vvhich the earth  has near it; and we perceive how much less dark the earth's
night-shadow is when the moon is so placed as to bear upon it.  The eclipsed
moon, again, is invisible to men on earth, because it receives neither sunshine
nor reflected light from this  earth,  for the  side turned towards us faces  the
shadowed part of the earth; but when the  moon is near the situation  in
which it is called new moon, or between us and the sun, the shaded side  of
the moon is then, in a degree,  visible to us  because facing the enlightened
side of the earth ; the bright crescent, or part of the moon illuminated by  the
sun, appearing to embrace the  non-illuminated  part,  and giving  occasion to
the popular  saying, that the new  moon holds the old moon in its arms.
   Many  persons  have doubted  whether the light of the moon could  be
altogether reflected light  of the  sun; the moon appearing to them more
luminous  than any body on earth merely exposed to the sunshine.  Their
error has  arisen  from contrasting the moon while returning direct sunshine
with  the  shadows of night on the earth around them.   But could they at
night see on a hill near them,  a white  tower or other object scattering light
as when it receives the rays of the sun, that object being nearer than the
moon, would appear to  them almost to  be on fire, and much brighter than
the moon.  The moon, when above the horizon in the day time, is perfectly
visible on earth, and is then throwing towards the earth just as much light as
during the night; but the day-moon  does not appear more luminous than any
small while cloud: and  although  visible   every day, except near the change,
many persons have  passed  their lives without  ever observing it.  The full
moon gives to the earth only about a one-hundred-thousandth part as much
light as the sun.

   "  Light  moves with great velocity."  (See  the Analysis, page 325.)
  The extraordinary precision with which  the astronomical skill of modern
days enables men to foretell the times of remarkable appearances or changes 
330                             LIGHT.
among the heavenly bodies, has  served  for the detection  of the fact, that
light is not an instantaneous communication between distant objects and  the
eye, as was formerly believed, but  is a messenger which requires time to
travel: and the rate of travelling has been ascertained.
  The eclipses of the satellites or  moons of the  planet Jupiter had been
carefully observed for some time,  and a rule was obtained which foretold  the
instants in all future  time when the satellites were to glide into the shadow
of the planet, and disappear, or when again to  immerge into view.   Now it
was found, that these appearances took place 16_i; minutes  sooner when  Ju-
piter was near the earth, or on  the same  side of the sun with the earth, than
when it was on the  other  side, that is  to say, more distant  from the earth
by one diameter of the earth's orbit; and at  all intermediate stations,  the
difference diminished from the 16| minutes, in exact proportion to the less
distance from the earth.   This proves, then, that light takes 16£ minutes to
travel across the earth's orbit, and 8| minutes for  half that distance, or to
come to us from the  sun.
  The velocity of light, ascertained in this way, is such, that in one second
of time,  viz:  during a single vibration  of  a  common clock pendulum, it
would go and  come  from  London  to  Edinburgh  200 times, the distance
between  these  being 400 miles.   This  velocity  is so  surprising that  the
philosophic Dr. Hooke, when  it was  first asserted that light  was thus pro-
gressive, said he  could more  easily believe the  passage to  be absolutely
instantaneous, even for any distance, than that  there should be a progressive
moment so prodigiously rapid.  The truth, however, is now put quite beyond
a doubt by many collateral facts bearing upon it.
  As regards all phenomena upon earth, they may  be considered as happen-
ing at the very  instant when the eye perceives  them ;  the difference  of time
being too small to be appreciated; for, as shown in the preceding paragraph,
if our sight could reach from London to Edinburgh, we should perceive a
phenomenon there in the four-hundredth part of a second after  its occurrence.
  It is hence usual and not sensibly incorrect,  when  we are measuring  the
velocity of sound, as when a cannon is fired, by observing the time between
the flash and the  report, to suppose that the event takes place at the very
moment  when it is perceived by the eye.
  In using a telegraph, no sensible time  is lost  on account  of light requiring
time to travel.   A message can  be sent from London to Portsmouth in a minute
and a half; and at the same rate  a communication might  pass  to Rome in
about half an hour,  to Constantinople  in forty minutes, to Calcutta in  a  few
hours, and so on.   A telegraph is any object which can be made to  assume
different forms  or appearances at the  will of  an attendant, and  so that  the
changes  may be distinguished  at a distance.   A pole with movable  arms is
the common construction, each position  of the arms standing for a  letter, or
cypher, or word, or sentence, as may be agreed upon.  Telegraphic signals
between  ships at sea are generally made by a few flags, the meanings of each
being varied by the mast on which it is hoisted, and by its combination with
others.

  " Light proceeds in straight  lines," Sic   (Read the Analysis, page 325.)
  We have scarcely a clearer notion of a straight line than that received from
the direction in which light moves :—but we can verify a line  so obtained by
other means, as by stretching a cord between the two extremes, or by  sus-
pending a weight by a cord, and making a movable solid measure to  corre-
spond with the cord, which standard may be used in any other case. 
LEAVING SHADOWS WHERE IT CANNOT FALL.         331
 _ W e can see through a straight tube, but not through a crooked one.  The
vista through a long straight tunnel is striking as an illustration of this fact,
and of the diminution of the apparent size of objects as they are more  distant.
lfaPer?jln enter one e"d of the canal-tunnel two miles long, cut through the
chalk-hills  near Rochester as part of the canal whicli joins the Thames and
Med way rivers, the opening at the distant end is seen as  a  minute luminous
speck, having the form of the general arch; and a person who has advanced
half way through the tunnel may see the luminous  speck at each end, then
appearing a little larger than in the former case.
  In taking aim with gun or arrow, we are merely trying to make the pro-
jectile go to the desired object nearly by the path along which the light comes
from the object to the eye.
  A carpenter looks along the edge of a plank, &c, to see whether it be
straight.
  Because light  moves  in straight lines, if a number of similar objects be
placed in a row from the eye, the nearest one hides the others.  In  a wood
or city,  a person sees only the trees or houses that are next to him.
  He who believes that a squinting person  can see round a corner, may also
believe that a crooked gun can shoot round a corner.
  All astronomical and trigonometrical observations are made oh the faith of
this property of light, the observer holding that any object is situated from
him in the direction in which  the  light comes  to him from it.  When the
mariner, after watching for hours in cloudy weather, has caught a glimpse of
the sun  or star through his sextant-glass, he has ascertained his place among
the trackless  waves, and boldly advances through the midst of hidden dangers.
And the beam darting from the light-house  across the stormy sea, would  be
useless if the light moved not in a straight line.


 " Leaving shadows where it cannot fall."  (See the Analysis, page 325.)
   The form of shadows proves that light moves in straight lines, for the out-
line of the shadow is always correctly that of  the  object  as seen from  the
luminous body.   If the light bent round the body, this could not  be.
   The shadow of a face on the wall is a correct profile.
   As a  wheel presented edgeways to the eye  appears only like a broad line,
but becomes  oval or round as it is  more turned, so  a wheel presented edge-
ways to the sun or other light casts a  linear shadow on the wall behind  it,
the shadow becoming oval or round as the position is changed.
   A globe, a cylinder, a cone,  and a flat circle, will all throw the same round
shadow if held with their axes pointing to  the luminous body, and, therefore,
by the shadow only, these objects could not be  distinguished.
   The fio-ure of a rabbit cut in pasteboard, will  throw the same  shadow  on
the wall as the animal itself; and,  again, that  shadow may be  well  imitated
by a certain position of the two hands joined, as is known to those who find
pleasure in witnessing the surprise and delight  of a child who beholds such
a shadow made to mimic the actions of life.
   A man under the vertical sun stands upon his little round shadow; but  as
the sun declines in the afternoon, the shadow juts out on the  opposite side,
and at last may extend across a whole field.
   A distant cloud which appears to the eye of an observer on y  as  a streak
alono- the  sky, may  yet be broad enough to shadow a whole  region; tor
clouds generally form in level  strata, and when viewed  by a spectator  on
earth at a distance are seen nearly edgeways. 
332
LIGHT.
  The velocity of th.e wind may be ascertained by marking the time which
the shadow of a cloud takes to pass over a plain  or  other space of known
dimension.
  A body held between a candle and the wall darkens a portion of the wall,
or casts  its  shadow there;  and the  whole space between it  and  the  wall
is a shadowed space, for any thing introduced there  is as much shadowed
as the portion of the wall.  Thus, also, all the heavenly bodies which re-
volve about the sun cast a shadow beyond them or away from the sun, as
is seen when one of them, before  brightly  visible, passes  where the shadow
of another is.   The satellites or moons of  Jupiter, when  they  suddenly dis-
appear to our glasses, or are eclipsed as  we  term  it, have  generally only
plunged into the shadow of the planet, and are not hidden by being then on
the other side  of the planet, as many suppose.   When  our own  moon is
eclipsed, that phenomenon  so awful  in the early ages of the  world, she is
only passing through the long shadow which the earth casts beyond it.
   When in the case of  a luminous centre and  a body casting a shadow, the
centre is larger than the body, then the cross section of the shadowed space,
or the shadow as thrown on a plane surface, will be less than the body, and
less, moreover, the farther the surface  is from the body, for the shadowed
space terminates  in a point.  This is true  of  the  shadows of all the planets
and of the earth, because they are less than the sun.—On the contrary, if the
light-giving surface is smaller than the opaque body, the shadow will be larger
than the body.  The shadow of a hand held  between a candle and the  wall
is gigantic;  and a  small pasteboard figure of  a man placed  near  a narrow
centre of light, throws a shadow as big as a real  man.   The  latter fact has
been amusingly illustrated by the  art of making phantasmagoric shadows.
   When the surface which receives a shadow is not directly exposed to the
light, the shadow may be much  larger than the object, even although the sun
himself be throwing the light;—as  is  seen when a slightly  projecting roof,
or a viranda, shadows from the  high sun of summer noon the whole front of
a house; or, as is proved by the  long  evening shadows of all countries; a
low wall will shadow from  the setting sun  a whole field.

" Light  passes readily through  some bodies—ivhicli are, therefore, culled
  transparent;  but when  it enters, or leaves their  surfaces  obliquely, its
  course is bent."  (Read the Analysis, page 325.)

  It may well excite  the surprise of inquirers that light, of  which the con-
stitution is so fine  or flimsy, should still be able to dart readily and in every
direction through great masses of solid matter, but such is the truth.   Thick
plates of solid  glass, blocks of  rock  crystal,  mountains of ice, &c, are in-
stantly pervaded by the  beam of the sun.
  What it is in the constitution  of one mass as compared with another, which
fits the one to transmit light, and the  other  to obstruct it, we cannot clearly
explain, but we perceive that the  arrangement of the particles  has  more in-
fluence than their peculiar nature.  Nothing is more opaque than thick masses
of the metals,  but nothing is more transparent than equally thick masses of
the same metals in  solution, nor than the  glasses of which a metal forms a
large proportion.   The  thousand salts formed by the union of  the  metals or
earths with the diluted acids, are all transparent, when, in  cooling  from  the
fluid to the solid state, their particles have been allowed to arrange themselves
according to the laws of their mutual attraction, that is to say, to form crys-
tals ; but the same substances in other states, as when reduced to powder, 
REFRACTION.
333
are opaque.   Even the pure metals themselves, when reduced to leaves of
great thinness, are transparent, as may be perceived by looking at  a lamp
through  fine gold leaf.   It is to be remarked, however, that even the most
transparent bodies intercept a considerable part of the light which enters them:
a depth of seven feet of pure water intercepts about one-half, so that  the bot-
tom of the sea is very dark.  And of the sun's light, when passing obliquely
through the atmosphere towards the earth, as when the sun  has lately risen
or is about to set, only a small part arrives.
   Light having  once entered a  transparent mass of uniform nature passes
forward in it as  straightly as in a  vacuum ; but at  the surface, whether on
entering or leaving it, if the passage be oblique, and if the mass be of a dif-
ferent density from  the transparent medium around it, a  very curious and
most important phenomenon occurs, namely, the light suffers a  degree of
bending  from its  antecedent direction, or a refraction, proportioned to the
obliquity.
   But for this fact, which to many persons might at first appear a subject of
regret, as preventing the distinct vision of objects through all transparent media,
light could have been of little utility to  man.  There could have been neither
converging lenses as now, nor any optical instruments, of which lenses form
a part, as telescopes and microscopes ;  nor even the eye itself, which has its
crystalline lens.
   Light falling from the air directly or per-
pendicularly  upon a  surface of  water,  glass,             Fl§- 136,
or any such transparent body, passes through
without suffering the least bending ;—a ray,
for instance, shot from a to the point c,  in the
surface of a  piece of glass g h, would  reach
directly across to o and b ; but if the ray fell
obliquely,  as from d  to  c, then, instead of
continuing in its first direction to  i and k, it
would at the  moment of its 'entrance be bent
downwards into a path c e, nearer to a line c
o, called the  perpendicular to  the surface at
the  point  of entrance,—and  then  moving
straightly while  in the substance of the  glass,
■ it would, when it passed out again  at e, in the
opposite surface, be bent just  as much as at first, but in the contrary direc-
tion, or away from a similar perpendicular at that surface, viz., into  the line
ef instead of e n,   A ray, therefore, passing obliquely through a transparent
body with parallel  surfaces, has its course shifted a little to one side of  the
original course, but still proceeds  in the  same direction, or in a line  parallel
to the first—as here shown in the line ef, parallel and near to the line i k; if
the surfaces of the body are not parallel, the ray is ultimately bent, as will be
explained some  pages hence.
   The degree of bending or refraction of light  in traversing a single trans-
parent surface is measured by comparing the obliquity  of its approach to  the
surface with  the obliquity of its departure after passing; and for this purpose
a line is supposed to be drawn perpendicularly through  the surface at  the
point where the  ray passes (as a b in the above figure drawn through c, where
the ray d c passes) and the relative positions of the ray to this line on both
sides of the surface, are easily ascertained.  Thus the line a d, drawn from
any point of the ray before passing to such perpendicular, is a measure or  tne
original obliquity or angular distance of the ray, and is called the sine of the
 
334
LIGHT.
angle of incidence, and the other line o c drawn from a corresponding point
of the ray after passing to the perpendicular, is a measure of the obliquity
after refraction, and is called the sine of the angle of refraction :—by com-
paring these two lines  in any case, the problem is solved.
  When light passes obliquely from air into water,  the refraction or bending
produced is such, that the line a d measuring the obliquity before refraction,
is always longer than the line o e measuring it after  refraction, by nearly one-
third of the latter, and the refractive power of water  is, therefore, signified by
the index H or 1,33;  in like manner the greater refractive power of common
glass has the index 1|, of diamond  the index 2^, and so on.  And it is im-
portant to  remark, that  for  the same  substance the  same relation  holds,
whatever the obliquity of the incidence ray may be.  If, for instance, where
the obliquity, as measured by its sine, is 40, and the refraction is half, or 20,
then in the same substance an obliquity of 10 will occasion a refraction of 5,
and obliquity of 4 will occasion  a refraction of 2; and so on.
  As a general rule, the refractive power of transparent substances or media
is proportioned to their densities.  It increases, for  instance, through the list
of air, water, salt, glass, &c.  But  Newton,  while  engaged in  his  experi-
ments upon this subject, observed that inflammable bodies had greater refrac-
tive  powers than  others, and he  then hazarded the conjecture, almost of
inspired  sagacity, which chemistry has since  so remarkably  verified, that
diamond and water contained inflammable ingredients.    We now know that
diamond is merely crystallized carbon, and that water consists of hydrogen
or inflammable  air and  oxygen.   Diamond has nearly the greatest  light-
bending power of  any known substances, and  hence comes in part  its bril-
liancy as a jewel.
   No good explanation has been given of the singular fact of refraction ; but
to facilitate the conception and remembrance of it, we say that it happens as
if it were owing to an attraction between the light and the refracting body or
medium.   The light approaching from d to c, for  instance (in the last figure,)
may be supposed to be attracted by the solid body below it, so as at the sur-
face to be bent into the direction c e; and, again, on leaving the body to be
still equally attracted  and bent back, so as to take  the  direction ef, instead
of en; and we  see why the attraction  and bending should be greater, the
greater the obliquity.
Fig. 137.
The following are familiar examples of this bending of light in passing from
  one medium to another.
  If an empty basin or other vessel b cfe, be in the sun's light, so that the
rays falling within it may reach low on the  side, as to d, but not to the bot-
tom, then, on filling the vessel with water, the sun  will be found to be shin-
                            ing on the bottom or  down to e, as well as on
                            the side.   The reason of this phenomenon is,
                            that water being a denser medium than air,
                            the light, on entering it at c, is bent towards
                            the  perpendicular (c/,) at the point of inci-
                            dence, and so reaches the bottom.  Again, if
                            a  coin  or metal  were laid  on the bottom  of
                            such a vessel  at e, it would not, while the ves-
                            sel were  empty, be seen by an eye at a, but
                             would be visible there immediatly on the ves-
                             sel  being filled  with water;—because  then,
                             the  light leaving the coin in the direction e c,
 
REFRACTION.
335
 lir!bedSbent atfvf ^ l""*' W°Uld 3t C' °n  ^S'^ from the ^er "to
 reach the ey   a7afr™h he P-P-d^lar, and  instead of going to g would
 the direction  /i    ♦   ,  ,  "' moreover' would appear  to the eye to be in
 Iblp to ,1\o   rf'uinStfad °f ,n lhe tr,le direction ce: ^ the eye not  being
 obiect to br't^r tht Hght had  been bent  in its course> woufd judge thS
 object to be in the line by which the light  came from it.            &
 /l'"8 bGCaufe obJects at ^e  bottom  of  water, when viewed obliquely,
 do not appear so low as  they really are, that a  person examining a river or
 pond, or any  clear water, from its  bank, naturally judges  its depth to be less
  han it is   Many a  young life has been  sacrificed to this error.  A person
 looking  from  a boat  directly down upon the  objects at the bottom of water,
 sees them in  their true directions,  but even then not at their true distances,
 as will be afterwards explained; and if he view them more and more ob-
 liquely,  the appearance becomes  more and  more deceiving until  at last it
 represents them  as at much less than half of their true depth.
   The ship in which the author sailed, once in the middle of the China Sea,
 where no danger was apprehended, entered by a  narrow passage a large
 horse-shoe enclosure of coral rocks.   When  the looker-out gave the  alarm,
 the predicament had become truly terrific.   On  every side, in water most
 singularly transparent, the rocks appeared to be almost at the surface of the
 water, and  the anchor, which in  the first moments had been let go to  arrest
 the ship, appeared to have been dragged to a  shallow place.  It seemed that
 if the ship, then  drawing 24 feet, or the depth of a two-storied house, moved
 but a little way in almost any direction, she must inevitably meet her destruc-
 tion.  On sending boats around to sound and  to search, the place of entrance
 was  again discovered,  and  was safely traversed a second time as  an outlet
 from that terrible prison.
   On account of this bending of light from objects under water, there is more
 difficulty in hitting them with a bullet or spear.  The aim by a person not
 directly over a fish must be made at a  point  apparently below it, otherwise
 the weapon will  miss by flying too  high.   The spear, sometimes used in
 this country for killing salmon, is a common weapon among the islanders of
 the Atlantic and  Pacific Oceans for killing the  albacore;  the use of it, like
 that of the fly-hook  in England, affording  to the fishermen sport as well as
 profit.  The author  once witnessed at  St. Helena this employment of the
 spear.  A small fish previously stunned, that it might not  try to escape, was
 every minute or two thrown upon the water as a bait, in the sight of perhaps
 a  hundred  great  albacores,  greedily  waiting for it  at  one side  below, and
 knowing the danger  to which they exposed themselves by darting across to
 seize it.  Some albacore bold enough, soon made at the mouthful, apparently
 with the  speed of lightning, but yet with speed which  did not save  him, for
 every now and then  the thrown spear met  him, and held him writhing there
 in a cloud of his death-blood.   After a victim so destroyed, the scene of action
 was changed.
   The bending of light when passing obliquely from water, is also the reason
 of the following facts.  A straight rod or stick, of which a portion is immersed
 in  water, appears crooked or broken at  the surface of the water, the portion
 immersed seeming to be bent upwards.  That part of a ship or boat visible
 under water, appears  much flatter and shallower than it really is.  A deep-
 bodied fish seen  near the surface of water, appears almost a flat  fish.   A
round body there  appears oval.  A  gold fish  in  a vase may appear as  two
fishes, being seen as  well by light  bent through the upper surface^ of the
water,' as by straight  rays passing  through the  side of the glass.   To see 
336
LIGHT.
bodies under water, in their true directions and nearly of  their true propor-
tions, the eye must view them through a tube, of which the lower end, closed
with a plate-glass,  is held in the water.
  As light is bent  on entering from air into water, glass, or other substance
denser Than air, so  it is also bent on coming from void space into the  ocean
of our atmosphere.   Hence  none of the heavenly bodies, except when directly
over our heads, are seen by us in  their true situations.   They all appear a
little hio-her than they really are, and more so the nearer they are to the hori-
                                       zon ; as when to a spectator at d,
              Fig. 138.                  suppose on the surface of the earth,
                                       a  star really at A appears to be at
                                       a, because its ray, on reaching the
                                       atmosphere at c, is bent downwards.
                                       In astronomical books  there is  al-
                                       ways introduced a table  of refrac-
                                       tion, as it is called, showing what
                                       correction must be made on this  ac-
                                       count for different apparent altitudes.
                                       This effect of our atmosphere  so
                                       bends the rays of the sun that  we
                                       see him  in  the morning  before  he
                                       is really above the horizon, and  we
see him in the evening after he is  really  below it—for the ray coming hori-
zontally from e to d, appears to come from b, although  in truth it  really
comes from the  lower situation B, and is bent into.the level line only at e.
Our atmosphere thus, by the bending of  light as well as by itself  becoming
luminous, lengthens at dawn and twilight the duration of the precious day.
As the atmosphere is denser near the surface of the earth than higher up,  the
light is more and  more  bent as it descends, and hence describes a course
which is sensibly  curved, and therefore unlike the course of light in water.
   Certain states of the atmosphere, depending chiefly on  its humidity and
warmth, change very considerably its  ordinary refractive  power; hence, in
one state, a certain hill or island may appear low and scarcely rising above  the
intervening  heights or ocean, while in another state,  the same object will be
seen towering above : and from a certain station, a city in a neighbouring
valley may  be  either entirely visible, or it may show  only the  tops of its
steeples, as  if the  bed on which it rested  had sunk deeper into the earth.  In
days of ignorance and superstition, such appearances occasionally excited  a
strange interest.
   Owing to the bending of light  in passing through the media of different
densities, a  beautiful phenomenon  is often observable in a day of warm sun-
shine.   Black or dark-coloured substances, by absorbing much light and heat
from the sun's rays, and warming  the air in contact with them, until it dilates
and rises in  the surrounding air, as oil rises in water, cause  the light, from
more distant objects, reaching the eye through the rarefied medium, to be
bent a little;—and owing to the  heated air rising irregularly under the influ-
ence of the wind  and other causes, these  objects acquire the appearance of
having a tremulous or a dancing  motion.   In a warm  clear day, the whole
landscape at last appears to be thus dancing.
   The same phenomenon is to be  observed at any time, by looking at an
 object beyond the top of a chimney from which  hot air is rising.   An illicit
 distillery has been discovered by the exciseman happening thus to look across 
REFRACTION.
337
ere was no
ve^ofsmok" ^^ alth°U«h charCOal WaS lhe fue1' and lh

oreTaution^l^ ^ V*6 *"?»* "tates of the atmosphere renders
m«^r n« k B T"7 r"1 makl^ very nice geometrical observations :-as in
measuring base lines for the construction of maps or charts.
N„' IS the obliquity with which a ray traverses the surface, which, in any
case of refraction  determines  the  degree of bending, a body seen through a
                  ir surface appears distorted according to the nature of that
140.
 surface.   It is because the two surfaces of common  window-glass are not as
 in the case of plate-glass perfect planes, and perfectly parallel to each other,
 that objects seen through a common window appear generally more or less
 out of  shape; and  hence come the elegance and beauty of plate-glass win-
 dow: and hence the singular distortion of things viewed through that swelling
 or lump of glass, which appears at the centre  of certain very coarse panes and
 which remains where the glass-blower's instrument was attached.
    The refraction or bending of light is interestingly exemplified in the effect
 ol the  glass called  a prism, viz., a
 wedge or  three-sided  rod of glass ;                 Fig. 139.
 such as that of which the end. is
 here  represented  at b c.   A ray
 from a falling  on the surface at b is
 bent towards  the  internal perpen-
 dicular, and therefore reaches c, but
 on  escaping again  at  c, it is bent
 away from the external perpendicu-
 lar, and thus with its original deviation doubled, goes on to d.
   The law of  light's bending, according to the obliquity
 with which it traverses the surfaces of a transparent body,
 is well elucidated by the effect of  what is called a multi-
 plying glass ; that is to say, a piece of glass  like a b c e,
 having many distinct faces cut upon it at angles with each
 other.  If a small object, a coloured bead for  instance, be
 placed at d, an eye  at e will see as many beads as there
 are  distinct surfaces or faces on the glass ; for first, the
 ray d a, passing perpendicularly,  and therefore straight
 through will form an  image as if no  glass  intervened;
 then, the rays from  d to the surface b, will be bent  by the
 oblique surface, and will show the object as if it were in
 the  direction e  b; and  the  light falling on the still more
 oblique surface, c, will be still more bent, and will reach
 the  eye in the  direction c  e, exhibiting a similar  object
 also in  that  direction—and so of all the other surfaces.
 If the eye were at d, and the object at e, the result  would
 still be the same.  A plate of glass roughened, or cut into
 cross furrows,  becomes a  very good screen  or window-
 blind, by disturbing the passage of light through it so that
 objects beyond it are not distinguishable.

 "And a beam of white light thus made to bend, is  resolved into beams of the
  various primary colours ; which beams, however, on being again blended,
  become white light as before"  (Read the Analysis, page 325.)
  The most extraordinary fact connected with the bending of light is that a
pure ray of white light from the sun admitted into a darkened room by a hole
 
338
LIGHT.
                                      in the window shutter, and made to
                                      bend by passing through transparent
                                      surfaces which it meets very oblique-
                                      ly (as the ray a, admitted and made
                                      to bend by passing through the prism
                                      of glass, b c, to fall upon the  wall at
                                      d,) instead of bending altogether and
                                      appearing still as  the  same white
                                      ray, is divided  into several rays,
which, falling on the white wall, are seen to  be  of  different most vivid co-
lours.  The original white  ray is said thus to be analyzed, or divided  into its
elements.
   This solar spectrum, as it is called, formed  upon the wall, consists, when
the light is admitted by a narrow  horizontal  slit, of four coloured patches
corresponding to the slit, and appearing in the  order,  from  the bottom, of red,
green, blue, and  violet.  If the slit be then made a  little wider, the  patches
at their edges overlap each  other, and produce, by  the mixture of their ele-
mentary colours, certain new tints.   Then the spectrum consists of the seven
colours commonly  enumerated and  seen in the rainbow, viz.,  red, orange,
yellow, green, blue, indigo, and violet.   Had red, yellow,  blue and  violet
been the four colours obtained in the first experiment, the occurrence of the
others, viz., of the  orange, from the mixing edges of the red and yellow—of
the green, from the mixture of the yellow and blue,—and  of the indigo, from
the mixture of blue and violet, would have  been anticipated.  But the facts
of the case not  being such, we  see  that they are not yet well understood.
When Newton first made  known the  phenomenon of the many-coloured
spectrum, and the extraordinary conclusions to which it led, he  excited uni-
versal  astonishment;  for the common idea of purity, the most unmixed, was
that of white light.   In farther corroboration of the notion of the compound
nature of light, he mentioned,  that if the colours which appear on the spec-
trum be painted  separately  round the rim  of a wheel, and the wheel be then
turned rapidly, the  individual  colours cease to be  distinguished, and a white
band  only appears  where  they are  whirling:  also,  that  if the  rays  of the
spectrum, produced by a prism, be again gathered   together by a lens, they
reproduce white light.  The red is the kind of light which is least bent in
refraction, and the violet that  which  is most bent.   It was at one time said,
as an explanation, that the differently coloured particles in light had different
degrees of  gravity or inertia, and were, therefore, not all equally bent.   It is
farther remarkable,  with respect to the solar spectrum, that much of the heat
in the ray is still less refracted than  even  the red light, for a thermometer
held below the red light rises higher than in any part  of the visible spectrum  ;
—and there is an influence  or  something in the beam more refrangible than
even the violet rays, and capable of  producing powerful chemical and mag-
netical effects.   The different  spots of colour in the spectrum  are not all the
same size, and there is a difference in this respect according to the refracting
substance.
  All  transparent substances in bending light produce more or less  of the
separation of colour; but it is an important fact, that the quality of merely
bending a beam, or  of refraction, and that of dividing it into coloured beams,
or of dispersion, are distinct qualities, and not  having the same proportions
to each  other in different substances.  Newton, from not discovering this,
concluded that a  perfect telescope of refraction  could never be made ; he sup-
posed that the bent  light would always become coloured, and so render the
 
REFRACTION BY LENSES.
339
objects indistinct.   We now know, however, that by combining two or more
media, we may obtain bending of light without dispersion,—thus, by oppos-
ing a  glass which  bends five degrees and disperses  one degree, to another
glass  which bends  three degrees and disperses one, the opposing dispersions
willjust counterbalance or  neutralize each other, while the two  degrees of
excess of bending will remain to be applied to use.
   The diversified colours of the  substances around us depend merely upon
their fitness, from texture or other cause, to reflect or transmit  certain modi-
fications of common light, and the colour is not a part or property  of the body
itself.  We shall soon find  that the vivid colours of the rainbow are merely
the white light of the sun, reflected to us after being bent and modified by the
colourless drops of falling rain; and that  the  sparkling with  appearance of
rubies and emeralds, which we see  in cut-glass  lustre, is  a phenomenon of
the same kind :—and  that by scratching the surface of a piece of metal so as
to have a given  number of lines in a given space, we can cause  the same
substance to appear of any  colour we please.

" Transparent bodies, as glass, may be  made of such form as to cause all
   the rays of light which pass  through them from any one point, to bend so
   as  to meet again in another corresponding point beyond them,—the body
   itself, from the required  form generally resembling that of a flat bean or
   lentil, being then called a lens.  (Read the Analysis, page 325.)

   The innumerable rays of light (of which five only are here represented,)
issuing from any point at c, towards any surface  in the situation a b, are said
to form a cone or  pencil of diverging light.   Now it is evident that to make

                                Fig. 142.
all such ravs converge or meet again in one place, as/, beyond a transparent
body placed at a b, it would be necessary, while the middle ray or axis of
the pencil c d did not bend at all, for the others to be bent more and more, in
proportion as they fell upon the body farther and farther from the  centre d.
Recollecting,  then, thelaw of refraction, that  light entering from air through
the surface of any denser medium, as glass, is bent there towards the perpen-
dicular at the  internal surface, in proportion to the obliquity of incidence, and
on leaving the opposite surface, is correspondingly bont away from its exter-
nal perpendicular, (see the case of the prism at p. 364,) we see that if a piece
of glass were placed at a b, of such  form that the rays falling upon it from c
should meet and leave its surfaces with greater and greater obliquity in some 
340
LIGHT.
 regular proportion, as the points of incidence  were more distant from the
 centre d, the purpose would be obtained.  And we have the satisfaction of
 knowing that a glass, of  which the surface is ground—which it easily may
 be—to have a  regular convexity or bulging, as if it were a portion cut off
 from the surface of a globe, can be shown to answer very correctly the re-
 quired condition.  Such a glass similarly ground on both sides, is here repre-
 sented edgeways between a and b, where the ray c d falling on its middle, of
 perpendicularly, and similarly leaving it, is seen going  straight through  to/,
 but the ray c e meeting the surface with a certain degree of obliquity, is bent
 down a little, first on entering the surface at e, and then as much more on
 leaving the opposite surface  with equal obliquity, and so arrives at f; then
 the ray c a, for corresponding reasons, is still more bent, and equally arrives
 atf;—-and the case would be  similar of any other rays  that might be exa-
 mined.  The point/is usually called a focus (meaning a  fire-place,) because
 when the light of the sun is  thus gathered, the heat concentrated with  it is
 powerful enough to make combustibles inflame.—-We have here to remark
 farther, that in  accordance both with calculation and experiment,  the direction
 in which a pencil of rays falls  upon a lens  does not affect the result of the
 convergence to a focus, only the focus is always in the direction of the cen-
 tral ray of the  pencil or beam;  it will be  at p, for instance, for light issuing
 from o, and at  z for light issuing from x.
   The lens represented at a b above or in the annexed diagram, at fig.  1
 having both sides convex, is called a  double convex lens.  A glass convex
 only on one side, and plane or flat on the other, as shown at fig. 2, would

                              Fig. 143.
            Fig. 1      2      3       4.56





            nwtf

 as effectually gather the rays, but with half the  power, and the  point of
 meeting or focus would be therefore  proportionably more distant.   Such  a
 glass is called a plano-convex lens.  Then the  gathering or converging
 power of any  glass, whether doubly or  singly convex, is in proportion to
 the degree  of  its convexity  or bulging  of surfaces, for  the less it bulges,
 the more  nearly does it  approach to a plane glass, and the more it bulges,
 the more obliquely will the rays, at any distance from  the centre, fall upon
 its surface, and the sooner, therefore, in consequence  of their  being more
 bent, will they all meet  the axis ray; hence  fig. 1 would converge much
 more quickly than fig. 3,  which represents nearly a common spectacle glass;
 and a very minute  globe  is  the form most powerfully  converging of all.
The surfaces of fig. 1 are portions  of a small globe; those of fig. 3 are
smaller portions, but of a  globe much larger.   Concave lenses as—fig. 4,
a double concave, and at  fig. 5, a plano-concave lens, in obedience to the
same law of refraction, spreads rays, or bends them away from  the axis of
the pencil, in the same  degree that similarly  convex  lenses gather  them.
A  concave lens, therefore, receiving  the  converging pencil of rays  from  a
convex lens, might restore them  to  their former  direction.    Very  useful
purposes, as will be afterwards explained, are served  in optics, by certain 
CAMERA 0BSCURA.
341
 combinations of differently formed lenses.   A lens may be convex on one
 side and concave on the other, as at fig. 6, called  a meniscus lens, because
 it resembles the crescent moon, and its effect will  be  according to the curve
 which predominates.
   A person collecting the case  of the " multiplying glass," described a few
 pages back, might say,—but is not a convex lens  merely a multiplying glass
 of a much greater  number of faces, and  if so, why, instead of one image,
 does it not  make thousands?  The answer is, that the multiplying glass, by
 every face,  bends a set of rays, capable of forming a distinct  and complete
 image; but the lens has no surface large enough to bend more than a single
 ray and it concentrates all the single rays into one  place, to form there one
 image of great vividness and beauty.

 "And when the light proceeding from every point  of an object placed before
   a lens is  collected in corresponding points behind it, a perfect image of
   the object is there produced.  When the image is received upon a suitable
   white surface  in a dark place, the arrangement is called, according to
   minor circumstances, the camera  obscura, solar  microscope or magic
   lantern."  (Read the Analysis, page 325.)

   Words are wanting to express the admirable consequences to man of the
 curious property of a lens that it can bring  together to focal points behind
 all the rays  of light which traverse it  from any points of an  object placed
 before it.   The following instance will lead to the understanding of others.
 If a lens as a, be placed so  as to fill
 up an opening made  in the window-
 shutter of a  darkened room, then, from
 anyobject before that opening—as the
 cross here  represented,  all the light
 which each point emits  towards the
 lens will be concentrated  or gathered
 together in a corresponding focal point
 behind  the  lens or within  the  room,
 and if a sheet of paper be  held  there at the distance  of the focal points, a
 beautiful image of the object will be seen upon the paper.
   In these few words, we  have described the interesting contrivance called
 the camera  obscura or dark chamber; and when a glass is chosen  of pro-
 per size and focal distance, and a screen or the wall of a chamber (if at the
 required distance,) is  properly  prepared to receive the  light, the most en-
 chanting portraiture is  instantly produced of the whole scene which the win-
 dow commands.    With what rapture does  the school-boy first view  this
 lovely picture drawn by nature's own pencil, and with  colours taken directly
 from the sun's bright ray—with  what rapture, as his eyes search over it,
 does he recognize,  perhaps, his  playmates  there, and  the river in which he
 bathes, and  where  he  sails  his  boat, and the wood in whose solitudes  he
 loves to wander, and the mountain heights which he climbs to meet the fresh
 breeze, and  at a distance from the world, to allow  his young fancy to work,
 beginning to shoot  far into time and space.   The  great  peculiarity of such
 a picture is, that it does not, like others, portray still-nature, but  every thing
 with appropriate motion or changes:  the  playmates  are all in  action: the
 leafy trees wave in  the wind, the clouds sail along, the sun may rise or may
set, and even the  lightning's  gleam may dart across:  or, again, commenced
enterprizes  may be brought to a close; the traveller may climb the  distant
    23
 
342
LIGHT.
hill and disappear; the fisherman may draw his net and secure his prize; the
contested race may be won or lost.   A Malayan chief in the island of Suma-
tra, was so surprised and pleased by a small portable  camera  obscura which
the author happened to have  among his  apparatus, that he seemed disposed
to give for it almost any thing he  possessed.
   It appears in the last diagram that the  image formed beyond a lens by the
gathered light, is in a contrary position to the object itself,—that is, inverted,
—because the light from the top  of the object darts through the opening or
glass in a descending direction, and that from  the bottom rises to the opening,
and  in  the same  direction  passes  beyond it.   It  is usual,  therefore, in a
camera obscura, to place a small mirror immediately behind the lens, so as to
throw all the  light which enters,  downwards,  to  a whitened table,  upon
which the picture may be conveniently contemplated.
   The camera obscura often  gives very useful assistance to young painters,
by enabling them to  trace correctly the outlines of the objects placed before
it, and also to study effects of light, shade and colour, more profitably than
they at  first can, by looking at the objects  themselves.   The laws of per-
spective are most intelligibly illustrated in this most true  picture.
   An effect, approaching in a degree to that of the complete camera obscura
now described, is produced by merely  making a small hole in the shutter of
a  dark  room, and letting  the light which  enters  by it fall  on any  white
surface  beyond.  The whole landscape is  then dimly portrayed upon the
surface.  Barry,  the  painter, while lying on a sick bed, mistook such a
scene appearing on the ceiling of his room  for a supernatural vision.  If a
cross be held before the opening  as in the last figure, it is evident that from
every.point of the cross-light  will enter by the opening, and  will fall on
corresponding parts  of  a sheet of paper held behind,—but as the light from
each point is not a single ray, but a spreading pencil or cone  of light, it will
fall on the paper, not  on one point, but on a surface at least as large as the
opening, and thus  the light from adjoining points will mix at the edges, and
will render the images misty and  indistinct, somewhat like those on the
back of tapestry.   If the opening  be very  small,  the picture  will be well
defined, but very feebly illuminated; and if  the opening be  of considerable
size, the mixing  of the  pencils  will be  so  great as  to  leave no particular
object  distinguishable.    But,  in  the latter  case, and  however  large the
opening be, if a lens be introduced, it will converge every pencil of light to
an exact point, and  the picture  will instantly be rendered perfectly clear.
A lens is never held up  in the light without forming beyond  it pictures such
as now described,  of every visible  object about  it, and the pictures are not
seen, only because there are no screens placed to receive them, and because
they are so numerous as to  confuse one another,—in other words, because
they are not admitted singly into  a  dark chamber.
   The distance from a lens at which an  image is formed or the rays of  the
light meet, depends first upon the refractive  or bending  power of the lens,
and therefore, on its form and on  the nature of its substance;  and, secondly,
upon the direction of the rays of light when they reach the lens, as to whether
they are divergent, parallel or convergent.   We have  already explained that
glass refracts about twice as much as water, and  that diamond refracts  about
twice as much as glass :  and we have considered the effect of different degrees
of convexity in lenses—arising equally whether the lens be of vvater enclosed
between glasses like watch-glasses, or  of solid glass, or of rock-crystal, or of
diamond itself. We now proceed to consider the joint effect of the refractive
power, and of the direction of the incident rays. 
                        CAMERA OBSCURA.                      343

Rays falling from a on a comparatively flat or weak lens at L, might meet

                              Fig. 145.
 only at d, or even farther  off: while, with a stronger or more convex lens,
 they might meet at c or at b; a lens weaker still might only destroy the diver-
 gence of the rays, without being able to give them any convergence or to bend
 them enough to bring them  to a point at all,—and then they would proceed all
 parallel to each other, as  seen at e and /;—and if the lens were  yet weaker,
 it might only destroy a part of the divergence, causing  the rays from a to go
 to g and h, after passing through, instead of to i and k,in their original direction.
   In an analogous manner,  light coming to the lens in the contrary directions
 from 6, c, d, Sic, might, according to the strength of the lens, be all made to
 come to a focus at a or at /, or in some more distant point; or the rays might
 become parallel, as m and n, and, therefore, never  come  to  a focus, or they
 might remain divergent.
   It may  be observed in  the figure above, that the farther an object is from
 the lens, the less divergent  are the rays which fall  from it upon the lens; or
 the more  nearly do they approach to being parallel.  From  b there is much
 divergence in the exterior rays, from c less, from d less still, and rays from
 a great distance, as those represented by e and/,  appear quite parallel.   If
 the distance of the radiant point be very great, they really are so nearly paral-
 lel that a very nice test is required to detect the non-accordance.  Rays, for
 instance, coming to the earth from the sun, do not diverge the millionth of an
 inch in a thousand miles.  Hence, where we wish to make experiments with
 parallel rays, we take those of the  sun.
  Any two  points so situated on the  opposite  sides of a lens, as that  when
 either becomes the radiant point of light,  the other  is the focus of such light,
 are called  congugatefoci.   An object and the image of it formed by a lens
 must always be in congugate foci, and as the one is nearer the lens, the other
 will be in  a certain proportion more distant.
  What is called the  principal focus of a lens, and by the distance of which
 from the glass we compare  or classify lenses among themselves, is the point
 at which the sun's rays,  or any parallel  rays, are  made by  it to meet; and
 thus, by holding the glass in the sun, and noting at what distance behind it
the  little luminous spot or  image  of the sun  is formed,  we  can at  once
 ascertain the focus of a glass—as at a for the rays e and/.
  It is a remarkable coincidence that the bending power of the common glass
used for lenses should be such, that the  focus of a double lens is  just where
the centre of the sphere would be, of  which the surface  of the lens  is a por-
tion.   This gives  us another fact with which to associate the recollection that
the  focus is nearer as  the convexity of the lens  is greater, that is to say, as
the  surface  is a portion of  a smaller sphere.   And  such being the law, it
may be proved by calculation as well as by the fact, that if a candle be held
in relation to a lens at twice the principal focal distance, suppose at c for a lens 
344
LIGHT.
with the focus at a, the image of the candle will be formed at /just as far on
the other side.   Thus, then, by trying with the lens until the image of a
candle is  formed at the same distance from  it as the object is, we  have a
second mode of ascertaining the focal distance of a  lens.  Other kinds of
glass and  other substances refract with different power; but the facts now
stated should be retained in the memory as standards of comparison.
   Because the focal point of light passing through a lens is at the same  dis-
tance  from the  centre of the lens, in whatever direction the  light passes
through, a surface placed  to receive the image of any broad object should
really be concave, that is to say, all parts of it should be at the same distance
from the centre of the lens, otherwise the image will be more perfect  either
at its middle than towards its edges, or vice versa—but it is not  found neces-
sary to attend to this in common practice, where the object and  its image are
not of great extent.
   The size of an image formed behind a lens is always proportioned  to the
distance of the image  from the lens, and the image is as much larger or smaller
than the object as it is farther from or nearer to the lens than the object.  This
                                will be  evident  from considering the an-
                                nexed figure,  c represents the place of a
                                lens, and the lens, according  to its power,
                                will form an image of the cross a b, in some
                                situation, as  at d, e, g, Sic   Now  when-
                                ever the image is formed and by whatever
                                lens, one end of it must be in contact with
                                the line a g, and the other end with the
                           b    line b h; and as these lines cross each
                                other at c, and widen regularly afterwards,
a line joining them (and the image is such a line,) must always be shorter the
nearer it is to c,  that is to  say, shorter in proportion to the converging power
of the lens.
   Many persons may not  have reflected, that the luminous circle called the
focus of a burning glass, is really but the image or picture of the sun formed
by the glass or lens.   The intensity of the heat and of the light is of course
in proportion as  the image is smaller than the glass which forms it, and the
nearer that the image  is formed to the lens, or the more powerfully convergent
that the  lens is, the smaller will the image be.   Mr. Parker's famous burning
lens, which cost £700, and is now the property of the Emperor of China,
was three feet in diameter and the diameter of the sun's image formed by it
was one inch: it concentrated the light and heat therefore about  1,300  times.
To render the effect still more powerful, a  smaller  lens was placed behind
the larger, farther reducing the size of the image to  one-sixth.  Very sur-
prising effects were produced by this lens,  in the melting of metals, inflaming
of combustibles, &c.   The size of burning  lenses, until  lately, was limited
by the difficulty  of obtaining the great pieces of glass required to form  them:
but they are  now built up of many pieces suitably united together.   Some
large lenses have been made of water, that is, of water enclosed between
meniscus glasses, like watch-glasses.  A common goblet of water, or a vase
holding gold-fishes,  has in  some cases  acted as a burning glass, setting  fire to
the curtains, near which it had been left in the sunshine.
   And the nearer that an object is brought  to a lens, the  more distant, and
therefore the larger will its image be;  for, as the rays  falling upon a lens are
divergent in proportion to  the nearness of the object, and therefore with the
same power of lens, must  meet farther behind (as seen in the figure at page
 
MAGIC LANTERN.                        345
343,) then the axis of the rays, as the lines c a and c b in the last figure, will
have separated far before the rays meet, and  will have made the  image pro-
portionally larger.  If we suppose little tf in the same diagram to be the object,
its image would be a b.   The sun is exactly as  much larger than  his image
formed1 by a burning glass, as he is more distant from it than the image; and
if we had a screen of sufficient size hung up  in  a distant space, a very bright
object of a quarter of an inch diameter might be made by a lens to form°an
image as broad as the sun.
   From all these considerations, we see that, in  a camera obscura, the screen
should be from the lens, at the distance of its principal focus for  distant ob-
jects, and a little farther than this for near objects.   Accordingly the lens is
generally fixed in a sliding piece, which allows  the distance from the screen
to be  adjusted to  circumstances.    If the representation be desired large, the
lens must be of a long focus ; if small, the lens must be of a short focus. Again,
when by the reversed use of the lens, a small object as d is to be  magnified
to such a size as a b, then the object must be placed a little beyond the focus
of the glass; for if placed  nearer, the pencils  of  rays from it would never  be
gathered to focal points  at all, and no image would be formed at any distance.
   When, as alluded to in the last sentence, a  small object is placed very near
a lens, and the image of it  is thrown upon the wall of  a  dark room, perhaps
a hundred times farther from the lens than the object is, the image is a greatly
magnified representation of the object, viz., it is a hundred times longer and
a hundred times broader, and therefore has ten thousand times as  much sur-
face as the object; but if in this  experiment the  object  be illuminated only in
an ordinary degree, the light from it is so scattered as not to suffice for dis-
tinct division.   Hence, to attain  fully in this manner the  purpose  of a micro-
scope, a very  strong light,  concentrated by a  suitable  mirror or  glass, must
be directed upon the object.   When the light of the sun is used  in such  a
case,  the complete apparatus is called the solar microscope, and serves beau-
tifully to display the structure of many minute objects.   When artificial light
is used, as of a lamp, the apparatus is called the lucernal microscope or magic
lantern.
   A good solar microscope becomes one of the most interesting presents which
science has made to  man, for aiding  him in his researches into the secrets  of
nature.   With the late improvements in the construction of lenses, by which
the dispersion of light or the rainbow-fringe, is prevented (as will be explained
under the head of Telescopes,) objects may be magnified two or  three hun-
dred thousand times, and still be  so luminous as to be beautifully distinct:—
thus a cheese-mite will appear of the dimensions of a hog, and creatures alto-
gether invisible to the  naked  eye, or perceived  by it only as minute  white
points, are discovered to be animated beings,  having the  perfect proportions,
and often the beauty of larger animals, and endowed with similar appetites,
passions, and apparent ingenuity, but with an  activity far surpassing that met
with in the more bulky creation.  A judicious selection of objects for the solar
microscope is  calculated exceedingly to surprise  the mind on its  first attend-
ing to them, and to fill it with high  conceptions of the  infinity of creation.
With  the common microscope only one person at a time can feast his wonder;
but with the solar, a whole roomful of company  may at once contemplate the
same objects and  witness the  same actions, and thus have their  admiration
increased by the consciousness of sympathy.
  The magic lantern, we have said, consists of a powerful lens, with objects,
highly illuminated  by lamp-light, placed so  near  it that their images are
formed far off, and are therefore proportionally larger.  For the magic lantern 
346
LIGHT.
the objects are generally paintings made on thin plates of glass with trans-
parent colours ; and  each plate is formed to slide through a slit or passage
behind the lens.  The lens itself, or what may be called half of it, (for there
are often two lenses joined to give greater power,) is movable with the tube
which is seen projecting from the lantern, so that its distance from the object
may be varied, and thus a corresponding  approach to or receding  from the
screen may be allowed, which will produce an  increase or lessening of the
magnitude of the visible picture on the wall.
   Some public lectures on astronomy and  other branches of natural history,
have had  the drawings and paintings  required  for  the  elucidation of their
subjects, made in  miniature upon glass, to be  magnified afterwards to the
degree desired, and shown upon any part of the lecture-room  by the magic
lantern.
   A thick fog or smoke at night will sometimes reflect the images of a magic
lantern so as to make them distinctly visible ; and there are several cases on
record, where persons, wickedly ingenious in this way, have  terrified igno-
rant individuals almost to death, by throwing spectres from a concealed lan-
tern.   Some years ago a sentinel in St. James' Park was thus persuaded that
he had seen supernatural beings near him  among the trees.
   A very charming illusion  is produced by a magic lantern manoeuvred on
one side of a thin screen, while the spectators, not aware of the existence of
the screen, are sitting on the other side.   The image—let us suppose it that
of a genius  flying in the air—may be first thrown  upon the screen from the
lantern while very near, and then it will be small, and, if desired, exceedingly
bright, because the light is much  concentrated.   If the  exhibitor  then gra-
dually recede from the screen, adjusting at the same  time the distance of the
lens from the picture, the image will become progressively larger, and to the
spectators will appear to be soaring and approaching, until at last the expand-
ed wings  and limbs  seem  hovering almost over their  heads.   An  endless
variety of most ingenious and  beautiful exhibitions of this kind have been
made, under the name of the phantasmagoria or raising of spectres.

"The eye itself is, in fact, but a small camera obscura." (Read the Analysis,
                               page 325.)

   Who could  at first believe that in describing the camera obscura, as we
have now done, we had in reality been describing only a large  model of that
most interesting of the objects  of creation, the living eye itself, the  inlet of
man's knowledge,—what may be called the visible  dwelling of the  soul,—
that from which the  fire of passion darts,  through which the languor of ex-
haustion is perceived, and in which life and thought seem concentrated ! Yet
the eye is nothing but a simple camera obscura, formed of the parts  described
above as essential to  the camera obscura: but in  its simplicity so perfect, that
they who delight to  find around them tangible evidences of the existence of
an  all-wise and good Creator, point to this in the midst of thousands, as one
of the most undeniable and triumphant proofs.   We shall now describe the
eye and its actions: and keeping present to us the idea of the camera obscura,
as already treated of, we shall find that the use  of the various parts  will be
declared by merely enumerating them.   This paragraph should be perused
while the reader has  the opportunity of observing either his own eye reflected
in a mirror, or the eye of some companion near him.
   The human eye, then, is a globular chamber of the size of a large  walnut,
having for its outer wall a very tough  membrane  called, from its  hardness, 
THE EYE--STRUCTURE.
347
the sclerotic coo!, in the front of which there  is one round  opening or win-
dow,  named, because  of  its horny texture,  the cornea.   The chamber is
lined  with a finer membrane or web, the choroid (having relation to colour,)
which, to ensure the internal darkness of the place, is covered with a black
paint, the pigmentum nigrum.  This lining is bordered at the edge of the
round window by a folded drapery, the ciliary processes, hidden from with-
out by being behind the curious contractile window-curtain the iris, (so named
for its rainbow variety of colour in different  persons,) through the central
opening  of which, called  the pupil, the  light enters.  Immediately behind
the pupil is suspended, by attachments among the ciliary processes, the crys-
talline lens, a double  convex,  most transparent body of considerable hard-
ness,  which so  influences the light passing through it from external objects,
as to form perfect images  of these' objects, in the way already  described, on
the back wall of the eye, over wrhich the optic nerve, there called the retina, is
spread as a second lining.  The eye is maintained in its globular condition
by a watery liquid which  distends its external coverings, and  which, in the
space before the lens, or the anterior chamber of the  eye, being  perfectly
limpid, is called the aqueous humour, and in the remainder or larger posterior
chamber, being enclosed in a pellucid spongy structure, so as to  acquire some-
what of the appearance of melted glass, is called the vitreous humour.
   The annexed figure  represents an eye of the common dimensions, supposed
to be cut through its middle, from above downwards, so as to show  the edges
of the coats, &c.  C is the outer or sclerotic coat, known  popularly, where
most exposed in front, as the white of the eye.  A is the transparent cornea
joined to  the edge of the  round opening of the sclerotic; it is more bulging
than the sclerotic, or forms a portion  of a smaller sphere  than the general
eyeball, so that, while it may be truly called  a bow-window, it, or rather the
convex surface of its contained water, is also a powerful lens for acting on the
pencils of entering light.  At B, the similarity all round the edge of the cor-
nea, is attached the window-curtain or iris, shown here  edgeways  immersed
in the aqueous  humour, and hanging inwards from above and below towards
its central opening or pupil, through which the rays of light  are passing to
the lens.  The iris has in its structure two sets of fibres, the circular and the
radiating, which cross and act in opposition to each other ;—when the circular
fibres contract, the pupil  is lessened, when the radiating contract, it is en-
larged ; and the  changes  happen according to the  intensity of light and the
state  of sensibility of the  retina, as may at any time be proved  by closing the
eyelids for a moment to make  the pupil dilate, and then opening them to-

                                Fig. 147.
 
348
LIGHT.
 wards a stronglight, to make it contract. Behind the pupil is seen the lens D
 with its circumference attached to the ciliary processes E: it is more convex
 behind than before.   The disease of the eye  called cataract  (from the Greek
 word implying obstruction,) is the circumstance of the lens becoming opaque,
 and the cure is to extract the lens entirely, or to depress it  to the bottom of
 the eye, and then to  substitute for it externally a powerful artificial lens or
 spectacle-glass.  The three lines marking here the boundary of the eye stand
 for its  three coats as they have been called, the strong sclerotic, and the double
 lining  of the choroid and retina.   The figure of a cross is represented upon
 the retina as formed by the light entering from the cross without (which cross
 has  to appear here  small and near, although supposed to be  large and dis-
 tant.)   The image of the cross is inverted, as  explained for the camera ob-
 scura : but we shall learn  below that the perception of an object may be equally
 distinct in whatever position the  image fall on  the retina.   It has been ex-
 plained above, that a lens  can form a perfect image  of considerable extent
 only on a concave snrfacei—and the retina is such a  surface.   The present
 diagram farther explains what is meant by the  anterior and posterior chambers
 of the  eye, namely, the compartments which  are before and  behind the crys-
 talline lens D.
   The nature  of the eye as a camera obscura is beautifully exhibited by taking
 the eye of a recently killed bullock, and after carefully cutting away the back
 part of the two outer coats, by going with it to a dark place and directing the
 pupil  towards any brightly  illuminated objects;  there may  then be  seen
 through the semi-transparent retina, left as  a  screen at the back of the eye, a
 minute but perfect picture of  all  such objects—a  picture, therefore, formed
 on the back of the little  apartment or camera obscura by the  agency of the
 convex cornea and lens in  front.   The picture  is inverted, for reasons ex-
 plained above.
   Understanding from all this, that when a man is said to be looking at an
 object, his mind is in truth only taking  cognizance of the picture or impres-
 sion made  on his retina, it excites admiration in us to think  of the exquisite
 delicacy of texture and of sensibility which the retina must possess, that there
 may be the perfect perception which really occurs of even the separate parts
 of the minute images there  formed.  A whole printed  sheet of newspaper,
 for instance, may be portrayed on the retina on less space than the surface
 of a finger-nail, and yet  not only shall every word and letter be separately
 perceivable, but in the centre of the  picture at least, even an imperfection of
 a single  letter.  Or, more wonderful still, when at night an  eye is turned up
 to the blue vault of heaven,  there is portrayed on the little concave of the
 retina the boundless concave of the sky, with every object in its just propor-
 tions.  There  a moon in  beautiful miniature may be sailinc among her white-
 edged clouds, and surrounded by a thousand twinkling stars, all in just propor-
 tion, so  that to an animalcule within  and near the  pupil, the retina might
 appear  another starry firmament decked in its  glory.   If the images in the
 human  eye be thus minute, what must they be in the little eye of a wren, or
 of other animals smaller still!   How wonderful are the works of nature !
  Because  the imna.es formed on the retina are  always inverted as respects
 the true position of the objects producing them—just as happens in a simple
 camera obscura—persons have wondered that things  should  appear upright,
 or in their true situations.  The explanation  is  not difficult.   It is  known
 that a man with wry neck judges as correctly of the  position of the  objects
 around  him  as anv other person—never deeming them to be inclined or
crooked, because their images are inclined in relation  to the  natural perpen- 
THE EYE—DOUBLE VISION.                  349
 dicularity of his retina; and that a bed-ridden person, obliged to keep his head
 upon his pillow, soon acquires the faculty of the person with wry neck ; and
 that an affected girl inclining her head while  trying  her various attitudes,
 learns from much practice, to judge  of the manoeuvres of a beau as conveni-
 ently in that way as in any other; and that boys who at play bend themselves
 down to look backwards through their legs, although a little puzzled at first,
 because the usual position of the images on the  retina is reversed, soon see
 as well in that way as in  any other.  It appears, therefore, that while the
 mind studies  the form, colour, Sic, of external objects in their images pro-
 jected on the retina, it judges of their position, not by the accidental "position
 of the images on the retina, but by the direction in which the light comes
 from the object towards the eye—no more deeming an object to be placed
 low because its image is  low in the eye, than a man in a room into which
 a sunbeam  enters by a hole in  the window-shutter, deems the sun. low be-
 cause its image is on the floor.  A candle carried past a key-hole, throws its
 light on the opposite wall, so as to cause the luminous spot there to move in
 a direction the opposite of that in which the candle  is carried; but a child is
 very young, indeed, who  has not learned to judge at once of the  true motion
 of the candle by the  contrary apparent motion of the image.   A  boatman,
 who, being accustomed to his oar, can direct its point against any object with
 great certainty, has long  ceased to reflect,  that to move the point of the oar
 in some one direction, his hand  must move in the contrary direction.  Now
 the seeing things upright by images which are inverted, is a phenomenon akin
 to those which we have here reviewed.
   Another question somewhat allied to the last is, why, as we have two eyes,
 and  an image of any object placed before them is formed  in  each—why the
 object does not appear to us to be double.  In answer to this, again, we shall
 only state the  simple  facts of the case.  As in two  chess-boards there are
 corresponding squares, so  in the  two eyes there must lie corresponding points,
 and when on those points  a similar impression is made at the same time, the
 sensation or vision is single ;  but if the impression be made on points which
 do not correspond, owing  to some disturbance of the natural position of the
 eyes, the vision becomes double. Healthy eyes are so wonderfully associated,
 that  from earliest infancy they constantly move in perfect unison.  By slightly
 pressing a finger on the ball of either eye, so as to  prevent its following the
 motion of the other, there is immediately produced the double vision; and
 tumours about the eye often have the same effect.  Persons who squint have
 always double vision ;  but  they  acquire the power of attending to the sensa-
 tion  in one eye at a time.   Animals which have the eyes placed on opposite
 sides of the head, so that the two can never be  directed to the same point,
 must have in a more remarkable  degree the  faculty of thus attending to one
 eye at a time.
   The corresponding points in the two eyes are  equidistant and  in similar
 directions from the centres of the retinae,  which centres are called the points
 of distinct vision, and at them the imaginary lines  named the axes of the eye
 terminate—but it is worthy of remark that these points, in  being  both  to
 the right or both to the left of the centres, must be one of them on the inside
 of the centre, as regards the nose, and the other  on the outside—that is to
 say,  a point of the left eye between the centre  and nose, has its correspond-
 ing point in the right eye between the  centre and the cheek—and from this
fact arise consequences meriting  attention.   When the two eyes are directed
 to any object, their axes meet at it, and the centres of the two retinas are oppo-
site to it, and all the other points  of the eyes have perfect mutual correspond- 
350
LIGHT.
ence as regards that object, giving the sensation of single vision;  but  the
images  formed at the same  time, of an object  nearer to or farther from  the
eye than the first supposed, cannot fall on corresponding points, for an object
nearer than where the axes meet would have both its images on the outsides
of the centres, and an object more distant would have both its  images on  the
insides of the centres, and in either case the vision would be double.  Thus
if  a person hold up one  thumb before his nose, and the other in the  same
direction, but farther off,  by then looking at the  nearest,  the more distant will
appear double, and  by looking at  the more distant, the nearest will appear
double.
   The reason for applying the term " point of  distinct  vision" to the centre
of the retina, is  felt at once by looking at a printed page, and observing that
only the one letter to which the axis of the eye is directed, is distinctly seen;
and, consequently, that  although the whole  page be depicted on the retina
at  once, the eye, in reading, has to direct its centre successively to every part.
   On examining a dead  eye, the point of distinct vision is  distinguishable
from  the retina around  by being  more transparent.   It might have been
expected that this point  would have been where the optic nerve enters  the
eye:  but, in fact, the optic nerve enters considerably nearer to the nose than
the point of distinct vision is;' and singularly, where  it enters, the part is
altogether blind or insensible.   Had the two optic nerves, therefore, entered
at corresponding points  of the retina, (in the sense explained above,) there
would have appeared a black spot on every object opposite to the insensible
points;  but as the case really stands, the part of any object from which  the
light  passes to the insensible or blind part of one eye must be opposite to a
sensible part of the other.  The existence of the blind spot, where the nerve
of the eye enters, is discoverable by placing in  a row three objects—wafers,
for instance—across a table, with intervals of about two inches between them,
and then looking with one eye, (the other being  shut) from a distance of about
eight inches, at the  wafer which is on the side of the nose;—the middle
wafer will be invisible, although the eye will see that on each  side of it;  and
if  the eye be then directed to the middle wafer, the external one will disap-
pear.  Another proof is  obtained by  shutting one eye  and looking with the
other at the points of two fingers held together before it;—if  one of the fin-
gers be  then  gradually  moved away laterally, its  point when at a certain
distance from  the other will disappear, but will be seen again when its  dis-
tance is still increased.
   It appearing, from the explanations now given, that there cannot be perfect
sight unless where a perfect  image  is formed on  the  retina, and the truth
having  been formerly explained, that images behind any lens will be at dif-
ferent distances from it,  according  to the various distances of the objects in
front, that is to  say, according as the pencils of light which fall upon it have
more or less of divergence in them, it follows,  that the  eye in being able, as
it  is,  to ,see distinctly objects  at different distances,  (the nearest is about five
inches,) possesses a  power of altering the  relation of its  parts to accommodate
itself to the circumstances.  We do not yet  perfectly know whether it does
this by lengthening  or changing  the  form of the ball through the action of
the surrounding muscles, or by changing the place  or  the form of the lens,
but that one or  more of these events occurs there can be no doubt.
   Among the eyes of the myriads  of mankind, however, it happens that all
do not originally possess these powers exactly in the  requisite degree, and
that many lose  them, as  life advances, from a natural or usual decay.
   Persons are called short-sighted whose eyes, from too great convexity of 
THE EYE--SHORT SIGHT —LONG SIGHT.
351
Fig. 148.
 the cornea or lens, have  so strong a bending or  converging power, that the
 rays of light entering them are brought
 to a focus before reaching the retina—
 at a, for  instance, instead of  at b: so
 that the  rays, by spreading again be-
 yond the focus, produce  on the retina
 that sort of indistinct image  which is
 seen in  the camera obscura, of which
 the screen is too distant from  the lens.
 This defect of sight obliges the indi-
 vidual  when using  the naked eye to
 hold objects very near it, that the consequent greater divergence of the rays
 may be proportioned to the unusual refracting power of the eye—or the per-
 son may find a remedy in placing concave lenses between the object and the
 eyes, which lenses, by rendering light from objects  at a usual distance more
 divergent, (as explained page  34.),) cause the perfect images in the eye to be
 formed farther from the lens, and thereby on the  retina itself.   Without con-
 cave  spectacles—as the lenses are called when fixed together  in a frame-
 persons with the defect now under consideration cannot  see distinctly any
 object that is distant, for the rays, coming nearly parallel, are quickly gathered
 to a focus.   This defect often diminishes with years, and the person who in
 youth needed spectacles,  in old age sees well without them.
    There is an opposite defect of deficient converging  power in the eye, de-
 pendent  on  a  too great flatness of the cornea or lens, and which is" much
 more common than the last-mentioned defect; indeed, the great majority of
 persons after middle age sooner or later  begin to  experience it.   In this case
 the rays of light are not yet collected into a focus  when they reach the retina:
 they  would  only meet  at b, for
 instance, instead of as they should
 do at  c,  and hence the  image is
 indistinct, in the same manner as
 in a  camera  obscura, when  the
 screen  is held too near  the lens.
 Persons suffering this defect  can-
 not, when using  the  naked  eye,
 see distinctly any object very  near
 to it, because the gathering or converging power of the eye cannot conquer
 the great divergence of rays coming from a near  point; and hence such per-
 sons always  remove  objects under examination  to a considerable distance,
 often to that of arm's length, so as to receive from them only the rays nearly
 parallel.  These persons, in contra-distinction to the last described, are called
 long-sighted persons; and after middle  age, most persons become more  or
 less long-sighted.   Their defect is remedied by the common convex specta-
 cles, which  do part of the converging  work, so to express ourselves,  before
 the light enters the eye, leaving undone  only that which the eye can  easily
 accomplish.  As this defect, like the last, is met  with in all degrees, specta-
 cles must be chosen accordingly.   Certain curvatures or strengths of these
 have been particularized  and  numbered as naturally belonging to different
 ages or periods of life, but each person should choose under the direction of
 an experienced judge, until that strength be found  which enables him to read,
 without any straining of the eyes,  at the  common distance of from twelve to
 eighteen inches.—We cannot apply the mind to this part of our subject with-
out feeling admiration at what science has accomplished for man in assisting
Fig. 149.
 
352
LIGHT.
and restoring his sight.   Now that in civilized society, the common employ-
ments and enjoyment of life require a visual power capable of distinguishing
such minute objects as written or printed characters, to deprive old men of
their  spectacles, would  be to condemn  many of  them to useless inactivity
and a listless blank of mind for the remainder of their lives.
  An eye much accustomed to examine  near and  minute objects, often loses
something of its pliancy, and becomes defeclive when tried at distant things,
as the watchmaker's  eye,  the engraver's,  Sic   On the  other  hand,  the
old seaman's, which  has  so often and  uninterruptedly been  bent on the
distant horizon, straining to catch the view of an expected sail, or of land,
has a power  of discovering distant things  which is wonderful;  but it often
experiences deficiency in regard to near  things.
  A man who uses  his eyes under water sees very indistinctly, because the
difference of density between water and the eye not being so great as between
air and the eye, the  bending or refraction of light entering from the water is
not so great as of light entering  from air, and the  internal structure  of the
human eye being adapted  to the greater refraction, perfect images are not
formed on the retina.   A man to see well under water, therefore, requires to
aid the usual power of his  eyes by strong convex spectacles.   It is to meet
the necessity now explained, that the lens  of a fish's eye is extremely con-
vex, or  almost round, as is every day seen in the white  round bead  which
issues from the eye of a boiled  fish—that little  globe  being the crystalline
lens of the fish coagulated or hardened like the while of an egg during cooking.

There are  many  important considerations  connected with  the sensibility of
  the retina, which regard  rather  the laws of life  than of light, but we must
  here glance at a few of thein.

  Any impression of  light made  upon  the retina lasts for about the sixth of
a second.   Hence when the burning end of a stick is made to sweep rapidly
across the view, its  path appears to the eye a line of light:  and if it be made
to revolve in a circle  six times in a second, as when moved by the hand or
fixed to a turning wheel, that circle will  appear to the eye a complete ring of
fire.  The polished end of an elastic wire, of which the other end is fixed
in a block of wood, when caused  to vibrate, similarly forms a line or a curve
of light.   A harp-string, while  vibrating  as it  sounds, appears  like a flat
transparent  riband.   Lightning  or  other  meteor darting across  the sky,
although in fact but a moving luminous point, is generally thought of as a long
line of light: the  term forked-lightning has reference to this prejudice.   The
same remark  applies  in  a  degree to a sky-rocket in its rapid ascent.  Two
or more colours painted separately on the rim of a wheel  which is made to
turn rapidly, appear to a spectator to be  as  completely united as if they were
really mixed;—it has been already explained how patches  of all the colours
of the rainbow, when mixed in this way, form white light.  If  on one  side
of a card a little bird be  painted,  and on a corresponding part of the other
side a cage, then on making the card turn rapidly by twisting  between the
fingers and thumbs  two threads fixed to  its opposite edges, the bird and cage
will be seen at once, and the bird will appear to be imprisoned in the cage;—
or, if a pensive Juliet sitting in her bower occupy one side of the card and a
longing Romeo the other, hy the magic turn of  the threads the lovers may
instantly be  brought  together.    Dr.  Paris displayed taste and an amiable
ingenuity in designing this toy with great variety of subjects.
   A  certain intensity of light is  necessary to distinct vision, but the  degree 
THE EYE--JUDGING BY APPEARANCES.
353
 vanes with the previous state of the organ. A person passing from the bright
,day into a shaded room, might for a time fancy himself in total darkness, and
 to persons sitting in the room, and become so accustomed to the less light as
 to see well with it, he might appear to be almost blind. The dawn of morning
 after the darkness  of  night appears much brighter than an equal degree of
 light in the evening.   When, as the night  falls, our lamps or candles are first
 introduced, the glare  is often for a time offensive to the eye; and a similar
 feeling,  but still stronger is experienced, when  in  the morning, bed-room
 window shutters or  close-drawn curtains are suddenly opened.  After the
 repose of  night,  the sensibility of  the eye, when first opened, is often such
 that the globules of blood moving in  the  capillary vessels of the retina pro-
 duce the impression there of little balls of light pursuing one another along
 the  tortuous vessels.  To a prisoner after long confinement in a dark dun-
 geon, the light of the sun is almost insupportable.   And a dungeon, which to
 common  eyes is utterly dark, still to its  long-held inmate has ceased to be
 so:—there are various  instances in the records  of the barbarous  ages, of
 prisoners confined for years  in  darkness, deemed  absolute, but who, after a
 time, could  see  in it, and made entertaining companions of the mice  and
 spiders which  frequented their cells.  The  darkness of a total eclipse after
 bright sunshine,  appears deeper than that of midnight, because of the sudden
 contrast.   The long polar night of months ceases  to appear very dark  to the
 polar inhabitants.  If an eye be directed for a time to a black wafer laid on a
 sheet of white paper,  and be then turned to another part of the sheet, a por-
 tion  of the paper at that other part,  of the size  of the wafer, will appear
 brilliantly  illuminated; for the ordinary degree of light from it appears intense
 to the part of the retina lately receiving almost none.   An eye directed long
 and intensely upon any minute object—as  when a sailor watches  a speck in
 the distant horizon, supposed to be a ship, or when a hunter on  the  brown
 heath keeps  his  eye  fixed  on  a bird nearly of the  colour of the heath, or
 when an astronomer gazes long at a little star—has the sensibility of its cen-
 tre at last exhausted, and ceases to  perceive the object; but on directing the
 axis of the eye a little to one side of the object,  so that an image may be
 formed only near the centre,  the  object may be  again  perceived, and the
 centre in the mean time enjoying repose, will recover its power.
   But the  most extraordinary fact connected with the  sensibility of the retina
 is, that if part of it be strongly exercised for a time by looking at some bright-
 coloured object, on the eye  being  then  turned  away or altogether shut, an
 impression or spectrum will remain of the  same form as the object lately con-
 templated, but of a perfectly different colour.   Thus if an eye be directed for
 a  time to a red wafer laid on white  paper, and be then shut or turned to an-
 other part of the  paper, a beautifully bright green wafer will be seen;  and
 vice versa, a green wafer will produce a red spectrum: an orange wafer will
 similarly produce a blue spectrum ; a yellow  one a violet spectrum, Sic ; and
 a  cluster of wafers will produce a similar cluster of opposite colours.   Then
 if the hand be held over the closed eye lids to prevent entirely the approach
 of light to  them,  the spectrum of bright objects will appear luminous sur-
 rounded by a dark ground, and when the hand is again removed, the contrary
 will be true.   Again, if the eye be in a degree fatigued by looking at the set-
 ting sun, or even at a window  with  a  bright sky beyond it, or at any very
 bright object, on then shutting it, the lately contemplated forms will be per-
 ceived, first of one vivid colour, and then  of another, until perhaps all the
 primary  colours  have passed in review.  These extraordinary facts prove
 that the sensations  of light and colour, although excitable by light, are also 
354
LIGHT,
producible without it.   This truth gave occasion to Darwin's ingenious theory,
that the sensation of any particular colour, of red, for instance, is dependent
upon a certain state of contraction of the minute fibres of the retina,—and that
the fibres, when fatigued in that  condition, seek relief when at  liberty, by
throwing  themselves into an opposite state,—as a man whose back is fatigued
by bending  forward, relieves  himself not by merely standing erect, but by
bending the spine backwards—which new condition in the eye, whether pro-
duced by light or  by  any other cause, gives the sensation of green.  He
applied his explanation similarly to all other cases of colour.   It is remarka-
ble that the colours which thus appear  opposites to  each other in kind are
those which, when the solar spectrum produced  by a prism, as described a
few pages back, is painted round a wheel or circle, are opposite to each other
in place.
   There are persons who, although having distinct perceptions of form, and
of light and shade, have not the power of distinguishing colours.  It is com-
mon for such persons to deem pink and pea-green (naturally opposites) the
same colour, and therefore, not to  distinguish difference of colour in a red
berry and the leaves around  it.   A man  with this defect, trusting to  his own
judgment, has, without knowing it, dressed himself like a parrot.

" The mind judges of external objects by the relative size, brightness, colour,
   &/c, of the minute but perfect images or pictures  of them formed at the
   back of the eye on the expansion of nerve called the retina;  and the art of
  painting is successful in proportion as it produces on a larger  scale such
  a picture, which, when afterwards held before the eye to re-produce itself in
   miniature upon the retina, may excite the same impression as on the ori-
   ginal object."   (Read the Analysis, page 325.)

   We now understand how an admirable miniature resemblance of the objects
before us  is  produced upon the retina of the  eye, by the light from them re-
fracted in  passing through the different parts of the eye; but after all, this is
only a picture, and the  inquiry remains—which many persons would suppose
so simple  as to be trifling, but which is in reality most curious and important
—how we are  thereby enabled to judge  of the magnitudes, distances,  and
other particulars respecting the things examined ?  Here it will be found, to
the surprise  of persons first entering upon the subject, that we learn the mean-
ing of a scene or pictorial signs only gradually, as we do of any other system
of signs,  and that a person whose eyes, although perfect, had  been kept
covered from infancy up to maturity, would no more  "see," in the complete
sense of the word, that is, understand, any scene or prospect on which he
first opened  his eyes, so as to have a perfect picture of it on his retina, than a
rhild understands or can read a printed page, when he first looks into a book.
Most interesting information has been obtained on this subject, by observing
the facts where blindness from birth has, by surgical operation, been suddenly
cured in persons arrived at maturity.
  If a man  were placed from infancy in an apartment fitted up as a camera
obscura, and had no means of becoming acquainted with external  nature, but
by watching the images appearing upon  the screen, he could learn  scarcely
any thing  of the universe around him; but if after a time he were  allowed to
walk  out,  and to  examine by the  touch and by measurement  the different
objects whose images he was in the habit of viewing, and to ascertain what
size, shape and distance of an object corresponded with a certain magnitude,
form, position, and brightness of image, the transient imagery mighf at  last 
THE EYE--VISUAL ANGLE.
355
 be to him a very clear indication of the real particulars: making him in ima-
 gination  present  to the objects, almost as if  he  went and  examined them
 with his  hands.   In the same manner, in  a degree, the  mind  may be con-
 sidered  as stationed in or  about the little camera obscura of the eye, whence
 it cannot itself escape to examine external nature ; but must learn the mean-
 er, \   'ma°es formed  on the retina, by commanding  the  services of the
 bodily limbs or members, and the other organs of sense.—The judging of
 things by sight, then, is merely the interpreting one set of si<ms, as judging
 by sounds or language is interpreting another,  and judging by hieroglyphics
 or any written character is interpreting a third.   The  common visual signs
, on the retina, however, are of all signs the most readily learned or understood,
 from having certain fixed  relations  in  form, magnitude and position to the
 things signified;  while words, hieroglyphics, and written characters, are quite
 arbitrary, and have no  such relations.
   Bodies differ and are distinguished among themselves chiefly by their com-
 parative dimensions, that is, their form  and magnitude, or shape and size;
 and to ascertain these and  the relative distances and positions, are the great
 objects which by the eye the mind seeks to accomplish.   Now it effects its
 ends by considering collectively,
   1st.   The space and place occupied by objects in the field of view, mea-
 sured by what is called the visual angle.
   2d. The intensity of light, shade, and colour.
   3d. The divergence of the rays of light.
   4th.  The convergence of the axes of the eyes.

    We shall treat of these particulars separately in the order now stated.

 1st.  The  space  and place occupied in the field of view, measured by the
                              visual angle.
   The term field of view is used to designate  that open or visible space be-
 fore the eyes, in which objects are  seen:  and it may mean either the small
 field visible in one position of the eyes,  or that which is perceived on direct-
Fig. 150. 
356
LIGHT.
ing them all around.  If a man  as at e, were surrounded by a  large globe
or sphere of glass as a, through which  his eye at the centre might view the
several objects around occupying certain situations and certain proportions
of the circumference,  and if the sphere had any equal divisions or degrees
marked  upon  it all around, he  would be able at  once to say exactly what
portion of his sphere or field of view was  shadowed or occupied by any
single object, as the cross here shown at i, and thus to describe very intelli-
gibly either for his own recollection, or to inform others, its relative magni-
tude and situation as  then appearing to him,—just as  he might say, on
looking at a tree in the garden through a common  window which  is a por-
tion of the field of view really divided by the cross  bars, whether he saw the
whole tree through one pane or through several, and through which pane or
panes he saw  it.   It may be remarked farther, that  whether the  supposed
sphere of glass were large or small, viz., were as b or c, the part of its surface
apparently occupied by any object either beyond or within it, would bear the
same proportion to the whole surface;—if a d were  a tenth of the small circle
or globe, c g would be a tenth of a larger.  Now as men have found it  con-
venient to  consider a circle (and every circle) as divisible  into 360 degrees,
(which are smaller, therefore, in a small than in a larger  circle, although in
each having the same relation to the whole,) the ready mode of comparing
the apparent magnitude of objects is to say how many of these degrees of the
field of view each object occupies: and this is really what  is meant by the
apparent size of an object.   And because the most  convenient way of mea-
suring a portion of a circle, of which  the whole is not seen,  is to measure by
a fit instrument the angle or corner formed at its centre by lines drawn from
the extremities of the portion,—as here the angle at e formed by  the lines c e
and g e, the object is said either to occupy a certain  number of degrees of the
circumference  of the circle, or to subtend an angle of the same number of
degrees at its centre, and this angle is called the visual angle, the  subject of
our present disquisition.
   The visual angle, then,  in regard  to  any object,  is that included between
the lines or rays, as a u and d i, which, from the extreme points of the object,

                               Fig. 151.
as a d, meet and cross in the lens of the eye, and go afterwards to  form the
extremes of the image on the retina, and, as  formerly explained, the angle is
the same on  each side of the  lens, viz., towards the object or  towards the
image.
   Now if all bodies were at the same distance from the eye, the magnitude
of their images formed on the retina, or in other words, of the visuarangles
subtended by them, would be an exact measure of their  comparative real
magnitudes, as is seen in i u, the image  of the great cross a d, and  in i  o the
image of the small cross b  d:  but it is evident here, that the cross c e, which 
THE EYE—APPARENT SIZE OF OBJECTS.           357
is twice as large as b d, makes, because twice as far off, an image of only the
same size as b d, and an image therefore  only half as large as that of a cross
a a equal in size with itself:  and the same  rule of proportion holds for all
other  comparative distances—at a hundred times the distance, an object ap-
pearing only the hundredth part as tall, and so forth. To judge, therefore, by
tne eye of the true size of an object, we must know its distance as well as its
apparent size or visual angle.
Many familiar facts receive their explanation from the law of the visual angle
   or  apparent size  being less always  in proportion as  the  distance of an
   object is greater.

   A man (instead of the cross here shown) at d, standing near the outside of
a window, as b c (here represented edgeways) may to the eye of a spectator
within the window at h, subtend the  same visual angle,  or appear as tall as
the window, the  light from the man's head passing through the  top of the
window, and that from  his
feet passing  through  the
bottom:   but  if  the man
then moves away from  the
window,  the  eye   of  the
spectator will be  able to see
his whole body  through a
smaller and a  smaller  ex-
tent of  the window,—as
through half its height or a c, when he is twice as distant, or at/, and through
the third or o c, when he shall be three times as distant, or at g,*and so forth,
for any other distance;  so that soon a small figure of a man  cut in paper, if
applied upon the  glass, would  exactly cover the part of it through  which the
light from him entered to the spectator's eye, and would then, by completely
hiding him from  view, be an exact measure of his apparent size :  and at last
a fly passing over the pane might equally hide  him, and  the  fly then would
subtend a larger visual angle than he,  that is to say, would be forming on the
retina a larger image than the man.   Thus it often happens in reality, that a
person sitting near a window, and intent upon  some subject of study or of
conversation, mistakes a fly  on the glass for a man at a distance;  or, on the
contrary, a man for a fly.  It  is ascertained that the eye, with an ordinary
degree of light, can see  an object which in the field of view occupies only
the sixtieth of a degree (or one minute.)   This space is  about the 100th of
any inch in  a circle of twelve inches diameter, the eye being supposed in the
centre of the circle.  Now, a body smaller than this at six inches from the
eye, or any thing, however large, placed so far from the eye as  to occupy in
the field of view less space than this, is  invisible to  ordinary sight.   At four
miles  off, a man becomes thus invisible.   A  pin-head near will hide a house
on  a  distant hill—nay,  will hide even the planet Jupiter,  although 1,000
times  bigger than this earth.
  In  accordance  with the principle now explained, a marine telescope has
been constructed, in  which the field of view is  divided by fine cross wires,
or otherwise, so that the person using it can say at once how much of its
field any object occupies.  When  ships are in  chase, it is common, by this
instrument, or some other which  will  detect a change  of visual angle, or
apparent size, to view  the  fleeing or pursuing ship; and if the apparent
size be observed to increase, the conclusion  follows that  the  ships  are
    24 
358
LIGHT.
 nearing each other;  if, on the contrary, the size diminishes, the chased ship
 is escaping.
   By applying this rule, whenever the real size of a distant object is known,
 the distance is  ascertainable, and,  vice versa, where the distance is  exactly
 known, the size is determinable:—for it is evident that if a body, as a ship,
 known to be 100 feet tall, occupy  or subtend in the field of vision the 360th
 part of a whole circle, or one degree, the whole circle must be in  circumfe-
 rence 360 times 100 hundred feet, or 36,000;  and the diameter of any circle
 being nearly one-third of its  circumference, while, in the case supposed,  the
 distance of the ship  is the  half-diameter, we learn that distance.  Again, if
 we know the distance of a ship  or other object to be a mile, and if we then
 find the visual angle  subtended  by the  object to be  the  1,000th  part of a
 circle, we know its true size to be the 1,000th part of a circle, of which  the
 half diameter or radius is one mile.  It is by applying this rule in a  manner
 to be afterwards explained, that  we  determine  the size of the  heavenly
 bodies.
   We now perceive that if the rays of light coming to the  eye through a
 plate of glass, from objects  seen beyond  it, could leave marks in the glass,
 at the points  where they passed, and marks capable of giving  out the same
 kind of light as the  objects, there  would be formed upon the glass  such a
 representation or  picture of the objects formed or viewed  through  it, that
 when held before the eye, it would form on the retina, the image or images
 the same in almost  all respects as the objects  themselves;  for  from  the
 different points of the glass, light  could dart to the eye of  the same kinds
 and in the very same directions  as that originally coming from the objects.
 Now the art of painting seeks so  to dispose lights, shades  and colours  on
 any plane surface, as to produce the sort of representation of objects here
 contemplated, while  the  picture-frame has to recall  the  window-frame, or
 edge of the plate of  glass through which  the true scene  is supposed  to be
viewed.  It is admirable how perfectly this art  now accomplishes  its ends;
and  although there are still trifling differences  between  the  effect upon  the
eye, of the  picture and of the realities—which peculiarities we  shall consider
presently, and  how  they may  be combated so  as to render  the illusion
 almost perfect,—it is  not one of them, as might be supposed from the small
extent of the canvas, that the picture  appears to the  retina  smaller than
 the  objects  themselves.   Few  people, before  studying this subject,  are
 aware that in a good  picture the  size of the figures is always  made exactly
 such, that at the distance from the eye at which the picture  is meant to be
 viewed, they produce on the retina the very same  size of image as  would
 be produced by the realities seen under the aspect represented  in the picture.
 To become sensible'of this, let a person look through a window-pane, with
the eye at the distance of eight inches from it, and let him  trace with a sharp
point upon the glass, previously coated with gum, the  outline of the scene
beyond—perhaps a street or square,—he will find, that the outline of a man
 seen there at the distance of  twenty paces, and appearing perfectly to coin-
 cide with the boundaries of the person, so that, if opaque, it would just hide
 the person, will be scarely half  an inch tall, while the  figure of the man a
 few hundred paces off, will  appear so small, that the minuter  features could
 not be distinguished, even if they could be drawn.
   Now as a person  who  reads the  description  of an elephant, does  not
 deem the animal larger or smaller  because of  the size  of letter used in  the
 printing, or  in the size of the accompanying  engraved representation; and
 as a man in a picture gallery viewing miniatures and larger portraits, does 
THE EYE--APPARENT SIZE OF OBJECTS.
359
not conceive of the originals according to the  size of the representations:
and as a man  viewing a well-executed picture of a Grecian  temple, never
dreams, unless his  attention  be particularly directed to the fact, that upon
the canvas, the distant pillars of the rows are  much smaller than the near
ones; but in all such cases  the mind merely uses the signs to help it to con-
ceive of the things according to previous knowledge, or to other principles
of judging: — so  in  any  common case of seeing, the  mind  takes  little
account of the apparent size of objects, but passes  instantly from the types
to the realities, which are, generally, more or less known,  and it soon ceases
to be aware that the apparent size of the same object  ever changes.   Few
persons, for instance, are  aware  that when two  friends shake  hands, each
appears to the mere eye of the other ten times taller than  when he has  walked
ten paces  away; or that a  chair at one  end  of a room appears to a person
sitting at the other, only half as large as a chair in  the middle of the room:
but such are the  facts; and  they may be immediately proved  by holding a
common  eye-glass  or  ring at a  certain distance from the  eye, and then
looking through it at any similar objects placed at different distances ;  then,
while of a chair  standing  near, only a  small part  will be  visible  through
the ring—of a distant chair  the whole  may be  seen;  and so  of any other
case.  At five miles distance, the fleets  which met on the  great day of Tra-
falgar might have been seen  through a  marriage-ring as  the  picture-frame.
There are occasions, however,  where the usual collateral helps to the imme-
diate recognition  of objects being  wanting,  the  observer's  attention  is
strongly aroused to the  facts of their diminutive  appearance  produced by
distance;  for  instance, when a man on a long sea-voyage first approaches a
land of which the features are in a degree new to him ; as when an English-
man arriving in India, scarcely believes that the little specks which he sees
scattered along the  shore are commodious dwellings, or  that what seem  to
him only luxuriant herbs or bushes, are magnificent  palm-trees.
   For the same reason that a distant body to the mere eye appears diminutive,
namely, the smallness of the visual angle subtended by it, so does a distant
motion to the  mere eye appear slow.   A carriage dashing past a pedestrian
in the street, may surprise  him by its speed; but if viewed at the same time
by a spectator at the top of a lofty tower near, it seems to be but crawling
along the  pavement.  A  ship driven before a tempest, seems to a sailor on
board almost to fly through the white foam which surrounds her; but if then
observed  by a spectator on shore, as an object on the distant horizon, she is
scarcely perceived to change  her place.   A balloon high in the air, and borne
alone- on the wings of the wind at the rate'of seventy or eighty miles an hour,
may°still for a time leave a spectator on earth doubtful as to  whether  it be  in
motion, or in what direction  it moves.   The moon in her orbit wheels round
the earth at the astonishing rate of about 2,000  miles an hour, yet, owing  to
her distance from it, her motion is not visible to the naked eye of the inhabi-
tants of the earth, except by comparing  her place  at considerable intervals.
In respect to  bodies still more distant than  the  moon, the  truth at  present
under consideration is still more striking.

Having now explained how  the apparent transverse measures or breadth  of
   bodies  and  of space, in  other words, the visual angle subtended by them,
   is affected by their distance from the eye,  we  proceed  to show how it is
   affected also by their position.
   A globe at a certain  distance from the eye, however turned, preserves the
same appearance  in the field of view, and its outline traced upon a plate of 
360
LIGHT.
glass held across  between it and the eye, is, like its direct  shadow upon a
wall, always a circle; but an egg  which, held  in  one position, produces a
circular outline or image, when  held  in  another, produces an  image nearly
oval. A wheel when viewed sideways appears a perfect circle, when viewed
edgeways it appears a broad  straight band or line, and in any intermediate
position it appears oval.  The apparent form, then, of a body, is only a hint
to the mind from which, by former experience or instruction, it guesses at the
true form.   If a man had never seen an  egg  but endways,  he never  could
have known that it was not a sphere.
   If any long straight object, as a beam, be placed with one of its ends
directly to the eye, that end only can be seen, and according to the case, must
appear a square  or  circle of the  diameter of the beam ; if it then be placed
with its side directly to the eye,  its  whole length will be seen; and if placed
in any intermediate position, it will  appear more or less shortened;—in all
cases, its outline on the retina being similar to that of its shadow on a wall
directly behind the person.   A man  has advanced on a spear pointed directly
to his eye without seeing it, or on  the end of  a  bar of  iron  carried on  the
shoulder of  a porter in the street.   A  common telescope held with its end to
the eye  appears a perfect circle, if then inclined a little, it seems to jut out on
one side, and as  the  inclination is increased, it juts out more  and  more, until
it displays its whole length.  A  great ship of war whose stern is towards a
spectator, appears a rounded building  with its rows of windows like those of
a peaceful habitation; but as it turns,  it  gradually reveals  the long batteries
of bristling cannon.  A straight row of athousand similar objects, as of soldiers
in rank, pillars, trees, Sic,  may  appear to a person at the extremity as only
one object of the kind, the nearest individual completely  hiding all the others;
but if viewed from the side and  at a certain distance,  the individuals may be
counted.
   The appearance now treated of is caWed foreshortening, and is  to be noted
wherever  surfaces or lines are not placed so as directly  to face the spectator.
   Perhaps the most important case of foreshortening is when the eye  looks
more or less obliquely along an extensive  plane  surface, the  general surface
of the earth, for instance, or of the sea, by estimating aright the foreshortening
of which, we judge of the distance or  situation of the objects placed upon it.
It will be readily perceived, that in  all such cases the more  distant portions
of the surface are progressively more foreshortened than the nearer ;—for a
man standing at a on a plain as a b, and with his  eye at c, looking down
                                              before him, sees a portion of
                  Fig-153-                      the surface a dalmost direct-
                                              ly, or with little foreshorten-
                                              ing, and  an  extent, as  a d,
                                              equal to  the height of the
                                              eye, will subtend in his  eye
                                              an angle of 45°, or half a
                                              right angle, viz., the angle
                                              a c d,  and  therefore rather
                                              more than half of all that can
be subtended by a  straight line  or  space from his feet  to the horizon, how-
ever distant; the next equal spaces  viz., d f, will subtend an angle of only
18°, viz., dcf, the next of 8°, viz., f c g, and  so on ;  and as he carries his
view more and more forward, the surface becomes to it more and more oblique,
until at last the light coming seems  more to skim along the level than to rise.
This explains why a person having a side view of a row of separate objects,
 
THE EYE--FORESHORTENING.
361
as of men in line, trees, pillars, &c, may see through or between the nearest
of them, but towards the extremes sees them as if standing in closest possible
array, or as if forming a continued surface.   The same remark explains why
masses  of cloud scattered uniformly over the sky, may allow a spectator Jo
see wide intervals of the blue heaven over head, while all  around there is a
dense cloudy mass appearing to rest on the horizon.
  If a man standing on a hill look down upon a field or  plain which is well
known  to him, and if he see some objects  near its side, and some near its
middle,  and some near its distant  border, he knows at once how far they are
from him and from  one another.   Similarly,  when viewing the ocean from a
lofty cliff, and seeing ships scattered over its  face, he may judge correctly of
their distance, for he can see only a certain extent of ocean which becomes
to him as a known field.  The man stationed at the flag-staff on the High  -
Knowl  of St. Helena, looks down upon  a circular field of the  Atlantic a
hundred miles broad, and can tell the distance of  any sail  in sight to within
a mile or two.   Now, although the ground-plan of an  extensive landscape
may not be so level as the face of the ocean,  there  is still an  approximation,
which very considerably assists a spectator's judgment of distances.
   Painters are  not only careful to foreshorten, according to  the  proportion
explained above, all the objects which they portray, but they often  avail them-
selves of the  principle to produce  most striking effects.  For instance, Martin,
in many of his beautiful designs, by judicious  foreshortening, has exhibited
miles in extent  of gorgeous architecture  and of armed  men, on a space  of
canvas  that would seem scarcely more  than  sufficient to receive  a very few
figures  ; he has made a single magnificent pillar or accoutred warrior in the
 foreground, become the type which first fills the mind with  admiration, and
then sends it along  the retiring lines of beautiful perspective,  where every tip
or edge renews the first impression;  A  man lying on  a high table or bed,
with his feet towards the spectator, is foreshortened  into a  roundish heap, of
 which the soles of the feet hide the greater part.  This is the description  of
 the painting  which has been called the " Miraculous Entombment," and it is
because an unreflecting spectator  moving  sideways with the expectation of
seeing  more  of the body, still sees only the soles of the feet, and may suppose
 the body  turned round so as to keep the feet towards him, that the painting
 has received its appellation.  For nearly the same reason,  the eye of a com-
 mon full-faced portrait may seem to follow a spectator to whatever part of the
 room he  o-oes,—for by moving to one side he cannot see the  side of the eye-
 balls.   It°is  related of a murderer, that he was impelled  to commit suicide by
 observing that the eyes of the portrait of his victim were always fixed upon
 him.   A rifleman portrayed as if taking aim directly in  front of the picture,
 will appear to  every spectator in the room to be pointing  at him especially.
 To  terrify young ladies, a  little  arch Cupid has been similarly represented
 with his arrow pointed directly at them, and just ready to let it slip from his
 bended bow.
   As the painter, availing himself of a  knowledge of the principles now ex-
 plained, by  which the eye usually judges of size and distance, may produce
 on his  canvas the most charming  illusions, so may the tasteful landlord, in his
 ornamental gardens and pleasure-grounds, by working his levels into artificial
 undulation o? hill and dale,  and clothing these with tree and edifice of magni-
 tudes to  correspond—make the eye of a spectator luxuriate  in the contem-
 plation of supposed extensive plains, lofty mountains, widespread lakes, and
 distant pagodas—all within the narrow space of an acre or two; thus, by other 
362
LIGHT.
means, producing on the retina the same impressions as Claude, Poussin, or
Wilson has done by the finest pictures.
   When any objects or mass of objects is foreshortened, by one part being
farther from the eye than  another, that part appears also in  a proportion
smaller than the other.   For example, in a straight row of similar  houses,
pillars, trees, &c, (see the next cut,) those nearest to the eye will, on a glass
held before the eye to receive their images, form the largest images, and there
will be a gradual diminution from the largest to the least, so that lines drawn
upon the glass along the  tops and bottoms  of the images would tend to a
point, called, for a reason to  be explained below, the vanishing point. Thus
a  person looking from  a window along a straight street, must, to  see  the
chimney of the nearest house, look through the top of the window, and to
see the street door must look through the bottom ;  but the most distant house,
both top and bottom, may be concealed from view by a little spot upon  the
glass at the height of the eye.  This remarkable tapering of foreshortened
objects  may of course be strikingly  observed on looking at any correctly
made drawing  or engraving  meant to represent a  retiring row of similar ob-
jects ;—such drawing being  in truth an  attempt to realize by art, on  the sur-
face of a sheet of paper, the appearance of  the  objects as  seen through a
window or aperture the  size of the paper.
   The art which gives rules for tracing objects on a plane surface,  as they
would appear to an eye  looking at them through that surface, if transparent,
with their various  degrees;  first, of apparent  diminution  on  account of dis-
tance : and, secondly, of foreshortening on  account of the obliquity of view,
is called, from the  Latin word signifying to  look through the art of perspec-
tive.  It regards entirely the two particulars now mentioned ;  and notwith-
standing the terror with  which, in the imaginations of many young painters,
the study of it is clothed, by reason of the mathemetical difficulties with which
it has usually been mixed up, it is in itself exceedingly simple.  We  hope
that a person capable of ordinary  attention, will, after what we  have  already
said, and after the few additional remarks which we have still to make on the
appearances of nature, be able readily to understand the great  rules of per-
spective.  Although, without a knowledge of these rules, a quick eye soon
enables its  possessor to  sketch from nature with  much truth ;  and although
the two instruments, the camera obscura, already described,  and camera lu-
cida, to be  described in a future page, give almost mathematical accuracy to
drawings from  nature, without requiring other skill in the draughtsman than
                                            to trace and make permanent,
                                            with ink  or pencil, the lines of
                                            light which he  sees  on  the
                                            paper;  still the subject  is so
                                            interesting to all who  attempt
                                            to  sketch,  and indeed to all
                                            who wish to look intelligently
                                            either at nature or at the works
                                            of art, that none who have the
                                            opportunity of   studying  it
                                            should negJect it.
                                              Supposing a straight row of
                                            similar objects, as of the  stone
                                            blocks or pillars  represented
                                            here from a or b to S, to be
                                            viewed by a person stationed
28
18 
THE EYE--PERSPECTIVE.
363
near the side and end of the row, as over the point C, then, because, as al-
ready  explained, objects to  the  eye appear smaller  in  exact  proportion to
their increased distance  from it,  the second block, if twice as  far off as the
first, would appear only half as large; the third, if three times as far, would
be only one-third as large, and so on to any extent and for any other propor-
tions ;  and if the 1,000th or any other block, owing to its distance, subtended
to the  eye an angle less than the sixtieth of a degree of the field of view, it
would be altogether invisible (as explained at page 333,) even  if nothing in-
tervened between it and  the eye.  Then, where the row ceased to be visible
from the minuteness of the parts, or from the fact of the nearer objects con-
cealing the more remote, it might be said to have reached its vanishing point.
—When a student of perspective has  learned  all that regards  the vanishing
point in relation to a line, and the corresponding vanishing line, in relation to
a surface, he has learned half of  his art.   The above  cut considered as the
representation of a street running directly south  to S, sketched from a win-
dow looking along its centre, will serve as a useful illustration.
   It is important, first, to remark, that in any case of a straight line, or a row
of objects thus vanishing from sight, as here the line or row a S, in whatever
direction it points, whether east,  west, north, or south, &c, in that direction,
too, will its remote or vanishing  extremity appear  to be from  the eye.   In
this sketch, the row a S is supposed to run directly south: and although the
eye to see the near end of it, would have to look towards the left hand, or in
a degree east, still every successive pillar  would  be more and more nearly
south, and the point in the heavens, or in a picture, or in a transparent plane
before the eye, where the line would vanish, would be so nearly  south  from
the eye,  and not to the east, because the pillars happened  to be  east of the
individual, that no ordinary measure would detect the little want of corre-
spondence ; then similarly, if there were more rows of objects, as of pillars,
houses, trees, Sic, parallel to the  first, but considerably apart from  each other,
as the lines here a S,b S, d S, Sic, still all would  vanish or seem to terminate
in the verv same point of the field of view.  The reason of this is easily under-
stood.   Let us suppose a line drawn directly south from the eye to the point
S, between the parallel lines of pillars, houses, and trees, a S,b S, d S, Sic,
also pointing directly south; and  let us suppose the two rows of pillars to be
100 feet apart, then evidently for the same reason as the space between the
top and bottom of the pillars, that is to say, their height becomes apparently
less and less as their distance from the eye increases, so  will the space be-
tween  each pillar and its opposite, or between it and the point corresponding
to it in the visual ray along which the eye looks, become apparently less, and
therefore the lines of pillars really 100 feet apart from  each other, and 50 feet
from the visual  ray, will, at a certain distance from the eye, (viz., where a
space of 50 or 100 feet is apparently reduced to a point,) appear to join, and
the three lines will appear to meet in that point, beyond which  none of them
can be visible, and which is therefore the vanishing point of all.   The  con-
ception of this truth may be  facilitated by supposing a planet to be visible in
the exact point of the  heavens at the moment of observation  ; then, if the three
parallel lines were continued on to the planet, and were visible all the wav, they
would  arrive there with the interval between them just as when they left the
earth:  but as a planet, although thousands of miles in diameter, owing to its
distance from the earth, appears on  earth only as  a point, much more would
two lines only 100 feet apart be there  undistinguishable  in place  by human
sight.  And what is  true  of a space  of 100  feet  between parallel lines, is
equally true of a mile or  of thousands of miles.   As a general rule, therefore, 
364
LIGHT.
it holds, that all lines in nature parallel  to each  other, when represented  in
perspective, tend towards an end in the  same vanishing point; and that point
is the situation  where the line  terminates, along  which the eye looks  when
directed parallel to any  one of the real lines.   And this  is true  only  of
lines all in the same level or horizontal plane, viz.,  such as might lie along
the surface of the sea, but also of Jines placed  one above another, as those
running along the tops and bottoms of the  pillars here, or along the walls,
roofs, and  windows of the houses, or along the roots and summits of the trees,
and indeed of all lines in whatever situation, provided they are parallel to the
visual  ray, and therefore to one another.   And  the truth holds equally with
respect to  lines  which do not vanish at the " point of sight," or centre of the
picture, as with respect to those which do.  When it is ascertained, therefore,
that a line or boundary of any  natural or artificial object has a certain inclina-
tion to the axis of the picture, or to what we have described as the principal
visual ray, then also is it known that all the parallels to that  line have their
vanishing point in the same spot of the field of view, and  a line  supposed to
be drawn  from  the.eye to the heavens, or  really drawn from the eye to  the
picture in that direction,  marks upon the picture the true vanishing point.
   It will  be understood why,  in a  long arched  tunnel, or a cathedral, with
many longitudinal lines on its floor, walls, roof, &c, all such lines seen by
an eye looking along from one end, appear to converge to a point at the other,
like the radii of a spider's  web; and why, similarly,  in the representation of
a common room, viewed from one end, all the lines of the corners, tops and
                                            bottoms of  windows, floor,
                 Fig. 155.                   stripes on a carpet, corners of
                                            tables, Sic, being parallel to
                                            each other, tend to the same
                                             vanishing  point as V.  The
                                             appearance of the lines in the
                                             floor of this room may recall
                                             that  of  the furrows in  a
                                             ploughed field  as seen from
                                            one end,  when  they appear
                                            like the ribs of a fan spread
                                             out towards the spectator.—
                                             The same considerations will
                                             explain the phenomenon often
                                             to be  observed,  of two little
                                            clouds seen  near  each other,
                                            and almost  motionless for a
 time in the  distant sky, but  which on approaching  the  spectator with the
 wind, appear gradually to separate, and in a corresponding degree to enlarge,
until one  of them sweeps past considerably to the right hand, and the other
 considerably to the left; after  this, they lessen and approximate as they before
 enlarged  and separated, and at last  beyond the spectator, appear as small and
 as near as when first observed.—Clouds being so mutable  and  uncertain in
their forms, persons have been led  to deem all apparent changes  in  them, of
form, size and  place,  to be real changes, and not, as they generally are, mere
 optical or perspective illusion.
   By far the most important vanishing point in  common scenes is the middle
 of the horizon  or level line, and in a picture properly  placed, it is at the exact
 height of the eye.   It is marked S in  the figure before the last, and  V in  the
 last figure.  Because in houses, the roofs, foundations, floors, windows, Sic,
 
THE EYE--PERSPECTIVE.
365
 are all horizontal, the vanishing points of their lines must be  somewhere in
 tne horizon, and if the spectator be in the middle of a street or of a building,
 and be looking m the direction of its walls, their vanishing  point will be in
 the centre of  the scene or picture;  if he be elsewhere, it will  be to one side.
 in holding up a picture-frame, through which to view a scene suitable for a
 mcture, it would be found most generally befiting  to raise it until the line of
 the horizon appeared to cross  it at  about  one-third from  the  bottom:—this
 tact becomes the reason of the rule in painting, so to place the horizontal line.
 in beginning a picture, this line is usually the first line drawn on the canvas,
 marking the place of the vanishing points of all level lines and surfaces.  And
 the eye of the spectator is supposed to be placed before the middle of it, and
 generally about as far from the picture as the picture is itself long, such bein^
 the extent of view which the eye at one time most conveniently commands.0
   Understanding now that the apparent or perspective direction of all lines
 in a scene is towards their vanishing points as above discovered, and the rule
 having been given for  determining these  points  in a drawing, we  proceed
 to show how  much of a line drawn to  any vanishing point  belongs to the
 known magnitude of any object  through which it passes ;  in other  words,
 how much an object is in perspective  foreshortened in consequence of its
 distance and obliquity of position.
   If we suppose A S  P to represent a plate of glass seen edgeways, and that
 towards the point S in it, an
 eye is looking  from  the                     Fig. 156.
 point D,  evidently  then, a                         ^
 line from  P continued  in the
 direction  of R and  beyond
 until it vanished from  sight,
 would have as its perspec-
 tive image or representation
 on the glass a line reaching
 from P   to  S,—S   being,
 moreover, the point  of sight here, and the pictorial  vanishing point of the
 line.   Now  to divide the representative line P S so as to correspond with
 any given portions of the original line P R, Sic, it would only be necessary
 to draw other lines from the place of the eye D to cut or touch the original line
 in the situations desired, and these lines would cut the perspective line S P
 as required:  for instance, the portion of true line a b would be represented
 by that portion of the image-line S P included between the  two lines a D,
 and 6 D, and so of any other portions.  There are figures drawn  on many
 mathematical  scales  by which such problems as this can be  at once approxi-
 matively solved;  and it would be possible, by trigonometrical  calculation, to
 solve them exactly in all cases: but the  most generally convenient mode  in
 practice is to sketch on the intended drawing (as that of vvhich the bounda-
 ries are given  in the  next cut) the kind of measure shown above, by setting
 off from the point of sight S, a distance on the horizontal line, as at D, equal
 to the distance of the eye from the picture, and then by oblique lines from  D
 drawn  upon the base line P R, to cut the perpendicular line P  S in the situa-
 tions  desired—as is  seen in the last figure,  which  differs  from fig. 161
 chiefly in  having  the point of distance marked before its point of sight, instead
of, as  here,.laterally.   And  the line P S being always cut by the oblique
line from  D in proportion to the  length of base-line between  P and the ex-
tremity of the oblique  line, a horizontal line drawn through any point in it,
cuts in corresponding proportions all the other lines which have their vanishing
 
366
LIGHT.
points in the horizontal line at S, for instance a S b S, &c.   Thus, to draw
in perspective, on the surface above represented and prepared, a chess-board
or board of squares, it is necessary to set off the breadth of the board on the
Fig. 157.
ID	S			
	~~ ~"~-—4^		AXjf	
			5sF^	d.
 base-line to the right and left of P, viz., at b and a, and  then  to draw to the
 point of sight as a vanishing point, the lines a S and b S, part of which lines
 will, therefore, represent the sides of the board, and then to draw  the diago-
 nal b D, which, for the reasons above stated, will cut the lines P S and a S in
 proportion to the length of base-line to the right of their extremities ; a efb,
 therefore, is  a square  seen  in  perspective,  and any number of smaller in-
 cluded squares are made by drawing lines from the vanishing point to equal
 divisions on the base, and making cross lines where the diagonal cuts these.
   Much of the delight which the art of painting is calculated to afford is lost
 to the world, because persons in general know not how  to look at a  picture.
 Unless  a spectator place himself where he  can see the objects  in true  per-
 spective, so that he may fancy himself looking at  them through a window or
 opening, every thing must  appear  to  him  false  and  distorted.   The eye
 should be opposite the point of sight of the  picture, and, therefore,  on a level
 with the line of the horizon, and it should be at the required distance, which
 is generally at least as great as the length of  the  picture.  But blame not
 unfrequently rests also with the artist, from his having neglected the study of
 perspective.   It is very common, for instance, to see miniature resemblances
 of architectural structures so foreshortened  and tapered,  that the eve, to see
 them in true perspective, would require to be  within  an  inch of the'paper;
 whence at the usual distance often or twelve inches they  are seen as hideous
 distortions.   The  specimens,  in the few preceding pages, necessarily exem-
 plify in a degree this  error, because the point  of distance had to be  marked
 where there was but a small page.  These figures, therefore, by any person
 studying the subject particularly, should be drawn on  a scale so much larger.
 as to allow the eye really to view them at the distance supposed.

 A means of judging of the dimensions of the bodies by the visual angle, but
   which depends neither on the absolute size  of the image, nor on the fore-
  shortening of the ground plane on which the body stands, is to use known
  objects in view as measures  for others near them" which are unknown.
  If any person of our acquaintance be standing  at some  distance from  us
near another person who is a stranger,  we know-how tall the stranger is  by
taking the acquaintance as a measure.
  In pictorial representations of objects little familiar,  as to many people are
the Egyptian pyramids, the bodies of the whale, the elephant, the camel, &c,
human beings  may be represented around them to serve as  measures for the 
THE EYE—PERSPECTIVE.
367
 less known  object.  The Colossus of Rhodes seen from  afar, might to a
 stranger have appeared but an ordinary statue of a man, but the exact magni-
 tude would have been  known as soon as a ship of known dimensions were
 seen sailing into port between his gigantic limbs.
   When an unpractised eye  is first directed from a distance to a great ship of
 war, it will on many accounts dwell upon the object with wonder and admi-
 ration; but it may not  j-jdge truly of the enormous magnitude  until  it be
 near enough to perceive the  sailors climbing on  the rigging, and appearing
 there,  by comparison, as flies or little  birds appear among the branches of a
 majestic tree.
   By  having  a measure of this kind presented to us, the  magnitude and eleva-
 tion of great  edifices are rendered  more obvious.  The magnificent pile of
 St. Paul's in  London becomes still more striking, when we  discover visitors
 looking from  the balconies near  the summit-cross.  They appear so minute
 among the surrounding huge masses that a person is for a while disposed to
 doubt  whether they be men:  but the fact once ascertained,  the grandeur of
 the temple is  most impressive.
   Many persons cannot distinguish between the little pilot balloon (some-
 times  dispatched before the great one to show the direction of the wind) and
 the great balloon itself, until with the last they perceive the aeronauts as little
 black  points suspended under the globular cloud.
   Strangers visiting Switzerland, on first entering the valleys there, are  often
 much  deceived as to their extent.  Being familiar generally with more lowly
 hills and shorter valleys at home, which, however, from being near to the
 eyes, form bulky images, and having at first no other measure, they almost
 universally underrate the Alpine dimensions:—they will wonder, for instance,
 in the  valley of Chamouny, that they should be travelling swiftly for hours
 without reaching the end,  where on entering they did not anticipate a drive
 of more than  half an hour.
   The author once sailed  through the Canary Islands, and passed in view
 of the far-famed Peak of Teneriffe.  It  had been in sight duringthe afternoon
 of the  preceding day, at a distance of more than 100 miles,  disappointing
 general expectation by appearing then  only as an  ordinary distant hill rising
 out of  the ocean, but next morning, when the ship had  arrived within about
 twenty miles of it, and while another ship  of the fleet, holding her course six
 miles nearer to the land, served  as a measure, it stood  displayed as one of
 the most stupendous single objects which on earth, and  at one view, human
 vision  can command.   The  ship in question, whose side, showing its  tiers
 of cannon, equaled in extent the fronts of ten large  houses  in a street, and
 whose  masts  shot up like lofty steeples, still appeared but as a speck rising
 from the sea, when compared with the  huge prominence beyond it, towering
 sublimely to heaven, and around  which  the masses  of cloud, although as lofty
 as those which sail  over the fields of Britain, seemed still to  be hanging low
 on its  sides.   Teneriffe alone of very high mountains, rises directly  and
 steeply out of the bosom of the ocean, to an elevation of 13,000 feet, and as
an object of contemplation, therefore, is more impressive than even  the still
loftier summits of Chimborazo or the  Himalayas, which rise from elevated
plains,  and in the midst of surrounding  hills. 
368
LIGHT.
It is because objects which  are nearly on a level with us, as contrasted with
  such as are either much above or much below, are in general more nume-
  rously surrounded by other known objects which serve  as  measures of
  comparison, that we judge so much more correctly of the size and distance
  of those near our level than of others.
  A man walking like ourselves on the sea-shore or  other level, is at once
fully recognized; and probably it may not occur to us, that he appears smaller
on account of the distance; but if the same man be  seen afterwards at an
equal  distance above us, collecting the sea-fowl's eggs on the face of a cliff,
or below us, gathering shells on the beach when we ourselves have reached
the height, he appears  no bigger than a crow; yet in all the cases he is where
the same bulk  forms the same magnitude of image on the retina.
  Even on a horizontal plane, if the general surface be bare and uniform,
single distant objects appear very diminutive.  This is true, for instance, of
a man seen apart from his caravan,  while journeying across  a sandy desert;
but a  man viewed at an equal distance, in the midst of a cultivated landscape,
appears of his natural size.   The same is true of a boat or ship seen out on
the high sea, as contrasted  with the  like viewed in a harbour  where other
known objects are near them.
  We may  now  understand  why  the sun  and moon, at rising or setting,
appear to us much larger than when they have attained meridian height—
although, if we examine them  by any measure of the visual angle, as simply
by looking at them through the same ring or  tube, we find that there is
scarcely a difference;  and singularly we find the difference to be, that the
orbs  seen horizontally appear even  less than when seen on the meridian,
owing to our being then about 4,000 miles more  distant from  them.   The
sun and moon as they appear from  this earth are nearly of the same  size,
each occupying in the field  of view about the half of a degree, or as much as
is occupied by a  circle of a foot in diameter when held 125 feet from the
eye—which circle, therefore, at that  distance, and at any time, would just
hide either of them.   Now when a man sees  the rising moon apparently
filling up the  end of a street, which he knows to be 100 feet wide, he very
naturally believes that the moon then subtends a  greater angle than usual,
until the reflection occurs to  him, that he is using as a measure, a  street known
indeed to be 100  feet wide, but of which the part concerned,  owing to its
distance, occupies  in his eye  a very small space.   The wridth  of the street
near him may occupy 60° of his field of view, and he might see from between
the houses many broad constellations instead of the moon only, but the width
of the street far off may not occupy, in the same field of view, the twentieth
part of a degree, and  the moon, which always occupies half a degree will
there  appear comparatively large.   The kind of illusion now spoken  of is
yet more remarkable when  the  moon is seen rising near still larger  known
objects, for instance, beyond a town, or a hill which then appears within the
luminous circle.  Any person who from the river-side terraces of Greenwich
has observed the sun  setting beyond London,  with St.  Paul's  Cathedral
included in the glorious picture, will recollect a most interesting example of
our present subject.—That our ocular judgment of the size of the  sun or
moon is thus influenced by  the presence or absence of objects of comparison,
and not by the place of the bodies  in the sky,  is  proved by the fact, that a
person viewing these bodies at any elevation from the bottom of some of the
Swiss valleys, where  he might almost suppose  himself placed at the centre
of the earth, and looking abroad along an endless extent of precipices—if he 
THE EYE—INTENSITY OF LIGHT.
369
can closely compare them with certain known magnitudes of ridge or forest
bounding his view, sees  them as large as they appear from other situations
when rising beyond a low horizon.  Another proof is afforded by the case
of a balloon at a great elevation seen crossing the disk of the sun or moon,
and then appearing, however large in reality, as an absolute speck within the
vast luminous area.   In a future paragraphit will be explained, that another
circumstance contributes to cause the sun and  moon when low to  appear
larger than when high, namely, their apparent dimness, owing to the obstruc-
tion of their light in traversing the low dense atmosphere.
  It may be remarked here, that the visual  estimate formed of the great
size of the sun and moon  when they are seen on  the  horizon  is  not  an
illusion,  as  is popularly supposed,  but an  approximation  to  truth, still
prodigiously short of the reality.  When we  see a distant  tree, or a house,
or a hill, apparently within  the circumference of one  of these orbs, it is
really true that  the  orb  is larger than  the tree, or house, or hill, just as
another  more  distant hill would be larger  than nearer objects similarly
surrounded by its outline: but the celestial body is so much larger, that even
if the whole British Isles could be lifted away from the earth, and suspended
near the  moon, as a map in the sky, they would  hide from a spectator  on
earth but a small part of  the disk of the  moon.

Having now shown that the visual angle or apparent size can be a measure
  of the distance of any object only when the true  size also is known, or
  of the true size  when the distance  is  known, we proceed to examine
  other means which the eye  commands for guessing at distances.

2d. Intensity of light, shade, and colour.   (See  the Analysis, page  325
                          and page 355.)

  It has already  been explained that light, like every other  influence radi-
ating from a centre, becomes  rapidly weaker as the distance from the centre
increases, being, for instance, only one-fourth  part  as  intense, at  double
distance, and in a corresponding  proportion for other distances; while  it is
still  farther weakened by the  obstacle of any transparent  medium  through
which it  passes.  Now the eye soon becomes sufficiently familiar with these
truths to judge from them, with considerable accuracy,  of the comparative
distances of objects.
  The fine Gothic pile of Westminster Abbey may break upon the view in
some siuations where nearer  edifices, and perhaps some  minor imitations of
its beauties, already fill the eye  with  their strong lights, but the misty or
less distinct outline of the venerable pile may warn the approaching stranger
of its true magnitude, and prepare him  for the  enjoyment  which a  nearer
inspection of its grandeur and perfection is to afford.
  A small yacht or pleasure-boat may be built according to  the same model
or with the same comparative dimensions  as a first rate  vessel of war, and
may be in view from the shore at the same time, only so much nearer than
the ship,  that both shall form  images of the same magnitude on the retina of
a spectator.   In such a case an unpractised eye might have difficulty to dis-
criminate, but to  an old seaman, the bright lights of the little vessel, con-
trasted with the softer or more misty appearance of the larger, would  declare
the  truth  at once.   A haziness occurring in the atmosphere between the
little vessel and the eye might considerably favour the illusion.
  In a fleet of ships, if the sun's direct  rays fall upon one here and there 
370
LIGHT.
through openings among the clouds, while the others remain  in shade, the
former in appearance  starts towards the spectator.  In  like manner, the
mountains of  an unknown  coast, if the  sunshine  fall  upon them,  appear
comparatively  near, but if  the clouds again intervene, they recede and mock
the awakened  hope of the  approaching mariner.
   A conflagration at night, however distant, appears to spectators, generally,
as if very near, and inexperienced persons often run  towards  it with the
hope of "soon  arriving,  but find, after  miles traveled, that they have made
but a little part of the way.
   A person  ignorant of astronomy deems the  heavenly bodies  so  much
nearer to the earth  than they are, merely because of their being  so  bright
or luminous.  The evening star, for  instance, seen in a clear sky over some
distant hill-top, appears as if a dweller on the  hill might  almost reach it—
for the  most intense  artificial light  which could be  placed  on the  height
would be dim  in comparison v/ith the beauteous star; yet to  a  dweller on
the hill it appears just as distant as to one on  the plain ; nay,  at a thousand
miles farther  west, and,  therefore,  nearer, the appearance  would still  be
nearly the same.
   The  concave of the starry heavens appears flattened  above, or as if its
zenith were nearer to the  earth than  its sides or  horizon, because the light
from above havipg to pass  through only the depth or thickness of the atmo-
sphere, is little obstructed,  while of that which  darts towards any place
horizontally through hundreds of miles  of dense  vapour-loaded air,  only a
small part arrives.
   The sun and moon appear larger at rising and setting than when midway
in heaven, partly, as already explained, because they can then be easily com-
pared with  other objects, of which the size  is known, but  partly, also,
because of the much less light arriving from them in the former  situation,
while their diameters are nearly the same.
   A fog or mist is  said to magnify objects seen through  it.   The truth is,
that because  it diminishes  the intensity of the light from them, it  makes
them appear farther distant without lessening the visual angles  subtended by
them ; and because an object at two miles, subtending the  same angle as an
object at one mile, is twice as large, the conclusion is drawn that the dim
object is large.  Thus,  a person in a fog  may believe that he is approaching
a great tree, fifty yards distant, when the next step throws him into the bush
which had deceived him.—Two  friends meeting in a fog, have  often mutu-
ally mistaken  each other for persons of much greater stature.—A row  of fox-
glove flowers on a neighbouring bank has been  mistaken for  a  company of
scarlet-clad  soldiers on the  more distant face  of the hill.—There are, for
similar reasons, frequent misjudgings in late twilight and early dawn.—The
purpose of a thin gauze screen interposed between the spectators in a theatre
and some person or object meant to appear distant, is intelligible  on the same
principle: a boy near,  so screened, appears to be a man at a distance.—The
art of the painter uses  sombre colours when his  object  is to  produce in his
picture  the  effect of distance.—On  the  alarming occasion of a very dense
fog  coming on at sea,  where the ships of a fleet are near  to  each other,
without wind, and where  there is considerable  swell or  rolling of the  sea,
much damage  is often done: but it is to be remarked in  such a case, that the
size of  each ship approaching to the shock, is always, in the apprehension
of the crew of the other ship, exaggerated.
   The celebrated Spectre  of the Brockcn, among the Hartz Mountains, is  a
good illustration of our present subject.   On a certain ridge, just at sunrise, 
THE EYE--INTENSITY OF LIGHT.
371
a gigantic figure of a man had often been observed walking, and extraordinary
stories were related  of him.  About the  year 1800, a French  philosopher
and  a friend went to watch the apparition ;  but for  many mornings they
paraded on  an opposite ridge in vain.   At last, however, the  monster was
seen, but he was not alone; he had a companion, and singularly he and his
companion aped all the motions and attitudes of the observer and his compan-
ion:  in fact, the spectres were merely shadows of the observers, formed by
the horizontal rays of the rising sun falling on the morning fog which hovered
over the valley between the ridges ; and because the shadows were very faint,
the figures were deemed distant, seeming men  walking on the opposite ridge;
and  because a  comparatively small figure seen near,  but supposed distant,
appears of gigantic dimensions, these shadows were accounted giants.
   While the comparative intensities of light coming from bodies considered
as wholes, or from their sides similarly exposed to the source of light—furnish
an indication of their different distances from  the observer,  the  comparative
intensities from their sides dissimilarly or  unequally exposed to  the source of
light, and which, therefore, reflect light to the eye, or are illuminated in dif-
ferent degrees,  furnish an indication of the forms and attitudes of the bodies.
In observing, for instance, a white house exposed to the sun, it is seen that
the side directly receiving the rays is highly illuminated or bright, while the
other sides are less so, and are said to be  in the shade—a shade which is more
or less deep in  proportion as there are few or many sources of reflected light
near it.  The different faces or walls of such a house are, to the sense of the
observer, as strongly distinguished from each  other, by the  mere difference
of shade, as if they were of different colours, or as if they were  examined by
the touch, or by walking round them.   If the object were a ball instead of a
square house, there would still be as great differences of shade in the half not
receiving direct rays, but the parts, instead of  forming abrupt contrasts like
the walls of a  house,  would appear  to melt  into each other,  marking  the
beautiful  round contour of the object.   The consideration of all such cases
forms the subject of chiaro-oscuro,  so interesting to the painter.
   Had there not been in nature the provision of light and shade now described,
the sense of sight would have been of comparatively little use, and a mass of
things in  the light, if  of the  same colour, would have been as  little distin-
guishable from  one another by a person  looking directly at them, as things
forming a mass or shadow are  distinguishable  by a  person looking  at  the
shadow.   It is  this provision, therefore, which enables us, independently of
colour, to distinguish the profile or outlines of different bodies placed near to
each other, and to distinguish in the same  body the protuberant or other form
of the surfaces  which is towards  the  observer.  But for this, it would have
been  impossible to distinguish, for instance, between a white  wall when
naked and when having various white objects  placed before it:  and  it would
have been impossible to distinguish between the rounded figures, if similarly
coloured, of a flat circle, a sphere and a cone, all directly exposed to the eye;
but in reality, by some difference of shade, the  white objects are distinguished
from  the  wall,  and in the three  geometrical figures, the uniformly bright
surface of the circle,  the soft rounded shadowing of the sphere and the shade
coming to a point on the cone, at once declare the true forms.    But for the
shadowed parts,  the  facade of a  white  palace of varied architecture would
have been an unmeaning sheet of  lights:  the  lights, however, and  shadows
produced by the juttings  and  recesses,  mark the variety  of surface  most
completely; and the round pillar is distinguished from  the square, and every
pediment, and  capital, and architectural  ornament,  stands  out pleasingly 
372
LIGHT.
conspicuous.    But for light and  shade, again, the "human face divine"
would have been  an unmeaning patch of flesh, for there  are few lines in it
but those made by different exposures to the lights, and yet its every promi-
nence  and  depression, and every momentary change, are  so  truly indicated
to the eye that it  becomes full of meaning or expression.   How well mere
light and shade serve to convey what the eye has to learn of a scene or object,
may be perceived by examining any of the admirable engravings which now
abound, and which, although made up entirely of degrees of shade, or of black
and white,  are scarcely inferior in expression to finished paintings.
  The student of painting soon learns that the lines called outlines, by which
he first sketches subjects, do not exist at all in nature, and have to be again
effaced in his  finished work : for they only mark the place where lights and
shades happen to meet.   Much may be conveyed to the mind, however, by a
mere outline, and particularly of lines if different breadth  and thickness are
used to mark the situation of the fainter and deeper shadows.
  The subject of chiaro-oscuro is not so simple as, from the  fact of the sun
being the great source of light,  might at first be supposed; for although  this
be true, still everybody which  reflects the sun's light becomes a new source
to those about it, and the shading of a picture must have reference to all such
sources, and to the colours of the body itself, and of the neighbouring bodies.
   In looking at an extended landscape, it is seen that the near objects consi-
dered as wholes, are comparatively bright, that their  shadows  are strongly
marked, and that their peculiar  colours are everywhere easily distinguishable
—as of flowers, fruit, foliage, <fcc, but of objects farther off, the colours, with
increasing distance become dim, the lights and shadows melt  into each other
or are confused, and the illumination altogether becomes .so faint that the eye
at last sees only an extent of distant blue mountain or plain—appearing bluish,
partly because the transparent  air through which the light must pass has a
blue tinge,  and partly because the quantity of light which can arrive through
the great extent of air is insufficient to exhibit the detail.  The ridge called
Blue Mountains in Australia, another of the same name in America, and many
others elsewhere, are not really blue,  for they possess all  the  diversity of
scenery which their climates  can give, but to the eye which first discovered
them, bent  on them generally from a distance : they all at first appeared blue,
and they have retained the name.
  In a good picture, where, upon canvas stretched on a frame, the artist has
disposed the lights, shades and  colours in  the very situations  and with the
intensities which they  would have had on coming from  the real  scene to
the eyes, through a plate of glass filling up the frame, all that we have now
been saying is strictly exemplified.   In the foreground, the objects are large
and bright, but as the scene is  supposed to be gradually more remote, the
size and brightness  correspondingly diminish,  until, at last,  there is only a
dim mixture of bluish or grayish masses  forming  the horizon and sky.
   A child,  during what may be called the education of the sense  of sight,
has a strong  perception of the  vast differences of appearance which things
assume according to their accidental distance  from the eye, their  position,
their  exposure to light, &c; for  many of these differences, being at  first
calculated  to deceive the j'oung judgment, have, from time  to  time, been
noted by him with surprise.   Thus, a boy when  he first discovers that a
ship which, at the quay, with  her white sails spread out, concealed from
him half the  heavens, is, in an hour  or two  afterwards, seen by him on the
distant horizon as a dark speck hardly big enough to hide one  star, has
his attention  strongly awakened, and he feels  surprise; or, again, when he 
THE EYE—PERSPECTIVE.
373
 discovers that the faint blue unchanging mass which he had always observed
 bounding in one direction, the view from the home of his  infancy, is a dis-
 tant mountain-side, thickly inhabited and covered with fields and  gardens,
 where, in succession, all the  bright  colours of the  different  seasons predo-
 minate—he is equally struck.  But as soon as experience  has enabled him
 to  interpret  readily and correctly,  the  visual signs under  every variety of
 circumstance, his attention passes so instantly from them to the realities—
 which alone are interesting to him—just as it might pass  from the paper and
 printing of a newspaper to the important intelligence communicated  by them
 that he  very soon ceases to  be  aware  that the sign, which, in every case,
 similarly suggests the object,  is not,  also, in every case similar to itseV, and
 the very same true and complete representation of the reality.  The prejudice
 that the sign is of this nature, becomes  quickly so strong,  that even a diffi-
 cult effort has been made by a grown person again to attend  to the  mere
 appearances, in any scene of  which the realities are known.
   This attempt to analyze mere appearances, and which, in one sense, is an
 attempt to unlearn something, or to retrograde, is called, as  already stated,
 the study of perspective.  When it regards the apparent reduction of size, and
 the foreshortening of bodies  under various  circumstances, it is called linear
 perspective;  when it regards the fading of light and  the modifying of colour,
 it is called aerial perspective.   As the art of painting depends entirely  upon
 the understanding of  these two  departments, the gradual progress  which it
 has made in different countries is a measure of the degree in which the com-
 mon prejudice that things appear as they are has, in them, been  overcome.
 Where this prejudice exists, any untaught person conceives a good  painting
 to  be merely  a miniature representation drawn according to a certain  re-
 duced scale,—as of an inch to a yard,—and in which all the dimensions of
 things are to be measured as simply  as in the reality—while the colours, as
 to vividness, Sic, should perfectly agree with the originals.   This statement
 is remarkably illustrated by the facts, that children in their  rude attempts to
 paint, always aim at realizing the notion of the art above detailed,  and that
 such has been the first stage of painting in  every country.  In Europe  now,
 owing to the labours  of men of genius, art in painting may be said almost
 to rival nature, producing  scenes as  lovely as the finest of nature's scenes,
 and scarcely distinguishable from them : but in other countries, as in China
 and India, among the native artist?, the first stage of the art is still in exist-
 ence.  In a  Chinese  picture, owing to the absence of perspective propor-
 tions, an extensive subject is only a collection of portraits of men and things
 drawn all on the same scale, and placed near one another, and where all the
 colours are as vividly shown as if the objects were only a few feet from the
 eye; there,  the figures at the  bottom or fore-ground are  supposed to repre-
 sent the objects nearest to the spectator,  while the figures higher up are  sup-
 posed to be of more remote objects, all appearing as they might be  seen in
 succession by a person who had the  power of flying over the country.  This
 kind of picture or representation, although not natural, if all viewed  at once,
 may communicate more  information than a single common painting, for it is
 equivalent to many such.  In  Europe lately, the principle has been  again
 usefully acted upon  for  certain purposes, as for representing on one  long
 sheet or on a succession  of sheets, connected in a suitable manner, the banks
 of a river or a line of road.  The banks of  the Rhine particularly have thus
been admirably portrayed, so  that the spectator directing his eye along the
 paper, feels almost as if carried in a  balloon to view in detail the whole of
the  real and enchanting  scenery.   The principle might, perhaps, with ad-
    25 
374
LIGHT.
vantage,  be acted  upon  still more extensively — for instance, to produce,
instead of common maps or charts of countries, true bird's-eye views, over
which the eye, moving from place to place, and at every new point of sight,
would see a certain portion of  the country, as a  bird or aeronaut  would,  the
sketch being supposed to be taken from that certain elevation deemed most
suitable for the ends in view.

   3d. Divergence of the rays of light.   (See  the Analysis, page 355.)

  This is the  next  circumstance to be  mentioned by which  the eye judges
of distance.  Supposing the line E F to  mark the place and breadth  of  the
pupil of the eye, the  light entering from an object at a which is near (it is
                                         here  placed nearer than an object
               Fig. 158.                  could be seen in reality,) is very
                                         divergent, or is  spreading  with a
                                        large angle;  from b the pencil of
                                         rays  is less  divergent, or opens
                                         with  a smaller angle ; from c it is
                                         less   divergent still, and  so  on.
                                         Now the eye, to form an  image
on its retina, requires to exert a bending  power exactly proportioned to the
divergence of  the received rays: and it appears  to have a sense of the effort
made, which becomes to the person a kind of measure of the distance of the
object.  This  divergence of the rays  entering the eye, is the chief circum-
stance in which the most perfect painting must  still differ  in its effect upon
the eye from a natural scene—for, first, in the natural scene, the  objects are
generally more distant than their representation can be; and,  secondly, while,
in nature, every object, according to its distance, is sending rays which reach
the eye with  different divergence, and which  rays, therefore, can produce
distinct images on the  retina at any one time, only of the objects  which
are at the same distance  from the eye, the rays from a picture, which is a
single plane surface,  come from every part with  the same divergence, and
the eye must feel a disappointment in not having to accommodate its power
of bending, to the different distances attempted to be portrayed on the can-
vas.  It might be expected  that this kind of disappointment would be more
felt on looking at a common picture placed a few  feet from  the  eye,  than at
the sort of picture  called panorama, which is on a larger scale and  propor-
tionately more distant, but such is not the case ; and the reason seems to be
that in the former the illusion is not intended to be complete, the fact of its
being but a picture not being at  all concealed, and the eye is therefore at
once told to expect a difference of feeling;—but in the panorama, the whole
circumstances  are  arranged to deceive the eye entirely, if possible,  and to
make it believe that the  images on the retina are formed by light from the
objects themselves,—then to the eye, really deceived in all other  particulars,
the non-accordance with nature in  this one is strongly, and, by some persons,
even  painfully felt, so as on their first  entering the place to cause headache
or giddiness.—The illusion and consequently the pleasure from viewing any
picture may be made more complete by the spectator  using lenses or spec-
tacles, such that the focal distance shall be equal to the distance of the paint-
ing from  the  eye; because such  lenses, as was formerly explained,  would
render all the  rays  entering the eye nearly parallel, and therefore  very nearly
such as would arrive  from objects  at a considerable distance. 
THE EYE—PERSPECTIVE.
375
Hth.  Convergence of the axis of the eyes.   (See the Analysis, page 355.)
    This is the last circumstance to be mentioned, by which a person, through
 the eye, judges of the distance of objects.   In  consequence of there being
 in the two eyes corresponding parts which must be similarly affected by any
 object, that the person may have a single vision of it, as was explained in a
 former page, the  axes of both eyes must point to the object,  and if it happen
 to be very near, they will meet and  cross each  other so near the face  as  to
 produce the appearance of squinting,—seen  when  a person tries to look  at
 the point of his nose,—but if the object be more distant,  the  obliquity will  be
 less, until at  last the eyes  directed to  a thing at a very great  distance, will
 have their axes almost  parallel.  The last figure  may serve also to explain
 this subject.   Supposing E  and F to mark the places of the two eyes if the
 object  looked  at  be  near  them, as at a, they must be very much turned in-
 wards, that their  axes may meet at a ;  if it be at b, they will be less turned,
 if at c  less still, and  so forth.
    When the eyes are not directed to any thing in particular, the axes gene-
 rally become parallel, or as if they were pointed to a very distant object: and
 because this happens generally when persons are  reflecting on  things vvhich
 are absent and seen only by  the mind's eye, it is an expression of countenance
 held to mark contemplation or thoughtfulness.
    The direction of the visual  axes is another particular, like the divergence
 of light, as to  which  a mere picture can  never  produce upon   the eye pre-
 cisely  the effect of the objects themselves.   To  see a picture, the axes must
 meet at it, and generally, therefore, at a few  feet from the eye;   while to see
 the  objects  of nature, they often do not meet  nearer than at  miles.  By a
 glass,  however, as will be explained a little farther on, it is possible to correct
 also this  defect,  and  to render the  optical illusion, as regards still  objects,
 absolutely complete.
    When  a picture has  to represent  objects  supposed far from the eye, the
 farther the picture itself is placed from  the eye,  supposing the  figures to  be
 made proportionately large, the more nearly  perfect will the illusion become,
 because the divergence of rays  and convergence  of the axes  (the two circum-
 stances in which the  effect of a mere picture on the eye must always differ
 from the  effect of a  real scene) will be in  proportion  more nearly natural.
 This explains  in part why the picture called panorama (from  Greek words
 signifying a view of the whole) is an exhibition  so charming; for usually the
 painting is far removed from the eye, and is drawn on a proportionately large
 scale,  and the eyes feel that the light comes from a considerable distance, and
 that their axes do not need to converge  very  much'; and when in such a case,
 the first impression of  the want  of absolute  conformity to nature has passed
 away, the illusion becomes  nearly complete.  But a not less important pecu-
 liarity in the panorama is, that instead of being a painting on a plane surface
 like common  pictures, and embracing only a small part of the  field of view,
 it is on a curved  surface which entirely surrounds the spectator, and on which
 all the objects visible in the various directions from the supposed place, are
 seen in the very  situations  which in nature  they  hold ;  and the spectator is
 enabled  to conceive  much  more distinctly of each particular by seeing it in
 relation to others around.   Few  persons can forget the impression made  on
 them by  the first panorama which  they may have seen; and after increased
 maturity of judgment, they will  discover still more and  stronger reasons  for
 admiring this almost miraculous  mode of instantly transporting them to any
 distance, beyond seas and other dangers,  to contemplate  at their ease  the 
376
LIGHT.
most interesting scenes on earth, represented under the most favourable cir-
cumstances of position, light and weather.   Hence few persons of good taste
neglect the opportunity now, in most great towns so frequently offered, of
obtaining at little cost so high a gratification.
   To correct the slight remaining optical defects of a common panorama, a
large lens may be used, of which the  focal distance is  equal to the distance
of the picture from the eye.  This has  the effect of diminishing the  diver-
gence of the rays until it becomes exactly that which belongs to the supposed
remoteness of the objects, and it also bends the light so  that the axes of the
eyes may be nearly parallel.  The author has found a convenient  mode of
using the lens for such a purpose to  be to cut out two round pieces from
opposite  sides of it, and to form them into a pair of spectacles :—'from one
lens three  pairs may be formed.  Panorama exhibitors should keep such
lenses or spectacles for the use of visitors.
  The effect of the magnitude and distance of the ordinary large panoramic
views  might, with  the  assistance  of proper  glasses, be had from even  the
smallest picture on engraved representation embracing the same field;  and it
is remarkable that  some enterprising person  has not undertaken to publish
sets of interesting views fitted to be used  in that way.  A  common panorama,
occupying a circular wall of 150 feet in  circumference and twenty feet high,
may be reduced—and still retaining the same truth of proportions, to appear
on a piece  of paper five feet long and  eight inches high or broad; and if
this were set up in a suitable frame, like a wall, round the head of a spectator,
while its edges were concealed  by drapery or otherwise, and the eye could
only view it through  fit glasses placed in its centre and made to turn round
so as to command  the whole, it could  not by any ordinary spectator be dis-
tinguished  from the large panorama.   With  the art of lithography, now so
well adapted  for producing soft representations  of scenery, the expense of
such views might be very moderate allowing them to form a common part of
library furniture.   When we reflect upon the expansion of mind obtained by
travelling, and  that not a few of the  advantages would follow a familiarity
with a good selection of panoramic views, it is not perhaps too much to sup-
pose that courses of instruction in geography, history, &c, may before long
be illustrated by this  most interesting mode of aiding the  conception and
memory.
  Common paintings  and prints may  be considered as detached parts of a
panoramic  representation, showing as much of that general field  of view
which always surrounds a spectator, as can be seen by the eye kept in one
place, and looking through a window or other  opening of moderate size.
The pleasure from contemplating these is much  increased by using a lens or
                     such spectacles  as above  described.  There is in the
                     shops such a lens, with the title of optical pillar ma-
                     chine, or diagonal mirror, fitted  up so  that the print
                     to be viewed is laid upon  a table beyond the stand of
                     the lens, and its reflection in a mirror supported diago-
                     nally over it, is viewed through the lens.   The illu-
                     sion  is rendered  more complete  in such  a  case  by
                     having a box, as a b, on the bottom of which the paint-
                     ing is laid, and at the top of which the lens and mirror,
                     fixed in a smaller box at a, are made to slide up and
                     down to allow of  a ready adjustment of the focal dis-
                     tance.  This box used in a reverse way becomes a
perfect camera  obscura.  The common  show-stalls  seen in the streets  are
 
            THE EYE--PAINTING REPRESENTING MOTION.        377

boxes made somewhat on this principle, but without the mirror; and although
the drawings or prints in them are generally very coarse, they are not unin-
teresting.   To children whose eyes are not yet very critical, some of the
show-boxes afford an exceeding  great treat.
   A still more perfect contrivance of the same kind has been exhibited for
some time in London and Paris under the title of Cosmorama, (from Greek
words signifying views of the world, because of the great variety of views.)
Pictures of moderate size are placed beyond what  have the appearance of
common windows, but of which the panes are really  large convex lenses
fitted to correct the errors of appearance which the  nearness of the pictures
would else produce.   Then, by adding various subordinate contrivances, cal-
culated to aid and heighten the effects,  even shrewd judges have been led to
suppose the small pictures behind the glasses to be very large pictures, while
all others have let their eyes dwell upon them with admiration, as magical
realizations of the natural scenes and objects.   Because this contrivance is
cheap and simple, many persons affect to despise it; but they do not thereby
show their wisdom :  for to have made so perfect a representation of objects, is
one of the noblest triumphs of art, whether we regard the pictures as drawn
in true perspective and colouring, or the lenses which assist the eye in exami-
ning them.
   It has already been stated, that  the effect of looking through such glasses
at near  pictures, is obtainable, in a considerable degree, without a glass, by
having the pictures very large, and placing them  at a corresponding distance.
The rule of proportion in such a case is, that a picture of one foot square at
one foot distance from the eye, appears as large as a picture of 60 feet square
at 60 feet distance.   The exhibition called the  Diorama is merely a large
painting prepared in accordance  with the principle now explained.  In prin-
ciple it has no advantage over the cosmorama or the show-box, to compensate
for the greater expense incurred, but that many  persons may stand  before it
at the same  time, all very near the true point of  sight, and deriving the plea-
sure of  sympathy in their admiration of it,  while a slight motion of the spec-
tator does not  make  his eye lose the right point of view.
   A round building of prodigious magnitude  has lately been erected in the
Regent's Park in London, on the walls of  which is  painted a representation
of London and the country around, as seen from the  cross on  the top of St.
Paul's Cathedral.   The real scene is unquestionably one of the most extra-
ordinary which the world affords, and  this  representation of it combines the
several  advantages of—the circular view of the panorama—the size and dis-
tance of the great diorama—and that from the details being so minutely paint-
ed, distant objects may be examined by a telescope or opera-glass.
   From what  has  now been said, it may be understood, that for the purpose
of representing still-nature, or  mere momentary states of moving objects, a
picture truly drawn, truly coloured, and which is either very large to correct
the divergence of light and convergence of visual axes, or if small, is viewed
through a glass, would affect the retina exactly as the realities.  But the
desideratum still  remained of being able to paint motion. Now this, too, has
been recently attempted, and in many cases  with  singular success, chiefly
by making the picture transparent, and throwing lights and shadows upon it
from  behind.   In the exhibition of the diorama  and  cosmorama there have
been  thus represented with admirable truth and beauty such phenomena as—
the sunbeams occasionally interrupted by passing clouds,  and occasionally
gilding the varied scene : perhaps darting through the windows of a venerable
cathedral and  illuminating the  interesting objects in its interior—the rising 
378
LIGHT.
and disappearing of mist over a  landscape—running waler, as, for instance,
the cascades among the sublime precipices of Mount St. Gothard, in Switzer-
land;—and one of the most striking scenes of all, a great fire or conflagra-
tion.  In the cosmorama of Regent Street,  the great fire of Edinburgh was
admirably represented; first, that noble city was seen sleeping in darkness
as the fire began, then the conflagration grew and  lighted up the sky, and at
short intervals, as the  wind increased, or as  roofs fell in, there were bursts of
flame  towering to heaven, and vividly illuminating  every wall or spire which
caught the direct light—then the clouds of smoke  were seen rising in rapid
succession and sailing northward upon the wind,  until they disappeared in
the womb of distant darkness.  So naturally was all  this represented, that
no stranger can have viewed the  appalling scene with indifference, while on
those  who knew the city, the  effect can  scarcely  have been weaker than if
they had witnessed the reality.   The mechanism  for producing such effects
is very  simple;  but spectators, that they may  fully enjoy  them, need  not
particularly  inquire about it.
   It is remarkable, when the imagination is once excited by some beautiful
or striking  view,  how readily  any  visual  hint produces clear and strong
impressions.  One day in the cosmorama, a  school-boy visitor exclaimed
with fearful  delight that he saw a monstrous tiger coming from its den among
the rocks;—it was a  kitten belonging to the  attendant, which by  accident
had strayed  among the paintings.  And another young spectator was heard
calling  that he saw  a  horse  galloping  up the mountain  side;—it was a
minute  fly crawling slowly along the canvas.  There is, in this department,
a very fine field yet open to the exercise of ingenuity, for the contemplation
of pictures  representing motion or progressive events,  may  be  made  the
occasion of  mental excitement the most varied  and intense.   For instance,
there  are few scenes on earth  calculated to awaken  more  interesting reflec-
tions  on the condition of human nature than  that beheld by a person who
sails along the river Thames  from London to  the sea, a  distance of about
sixty miles, through the wonders which on every side there  crowd on the
sight—the forests  of  masts from all parts of the world—the glorious monu-
ments of industry, of philanthropy, of science—the endless indications of
the riches, the high  civilization, and  progressive happiness of the  people.
Now this scene  was lately, in one of our  theatres, strikingly portrayed by
what was called a  moving panorama of  the southern bank of the Thames.
It was a very long painting, of which  a part only  was seen at a time gliding
slowly  across the stage, and the  impression made  on the spectators  was  that
they themselves were sailing down the river in a steamboat, and viewing the
fixed  realities.  In the same manner might  be most interestingly represented
the whole coast of Britain, or any other coast, or  any line  or road, or even a
line of balloon flight.   There was another moving panorama exhibited about
the same time at Spring  Gardens, aiming at an effect of still greater difficulty,
viz.,  to depict a course of human life ; and  the history chosen was that of the
latter part of Bonaparte's  career.  Scenes representing the principal events,
were, in succession,  made to glide across  the field  of  view, and  were so
designed that the real motion of  the picture gave  to the spectator the feeling
that the events were then in progress; and  with the accompaniments of clear
narration and suitable music, they produced on those who viewed them the
most  complete illusion.   The  story began by recalling the blow  struck at
Bonaparte's ambition  in the battle of Trafalgar; and to mark how completely,
by representations of  various moments and situations of the battle, the spec-
tators were  in imagination made present to it, the author may mention that 
THE EYE--PAINTING REPRESENTING MOTION.        379
on the occasion of his visiting the exhibition, a young man seeing a party of
British represented as preparing to board  an enemy's ship, started  from  his
seat with a hurrah, and seemed quite confounded when he discovered thathe
was not really in the battle.   To the  views of Trafalgar succeeded  many
others, similarly introduced and explained, in each of which the hero himself
appeared: there were his defeat, at  Waterloo—his  subsequent flight—his
delivery of himself to the British admiral—his  appearing at the gangway of
the Bellerophon to thousands of spectators  in boats around, while in Plymouth
harbour, previous to  his departure for ever from the shores of Europe—his
house  and habits  during his  exile, with  various  picturesque  views of St.
Helena;—and last of all, that solemn  procession, in which the bier with his
lifeless corpse was moving slowly on  its way to the grave under the willow-
tree.   The  exhibition now spoken  of might have been made better in all
respects, yet in its mediocrity it served to  prove how admirably adapted such
unions of painting,  music and narration are to affect the mind, and therefore
to become the means of conveying most impressive lessons of historical fact
and moral principle.
   Painting, whether employed to portray scenes of entirely  still nature, or
scenes involving some kind of motion as above described, has still, as its great
aim or end, merely to represent interesting subjects, and to give to the spec-
tator as much as possible that clear conception of them which is obtained by
ocular examination  of realities; and thus, as a system  of visual signs of
thought, it becomes, like language, which is a system  of audible signs, a
means of expanding the  boundaries of individual human existence  into wider
space and time, and thus of elevating human nature.  While it portrays only
strict matters of fact, whether of past or present time, as  particularly  human
individuals, objects of natural  history, the beautiful and magnificent scenes of
nature, interesting events which  the artist has the means of faithfully repre-
senting, &c, it may be called truly historical painting, embodying the mate-
rials of true history, both natural and civil, and then it is of singular value.
But even when applied to other purposes, it may still be fraught with delight;
and just as  language, of which the grand object or use  is to  express strict
truths, has still been admirably employed in giving a permanent existence to
a variety of fictions, from the wildest fables and rhapsodies to the historical
plays and novels of modern times, as those of Shakspeare and of Scott—which
plays and novels, although not furnishing true portraits of individual human
nature, are yet most correct portraits of general human nature—so may pain-
ting be employed in  embodying fictions  adapted to its peculiar powers,  and
it may do so in a manner to prove the artist endowed with the highest degree
of human o-enius.   It should  always be  recollected, however, that what is
usually dignified with the name of historical painting, really bears to  histori-
cal truth only the kind of relation which novels and plays bear to it, and often
approaches even less nearly to the truth; for it  pretends to relate a thousand
minute circumstances which no history has preserved, and which, therefore,
only the imagination of the artist can supply.  Thus when a painter, know-
ing that Lucretia  stabbed herself in  the  presence of her father  and  others,
after the crime of Tarquin, exhibits a woman dying, and  a certain number of
persons around her in horror and astonishment, he no more  represents the
real Lucretia and her  friends than he represents any other  particular young
woman and her friends;  for he is quite assured  that not one of the figures in
such a picture is a portrait of the individual whose name it bears ; his picture,
therefore, in so far, is as untruth or fiction, while it very probably has some of
the additional errors and even absurdities  so common among historical paint- 
380
LIGHT.
ers, in respect of national usage in costume, religion, manners, Sic, and in
respect to general personal appearance,—as when a Rubens wishing to repre-
sent Sabine or other ladies, gave  them the Dutch corpulency deemed comely
in his own  country, although it strikingly  contrasted  with the  true forms of
Italian or Grecian nymphs.  From all  this it appears that historical pictures
may often be regarded as portraitures, not of the  realities, but of comedians
acting scenes in historical plays intended to represent the realities.
   In dealing with the events  of ordinary history, there  is  no  strong reason
why artists may not please themselves  and their spectators as we have now
been describing; but it may  admit of doubt whether  similar liberties should
be allowed  with respect to religion.  Yet  any painting of the last supper, for
instance, or of the ascension, is not more true than a theatrical representation.
To judge of the nature of such a picture we have only to suppose any of the
events recorded in  the  New Testament to be represented by a painter in
China with the countenances seen on Chinese tea-boxes: such a  representa-
tion would  appear in  Europe revoltingly absurd;  but the common practice
here is only a degree better, Italian countenances  being usually substituted
for the Jewish; and twenty painters, undertaking the same subject, generally
put different persons into all the situations.  Then it  can produce no pleasing
impression  on a Christian's mind to be told, that an admired painting  of the
crucifixion  was made  chiefly from the body of an executed murderer; or that
for a praised representation of the triumphal entry into Jerusalem, the painter
had deemed his own physiognomy the  most  befiting for the principal figure,
while he copied the portrait  of Voltaire as a specimen of the  bad Jews, of
Newton  as a specimen of the good, and of wife, cousins, acquaintances, and
old clothes-men, to  make up the remaining groups.  With the knowledge
that such things have  often been,  it need not  surprise that  many persons of
correct feeling turn  with horror from  all these mimicries and  falsehoods, to
seek their idea of God and his providence in  the sublime descriptions of his
attributes, which written language conveys, and which all creation, in a mute
language not less impressive, so  strongly confirms.  When men generally
could not read, and as  a mass were  extremely ignorant, various means of
fixing their attention upon religious subjects  might be  useful,  and therefore
proper, as sacred plays, certain possessions, pictures, &c,  which have now
in many  countries ceased to  be either;  but a person of good sense will con-
tinue to regard  with a certain  respect whatever at any time may have con-
tributed to reclaim portions of mankind from barbarism and wickedness to
the just appreciation of the divine charities of a pure  religion.
   There are in painting other classes  of fictions, which  pretend  to nothing
beyond fiction, and  which are yet truly admirable; such are personifications
of the virtues and vices, serving to recommend the practice of the former, and
to deter from that of the latter—almost  all Hogarth's works are of this charac-
ter, and evince the highest mental acumen  and genius :—then  may be  men-
tioned the personifications of what have been called the elements  and powers
of nature, including  many of the personages of the heathen mythology—then
other generalizations of the characteristics of human or other nature, as scenes
of domestic affection,  of the play of the  passions, &c, &c; and because many
subjects when so sketched, are intelligible to the eye with the suddenness of
lightning., where longest verbal description would convey the idea but imper-
fectly, the  art of painting, in regard  to them, possesses a truly magical and
inestimable power.
   As painting,  whether employed to represent matters of fact  or of fiction,
can accomplish its ends  only through the means of drawing or  linear per- 
                           THE TELESCOPE.                       381

spective, and of shading and colouring, or aerial perspective, these  subjects
require to be studied by every artist with great attention ;  but it is important
for all to be aware that the greatest mastery over these, which are merely the
mechanical parts of the art, will go a very short way towards producing good
performances, unless there be present also the genius to select or to compose
subjects worthy of being represented,—indeed, will go little farther  to make
a painter than the learning of mere penmanship goes to make a historian or
a poet.   This remark seems the more necessary, because there is in human
nature a disposition to  value so much the means by which important ends are
attained, that often the end itself is forgotten in the contemplation of the means,
—as when a person, perceiving that money  will procure all desirable things,
at last becomes the insane miser, and dies from  want of the  common neces-
saries of life rather than touch his hoarded treasures :—while among painters,
as among persons of other  occupations,  the  talent for the inferior  or  more
mechanical departments of the art, is more common than for the higher.  Do
we not see  the  subordinate  accomplishments of the painter, by not a few,
both artists and pretended connoisseurs, supposed to  be the principal?   But
this is evidently to value the dress or clothing, instead of the person; or like
the bibliomaniac, to regard the type and binding of books more than the sub-
ject.  To prove how unessential what is called high-finishing in painting,  is
to the complete attainment of the  purposes of the art, we may instance the
cartoons of the immortal Raphael, which to the mere mechanic in art appears
almost daubs, although exciting such enthusiasm in  the superior mind: and
many of the mere sketches of genius are to a true taste more precious than
some of the most laboured pieces in our galleries.   As it is of no importance
to a man who sees approaching the  friend of his heart, whether it be by day-
light or candlelight, or  with the source of light above or below, &c, provided
there is light enough for him to distinguish clearly the friend of his  heart, so
is it of no importance  how  any interesting subject  is  represented, provided
the picture vividly excite a true conception  of  the subject.   A painter will
discover the difficulties which  a brother artist had to  surmount in represent-
ing an object in some particular predicament, as regards the  light, Sic, and
may estimate the talent accordingly: but the great proportion, even of the
most accomplished ordinary spectators, will generally be looking beyond the
sign to the thing signified, heedless of the artist's difficulties.  In consequence,
however, of the prejudice in favour of " a sweet or adorable bit of colouring,"
as it  will sometimes be called—and which in truth may have the merit of most
natural colouring, there are preserved in many galleries pictures disgusting in
almost all othe/respects, as  of drunken  Dutch boors, with  fiery noses and
physiognomies degrading to human nature, &c, Sic ; on seeing which, the
man of°taste deplores that the art of painting should so often have been prosti-
tuted by clever men to  the vile purpose of representing things of worse than
no interest.

"When the image formed, as above described, beyond a lens, is viewed in
   the air by an eye placed  still farther beyond in the same direction, the
   arrangement, according  to  minor circumstances, constitutes either the
   common telescope  or microscope."   (Read the  whole second paragraph
   of the Analysis, page 325.)
   The name of telescope (a compound Greek term, signifying to see far, as
microscope signifies to  see what is small,) applies to that wondrous instru-
ment of modern invention by the use of  which the intelligent soul may be 
382                             LIGHT.
said, on the beams of light as its path, to dart widely into space for the pur-
pose of contemplating the distant glories  of creation ;  or again, by which it
can command distant objects instantly to approach, for the purpose of conve-
nient inspection.  In ancient times, a man, while looking with admiration on
the bright face of the moon, might have exclaimed, "How pleased would  I
be, had I the power to fly upwards to that celestial orb, the better to under-
stand its nature and beauties;" but he could little have  anticipated  that the
day was coming when human ingenuity would find means in a great measure
to satisfy the wish:—now the telescope is this means, for one which merely
doubles apparent magnitudes, shows the moon exactly as she would appear
to a person who had ascended towards her from theearth a distance of 120,000
miles,  while one of greater power produces effects correspondingly great.
—But  to examine the heavenly bodies is by no  means the only use of the
telescope, man  being often extremely interested to discover what  is  passing
at a distance on the surface of the earth around him.   Thus, by a telescope,
the military chief obtains  a close view of approaching friends or foes, while
they are yet concealed from the naked eye, in the blue mist of distant moun-
tain  or plain—and similarly, the sea-captain, while persons around him per-
eeive only a little speck on the far horizon, discovers there a ship of class and
nation  at once  evident to him, and  with the crew of which, by the additional
use of  signal flags,  he is  enabled  readily to converse.   At midnight a tele-
scope directed to a distant cathedral, may so effectually call it into the presence
of the  observer, that on the clock-turret may be watched the progress  of the
slow hands which tell of  the unceasing lapse of time.   A man, in the midst
of a wide plain, or on a lofty hill-top, or far on the face of a lake,  who might
suppose himself quite alone and unseen,  would yet, by a telescope, be instantly
placed under the observation of whoever chose to watch him.  And the same
might  happen to a man within  the high walls of his own  garden, or even
within his b/)use near an open window, if a straight line could reach him from
some station where an observer was.   Some remarkable cases  of actions,
imagined by the parties to have been done in perfect secrecy, have thus been
brought to light.
   Now the telescope, with its extraordinary powers, exhibits but another mo-
dification of the simple case (described at page 341, and exemplified in the
camera obscura, Sic,) of an image formed for visual inspection beyond  a lens.
And we shall here explain that its powers depend altogether on the two cir-
cumstances, first of its large lens  collecting, for  the formation of the  image
(subsequently transferred  to the observer's  retina) a thousand times  or more,
the quantity of light which the naked pupil could receive; and second, of its
forming by this light an image, to which the eye may be brought very near,
so as to examine it with magnifying glasses of any power.
   To  understand this well, we must recall, that  the nature of the bending  of
light in passing through a lens is such, that all the rays reaching the lens from

                                 Fig. 160.
any point of a visible object in front, and forming what is called a pencil of
light—as that spreading  from the  point A of the cross here represented, to 
                           THE TELESCOPE.                       383

 the lens L—are rn\]t>ntaA ;~            ,•
 bevond  the fpl      6d,ln a ^responding point, as a, at the focal distance
 fffie direct liie A "V * Wlys l° meet the central  ra? of the Pencil ^re,
 centre of thP ?p  °5  a"d',therefor^  when the light comes from above the
 comes from hiln   ^ f°Cal- meetin^ is belovv' as shown  here 5 and "-hen  it
 ev^rv vSL nn Vf' r6 u^8 *S ab°ve ; then the same happening as regards
 TanVB are hlT °f  ^ °h}T ^e  ^ from ™lY the ^°  extrfi™ Points
 thesol IIII ^^TTV 3t corresP°nding  points beyond  the lens in
 n LedXre I   ?, and *' the uC°lleC"ted ^ht' if received ™  a white screen
 teJllt   tlf     came;a. «*«*««,  will make apparent to an eye in any
     vl,       ln7rted lmagG °f the 0bJect-  ^ow in the place where
 SonTnSfn   ° T!    imuge'  lf n° SCreen be interposed, the rays, al-
 tZtrT.     °l deStr7ed' but.mereI.v cross each other in the air, without
 hlnnr\Z f  T*7-" they P™10"*1*  crossed « the lens, and spread again
 beyond the foca points, or towards c, as here shown, as they originally spread
 from the several  points of the object itself;  an eye, therefore, placed  any-
 where beyond c, must  receive  portions of the  pencil from every point of the
 image, and may see the  image in the air as it would see an  object situated
 where the image is, in the  focus of the lens—This may be observed at once
 by holding a spectacle-glass or any lens at a proper distance between an ob-
ject and the eye.
  Now a telescope is  merely a long tube, blackened within  to exclude and
destroy useless  light, and having a large lens, called  the object-glass, filling
its distant end,  to gather the light  from the objects  in front,  and with that
light to form images towards the other or near end of the tube,  where the eye
may conveniently inspect them.   These images, for a purpose to be imme-
diately explained, are  examined through another  lens called  the eye-glass,
which is fixed in a small  tube made to"slide backwards and forwards in the
larger, so as to  admit of the focal  distances being adjusted  to the power of
different eyes, &c.  The accompanying sketch  shows the progress of the
 light from the object A, through the object glass  L, to form an image b a,
 and afterwards to be bent by the eye-glass D, so as to  enter the pupil of the
 eye at E, where the rays cross,  to form the last image on the retina.
   In the simple telescope with only two lenses, as above represented, called
 the astronomical telescope or the  night telescope, because chiefly used at night,
 the image is inverted; but this is a circumstance of no importance in viewing
 the heavenly bodies ; to fit the  instrument, however, for viewing terrestrial
 objects, it is necessary to place in the tube another  simple or compound lens,
 which shall form a second image from  the first, and by inverting a second
 time, shall produce an image really upright.
   To determine how much larger an object will appear when viewed through
 a certain telescope,—for instance, through one with an  object-glass of three
feet focus,—than  when  viewed by the naked eye, we must recollect  that the
image is formed in the focus of the object-glass, or at b a, in the last figure,
and subtends  from the centre of  that glass or lens,  the same visual  angle as
the object itself (a fact explained page 357,) and to an eye placed there, would 
*
 384                             LIGHT.

 appear of the same size as the object, but if the eye be brought nearer to  the
 image than the centre of the object-glass is, the image will appear by so much
 taller and broader, and thus, as compared with the object, may be called so
 much  magnified.   Now as  the naked  eye cannot see distinctly an object
 nearer to it than at about six inches, because of the great divergence of light
 from a nearer radiant point,  the  telescope in question, without an eye-glass,
 would allow the eye  to come only six times nearer to  the  image than when
 at the centre of the object-glass, and would  only magnify the diameter  six
 times ;  but if  then an eye-glass, as D, of half an inch focus were placed half
 an inch from the image, so as to render  the rays of every pencil parallel, and
 therefore fitted to the powers  of the eye, while the different  parcels would
 cross each other a little way beyond the glass, as shown above, an eye placed
 to receive in its pupil the  crossing parcels,  would see the image as large as if
 at half an inch from it, and therefore 72 times nearer than if viewed from  the
 object-glass, and therefore again as of 72 times greater diameter.   Now, as in
 all cases, the image in a telescope is in  the focus both of the object-glass and
 eye-glass, and is therefore nearer to the  latter than to the former in proportion
 as their focal distances differ, the magnifying power is measured  by that dif-
 ference—in the case at present supposed, the difference is  as 72 to l,and 72
 is the  magnifying power  of the telescope.  The  rule is  generally thus  ex-
 pressed, " divide the focal distance of the object-glass by that of the eye-glass,
 and the quotient is the magnifying power."   It is  always to be remembered,
 that if the diameter of an  object  be  magnified 10  times, the surface is mag-
 nified 100 times, and  so in proportion for other numbers.
   With  such  means  of aiding the  sight,  then,  it is  that we discover  the
 mountains of our  moon, and can even measure their altitudes; that we can
 see the  four  beautiful moons  of the planet  Jupiter;  that we can perceive
 marks and irregularities on the surfaces of  the other planets, enabling us to
 say at what  rate  they severally whirl  round their axes,  experiencing  the
 phenomena of day and night;—and that we can determine many other inte-
 resting particulars.
   The discovery of the telescope is said to  have been first made accidentally
 by the children of  a Dutch spectacle-maker, while playing with their father's
 work; but it was turned to no  use until Galileo, led by science, fell upon it
 again, and with the knowledge of its worth, obtained  from it  the most sub-
 lime results.   The human heart can rarely have throbbed with such delight
 as Galileo's, when  he  first  directed his optic tube to the heavens, and through
 it contemplated so  many glorious objects before unseen by human eye;—°as
 the planet Venus,  our  beautiful morning and evening  star, appearing not a
 circle, but a crescent like our moon in her quarters—as the satellites of Jupi-
 ter—the  rings of Saturn—myriads of stars  until  then invisible to man ;  and
 in a word, when he beheld the undoubted  proofs  of the true  system of the
 universe, as his genius had before conceived it uniting the greatest simplicity
 with unspeakable grandeur.
  The Galilean telescope was  simply a large object-glass to  collect much
light, with a small  concave eye-glass placed so as to intercept the converging
rays before they reach their  focus, and to change  their convergency into°the
 parallelism which  the eye  could command.  This telescope, although mag-
 nifying  less than  that made  of two convex glasses, as  above described, still,
 from occasioning no loss of light by the crossing of rays in forming an ima<re,
 was of considerable power.  The common opera-glass is a telescope made
 on this principle.
   It was explained at page 338, that a ray of light, in being bent or refracted 
THE TELESCOPE.
385
  b>  transparent media, as by a lens, is also divided into  rays of the different
  colours seen in the rainbow.  Hence an image formed  behind a simple lens
  has coloured edges or fringes.   This fact rendered the images of small ob-
  jects much  magnified, in the first made telescopes, very indistinct: and, but
  tor the important discovery made by Dollond,  the  optician, that different
  kinds  ot  glass have dispersive and refractive powers of different relative
  lorce,  so that a concave  lens of  a certain  curve applied  to a convex lens
  might  completely counteract the dispersion of colour by the latter, while it
  left enough  of  the convergence of the rays  for the formation of an image-
  refracting telescopes would have  always  been  very imperfect.   Dollond
  called  his telescopes  achromatic, or not-colouring.   It is  very remarkable,
  that he had  the fortune to obtain some glass for  his purposes more suitable
  than any which has been procured since, or which could be made by known
  rules, until the late improvements  in the manufacture  suggested by the inge-
  nuity of Mr. Farraday.   The author of this work  carried  abroad  with him
  a small telescope  of old Dollond's, which often gave  more correct informa-
  tion respecting minute coloured  objects at a distance, as signal  flaas at sea,
  than much larger glasses of modern make.
   The Microscope of greatest power and with  the form called compound,
  in its structure  approaches very closely to the telescope, the chief difference
  being,  that while  in the telescope a large distant object forms in  the focus of
  the object-glass an image exactly as much smaller than itself  as the distance
  of the  image from the glass is less—in the microscope conversely, a  small
  object  placed  near the focus of the object-glass produces a more distant
  image,  as much larger than  itself  as the image is more distant than it—and
  in the one case as in  the other, the image is viewed through an  appropriate
  eye-glass.  The object-glass in the telescope is large,  in the microscope it is
  generally very small.  If, in the latter, an  object-glass be used of one-eighth
  of an inch focal distance, and the object be so placed that its image  is formed
  at six inches, the  image will be of diameter 48 times  as  great as the object,
  or will  have nearly 2,500 times as much surface:  and if that image be viewed
  through an eye-glass of half an inch focus, the image will appear still twelve
  times larger, or 30,000 times larger than the object.
   A simple  convex lens is called  a single microscope,  and it magnifies, as
  already explained, chiefly by allowing the eye to be brought much  nearer to
  the  object than the  distance at  which the object  could be  seen without the
 glass;  but even where the distance of the eye and object is not changed, a
 lens interposed  will  still magnify by bending the light, as at d and/, making
 that which comes to  the eye at e from
 the top  of such  an object, as the little                Fig. 162.
 cross a, to appear to come from  b,
 and  that  from  the  bottom to come                        ci    .......<&
 from c, thus magnifying the cross          ^_________- fy~—-^~T
 here represented by the black lines,     ^ "c=~~~~---~-____.    \ ]     __X
 to appear of  the size represented by                      ~p/<r^.    \c
 the dotted lines.  A concave lens mini-
 fies for  the contrary  reason.
   Perhaps there is not a greater treat for a person"who has feeling for the
 beauties of nature, than to explore with the  microscope.  While the telescope
 lifts  the mind to the contemplation  of boundless space occupied by myriads
 of suns, and exhibits this globe of ours as less, compared with the  universe
 around it, than a leaf is, compared  with a  forest, or one grain of sand com-
pared with all which lies on the ocean's shore; the microscope again excites 
386
LIGHT.
new astonishment by showing on a leaf, or in a single drop of some water
in which the leaf  has  been infused, thousands  of living  creatures,  and of
creatures not imperfect because thus small, but endowed with organs  and
parts as complex and curious as those of an elephant.   And he who admires
the curious structure of a honey-comb, may bend his eye through  the micro-
scope upon the cut surface of a willow branch, or of other wood, there to
see a similar structure  more wonderful still:  or he may compare the  lace of
a fly's wing with the most perfect which human art can weave;  or the beau-
tiful proportions and perfection of  the limbs and weapons  of an insect, invi-
sible, perhaps, to the naked eye, with any larger objects of the kind already
known to him.
   Telescopes  and microscopes might with propriety be both called  micro-
scopes, for often the telescopic object subtends to the naked  eye even a smaller
angle than the objects which the microscope examines.  The minutest visible
insect at hand may hide from the eye a planet at a distance.  The image in
the telescope, however, is  always much smaller than  in the  microscope,
because the rays from a distance being nearly parallel, must form the image
nearly in the principal  focus of the  object-glass; while for the  microscope,
the rays from the near  object being very divergent may be made to form the
image far beyond that focus, and therefore proportionately  larger.
Fig. 163.
 ; Light falling on very smooth or polished surfaces, is reflected so nearly
  in the order in which it falls, as in many cases to appear to the eye as if
  coming directly from the  objects  originally emitting it,—and such sur-
  faces are called mirrors ; the surface which is flat as well as polished, is
  called a plane mirror."   (Read the Analysis, page 325.)

  If, on a marble  slab, or other flat surface, (represented here at M R, with
                           the edge supposed towards the spectator,) a ball
                           were  projected from A perpendicularly towards
                           D, the ball would rebound directly  back to A,
                           but if projected obliquely, as from  B to  D, it
                           would not return  to the first situation B, but to
                           b, a corresponding situation on the opposite side
                           of the perpendicular,  thus  making the angle of
                           the return  or reflection equal to the angle of
                           approach or incidence ; the same would be true
                           of a ball approaching obliquely from any other
                           point, as C, and rebounding  to  c.   Now light
                           is reflected from polished surfaces according to
                           the same law, so that an eye at A  would see
                           itself as if placed at  d, an eye  at b would see an
                           object really at B as  if it were at e, and so forth.
Where the existence of a  mirror is not suspected, the objects reflected from
it are held to be realities placed beyond where it is.  A wild animal will
attack its image in a glass; and the fable says  that a  dog  crossing a brook,
quitted the piece of meat in its mouth to catch the  tempting image which he
saw in the water below.  The  reason that an object seen in a plane mirror
appears to be just as far beyond the mirror as its true distance on  the side of
the spectator, is, that the diverging rays of a pencil of light have  the  same
divergence after as before reflection.
  Any plane very smooth surface  reflects light  as  now described, and is a
mirror: but different substances send back very  different proportions of the
 
MIRRORS.
387
light which  falls  on them.   A highly polished metallic surface  is the best
mirror, often returning three-fourths of the whole light.  Hence, in reflecting
telescopes, the mirrors are made of polished metal.
  Our common looking-glasses are really metallic mirrors, for it is the smooth,
clear surface of the  quicksilvered tin foil  behind the glass which reflects the
light, the glass itself merely serving the purpose of preserving  the metallic
surface perfectly  clean and  flat.  There is always an imperfection  in  such
glass mirrors, when used for viewing objects obliquely, because the external
surface of the glass acts also as a mirror, although so much more feebly than
the metal behind, and forms a separate  image not quite  coinciding with the
other, and therefore mixing with and confusing it.
  The mirror-power of glass unaided is seen from the panes of a plate-glass
window, which make objects in front very visible, although by no means
with clearness comparable to that  from a"  metallic surface.  All common
panes of  glass in windows, or in print-frames, &c, reflect as much  light as
plate-glass,  but the reflection  being irregular because the surface is irregular,
scarcely attracts notice.
   The smooth surface of a fluid is a mirror, which is, moreover, horizontal;
and when that surface is metallic, as of mercury,  the mirror is most perfect.
In  water, spirits, oil, or any other liquid,  it is also perfect, but feebler.
   The mirror of liquid quicksilver is sometimes used by astronomers in observ-
ing the apparent  altitudes of the heavenly bodies,  for the image in the mirror
appearing exactly as much below the horizon as the object is  really above it,
half the distance  between them is the true height.
   A varnished picture, or any japanned  surface, is  a  mirror:  nay, even a
polished table of mahogany or other wood—as it is well known among play-
ful children.  The author, while writing this, has before him  a table covered
with black leather, and in that covering, as a  mirror, he clearly sees the bright
objects beyond the table.   Polished stones, as marble slabs, &c, reflect as
much as glass.   Even a surface of air  may be a mirror, as where a cold and
dense stratum happens  to lie in contact with a warmer and rarer stratum.
In  such cases, where particular causes  have unequally heated different levels
of the atmosphere, the trees, islands, Sic,  happening to be below, are reflected
from above, and appear as if in the sky.  This phenomenon is called mirage.
It is often to be observed over the burning sands of Africa, where the air is
much heated; and elsewhere certain kinds of mist and thin clouds produce  a
similar effect, causing, for instance, a  ship to appear as if suspended aloft,
 with keel uppermost.
   In certain cases, an object seen by the light reflected from a mirror appears
reversed, as when the right hand of a person standing before a glass becomes
the type for the left hand of the image; or when a tree, or rock, or mountain,
seen in the mirror of a lake, has  its top downwards.
   It is on this account, that a man painting his own portrait  from a mirror,
is apt to reverse all the accidental characteristics of the countenance or person,
not the same on  both sides; and if, as  is generally true, one eye be  higher
than the other, or the nose be a little to one side, a very incorrect resemblance
will be produced.   Hence also  a person whose  countenance is at all  thus
peculiar, never sees himself in a mirror as he  appears to others; and a belle
or beau,'who has decided  that a curl is more graceful on the left temple, may
unconsciously leave it on  the right.
   Bv an image,  however, reflected from  a first mirror to a second, and from
that "to the eye, persons  may see the object, or themselves,  if they choose,
as others see them.  What a pity that there are not some moral mirrors to
answer an analogous purpose! 
3S8
LIGHT.
Fig. 164.
    A candle placed between two parallel mirrors fixed on opposite sides of a
 room, makes visible in either glass to a spectator near the middle of the room
 an endless straight line of lights.  If the glasses be  inclined to  each  other,
 the lights will appear  as if placed in the circumference of a circle, of  which
 the centre  is where  the  prolonged mirrors  would  meet:  this fact is well
 illustrated in  the beautiful  toy called the kaleidoscope. It is possible to place
 a few mirrors in  such situations around  an apartment, that a man entering
 it, may see himself multiplied  into  a  crowd, and a  few ornamental  pillars
 may produce the  effect of thousands formed into long colonnades of retiring
 lines.
   The sun or moon reflecting in a still lake, appear as they do  in the sky;
 but if the surface of the water become at all ruffled by the breeze, instead
 of one distinct image,  there will be a long line of bright tremulous reflection.
 The reason of this appearance is, that every little wave, in an extent per-
 haps of miles, has some  part  of its rounded surface with  the  direction  or
 obliquity which,  according to the required relation  of the angles  of inci-
 dence and reflection, fits it to reflect the  light to the eye, and  hence  every
 wave in that extent sends its momentary gleam, which is succeeded by others.
   Although the external surface of glass reflects but a small part of the light
 which falls upon it, being, therefore, a feeble mirror,  still curiously, if light,
                         which has entered a piece of glass, fall very oblique-
                         ly  upon the  back or internal surface, instead  of
                         passing out there, it is more perfectly reflected than
                         it would be by the best metallic mirror. This light
                         from  A entering a piece of  glass at B, is entirely
                          reflected at C, the back of  the piece,  and escapes
                         at D towards E.   The back of a wedge of  glass,
                        or common prism, thus  becomes a perfect mirror.
   It is this fact which  enabled Dr. Wollaston to construct that beautiful little
 instrument called  by him  the  Camera Lucida.   The  two surfaces  at the
                          back of the  small prism of glass  A become mir-
                          rors,  the first reflecting to the second, and the
                          second to the eye  at  E, the objects in the  land-
                          scape before it, while  the eye also sees through
                          the  glass to the paper  below at B, and may sup-
                          pose the  imagery to  be feebly portrayed on the
                          paper: with a  pencil  that appearance is  made
                          permanent, and a correctly-drawn outline of the
                          scene is at  once obtained.  The  instrument for
                          assisting draftsmen is  still simpler than the ca-
                          mera obscura.   Other modifications of the instru-
                          ment have since been contrived.
   The same fact  of the internal  surface of a  transparent mass becoming a
 mirror, gives  us  the   explanation of that apparition  or phenomenon so
 admired before it was understood, and not less admired since—the rainbow,
 or arc in the sky, as in France and elsewhere it is named—an object which
 the poets of nature have almost worshipped for its beauty, and which few of
 us can cease to remember as one of the delights of our  boyish days, when
 we saw it stretching over the haunts of  our young pleasures, and may have
pursued it in the hope of catching some of the falling rubies and emeralds,
or bright-coloured dew  of which it might be composed.
   When a partial shower of rain  falls on the side of  the landscape opposite
to  where  the  sun is shining, there  immediately appears in the shower a
Fig. 165.
E
3c
B 
MIRRORS.                           339
variegated arch, red at its external bord
Fig. 166.
  orange  vp1|„ ' '     * ? GXternal border or confine' and the* successively
  Sbed aoaJ^t "'  ^ °X^ °f ^ C°loUrS °f the Prisraatic sPect™m
  II  n  tip P  g     7'} !°Wards lts lnner border-  Its centre i3 directly oppo-
  hsmi through tb?1 ^V^ °f * Strai°ht Hne SUPP0Sed t0 be dra™ *°>»
  the sun through the eye of  the  spectator  towards the opposite horizon;  and
  Tffii  m't    c a[ways,,lnder the horiz°"' the bow  is less than a semicircle.
  I he diameter of the circle of which the bow is a part, occupies  nearly 82°
  1%l£eld °f tVMT',iha- *8  to sa^ the b™ always coincides with a hoop of
  one foot d ameter-held eight inches from the eye.  There is a second bow of
  much fainter light  than the first, and with  the colours  in reverse order: it is
  ot 108  diameter,  and therefore external to the other
  w??Vhe exPu,anation of  lhis  mi^cle  of  beauty is  simply as  follows.
  While the sun  shines upon  the  spherical  drops of falling rain, its light falls
  upon the whole central part  of any drop, passes completely through, but that
  portion  which enters near the edge of the drop, as at a, is refracted or bent,
  and reaches the back  surface  of  the drop at  y so
  slantingly,  or at an angle so great, that it suffers there
  an entire reflection instead of being transmitted; the
  ray, therefore,  is  turned to b, where it  escapes from
  the drop, and as here shown, descends  to the earth
  or eye in the direction b e.  Thus every drop of rain
  on which the sun shines is  a little  mirror suspended
  in  the sky. and is returning at a certain angle all round
  it,  viz., at an angle  of 41°, a portion of the light which
  falls on it;  and an  eye placed in the required direc-
 tion receives that reflected light.   If in this case, however, there were reflec-
 tion only, and  not also refraction  with separation of colours, the  rainbow
 would be only a very narrow resplendent arc of white light formed of millions
 of  little images of  the sun;  but in truth,
 because  the light,  vvhich enters near the
 edge  of the drop,  traverses the surface
 very obliquely, it is much bent or refracted
 before its reflection, as  seen at  a, and is
 divided into rays of its  seven colours, as
 it would be on passing through a prism (as
 explained at page 337 ;) and  this division
 or  separation  continuing  after  the light
 again escapes from the drop at  b, instead
 of one white ray  descending from each
 drop to a certain point of the earth, seven
 rays descend (here  marked by dotted lines from the figure 1 on the left hand
 to 7, 6, 5, Sic, on the right,  and  with separation greater than occurs in reality
 to make  it very  evident,) and of  these rays, an eye can only receive one at
 a time from the  same drop, which drop will then appear of the colour of the
 ray: but for the same reason that seven eyes  placed  in a line from above
 downwards, as at  7, 6, 5,  Sic  on the right would be required to  see  the
 seven colours from  one drop  in the centre of the bow, so one eye looking in
 the  direction of  seven drops situated in a corresponding row, as from  1 to 7
 on the left, will catch the lower or red ray of the upper, the orange or second
 ray  of  the next, the yellow  or third ray of that which follows, and  so on,
 while it will lose all the others, and thus will see the several drops as if they
 were each of one colour only.   Of such elements, then, found in the same
relative directions all around  the  eye, the glorious arch is formed.   No two
    26
Fig. 167.
 
390
LIGHT.
eyes can see the same rainbow, that is, can receive light from the same drops
at the same  time; and the same eye does not for  two instants receive light
from the same drops.  This rainbow can never appear to a person on a plain,
unless when the sun is within 41° of the horizon, for otherwise the centre of
the rainbow would  be  more than  41° under the horizon, and therefore the
whole circumference would be  below it too.
                             We  have described  above what  is called the
         Fig. 168.          principal bow, formed in the drops by two re-
                          fractions, and one reflection of light.  To pro-
                          duce the fainter second or external bow, men-
                          tioned  above, and of which the colours  are in
                          reverse order, the light which enters on the under
                          side of the drop, as at a, is reflected first at y,
                          then again at  b, and  escapes at c towards the
                          eye : after two reflections as well as two refrac-
                          tions.   As the semi-diameter of this bow is 54°,
it may be visible when the internal bow is not.
  An artificial rainbow may be  produced in sunshine at  any time by scatter-
ing water-drops from a bush or otherwise ; and a rainbow is often seen among
the spray of a lofty waterfall, or of a stormy sea.  The cut-glass ornaments
of chandeliers, Sic, produce colours on the same principle as rain-drops; as
do also mist and particles of frozen water between a luminous body, and the eye
exhibiting the circular coloured halos often observed  around the sun and  moon.
A white halo is light reflected from the external surfaces of drops or particles.

" Mirrors may be plane, convex, or concave; and certain curvatures will
  produce images by reflection, just as lenses produce images by refraction;
  in consequence, there are reflecting telescopes, microscopes, Src, as there
  are refracting instruments  of  the  same  names."   (See  the Analysis,
  page 325.)

   While a plane surface reflects light,  so that what is called the image in it
of a known object may readily be mistaken for the reality, convex or concave
mirrors reflect as if every distinct point of them were a  separate small plane
mirror, and  their effects on light correspond with a relative inclination of the
different  parts.  The only forms of much importance are the regularly sphe-
rical or parabolic concave and convex mirrors.   We shall now find that these
produce on light similar effects with lenses, only the concave mirror answers
to the convex lens, and the convex mirror to  the concave lens.  It is the
concave mirror which gathers the  light to form  images in the  most perfect
telescopes that exist, as those of Herschel  and others.  Admirable as is the
refracting telescope, it still falls short in certain respects of the telescope acting
by  reflection.
                            In a hollow sphere,  or  part of a sphere with
                          polished internal surface, if rays radiate from the
                          centre  in  all directions,  they reach every  part
                          perpendicularly, and therefore are thrown back
                          to  the  centre.   Thus  if  A B were a  concave
                          spherical mirror, of which  C were the  centre,
                          rays issuing from C would, in obedience  to the
                          law that the angles of incidence and reflection are
                          equal,  again meet at C.
                            It can be proved  also, that any ray parallel to
 
CURVED MIRRORS.                       391
Fig. 170.
 the axis falling upon such a mirror, will be
 reflected inwards so as to cut the axis half-
 way between the mirror and its centre, viz.,
 at D, the centre being C.  Then as all pa-
 rallel  rays must  meet in the  same  point,
 that point  becomes a focus, as already ex-
 plained for lenses, and there an image of the
 sun will be formed when the mirror is held
 threctly towards  the sun.  This point is called  the  principal focus of the

   For the  same  reason that parallel rays meet in the focus, so will rays,
 issuing from  the focus  towards the mirror, become parallel, after reflection
 as  seen above or in the figure at  page 271; and if they be then caught in  a
 second and opposite mirror, as also represented at page 271, corresponding
 effects will follow.                                              t      &
   Now, for a concave mirror, as already explained for a lens, when rays fall
 on it obliquely from one  side of  the axis, their focus will be  on the opposite
 side,  and therefore the mirror will form an inverted image of any body placed
 before it, just as the lens does ;  and the  image will be near or distant, and
 large or small, according to the divergence of the  approaching rays, exactly
 as  happens with lenses;  and  thus the camera obscura, magic lantern, tele-
 scopes and microscopes, may all  be formed by mirrors, as they may be by
 lenses.   Moreover, concave mirrors magnify, as concave lenses of the oppo-
 site names do.  The two subjects of images by refraction and by reflection
 run so nearly  parallel, that it would be useless repetition here to enter upon
 the detailed consideration  of the latter subject, and  we shall therefore content
 ourselves with showing why a  concave mirror  magnifies and why a convex
 mirror minifies.
   A concave mirror magnifies be-                 Fig. 171.
 cause the light from the top of the
 cross at A, reaching  the mirror    d
 where it can be reflected to an eye     ■
 placed at F, viz., at E, seems to     j
 the eye to  come from C,  and  the     i
 light  of B similarly appears  to   -j-v.   _..---
 come from D, so that the cross A
 B, by the reflection, seems to the
 eye to be of the greater dimensions C D.
  In the convex mirror, again, for corresponding reasons, the cross A B ap-
 pears  only  as C D, and therefore  much smaller  than the reality.
  Concave or magnifying mirrors are often
 used by persons  in shaving.
  A convex mirror is a common  ornament of
 our apartments,  exhibiting a pleasing minia-
 ture of the  room and its contents.
  Any polished convex body is a mirror, and
 therefore the ball of the human eye is one, in
 which we may contemplate most perfect mi-
niatures  of surrounding  things.    It  is the
image of the window or  of the  sun in the
convex mirror of the  eye, which painters usually represent by  a  spot  of
white  paint there ; and a similar luminous spot or  line  must be made when
Fig. 172.
C
 r--
 
392
LIGHT.
they have  to represent almost any of the pieces of furniture which have
rounded polished surfaces as bottles, glasses, smooth pillars, &c.
   Convex lenses thus are also mirrors to all the objects  around them, and
very strikingly so, owing to  the  perfection  of the form of a  lens.  The
polished back of a watch, often, in the same way, attracts the attention of a
child, who wonders to see there so clearly " the little baby."
   It has been a  mathematical  amusement to calculate what  kind of distor-
tion mirrors of unusual forms will produce, and then to make distorted draw-
ings, which, when reflected from such mirrors,  might produce in the eye the
natural image of the objects.
   When a concave mirror is used for a telescope, the  image formed in front
of it, and to be examined  through the magnifying eye-glass, may be viewed,
—first, as in Herschel's telescope,  by the  spectator  turning  his  back  to the
real object, and looking in at  the mouth of the. telescopic tube, near to the
edge of which the image is thrown by a slight inclination of  the mirror at its
bottom:—or, secondly, as in the Newtonian telescope, through an opening
in the  side of a tube, after being reflected by a small plane mirror, placed
diagonally in the centre of the tube:—or,  thirdly, as in the Gregorian tele-
scope,  through an opening  cut  in  the principal  mirror or  speculum, after
being reflected towards that opening by a smaller mirror placed in the centre
of the  tube; this last arrangement is that preferred for smaller telescopes,
because the spectator, while seeing the image, is also looking in the direc-
tion of the object.
   Reflecting telescopes have the advantage of being perfectly achromatic,
that is, of producing no coloured or rainbow edges to the  images; for com-
pound  light is reflected, although not  refracted entire, all the  colours follow-
ing the same law of equal angles of incidence and reflection.
   Herschel's largest  telescope had a mirror of 48 inches in diameter, and
therefore received about 150,000 times  more light  than an unassisted eye,
forming with that light, at a focal distance of 40 feet, a large  image admira-
bly distinct.  It  was with such a telescope that, in  the  obscurity of remote
space,  Herschel  discovered the immense  planet rolling along, which, in ho-
nour of his royal patron, he called  the  Georgium Siclus, but which  now,
by the decision of the scientific world, bears his own name;—and with such
he  discovered moons before unseen, of other  planets, and  he  unravelled the
celestial nebulas  and  clustered  stars of the milky way, and, in  a word, un-
veiled vastly more than h;ul before been  done,  tfie  system of the boundless
universe.   If this world were  to last for  millions of years, the discoveries
made by Herschel's  telescope would mark a memorable epoch  of its  early
history.

" Light returned from, or passing through bodies of rougher or irregular
   surface, or which have  other peculiarities, is  so modified as to produce all
   those phenomena  of colour  and varied brightness seen among natural
   bodies, and giving  them their distinctive characters and beauty."  (See
   the Analysis,  page  325.)

   General remarks on this part  of our subject  were made in the beginning
of  the section, in the  explanations of  how objects not self-luminous become
visible by reflecting  the light  of other bodies,  and  of how  the prism  sepa-
rates a ray of white light into  rays  of the several colours which are seen
also in the rainbow—which rays, on being again  mixed, become white light 
COMPARISON OF LIGHT AND  SOUND.
393
 now ZceTdin^ t1 bey°nd fth,fSe remarks we have ^ the intention of
 wimin the tot Jf°Jl>  * faU aCC°Unt °f the matters that  miSbt co™
 "lume  for  there  would ST*****  "°™ °CC.l,Py  *'  P'dgeS °f * ^
 have existed on it  • ,-   f  ° paSS in review~the various opinions which
 has been called t    mima(e™t"re of light,—the facts connected with what
 <^to,^ih»eff  nSatWn o{US^-the relation of light in its double
 refaction, to the ultimate structure of material masses, Sic, all which sub-
 jects are in certain respects highly interesting, but-as some of them are not yet
 completely invest,gated_as  respecting  others various  opinions  prevail.-as
 they involve few matters yet applied  to  common use.-as the reasonings
 about them  are far removed from ordinary trains of thinking, and  refer To
 facts altogether unknown to  common observation,-we hold Them not to be
 fit parts of a popular treatise on light.   We may state, however, that per-
 sons who have the leisure and mathematical  preparation  necessary for pur-
 suing the study, will find their labour in it richly rewarded.
   What we deem necessary  here  to add, is, that white light, in falling upon
 any transparent substance, as air,  water, glass, Sic, reduced to thin plates of
 films, is so affecte.d, that for certain degrees of thinness, different for each
 substance, it is decomposed,  and  is reflected or  is transmitted, not  as white
 light, but as some of the colours  of the rainbow, and the colour reflected in
 any case, is always the  opposite or complement of that which is transmit-
 ted, that is to say, is such that the two brought together make white  light as
 before.  The facts may be studied as Newton originally studied them, m  the
 thin plate of air which occupies the space between a convex lens and a plane
 surface of glass  upon which the lens is laid,—in which plate, as the distance
 from the point of the apparent  contact of the  glasses increases, there  are
 all degrees of thinness, and with  these  appear successive rings of vivid  co-
 lours.  The  same truth is exemplified in the colours of a soap-bubble, which
 brighten as the bubble swells and becomes of  thinner substance,  and  are
 different as the thickness is different and greater from above  downwards;—
 and  it is exemplified also in the colours seen  in the fissures of cracked
 ice or crystalline spars, and in numerous other common facts.  Now, what-
 ever be the reasons of such decomposition of light—and the  explanation is
 not  yet  complete—we cannot doubt that  in  natural bodies generally, the
 colours,  opacity, transparency, &c, depend entirely  upon the volume and
 arrangement of the minute fibres or  plates, with included interstices, which
 constitute the volume or structure of  each mass.   Accordingly, whatever
 changes  that arrangement may change also the  colour of the mass.  Thus,
 by drawing a certain number of minute  lines on  a certain extent of any me-
 tallic surface, we may make it of what colour we please; and mother-of-pearl
 owes its  vivid colours and beauty entirely to its furrowed or striated  surface,
 as is proved by our making an impression of that surface on sealing wax and
 perceiving that the wax then  exhibits similar colours.
   The investigations in progress respecting the phenomena of light, are fur-
 nishing new proofs of the extreme simplicity of nature, amidst the boundless
 extent and most curious variety.  When men thought of the sense of touch
 only as it exists  at the tips of the fingers, or on  the  general  surface of the
 body, they were far  from suspecting that the sense of hearing had the near
 relation to it which subsequent discoveries  have  proved, and  still less, that
 the sense of sight  was  only yet a finer touch than  hearing.  But  step by
step  they have ascertained, 1st, in relation to sound, that  the air through
which it  usually reaches the organ of hearing, is  a material fluid as much as 
394
LIGHT.
water, consisting of the same or smaller particles, only more distant among
themselves,—and that a motion or trembling in the air, by affecting nerves
exposed in the ear, produces the sensation of sound by slight repeated pressures
on these nerves, as the trembling in a log of wood caused by the action of a
saw produces a peculiar sensation of touch in the nerves of a hand laid on the
log;—and, moreover, that sound in all its varieties, is merely such trembling,
affecting a structure of nerve in  the ear, which  nerve is made as much more
readily excitable than the nerves in the fingers or general cutaneous surface,
as the action or impulse of trembling air  is more delicate than the  stronger
pressures of common occurrence.—And, 2dly, in the investigations  respect-
ing light, this kind of comparison is  carried a  step farther, for  it is become
matter almost of certainty that the sensation of  light is produced in the suita-
ble nervous  tissue of the  eye, called  the  retina, by a trembling  motion  in
another fluid than air, which fluid pervades all space, and in rarity or subtilety
of nature surpasses  air yet more than air does  water or solids;—and  that,
while in sound, different tones or notes depend on the number of vibrations
in a given time, so in light do different  colours  depend on  the number and
extent of the vibrations.  Can human imagination picture to itself a simpli-
city more  magnificent and  fruitful of marvellous  beauty and utility than  all
this?—But yet farther, as air answers in the universe so many important pur-
poses besides that of conveying sounds—although this alone  comprehends
language, which almost means reason and  civilization—so also does the ma-
terial  of light minister in numerous ways, in the phenomena of heat, electri-
city and magnetism.
   The truths now positively ascertained with  respect to  the nature of  light
and vision, are  among those in the wide  field of human  inquiry, which,
acting on ordinary apprehension,  most forcibly place the student as it were,
in the very presence of  Creative  Intelligence, awakening in him the  most
elevated thoughts of  which the human mind is capable.   Had there been  no
light in the universe, all its other perfections in regard to  man had existed in
vain.  This earth would have been to its  human inhabitants what  any un-
known shore would be to exiles abandoned upon it after their eyes were put
out:  every movement might be to their destruction, for their perceptions,
being limited by the length of their arms, and  of their fearful groping steps,
the wretched beings  separating when impelled  by hunger4o search for food,
would probably scatter to meet no more.   But the  material of light exists,
pervading all space,  and  certain impressions made upon  it in  one place ex-
tend rapidly over the universe, the progressive  impression being called a ray,
or beam of light.  The beams of light, then, from all parts  coming to every
individual, may be regarded as  millions of supplementary arms or feelers
belonging to the individual, and which reach to the end  of the universe, so
that each  person, instead of  being as a blind point in space, becomes nearly
omnipresent; then these limbs  or feelers have  no weight, they are  never in
the way, they impede nothing, and they are only known  to exist when  their
use is required!
   But this miracle of Light would have been totally useless, and the  para-
dise of earth would have been toman still a dark and dreary desert, had  there
not been farther the  twin miracle of the Eye, an organ of commensurate deli-
cacy to perceive the light.  In the Eye we have to admire the round cornea,
of such perfect  transparancy,  placed exactly in the anterior of the ball, (and
elsewhere it had been useless,) then exactly behind this, the beautiful curtain,
the iris, with its opening, called the pupil, dilating and contracting to suit  the 
PERFECTION OF THE EYE.
395
intensity of light—and exactly behind the iris, again, the crystalline lens pos-
sessing  important and remarkable properties, and which, by acting on the
entering light, forms of it on the retina  beautiful  pictures or images of the
f 11C  ft? .,ront;~the most sensible part of the retina being where the images
tall.   U these parts and conditions, had any one been otherwise than as it is,
the  whole eye had been useless, and light useless, and the great universe use-
less to man, for he could not have existed in it.—Then, farther, we  find that
this precious organ, the eye, is placed in the person, not as  if by accident,
anywhere, but aloft on a befiting eminence, where it becomes the  glorious
watch-tower of the soul; and, again, not so  that  to alter its direction the
whole person must turn, but  in the head, which, on a pivot of admirable
structure, moves while the body is at rest; besides that, the ball of the eye
itself can roll in its place, and is  furnished with muscles which, as the will
directs,  turn it with the rapidity of lightning to sweep along the  horizon, or
take in the  whole heavenly concave;—then is the delicate orb secured in  a
strong socket of bone, and there is over this the arched and padded  eyebrow
as a cushion, to  mitigate the shock of blows, and   with its inclined hairs to
turn aside any descending perspiration or other moisture which might incom-
mode;—then is there  the soft and pliant eyelid with  its  beauteous fringes,
incessantly wiping the polished surface, and spreading over it the pure mois-
ture poured  out  from the  lachrymal  glands  above, of which moisture the
superfluity, by a fine mechanism, is  sent into the nose, there to be  evapora-
ted by the current of the breath;—still farther, it is to be noted,  that instead
of there being only one such precious organ, there are two, lest one, by acci-
dent, should be destroyed, but  which two have so entire sympathy,  that they
act  together as only one more perfect;—then the  sense of sight continues
perfect during the period of  growth from birth to maturity, although  because
the eye  then increases in size, the-distance between the lens and the retina is
constantly increasing;—and  the pure  liquid  which fills the eye, if rendered tur-
bid by accident or disease, is by the actions of life,  although its source be the
thick red blood, gradually restored to transparency.—The mind which can
suppose or admit that, within any limits of time,  one single such apparatus
of vision could  have  been  produced  by accident, or without design,  must
surely be of extraordinary character, or must have  received unhappy bias in
its education; but the mind  which can still farther admit that the millions of
human eyes which now exist on  earth, all equally perfect, can have sprung
from accident—and that the  millions  of millions of  other eyes throughout the
almost innumerable species of the living creation,  where each is adapted to
the peculiar nature and circumstances of the  animal which  bears it, can be
accident; and, lastly, that the countless millions  of  all these well  adapted
kinds, which have existed in  past  ages, were all but accidents—the mind
which can admit this, must have some of its highest faculties either benumbed
or destroyed.
  As a concluding reflection with respect to vision, we may remark, that all
the provisions above considered have mere utility in view, for any one of
them wanting would leave a necessary link in the  chain of creation  wanting:
but we have shown, in a preceding part of the work, that if  there had been
white light only, susceptible  as  now of different  degrees of intensity and
shade the merelv useful purposes of vision would  have been answered about
as perfectly as with all the colours of the rainbow—a truth instanced in the
facts, that many  persons do not  distinguish colours, and that it  imports not
whether a person view objects in the morning, or  at midday, or at even-tide, 
396
LIGHT.
or through plain glass or coloured glass, provided there be light and shade
enough to show them clearly.  While, therefore, the existence of light gene-
rally, and of  the eye, speaks of Creative Power and Intelligence, the exist-
ence of colours, or of that lovely variety of hues exhibited in flowers, in the
plumage of birds, in  the endless aspects  of the earth and heavens—because
appearing expressly  planned to give delight to animated beings, speaks of
Creative Benevolence, and may well excite in us towards the Being in whom
these attributes reside, the feelings associated in our minds during this earthly
scene, with the endearing appellation of  "Father." 
Fig. 173.
 
 
MECHANISM OF THE HUMAN SKELETON.
399
PAET  Y.
ANIMAL  AND MEDICAL PHYSICS.
                               Section I.

                  Mechanism of the Human Skeleton.

   Having now completed our study of general mechanics, we shall proceed,
 with the light thence derived, to examine that most interesting illustration of
 many of the truths—the solid frame-work of the human body—a perfect
 work of an unerring Engineer!
   There is scarcely a part  of the animal body, or an  action which  it per-
 forms, or an accident that can befall it, or a piece  of professional assistance
 which can be given  to it, that does not furnish illustration of some truth of
 natural philosophy ;  but were we  here to enter into much detail, we should
 be giving minute  lessons in medical science, instead of explaining general
 laws.  We shall therefore only touch upon as many particulars as will  make
 the understanding of all  the others easy;  trying to include, among our illus-
 trations, such matters of importance as would be likely to escape the notice
 of a hasty student.
   The cranium or skull has been already mentioned as an instance  of the
 arched form answering  the  purpose of giving strength.  The brain,  in its
 nature, is so tender  or  susceptible of injury, that  slight local pressure dis-
 turbs its  action.    Hence a solid covering like the skull was required with
 those parts made  stronger and thicker which  are most exposed to injury.
 An architectural dome is constructed to resist one kind of force only, always
 acting  in  one direction, viz., gravity; and therefore its strength increases
 regularly towards  the bottom, where the weight and horizontal thrust of the
 whole are to be resisted; but in a  skull, as in a barrel or egg-shell, the mere
 tenacity of the substance is many times greater than sufficient to resist gravity,
 and therefore the form and securities are  calculated to resist  forces of other
 kinds operating in all directions.  When we reflect on the strength displayed
 by the arched film of an egg-shell, we need not wonder  at  the severity of
 blows which the cranium can withstand.
  In the early foetal state, that which  afterwards becomes the strong bony
 case of the brain  exists only as  a tough  flexible  membrane.  Ossification
 commences in this membrane long before  birth, at a certain number of points
 from which it spreads,  and  the portions  of the skull formed around  these
 points soon acquire  the appearance of so many scales  or  shells applied on
 the surface of the  brain,  and  held together by the  remaining  membrane not
 yet ossified.  They  afterwards become firmly fixed together,  by projections
 of bone from each, shutting  in among similar  projections  of the adjoining
ones, until  all mutually cohere by perfect dove-tailed joints, like the work of
a carpenter.  These joints are called sutures of the cranium, and are visible 
400
ANIMAL AND
to extreme old age.  Through  early childhood, the  cranium remains to a
certain degree yielding and elastic, causing the falls and blows, so frequent
during the lessons of walking, Sic, to be borne with comparative impunity.
The mature skull consists of two layers or fables, with a soft diploe between
them;  the outer table being very tough, with its parts dove-tailed into each
other as tough wood is joined by human artificers; while the inner  table is
harder and  more brittle, (hence called vitreous) with its edges merely lying
in contact.
   A very severe partial blow on the skull generally fractures and depresses
the part, as  a pistol-bullet would:  while one less severe, but with  more ex-
tended contact, being  slowly resisted  by the arched form, often injures the
skull by what is  correspondent to the horizontal thrust in a bridge,  and
causes a crack at  a distance from the place struck—generally half way round
to the opposite side.   The French, in speaking of this effect, use the term
contre-coup.   Sometimes in a fall with the head foremost,  the skull would
escape injury, but for the trunk which falls upon it, and drives the  end of
the spine against  or even through its base.
   In the lower jaw we have to remark the  greater mechanical advantage,  or
lever-power,  with which  the  muscles act, than in other parts  of animals.
The temporal and masseter muscles pull almost directly, or at right  angles
to the  line  of the jaw, while in most other cases, as  in that of the  deltoid
muscle lifting the arm,  the  muscles act very  obliquely,  and with  power
diminished in proportion  to the obliquity.   An object placed  between the
back teeth is compressed with the  whole direct power  of the strong muscles
of the jaw.   Hence the  human jaw can crush a  body which  offers great
resistance,  and the jaws  of  the lion, tiger, shark, and crocodile, Sic, are
stronger still.
   The teeth  rank high among those parts of the animal body which appear
almost as if they were severally the results of distinct miraculous agencies—
so difficult  is it to suppose a few simple laws of life  capable of producing
the variety of form and fitness which they exhibit.   They constitute a beau-
tiful set of chisels and wedges, so arranged as to be most efficient for cutting,
and  tearing, and grinding  the food, with  their exterior enamel so hard, that
few  substances in nature can make an impression upon it.   In early states
of society, teeth were used for many purposes for which steel is used now.
It seems, however,  as if the laws of life, astonishing to human intellect  as
they are, had still been inadequate to cause teeth cased in their hard and
polished enamel,  to grow as the softer  bones grow; and hence has arisen a
provision more extraordinary still.  A set  of small teeth appear soon after
birth, and serve the  child until  six or seven years of age: these then fall
out,  and are  replaced by larger ones, which  endure for life; the number  of
the latter, however, being completed only when the  man or woman is full
grown, by the four teeth, called wisdom teeth, from coming with the person's
maturity, to fill up the then spacious jaw.
   The spine or back bone, in its structure, has as much of beautiful and varied
mechanism as any part of our wonderful frame.  It is the  central pillar  of
support and great connecting chain of all the other parts; and has, at the same
time, the office of containing  within itself, and  of protecting from  external
injury, a prolongation of the brain, called the spinal marrow, more important
to animal life than the greater part of the brain itself.   It has united in it the
apparent incompatibilities of great elasticity, great flexibility in all directions,
and  great strength, both to support a load and to defend its important contents,
—as we shall now perceive. 
MEDICAL MECHANICS.
401
 as *^bodv^n         feStS °" the elaStic column of the sPine' as softly
 ?Ln?  r  y      uarnage rests l,P°n its  sPri»gs-   Between each two  of the
 twenty-four vertebne or distinct bones of which the spine consists, there is a
 wh ch v ebl?   'nf ^^ SUb8tance* ab0Ut half as bulkv as a vertebra!  nd
  n   alio V as T  7  t0 a?y'8U*len Jar: tben the spine is  waved or beni like
 rp, In  &   •   Pe~eived on sewing it sideways, or in  profile, and  by this
 reason, also, it yields to any sudden  pressure operating  against either end.
 rhe bending might seem a defect in a column intended  to support weight
 but the disposition of the muscles around is such as to leave all the elasticity
 of that form, and a roomy thorax, without  any diminution of strength.
   flexibility.—The  spine has been compared to a chain,  because it consists
 of many distinct pieces (twenty-Tour.)   They are in contact by smooth rub-
 bing surfaces, which allow of a degree of motion in all directions ; and a little
 motion  comparatively between each two adjoining pieces, becomes a areat
 extent oi motion in the  whole  line.                                   to
   The strength—of  the spine as a whole,  is shown in the fact of a  man's
 easily carrying upon  his head  or back a weight heavier than himself; and the
 strength of each separate vertebra surrounding the spinal marrow, is evident
 in its being a double arch, or strong irregular ring.  The  spine increases in
 size towards the bottom, in the justest proportion, as it has more weight to
 bear.  The articulating surfaces of the spine are so many, and so exactlyfitted
 to each other, and are connected by such number and strength  of ligaments,
 that the combination of pieces,  becomes,  in reference to°motion,Da much
 stronger column than a single  bone of the same size would be.
   Considering the great number  of parts forming  the  spine,  and their nice
 mutual adaptation, it might be expected that injuries and diseases of the struc-
 ture would be very frequent.   The reverse,  however, under natural circum-
 stances, is true; so that while hundreds and thousands of works  have been
 published on the diseases of almost every other part of the body, hardly any
 have been  written on  spine-affections, and  what  have appeared are of very
 recent date.  One reason of this  is, that whatever regards health and disease
 is now much more completely analyzed than  formerly; but another and the
 chief reason is, that from a change in modern times introduced into the system
 of education for young ladies, a considerable proportion of them have grown
 to womanhood with weakened and crooked spines.—The  subject merits fur-
 ther consideration here.
  To the well-being of the higher classes of animals, a certain degree of exer-
 cise of their various parts is not less necessary than their nourishment, and if,
 during the period of growth,  such exercise  be withheld  by any cause, the
 body never acquires   its due proportions and  strength.   To prompt youno-
 creatures to the required exertion, nature has given them an overflow of life
 and energy, as  evinced in the ever-changing occupation of a child in the quick
 succession  of its ideas, in its jumping and skipping, and using all the modes
 of roundabout action to expend  muscular energy, instead of seeking, as in after
 life, to accomplish its  ends  in  the shortest  ways;—and as seen among the
 inferior  animals, in the play of  kittens, puppies, lambs, Sic  But, strongly
 as nature has thus expressed herself, tyrant fashion, with a usual perversion
 of common sense, had of late  times, in  England, for young women of the
 higher classes,  formed a school discipline, directly at war with nature's dic-
 tate; so that a stranger arriving from China, might  almost suppose it our
desio-n to make crooked and weak spines by that discipline, as it is the design
in China to make little feet by  the iron shoe.  The result is the more striking,
when the brothers of the female victims, and who of course have similar con- 
402
ANIMAL AND
stitutions, are seen to be robust, healthy, and  well-formed.   A peasant-girl,
when her spirits are buoyant, is allowed to obey her natural feeling, and  at
proper times to dance,  and skip,  and run, until healthy exhaustion  asks that
repose which is equally allowed ;  and thus she grows up strong and straight:
but the young lady is receiving constant admonition to curb all propensity  to
such vulgar activity, and  often, just in proportion as she subdues nature, she
receives the praise of being well-bred.  The multifarious studies, also, of the
latter come powerfully in aid of the admonition, by fixing her for  many hours
every day to sedentary employment; and  the consequences soon  follow,  of
weakness in the body  generally from the  want of the natural quantity and
variety of muscular  exertion, but weakness of the back particularly, from the
manner in which  the sitting is usually  performed.   It  would be  accounted
great cruelty to make a delicate girl stand all day, because  her  legs would
tire, but this very cruelty is in almost constant operation against her back, as
if backs could not tire  as well  as  legs.   When she is  allowed to  sit down
because she has been long standing, great care is taken  that the muscles  of
the back, which still remain in action as she sits, shall not be at all relieved;
for, from the idea that it is ungraceful to loll, she is either upon a stool which
has no back at all, or upon a very narrow  chair with a  perpendicular back.
Now neither of these seats relieve her spine, the stool,  however, being less
hurtful than the chair, because it allows the spine to  bend  in different  ways
so as to rest the different sets of muscles alternately, while  the  chair keeps
the spine  constantly upright and  nearly unmoved.   This excessive fatigue
soon causes the spine,  somewhere,  to give way and to  bend, and the curva-
ture often becomes permanent.   And, as when a bend takes place in one situ-
ation, there immediately follows an  opposite bend above or below, to keep the
centre of gravity of  the body always directly over the base, the curve thus
becomes double, like an italic/, and the distortion is rendered complete.—
In bending the spine is sometimes also partially rotated  or twisted, so  as  to
show from behind that waving profile which should be seen only from the side.
   When owing to such discipline the inclination of the back has once  been
begun, it is often rapidly  increased by the  means used to correct  it.  Strong
stiff stays are put on to support the  back, as is said, but which in reality, by
superseding the action of the muscles placed there by nature as the supports,
cause these to lose their strength,  and to be unable, when the stays are  with-
drawn, to support the body.  Longer sittings in the narrow upright chair are
then recommended, and sometimes  the back is forcibly stretched by pullies,
so the patient is kept all day and night lying on an  inclined board, losing her
health, &c.;—the only things guarded against being, the patient  should take
due exercise and air, and should rest properly when she is not taking  exer-
cise.  With many persons the prejudice had at last grown up, that strong
stays should be put  on  at a very early age, to prevent the first approach  of
the mischief, and that  children should  always be  made to sit  on straight-
backed chairs, or to lie on hard planes : and it is probable, that if these cures
and preventives had been adopted as universally and strictly as many deemed
them necessary, we should now scarcely have in England a youn_>; lady  of
healthful form.   What would be said of the person  who  should try to im-
prove the strength and  shape of a young race-horse or grayhound, by  bind-
ing tight splints or stays round its beautiful young body,  and then tying  it
up in a stall!  But this is the kind of absurdity and cruelty which  has  been
so commonly practised in this country towards beings than whom, as nature
offers them, the universe surely contains none more faultless.
  A pernicious prejudice, with respect to  such curvature or distortion of the 
MEDICAL MECHANICS.
403
 mo^rsconceaW               U W3S a SCr°ful°US affection' and  man>'
 for from  hZ   I   ** mUch 3S P°Ssible' and S0USht remed>'  from 1»«*»
 „S ,W       consequence, until within a few years, the  management
 tnFZtt      ,  WaS Cuhieflv in the hands of so'ne irre§ular members of the
 protession,--and  a  rich source  of  wealth it became to them, from many of
 their remedies being calculated rather to prolong than to cure the evil.   The
 practice in  such  cases, however, has now fallen  into the  hands of the pro-
 fession generally  ; the science having detected the true cause of the evil, its
 frequency is  already diminished.   It has been shown  that to  prevent the
 disease is easy, and that the best cures are  those conducted on the general
 principles of improving the health of the patient by fit regimen, of prescribing
 such exercises as may directly strengthen the affected part, and of causing
 the patient, when reposing, to assume positions which directly counteract the
 morbid tendency.
   Some might expect here a long description  of  machines employed in the
 treatment of spine affections : but the list of those  which are useful or safe is
 very short:—a sofa to  rest upon  during the day and a  fit  bed for the night;
 (the " hydrostatic bed," proposed by the author of this work, and described
 in the next chapter, has certain  advantages ;) choice  of pleasant means of
 taking exercise, such as the  skipping-rope, shuttle-cock, dumb-bells, a rope-
 ladder to climb, a winch to turn, Sic:—and where it is much desired that the
 young lady should employ  herself in  the sitting attitude, as in practising music,
 a chair may be used, with crutches rising from its  side, or with straps de-
 scending from pullies in an overhanging canopy or crane, and kept tight by
 proper weights at  their distant ends, to support the head and shoulders.  The
 author has had a  small  crane of  wood made, which well answers the last-
 mentioned purpose,  and may be  attached to  a common chair.   It would be
 out of place here to detail those particulars of constitutional treatment which,
 in peculiar habits, may  be  required  to aid the effects of the means above de-
 scribed.
   The ribs.—Attached to  twelve vertebras in the  middle  of the back, there
 are the ribs or bony  stretchers  of the cavity of the  chest, constituting a struc-
 ture which solves, in the most perfect manner, the difficult mechanical  pro-
 blem of making a cavity with solid exterior, which shall yet be capable  of
 dilating and  contracting itself.   Each pair of corresponding  ribs may be
 considered as  constituting  a  hoop ;  which hangs  obliquely down from the
 place of attachment behind, so  that when the forepart of all the hoops is lifted
 by the muscles, the cavity  of the chest is enlarged.
   We have to remark the  double connection of the rib behind, first to the
 bodies of two adjoining vertebras, and then to a process or projection from
 the lower, thus affecting a  very steady joint, and yet leaving the necessary
 freedom of motion : and we observe  the forepart of the  rib to be joined in the
 breast-bone by flexible cartilage, which allows the  degree of motion required
 there  without  the complexity of a joint, and  admirably guards, by its elasti-
 city against the effects of sudden blows or shocks.
   The muscles, which have their  origin on the ribs and  their insertion  into
 the bones of the arm, afford us an  example worth remembering of action and
 reaction being equal and contrary.   When the ribs are fixed, these muscles
 move the arm ; and when the arm is fixed, as by resting on a chair or other
 object, they with equal force  move the ribs.   The latter occurrence  is seen
in fits of asthma and  dyspnoea.
   The human skeleton, with its naked ribs, is so  associated in the common
mind, with ideas of death and loss of friends, and  all the terrors of doubtful 
404
ANIMAL AND
 futurity, that to most persons it is an object of abhorrence : but to the philoso-
 phic mind, which rises superior to place and time, the so admirable adapta-
 tion of all the parts to their purposes, and of parts which, being purely me-
 chanical, are perfectly understood, makes it, independently of all professional
 considerations, an object of the most intense interest.  Such  mechanism
 reveals, by intelligible signs, the hand of the Creator; and a man may be said
 sublimely to commune with his Maker, who contemplates and understands
 the structure aright.
   The shoulder-joint is remarkable for  combining  great extent of motion
 with great strength.  The round head of the shoulder-bone,  that it may turn
 freely in  all ways, rests upon a shallow cavity or  socket in  the  shoulder-
 blade ; and the danger of dislocation from this shallowness is guarded against
 by two strong bony projections  above  and behind.  To increase the range of
 motion to the greatest possible degree, the  bone called'the  shoulder-blade,
 which  contains the socket of the arm, slides above itself upon the convex
 exterior of the chest, having its motion  limited in  certain directions by its
 connection, through the collar-bone or  clavicle, with  the sternum.
   The scapula or blade-bone is extraordinary as an illustration of the mecha-
 nical rules for combining lightness with strength. It has the strength of the
 arch from being a little concave, like the dished wheel already described, and
 its substance is chiefly collected in its  borders and spines, with thin  plates
 between, as  the strength of a wheel is collected in its rim, and spokes, and
 nave.
   The bones of the arms, considered as levers, have  the  muscles which
 move them  attached very near to the fulcra,  and very obliquely, so that the
 muscles,  from working  through a short distance, comparatively  with the
 displacement of the  resistances at the  extremities,   require  to be of  great
 strength.   It has  been calculated that  the muscles of the  shoulder-joint, in
 the exertion of lifting a man upon the hand, pull with a force of two thou-
 sand pounds.
   Notwithstanding all the  securities to the shoulder-joint now described, in
 the infinite variety  of twists, and falls and accidents to  which men, in the  busy
 scene  of  society,  are liable, the joint is frequently dislocated, that is, the
 rounded head of the humerus or arm-bone slips from its socket, with instant
 lameness  as a consequence.
  In the treatment of dislocations and fractures of the frame-work of the human
 body, the surgeon  cannot avoid  displaying strikingly either his professional
 skill or ignorance.   With what ease does the  displaced arm or thigh-bone
return  to its socket, under the guidance of the  skilful hand; and to what horri-
ble, and often unavailing torture, is the patient subjected, when, in such a case,
ignorance dares to act!   It is very painful to allow the  imagination to dwell
upon the records of ancient surgery, and to be made present, as it were, to
the stretching of patients on the  rack with pullies and powerful engines, to
do what better information could have accomplished with  such gentleness.
And would that the records of  modern  times  contained no instances of  indi-
viduals crippled for life by bad practice.   To  a  practitioner in this branch,
impunity and  a quiet conscience can  now be secured only by his having a
perfect knowledge  of  anatomy, and familiarity with the  laws of mechanical
philosophy.
  With our present information on these subjects,  we are surprised at the
detail of the practices and errors promulgated in former times, owing to im-
perfect knowledge of mechanics, even by authors of  the highest credit.  It
would hardly be believed that  so distinguished an ornament of English sur- 
MEDICAL MECHANICS.
405
  gery as Mr. Pott, should assign as one reason for not pulling by the hand or
  foot, in reducing  a dislocation of the  shoulder or hip, that the intervening
  joints prevented the strain from reaching the part desired.
    Some surgeons, possessing a certain degree of knowledge in mechanics
  but  only that  degree  which  is dangerous, having heard that the lever was a
  Powers engine,  have tried to replace bones solely by leverage,  as it was
  called,   rims, a man's dislocated arm has been placed over  the  back of a
  chair as a fulcrum, or over the  top of a door, and while the weight of the
  suffering body was hanging to it on one side as the resistance, force has been
  applied to  the other  side, enough  sometimes to break the bone,  or to tear
  away the ligaments and soft parts about the joint.
    Other surgeons, after learning in the same way the effects of the pulley,
  have wished to do all by irresistible extension, and instead of borrowing the
  moderate assistance which might be useful, have  torn muscles and  ligaments
  from their attachments.
    It is not the object of this work  to enter into an extended examination of
  the accidents which befall the body requiring mechanical skill for their proper
  management, for this would be to deliver a course of instruction on practical
  surgery ; but it is  wished to awaken the  attention of the medical student to
  those valuable general principles which may furnish direction  in most diffi-
  culties.   Knowing these principles, and possessing good sense, he  will  often
  be a more effective minister of  his art than a man full of learned precedents,
  who knows them not.   To  make this lesson more impressive  to his youn°-
  readers, the author may take the liberty  of adducing  his own  experience"
  When he was  himself very young,  and had not yet had extensive  practical
  experience, he was thrown into a situation where a heavy medical charge de-
  volved  upon him, and where, through accidents  among a numerous crew,
  during  a very eventful voyage, vvhich led to intercourse with the savao-e
  inhabitants of unfrequented coasts, he had, within twenty-six months, more
  practice in  singular wounds, dislocations, and fractures, than falls  to the lot
 of many practitioners  during a life:—in that time he  became  strongly im-
 pressed  with the importance to the medical man of such knowledge as he now
 recommends : and he had reason to rejoice that although Natural Philosophy
 was  not then much insisted upon in  the  course  of professional education,
 circumstances had led him to look carefully at the body through that  medium.
   The os humeri, or bone of the upper arm, is not perfectly cylindrical, but
 like most of the other  bones  called cylindrical, it has ridges to give  strength,
 on the principle explained  in the chapter "on strength of materials."
   The elbow joint  is a correct hinge, and so strongly secured that it is rarely
 dislocated without fracture.
   The fore-arm consists of two bones with a strong membrane between them.
 Its great breadth,  from  this structure, affords abundant space for the origin of
 the many muscles which go to move  the  hand  and  fingers: and  the very
 peculiar  mode of connection of the' two bones gives  man that  most useful
 faculty of turning the hand round, into what are called the positions of prona-
 tion  and  supination,—exemplified in the  action of twisting or of turning a
 gimblet.
  The old surgeons, who acted frequently by rules of routine rather than by
 reasons, in the accident of fracture to one or both bones of the fore-arm, often
 applied a tiodit bandage, which pulled the bones at the fractured part close to
each other, and thus injured the future shape and strength of the arm.
  The wrist.   The many small bones forming the wrist have a signal effect
    27 
406                          ANIMAL AND
of deadening, in regard to the parts above, the shocks or blows which the
hand receives.
   The annular ligament is a strong band passing round the joint, and keeping
all the tendons which pass from the muscles above to the fingers, close to the
joint.  It  answers the purpose of so  many fixed pullies  for directing the
tendons:  without it, they would all, on  action, start out  like bow-strings,
producing deformity and weakness.
   The human hand is so admirable, from its numerous mechanical and sensi-
tive capabilities, that an opinion  at one time prevailed, that man's  superior
reason depended on his possessing such  an instructor and such a servant.
Now, although reason, with hoofs instead of fingers, could never have raised
man much above the brutes, and probably could not have secured the conti-
nued existence of the  species,—still the hand is no more than a fit instrument
of the godlike mind which directs it.
   The pelvis, or strong irregular ring of bone on the upper edge of which the
spine rests, and from the sides of which the legs  spring, forms the centre  of
the skeleton.  A broad bone was wanted here to  connect  the central column
of the spine with the lateral columns of the legs, and a circle was the lightest
and strongest.  If we  attempt still farther to conceive how the circle could be
modified so as to fit it—for  the spine to rest  on, for the thighs to roll in, for
muscles to  spring from, both  above and  below,  for the person to be able  to
sit, Sic, we shall find, on inspection, that all our anticipations are realized  in
the most  perfect manner.   In the pelvis, too, there are the thyroid hole and
ischiatic notches, furnishing  subordinate instances  of  contrivance  to save
material and weight:—they are  merely  deficiencies of bone where  solidity
could have given no additional strength.   The broad ring of the pelvis pro-
tects most securely the important organs  placed within it.
   The hip joint exhibits the  perfection  of the ball and  socket articulation.
It allows the foot to move  round  in a circle, as well as to have the great
range of backward  and  forward  motion, exhibited in the action of walking.
When we see the elastic tough smooth cartilage which lines the deep socket
of this joint, and the  similar  glistening covering of the  ball or head of the
thigh-bone, and the lubricating synovia poured into the cavity by appropriate
secretaries, and the strong ligaments giving strength  all around,  we feel how
far the most perfect of man's  works falls short of the mechanism exhibited
in nature.
   The thigh-bone is remarkable for its two projections near  the top, called
trochanters, to which  the  moving muscles are fixed ; and which  lengthen
considerably the lever by which the muscles work.   The shaft of the bone
is not straight, but has a considerable forward curvature.   Short-sightedness
might suppose this a weakness, the bone being a pillar to support a weight;
but the bend gives  it in reality the strength of the  arch, to bear the  action
of the mass of muscles called vastus, which  lies and swells upon its fore
part.
   The knee is a hinge joint of complicated structure,  claiming  the most
attentive study of the surgeon.  The rubbing parts are flat and shallow, and,
 therefore, the joint has little strength from form ;  but it derives security from
 the numerous and singularly strong ligaments which surround it.  The liga-
 ments on the inside of the knees  resemble, in two circumstances, the annular
ligaments of joints, viz., in having  a  constant and  great strain to bear, and
 yet in becoming stronger always as the strain increases.  The line of the leg,
 even in the most perfect shapes, bends inwards a little at the knee, requiring
 the support of the ligaments ;  and in many persons it bends very much ; but 
MEDICAL MECHANICS.
407
the inclination  does not increase with age.  The legs of many weakly in-
Kneed children become straight by exercise alone.   This inclination at the
middle joint of the leg, by throwing  a certain strain on the ligaments, gives,
in such actions  as jumping, running &c, an increase of elasticity to the limb.
   In the knee there is a singular  provision of loose cartilages between the
ends of the  bones.  They have been called  friction-cartilages, from  a  sup-
posed relation  in  use to friction-wheels, but  their real effect seems to be, to
accommodate, in  the  different positions  of  the joint, the surfaces  of the
rubbing bones to each other.
   Under the head of Pneumatics, we shall find that the  bones forming the
joints are held together, independently of their ligaments, by a constant  pres-
sure of the atmosphere, amounting in the knee, for instance, to upwards of
sixty pounds.
   The great  muscles on the fore-part of the thigh are contracted  into a ten-
don a little above the knee, over and in front of which the tendon has to pass
to reach the top of the  leg,  where its  attachment is.  The part of the  tendon
over the joint becomes bony, and forms the patella or knee-pan, often called
the pulley of the  knee.  This peculiarity enables the muscles  to act  more
advantageously, by increasing the  distance of the rope  from the  centre of
motion.  The patella is, moreover, a sort of shield or protection to the  fore-
part of this important joint.
   The leg below the  knee, like the fore-arm already  described, has two
bones.  They  offer spacious surface of origin for  the  numerous muscles
required  for  the feet, and they form a compound pillar  of greater strength
than the same quantity of bone as one shaft would  have had. The individual
bones also are angular instead of round, hence  deriving greater  power to
resist blows, Sic
   The ankle-joint is a perfect hinge of great strength.  There is  in front of
it an  angular ligament, by  which the greater part of the tendons passing
downwards to the foot and  toes are kept in their  places.   One of these ten-
dons passes  behind  and  under the bony projection of the inner ankle, in a
smooth, appropriate groove, exactly as if a little fixed pully were there.
   The heel, by projecting so far backwards, is a lever for those strong mus-
cles to  act by, which form the calf of the leg and terminate in the tendo-
achillis.  The  muscles, by drawing at it, lift the body, in the actions of
standing on the toes, walking, dancing, Sic   In  the foot of the negro, the
heel is so long as, in European estimation, to appear ugly; and its great length
rendering the effort of smaller muscles  sufficient for the various purposes,
the calf of the  negro's leg is smaller than of other races of men.
   In a graceful human step the heel  is always raised before the foot is  lifted
from  the ground,  as if the  foot were part of a wheel rolling forward; and
the weight of the  body, supported by the muscles of the calf of the leg, as
just described,  rests for the time on the fore-part of the foot and toes.  There
is at that time a bending of the foot in a  certain degree.   But where strong
wooden shoes are used, or any shoe so stiff that it will not yield and  allow
this bending  of the foot, the heel is not raised at all until the whole foot rises
with it, so That the muscles of the  calf are scarcely used,  and in consequence
soon  dwindle in size,  and almost disappear.  Many of the English farm
servants  wear heavy stiff shoes, and in London  may constantly  be seen as
the drivers of country wagons, with fine robust body and arms, but with legs
which are fleshless spindles, producing a  gait most awkward and unmanly.
The brothers of  these men, otherwise employed, are  not soi mis-shapen;
and even they themselves, when  they choose to  become soldiers, and are 
408
ANIMAL AND
trained in military exercises, lose their peculiarity.  What a  pity that, for
the sake of a trifling saving, graceful  nature should  be  thus deformed.   An
example  of an opposite kind is seen in Paris, where, as the streets have no
side pavements, and the ladies are obliged  consequently to walk almost con-
stantly on  tiptoe, the great action of the  muscles  of  the calf has given a
conformation of the leg and foot, to match  which the Parisian belles proudly
challenge all the world,—not aware, probably, that it is a defect of their city to
which the boasted peculiarity is mainly due.
  A person confined to his bed for a week  or two by sickness, has generally
to remark a much greater wasting of the legs than of the  arms:  the  reason
of which is, that the muscles of the leg, in  ordinary cases, being more in use
than those  of the arms, their ordinary bulk is more dependent on use, and
thev suffer  a corresponding change from inaction.
  Such facts as now  mentioned,  bear directly  on  the subject so near the
hearts of many English mothers, viz., the  weak and crooked backs of their
daughters.   From  such they may understand that strong stays, which in
part supersede the action of  the muscles placed by nature  around the spine
to support it, cause these muscles to dwindle, and afterwards, when the sup-
port of the stays fails or becomes unequal, leave the back to bend or twist.
Stays, therefore, can  neither help to make strong and well-formed backs
originally, nor can they be  a remedy after the weakness  has  commenced.
A healthy young woman from the  country, with spine lying deep between
the firm "cushions  of muscle which support it, if, according to town fashion,
braced up in tight stays, will frequently, at the  end of  a short  time, exhibit
such a wasting of the flesh, that the  points  of bone  in  the spine may be
counted by the eye, all the  way down.
   The arch of the foot is  to be noticed as another of  the many provisions
for saving the body from shocks by the elasticity of the supports.  The heel
and the ball of the toes are the two extremes of the elastic arch,  and  the leg
rests between them.
   Connected with  elasticity, it is interesting to remark how  imperfectly a
wooden leg answers the purpose of a natural leg.  The centre of the body,
when supported by  the wooden  leg, which  always   remains of the same
length, must describe, at each step, a portion of a circle of which the  bottom
nob of the  leg is the centre; and the body  is, therefore, constantly rising and
falling;—but with the natural legs, which, by gentle  flexure at the knee,
are made shorter or longer in different parts of the step  as required, the body
is carried along in  a manner perfectly or nearly level.   In like  manner, a
man riding on horseback, if he keep  his back upright and stiff, has his head
jolted by every step of the trotting animal; but  the experienced  horseman,
even without rising in the stirrups, by letting  the back yield a little  at each
movement, as a bent spring yields during the motion of a  carriage, can carry
his head quite smoothly along.

   In a  general review  of the skeleton, we have  to'remark,  1st, the  nice
adaptation  of all the parts  to one another,  and to the strains which they have
respectively to bear;  as—in the size of the spinal vertebras increasing from
above downwards—the bones of the  leg being larger than those  of the arm,
and so on.   2dly, the objects of strength and lightness combined;  as by the
hollowness of the  long bones—their angular form—their thickening and
flexures in particular places  where great strain has to be borne—the enlarge-
ment of the  extremities to which the muscles are attached, lengthening the
lever by which these act, &c.  3dly, we have to remark  the  nature and 
MEDICAL MECHANICS.
409
 strength of material in different parts, so admirably adapted to the purposes
 which  the  parts serve: there is  bone for instance, in one place nearly as
 hard as iron, where, covered with enamel, it has the form of teeth, with the
 office of chewing and tearing all kinds of matter used as food;  in the cranium
 again, bone is softer but tough and resisting; in the middle of the long bones it is
 compact and little bulky, to leave room for the swelling of the muscles lyin<r
 there;  while, at either end, with the same quantity of  matter, it is large  and
 spongy, to give a broad surface for articulation: and in  the spine,  the bodies
 of the vertebras, which rest on an elastic bed of intervertebral  substance, are
 light and spongy, while their articulating surfaces and processes are  very
 hard.  In the joints we see  the tough elastic smooth  substance called carti-
 lage covering the ends  of the bones, defending and padding  them, and de-
 stroying friction.   In infants  we find all the  bones  soft  or  grfstly, and there-
 fore calculated to bear with impunity the falls and  blows  incidental to their
 age; and we see certain parts, where elasticity is necessary or useful, re-
 maining cartilage or gristle for life, as at the anterior extremities of the ribs.
 About  the joints we  have to remark the ligaments  which bind  the bones
 together, possessing  a tenacity scarcely equalled  in any other known  sub-
 stance;  and we  see  that the  muscular  fibres whose  contractions  move
 the  bones and thereby  the  body,—because  they  would  have rendered  the
 limbs clumsy  even to  deformity had they all passed over the joints to the
 parts which they have to pull,—attach themselves, at  convenient  distances,
 to a strong cord called a tendon, by means of which, like a hundred  sailors
 at a rope, they make their effort  effective at any distance.   The tendons are
 remarkable for the  great strength which resides in their slender  forms,  and
 for the  lubricated smoothness of their surfaces.  Many other striking particu-
 lars  might be enumerated, but these may suffice.—Such, then, is the skeleton
 or general frame-work  of the human bod}'; less  curious and complicated,
 perhaps, than some other parts of the system which we have yet to examine,
 but so perfect and so wonderful,  that the mind which  can attentively consi-
 der it without emotion is in 3 state not to be  envied.*
   This living  force of man  has been  used as a  working power  in various
 ways, as in turning a winch—pulling at a rope—walking  in the inside of a
 large wheel to  move it, as a squirrel or turnspit dog moves  his little  wheel,
 &c.   Each of  these has some particular  advantage: but the mode in which,
 for many purposes, the greatest effect may be produced, is for a man to carry
 up to a height his body only, and  then  to let it work  by its  weight in  de-
 scending.   A bricklayer's labourer  would be less fatigued  to  lift twice as
 many bricks to the  top of a house in the  course of a day, by ascending the
 ladder without  a load, and raising bricks of  nearly his own weight  over a
 pulley each time in descending, than, by carrying  fewer bricks and  himself
 up together, and descending again without a load, as is still usually done.
   Reflection, independently   of experiment,  would naturally  anticipate  the
 above stated result, for the load which a man should be best able to carry, is
 surely that  from  which  he  can never  free  himself,—the load of his own
 body.  Accordingly the strength of muscles  and disposition of parts are all
 such as to make his body appear very light to him.
  The question which was agitated  with such warmth  some time  ago as to

  * In the second and third editions of this work a criticism was introduced at this place,
of a treatise on "Animal Mechanics," published as part of the Library of Useful Knov>-
ledo-e ■ in which treatise the author, from imperfect acquaintance  with Natural Fnilosopli)
had fallen into many grave errors.  That note, having answered its purpose, is not repeated
 here. 
410
ANIMAL AND
the propriety of condemning  men  and women to work on  the  tread-mill,
receives an easy decision here.   They work by climbing on the outside of a
large wheel or cylinder, which  is turned by their weight, and on which, to
avoid falling from their proper situation, they must advance just as fast as it
turns.   There are, on the  outside of the cylinder, projections or steps for
the feet, and the action to the workers is exactly that of ascending an accli-
vity.   Now,  as nature has fitted the human body for climbing hills as well
as for walking on plains, the work of the tread-mill, under proper restrictions
as to  duration, must be nearly as  natural and healthful as any other.—Its
effects  have ultimately proved it to  be so.
  Animal power being exhausted in proportion, as well to  the time during
which  it is acting,  as to the intensity of force exerted, there may often be a
great saving  of power by doing work quickly, although with a little more
exertion during the time.   Suppose two men of equal weight to ascend the
same stair, one of  whom takes  only a minute to reach the top, and the other
takes four minutes, it will cost the first but a little more than a fourth part of
the fatigue which it costs the second, because the  exhaustion has  relation to
the time duriug which the muscles  are  acting.   The quick mover may have
exerted, perhaps,  one-twentieth more  force in the first instant, to give his
body the  greater  velocity  which was afterwards  continued, but the sloth
supported his load  four times as long.
  A healthy  man  will run rapidly up a long stair, and his  breathing will
scarcely be quickened  at the top;  but if he walk up slowly, his legs will
feel fatigued,  and he will have to wait some time before he can speak calmly.
  For the same reason coach-horses are much spared by being made to gallop
up a short hill, and then being  allowed to go more slowly for a little  time,
so as to rest at the  top.
  The rapid  waste of muscular strength which arises from continued action,
is proved  by  keeping  the arm extended horizontally for some time.  Few
persons can  continue the  exertion beyond a minute  or two.  In animals
which  have long horizontal necks, there is a wonderful  provision of nature
in a strong elastic  substance on  the back or upper part of the neck, which
nearly  supports the head independently of muscular exertion.
  In farther illustration of the truth that strength is saved in many cases by
doing work quickly, we may recall  the fact explained at page 60, that a body
thrown or shot upwards with double velocity, rises four times  as far as when
shot with  a single  velocity, or half  of  the other.

                            "Instruments."

The following remarks regard some instruments used by medical men, and
         which range under the present division of "Mechanics."

   The obstetric forceps.—As  the  blades beyond the joint or  fulcrum  are
longer  than the handles, the pressure on the head included in them is less
than that exerted by the hand  that  uses them, but its degree should always
be kept present to  the mind of the  operator.
   The vectis, or lever used  instead of the forceps just mentioned, is a dan-
gerous instrument  in  unskilful hands.  In fact,  whenever it is used as  a
lever, in the common  acceptation of the term, with some part of  the pelvis
as the fulcrum, the use of it is a piece  of unskilful cruelty; for the soft parts
between the bone  and  the instrument are bruised not only with the whole
force of the hand,  but with twice or thrice as much, according as the resist- 
MEDICAL MECHANICS.
411
ance is nearer to  the fulcrum  than to the hand.  The instrument is safely
used, only when the operator makes  one of his hands the fulcrum, and uses
the other as the power, or makes different parts of  the same hand  answer
both purposes; and then  there is a  resemblance  between the  action of  the
vectis and of a hook.
   The levator, or lever for raising the broken and depressed portion of  the
skull in trepanning, has a  fulcrum attached to it by a joint.   Care should be
taken to place this fulcrum where its pressure cannot be injurious.
   The circular-saw or crown-saw of the trephine, should be worked with
a quick motion and gentle pressure, for the reason given at page 107,  when
treating of cutting instruments.  The purpose is thereby attained, both sooner
and better, and the head of the patient is less shaken.
   For the same reason, in  amputation, a light and quick motion of the straight
saw causes less jarring, and effects an easier division of the bone.
   In using the amputation knife, the speed, neatness and success  of the opera-
tion are all favoured by blending the drawing or saw  motion of the knife,
with the pressure towards the bone.
   These last observations are of a hundred similar, which might be  made to
prove  the extreme importance to a  surgeon of having familiarity with  the
use of tools and  instruments.  Perhaps a person cannot better acquire  this
than by practising, while young, some amusing work of carpentry.  Manual
dexterity, and  a  little readiness at  mechanical contrivance, so  frequently
prove of importance to persons in all stations, that it is a great defect in the
prevailing system of  general education, not  to  cultivate them  with greater
attention.
   The tooth-key  is an instrument found in many hands, and persons who do
not pretend to more than the lowest degree of skill in the  healing art, are
not afraid to use it. The  consequence is, that perhaps scarcely a day passes,
in which teeth are not broken and jaws splintered,  and gums  bruised even
to sloughing, by  the unskilful or awkward use of it.  The common  tooth-
key may be compared to  a wheel and axle, the  hand of the operator  acting
on two spokes of the wheel  to work it, while the tooth is  fixed to  the axle
by the claw, and  is drawn  out as the axle turns.   The gum  and  alveolar
process of  the jaw form the  support on which the axle rolls.   The common
errors in tooth-drawing by the key, are these :
   1st.  Turning the key towards that side where the adjoining teeth are so
close that the tooth to  be drawn cannot pass, without either breaking one of
them, or being itself broken.   Sometimes two or even three teeth  are thus
unfixed instead of one.
   2d.  Neglecting the  natural inclination  of the tooth.   By winding it round
in the  direction in which  it already inclines, and  in accordance  with a bend
which  is generally found in it, the operation is easy and safe;  but by draw-
ing it in the opposite way, it not unfrequently is broken, or it  splinters the
part of the jaw-bone in which it was set.
   3d.  If the tooth-claw be blunt, its point may slip upon the tooth, so as to
produce ajar which is very apt to break the tooth.
   4th. Unless the  axle or fulcrum of the key be made to rest as evenly as
possible  on the gum,  it will tear or otherwise injure the  gum.  It should
rest, if possible,  over  the part of the bone in which the tooth  is set, for if
not—as when a back  tooth is drawn by an instrument resting on a part con-
siderably anterior to it—the twist produced is painful, and there is danger of
splintering. 
412
ANIMAL AND
  A man who, after making these reflections, operates leisurely a few times
on the dead subject, will be  able to give instant and safe relief to very com-
mon and most intense suffering.   And it is hardly excusable in any medical
man who  may be placed where  a professed dentist cannot  be procured, to
neglect acquiring a talent so  easy.
  Some dentists, by a strong forceps made for the purpose, pull teeth directly
out; others by a simple sharp-pointed lever push them out; and  others use
a forceps in the  manner of  the tooth-key, by resting  one side of it on the
gum as a  fulcrum, and  then giving  it a  twisting  motion:  in the latter case,
the resting side of the forceps is  formed  like the bolster of a  tooth-key.  But
much more in all cases depends on the dexterity  of the operator than on the
form of the instrument.
  Steel trusses for  ruptures are among the blessings to suffering humanity
which modern ingenuity has supplied.  From the unhealthy  employments
of some men, and the early  dissipation or  unnatural modes of  life of others,
debilitated constitutions  are frequent, and are often transmitted to offspring;
and one of the lamentable effects is that weakness of the flesh forming the
sides of cavities which, on  occasions of strong  effort, allows, at particular
points, the living parts to protrude from  within, so as to form tumours under
the skin.   A  person  perfectly healthy may suffer the same injury from a
very violent strain or pressure  on the abdomen.   The occurrence is called
hernia or rupture:  the  most common hernia is that  of the small intestine
through the groins.
  Formerly this occurrence  disabled for life.  A  man who  had hernia was
discharged from  the army or navy ; he could not  ride  on horseback, or take
usual exercise ;  he could not lift a weight;  and in a word, he often became a
miserable burden to himself  and others.   Now, by fitting the pad of a good
steel truss to the part, the rupture is as perfectly restrained, as  if the hand of
a skilful surgeon were constantly there.   The truss may be put on  and off,
with  as little  reflection or trouble, as a part of the  ordinary dress, and the
man becomes again almost as fit for all the  duties of life as if he were with-
out his ailment.
  The old and still  common form of the steel truss is that of a half or three-
quarter hoop, so bent and tempered, that when put upon the patient, one end,
which has a pad upon  it, presses with  a given force, over the opening by
which the rupture protrudes, and the remainder tightly embraces the body.
The  objections  to this kind of truss are, the difficulty of making  it to fit
exactly; its being rather troublesome to put on and off; and its pressing disa-
greeably round  the  body.
  Another kind  of truss, free from these objections, consists of a little more
than half a hoop, with a pad at each end : one of the pads supports the weak-
ness, and  the other rests upon the  centre of the back, to bear  all the strain
there, while the hoop itself passing round the hip farthest from  the hernia
reposes loosely on the hip.  This  truss may be  called self-adjusting, for it
almost of itself falls into its  place, and needs no fastenings; the same truss
fits all persons of one size, whatever their shape; and the  strength may be
adjusted by changing the number of plates  in the  spring-hoop.
   Tourniquets, crutches, splints, <y-c.  fyc, are so  simple in all respects as not
to merit special  notice here.
   This section contains some of the reflections which, in  contemplating the
human skeleton, occur to a person  familiar with mechanical philosophy;
and the more complete such  a person's knowledge is of anatomy, physiology, 
MEDICAL MECHANICS.
413
surgery  and medicine, the more numerous will be the professional objects on
which  his philosophy will shed a light, dissipating doubt  and error.   Th"
author has not entered into more minute detail, because it would have been
encroaching upon  the office of the teachers  of particular departments, and
because  he thinks  that any one who is not enabled, by the examples  here
given, to make the applications of the general laws  to  all possible cases,
may account the study of the healing art unsuited  to the faculties with which
he is endowed. 
414            FLUIDITY IN RELATION TO ANIMALS.
PART   Y.

   (continued.)
SECTION II.—DOCTRINES OF FLUIDITY  IN  RELATION  TO
                             ANIMALS.
In the preceding sections on the laws of fluidity,  occasional illustrations
  have been taken from the animal economy ;  but there are many other par-
  ticulars of the same class and  of great  interest, which it is  convenient to
  consider apart under the four heads of,  1st.  The circulation of the blood ;
  2d. The respiration and voice; 3d.  The digestion,  and 4th.  The pelvic
  phenomena.  It is important to remark here, that this section cannot be
  understood by a person ignorant of those which precede.

                    THE CIRCULATION  OF THE BLOOD.

  Perhaps there are few points more remarkable in the history of the pro-
gress by which man  has  arrived  at his  present knowledge of the universe,
than that he was so long ignorant of the fact that the blood in his own  and
in other animal bodies is constantly circulating.   England claims as one of
her sons, the man whose powerful intellect at last established this truth, in
opposition to strong appearances, and  to the  most fixed prejudices.   Dr.
Harvey published his proofs in the year 1619.   A person who tries to ima-
gine what the science of medicine could have been while it took no account
of this fact, on which, as a basis, much of the certain reasoning about the
phenomenon of life must rest, is prepared for what old medical books exhibit
of the writhings of human reason, in attempts to explain and to form theories,
while a fatal error was mixed with every supposition.—The chief  circum-
stance which prevented  the earlier discovery of the  circulation was, that on
examining  dead bodies, the arteries were always found empty of blood;—
which was  the reason, also, of  those vessels being called arteries, or air-
tubes.
  We now know, that as the Thames water spreads over London in  pipes,
to supply the inhabitants generally, and to answer the particular purposes of
brewers, bakers, tanners, and others, and is then in great part returned to
where the current  sweeps away the impurities, so, nearly  in  the human
body, does the  blood spread from the centre, through the arteries, to nourish
all the parts, and to supply material of secretion to the liver, the kidneys,
the stomach, and other viscera; and returns from these by the veins towards
the heart and lungs, to be purified and to  have its waste replenished, that it
may again renew its course. 
THE CIRCULATION.
415
   The circulation may be more particularly described thus.  From the left
 chamber or ventricle of the  strong  muscular mass, the heart, a large tube
 arises, called the aorta ;  and by a continued division or ramification,  opens
 a way for the bright scarlet blood to the very  minutest  part of the  living
 frame,—the extreme divisions of  twigs  being so  small, that thev are  called
 capillary or hair-like tubes.   At the terminations of these vessels", the blood,
 after answering the purposes of general  nutrition, &c, by which it loses its
 bright colour, enters the commencements of the venous tree or  returning
 channel;  and  gliding successively from  smaller to larger branches, returns
 towards the right chamber or ventricle of the heart, requiring purification and
 partial renewal.  Considering the great  arterial and venous systems of the
 body as twin  trees—the scarlet  and the purple, with  corresponding  and
 meeting branches, and with trunks vvhich touch each other at the  heart—it
 will appear that  they again similarly meet  or  inosculate by their extreme
 roots, and thus form a continued or circular channel.  The root of the venous
 tree, by which  the blood spreads from the right chamber of the heart through
 the lungs, is called  the pulmonary  artery, and that  of the arterial tree, by
 which the blood returns to the left  chamber, is called  the pulmonary vein.
 Both of these ramify in the spongy  masses of the  lungs forming a great part
 of the pulmonary substance.    Fresh material for the blood  is brought from
 the digestive organs by the lacteal absorbents and thoracic  duct, and is con-
 stantly pouring into a large vein near the heart, to be  completely mixed with
 the dark or returning blood  by a  violent  agitation or  churning during its
 passage through the heart.   The  mixture, on leaving the right ventricle, is
 strained through the minute ramifications of the vessels in  the lungs,  and at
 the same time  is exposed to the  action  of the air entering the  cells of  the
 lungs in respiration, by which exposure the dark purple blood becomes again
 pure scarlet, and when it reaches the left  chamber or ventricle, is ready to set
 out on  its journey as  before,  charged with new life and nutriment.   The
 two chambers or ventricles of the heart have each an anti-chamber or auricle,
 (so called from  an external resemblance to a  dog's ear,) into which  the blood
 is first received from  the veins; and there are  valvular doors between  the
 auricle and ventricle, which allow  the blood to pass readily into the  ventricle,
 but oppose  its  recoil during  the  ventricular contraction.   Similarly  acting
 valves are placed between the ventricles and great arteries.  There are valves
 also in many of the veins, over the body, to  secure the natural course of the
 circulation.   Besides the important  change of purification which the  blood
 undergoes in passing through  the  lungs,  its composition is  much influenced
 by the action of the  kidneys, of the  exhalent of the skin, and of the liver,—
 the two former relieving it from superfluous  moisture and salts, the  last from
 a large quantity of matter in the form of bile.
   The description given above of  the circulation of the blood is only an out-
 line ; and yet by showing the manner  in which fresh material enters, it contains
 more than Harvey knew of the subject.   In this department of  knowledge,
 as in most others, we have advanced from the very general and vague,  to the
 more particular and  precise :—and just  as the general  nature of steam was
 known long  before it served in steam-engines, and as the period of the moon's
 revolution had  been accurately observed for thousands of years before  the
 fluctuations  in her velocity could  be calculated so  as  to make her the  mari-
ner's best guide in his courses across  the ocean,  so, when Harvey had proved
the general fact of the circulation  of the  blood,  he had left much yet  to be
done  by observing and raising from subordinate facts, to render the know-
ledge available for the  many useful purposes which it is calculated  to serve. 
416
FLUIDITY IN RELATION TO ANIMALS.
Wfthin a few years only, has the importance of the subordinate circumstances
been fully appreciated,—as  is evinced by the  numerous works composed to
elucidate them ;  but many of which works have served only to prove, that if
the difficulties were to be solved by natural philosophy, medical men in gene-
ral  had not  yet studied  it sufficiently to be  able  to use it  successfully.'-  In
this section it will be attempted to place certain important points of the subject
in a clear light:  and by referring directly to the general laws of nature, as
explained in the  body of the work, to settle existing disputes on some of these
points, to remove remaining doubts on others, and to suggest some important
new applications.
  The fact of the circulation of the blood being once admitted, an  inquirer
who contemplates the apparatus by which it is effected, is led by the general
analogies of nature to conclude—1st.  That the ventricle of the  heart, at  each
contraction,  empties  itself into the great artery,  as a  forcing-pump at  each
stroke empties itself into its pipe ;—2d. That the consequent jet causes a wave
to pass along to the extremities of the arterial tree, (accounted simply elastic,)
so as to produce everywhere what is called  the pulse; 3d. That the force of
the heart, acting along the arteries, forces the blood through their open capil-
lary extremities into the  commencing veins, and  along the  veins back to the
heart again.   Now these assumptions, which Harvey believed completely to
describe  the circulation,  are all nearly true: but are  still so far from being
either the exact or the whole  truth, that they leave important facts unex-
plained.  Thus :—1st. The pulse, instead of being the wave, as slowly pro-
gressive as the view above given anticipates, is almost as instantaneous over the
whole body as a shock of electricity ;  2d. The arteries are all found empty
after death, although they have no power of emptying themselves ; and if an
artery be tied in the living body, the part beyond  the ligature, and cut off
therefore from the influence of the heart, is equally  emptied ;—3d. The rapi-
dity of the blood's passage through the capillaries varies very much, but it
does not vary in exact accordance with the  changes in the  rapidity or force
of the heart's* action.—These and other facts, ascertained since Harvey's day,
not exactly squaring with his views, have rendered further  investigation ne-
cessary; and it is now additionally known—1st. That the coats of the arteries
are not simply elastic but actively contractile ; and 2dly. That the capillary
vessels can move the blood independently of  the heart.  In analyzing this
subject, it is convenient to follow  the blood round from the heart to the heart
again, through the three stages of 1st. The arteries ; 2d. The capillaries ; 3d.
The veins.

                   Motion of the blood in the arteries.

  The contractions of the heart inject the blood into the arteries with a force
maintaining such a tension in them, that, according to the interesting experi-
ments of Dr. Hales, recorded in  his  Statistical Essays, if any artery of a
large animal like a horse be made to communicate with an upright tube, the
blood will ascend in the tube to a height of about ten feet above the level of
the heart, and will there continue, rising and falling a few inches  with  each
pulsation of the heart.   Now a column of ten feet, as explained at  page  120,
indicates a pressure of about four and a half pounds on the square ?nch of
surface : this, therefore, is  the force of the heart  urging the blood along the
arteries into the veins.—The opposing tension of the veins is much less, be-
cause, as will be explained under the proper head, the  blood readily escapes
from them into the heart: Hales found that  in a tube communicating with a 
THE CIRCULATION,
417
vein, the blood stood only a few inches higher than the level of the heart
In small animals he ascertained the tension of artery and vein to be less than
in large ones ; and the ratios deduced for the human body, under ordinary cir-
cumstances, were an eight feet column, or nearly four pounds per inch for the
arteries ; and half a foot column, or a quarter of a pound per inch for the veins.
   Arteries examined after death are found  to  consist of,—1st, an outer coat
of strong elastic substance; 2d, a middle coat of circular fibres ; and 3d, an
invex inner coat of smooth lining membrane.   Their elasticity or  power of
resisting change of dimension, and of returning to a middle state from either
dilatation or compression, because remaining in the dead artery, was the most
obvious property, and was that first attended to.   Minute observation of the
phenomena of life has since determined the following facts, proving and illus-
trating a contractility resident in the fibrous coat.
   1.  A small living artery, cut across,  soon contracts so as to close its  canal
and arrest hasmorrhage.
   2.  While an animal is bleeding to  death, the arteries, in  accommodating
themselves to the decreasing quantity of blood, contract far beyond the degree
to which their simple elasticity would carry them ; and they relax again after
death.  Dr. Hales took seventeen quarts of blood  from a horse before it died,
in whose  body only three quarts more were found altogether, and  yet the
moment before death the tension of the arteries sustained a column of two feet
of blood in his experimental tube.
   3.  The  artery of a living animal, if exposed by  dissection to the air, some-
times will  contract in a few minutes to a great degree: and in such  a  case,
only a single fibre of the artery may be affected, narrowing the channel like
a thread tied round it.   (.See Parry on the  Pulse.)
   4.  When a living artery is  tied, the part between the ligature and the
nearest branch on the side of  the  heart gradually  contracts, and  becomes at
last a solid or impervious cord.
   5.  Fluctuation in the vital  action  of parts, is often  attended with  sudden
increase or diminution of calibre in the arteries concerned.
   Although these facts prove  indubitably a contractility in  the coats of arte-
ries distinct from their elasticity, still, because the circular fibres do not resem-
ble common muscles in colour or in chemical composition, or  in being imme-
diately obedient to the stimuli of electricity, pricking, great  heat, Sic, their
contractility was by many persons for a long time denied.  The dispute, how-
ever, was  often  more about  the words contractility and muscularity, than
about facts.
   The pulse in the arteries, chiefly as regards its almost instantaneous occur-
rence over the whole system, in all states of arterial dilatation, and its great
strength and sharpness in very small and remote branches, points also to the
active contractility of the arterial coats: for,
   1.  Were the arterial tree in  the living body a system of simply elastic tubes
as readily admitting of farther dilatation as in the dead body, the  first part or
trunk would affect the motion of the blood beyond it, nearly as the air-vessel
(see page 116) placed at the commencement of artificial arrangements of water-
pipes affects the motion of the water in them ;—that is to say, as the air-vessel
converts the sudden and interrupted jets of  water from pumps of fire-engines,
town-supplying pipes, Sic, into a uniform  stream  with scarcely a remnant of
shock, so,  in the arterial branches, simple elasticity would cause a more tran-
quil flow than indicated by the  remarkable gushes from a wounded  artery,
and a quieter beat  than that bounding pulse of life felt in the  remote artery
of the wrist, as  sensibly, in proportion, as  near the heart itself. 
418
FLUIDITY IN RELATION TO ANIMALS.
   2.  Were the pulse a wave advancing in tubes that yielded  as  readily as
the dead arteries in their middle states of dilatation, it would be more slowly
progressive from the heart to the extremities;  but it is felt so instantly over
the whole body, as to be commonly compared to a shock of electricity.
   3.  A pulse may be produced artificially in the arteries of a body recently
dead, by filling them with water to the tension of life, and then injecting at
intervals, by a syringe, as much water as the heart throws of blood at a beat;
but although the artery is then distended nearly to the limit of its dilatability,
and is, therefore, rendered rigid, the beats are weaker than those of the living
pulse.  A similar experiment, tried by connecting the artery of a dead animal
with the corresponding artery of a living one, has a similar result.
   4. A tube, extensively elastic, that it might convey a wave of liquid  with
a velocity approaching to that of the  pulse, would  require  to be so  tense,
from fulness, as to be discernible always by the  touch, through any imbed-
ding  medium, such as flesh, like a hard cylinder or cord ;  and it would be
acting constantly as a spring tending to straighten itself, and, therefore, would
be stiffening the parts through which it passed.  Now the living arteries,
between their pulsations, are almost as soft and compressible as the surround-
ing flesh, and they offer no perceivable opposition to bending, in  any move-
ment  of the parts.   This  may  be verified by examination of the lips, for
instance, or of the fingers; but when a person sits cross-legged, the well-known
shaking of the suspended foot, in unison with the pulse, shows the recurring
efforts of the artery to straighten itself, during the moments of greater tension.
   5.  A bulky wave in elastic vessels would have to recoil from the extremi-
ties, or to pass through them as a gush; and the recoil would be particularly
observable near the ligature of a tied artery : but examination has not detected
such effects in the living body.   The operation for aneurism,—in  which the
artery is tied beyond the  tumour, instead of, as  usual, on the side next the
heart,—if it checked a strong wave, would almost certainly produce bursting;
yet Mr. Wardrope and others have lately performed this operation with suc-
cessful issue.
   6. The wave would be more interrupted by the bandage in the operation
of bleeding, than the living pulse is.
   7. The pulse of a paralytic limb often seems more affected, than mere change
of size in the artery will account for.   The same is true, in an opposite way,
of the pulse in an artery leading to an  inflamed part.
   8. If the  abdomen of a living animal be opened, the mesenteric artery, in
all its ramifications, is seen stiffened and raised up suddenly with every  pulsa-
tion, in a manner which the spreading of newly received blood  in a very
yielding vessel does not accouut for.
   9. In the interesting experiments of Bichdt, Parry and others, to ascertain
the exact extent of the supposed dilatation and contraction of arteries, during a
pulse, not the slightest degree of either was  discernible,  even when  sought
for with microscopes.
   To explain these and other phenomena, then, it seems necessary to admit,
as occurring throughout the whole body, and almost simultaneously with the
contraction of the heart itself, such an action of the  contractile fibres  of the
arteries, so as to  modify the elasticity of the arteries, and to render them rigid
enough, in all degress of dilatation, for the heart to produce its effect through
them  almost as  it would through tubes of metal.—Dr. Young, in  a  paper
published in the Philosophical Transactions for 1809, and charcterized  by
the usual elegance  and precision of his writings, has  adduced experiments 
THE CIRCULATION.
419
and calculations, to  show that waves in elastic vessels advance more quickly
than was before imagined ; but the  spreading of the pulse seems to-be yet
more rapid than his calculation anticipates.—It is evident, that when arteries,
in consequence of depletion, are contracted beyond the middle station of their
elasticity, their tension  and power of quickly conveying the pulse must be
dependent altogether on the condition of their contractile fibres.
   1 he careful experiments which could detect no change of size in the arteries
during the pulse, while they disprove the ancient belief of a considerable tume-
faction or wave passing  along, or of a considerable  filling and  emptying of
arteries, like what occurs in  the heart, might also be supposed  positively to
disprove the  occurrence of any general constriction of the vessels  on  their
contents—but erroneously :—for if a man's arterial system, considered as one
cavity, be supposed to contain five pounds of blood (which is near the truth,)
and if the vessels be thought to embrace their  contents, even  between  the
pulses, with force enough to have all a rounded or cylindrical form, although
remaining soft and yielding to the pressure of the  finger; and if we suppose
their coats, during the pulse,  to be thrown into a sudden contraction, as if in
obedience to an electrical shock, still, because blood is incompressible, and
because just as much enters  the arteries with every pulse  as escapes from
them before the next, their bulk would not sensibly diminish  by  the strongest
conceivable action of their coats ; of which  action the only sensible effects
would be, that the soft, yielding, and, in some places, compressed tubes would
be suddenly converted into hard or resisting cylinders; and that wherever,
by any accidental pressure, an artery had been flattened, it would, in regaining
its cylindrical form, strike or pulsate against the compressing body.—Whether
such an action as this contributes to produce the arterial pulse will be consider-
ed under the head of " the pulse," after we  have seen how the  blood moves
in the capillaries and veins.
   In any admissible view, however, of arterial agency, we find that the arteries
contribute to the circulation of the blood, but as tubes  which convey it, their
own permanent  tension, and, therefore, the  force with which  the blood is
pressed into  the capillaries, being  derived  from  the  heart  alone.  Even if
there be a  momentary  arterial contraction,  such as alluded to  above, at the
instant of the pulse, it is of too short duration to have an appreciable effect,
and probably any effect would be counterbalanced by the same  action perva-
ding the capillaries.  Many physiologists have had a confused belief that the
arteries aided  very  actively  in  propelling the blood;  but a  little reflection
would have shown, that as they have no vermicular or progressive contrac-
tion, like  the  intestines, they no more propel the fluid  within them,  than any
other tubular  conduits do.—Although they be thus in no degree instrumental
in the propulsion of the blood, still, by more permanently enlarging or dimin-
ishing their calibre, that is, by merely becoming larger or smaller conduits,
they may much influence its local distribution, and the  speed of its transmis-
sion.*
  * It has long been a subject of dispute whether the arteries exercise any  active power in
the circulation of the blood, and many ingenious experiments have been  instituted to deter-
mine the question.  Our author, in denying that the arteries are in any degree instrumental
in the propulsion of the blood, is not borne out by recent investigations.  M. Poiseuille,
indeed, seems satisfactorily to have proved that the contractility of the arteries does assist in
the propulsion of the blood.  By a series of well devised, and apparently accurate experi-
ments M. Poiseuille arrived at the unexpected result, that the force  of the blood in the
arteries will support a column of mercury of the  same height with whatever part of the
course of the arterial circulation the column is placed in connection—whether for example
it is connected with the origin of the carotid, or with a branch derived by repeated subdivi-
sion from the crural artery : and he, therefore, concludes that the force with which a mole- 
420
FLUIDITY IN RELATION  TO ANIMALS.
   The nature of  this work does not allow us to record historically the  vari-
 ous errors  into which  even able  men have  fallen, in attempting to  explain
 the office of the arteries, but we shall glance at the  following.—Dr. Monro
 and  John Hunter,  two of  the  most able physiologists  that  the world has
 seen, believed  that the arteries did almost as  much in propelling the blood as
 the heart itself.   We  need not repeat the  refutation of this  opinion.   The
 ingenious  Bichat, again,  unable to  detect  either  momentary contraction or
 dilatation in the arteries, thought that the blood  was pushed  along  them by

 cule of blood moves, is the same throughout the whole arterial circulation.   Following out
 these researches, he has investigated the manner in which the original impulse communicated
 by the heart, is transmitted  unimpaired to distant parts of the circulation, notwithstanding
 the retarding tendency of friction, and the yielding of the parietes of the vessels.
  His first object  here was to ascertain whether the arteries are dilated by the stroke of the
 heart, and impulse communicated to the blood,  and what the amount  of the dilatation may
 be.  By many physiologists a very extravagant idea used to be entertained of the amount of
 their dilatation 5 on the other hand, the later researches of Parry, and other experimentalists
 have  assigned exceedingly narrow limits to it; nay, by one eminent physiologist, Bichat, it
 has been  denied  altogether.  M. Poiseuille has determined  the point by means of a very
 satisfactory set of experiments with  an apparatus of his own invention, and has ascertained
 that dilatation is affected ; but that it is so small  as certainly to be  indistinguishable in an
 artery by the unaided  senses.   This apparatus  cannot be thoroughly described without a
 diagram ; it will be sufficient, therefore, for us to mention, that it is so contrived as to con-
 tain about eight inches of the carotid artery of the horse in a vessel filled with water, and
 made water tight, except at one point, from which a small horizontal glass-tube issues, about
 an eighth of an inch in diameter.   At each contraction of the animal's heart it was  found
 that the water in the small tube abvanced two inches and eight-tenths, and that it retired to
 its former place during the  diastole of the heart.  The diameter of the artery was seven-
 twentieths of an inch.  Hence it may be calculated that at each  pulsation its capacity was
 increased by about a thirtieth part.
  Having ascertained this fact,  M. Poiseuille goes on to inquire, whether the power vvhich
 is expended  by the blood in causing this dilatation is restored by the subsequent contrac-
 tion of the artery.  For this purpose a portion of the common carotid artery of the horse,
 ten inches long, and seven-twentieths of an inch in diameter, taken immediately after death,
 was connected with the end  a, of the tube, (see Figure ;) a stop-cock, however, being pre-
 viously fitted between a and b.  The other end of the artery was fixed on a tube of analogous
 construction, different in fact only in  so far as the limb  c d was  inclined at about half a right
 angle instead of being vertical, and the stop-cock  was placed near the end d.  The whole of
 the first tube, the  artery and part of the descending limb b c of the second tube was filled
 with water, a little mercury then filled the curvature of the second tube, and the ascending
 inclined limb of that tube above the mercury was filled with water.  The stop-cock of the
                   last tube being closed, and that on the first tube being opened, mercury
    Fig. 174.      was poured iuto the former by its end d, till the pressure on the artery-
                   amounted to ninety-five millimetres or about 3.8 inches.  The stop-
              -\(X  cock of the first tube was then closed and that on the second  tube was
                   opened ; upon which  the water  rose  instantaneously in the latter, a
                   portion flowed out at the top, and the remainder then'sank a  little, and
                   assumed a fixed level.  On making the necessary computations, M.
                   Poiseuille found that the point to which the mercury was raised in the
                   second tube at the moment of the contraction of the artery indicated an
                   elevation of one hundred  and ten millimetres or 4.4 inches.   Hence
                   the pov>er -with -which the arterial coats  contract upon themselves after
                   being dilated, exceeds that -which  is expended in dilating them.  In the
                   present experiment the excess was equivalent to  six-tenths of an inch
                   of mercury, or three-nineteenths more than the dilating force. In three
               g.    other experiments, the excess of the column of mercury was 9.20, 14.
                   20, 19.20 of an inch.   When repeated with the artervofan animal
                   which had  been killed four days  before, the excess was*less than 4.20.
                   It is evident, therefore, that whatever force the blood after issuing from
                   the heart loses in consequence of its acting on yielding vessels, is com-
                   pletely restored hy the elastic contraction of their parietes.  The me-
                   moirs of M. Poiseuille will be found in the Repertoire General d' ana-
tomie et de Physiologie, Tom. VII.
  Dr. Badham in an interesting  paper in the London Medical Gazette  (vol. viii. p. 549) has
adduced some strong evidences of the existence of independent arterial action, and gives a
sketch of various pathological phenomena, which appears explicable on such an admission,
and inexplicable without it.—Am. Ed. 
THE CIRCULATION.
421
the heart, instantly through their  whole extent, as a solid rod  of  metal  or
wood  is advanced by an impulse  at  one end.  Dr. Parry took nearly the
same view of the subject, and illustrated his idea  by referring to the experi-
ment of moving a whole  line of billiard-balls  by  striking the extreme one.
Both these authors erred by neglecting the hydrostatical truth, that pressure
in a fluid operates equally in all directions, and, therefore, that fluid pressed
into a tube tends to dilate the tube, just as powerfully as to  drive the fluid
forward : and they did not advert to the fact that the progress of  the blood in
the small arteries is not by waves  or successive jets, but is nearly a uniform
stream.  The blood  could only advance, as they supposed, by the arteries
becoming, for an instant,  absolutely rigid, in consequence of a strong action
of their contractile fibres.
  It merits notice here, although not strictly a  mechanical fact, that arteries
permanently  increase or diminish in size when a permanent change takes
place in the demand for their service.  The arteries of the gravid uterus, or
of an increasing tumour, grow with the part supplied, while, on the contrary,
those of the  stump left after  amputation soon  remarkably diminish.  If the
chief artery of a limb  be obliterated by any cause,  as after the operation for
aneurism, the small collateral  anastomozing branches increase in size to do
its duty.
  It is farther remarkable, that when arteries are  called upon to carry an in-
creased quantity of blood, they often become tortuous or serpentine, as well
as larger; and that arteries leading to parts whose  actions are  naturally inter-
mitting or fluctuating, have  generally the tortuous form.  Of these  truths,
the arteries leading to rapidly growing  tumours,  or to  varicose aneurisms,
and the arteries of the uterus and testes, may serve as instances.   This bend-
ing of arteries,  and  the  very curious division into many branches  which
again re-unite, found  in those  leading  to the  brains  of some animals, do not
seem intended to slacken the rapidity of the sanguineous current, but  to give
the artery a greater control over the supply.

               Passage of the blood through the capillaries.

  We have seen that  the heart keeps up a tension  or pressure in the arteries
of about four pounds on the square inch of their surface : and with this force,
therefore, is propelling the blood into the capillaries.  Tf these last were pas-
sive tubes, constantly.open, such force would be sufficient to press  the blood
through them with a certain uniform velocity:  but they are vessels of great
and varying activity;  it is among them that the nutrition of the different tex-
tures  of the  body takes place, as of muscle, bone, membrane, &?c; and that
all the secretions from the blood are performed, as of bile, gastric juice, or
saliva, Sj-c;  and to perform such varied and  often fluctuating offices,  they
require to be able to control,  in all ways, the  motion of  the blood passing
through them.   The  capillaries  of the cheek, under the influence of shame,
dilate°instantly, and admit more blood, producing what is called ablush;—
under  the influence of anger or fear,  they suddenly empty themselves, and
the countenance becomes pallid—tears or saliva, under certain  circumstances,
gush in a moment, and in a moment again are  arrested—if a  person  having
inflammation in  one hand be blooded from corresponding veins in both arms
at the same time, twice or thrice as much blood will flow  from the diseased
side  as from the  other.   Similar changes occur  in many other instances.
Now  the only action  of cylindrical vessels, capable of causing these pheno-
mena, is contraction and dilatation of their coats; and with reference to such
    28 
422
FLUIDITY IN RELATION TO ANIMALS.
action it merits notice, that arterial branches have more of the fibrous or con-
tractile coat in proportion as they are smaller.
   A muscular capillary tube strong  enough to shut itself against the arterial
current from the heart, is also strong enough to propel the blood to the heart
again through the veins, even  if the resistance on the  side of the veins were
as great as the force on the side of the arteries.   For if we suppose the first
circular fibre of the tube to close itself completely, it would, of course, be ex-
erting the same repellent  force on both sides,  or as regarded both the artery
and vein.  If, then, the series of such fibres forming the tube were to contract
in succession  towards the vein, as  the fibres of the intestinal canal contract
in propelling the contents of that canal, it is evident that all the blood in the
capillary would thereby be pressed into the vein towards the heart.   If after
this the capillary again relaxed on the side of the artery, so as to admit more
blood, and again contracted  towards  the vein  as before, it would  produce a
forward motion  of the blood, first towards  the  vein,  and then in it inde-
pendently of the heart.   As capillary action,  however, is not visible,  its na-
ture has not yet been positively ascertained :—some persons have deemed it
electrical.
   It is capillary action which  absorbs and moves the  fluids of the classes of
animals which have no heart.   It must also be the power which moves the
blood in warm-blooded monsters formed  without hearts.  There are cases of
apparent death among human  beings, where the heart remains inactive for
days, and yet a degree of circulation sufficient to preserve life is  carried on
by the capillaries.   In illustration of capillary action, we have also the absorp-
tion, by the lacteals, of nutriment from the alimentary canal; and, perhaps,
to a certain extent,  the circulation of the blood in the livers of animals.  In
this last case, the blood collected by veins from the abdominal viscera, instead
of going directly to the  heart, is again distributed  through the liver  by the
branches of the vena portse,  and is then again  collected by the ordinary veins
of the liver, and carried to the heart: it thus moves through two sets of capil-
laries in passing from the arteries to the heart again.
   The action  of the capillaries is the cause of that singular fact which pre-
vented the ancients  from  discovering the circulation  of the  blood, viz., the
empty state of the arteries after death.  All the muscular parts of an animal,
including, therefore, the contractile coats of vessels, retain their life, or  power
of contracting, for a  considerable  time after respiration has  ceased,—as  is
seen in the recovery of persons apparently drowned or suffocated;  in the
leaping of a heart taken from an animal recently killed ; in the actions resem-
bling life which can be  produced,  by the agency of galvanism, in a body
recently dead: but  the fact is  seen  still  more  aptly for our purpose, in the
total disappearance  of a local inflammation after the death of the patient,—
for inflammation involves a gorging or over-distension of the capillaries, into
which, when the heart has ceased to press blood, the contractile force remain-
ing in  them, even under disease and in a dead animal, is sufficient to squeeze
the blood  out of them, and often to remove all trace of the malady which has
been  fatal.—In ordinary oases, then, the capillaries throughout  the body re-
main  alive and active for a considerable time after breathing has ceased, work-
ing like innumerable little pumps, and emptying the arteries into the  veins.
As the red blood is  their  proper sustenance as well as stimulus, they work as
long as there is any of it comma- into them from the arteries behind; except,
however,  the capillaries of the lungs, which soon cease to act, because, after
breathing  has ceased, they receive only black blood, and are  moreover com-
pressed by the collapse of the chest; and all  the blood accumulates behind 
THE CIRCULATION.
423
them.   The capillaries may continue to be filled from the  arteries, either in
consequence of  their elasticity opening them with  what is called a suction
power,  or of an absorbent power dependent on  life, like that of the lacteals
and of the absorbents all over the bddy, and, perhaps, of the vessels in the
roots of vegetables.   When death is produced by lightning, or by the poisons
which destroy all muscular irritability, and therefore that of capillaries, the arte-
ries after death are found  to contain blood like the veins.   In a living  body,
if an artery  be tied, the part beyond the ligature is soon emptied into the veins,
and becomes flat.—The experiment has been made even upon the aorta itself.
  The  empty state  of the arteries  after death  has  been ascribed, by some
teachers, to  the momentum with which they supposed the blood to  be thrown
out from the heart in its last contraction—sufficient, said they, to squirt it fairly
through the most distant capillaries; a doctrine exemplifying the carelessness
with which  men sometimes receive and repeat opinions, to which their atten-
tion has never been fully awakened.   Such an effect would not follow, even
if the action of a dying heart  were the strongest possible; while,  in reality.
it is in most cases so feeble, that the pulse for some  time  ceases to be per-
ceptible at the extremities, and the diminished circulation lets them become
cold.—Other physiologists have taught that an artery is capable of contract-
ing directly  upon its contents, so as  to expel even the last drop;—but  large
arteries, when emptying,  do not contract roundly like an intestine:  they be-
come flat like elastic tubes of leather sucked empty, and no contractile action
of the vessel itself could bring its sides together in such a manner.  If arte-
ries emptied themselves by their own action, the pulmonary  artery should be
more certainly empty than the aorta, because it is shorter: yet it  is  always
full; for the reason already stated, that the pulmonary capillaries cease to act
after respiration  has  ceased, on account of the blood in them  being venous or
dark blood,  and  therefore  not life-supporting or stimulant to them.

                   Passage of blood through the veins.

  The veins have much thinner coats than the arteries, and if taken altogether
have much greater capacity: for besides being larger than the corresponding
arteries, they exist,  in  many  situations, as double sets, an  exterior  and an
interior: they have also very frequent inosculations or communications with
each other throughout their whole course.
  The simple weight of the column of blood in any descending artery is just
sufficient to raise the blood through  open capillaries to an equal height in the
corresponding vein,  according to the hydrostatical law, that  fluids attain the
same level  in all communicating vessels;  and therefore, as  the arch of the
aorta rises  considerably above the heart, the  gravitating pressure of the de-
scending arterial column of blood  would be sufficient to lift that in  the veins
not only up to the heart, but considerably beyond it.  In addition  to this in-
fluence  of gravitv on the venous current, the blood is  pressed into the arteries,
and from them, therefore, towards  the veins, with a force from the heart itself,
as stated above,  of about four  pounds to the square inch, or, in other words,
as if there were  a column of blood eight feet higher than the  heart  urging the
current.   It mi-rht be expected from the law of equal diffusion of pressure in
fluids that these causes would soon produce a tension in the veins  as  great
as in the arteries:  and  this does  not  happen, only  because  the blood has a
ready escape from the veins through the right ventricle of the heart   Under
ordinary circumstances, there can be no greater tension in the veins than what
is sufficient to lift  the blood to the heart and to overcome the friction;—as in 
424
FLUIDITY IN RELATION TO ANIMALS.
an upright leathern tube, open at top, and receiving water at its bottom from
a powerful forcing pump, there never can be greater tension or pressure than
what corresponds to  the height of the fluid column in the tube, and  to  the
friction  between the fluid and tube.   In Dr. Hale's  experiments, already
alluded  to, a tube connected with a  vein  so as to receive its blood, became
filled with blood to a height only of about six inches above the level  of  the
heart.   As Dr. H. generally cut the vein completely across, and inserted  the
tube into the portion leading from the capillaries, he  would have  discovered
the whole power with which the blood is pushed along  the  veins from  the
capillaries, but for the free lateral  communication of veins  with each other,
which reduces the tension even in an obstructed branch, to the degree exist-
ing in the system generally.  When, from agitation of the animal, or any strain-
ing exertion, the  passage  of the blood into the  heart  was impeded, all  the
veins became tense, and a tube inserted into the returning jugular had blood
running over, at a height of three feet above the heart.
  If the blood  did not escape from  the veins, as above described, the only
cause which could prevent the venous tension from becoming as great as  the
arterial, would be obstruction in the  connecting capillaries : but the following
facts and considerations prove that these vessels, which, in  the dead body,
allow the  passage of injections, in the living body  freely allow the passage
of blood.  1st.  Magendie laid  bare the chief artery and vein of a living limb,
and at the part, detached  them from the flesh underneath, so that he could
apply a tight bandage round the limb without  including them, and could thus
render  them the only channels of circulation  for  the limb beyond the band-
age.  He  then found, that when a separate ligature was  put upon the vein,
to prevent the return of  its blood  to the heart, and a puncture was made
beyond  the ligature,  the flux of blood from the puncture was rapid or slow,
according  as the  heart was  allowed  to produce  a greater or less degree of
tension  in the artery:—this tension was  regulated  by his  compressing  the
artery between the fingers.  2d. After a similar  preparation of the parts,  the
blood will ascend in  a tube from the obstructed  vein very nearly as high as
from the artery.  3d. In the common operation of  bleeding, at the moment
of puncturing  the  vein, the  blood often jets from it  as  from an  artery, stain-
ing even the  top  of  the  bedstead.   4th.  The microscope  discovers  in  the
capillaries, a uniform forward  motion of the blood, as if it were obeying  the
steady pressure of the arterial tension, and not any intermitting action.  5th.
Disturbed action of the heart, by obstructing the passage of the blood through
it, is very soon attended with a tumefaction  of  all the veins leading to  the
heart:  the tumefaction becomes very visible  about  the neck and head,  and
in the liver produces swelling and acute pain.   6th.  Dr. Young, from experi-
ments made by him,  and reported in the Philosophical Transactions for 1809,
concluded that  perfectly open  capillaries, of the  size  existing  in the living
body, should just retard a flow of blood  urged by the  usual arterial tension,
in the degree which really occurs:—a correspondence proving that they must
be  open;  and  open  vessels, however small, and how slowly soever they
transmit the blood, still, if the escape of  blood from the veins were arrested,
would  transmit the arterial tension  without diminution.  7th. The fact  that
after death the capillaries empty the arteries into  the veins, proves that, under
certain circumstances, the venous tension may become even greater than the
arterial.—These facts, then, and others that might be mentioned, prove incon-
testably that the blood is pressed into the veins from the arteries and capilla-
ries, with force sufficient to lift it, not only to the heart again, but many  feet
farther, viz., about as far as it would ascend in a tube  rising  from the tense 
THE CIRCULATION.                       425
arteries themselves.   So little, however, has this important truth been under-
stood, that in elementary works of authority lately published, the venous
current is  treated of as a very obscure subject; and some authors, in their
anxiety to explain it, have assigned causes for it, which, as will appear here-
alter, are  positive absurdities  in physics.  The  difficulty in the question
seems to have arisen from the great disparity observed between the tension
in the arteries and in  the veins, while the reflection did not occur, that the
disparity was owing to there being  a free  passage or outlet from the veins
through the heart.
   The  illustrious Bichat, with  an inattention  to facts, extraordinary in  him,
persuaded himself that the influence of the heart ceased entirely at the capil-
laries and that the blood was returned through the  veins by the action of the
capillaries alone.   How could he avoid the single  reflection, that,  if the pur-
pose of the arteries had been merely to convey the blood  to the  capillaries,
and not also to bear the force which pressed  it into and  through  them, the
extraordinary strength of the arterial coats, and the great power°of the heart
to fill them and keep up the tension described, would have been quite super-
fluous?—and  he  knew that  nature  does nothing  in vain.*  The reflection
applies strikingly to the pulmonary artery, of which  no branch  exdeeds a
few inches in length.
   The  uniform current of blood along the veins, so apparent in the operation
of bleeding, and produced, as now  explained, by the combined influence of
the heart and  capillaries, suffers a considerable disturbance in the neighbour-
hood of the heart from three causes.   1st. As there is no valve between the
veins and the auricles of the heart, each contraction of the right auricle tends
to throw the blood back into the veins, as well as forward into the ventricle,
and thus produces the venous  pulse often felt in  the neighbourhood of the
chest.   2d. When  the chest is  expanded by inspiration, it  is more roomy
than during  the collapse of expiration, and  the blood then enters it  more
readily.   3d.  While the  chest is inhaling or drawing in air, that is to say,
expanding so  as to diminish  the tension or pressure of the air within it, (see
Pneumatics,) it is by the same action favouring the entrance of blood through
the veins towards the heart placed in it;—on the contrary, while it is exhaling
or throwing out air, it is, with  equal force, resisting the  entrance of blood,
and  slackening, or even causing recoil of the inward current.   This favour-
ing or resisting force, however, as will  be hereafter  shown, is only such  as
to lift or support a column of blood of about half an inch  in  height.—It ap-
pears, then, that the entrance of blood into the chest fluctuates by  reason  of
the respiration, Sic, as the entrance of a river stream into  the sea  fluctuates
by reason  of the ebbing and flowing of the tide.  An eye watching the jugular
vein, under favourable circumstances, may see  it tense or slack in accordance
with the opening and shutting of the chest.
  It still remains to be ascertained whether or not veins have in themselves
any contractile  power, such  as can  partially empty  a  lower portion into a
higher portion beyond an adjoining valve.  If so,  the valve by then bearing
the pressure, would let more blood be easily raised from below into the por-
tion  so relieved: and the action, without being equal to the  office of  com-
pletely emptying any portion of a vein, would still  have the effect of dividing

  * This incorrect and inconclusive mode of reasoning is so common that we may be per-
mitted to protest against  it.  The influence of the heart may cease with the  capillaries,
and yet nature has done nothing in vain.  Before we would be justified in making such a
charie against nature we must possess an infinitely more precise knowledge of the circula-
tory forces and of the functions of the arterial system than we do at present.  Am. Ed. 
426
FLUIDITY IN RELATION TO ANIMALS.
a long heavy column into a number of short columns of comparatively little
resistance.  It is certain, at least, that the valves in the veins, by preventing
the falling back of blood which has once  passed towards the heart, must
affect its flow during  bodily exercise; for every time  that pressure is made
on a vein by a swelling muscle or otherwise, the blood  in the  part must be
forced forward, and cannot return.
  The veins which are surrounded by muscles are thinner and weaker than
those  supported  only by the  skin.  The  external veins of the legs  are
almost as strong  as arteries.   Proving, however, that the  fabric of veins is
much  weaker than that of arteries, any vein in the  living  body, made  to
communicate directly  with an artery, soon exhibits what is called a varicose
aneurism, and swells  to bursting.  Veins possess power, to a great extent,
of adapting themselves to the varying quantity of blood.
  Some  recent authors, as stated above, either not aware of the facts which
prove that the blood is everywhere  pressed  into the veins with  force much
more than sufficient to raise it  to the heart again; or, being  unable, from
their little  familiarity  with mechanical  science, to draw exact conclusions
from the facts, or to avoid errors in their  own hypotheses, have promulgated
the opinion  that the progression of the blood  in  the veins is greatly owing
to a partial vacuum or  a suction power in the heart or chest; that is to say,
to the atmospheric pressure remaining constant on the body generally, while
it is, at intervals, lessened about the heart.  Now  the whole influence of
this effect or circumstance, as stated  above,  is merely a slight disturbance of
the  uniformity of the venous current near the chest.   Such a doctrine could
not be  proposed or entertained for a moment by a person understanding the
principle of a common household pump;  and its having been published, and
tolerated by certain professional men in the present time, will remain a proof
to posterity  of the deficiency, as regards  fundamental science or natural phi-
losophy, now existing  in the ordinary medical education.  Much  ingenuity
has been wasted upon  it,  particularly by Drs.  Carson  and Barry, the latter
of whom, after making laborious experimental investigations on living ani-
mals, has even attempted to build upon it a superstructure of  medical theory
and practice!  To say that the  influence of  the heart  or chest is the power
which  draws the blood to  the  heart  from the  general  system, is as if one
asserted that the rising and falling of the  tide at the mouth  of a  river is the
power which collects the tributary streams in the interior country.
  We  shall enter into a little  detail  on this  subject, because  the discussion
will elucidate some minor points connected with the circulation.
  Presuming, then,  that  the  reader perfectly understands  the  theory  of
pumps, and therefore of atmospheric pressure, as explained under  Pneu-
matics, he will readily understand the two following  propositions, either  of
which  proves  it to be a physical impossibility, that a  sucking  action of the
heart or chest can be a cause  of the  blood's motion  along the veins.   1st.
The veins  are pliant  tubes free to  collapse,  and no  pump can  lift liquid
through such.  2d. The suction-power of the chest in healthy respiration
is too weak  to lift liquid even one inch through tubes of  any  kind.
  A practical  illustration of the  first  proposition is  afforded by putting the
point of a syringe into a peice of gut, or eel-skin, or vein filled with water,
and then trying to pump up the water.   The result will be, that the  fluid
close  to  the mouth of the syringe  will  enter it, and then the sides of the
pliant tube will collapse against the  syringe, making  an end of the experi-
ment.  In exact proportion to the rigidity of the tube will be  the distance to
which  the influence of the syringe will extend in it; if, for instance, half an 
                          THE CIRCULATION.                      427

ounce of pressure on the square inch of its surface be  required to  make it
collapse, then the pump  will draw up one  inch  of water, and so for other
proportions.   If, during the action of the  syringe, the tube were allowed to
open freely at the bottom into a vessel of vvater, instead of the syringe then
drawing any  more water from the vessel into the tube, the original contents
of the  tube would straightway be discharged downwards into the vessel.
  The explanation of all these facts is  found  in  the pressure of the atmo-
sphere, (see from page 153  to  page 158) seeking entrance everywhere at
the surface of the earth, with a force of fifteen pounds  per square inch, and
overcoming any opposing force less than this ;—a pressure which  is suffi-
cient, therefore, to  push  a column of water of thirty-four  feet in height, up
through a rigid tube into the vacuum of a pump, but will  cause the  sides of
the tubes to collapse, unless able to sustain its force.  When nature  intends
a tube  to resist any degree of suction, the tube is made  rigid in proportion;
—witness  the wind-pipe and its branches, which are  the only instances in
the human body.   And  if  tubes prepared for  sucking light air only have
received such rigidity, how much stronger would tubes have been made for
sucking blood.
   Some bad  reasoners on this subject have believed, that  if a suction power
exist, capable of lifting  one inch of a  column of liquid,  any column, how-
ever long, must  follow the first inch when  acted upon by  the power;  for,
say they, the  atmospheric  pressure, by  preventing a vacuum,  will  prevent
separation  of the liquid.  Now, in the first  place, this  reasoning  is quite
inapplicable to pliant  tubes, because the ready collapse  of their sides will
both allow the separation and prevent the vacuum; and, in the second place,
with respect  to rigid tubes, it is equivalent to asserting  that a force just capa-
ble of  lifting  one link of a chain, must,  therefore, be able  to  lift  any  number
of connected links.  Water, in a rigid tube, to which air  has no admittance,
may truly be considered as  a chain, for it is held  together by a force of fif-
teen pounds per inch, pressing inwards at the two ends: and  by force inferior
to this, cannot lift one  portion of it away from another,  and  therefore cannot
draw out a drop but by  lifting the  whole.  A man  cannot  suck any water
from a rigid tube which is closed at the  bottom; and if  the bottom be open,
and he has not power to support the whole contained fluid, it will sink from
his tantalized lips to stand at an elevation corresponding  to  his suction power.
   To illustrate the second  proposition respecting the trifling suction power
really residing in the chest, we may state that a person of ordinary strength
using the whole power of the chest, (but not of the mouth separately, which
is a smaller and much  more powerful pump than the chest,) cannot, through
a rio-id tube,  suck water from more than about two feet below his lips, and
therefore not half way so far as from the extremities of his body; while, in the
opposite action  of blowing  outwards, as in the  attempt to  blow through a
tube which is dipping into  water, he finds nearly the  same limit.   But in
ordinary breathing, instead of force corresponding to a  liquid column of two
feet, or a fifteenth of the atmospheric pressure, the increase and diminution
of air-density in the chest are measured  by a column of less than one inch or
about  a five-hundredth  of  the  atmospheric  pressure.   This  fact is easily
shown by breathing through the nose, while holding in the  mouth  one end
of a o-lass  tube,  the other end  of  which  is  immersed in water, and' then
noting  how  much  the water in the tube rises above the surrounding level
during  inspiration, and sinks below it  during expiration.  The  mouth,
durinff this experiment,  may be considered as a part of the general cavity of
the chest, to  and from which air is passing by the narrow openings  ol the 
428
FLUIDITY IN RELATION TO ANIMALS.
 nostrils.  In tranquil breathing, with both  nostrils open, the fluctuation in
 the tube is less than half an inch each way; with one  nostril closed and  the
 other a little compressed, it may amount to a whole inch; and with hurried or
 convulsive breathing, like  that of  an animal in terror and  in  pain, it may
 exceed twelve  inches.  Although  the  measures thus  obtained  from  the
 mouth are somewhat too small for the changes in the chest itself, because  the
 chest is more remote from  the opening by which the external air enters,  the
 difference is very trifling, as may be proved during such experiments, by stop-
 ping the nostrils altogether, while the same respiratory  efforts are continued;
 and as is also proved by the agreement of the  results  with strict calculation
 found on the inertia and velocity of the air respired—a calculation similar to
 that required in  adjusting the index to the machine  mentioned at page  215,
 for measuring water-currents.   In common healthy breathing, then, while  the
 mouth is open, the fluctuation of pressure in the  chest would be measured
 by less  than half an inch motion each way of the liquid column.   Dr. Barry,
 not aware that this point could be so easily determined by the bloodless experi-
 ment described above, or even by a simple calculation, sought the solution by
 numerous trials  on living animals, into  some part of whose  chest he forced
 a tube.   But even if farther experiments had been at all  necessary, those of
 Dr. B. could not have decided  the question, first, because the pain and agita-
 tion of  the dying animals rendered the breathing violent  or  unnatural;  and,
 secondly, because  his experimental tube often or always became a syphon,
 and he, not adverting to this fact, has not recorded the difference of level in
 the liquid at the  two ends.   That the external level was, for the most part,
 higher than the  internal, is proved by his having noticed, almost solely, the
 inhaling action of the chest, although the exhaling is often a more powerful
 effort.
   Calling an inch  column of blood, then, the measure of the greatest suges-
 cent and repellent powers of the chest during ordinary respiration, we see
 that the force which really  sends  the blood from below to the heart, may
 have to lift a column one inch shorter during inspiration,  and one inch longer
 during expiration: but  this is  the  full and  true measure and nature of the
 influence of the inspiration on the blood's return to the  heart. To say, then,
 that the atmospheric pressure, modified  by  respiration, is the  great power
 which moves  the  venous blood, is just as if we said, that a boy  standing
 near the fly-wheel of a steam engine of a hundred horses power, and giving
 it his  Lilliputian thrust, alternately backward and forward, were the prime
 mover of the machinery.
   The  truth explained above, that no kind of pump  can lift fluid through
pliant tubes, free to collapse, like the veins, renders it unnecessary farther to
speak here of the pumping action of the heart itself, insisted on  by Dr.  Car-
son; or of that other action, mentioned in a subsequent part of this work,
to which  also he  attributes great  influence, viz., the tendency towards a
vacuum external to the lungs and around the  heart, produced by  the disposi-
tion of the lungs to collapse.  It may be remarked, however, that  this last
influence is more considerable than the simple inspiratory action dwelt on  by
Dr. Barry, and that  it  operates during expiration nearly as  much as during
inspiration, varying in force with the degrees of expansion of the chest.  It
is weaker in the living  than in the  dead body, because  the rigidity of the
distended pulmonary arteries helps  to support the.weight  of the living lungs.
  Were it necessary to give proofs  to persons unable to follow the above
argument, that a suction-power in the  heart or chest is not  the force which
draws the blood  from the extreme veins, reference might be made to many 
THE CIRCULATION.
429
 notorious facts quite incompatible with that supposition;  such, for instance,
 at those  recorded at page 424, and others.  A vein tied,  fills tensely below
 the ligature—a vein cut across bleeds from the orifice which is distant from
 the heart, .and  will fill a fofty tube connected with it—the  circulation goes on
 in persons holding  their breath—the veins  of fishes, which do not breathe,
 return the blood as  well as  those  of men, &c.  Sic*

   After the explanations now given, it is  almost  superfluous  to remark  that
 absorption in animals cannot depend on  atmospheric pressure, and that the
 effect of  cupping-glasses applied  to extract blood, or to prevent the absorption
 of  poison m wounds, in no way  depends upon the fluctuating density of the
 air m the chest.t   Dr.  Barry's reasonings  upon  these  subjects  involve the
 same fallacies  as his reasonings on the  venous  current.  With respect to
 absorption,  they neglect the  fact of fluids having weight;  and with respect
 to cupping glasses,  of which the true  action is explained at page  175, they
 are  equivalent to asserting that  the action^of  pumps drawing water from a
 river among the hills is influenced by tides, or pumps operating at its mouth
 in the sea.

   If the fluids in animal vessels had no weight, it is true, that in absorption,
 an external  atmospheric pressure of fifteen pounds per inch might force new
 matter into a  receiving orifice, at  the instant during inspiration,  when the
 opposing pressure in the chest, at the other ends of the  vessels, were half an
 ounce per inch less,—there would be no physical absurdity in supposing this,
 although there are physiological facts  that disprove it—but when we reflect,
 that  in all vessels under the level of the heart, the  weight  of the  contained
 fluids causes an additional outward pressure of about  half an ounce troy for
 every perpendicular inch of fluid  column, making an excess of outward pres-
 sure  at the toes, for instance,  even at the most favourable time for absorption,
 of about two pounds  per inch, we see that absorption must be a strong action

  * The influence of inspiration  of the cavity in the chest exterior  to the lungs, and the ex-
 pansive power of  the heart, on the circulation of the blood in the veins, have no doubt  been
 greatly over estimated by Drs. Barry, Carson and others, but our author appears to us to
 have  undervalued their effect.   Their  joint  power is more considerable than  the reader
 might be led to suppose from the perusal of the preceding pages.
  The influence of inspiration has been estimated by our author, perhaps justly, as  only
 sufficient to raise  a column one  inch ; if this force acted through rigid tubes of the length
 of the veins, it would produce no movement of ihe contained fluid ;  but acting through pliant
 tubes, it would rise one inch of the blood out of the vein nearest the heart, and if this power
 acted alone, its effect would here  cease.  But the vis a tergo, produced by the propulsive
 power of the capillaries and, perhaps, also of the  heart, prevents the collapse ;  the vein is
 kept full, and at every inspiration this power is renewed.
  The influence of the tendency towards a vacuum external to the lungs, and  around the
 heart, from the contractile disposition  or the resilience of the lungs, is admitted  by our
 author to be more considerable than the inspiratory effort, and it in  fact is, we think, greater
 than is suspected.  There are reasons  for  believing that  the lungs do not entirely fill the
 cavity in which they are contained  ; the influence of this space is therefore constant, though
 greater during inspiration, and of course diminished during expiration.
  The capillaries, our author has most  satisfactorily shown, have a vital expansive power ;
 and though he does not assert that the  heart has no such power, he denies that it can  have
 any influence on the movement of the venous blood, since it must act through pliant tubes.
 This would be the fact if the expansion of the heart were the only moving power, but the
 vis a tergo prevents their collapse,  and the effect of the expansive power of the heart, what-
 ever that may  be, is allowed to act.                       _.,,_.     . ,   r ,  .    .
  While therefore, the action of the capillaries and perhaps of the left ventricle of the heart,
must be considered as the main  forces  by which the blood is propelled through the veins,
the expansive  power of the heart—respiration and the resilience of the lungs, or atmo-
spheric pressure—ought to be viewed as accessary forces, though their precise power  cannot
readily be estimated.—Am. Ed.                                 _             svinWn
  +  The effect of cupping-glasses in preventing the absorption of poisons has been  snown
by Dr Pennock to be owing to mechanical pressure.  See his interesting experiments i»
the American Journal of the Medical Sciences, Vol. II.—Am. Ed. 
430
FLUIDITY IN RELATION TO ANIMALS.
 of life, able to overcome a great excess of mechanical resistance, instead of
 a passive phenomenon obeying an excess of mechanical force.   If a mere
 balance of pressures  acted at the orifices, as Dr. B. supposes, the blood and
 other fluids would be constantly oozing out from all orifices below the heart,
 as blood really does from an artificial opening, with force that  would fill a
 tube reaching as high as the  heart.   It would be good news for proprietors
 of mines, and other persons  having to raise water, if by taking off an ounce
 or two per inch of the atmospheric pressure at the top of a full pipe, the atmo-
 speric pressure continuing elsewhere would then force water in  at openings
 below and cause an upward current:—but in truth, to make the atmosphere
 efficient below, powerful steam-engines or other means must be used to take
 off a pressure above, of at least half an ounce per square inch, for every  inch
 in height the water has to rise.
   Another erroneous conception of atmospheric pressure, akin to that which
 we have been  considering, is expressed in the following reasoning on  the
 progress of blood in the veins.   *' The atmosphere presses 15 lbs. per square
 inch on all things; the blood  therefore, in a vein which has 20 inches of sur-
 face, is pressed upon, through the flesh, with a force of 20 times 15, or 300
 lbs., while a cross section of  the vein near the heart would measure less than
 one inch.  The blood, therefore, is  always running towards  the heart, to
 escape from a powerful excess of atmospheric pressure."—This paradox is
 solved by the law of fluid pressure, explained at page 131.  The same  rea-
 soning would prove that an eel-skin suspended by its lip, and filled with water,
 when exposed to the pressure of the atmosphere, should quickly be emptied;
 and nearly the same would prove  that a long sharp wedge thrown into water,
 should be always moving in a direction away from its point; and that a ship
 formed like the wedge, should make quick speed across the sea without either
 oar or sail.

 A knowledge of the facts detailed under the three heads of arteries, capillaries
     and veins, prepares us for the discussion of the following subjects.

                         The force of the heart.

   The arterial tension of four pounds to the square inch, marked by its sup-
 porting in a tube connected with the arteries, a column of blood ei«ht feet high,
 (see page 417,) is produced by the action of the  heart; but as the heart, while
 injecting the blood against this resistance, has moreover to overcome the inertia
 both of the quantity injected  and of the mass in the great artery, first moved
 by the injection, as also the resisting elasticity of the vessel which yields  to
 momentary increase of pressure, the heart must act with a force exceeding
 four pounds on the inch.   And as the left ventricle of the human heart, when
 distended, has about ten square inches of internal surface, the whole force ex-
erted by it is a matter of simple calculation.  It is remarkable, as  there is this
easy means of solving the question, that the accurate  Magendie, in his recent
elements of physiology, should speak of it as undetermined ; and  should cite,
as the best approximation, an estimate made from the obscure circumstance
 of a loaded foot shaking  in unison with the  pulse, when suspended  in the
cross-legged sitting attitude.
   Some physiologists have expressed surprise that the force of the heart should
 be so great as it is, remarking that much less would have sufficed  to propel
the blood to the most distant capillaries ; but they did not reflect that the heart,
besides carrying on the general circulation, has to force blood into those parts 
THE CIRCULATION.
431
of the flesh which, in the various positions of sitting, lying, standing, &c, are
for the time compressed by the whole weight of the body; for that, if it were
not strong enough  for this  purpose, either the compressed  parts, deprived
of their nourishment,  would quickly die, or the person, oblioed to be every
moment changing position, could obtain no lengthened repose.  In illustra-
tion of this point, we may advert to the frequent occurrence, in diseases where
the power of the heart is for the time weakened, of sloughings, or bed-sores
in the bearing parts, causing many cases of illness to terminate fatally which
would otherwise soon have terminated in health.—The author of this work
has had great satisfaction in suggesting a means  of entirely  preventing such
deplorable termination, namely, that vvhich he is now about to describe under
the title of

                   The Hydrostatic  Bed for Invalids.

  In many of the diseases which afflict humanity,  more than half of the suf"
fering and danger is not really a part of the disease, but  the  effect or conse-
quence of the confinement to which the patient is subjected.   Thus a fracture
of the bone of the  arm is as serious a local injury as a fracture of one of the
bones of the leg; but the former leaves the patient free to go about and amuse
himself, or attend to business as he wills, and  to eat and drink as usual—in
fact, hardly renders him  an  invalid; while the latter imprisons the patient
closely upon his bed, and brings upon  him, first, irksomeness of the conti-
nued position, and  then the pains of the unequal pressures borne by the parts
on which the body  rests.  These, in many cases of confinement, disturb the
sleep and  the  appetite, and  excite fever, or such constitutional  irritation as
much to retard the cure of the original disease, and not unfrequently to pro-
duce new  and more serious disease.   That complete inaction should  prove
hurtful to  the animal system, may by all be at once conceived; the operation
of the continued local pressures will be understood from the following state-
ments.  The health, and even life, of every part of the animal body, depend
on the sufficient circulation through it of fresh blood, driven in by the force
of the heart.   Now when a man is sitting or lying, the parts of his flesh com-
pressed by the weight of the body, do not receive the blood so readily as at
other times; and if  from any cause the  action of his heart has become weak,
the interruption will both follow more quickly and be  more  complete.  A
peculiar uneasiness soon arises where the circulation is  thus  obstructed, im-
pelling the person  to change of position ; and a healthy person  changes as
regularly,  and with as little reflection, as he winks to wipe and moisten his
eyeballs.   A person weakened by disease, however, while he generally feels
the uneasiness sooner, as explained above, and therefore becomes  what is
called restless, makes the changes with  much  fatigue; and should the sensa-
tions after a time become indistinct, as in the delirium of fever, in palsy, &c,
or should the patient have become too weak to obey the sensation, the com-
pressed parts are kept so long without their natural supply of blood that they
lose  their vitality, and become  what are called sloughs or mortified parts.
These have afterwards to be thrown off, if the patient survive, by  the pro-
cess  of ulceration, and they leave deep holes, requiring to  be filled  up  by
new flesh during a tedious convalescence.  Many a fever, after a  favourable
crisis, has  terminated  fatally from this occurrence of sloughing on the back or
sacrum; and the same termination is  common in  lingering consumptions,
palsies,  spine diseases, Sic, and  generally in diseases  which confine the
patients long to bed. 
432   ,
FLUIDITY IN RELATION TO ANIMALS.
  It is to mitigate all, and entirely to prevent some of the evils attendant on
the necessity of remaining in a reclining posture, that the hydrostatic bed is
intended.   It was first used under the following circumstances.
  A lady after her confinement, which occurred prematurely, and when her
child had been  for some  time  dead, passed through a combination and suc-
cession of low fever, jaundice, and slight phlegmasia dolens of one leg.  In
her state of extreme depression of strength and of sensibility, she rested too
long in one posture, and the parts of the body on  which she  had rested all
suffered:  a slough formed on the sacrum, another on the heel; and in the
left hip, on which she had lain much, inflammation began,  which terminated
in abscess.  These evils  occurred while she was using preparations of bark,
and other means, to invigorate the circulation,  and while her ease and com-
fort  were  watched over  by  the affectionate assiduity of  her mother,  with
numerous attendants.  After the  occurrence, she  was placed upon the bed
contrived for invalids  by  Mr. Earle, furnished  for this case with pillows of
down and of  air of various sizes, and out of its mattrass portions  were cut
opposite  to the sloughing parts;  and Mr. Earle himself soon afforded his
valuable aid.   Such, however, was the reduction of the powers of life, that
in spite of all endeavors, the  mischief advanced, and about  a week later,
during one night, the chief slough on the  back was much enlarged, another
had formed near it, and a new  abscess was proceeding in the right hip.  An
air-pillow  had pressed where these  sloughs appeared.   The  patient was at
that time so weak that she generally fainted when her wounds were dressed;
she was passing days and nights of uninterrupted suffering, and as all known
means seemed insufficient to relieve her, her life was in imminent danger.
  Under these circumstances, the idea of the hydrostatic bed occurred to me.
Even the pressure of an  air-pillow had killed her  flesh ; and  it was evident
that persons in such a condition could not be saved unless they could be sup-
ported without sensible  inequality  of pressure.  I then reflected, that the
support of water to a floating body is so uniformly diffused, that every thou-
sandth of an inch of the inferior surface  has, as  it were, its own separate
liquid pillar, and no one  part bears the load of its neighbour—that a person
resting in a bath is  nearly thus  supported—that this patient might be laid
upon the surface of a bath over which a large sheet of the water-proof India-
rubber cloth were previously thrown, she being rendered sufficiently buoyant
by a soft mattress placed  beneath  her—thus would she repose on the  face of
the water, like a swan on its plumage, without sensible  pressure anywhere,
and almost as if the weight of her body were annihilated.   The pressure of
the atmosphere on our bodies is of fifteen  pounds  per square inch of its sur-
face, but because uniformly  diffused, is not felt,   The pressure of a water-
bath of depth to cover the body, is less than half a pound per inch, even on
the under side where it  is greatest, and  is similarly unperceived.  A  bed,
such as then planned, was immediately made.  A trough of convenient dimen-
sions (6 feet long, 2 feet 8 inches wide, and 11 inches deep,  are good com-
mon dimensions) was lined with metal, to  make it water-tight; it was about
half filled with  water, and over it was thrown a sheet of the India-rubber
cloth as large as would be a complete  lining to it if empty.   Of this sheet
the edges, touched with  spirit varnish to  prevent  the water creeping round
by capillary attraction, were afterwards secured in a water-tight manner all
round  to the upper  border  or top  of the trough, shutting in the water as
closely as if it had been in bottles, the only entrance  left being through an
opening at one corner, which could be perfectly closed.  Upon this beautiful
dry sheet a suitable mattress was  laid, and constituted a bed ready to receive 
THE CIRCULATION.
433
its pillow and bed-clothes, and  not distinguishable from a common bed but
by its most surpassing  softness or yielding.  The bed was  carried to the
patient s house, and she was laid upon it;  she was instantly relieved  in  a
remarkable degree:  sweet sleep came to her;  she awoke refreshed; she
passed the  next night much better than usual; and on the following day, Mr.
Earle found that all the sores had assumed a healthy appearance ; the healing
from that time went on  rapidly, and no new sloughs were  formed.  When
the patient  was first laid upon the bed, her mother asked her where the down
pillows, which she before had used, were to be placed; to which she answered,
that she knew not,  for that she felt no pain to direct: in  fact, she needed
them  no more.
  It may be here recalled to mind, that the human  body is  nearly of the
specific gravity of water, or of the weight of its bulk of water, and therefore,
as is known to swimmers, is just suspended  or upheld in water without exer-
tion, when  the swimmer rests tranquilly on his  back with his face upwards.
He then displaces vvater equal to his own body in weight as well as in bulk,
and is supported  as the  displaced water would have been.   If his body be
two and a half cubical feet in bulk, (a common size,) he will  just displace two
and a half  cubic feet of water,  equal in weight  to  his body.  If, however,
instead of displacing the water with his mere body, he  choose to have some-
thing around or under him which is bulky with little weight, as the mattrass
of the bed  above described, then,  after his weight has forced two cubical feet
of that under the level of the water around, he will float with four-fifths of his
body  above the level, and will sink much less into his  floating mattrass than
a person sinks in an ordinary feather-bed.   It thus appears  that by choosing
a certain thickness of mattrass,  and if unusual positions  are required, by
having different thicknesses in different parts, or by placing a bulk of folded
blanket or  of pillow over or under the  mattrass  in certain situations, any
desired position of the body  may  be  easily obtained.  If  the water be about
six inches-deep, which in general will  suffice, the person standing upon any
part of the  bed, or sitting with  the knees  raised, will  cause the  part of the
mattrass on which he rests gently to touch the bottom, because a narrow end
of  the body cannot displace  water equal to the  bulk of the whole, but  even
then the person is as if standing or sitting on a soft sofa.  If it be desired to
prevent the mattrass, when 'used as a seat, from  touching the bottom, the
object may be attained by having under its middle a broad band or  strap fixed
to one edge of the trough, and connected with the  other by buttons or other-
wise, so as to be tightened to allow  the mattrass to descend just  so far, and
no  farther.
   This bed is a warm  bed, owing to  water being nearly an  absolute non-
conductor of heat from above downwards, and owing to its allowing no pas-
sage of cold air from below.   From this last fact, however, less of the per-
spiration, sensible and insensible, is carried off by the air than in  a common
bed, and unless the patient can leave  the bed daily to let it be aired like a
common bed, there  will be a  necessity for ventilation  to prevent  the perspi-
ration from being condensed  on the  water-sheet below.  This ventilation  is
perfectly obtained by placing under the mattrass, arranged like the bars of a
gridiron  small flexible tubes  of tinned wire, wound spirally, with their ends
open to the atmosphere, either directly or  through two larger tubes crossing
and connecting their extremities near the  ends of the  mattrass, and then
issuing at the corners of the bed from under the clothes,    rh.s.bedI  is; in
itself as dry as a bed can be,  for the  India-rubber cloth  (of which  bottles can
be made) is quite impermeable to water, and  the maker is now  preparing 
434
FLUIDITY IN RELATION TO ANIMALS.
cloth expressly for this purpose.   Then, as Sir Humphrey Davy recommend-
ed that his safety lamp should be double,  some persons may prefer a double
sheet, to obviate the possibility of accident.   Unlike any other bed that ever
was contrived, it allows the patient, when capable  of only feeble efforts, to
change  his position, almost like a person swimming, and so to take a  de-
gree of exercise, affording the kind of relief which, in constrained positions, is
obtained by occasional stretching, or which an invalid seeks by driving  out
in a soft-springed carriage.  It exceedingly facilitates  turning for the purpose
of dressing wounds, for by raisingvone side of the mattrass or depressing  the
other, or merely by the patient's extending a limb to one side, he is  gently
rolled over, nearly as if he were simply suspended  in water;  and it is pos-
sible  even to dress  wounds, apply poultices, or place vessels  under any
part of the body, without  moving the body at all; for there are some  inches
of yielding water under the body, and  the elastic  mattrass may at any part
be pushed down, leaving vacant space there, without the support being less-
ened for the other parts.  Then, with all  the advantages which other  inva-
lid beds possess, and with those  which are entirely its  own, it may  yet be
made so cheaply, that even in hospitals, where economy must prevail,  it may
at once  be adopted for many of the bed-ridden.   Mr. Earle, within  a few
days of seeing the first one, had others made for patients in St. Bartholomew's
Hospital, and  has  been as much  pleased  with the results of them as  of the
first.  The bed has since  been introduced  into St. George's Hospital by Mr.
Keate, and elsewhere.—The author has now seen enough of the effects of
this bed to make him feel it a duty  at once to publish a notice of it.   With
it, evidently, the fatal termination called sloughing, now so common, of fevers,
and other diseases  need never occur again.   And not only will it prevent that
termination, but by alleviating the distress through the earlier  stages, it may
prevent many cases  from even reaching the degree of danger.   Then it is
peculiarly applicable to cases of fractured bones, and other  surgical injuries;
to palsies, diseases of the  hip joint, and spine; and universally, where per-
sons are obliged to pass much time in bed.  And in all cases of curvature of
the spine, either actually existing or threatened, it affords a means of  laying
a patient in any desired position, and with  any degree of pressure incessantly
urging any part of the spine back to its place.  .If used without the mattrass,
it becomes a warm or a  cold bath, not allowing the body, however, to be
touched by the water;  and in India, it might be made a cool bed for persons
sick or sound, during the heats  which there prevent sleep and endanger
health.  There are numerous  other professional adaptations and modifica-
tions of it, which will readily occur to practitioners  sufficiently versed in  the
department of natural philosophy (hydrostatics) to which it belongs.  Before
reflection, a person might suppose a resemblance between  it  and an air-bed
or pillow, calling this a water-bed or pillow; but  the principles of the two
are  perfectly distinct or opposite.  An air-pillow  supports  by the tension of
the surface which  encloses the  air,  and is  therefore like a  hammock  or  the
tight sacking  under  the  straw mattrass of a  common bed, and  really is  a
hard pillow ;  but in the hydrostatic bed, there is no tense  surface or  web at
all;  the patient is floating upon the water, on which a loose sheet is lying,
merely to keep the mattrass dry, and every point of  his body is supported by
the water immediately beneath it.  To recall the difference here described,
and which is of great importance, the bed  is better described by the appella-
tions of hydrostatic bed or floating bed than of water-bed.
  The author has  given no exclusive right  or  privilege to  any person  to
make this bed.  He  has hitherto employed  the carpenter nearest to him, 
THE CIRCULATION.
435
Mr.  Smith,  253 Tottenham-court Road,  at the back of Bedford  Square;
Mackintosh Si Co., 58 Charing Cross, the manufacturers of the cloth ;  and
Mr.  Williams, 25 Cleveland  Street, Fitzroy Square; but any carpenter or
upholsterer may learn to supply them, and he gives free  permission to all.
He hopes, for the sake of the poor, that a trough, without metallic lining, and
with a cheaper water-proof cloth, may be found to answer satisfactorily.
  The principle of the hydrostatic  bed is applicable, also,  to couches for
invalids, and with  certain considerable modifications, to the construction also
of chairs ; and there are other means than the water-proof sheet of  adopting
the hydrostatic principle for all—but the  subject has already occupied its full
share of this  volume.
  The preceding paragraphs are intended as much to direct in the choice and
use of common beds  for the sick, as to announce and describe the hydrosta-
tic bed for the cases  in which it may be required.   At present the "medical
attendant generally leaves whatever regards the bed to the judgment of friends
or nurses; but evidently, he  who has been  led  to reflect how  much the
course and event of a malady may depend  on the  patient's being supported,
so that no pain shall  arise  from local pressure, and as little muscular weari-
ness as possible from constrained position, will deem the bed-management
worthy of his own attention, and will be able more judiciously both to choose
and  to use beds.   There is a bed constructed of spiral springs, which may
be made so as to  diffuse  the support  more equably than any except the
hydrostatic bed; and  had professional men generally been acquainted  with
it, it would have been more used than it  is, and would have received various
modifications, of which it is  susceptible, for medical  purposes.  It  has long
been known, chiefly, however, as a mechanical  curiosity, or an object of
luxury, and  was introduced into  this country about seventy years ago by
Mr.  Merlin ;  but it has been so little known, that a few years ago an English
tradesman thought he might  appropriate the manufacture by taking a patent
for it.   It is  now made by  upholsterers generally, and the same principle is
applied in the construction  of sofas, chairs, and carriage cushions.

                 The velocity of the circulating blood.

  This has been much  overrated.  1st.  By  assuming that the ventricles of
the heart are  both completely filled from  the auricles and emptied towards the
arteries at each  pulsation:—an assumption  disproved by inspection of the
exposed heart of a living  body, and by the fact of the valves between the
auricles and ventricles not closing so perfectly as quite to  prevent regurgita-
tion.   2d. By supposing the  issue of blood  from  a wounded artery or vein
to be the measure of  the usual velocity.   Now it would  be as reasonable to
suppose the issue of water from  a wounded  pipe connected with any reser-
voir  to be the measure of a continued current  in that pipe,  although,  in truth,
the issue would be the same  even if the water in  the pipe were usually at
rest.  3d.  By supposing the frequency of the pulse to be a measure.  Now
we know, that in diseases of debility, and  in animals bleeding to death, the
pulse usually becomes more frequent as it becomes more feeble, and as  there
is le<s blood  moving: viz., the heart very  partially discharging its  contents
at each contraction.   4th, and lastly.  By supposing the strength of the
pulse to be the measure.    Now we find  that  the pulse in an artery just tied,
and in which, consequently, there is no current  at  all, is scarcely weaker
than  in an open artery.  The  common fact of a .arson's feet remaining stone-
cold  for hours, although the arteries leading to them pulsate nearly as usual, 
436
FLUIDITY IN RELATION TO ANIMALS.
is a proof that exceeding little blood is passing through the capillaries at the4*1
time,  and that the strength of the pulse, therefore, is no measure of the speed
of the blood.
  The ventricles of the heart appear, under common circumstances, to throw
out about an ounce and a half of blood at every contraction—or about seven
pounds per minute.  Now if the body  contain about twenty pounds alto-
gether, as seems to be  the  case, the whole would circulate twenty times in
an hour.   This would give an average  velocity of about eight inches per
second in the aorta, but gradually less in  the smaller  arteries, because when-
ever  a vascular channel subdivides,  the branches taken collectively have
considerably  greater area than the trunk from  which  they  arise, and the
current diminishes  in  a corresponding proportion,—just as  the speed of a
river  is always less in the parts of the  channel which are deeper and broader.
The velocity in  the extreme capillaries is  found to  be often less than one
inch per minute.  In  the veins, the blood must  move more  slowly  than in
corresponding arteries, in proportion as  the veins are more  capacious than
the arteries.

                              The pulse.

  The opinion  which the ancients  held, that the arteries  contained vital
spirits or air and not blood, rendered the pulse, to them, a very mysterious
phenomenon; and  many  curious hypotheses  were  framed   to explain it.
These it would  now be unprofitable  to  detail.  Even  Harvey's grand dis-
covery of the circulation, however, has not rendered the subject so simple as
might have been anticipated.   The  following opinions now  exist, or have
lately existed, with  respect to the pulse.
  1st. The great majority of physiologists have believed that a tumefaction
is produced in  the  aorta by each jet of blood from  the heart, and  spreads
afterwards  as a wave  into  all  the arterial branches.   2d. Many have sup-
posed an extensive contractile action of the arteries themselves, corresponding
to that of the heart.   3d. Bich&t,  unable  by any means to detect the slightest
change of  diameter in the arteries  during pulsation,  but perceiving that in
many situations they were  at the time somewhat lengthened,  so that straight
portions became bent, and  portions originally bent, were bent  still more, held
that this  locomotion, or changing of place in the arteries, was  the cause.
4th.  Others have supposed the impulse of the heart's contraction to be trans-
mitted through  the  fluid  blood, somewhat  as  sound  is transmitted through
bodies generally, or as a blow struck on one end of a log of wood, is felt
distinctly by a hand applied to the other, although there be no visible locomo-
tion.  5th. Dr. Young, in the paper in the  Philosophical Transactions already
alluded to,  has shown that a sudden rush forward of the blood in the artery,
such as might be produced  by injection at one end of a rigid tube,  would be
felt by a finger applied to the artery, quite as distinctly as a tumefaction; and
he deems this occurrence to be a chief.cause of the pulse.   Dr. Parry, in
his work on the pulse, points to this almost exclusively as the cause.
  Now the truth  is,  that the pulse in the living  body does  not depend
exclusively upon any one of the particulars just noticed, but has all of them
as elements;  and its fluctuations  and varieties depend upon  proportions in
which these elements  are combined.   We shall review them  again to prove
this.
  1st. At each  jet of blood thrown  into the aorta, a tumefaction or  wave
must  spread from the  heart to the extremities; for it is evident, that if blood 
                           THE CIRCULATION.                      437

be at all  pushed into  the arterial system, it either must dilate it, or cause an
equal quantity to be expelled at the same instant from the distant extremities-
now as the passage of blood through the capillaries appears perfectly uniform',
there  must  be  an intermediate dilatation.  Dr. Parry and others should not
have denied this dilatation because they could not see it: for even if its advanc-
ing front were more considerable than it is, it passes with such velocity that,
like a cannon-ball crossing before the face, it would not be perceived.
  2d. Contraction of the arterial coats certainly does not take place in the
manner and to the extent supposed by some, who have spoken of it as resem-
bling the contraction of the heart itself,  and as what might be a substitute for
the action of the  heart in  propelling the blood ;  but, as  shown at page 414,
the rigidity of tube which in all degrees of arterial dilatation causes the pulse to
be transmitted so quickly, can  depend on  nothing but a  contractile action of
the fibres.   There are some reasons for doubting whether this rigidity may
not increase at the moment  of the pulse.
   3d.  Unless the arterial tubes were absolutely inelastic, which they are far
from being, they  must be lengthened a little by a sudden injection of blood,
and, therefore, at  all  the curvatures particularly, there must be  a degree of
the locomotion described by Bichat, often  sensible to a finger  applied.0
   4th. That a tangible shock is conveyed through a fluid without any appa-
rent accumulation of the fluid or change of velocity, and  much in the manner
of  sound, is proved  by the facts, that we may  discover the working  of  a
water-pump at very great distances, through iron pipes connected with it, and
even through elastic pipes of leather, as those of a common fire-engine, from
which the water is spouting, nevertheless,  in a uniform stream.   The pulse
in a tied artery, in which there is no current or rushing wave, must be chiefly
from this cause, and from the locomotion of the artery.
   5th. That any  additional quantity  of fluid injected  into elastic  vessels
already full, must spread all over with a forward rush, affecting the finger of
an  examiner, as described above, is also most certain.  As the heart,  how-
ever, often beats  without discharging  much of  its  blood, and as in  many
arteries,  from inaction of the  capillaries,  or  pressure, the  blood for a  time
makes little or no progress, while the pulse, however, remains very distinct,
the pulse in such cases must be produced independently  of the  forward rush.
An animal intestine prepared,  and filled with water or air, and laid upon  a
table—or a full vein in the living body, carries a rapid and distinct pulse to a
great distance when gently tapped  by the finger.  The  cause  of the sensa-
tion, then, cannot be the  simple forward rush without tumefaction, described
by  Dr. Young and Dr. Parry.
  In whatever  proportions  these particulars combine to form  the pulse, its
force will be proportioned to the size of the artery.  Hence as  an artery lead-
ing to an inflamed part becomes of greater calibre,  its  pulse also becomes
stronger.
  It is a remark respecting the  pulse, appearing to the author worthy of deep
consideration, that if  the purpose of the heart and arteries were merely the
propulsion and conveyance  of the blood, their  structure and  action would
form most signal deviations from the ascertained rules of fitness in mechanics.
In machines of human contrivance, it  is one of the most important  maxims
" to avoid shocks, or jerking motions ;" and in former parts of this work, we
have described fly-wheels, air-vessels, springs, &c, as means of accomplishing
this object, and thereby of preventing the tearing and straining  of parts which
would else  happen.  In the human bodv, also, we have to describe the ad-
mirable elasticity  of the spine, of the arch of the foot, of the cartilages of joints,
     29 
43S
FLUIDITY IN RELATION TO ANIMALS.
&c, as contrivances answering the same end ;  and to  remark that, in  other
cavities than the heart, which are alternately filled and  emptied like it, as the
stomach, bladder, uterus, &c, there is a smooth and  gradual action.   The
heart alone is the rugged  anomaly, which, from before birth unto the dying
moment, throbs unceasingly, and sends the bounding  pulse of life to every
part ; and which, moreover, instead of being  secured  and tied down  to its
place, is attached at the extremity of the aorta, like a weight at the end of
an  elastic branch of a tree, and every time that it fills the aorta, is thrown
with violence, by the consequent sudden tendency of that vessel  to become
straighter, against the ribs, in the place where the hand  applied, feels it so
distinctly beating.                            .
  Now one  use of the pulsation of the heart probably is, by the agitation and
churning which the blood suffers in passing through it, to keep in complete
mixture all the heterogeneous parts of the blood, which so  readily separate
when left to repose :—but this  cannot be the only use, for the  object might
have been more simply attained ; and we may conclude that the phenomenon
has relation  to some important law of life still hidden from  us.   The cause
commonly assigned for the heart's contraction  is the peculiar stimulus of the
blood ; yet if we reflect that the heart will beat after removal from the body,
and when it contains only  air, and that during life it beats with extraordinary
regularity, whether the state of the circulation allow it  to empty itself at each
beat or not,  we perceive  that the cause is more obscure.   We cannot con-
template this subject attentively without perceiving a strong analogy between
the action  of the heart and some electrical phenomena in which there  are
successive  accumulations and  exhaustions of power;  and, recollecting the
important relations  which late researches have shown to exist between elec-
tricity and certain other actions of life, the inquiry becomes  very  interesting.
Galvanism can excite the muscles to their usual actions ; it powerfully affects
the secretions and the digestive function ; and the breathing in asthma : strong
animal passion seems to produce  electrical excitement: and  certain animals
have the faculty of  stunning  their enemies by an electrical discharge.   The
pulse, then, in its sudden, strong and regular recurrence, may be a kindred
phenomenon.  In this view, there would be less difficulty in  supposing a
momentary  stiffening or slight contraction of the whole arterial system, such
as the sudden rising of the mesenteric arterial tree so readily suggests: if there
be such, however, it is still closely connected with, and proportioned to, the
action of the heart; for it occurs only with that action, it indicates any dis-
turbance in the action, and  as death approaches, it ceases in the remote extre-
mities  first.
  The  preceding considerations exhibit the pulse as a complex subject, and
one on which professional opinions are not yet settled.   By showing its close
relation to the powers of life, they also prove it to  be an  object of high im-
portance to  the medical practitioner.  This last truth has scarcely been ques-
tioned but by persons either utterly uninformed or singularly deficient in the
power of tactile discernment;  yet, because no simple and good analysis of
the pulse, and detail of its relation to morbid states, has been made and pub-
lished, the degrees of skill acquired by individual practitioners with respect
to it are very various,  and in a great measure accidental.  Some practitioners
try the  pulse merely  for form's sake, because patients expect it; many exa-
mine it only to count its frequency; but others read  in it, with  confidence,
much of the  history and probabilities of the disorder, and decide on  the treat-
ment accordingly.   Few who have attended to the subject at all, can confound
the pulses of such diseases, as  acute  rheumatism, gastric inflammation, the 
THE CIRCULATION.
439
 fits of ague, &c.  The author remembers to have conversed with a Chinese
 practitioner who had only the scanty medical information of his countrymen,
   \Z   lU S    Y the pulse with sing^ar penetration.
   1 he changing circumstances  in the state of the circulatory system, con-
 nected  with health  and disease, and discoverable by a finger watchino- the
 pulse, seem to be chiefly the following ; and the epithets added in italics, are
 those which seem best to indicate the sensations  perceived.  The artery  at
 the wrist is that generally chosen for examination, because it is not like others
 imbedded in soft  parts, having only the skin over it, and nothing between it
 and the bone below.
   1st. The number of the contractions of the heart in a given time, and the
 regularity of  their recurrence.—Pulse, frequent, slow, intermittent,  equal,
 regular,  of varying force.
   2d. The degree of the heart's contraction, or the quantity of blood ejected
 at each time; and the corresponding state of the capillaries  as to the quantity
 of blood  passing  through them.—Pulse, full, long,  labouring,  bounding,
 feeble.
   3d. The force of the heart's action, with the correspondent arterial tension
 or  rigidity.—Pulse, hard, sharp, strong, wiry, iveak, soft, yielding.
   4th.  The suddenness of the individual contractions of the  heart, and the
 rigidity of the vessels in conveying the shock.—Pulse, quick, tardy.
   5th.  The size of the artery for the time, whether larger or smaller than
 usual.—Pulse, large,  small.
   Superficial as is this sketch, it may show that a good treatise on the sub-
 ject of the  pulse, as  connected with  disease,  is yet a desideratum  in medi-
 cine.  The sort  of empirical but useful  tact  which  many persons acquire,
 is not fitted to satisfy  the physician  who reasons deeply,  and  whose mind
 should have always present to it the various  constituents  of the pulse, and
 all the important circumstances of health or disease related  to its indications.
 The  laboured  treatises of  Solano,  Bordeu, Boerhaave, Sic, may  treat  of
 what were clear  ideas to  their authors, but  by not  referring  the  physical
 causes  of many varieties, they become so obscure to others,«that  many  of
 the divisions  and denominations appear altogether fanciful.   Dr.  Young's
 excellent  paper in the Philosophical  Transactions, details important facts,  as
 far as it goes, but it was not intended to  point  out all the  pathological rela-
 tions.   Dr. Y., guided by general principles,  asserted a progressive motion
 of the pulse, while other authorities were holding it to be quite simultaneous
 over the  whole system.  He  might have  mentioned in  proof, that careful
 examination can practically detect a  succession of beats at different distances,
 particularly at the four stations; 1st, of the heart; 2d, in the lip; 3d,  at the
 wrist;  4th,  at the ankle:—but the interval  of time,  even  between  the
 extremes, being only  a  small  part  of a second, persons will  often  fail  to
 make their first experiment satisfactorily.   Dr. Parry's treatise on the pulse
 which is  the  last one of note, although  having excellences, errs—in attri-
 buting the phenomenon to one  cause too  exclusively—in denying arterial
 dilatation, because it was not discovered by his mode of  searching for it, in
 supposing that a liquid column in an elastic tube, can be made to  advance
 like a solid rod, or a line of billiard-balls.  The too common neglect of me-
 chanical  philosophy by medical  men, is  signally  proved, by our  finding  in
 works of  authority, published at  the present day, such statements as that the
arterial pulse may be more frequent or less frequent than the beatings of the
heart   Dr  Good (Study of Medicine) says, that there may be various fre-
quency of pulse in various parts of  the body at the same time: Richerand 
440
FLUIDITY IN RELATION TO ANIMALS.
(Physiologie) says, the pulse is  more  frequent in the artery leading to a
whitlow than at the same time elsewhere;  and  many practitioners share
these notions.  What a satire on  the medical profession is this disagreement
on a point which, to common observers, seems, above all others, to occupy
the attention of the attendant on the  sick!

Having now explained the circulation of the blood in  general, we proceed to
      consider some cases where  mechanical circumstances  modify it.

                         Circulation in the head.

  The head may be considered as an air-tight vessel or cavity of bone, con-
taining  chiefly brain and blood,  and having  openings occupied by blood-
vessels, leading  to and  from the  heart.  The  atmospheric  pressure, there-
fore, always keeps the head full,  as it keeps the top of a syphon full;  and
because the substance of the brain itself does not, more than  water, sensibly
change in bulk by any ordinary degrees of pressure, there  must always be
the same quantity of blood in the head,  how much soever the quantity may
vary in  the body generally.  Regard to  this important truth,  a knowledge of
which has followed the discovery  of the true nature of atmospheric pressure,
enables  us to explain many hitherto obscure facts, both in health and disease;
—as the following instances will  show.
  If, from any cause, the arteries  in the  head become too full of blood, in the
same proportion the veins must become too empty; or, if the veins  become
too full, the arteries must  be too empty; and in either case, the  circulation
in the head will  be in a corresponding degree impeded, because, when  one
part of a channel is narrowed or diminished, the  current  throughout the
whole is slackened.   Now, as  insensibility supervenes when the  supply of
fresh blood to  the brain  is interrupted, and  death follows if  the interruption
continue long, it seems evident that in many of the cases of apoplexy, where,
on inspection, there is found nothing but a fulness of the arterial or of the
venous  system of the  head, death has  happened merely because the circu-
lation was  arrested in this way.   In other parts  of the body, not  circum-
stanced  like the brain, an excess of blood in one set of vessels  may happen
without inducing deficiency in another,  and therefore with  perfect impunity
to the individual.
  Simple increase of pressure produced by the blood  on the brain, provided
the proper balance exist between  the quantity in veins and  arteries, has no
injurious effect.  This is proved by the  safe descent of a person in a diving-
bell, where, at thirty-four feet under  the surface of the  water, the  body is
bearing  an additional pressure of fifteen  pounds on  the square inch (see page
163,) which pressure  through the blood-vessels affects the brain as much as
any other part.—On the other  hand, when a man climbs a mountain, or is
lifted  in a balloon,  the brain is  less pressed than  usual; but the proper
balance in artery and vein being maintained, no inconvenience is felt.  The
inhabitants  of  some of the valleys among the Andes are as far above the sea
as they would be at the top of Mont Blanc, where the atmosphere presses
only half as much as on the sea-shore; but  they enjoy good  health.
  As the box of the cranium encloses  the brain  so as  to  leave  no vacant
space, it is evident, that when the heart injects blood  with unusual violence,
the  strain at first is borne cheifly by the cranium, and not by  the coats of  the
blood-vessels.   Hence, the arteries of the  brain need not be, and are not,
nearly so strong as those of other  parts of the body. 
THE CIRCULATION.
441
  The veins of the brain are also  peculiar.   Common  veins in the head
would, for  the reason above  given, collapse  by any sudden  tension of the
arteries there,  and if they did, insensibility or death would ensue, on account
of the consequent stoppage of the  circulation.  The chief channels, there-
fore, for the refluent blood, instead of being common compressible veins, are
what have  been  called sinuses, or grooves in the bone itself, with exceed-
ingly strong membranous coverings, supported so  that the channels become
in strength, and as to maintenance of their capacity, little inferior to complete
channels of bone.  This singular deviation in the  structure  of the  cerebral
veins  from  what  is found  elsewhere, and  without which deviation, animal
existence could not be continued, is one of those  particulars which powerfully
affect the contemplative  mind, as proofs of the designing intelligence which
has planned this glorious universe.
   From not adverting sufficiently to the fact  now explained,  of the cranium
being a vessel always full,  and which will hold only a certain quantity, mis-
conception has prevailed among medical men with respect  to  many of the
affections of the brain.
   It has been  said, for instance, that the substance of the  brain cannot bear
pressure with impunity, for  that stupor immediately follows pressure, how-
ever produced.   Now the  truth is,  that pressure produces stupor only when
it interferes with  the circulation.  In wounds with loss of a large  piece of
the cranium, the  brain will bear very rough handling, because, if compressed
at one part, it may bulge in another, and leave the circulation free ; but if the
wound be small,  pressure made through it instantly affects the whole brain,
and the blood is prevented from entering from the heart.   Let one reflect, for
an instant, on what happens to the foetal head during parturition—how often
it escapes elongated and bent, almost as if it were of soft clay—yet the child
lives  and thrives, and  the  natural form is soon recovered.   The reason is,
that the fcetal  skull is soft, and pressure in one  part is compensated for by a
bulging or extension in another, and the blood is not expelled.
   Water in the head, again, is said to kill by this fatal pressure on the tender
brain; but, in reality, it kills  by keeping out the blood, and  so mechanically
 arresting the circulation.   Accordingly we see,  that where the fontanelle still
remains open, or where the sutures or joinings  of  the skull will yield, water
 may  accumulate  to a great degree without causing  much disturbance.
   A tumour in the brain, which would be of no  consequence if the brain were
 unconfined, soon becomes fatal by occupying  room in  the  skull, and to the
 extent of its size excluding or checking the supply of blood.
   If the substance of the brain at all increase and diminish in bulk, as muscles,
 &c, under certain circumstances, do, in the body below, all such changes must
 produce a considerable effect on the cerebral circulation and functions.

                    Effects of position on the circulation.

   While a man  is in a standing attitude, the  heart and  arteries have to send
 the blood  up the head against gravity:  but in the horizontal position, the
 blood, if  equally propelled, must  arrive with greater force, because  gravity
 then does  not resist.  Hence headache, or other symptom arising from fulness
 of blood in the arteries of the head, is often relieved by the  upright position,
 and is increased by lying down.
   Manv people  who have  had a slight degree of toothache during the day, find
 it intolerable when they lie down at night, and are relieved again by rising
 and  walking about.   Commonly they suppose that it is the cold of Ae night 
442
FLUIDITY IN RELATION TO ANIMALS.
which then lulls the pain; but it is in fact the change of position.  The author
knew a lady who was  obliged  to sleep for months in  the  sitting posture,
because she had a tic  doloureux in the face whenever  she lay down; and
another who was under the same  necessity for a considerable period after an
inflammatory affection  of the  brain,  because if her head fell low during sleep
she was immediately assailed by a  terrific dream of swords driven into the
brain.
  Delirium in fever is sometimes checked  at once by elevating the head.  On
account of the great relief thus  obtained, some  continental practitioners had
proposed to support the patients occasionally in an upright posture.
  Apoplexy has often been brought on by a man bending his  head down in
the act of tying his shoe,  or of pulling on his boot.
  Children and professed  tumblers  being  much in the habit  of  placing their
bodies in all positions, feel no inconvenience from  having  the head down-
wards ; apparently, because arteries  and  veins usually become strong enough
to bear the pressure to  which they are habitually exposed; but  to many old
people, accustomed to keep the  head always up, the attempt would be fatal.
  Ulcers on  the legs  are often obstinate  and will bleed, because  the veins
about them are too weak to support  the lofty columns of blood above.  Hence
the frequent counsel given in  such cases to keep the feet raised upon a chair,
and the utility of certain modes  of bandaging.
  Many inflammations of the legs and feet become exceedingly painful when
the limbs  are in a hanging position, and the pain  is relieved  by laying  them
horizontally.
  Many anasarcous or  dropsical affections of the legs increase towards night,
because, during the dependent position of the legs through the day, the absorb-
ents have not power to lift the fluid.  The swelling  disappears again before
morning.
  When the heart has  to send blood upwards, it requires to act more strongly
than when the body is  horizontal, and the  pulse increases five or six beats in
the minute: hence the  common rule to make a patient with haemorrhage lie
in the horizontal position, that  the  heart may become tranquil and allow the
bleeding to cease.

               Fainting from  diminished arterial tension.

  Fainting, which is  a  temporary cessation of the action of the heart, and
hence, as explained above, of the action of the brain for want of blood, is pro-
duced by several causes, and  among others, by any occurrence which renders
the blood-vessels about the heart suddenly less full or tense  than usual.  It
would appear that the heart being accustomed, when  ifcontracts, to a certain
degree of resistance, has its  action disturbed when  the  resistance is much
diminished.
   Thus haemorrhage, from any cause, by lessening the general  tension of the
sanguiferous system, often  causes fainting.  The state  is relieved by lying
down; probably because the still remaining weaker action of the heart is
sufficient to send blood to the head along a horizontal course, until the gradual
contraction of the whole vascular system reproduces the  tension necessary to
perfect action.  A small quantity of blood taken  away suddenly, affects the
circulation as much as  a larger  quantity taken gradually, apparently because
a certain space of time  is required for the gradual lessening of the vessels.
  The operation of tapping for dropsy in the abdomen would often bring on
fainting, but for the precaution of tightening a broad  bandage upon the body 
THE CIRCULATION.
443
as the water flows.   The reason is, that the sudden removal of a large quan-
tity of fluid which had been compressing all the abdominal vessels, and keep-
ing them perhaps only half full of blood, allows them again suddenly to receive
their natural quantity, and thus produces  a relaxation  of the other parts of
the  vascular system.
  Sudden parturition often causes faintness for the same reasons.
  Even rising up suddenly from a horizontal  position will cause an  approach
to fainting in weak people, or in those who have been long bed-ridden: pro-
bably because the heart having for a time been accustomed to send blood only
in a horizontal  direction to the head, does not in an instant exert  the addi-
tional power required to lift an  upright column with equal force;—besides,
that the blood does  not  then  return to the heart by the veins, from  the infe-
rior parts of the body, so readily as before.
  These various facts,  now  easily understood, from the reason of  a rule
which  is a great modern improvement in the  practice of the healing art, viz.,
in bleeding for  the cure of inflammation,  to take  the blood  away as quickly
as possible.   This subject deserves a little farther consideration.
  A great proportion of dangerous diseases  involve inflammation  of some
vital organ ; and inflammation consists chiefly, as already stated at page 422,
of a gorging or over-distension of the capillary vessels in the part.  The
nature of the capillaries, again, is such  (page 422) that when not maintained
constantly full by the pressure  of the heart behind  them, they gradually, by
their own  action, empty themselves towards the veins—as is seen  in the
disappearance of a local inflammation soon after the death of the person, or in
the fact of the  arteries  being emptied of blood after breathing ceases, Sic
Now ever since medicine deserved the name of  an art, practitioners have
accounted the lancet their sheet-anchor in inflammatory disease; but it is only
in late  times, since  the  circulation  of the blood was understood,  that they
have known the rationale of the remedy, viz., that it acts by diminishing
vascular tension, and hence the action of the  heart, and so allowing the small
vessels to empty themselves  by their own force, and to  recover sufficiently
to resist the return of an excessive load.   It is still more lately that  they have
understood  how much  more suddenly  and  completely the  disease is cured
by  abstraction of a small quantity of blood so rapidly as  to produce fainting,
than of a much larger quantity  so slowly that only weakness follows.   Judi:
cious treatment now cures inflammation much more certainly and completely
than was done  formerly, yet with much smaller loss of  the precious blood,
and with less  danger of those diseases  of  weakness, or of  that  complete
breaking-up of the constitution, which  often  follow great depletion.   To
induce faintness, large openings are to be made into  the veins—sometimes
into two veins  at once, and the patient is kept in the upright attitude.  Often
thus an inflamed  eye, which was  red as scarlet before bleeding, in a few-
minutes is rendered nearly of  the natural appearance; and intense internal
inflammations, as of the brain,  lungs, bowels, Sic, which if neglected would
be  shortly fatal, are removed in the  same manner.  In all these  cases  the
faintness seems to be  almost  equally  efficacious, whether it happens after
the loss of ten ounces of blood, or of  fifty; or even, as sometimes occurs,
when  it happens without bleeding at all,  after merely tying the arm  in pre-

P Reflection upon these circumstances  led the author to think that, in certain
case*  the beneficial effects of blood-letting might be attainable by the simple
mean's of extensive  dry cupping, alluded to at page 176; that is to say, by
diminishing the atmospherical pressure on a  considerable part of the body, on 
444
FLUIDITY IN RELATION TO ANIMALS.
the principle of the cupping-glass used very gently, and thus suddenly remov-
ing for  a  time from about the heart, a quantity  of blood, sufficient by its
absence to produce faintness.   The results of trial have been such as to give
great interest to the inquiry, and  the author's leisure will  be devoted to the
prosecution of it.—An air-tight case  of copper or tin-plate, or of air-tight
cloth kept extended by hoops,  being put upon a limb, and made close  by a
suitable collar  tied at the  same time round its mouth and the limb,—on part
of the air  being then extracted  by a suitable syringe, in an instant the vessels
all over the limb become  gently distended with blood; and  as the blood is
suddenly taken from the centre of the  body, faintness is produced, just as by
bleeding from a vein.   The excess of blood may be retained in the limb as
long as  desired;  for the circulation is not impeded.  To produce a powerful
effect with a slight diminution of pressure,  more than one limb must be
operated upon at the same time.
  An instrument resembling the  contrivance now described, was proposed
about twenty years ago by a non-professional person, as a means of drawing
all sorts of diseases out of the  body through the pores of the skin.  He en-
closed  a leg in an  air-tight case;  he then admitted steam to heat  the  limb,
and relax  the pores of the skin, as he  said, and  then he worked an air-pump
to draw out the disease.   He  called the engine the air-pump vapour bath.
In various cases where its true action was desirable, although not understood
by the  proposer, nor judicially managed, it proved beneficial.
  The operation of applying tourniquets or bandages round the limbs, so as
to influence the transmission of the blood,  affects  the action of the heart.  It
:s said  sometimes to have prevented the accession of ague.  It is a means
akin to  those above described.
  Because arteries are stronger and more tense than veins,  a  bandage may
be put  round a limb, tight enough to close the veins but not the arteries, and
the limb will then swell beyond the ligature.   By thus putting tight elastic
bandages round all the limbs at once, and immersing them  in warm water to
favour the dilatation of their vessels, so much blood may be suddenly detained
in them as to cause the person  to faint.  Such a means, therefore, might also
be used remedially.
   In the same way, a tight handkerchief, or stock round the  neck, will often
retain the  venous blood in the head, and cause  apoplexy.—Strong pressure
made on the jugular veins kills as certainly as if made on the windpipe.
   When a hernia  or other tumour is  strangulated, it swells in the  manner
above described, and if not relieved, soon mortifies.
   Diffused pressure, like that made by rolling a bandage round  a whole limb,
or by immersing the limb in fluid,  must affect the  circulation.   The veins
will be  more compressed than  the arteries, by reason of the distendino- force
in them being less.   Varicose veins, therefore, are  usefully supported by a
bandage or  laced  stocking.   The reason why this  manner of  supporting
assists  so powerfully in the healing of ulcers on  the legs, may be,  that the
support affects the capillaries and absorbents as well as the larger vessels.
   Poultices, by  their weight,  produce a soft compression of the parts on
whicli  they are applied;  and in certain cases, may  benefit by mechanically
squeezing the excess of blood  out of weakened vessels.
  The author  has  relieved the chronic inflammation of a sprained ankle, by
ordering the foot and leg, covered with an oiled-silk stocking, to be enclosed
in a boot strong enough to support the pressure  of quicksilver, which was
then poured into the boot.  The effect is a pressure by the fluid metal on the
weak vessels, of one pound  to  the square  inch, for every two  inches of the 
                       RESPIRATION AND VOICE.                    445


Seated ^ *! Pait-4 heiSht 0f four or five inches giv<* the
Aether  To htnH     Ch grCater elevation would st0P the circulation alto-
gether.   No bandage can press with uniformity approaching to this action of

is Jxplicatle^^ri111^ Pressure>in removing tumours of various  kinds,
^iwu*™™?* PnnC,P>  The aUth°r d0ubtS "0t that in ^
^at Present  in P n Y man^ed: would P™e a more valuable remedy than
is at present  generally  supposed.   The elastic steel half-hoop, with  one
irs^rean,an°lh,er behiml' latdy introdllced f°' ^e relief ofheZ,
nfh „ ♦      "I1' mrd6' ? C6r,ain Cases' of ProducinS a uniform pressure
of the nature spoken of, and of any desired force.
  When a man stands in  a bath, with the water up  to his chin, there  is a
pressure ofthe watei-upon his body, proportioned every where to the depth,
(see page idU.)   1 his pressure  must produce a considerable effect on the
blood-vessels of the lower parts  of the  body.  We  see in  this that  a bath
must prope. the blood from all other parts of the body, towards the cavity
of the chest, which  the pressure cannot reach.   It  is this effect which in
part causes  the feeling of thoracic oppression  experienced by persons on
first plunging into water, which feeling is usually attributed altogether to the

  The old practice of placing a  patient  in a  pit, and surrounding his body
with earth or sand, must have had a mechanical action of the kind'now con-
templated, in addition to any other influence.
  Transfusion of blood from a  vein of a  healthy person into  that of one
fainting or dying from haemorrhage, is an operation the converse of some of
those mentioned  above.   It  has  been frequently performed  with success.
The cases to which it seems best  fitted, are those of flooding after parturition,
and of wound; and there  can be no  doubt that  many of the lives lost from
these so frequently recurring causes,  might be saved  by its adoption.  The
blood to be injected is received into a vessel, as in common bleeding, from
which vessel, by a fit syringe, (to be described in a future  page,) it is trans-
ferred, as it flows, into an opened vein of the patient.   The admission of air
with the blood would be fatal, and has therefore to be  most carefully guarded
against.  The last interesting report upon this subject is that of Dr. Blundell,
in his Physiological Essays.
                         RESPIRATION  AND VOICE.

 The doctrines of fluidity, illustrating  and illustrated by the  animal respi-
                            ration and voice.

   As the motion of a windmill depends altogether on the breeze to which its
 vanes are exposed, so does the motion and the life of that most wonderful of
 structures, the animal body, depend on the supply of air for  its breathing.
 If this supply be withheld but for a few  moments, painful convulsions ensue;
 and if for a still longer period, the  body, however perfect and beautiful, is
 made a lifeless corpse, soon to  putrefy and be decomposed.
   The mechanical nature of air, as  to its lightness, elasticity,  &c, and  the
fact of its forming an ocean around the earth of about fifty miles high, are
now well understood, and have been fully explained under pneumatics ;  but
the precise  nature of its life-sustaining action has yet to  be elucidated by
farther  research of chemists  and  physiologists.   Thus far,  however,  we 
446
FLUIDITY IN RELATION TO ANIMALS.
know—that the ingredient called oxygen, constituting a  fifth of the  atmo-
sphere, is the most essential part—that air, by being breathed once, is rendered
unfit  for farther respiration at the  time—and  that a man requires  about a
gallon per minute.  The enterprizing Mr. Spalding, who introduced the use
of the diving-bell, descended for the last time with a companion on the coast
of Ireland, when, owing to the  signal cord becoming entangled round the
great rope of the  bell, which had turned in descending, he could not  make
their want of air known above, and  both were found dead when the bell was
drawn up soon after, although  the water had not touched them.   Of a hun-
dred and forly-six Englishmen, who, in the year 1750, were made prisoners
at Calcutta, and were  thrown into the close dungeon, since  called  the  black-
hole, only twenty-three survived the few hours of their confinement, and one
of the most appalling  recitals of human suffering existing on record, is what
these persons  had afterwards to make.
   We know generally of the life-supporting action of air, that it consists  in
some  change operated by the air on the blood; and we know that the function
of respiration  has merely to bring air and blood together in the cavity  of the
chest, that this change may take place.   The  blood while in the chest is
moving along a part of its circle, in  vessels of extreme  minuteness and thin-
ness, and the  air at each  inspiration rushes in among these,  so that  every
globule of blood  passes within its  influence.   And the blood, which, after
having served  the  purposes of the  body,  arrives  at this part  of its course,
black  and impure, immediately after its  exposure  to the  air, enters the left
chamber of the heart, of a beautiful scarlet colour, and  thence  departs to
carry  new life to the general system.
   The minute vessels through which  the circulating blood is strained in the
chest, do not hang loose in the cavity, but  are supported by running through
spongy masses, called  the lungs, which  consist chiefly of these vessels and
of thin membrane formed into  cells.   The cells, at every inspiration, receive
fresh air through  the  cartilaginous windpipe which branches into  them, and
at every expiration, they return the changed air  by the same channels  to the
atmosphere.  The lungs of a  child, before birth, are pefectly collapsed, or
without the least air in their structure, and hence are dense  enough to sink in
water ;  but after breathing, they retain a portion of air,  and will float.  This
fact has been accounted a test of whether a child  had  been born dead or  alive ;
but because putrefaction, Sic,  will cause air to be  in lungs which have never
breathed, the criterion  may be fallacious.
   The chest is a large cavity, of form  approaching to that of a  common bee-
hive, bounded laterally by the  encircling ribs, behind the  spine, and  before
by the sternum, and divided below,  from the abdomen  or belly, by a strong
membranous  and  muscular expansion, called the  diaphragm.  The ribs, in
the natural state, hang obliquely downwards from their posterior attachments,
and on being raised in  front, they widen or increase the size of the cavity, as
already explained at 403.   The cavity is farther enlarged  by the  descent of
the diaphragm, which  may be regarded as both the floor of  the chest and the
roof of the abdomen, and which,  being convex upwards like a dome, by con-
tracting itself to a more flat condition, sinks out of, and enlarges the  chest,
while it descends into,  and diminishes the abdomen, or at least causes pro-
trusion of its sides.
   Now on the chest being enlarged  by the rising of the ribs  and descent of
the diaphragm, or by  either singly,  the  air rushes into it  through the  mouth
and windpipe, exactly  as air rushes  into a  common bellows through its pipe,
when the valve is shut and the two  boards are drawn apart; and air is again 
RESPIRATION AND VOICE.
447
expelled from the lungs by the contraction of the chest, as from the bellows
by the approximation  of the boards.  Into both cavities air enters, because
with the enlarging dimensions, the air which was within dilates, and becomes
less powerfully tense or resisting against the  external pressure of the atmo-
sphere, and so allows more air to rush in to restore the equilibrium.  The air
is expelled again  by the contraction of the cavities, because, by being com-
pressed, its elastic force or tension becomes greater than that of  the external
air, which it therefore easily repels, and so in part escapes.—By immersing
in water an India  rubber bottle, and then opening and shutting it, the entrance
and exit of fluid in this manner may be rendered very apparent.
  That the air admitted into  the chest should  have the fullest action  on the
blood passing there,  it was necessary that  the spongy mass of lungs in which
the blood-vessels  ramify, should occupy the  whole of the cavity, and be
equally distributed.  Now while the equable distribution is effected  by the
uniform elasticity or resilience which belongs to the structure of the lung, the
complete filling of the cavity is obtained,  not by general attachment between
the lungs and the ribs  or sides of the chest, as might be expected, but  by the
following  means, equally simple, and still more perfect.   The spongy mass
of the lungs is completely covered by a strong adherent membrane, called the
pleura, through which air cannot pass ; between this  membrane and a similar
lining of the  chest there is no  air or empty space, and therefore in the raising
and falling of the  ribs during  respiration,  this membrane remains always in
contact with  the  lining of the ribs, just as a bladder put into a bellows as a
lining, with its mouth secured around the nozzle, is filled  and emptied, and
remains in contact with the interior of the bellows, in all the states of dilata-
tion, as if there were  attachments in a thousand places.   This construction
allows the lungs  to  have a singular freedom of play during all the motions
of the body ; a freedom farther provided for  by their being  divided into five
portions or lobes, which slide  upon one another: of these, three occupy the
right side  of  the chest, and two with the heart occupy the left.   The right
and left sides of the chest are rendered cavities  quite distinct from each other
by the mediastinum, a strong membranous partition.  The mechanical dis-
position of the contents of the  chest, as now described, is productive of certain
consequences which it is important to understand;—for instance,
  If a wound  be made in one side of the chest so as to admit air, the lungs of
that side collapse in obedience to their weight and elasticity ; and as the chest
afterwards enlarges and diminishes in respiration, air more easily enters and
leaves the space around the collapsed lung, through  the wound, than it can
enter or leave the lung itself through the windpipe ; because, in the first case,
it has  no  force to overcome,  and in the  second, the elasticity, weight and
inertia of the lung oppose.  Thus the lungs of the wounded side become col-
lapsed  and useless.  If such a wound, therefore, were made in both sides of
the chest  at once, even without hurting any part within, the person,  unless
assisted, would die of suffocation.   The kind of resistance required in  such a
case, is first  to press the ribs  down so as  to empty the chest of  air as much
as possible, and then to keep the  wounds close  or covered while the ribs rise
again ; the air, of course, will then enter by the  natural road, the only one left,
to fill the chest, and  will distend the lung, and reach  the blood in the pulmo-
nary vessels as usual.   Then  by straining with the muscles of the chest, as
in the action  of blowing, and  at  the same time preventing  the  breath from
escaoino- by the mouth or nose, all the air which had entered by any wound
in the chest may be  expelled.   In Benjamin Bell's system of surgery  which
was long the manual of practitioners, counsel on this head was given the very 
448
FLUIDITY IN RELATION TO ANIMALS.
contrary of that required, and, of course, any patient treated according to  it
must have been lost.
  In cases of dangerous haemorrhage from a lung, caused by a wound in the
side, the proper practice is to allow the lung to collapse, as  now explained,
that  the haemorrhage may be checked;  and when the  danger is  past, the
treatment above described is to be adopted  to restore the natural play of the
lung.  Life may be supported for a long time by the lung in one side of the
chest.
  In cases of haemoptysis, or spontaneous bleeding from the lungs,  a disease
so often fatal, life might sometimes be saved or prolonged by making an open-
ing between two of the ribs, and  allowing the lung to collapse.  The affected
lung is often pointed out by the circumstances ; and the opening, when pro-
perly made, would be no more dangerous than in the case where, by a similar
opening, water or pus is discharged from the chest.
  The same operation has been tried as a forlorn hope in pulmonary con-
sumption.   This disease is often limited to the lung of one side, and as the
alternate stretching and collapse of the diseased lung during respiration, toge-
ther  with the contact of the air, powerfully prevent an ulcer there from heal-
ing,  or inflammation from subsiding, a new chance of recovery is  given by
allowing the diseased lung to collapse and remain at  rest.—Some cases are
recorded where cure is said to have followed this operation, and certainly,
where the circumstances are favourable for it, and where death must ensue
unless it can save, it is worth trying.
  When ribs are fractured, it is the practice to put a bandage round the chest,
so as for the time to prevent almost entirely the respiratory motion of the ribs,
and the breathing is then performed chiefly by  the rising and falling of the
diaphragm or floor of the chest, as above described.  Although a person with
broken ribs is wisely for a time subjected to the unnatural restraint, it is surely
the height of folly to inflict the same on healthy beings, as is yet, however,
so commonly done among young women, and often to the destruction of their
health, by the fashion of bracing the body  in tight stays.
  The force of a healthy chest's action in blowing is equal, as stated in last
section, to about one pound  on  the inch  of its surface; that is to say, the
chest can condense its contained air with that  force, and can, therefore, blow
through a tube,  the mouth  of which is  two feet  under the surface of the
water.   In the opposite action of sucking or drawing in air, the power is
nearly  the same.—In  both  actions it  is possible  to  use the cavity of the
mouth separately from that of the chest; and the mouth, being smaller, with
stronger muscles about it in proportion to its size, it can act more  strongly.
Some men can suck with the mouth so as to make nearly a perfect vacuum,
or to lift  water nearly thirty feet.   An expert operator with the blow-pipe
can keep up an uninterrupted blast by shutting  the mouth behind, while he
inhales, and replenishing it as is required in the intervals.
  In coughing, the glottis or top of  the  windpipe, by a curious sympathy
of parts, is first closed for an instant, during which the chest is compressing
and  condensing its contained air,  and  on the glottis  being then opened, a
slight explosion, as it  were, of the compressed air takes place, and  blows
out any irritating matter that may be in the air-passages; just, only with
inferior force, as the burst from the  chamber  of  an  air-gun  discharges its
bullet.—This  shutting of the glottis to allow the compression of the afr, and
the subsequent opening to  allow  the discharge, may  recur at very minute
intervals, and  many times for one fill of the chest, as is instanced in hooping-
cough.—The  action of coughing is often produced by irritation from a cause 
RESPIRATION AND VOICE.
449
which cannot be removed by coughing,  as inflammation of the chest,  or
tubercles; or even by irritation in a distant part, as when children are teeth-
ing, or when the stomach is overloaded.
   Sneezing is a phenomenon resembling cough,  only the  chest empties
itself  at  one throe,  and chiefly through  the nose, instead  of  through  the
mouth, as in coughing.   The irritation that produces sneezing is generally
in the nose; but, as  in the  case of cough, sneezing may occur from distant
sympathies ; witness that from worms in the bowels.
   Laughing consists of quickly repeated expulsions of air from the chest,
the glottis being at the time in a condition  to produce voice;  but there is not
between  the gusts, as in coughing, complete closure of the glottis.
   Crying differs from  laughing almost solely in the circumstance of the
intervals  between the gusts of air being longer.   Children laugh and cry in
the same breath, and it is often difficult to mark the moment of change.
   Hiccup is the sudden stopping, by a closure  of the glottis,  of a strong
inspiration at its commencement.   If  the inspiratory effort be afterwards
continued, it may cause air from the atmosphere, or half-digested food  from
the stomach, to enter the oesophagus.
   In  straining to lift weights, or to make any powerful effort, the air is shut
up in the lungs, that there may be steadiness and firmness  of the person.
At such a time, by the compression and condensation of air around the  heart
and large blood-vessels, the blood is determined violently outwards from the
chest, and often rises to the head, with force that produces giddiness, or even
apoplexy,—and the  eye will  sometimes become suddenly bloodshot, from a
small vessel giving  way; and  leech-bites  will break out afresh.—The  force
of this pressure outwards is measured, as  already stated, by  a column of
about two feet of blood; and this is, therefore, the measure of the additional
arterial and  venous tension in the body generally.
   Suffocation is the name given to what happens when the supply of air to
the lungs is, in any way,  prevented.   The blood not then refreshed by the
approach of the air,  rises to the brain unfit for its purpose, and  confusion of
thought is immediately produced, soon followed by convulsion and death.
   When this happens from mechanical obstruction at the narrow entrance
of the windpipe, as in croup, by the tenacious  films  thrown  off from  the
inflamed lining of the air-passages,  life  may be saved by  making a new
entrance for air through the windpipe, lower down in the neck,  and keeping
it free by a little tube  inserted, until the  obstruction above  be removed.
Where children die  with croup, it is frequently not from the  violence of  the
constitutional  disease, but  from detached  films thus accidentally sticking in
the narrow entrance of the air-passage.
   In  the cases  of strangling and  hanging, the tight  binding of the rope or
ligature crushes inwards the  cartilaginous  rings  of the  windpipe, and shuts
the air-passage.  It may also cause  apoplexy, by arresting the passage of
blood to  and from the head; and  there may be  dislocation  of  the cervical
vertebra? of the spine.                               .
   In  drowning, communication with the atmosphere is cut off altogether
by the supernatent water.  If, during  submersion, the chest  expands, it can
receive water only, instead of air.   The nerves and muscles, however, at the
entrance of the wind-pipe, are so  irritable, as to  be immediately excited by
the contact of anv unusual matter, and, for a considerable time, they keep  the
passage shut against the liquid seeking entrance.  It is partly  on this account
that  the body of a  person,  after submersion  in  water  and  apparent death,
may  often, if recovered within a moderate time,  be restored to life. 
450
FLUIDITY IN RELATION TO ANIMALS.
   The apparatus of the Humane  Society for the recovery of persons appa-
 rently drowned, includes a bellows for producing artificial respiration.  This
 bellows resembles a common bellows, except that its flap or valve, instead
 of being internal, is external, like  a large flute-key, and has a spring to close
 it, obedient to the finger of the operator.   The bellows receives  its  charge
 of fresh air on being expanded, while the valve is open; it sends the charge
 into the lungs on being compressed while the valve is shut;  it withdraws
 the charge again on being expanded with the valve shut; and the impure air
 is thrown out to the atmosphere on  its being compressed with the valve
 open.   These changes, repeated and continued, produce the artificial respira-
 tion required.  It is most important to  remark here, that if air be "injected
 into the lungs, either in too large  quantity or very suddenly, instead of
 recalling or sustaining life, it is as  certain a means  of killing as a dagger
 driven through the heart. This truth has been but lately known, and igno-
 rance  of it has probably decided  the fate  of many persons, treated with  a
 view to recovery after  submersion.  The operator should  reflect  that he is
 dilating the  delicate  air-cells of the lungs with the force of an hydraulic
 press; and that if he does so very suddenly, although to a small extent, he
 still may  rupture many  small blood-vessels, before  they can empty  them-
 selves  so as to yield.   In a bellows for the purpose of  artificial respiration,
 there should be the means of checking its opening to  suit the capacity of the
 patient's chest, and there should be a cock in the pipe or nozzle to regulate
 the speed of the passing air.
   In addition to the artificial breathing for the recovery of  suspended ani-
 mation, it is  often necessary to restore natural  warmth to  the body, to  rub
 the limbs  in aid of the circulation,  to administer stimulants by the  mouth, to
 excite  by galvanism, &c.
   It seems to be an error, and probably often  a fatal error, in  the  present
 mode of treating  persons apparently drowned, to use cold instead of warm
 air for the artificial respiration.  Thus while the important object of restor-
 ing the temperature of life is  sought by all  external means, the  great  incon-
 sistency is committed of blowing  cold air upon an  internal  surface  of  the
 body more extensive than the external; and until thatTecrprocal action of the
 air and blood begins,  which constitutes the slow combustion of natural re-
 spiration, every bellows-full of cold air admitted, brings back with it a portion
 of the  remaining  central warmth,  and  may thus chill so as to  make the re-
 covery impossible:—as a fire which has fallen very low  may be immediately
 extinguished by the  same action  of a bellows, which a little before  would
 have made it blaze.  Air might easily be heated for the purpose of respira-
 tion by pouring boiling water into a vessel containing it, and then connect-
 ing the bellows with  that vessel  by a fit pipe, or by  making  the bellows
 draw through a pipe  partially immersed in hot water:—a quart of boiling
 water  has  heat enough  in it to warm many gallons of air to blood-heat.
 This plan would not only avoid the mischiefs arising  from  the  cold air,  but
by affording the means of applying warmth even higher than that of life, it
might probably furnish the  most useful of all stimulants to the  parts about
the heart.   A healthy man can breathe, with impunity, air that is much hot-
 ter than boiling water.
   Late physiological investigations have shown that the breathing,  or me-
chanical action of the chest in respiration, is so dependent upon the influence
of the  brain, as to be disturbed and even stopped when the brain is  embar-
rassed : they have shown farther that the action of the heart is dependent on
the breathing, but not on the  brain, except as the cause of the breathing— 
VOICE AND SPEECH.
451
lifefor^^onSJ0"^1-^ arrtificiall-v' wiU  Preserve ^ circulation and the
lite lor a considerable  time after the  brain  has  altogether ceased to act  or
nfCMr " Rrn ? T?moyed fr°"?  the body-  Now' some interesting experiments
Iti'f    'e Pr°Vued  lhat Certain  P°isons are fatal' merely because
 HpLTS  2    V\me !he aCU°n 0f  the brain-through which suspension,
the actions, first of the chest, and then of the heart, cease, and death ensues:
but that in  such cases,  if the action of the chest be maintained artificially, the
circulation  arid  life of the body will be for a time continued, and the brain
may gradually  recover from the effect of the poison, so as to resume its office.
Thus certain cases of poisoning, which formerly would  have been fatal, may
now end in recovery.                                                  J
  An important application of this discovery is  to the treatment of cases of
convulsion, particularly those occurring from teething or other  irritations in
infancy.   The respiration ceases in these cases often  only because the action
of the brain is suspended : and if the respiration  be continued artificially, the
circulation  and life will also continue for a time,  during which the brain may
recover itself, either spontaneously, or in consequence of remedies employed,
and life may be saved.—The chest of an infant is comparatively so small,
that it may be filled from the mouth and windpipe of a grown  person, with
air which has not descended to that person's lungs, and therefore has not been
rendered unfit for  respiration;  and on the little chest being  afterwards com-
pressed by the  hand, the air will return.   The air may be blown directly in-
to the child's mouth  through a thin  handkerchief laid over  the mouth,"or it
may pass through  a tube inserted into the nostril or  trachea:—to prevent it
from passing into the stomach, the  larynx must be pressed  against  the oeso-
phagus  during its  entrance.   Let all who try this remedy,  keep present to
their minds the danger of inflating too  much.
  Any medicated air is generally inhaled by a patient from  an  oiled-silk, or
other air-tight  bag, or from a light gasometer (see page  211.)   When the
compound nature of  our  atmosphere was first  discovered, great  advantages
were  anticipated to medicine from the use of pneumatic or  aerial mixtures.
These expectations  have not been realized, but the subject still  remains
highly deserving of research.
THE  VOICE AND  SPEECH.
The chest and air-passages, with certain additional parts, constitute the organs
                          of voice and speech.

  An inquirer into the constitution of the universe around him, meets with
few  things calculated  more to  surprise him than  that faculty in the human
mind by which it can associate the ideas of objects with any arbitrary signs,
so closely that the ideas are afterwards excited by the signs almost as vividly
as by the objects themselves.   The inhabitants of  China, for instance, have
contrived many thousand grotesque characters, and determined what  object
each shall  recall; when a Chinese by study becomes  familiar  with  these,
he may have his bodily eye poring  over pages  of crooked and unseemly
scratches, while his mental eye through them sees  only a pleasing succession
of the most beautiful imagery of nature :  and the characters may be rendered
intelligible to the deaf and dumb man as well as to him who speaks; and they
serve as media of thought and communication  through  many provinces and
countries of which the spoken languages have no common resemblance. 
452
FLUIDITY IN RELATION TO ANIMALS.
   But if the ready remembrance of visible signs be wonderful, which have
a permanent existence, and which often may have some resemblance in form
to the things signified, how much more wonderful is it that an audible sign,
that is, a passing  sound or fugitive breath, called by man a word, should
serve as well; and  that by a succession  of  mere sounds, having so little
natural connection  with  the things signified, that they are totally different in
different countries, and are changing with fashion  from age to age, any train
of thought may be  made to pass through the minds  of an audience, so as to
excite  and to leave  impressions  almost as vivid as from  realities!  Such,
however, is  the fact, and it is greatly owing to  this and to a  correspondent
faculty of producing at will  a sufficient number of distinguishable sounds,
that man owes his  elevation above the  brutes of the field.   His godlike pow-
ers of intellect would have remained dormant and unknown, had he wanted
the faculty of comparing his invisible thoughts with those of his fellow men,
and of arranging and recording them by means of signs.—Written language
is a double remove from the objects themselves, being visible signs not of
things, but of the audible signs.
  The admirable apparatus by which man  is enabled to produce a sufficient
variety of sounds to answer his purposes, passes generally under the denomi-
nation of the organs  of'speech; because the act  of using sounds which have
meanings assigned to them  is called  speech.   It consists of the chest  for
containing air;  of  the larynx or cartilaginous box, with its narrow aperture
called the glottis, at  the top of the  windpipe, for producing the voice, and
varying its pitch; and of the short tube of the mouth,  with the tongue and
lips, for farther modifying the voice.
  In the chapter on acoustics, we explained that sound is the name given to
the effect produced upon the ear by certain tremblings conveyed to it gene-
rally through the medium of the air; and we explained how air, forced from
the human lungs through the opening at the top of the windpipe, causes  the
elastic lips of that opening to vibrate, and to excite the tremblings.  We have
now to show that this sound, in passing forward from the top of the wind-
pipe, may be modified at the will of the individual, in a great variety of ways
—a variety,  however, which is still very simple.
  The modifications of voice easily made, and easily distinguishable by  the
ear, and therefore fit  elements of language,  are about fifty in number ; but no
single language contains more than about half of them.   They are divisible
into two  very distinct and nearly equal classes, called, for reasons now to be
explained, vowels and consonants.
  Those of the first class are the simple voice issuing through the open mouth,
and influenced  only  by the degrees  in which  the mouth is  opened and elon-
gated.   They may be continued  as  long as there is  breath to issue from  the
chest, and it is for this reason that they are named vowels or calling sounds.
The Roman letters, A, E, I, O, U, as generally pronounced on the continent
of Europe, and of  which the sounds correspond nearly to  those aw, a, e, o,
and oo, of English writing, indicate  the  most  easily distinguishable vowels'.
Sound  passing through the  mouth in its most natural state of relaxation, is
heard as the modification expressed by the  Italian E, (or the a of the English
word care ;)  if the mouth be then widened, the sound becomes the A (as in  the^
English word bar;) if the mouth be  narrowed, we hear the  I (or the e of the
English tedious;) if  the mouth be elongated and at the same time widened
we  hear  the 0  (as in the English word bore;) and if more  elongated  but
narrowed, we hear  the U (as in the English rude.)  The possible mimber of
vowels, however, is as great as the possible degrees in which the dimensions 
THE VOICE AND SPEECH.
453
of the mouth may he altered.  About twenty of them are sufficiently distin-
guishable, but few languages comprehend  more than twelve.   Modern art
can produce  the  vowel sounds  mechanically  by means of tubes of certain
dimensions.
  The alphabets of Europe are very faulty in  not all using the same charac-
ters for the same sounds, and in not having, according to the true intent of an
alphabet, a character for each distinct sound.   In English one letter is  used
for several sounds, as A, in water, far, fat, fate, where it indicates four sounds
perfectly distinct.  In repeating  the English alphabet, the  A is pronounced
as the broad E of the Italians and of continental Europe, and the  E as the I;
and the I (in  tide, for instance,) as the diphthong AI of more correct alphabets;
and the U (in muse,) as  the diphthong IU.  In consequence of the changes
which the English have made in the meaning of the Roman letters, they now
experience increased difficulty in learning modern continental languages; and
their  own  pronunciation of the  ancient languages, to all but themselves, is
ridiculous, and almost unintelligible.  The same cause renders the pronuncia-
tion of English difficult to foreigners, and thus restricts much in other coun-
tries,  the cultivation of English literature.
   To explain the second class of the modifications of sound, called conso-
nants, we may remark that while any continued or vowrel sound is  passing
through the mouth, if it be interrupted, whether by a complete closure of the
mouth, or only by an approximation of parts, the effect on the ear of a listener
is very remarkable, and is so exceedingly different, according to the situation
in the mouth where  the interruption occurs, and  to the manner in which it
occurs, that many most distinct modifications thence arise.   Thus any con-
tinued sound, as A, if arrested by a closure of the mouth at the external confine
or lips, is heard to terminate with the modification which we choose to ex-
 press by the  letter P,  that is, the syllable AP has been pronounced; but if,
under similar circumstances, the closure be made  towards the  back of the
mouth by the tongue rising against the  palate, we hear the modification
expressed by the letter K, and the syllable AK has been pronounced; and if
the closure be made in the middle of the mouth by the tip of the tongue rising
against the roof, the  sound expressed by T is  produced, and the  syllable AT
is heard,—and so of others.  It is to be remarked, also, that the ear is equally
sensible of the peculiarities whether the closure precedes the continued sound
or follows it: that is to say, whether  the syllables pronounced are as above,
AP, AT, AK, or  on the contrary, PA, TA, KA.—The modifications of which
we are now speaking, appear, then, not so much to be sounds, as distinguish-
able manners of beginning and ending sounds ; and it is because they are thus
only  perceivable in connection with vocal  sounds that they are called conso-
nants
   Now in the mouth, considered as a vocal tube, there are three situations,
in which interruption of voice or breadth may most conveniently be made, and
there are six  modes of making it at each ; so that eighteen distinct interruptive
modifications or consonants hence arise.   These we shall now describe.

The three great oral positions, as they may be called, are,

   1st. At the external confine of the  mouth, or lips, giving the labial articu-
lations of which  P is an example.
   2d  In the middle of the mouth, where the tip of the tongue approaches the
palate behind the teeth, producing the palatal  articulations, of which 1  is an
example.
      30 
454
FLUIDITY IN RELATION TO ANIMALS.
   3d.  Near the back of the mouth, where the body of the tongue approaches
the palate, giving the guttural articulations, of which K is an example.

And the six modes in  which the voice or breath  may be affected in passing
   through each of the three positions of the mouth,  are,

   1st. A  sudden and complete stoppage, producing what  may be  called a
mute articulation:  viz., P, in the labial position; T, in the palatal; and K,
in the guttural.  (See here the general table of articulations next page  which
table may be considered as representing the tube of the mouth, with the  letters
so placed  as to show in what  situations in the mouth the sounds represented
by them are severally produced.)  A mute may also be made by stopping the
breath exactly at the teeth, viz., a dental-mute; but  it is hardly distinguisha-
ble from the palatal mute, produced just behind it, and being less perfect, is
not used.—Some awkward speakers, substituting  it for the  proper mute, are
said to speak thick.
  2d.  A sudden shutting, as in the last case, but  the voice being allowed to
continue until the part of the mouth behind the closure be distended  with air.
—This produces  the semi-mutes, B, D, and  G, (as heard in the syllables
AB, AD,  AG,) for the three positions.  There might be a dental  half-mute,
but it is no more used than the  dental-mute, and for the same reasons.  If
the sides of the tongue be depressed,after it has taken the position  required
for  T or D, the sound L is produced ; and the letter is, in the table, placed
below D,  although the sound, from being continuable, is not  in any sense a
mute.
  3d. The positions closed as for the mutes, while sound is allowed to pass
by  the nose.—Thus arise the  semi-voivels or nasals, M, N, NG, for the
three  positions.—NG (as in king) is a simple sound, although our imperfect
alphabet has no single letter for it.  The nasal sound of the French language,
which gives it so great a peculiarity, approximates to the English NG, but
differs from it in sound  being  allowed to pass  by the mouth, as well  as by
the nose.   It  is pointed  at by the small n in  the table, and  like the other
sounds which do not occur in  the English language, is here pointed  in the
Italic character.
  4th. Breath  only (or whisper)  allowed  to pass at the three oral positions
nearly closed.—Hence come the sounds which we call aspirates, viz.,  F, for
the labial  position,  TH and S, for the palatal, and CH (heard  in the Scottish
word loch,) for the guttural ;  the  TH and CH are simple  sounds, although
each  expressed in  Britain by two letters.   The  TH is heard in the word
bath,  and  is the sound expressed by the single letter 0  of the  Greeks.    The
CH is heard in the German ich, and is the x of the Greeks.   The soft aspi-
rate TH  is more  easily  made by  pressing the  tongue gently against the
teeth, and  allowing the  breath  to pass all round, than by the true palatal
approximation  of  parts, and the  soft dental aspirate,  therefore,  is used in
preference to the palatal.  The  letter S  is the hard palatal aspirate, and
differs from  the soft palatal  aspirate TH, in  the breath being made to
issue  with greater  force, and only by a  narrow space over the centre  of a
rigid tongue, instead of on all sides  of a soft  tongue, as for TH.  French
people, on first attempting to  pronounce TH, substitute for it the D,  or the
S, or the  Z (which is nearly related to S, as explained below.)  The author
has found it easy to enable ihem to pronounce the  TH at once, and perfectly,
by explaining its nature as above.  If we depress the  sides of the tongue while
pronouncing S, we produce the simple sound expressed by the English double 
THE VOICE AND SPEECH.
455
letter  SH, just as by depressing the sides of the  tongue while making D,
we produce L.
   5th. Using voice in the same manner as breath or tohisper is used for the
aspirates.—This produces the sounds called vocal aspirates, viz., V, TH, Z,
J, and GH.   TH vocal aspirate, is heard in bathe, as contrasted with the
simple aspirate in bath; Z comes  from the S position,  only  with  sound
instead of breath ;  SH pronounced with voice, becomes the J of the French
in the word^e, or the sound heard in the middle of the English word vision.
GH is a simple sound used in German, but not in English.

                        Table of Articulations.
S
s
CD
m
 o
>
p	B	M	F	V	pr
•	•	•	th	th	•
T	D	N	S	Z	R
•	L	•	sh	J	•
K	G	n	ch H	gh	ghr
Labial.
Dental.
Palatal.
  with the edges of
the tongue depressed.
Guttural.
  6th. Shaking the approaching parts in the three positions.—We thus make
vibratory sounds, of which  the  middle position gives the common R, the
only one of them used in England.  Some bad speakers of English, however,
make  the labial vibratory by shaking the P in such words as property; and
many  use the mitlural, which  is the burr of Northumberland,  and  the com-
mon affectation in Parisian speech, termed parler gras, or grasseyer.

                         Additional Remarks.
  ThP »ound of H is an aspirate produced even behind the situation of the
Jtural aspirate ch: it is, indeed, merely a  forcible  passing  of the breath
through the very back part of the mouth or throat. 
456              FLUIDITY IN RELATION TO ANIMALS.
   CH, in such words as chain, means T before sh.
   J, as heard in the English name John, is a compound sound, viz., D before
 the simple J of the table, which is the S of vision.
   LL.  The liquid or double LL of the French, as heard in the  word paille,
 is merely L with the letter Y  begun  to be pronounced  after  it.   It is heard
 in the English words billiard and halyard,  and would  be their terminating
 liquid were the syllable ard not pronounced.   The double LL of the Welch,
 as in the name of Lloyd, has the first L pronounced as an aspirate,  that is, as
 a whisper, and the second in the ordinary way.
   GN. The soft GN of the Italians  and French, is the English  N  with Y
 begun to be pronounced  after it.  It  is heard in our word lanyard; and in
 the Italian words pegnio bagnio ; and in the French word craignent.
   C, in English, stands  always  either for S  or K, as in the words  certain
 and car, and has no sound proper to  itself.
   Q in English expresses the sound of the letter K, with  U  following it;
 and yet, uselessly, U is always written after Q.
   X in English  means  either KS, as in the  word axle, or  GZ  as  in  the
 word example.
   The  consonants are best heard by sounding  them with voice before them:
 that is  to say, by making them rather terminate a syllable  than  begin it;
 pronouncing B, D, G, thus eb, ed, eg, rather than their  common alphabeti-
 cal names be, de, ge.
   The  labial sounds may be made either by the two lips, or by one lip  and
 the opposite  teeth.   F may be pronounced, for instance, by the  lips only,
 or by the lips and teeth :  and some persons awkwardly make it by the under
 teeth and upper lip.
   The  letters Y and I, in most modern languages, stand for nearly the same
 sound.   In English, for instance,  bullion  and  minion  might be  written
 bullyon and minyon, without suggesting a change of pronunciation.  In the
 words yard, you, yes, Sic,  the Y is a short I, very  closely  joined  to  the
 following sound.—W is also thus a short U, as perceived in the words war,
 we, Sic
   The author believes the analysis of articulations to be the best basis for a
 system  of short-hand written characters.  He has tried such a system,  and
 has found it exceedingly convenient.
   Lisping is chiefly the  habitual  substitution of the aspirate TH  for the S
 and SH.
   Whispering is articulation without voice ;  that is to say, articulation while
 breath only is passing.
   Stuttering, stammering, or  hesitation of speech, are terms implyino- an
 interrupted articulation, accompained  generally with more or  less of stain-
 ing and distortion of feature.  It is remarkable, with  respect  to this defect,
 that when the present work was first published, scientific or regular medicine
 had taught as yet no certain cure for it, although the frequent success of non-
 professional  and often ignorant  individuals, by a mode  of treatment which
 they solemnly bound their patients not to divulge, proved the cure, in  certain
 cases, to be both possible and not difficult—The author's attention  had been
 drawn to the  subject some years before, by an  interesting  case submitted to
 him, of stuttering connected with other disease; and it was in analyzing the
 subject  with a view to the treatment of that case, that he  framed  the analysis
 of articulation  contained in the preceding pages, and drew up a part  of  the
additional observations  which  are now to follow.  A cure  was obtained •
 but as the case possessed a  favourable peculiarity in the powerful mind of 
THE VOICE AND SPEECH.
457
the individual, to which the author attributed great importance, as  he had
nine leisure from his ordinary professional duties, to pursue the subject, or to
ascertain in what respects his plan might  differ from that employed by the
most successful of the practitioners  who concealed their proceedings, he
gave  his remarks in former editions of this  work, merely as continued elu-
cidation of the subject  of  speech.  He is now,  however, enabled to  state,
that his analysis has completely detected  the nature of the morbid affection,
and that it  directs simple and effectual  means of relief.  He declined med-
dling with  many cases  offered to  him after the original publication  of his
work, from the impression  that the cure  in the  instance mentioned above,
was owing at least as much to the ingenuity and perseverance of the patient,
as to his suggestions, and, therefore, that his professional superintendence of
the discipline required  for ordinary cases  would demand care and attention
which he could  not spare; but subsequent experience has proved to him that
the business is  altogether very simple and  easy,  and  as regards children,
may be managed by any intelligent instructor of youth who chooses to devote
attention to it, while grown individuals will often be able to relieve themselves
by the study of the  present section; and he hopes that in  very few cases
will the counsel of a person familiar with  the anatomy and actions of the or-
gans be found to fail.
   Command over the organs of speech is acquired in the same way as over
all the other muscular organs  of the body; those, for  instance, used in
walking, skating, fencing, performing on musical  instruments, &c: that is to
say, at first,  a distinct  act  of volition is required  for every individual move-
ment ; but the law of  association or habit rendering  the actions easier with
each successive repetition, they are at last formed into  connected tribes or
trains, which appear as obedient to a single wish  as the separate elements
originally were.  A child  at first exerts as distinct and powerful a volition to
pronounce the syllable pa, as after some practice to double the syllable and
make papa ; or after still more practice, to pronounce the longest and hardest
word of the language :—nay, at last, where there is strong and healthy power
of association, complete sentences, and even  rounded periods  of  eloquence,
are poured out like single  words, the  mind of the speaker seeming at liberty,
after each sentence or period is begun, to  meditate  and prepare that which is
next to follow.   As the faculties of locomotion and of speech are acquired in
infancy and  early childhood, persons no more recollect  how they gradually
acquired them than how their limbs grew ; but the  progress, described above,
may be watched by any individual of mature years in his own  person, while
he is learning such an'art as that of playing on  a  musical instrument.  He
will find, that at first,  every finger which is moved to produce a note, obeys
a distinct  thought and  volition ; that soon short trains of connected notes
become obedient to the will almost like a single note; that then  by degrees,
longer and longer trains or passages become  familiar, until at last the instru-
ment is obedient to the practised player, as voice is to the singer, or speech
to the orator.                                  .
   There is a great original diversity among individuals as to their powers ot
muscular association,  and therefore,  also, as to  their aptitude for acquiring
the various faculties of which we have  been  speaking.   Thus  some children
walk well before a year, others require a much longer time, and some never
succeed perfectly until they have had lessons  from the dancing-master or
drill serieant.—So, again, many people, by  ear and imitation alone, learn
Psmilv to Play on musical instruments;  but others must begin  by studying
the written  notes, and the orecise fingering by which each note is produced 
458
FLUIDITY IN RELATION TO ANIMALS.
on the instrument; and many, unless the notes be constantly before them,
cannot play at all.—So again, all persons may be said to learn to speak  at
first by ear and imitation; but many grow up to a certain age with defects,
which judicious lessons from parents or other tutors are required to remove;
and  there are some, as stutterers, who, owing to a naturally weak or irregu-
lar association, or to some accident in early life, which has strongly affected
their nervous system,  retain defects which no ordinary teaching can correct.
It appears, then, that an analysis and scale of articulate sounds, with minute
description of the  organic actions required to produce them, like the  scale
which we possess for music, in the gamut and rules for fingering, should
give nearly the same assistance to the speaker, which the gamut gives to the
player.   The table and analysis  contained in the preceding pages is inten-
ded to supply this  information.   It is constructed from minute consideration
of the organs of speech  while  in action.  It agrees in many respects with
the common grammatical divisions of elementary sounds, but in others  it
pursues the  analysis in a different way, and considerably  farther.  A person
who understands it well, will have, while he speaks, an  intelligent percep-
tion  of what he is doing, in addition to the parrot-like faculty of  habit, or of
repeating by rote, and  will thus command any desired sound by two powers
instead  of one.  And, as  a musician, when  his  musical  memory fails  him,
finds help by thinking of his written notes and their relation to his instru-
ment, so may a stutterer, when hesitating at any sound, receive benefit by
thinking of the letter which represents it, and of the position of the organs
required for that letter.  Then, by frequent practice in making the particular
combinations of sound which  are  difficult  to him, he  may strengthen the
useful habit, and ultimately overcome his defect.
  The most common  case of stuttering, however, is not,  as has been almost
universally believed, where the individual has  a difficulty in respect to  some
particular letter or  articulation, by the disobedience of the parts of the mouth
which should form it to the will or power of association, but where the spas-
modic interruption occurs altogether behind or beyond  the mouth, viz., in
the glottis, so as to affect all the articulations equally.  To a person ignorant
of anatomy, and therefore knowing not what or where the glottis is, it may
be sufficient explanation to say,  that it is the  slit or narrow opening  at the
top of the windpipe, by which  the air passes  to and  from the lungs, being
situated just behind the  root of the tongue.   It is that which is felt to close
suddenly in  hiccup, arresting the ingress of air, and that which closes, to
prevent the egress of air from the chest of a person lifting a heavy weight or
making any straining  exertion ; it is  that also,  by the repeated shutting of
which a person divides the  sound in pronouncing several times, in distinct
and  rapid succession,  any vowel, as o, o, o, o.   Now the glottis, during
common speech, needs never to be closed, and an ordinary stutterer is instant-
ly cured, if, by having  his attention properly directed to it,  he can keep it open.
Had the edges or thin lips of the glottis been visible, like  the external lips of
the mouth, the nature of stuttering would not so long have remained a mystery,
and  the effort necessary to the cure would have been suggested to the most
cureles, observer:  but because they were hidden, and professional men had
not detected, in how far  they were  concerned, and the patient himself had
only a vague feeling of some difficulty, which,  after straining, grimace,  gesti-
culation, and sometimes almost general  convulsions of the body, gave way,
the uncertainty with respect  to the subject has remained.   Even  many per-
sons who, by attention and much labour, had overcome the defect in themselves,
as Demosthenes  did, have not  been able to describe  to others the nature of 
THE VOICE AND SPEECH.
459
their efforts, so as to  ensure imitation; and evidently the quacks who have
succeeded in relieving many cases, but in many also have failed, or have given
only temporary relief, have not really understood what  precise end in the •
action of the organs their imperfect directions were accomplishing.
   Now, a stutterer, understanding of anatomy only what is stated above, will
comprehend what he  is  to aim at, by being farther told, that when  any con-
tinued sound is issuing from his mouth, as when he is humming a single note
or a tune, the glottis is necessarily open, and therefore, that when he chooses
to begin pronouncing or droning what we have  already described to be the
simplest of vocal  sounds, namely, the vowel e, and in its  less distinct modifi-
cation, as heard in the  English word  certain or in the French word que (to
do what at once no stutterer has difficulty,) he thereby  opens the glottis, and
renders the  pronunciation of any other sound easy :—or if, when speaking or
reading, he joins  his words together, nearly as a person joins them in singing,
(and this may be  done without its being at all noted as a peculiarity of speech,
for many persons do it in their ordinary conversation,) the voice never  stops,
the glottis never closes, and there is of course  no stutter.  The author has
given merely this explanation or lesson,  with  examples, to persons who be-
fore would have required half an hour to read a page,  but who immediately
afterwards read it quite smoothly ;  and who then, on transferring the lesson
to the speech, by continued practice and attention, obtained the same facility
with respect to it.  There  are many persons not accounted peculiar in their
speech, who in seeking words, to express themselves, or while coming to a
decision, often  rest between their words on the simple sound of e mentioned
above, saying,  for instance,  hesitatingly, " e......I e......think
e......I shall," the sound never ceasing until the end of the  sentence,
however long it may  be delayed.   Now a stutterer, who, to open his glottis
at the beginning of a phrase, or in  the  middle  after any interruption, uses
such a sound, would  not even at first be more remarkable than a  drawling
speaker, and he would  only require to drawl for a little while, until practice
facilitated his command of the other sounds.  Although producing the simple
sound  mentioned is  a  means of opening the  glottis, which by stutterers is
found very generally  to answer, there are cases in which other such means
may be  more suitable, as the intelligent preceptor will soon discover.—Were
it  possible to divide the  nerves of the muscles which close the glottis, without
at the same  time  destroying the faculty of producing voice, such an  operation
would be an immediate  and certain  cure of stuttering.
   While the spasmodic closure of the glottis, as above  described, is the com-
mon cause of stuttering, there are also cases in which the cause is a spasmodic
prolongation of some  of the aspirates or semivowel sounds, as of s,  m, I, Sic
Fortunately, however, the substitution of the simple sound is equally the cure
for all.                                                           .
   While the cure of  many stutterers has been accomplished  by their own
efforts, after the studv of what is written in this section,  for others, and par-
ticularly for young people, the  following  have been found to be farther useful
rules  or forms of direction; and a commentary upon them  making them
perfectly intelligible, would seem to comprehend all that can be communicated
upon the subject.—1.  Familiarize yourself with the idea of a continued sound,
as of  the roar of the  sea or waterfall, or  the note of an organ-pipe, and feel
that your speech  is to be as uninterrupted.—2.  Then never stutter  more, but
substitute always the  simple continued  sound for any threatened defect, and
rest ™on it until  power be felt to overcome  the difficulty—3. Never repeat
words or syllables.—4. The simple sound must become the  first syllable 
460
FLUIDITY IN RELATION TO ANIMALS.
[closely joined) of every difficult word, until the morbid habit be weakened.
The object of all these directions is to enable  the patient, first, to substitute
universally the drawl for the stutter, and then, as soon as possible, to discard
the drawl too.
  The view given above of the nature of stuttering and its cure, explains the
following facts, which to many persons have hitherto appeared extraordinary.
—Stutterers often can sing well,  and without  the least  interruption; for the
tune being continued, the  glottis  does not close.—Many stutterers  also can
read poetry well, or any declamatory composition, in which the uninterrupted
tone is almost as remarkable  as  in singing.—A person who draws a deep
breath before beginning to speak, as  he cannot long retain  the air, and the
glottis must be open to let it escape, is to a degree secured against the occur-
rence of  stuttering.   The  secret remedy of an  American quack, who years
ago got much money from Englishmen, was the direction thus to fill the chest
before beginning to  speak.  A Dr. McCormac, also, who published  a work
on this subject, founded on the erroneous idea, that stuttering was an effort
to speak  while inhaling air, instead of while exhaling, gave the same direc-
tion.—The cause of stuttering being a weak and easily disturbed association
of certain muscular actions,  we have the reason  why any degree of anxiety
or dread  as to speaking well, exceedingly increases the defect; and why many
stutterers,  who cannot make  themselves  intelligible in society, still, when
alone, can speak and read  as perfectly as any other person.  This  explains
also why many stutterers, who have gone to live for a time at the houses  of
pretended curers of their defect, have felt themselves singularly relieved from
the moment of entering the house ; because, knowing that they were  expected
to speak ill, they had no fear of disagreeably attracting attention, and therefore
had their powers much more at command.   These  persons, on returning  to
the world, have generally stuttered as badly as ever, but many of the asserted
cures of stuttering, with certificates obtained from the parties at the time, have
been of the nature now described.—The cause of stuttering being so simple,
as above described, one rule given and explained may, in  certain  cases, in-
stantly cure the defect, however  aggravated, as has been observed  in not a
few instances; and this explains also why an ignorant pretender may occa-
sionally succeed in curing, by giving a rule of which he knows not the reason,
and which he cannot modify to the peculiarities of other cases.—The same
view of the subject explains why the speech of a stutterer has been  correctly
compared to the escape of  liquid from a bottle with a  long, narrow neck,
coming—"either by hurried  gushes or not  at all:" for when the  glottis is
once opened, and the stutterer feels that he has the power of utterance, he is
glad to hurry out as many words as he can, before the interruption recurs.
   The study of the table of articulations leads to the immediate correction of
many minor defects in utterance, and  is  calculated to facilitate the acquire-
ment of foreign languages.  A lisping person, for instance, is cured  at once,
by being told that the tongue must not touch the teeth in pronouncing the letter
S; and a Frenchman who deems it impossible for him to pronounce  the Eng-
lish sound of TH, discovers that he cannot avoid doing so if he rests his tongue
softly against his teeth, opened a little, and then forces breath or sound to pass
between the  tongue and teeth.
   Several of the modern languages of Europe consist of nearly the same ele- ■
mentary or radical words, and differ among themselves chiefly by the preva-
lence in  each of certain terminations and of one or other of the "related and
convertible sounds classified in the analysis given above.   A student, there-
fore, who, by analytical investigation, or considerable practice, has become 
THE VOICE AND SPEECH.
461
impressed with the peculiar genius of a language, may invent, or  determine
by analogy, even before minute study, the majority of those words belonging
to each which have sprung from a common origin.  This  remark is so true
with respect to the languages of Italy, Spain, Portugal, and even France, that
to persons familiar with them, they are at last listened to rather as the same
language spoken  by different individuals, than as languages in themselves
different.
   Ventriloquism  is the name commonly given to the art by which an indi-
vidual  can assume characters of voice and speech which are  not  natural  to
him, and  thus, although alone, can imitate closely a  conversation held  be-
tween  two or more  persons.
  The most remarkable diversity is obtained by speaking during inspiration,
instead of, as usual, during expiration.   The voice so produced is more feeble
than the ordinary voice, and when accompanied by other circumstances favour-
ing the illusion, it may suggest very completely the idea of  a boy calling from
the bottom of a pit, or  from the interior of a chimney, &c.   An unsuspecting
peasant may be tricked into unloading his hay-wagon by an expert ventrilo-
quist, who makes him  believe that there is a poor child  packed  under  the
heap and ready to be smothered there.
  A person, by a little  practice, may acquire the power of producing, without
the slightest apparent motion of the lips or countenance, all the articulations
except the labial, and  of them the F, V and  M may be  tolerably imitated
by  parts behind; hence by avoiding  words in  which P and B occur, such
person may speak without visible movement of the organs, and if  he assume
the attitude of a listener, he may make  the deception of ventriloquism com-
plete.   The idea which some authors have had (see Good's Study of Medi-
cine, Sic) that the articulations of the ventriloquist are not produced by the
tongue and mouth,  as in common speech, is altogether an error.   The  art,
carried to a certain  degree, is not very difficult, as any person may ascertain
who tries it after considering minutely the nature of common  speech.
  There are also striking varieties of voice producible by speaking  with a
more acute or grave pitch than usual,  and with different degrees of contrac-
tion of the mouth ;  but these may be more properly called imitations than
ventriloquism.
  The variety of effect in sound which the human organs are capable of pro-
ducing is truly surprising.   There are adepts  in  the  art of imitations, who
not only mimic the speech of all ages and conditions of the human race, but
the songs of birds, the  cries  of animals, and even not a few  of the sounds pro-
duced  among inanimate things.   Many of these performances become in the
highest degree ludicrous, and furnish favourite amusements in our theatres.
A Mr. Henderson,  of  London, about  the end of the eighteenth century, used
to kill  his calf, as he called it, to crowded houses every night.  After drop-
ping a screen between  him and the audience, he caused to  issue from  behind
it all the sounds, even  to the minutest particular, which may be heard while
a calf  is falling a  victim  in the slaughter-house;—the conversation of the
butchers, the struggling and bellowing and quick breathing of the frightened
animal, the whetting of the knife, the  plunge, the gush,  the agony;—and,
revolting as the occasion is in  itself, the imitation was so true to nature, that
thousands eagerly went to witness the art of the mimic.
  The following  cases of inanimate  sound may be closely imitated  by  the
mouth: The working  of a grindstone, including the noise of the  water into
which  it dips, the rough attrition of the steel upon it, and the various changes
occurring with the change of pressure ;—the working of a saw cutting  wood ; 
462
FLUIDITY IN RELATION TO ANIMALS.
—the uncorking of a bottle, and the gurgling noise of decanting its contents ;
—the sound  of air rushing into a room in a winter night  by a crevice or
key-hole—and many others.
  It has already been explained, that voice depends  on the vibration of the
two edges or lips  of the  slit-like  opening of the  glottis, by which the air
passes to and from the chest.  The number of vibrations in a given time, or
the pitch  of  voice, depends, of course, on the length and tension of these
edges.  The length is varied by the positions of the arytenoid cartilages, and
the tension by the action of small muscles which act on these ; and the cavity
of the mquth is  enlarged or lessened to accord with  the number of vibrations,
by the rising or falling  of the tongue  and larynx which form its bottom.
The  peculiarities of individual voices must depend  chiefly on the size and
firmness of the cartilaginous box of the larynx, the strength of the muscles
of the chest vvhich force the air through the  glottis,  and the  pliancy of the
moving parts.
  The  glottis is smaller in  women than in  men,  and hence their pitch of
voice is higher :—with reference to music, the difference  is  generally of an
octave, or eight notes.
  The voice of a boy, in  regard to pitch, is generally the same  as that of a
woman ; but at the age of puberty, the sounding organs in the male enlarge
suddenly, and render the voice stronger than  before, and by nearly an octave
graver.   The voice of a eunuch is the voice of the  boy continued, because
the change called4 puberty  does not take place in him.
   Complete loss of voice, for longer or shorter periods,  is often experienced
by persons while  in feeble  states of health.   The vibrating, and,  therefore,
sounding  edges of the glottis,  which are usually kept tense by the operation
of certain  muscles, on these  ceasing to act, owing to the  state of their nerves,
will not vibrate as required, and the  voice is lost.  Slight  colds  suffice in
many people to produce this effect:  in others of morbidly sensitive or deli-
cate nervous temperament, it follows fatigue, or any  other cause of debility.
Articulation  is not destroyed by loss of voice;  and  whispering answers  pass-
ably the end of vocal speech.
   No intelligent mind can meditate on human speech and its influence in the
world, without  being roused to vivid  admiration.   But  for speech, the most
gifted individuals who have lived, had  they  existed  at all, could  have  been
little  superior in their worldly state to the leading oxen of our herds, or to
leading monkeys  in  the woods.   As regarded the rest  of mankind, Homer
and  Newton would  have lived in vain.   At the present day, among the
natives of Australasia, where language may be  said scarcely yet to exist,
human nature is  seen thus  brutishly debased; while, on the other hand, in
the history of the world,  we  may trace, as a consequence of more  perfect
speech, all the  progress which has been made in  arts and civilization.  By
language, fathers have communicated their gathered  experience  and reflec-
tions to their children,  who in their turn become fathers, have transmitted
them to succeeding  children, with  new accumulation;  and when, in the
course of ages, the precious store had  increased,  until  mere memory  could
retain no  more, the  art of writing arose, making language visible and  per-
manent, and enlarging without limit the receptacles of knowledge; and then
the art of printing came, which now rolls the still swelling flood into every
hamlet and every hut.   Language thus, at the present moment of the world's
existence, may be said to  bind the whole human race of uncounted millions
into  one gigantic  rational being, whose memory reaches  to  the beginnings
of written record, and retains imperishably the important events that have 
THE DIGESTION.
463
occurred; whose judgment, analyzing the treasures of memory, has already
discovered many of the sublime  and unchanging laws of nature, and  has
built on them the arts  of life, and through  them, piercing far into futurity,
sees distinctly many events that are to come; and whose eyes, and ears,  and
observant mind are, at this moment,  in every corner of the earth, watching
and recording new phenomena, for the purpose of still better comprehending
the magnificence and simplicity and beauty  of creation.
                             THE  DIGESTION.

 The  doctrines of fluidity, illustrating and illustrated by certain phenomena
                               of digestion.

   The animal body may be seen at first, in  the  maternal ovary, as a single
 speck of mucus;  but from possessing life—wonderful life—the little nucleus,
 placed in new circumstances, begins to gather itself substance from  around,
 and it increases in bulk.  For a certain time  it remains  attached to the body
 of its parent,  and draws the material of  its increase  from its parent's blood;
 but after  that time it is alone  and entirely dependent on its own resources.
 Then we see brought into  play that extraordinary apparatus now to be  de-
 scribed under the name of the digestive or assimilating organs; which,  un-
 der the direction of a nervous energy, can, out of almost any kinds  of dead
 animal or vegetable matter, build up the  beautiful living body to perfect matu-
 rity of size, and form, and faculty.   And it is not only while their bodies are
 growing that  animals require to take in  and  assimilate  new matter, but also
 after  maturity, in order  to repair the  waste  of constant action.  Supply of
 fuel and water to the steam-engine is not more necessary than of ailments to
 the living body.
   Some of the less  perfect animals take in sustenance almost like vegetables,
 by absorbent  tubes  that open  on their  surface;  but by far the greater part
 receive it first into an interior cavity, where it undergoes certain preparation,
 and is then  offered to internal absorbents, which  drink  up what is required,
 and carry it into the circulating blood.   This internal cavity is called a sto-
 mach. Its form and appendages differ exceedingly in different animals, accord-
 ing to the nature of the substances which serve for their  sustenance, and to
 various other circumstances.
   In  man, the process of  digestion has  the  following  steps.  The  food is
 first received  by the mouth.  It is  there broken or torn into small portions
 by the cutting and  grinding wedges, called  teeth, with which the jaws  are
 armed; at the same  time a fluid called saliva is  mixed with it, poured  out
 from  glands around, so as to reduce it into a pulpy mass :  this  mass  is then
 pushed backwards  by the tongue to enter the long tube called  the gullet or
 (Esophagus, which, by  successive contraction of circular  fibres, propels it
 down to the  pouch of the stomach, placed under the edge of  the left ribs.
 From the internal  surface of the stomach a liquor oozes, called the  gastric
juice,  the most general solvent in nature, and which, attacking the received
 food,  soon reduces it, of whatever kind, to the state of a  pultaceous mass,
 named chyme; in this state it enters the narrow  intestinal canal which is
 continued from the stomach, where it almost  immediately receives a mixture
 of bile and pancreatic juice poured out from  the  liver and  pancreas.  After
 this mixture, as it gradually passes on, a  chemical decomposition and separa-
 tion of parts takes" place, and the pure  nutriment of the body  assumes the 
464
FLUIDITY IN RELATION TO ANIMALS.
state of a milky fluid floating among refuse.  This milky fluid, called chyle,
is taken up all along the  canal by the numberless  absorbent mouths of the
vessels called lacteals, and is then carried to the thoracic duct, and by it into
the blood to supply the waste.  The intestinal canal is about six times as long
as the body, affording, therefore, a very extensive surface from which ab-
sorption may take place.   That remnant of the chyme which the absorbents
refuse, mixed with various  depositions or secretions, continues its  journey
onwards, and is perodically discharged.
  Much of the process which we have now described is mechanical, as will
appear immediately; other parts of it are chemical, such  as the solution of the
food by the gastric juice, the separation of the milky chyle, &c.: and parts
are vital, such as the afflux, just when wanted, of  saliva, gastric juice, bile,
&c, and  the muscular and absorbent actions.   He  who neglects the study of
any one of these three classes of particulars, must have a very incomplete
acquaintance with the function.—We proceed now  to explain the mechanical
or physical circumstances connected with digestion.

The abdomen may be considered as a vessel full of liquid, in which, therefore,
  there is pressure  in all directions, increasing with the depth, (see hydro-
  statics,) and increased also by the action of the surrounding muscles which
  form the sides of the cavity.

  The justness of this view of the abdomen becomes evident, when we con-
sider that only moistened  or semifluid food descends into the stomach, that
drink follows, and  that gastric and other juices are poured out to  mix with
the food as it passes on to occupy the  long intestinal canal; and that then the
intestines externally are perfectly smooth, and are moistened by the constant
secretion of lubricating serum, so that they slide among each other, without
sensible  impediment from friction.   The  abdomen, therefore, is  in  fact a
roundish smooth vessel filled with a thick fluid, which is farther contained in
a perfectly pliant and smooth-coated tube.
   Thus any  part of the contents of the stomach and bowels, in a living man,
is supported like  water in surrounding water,  and therefore, if the  whole
contents be of equal specific  gravity, no  part can descend or advance by its
weight.  Neither can any general pressure, nor contraction of the surrounding
parietes, hasten, except at  the moment of expulsion, the motion of any con-
tained matter—as  has, however, often been supposed ;  nor can it help to
empty one part into another—the stomach, for instance, or the gall-bladder,
into the small intestine.
   For the  same reason, however, the very  slightest contractile action of any
containing part is sufficient to dislodge its  contents—gravity as a  resistance
being neutralized by the surrounding fluid.   And  when  the  gall-bladder, or
 stomach, or any part  of the intestinal  tube, becomes  so full as to put  the
 elasticity of the  coats ever so little upon  the stretch, this circumstance alone,
 unless some muscular action oppose, will cause a discharge of the contents.
—The natural action of the intestinal canal is a  successive contraction of its
 circular fibres from above  downwards, propelling the contents, just as if  a
 small ring or tube were put round the canal and pushed forwards.
   These considerations  make  evident the common error of supposing that
 vomiting can, by the sudden compression of the abdominal viscera, mechani-
 cally emulge or clear the obstructed biliary ducts.  If general pressure of the
 abdomen could  produce this and similar effects, a descent in the diving-bell
 should be  a  powerful  remedy in human maladies; for nearly fifteen pounds 
THE DIGESTION.
465
feeUn ^ ^ ^^ l° the ordinar>r abdominal pressure, at a depth of thirty

  We hence see  also the kind of error into which our predecessors fell so
generally, when they attributed much of the digestive power of the stomach
to its simple pressure upon the food.  The  idea  probablv arose  from the
contemplation of the stomach or gizzard of a fowl, which is a powerful bristly
substance,  answering the purpose almost of a mouth and teeth, as wel! as of
a stomach.
  It is an error also  to suppose  that quicksilver, which is sometimes swal-
lowed to remove obstruction, runs through the bowels simply  by its weight.
On first entering the loose small intestine, it must drag the part containing it
to the bottom of the abdomen, and in that situation, the whole intestine must
pass round it, nearly as a rope passes through  a ring fixed to the floor. When
the mercury arrives at the part of  the intestine called the  caecum, where the
farther course lies upward along the fixed arch of the colon, it probably can
be  dislodged only by the patient's lying down.   Any useful operation of
quicksilver, in such cases, may be in its stimulating the bowels, by dragging
or displacing them, in the manner above described.

When the  abdominal muscles, which are the  containing sides of the cavity,
  become tense, whether from unusual fulness of the cavity, or from their own
  action in any of the straining exertions, a variety of important mechanical
  effects ensue.  Thus,

  A full stomach produces—tension and projection of the belly—projection
of the diaphragm into the chest, causing hurried  breathing,  and  impeding
speech  and singing—expulsion  of blood from the  abdominal vessels, and,
therefore, congestions elsewhere, as  in  the arteries  of the head, sometimes
producing  apoplexy.
  Abdominal fulness, as in dropsy, tympanitis, corpulency, pregnancy, Sic,
produces  most of the effects  now mentioned in an aggravated degree.   If
dropsy be allowed to proceed too far without  tapping, the patient will die of
suffocation from the  rise  of the diaphragm.—The external veins of the legs
and abdomen of a dropsical person are generally turgid, because the blood is
pressed into them out of the abdominal cavity, and because the passage  of
blood through the abdomen is impeded.  In  tympanitis, or  windy dropsy,
as it has been  called, the viscera hang down  in the abdominal cavity, while
the air occupies the upper part.   In common dropsy, the viscera float about
and are supported.
  Straining or strong action of the  abdominal muscles, and therefore also
pressure on the abdominal  contents, occur with almost every considerable
bodily exertion; for the abdominal muscles are the antagonists of the  great
muscles on the back and  about the spine, and must always come  into play
with  them, to give firmness and rigidity to  the trunk of the body.  This
may  be seen remarkably in the actions of lifting,  running, wrestling, &c.
As  the abdominal muscles cannot act in a continued way and strongly, unless
the  ribs, from which they arise,  become nearly fixed, the  ribs  are supported
durino- exertion by the intercostal muscles, and by the air  in the chest, then
confined by the closure of the air-passages : hence there is generally compres-
sion in the chest also when the abdomen is  compressed, and the blood  is
squeezed towards the extremities from both  cavities at once.  It is important
to remark, also, that  in what are called the strong  actions  of the  chest, as 
466
FLUIDITY IN RELATION TO ANIMALS.
 coughing, sneezing, blowing,  Sic, the abdominal  muscles are at least as
 active as the pectoral:  by pulling  down the ribs to which they are attached,
 they narrow the chest, and by compressing the abdominal contents, and thus
 raising up the diaphragm, they shorten  the chest.
   The following cases exemplify the effects of straining.—The  lifting of a
 great weight, or making any great  exertion, drives the blood up to the head ;
 as is marked by the sudden redness of the face.—Coughing or vomiting will
 cause closed leech-bites to bleed afresh, and  sometimes will overcome the
 action of the sphincter of the bladder or rectum : coughing will also produce
 vomiting.—Straining to empty the bladder, rectum, or  womb, or the effort
 of vomiting, will cause the rupture of a  blood-vessel in the white of the eye,
 with consequent effusion of blood there.—Apoplexy often happens under the
 same circumstances, from the breaking of a vessel in  the brain.—The rupture
 of a varicose vein, or of aneurism,  generally happens during exertion.—And
 during exertion, the  protrusion  is  likely to occur at any weak  part of the
 abdominal cavity, of some portion  of its contents, producing  what  is called
 hernia or rupture.
   Vomiting is produced, not by the forcible  contraction of the  stomach, as
 was long supposed, but chiefly  by  the action of the parietes of the abdomen.
 This is proved by the fact that the  stomach has been removed from  a  living
 animal, and a sheep's bladder containing liquid has been substituted  for it, in
 connection with  the gullet above and the intestines below ; and on then inject-
 ing an emetic drug into the veins of the  animal, vomiting has taken place, as
 if the stomach had been there and  unhurt.*   From this  we see why, to pre-
 vent regurgitation of the food, during exertion, the upperorifice of the stomach
 requires to be almost as strongly closed  as the sphincters below.
   A small pump—in this  application called the stomach-pump—has  lately
 been used in  medical practice, for  removing  poisons from  the stomach  in
 cases where the action of vomiting could not be excited.  It has already saved
 many lives.   It resembles the common small  syringe, except  that there are
 two apertures near the  end, instead of one, which, owing to valves in them,
 opening different ways, become what are called a sucking and  a forcing
 passage.  When the object is to extract from the stomach, the pump is worked
 while its sucking orifice is in connection with  an elastic  tube passed into the
 stomach,  and the discharged matter passes by  the forcing orifice.   When it
 is desired, on the contrary, to throw cleansing water or liquid into the stomach,
 the connection of the apertures and the tubes is reversed.
  As a pump may not be always procurable when the occasion for it arises,
 the profession should be aware, that in many cases a simple tube will answer
 the purpose as  well, if  not  better.   Such  a  tube being  introduced,  and the
 body of the patient being so placed that  the tube forms a downward channel
 from the stomach, all fluid matter will escape from the stomach by the tube,
 as water escapes from a funnel by its pipe; and if the outer  end of  the tube
  * The mechanism of vomiting is still a moot point in physiology.  Mr. Hai°hton, a cele-
 brated English physiologist, opened several animals during the effort of vomking'and he
asserts that he distinctly saw the contractions of the stomach. The more recent experiments
 of M. Magendie, which were repeated in the presence of a committee of the French Insti-
 tute, are, however, entirely contradictory of those of Mr. Haighton, and seem to show that
 the stomach is entirely quiescent in the act of vomiting.  M. Main«-ault, nevertheless has
been  led to results opposed to those of M. Magendie, and he is supported by  Professor
Portal and  M. Mom-don, both of whom appeal  to experiments, and to some pathological
facts, which are very imposing.   It appears to us probable that vomiting is usually the joint
 effect of the contraction of the stomach, and of the diaphragm and abdominal muscles, though
 either is of itself occasionally sufficient for that purpose. Am. Ed. 
THE DIGESTION.
467
 be kept immersed in liquid, there will be during the discharge a syphon-action
 01 considerable force.   On then changing the posture of the body, water may
 be poured in through the tube to wash  the  stomach, and may, by the same
 channel,  be  again  discharged.   Such a tube, made long  enough, might, if
 desired, be rendered a complete bent syphon, the necessary preliminary suction
 being produced by a syringe, or by an assistant, who acts through an inter-
 posed vessel.
   But there  is a  still easier mode than either of these now described, of dis-
 lodging poison from a torpid stomach,  viz., merely to  place the patient so
 tha,t the mouth shall be considerably lower than the stomach,—as when the
 body lies across  a chair on a sofa, with the face near the floor,—and  then, if
 necessary, to press on the stomach with the hand.   The cardiac orifice opens
 readily in such a case, and the stomach is emptied like any other inverted
 vessel.
   Useful as  the pump may prove, upon occasions,  in evacuating the stomach,
 its more ancient office of injecting the enema is still the more important__and
 recent experience seems to show that such injection may become a  remedy
 of more extensive utility than had yet been suspected._  From an erroneous
 opinion, that what had been called the valve of the caecum  acts as a perfect
 valve, allowing passage downwards only, few practitioners have ventured to
 order much liquid  to be injected, for fear of  overstretching  the lower part of
 the intestine ; and the possibility of thus, by injection, relieving disease situ-
 ated above the supposed valve, has scarcely been contemplated.  It  is now
 ascertained, however,  that fluid maybe safely  thrown in even until itreach
 the stomach.—Perhaps few, if any cases of obstruction  of bowels, could
 resist the gentle force of penetrating water, so that a mechanical remedy of
 certain effect may, in  many  cases, be substituted  for the  drastic purgatives
 and pernicious bleedings now used, and  often used in vain.—From what has
 been said above of the abdomen and  the  intestinal canal, it appears  that an
 injection tends to spread itself with singular uniformity over the whole.  This
 tendency  may be rendered obvious to sight, by throwing a sheep's intestine,
 recently extracted, into a bucket of water, and then pumping water in at one
 end;—a  stream  will issue strongly  at the other end,  although  several feet
 distant, almost immediately, and without any intermediate part having become
 very sensibly tense.—Of course, in the living body, in cases of spasm or ob-
 struction, the liquid must be thrown in against resistance very gradually.
   That case  is called introsusception of the bowel,  in which an upper portion
 falls, or is received into a portion below, (as one part of the finger of a glove
 may be received  into another part,) and the receiving portion of the bowel,
 mistaking the received  for descending food, holds  it fast.   This occurrence
 forms a complete obstruction, and generally proves fatal. Many infants, with
 irritable bowels, die of it.—Now a copious enema,  such as we have described
 above, is almost a certain cure.   The liquid advances until  it reaches the part
 where the portion of gut has been swallowed by gut below ; and as it cannot
 pass without pushing the introsuscepted portion back to liberty, it effects the
 cure.*
   The perpetual syringe, or little  valved pump, of which we have  been
 speaking as lately used  in applications to the animal body, can inject or with-

  * It should be remarked, however, that this measure can succeed only whilst the intro-
 suscention is recent- at least before inflammation has occurred and adhesions formed between
the introsuscepted portion and that portion of the bowel in which it is received.  Common
or atmospheric  air, from its great elasticity, lightness, &c, is the best fluid for the injection.
Am. Ed. 
46S
FLUIDITY IN RELATION TO ANIMALS.
draw any quantity, and is therefore very superior, for almost every purpose,
to the old large  syringes which  had no  valves, and which, without being
removed, could inject only once their fill.   With  well-adapted additional
apparatus, the same instrument will answer for many purposes, as for throw-
               ing up the enema, clearing the stomach, transfusing blood, ex-
               hausting cupping-glasses, relieving the over-distended breasts;
               for the lotio vesica?,  vaginae vel  urethras, Sic   No surgical
               apparatus is now complete  without one.   The  annexed out-
               line represents such a syringe.  The aperture c is rendered a
               sucking  orifice, by a  valve  at it,  opening inwards; and a is
               the forcing orifice, inconsequence of having its valve opening
               outwards:  b is  the piston, with its handle.   The  valves may
               be variously made,  or a single double-way cock may be used
               instead of both.  Convenient dimensions for the  syringe are
               from four to six inches for the length, and from three-quarters
               of an inch to an inch and a quarter for the diameter.
                 For a case of diseased rectum, where  it was necessary to
               use an enema daily, or oftener, the enema funnel,  represented
here, (from the funnel-shaped mouth  a downwards, and exclusive of the part
above a) was contrived, and was found more manageable by  the patient than
any other instrument.  If the tube a b be about two  feet long, (it may be of
metal, oiled  silk, caoutchouc cloth, Sic,) the liquid column contained in it
                   suffices to overcome the ordinary abdominal resistance;
                   but if a very short tube be used, the open funnel a must
                   be converted into a close vessel,  as represented here by
                   the dotted  line above  the funnel, having a bladder, or
                   other air-tight bag, d, connected with  it, and  a  bottle-
                   neck and cork, or a cock, at c, for admitting the enema.
                   On pouring in the liquid at c, the  air in the vessel c a
                   is forced into the bag, and on then  closing the opening
                   at c,  and  compressing the bag,  it is evident, that any
                   desired degree of injecting pressure may be exerted on
                   the enema.   This apparatus is both cheaper and more
                   simple than any syringe, and is equally effectual; and
                   the bag never being wetted,  lasts long: b is a cock kept
                   shut until the moment of injection.—The principle of
                   substituting, in an injecting apparatus, the pressure of a
                   liquid column for that of a piston, was first suggested in
               J   this work; and  yet, since the publication of the work,
                   more than one patent has been taken for it, for  parties
                   seeking to convert it to their profit.
  By viewing  the  abdomen in the true  light of a vessel or bag filled with
liquid which is seeking to escape in all directions, we have the explanation
of several circumstances connected with hernia or rupture; in which accident,
the containing sides of the abdomen in some part have given  way, allowing a
portion of the viscera to escape, so as to form a tumour under the skin.
  Hernia may be produced by all causes which strain or weaken the muscles:
as by leaping, lifting great weights, coughing  and  sneezing, lying with the
belly across a bench or  yard, as sailors do on ship-board, over-distension of
the belly by eating and drinking, corpulency, dropsy, pregnancy; debility of
muscle from dissipation, &c.
  The reason that a rupture increases so rapidly after it has once begun,  is,
that the protruding part is truly a fluid  wedge, of which, therefore, the openiDg
b 
PELVIC APPARATUS.
469
force  increases with the diameter.  (See Hydrostatics.)  This shows the
singular importance of arresting the accident at its very commencement.  The
trusses used to repress rupture were described at page 412.
  In attempting to return any part of the abdominal contents which may have
escaped as a rupture, we should recollect, that a soft uniform compression or
squeezing exerted upon the tumour by the hands of the operator, if greater
than the internal pressure of the abdomen, is slowly pushing back again any
fluid matter that  can  ooze inward  from  the tumour; and by thus gradually
lessening the  size of the  tumour,  may effect the desired object, without the
adoption of the last resource, of cutting parts to widen the inlet.  When, in
such a case, the operator sees clearly with the  mind's  eye what is passing
under his fingers, his efforts may often be successful, where a less intelligent
individual would fail.   No man practises medicine long, whatever his nomi-
nal department, without having opportunities of saving life, or  of preventing
a serious operation, by judicious management of recent hernia.  The barba-
rous old fashion of lifting the patient by the heels and shaking  him, that the
weight of the bowels might drag back again the part vvhich had  escaped, was
founded on ignorance of the fact, that the weight of the bowels in all positions
of the body, is supported almost entirely, not  by  their attachments, but by
the surrounding parts.
  The function of digestion or assimilation sketched in the preceding  para-
graphs, by which the animal body  assumes foreign matters from around, and
converts them into its own substances, is a subject of study little inviting in
some of its details, but taken altogether is one of the most wonderful which
can engage the human attention.   It points directly to  the curious and yet
unanswered question—what is life?  The student  of nature  may analyze
with all his art those minute portions of matter called seeds and ova, which
he knows to  be the rudiments of  future creatures, and the links by which
endless generations of  living creatures hang to existence:  but he cannot dis-
entangle and  display apart their mysterious life!  that  something under the
influence of which each little germ, when placed in  due circumstances, swells
out, to fill an invisible mould of maturity which determines its forms and pro-
portions.  One such  substance thus expands  into a beauteous rose-bush;
another into a noble oak ;  a third into an eagle; a fourth into an elephant—
yea, in the same way, out of the rude materials of broken seeds  and roots,
and leaves of plants, and bits of animal flesh, is built  up the  human frame
itself, whether of the active man, combining gracefulness with strength, or of
the gentler woman, with beauty around her as light.   How passing strange,
that such should be the origin of the bright eye, whose glance .pierces as if
the invisible soul  were shot with it—of the" lips which  pour forth sweetest
eloquence—of the larynx, whose vibrating fills the  surrounding air  with
music; and, more  wonderful than  all, of that mass shut up within the body
fortress of the skull, whose  delicate and curious texture is the   abode of the
soul, with its reason which  contemplates, and its sensibility which  delights
in these and endless other miracles of creation.
                           PELVIC APPARATUS.

                   The Secretion of the Kidneys, 8,-c.

  Of the large  quantity of fluid  daily taken  into  the  human body, much
escapes with the breath, as  is proved  by the visible condensation of it in
     31 
470
FLUIDITY IN RELATION TO ANIMALS.
frosty air, or on any cold polished surface held near the mouth ;  part escapes
by the skin in perspiration;  but the greatest part, after having answered the
purposes of the constitution, is separated from the blood by the two secreting
organs, called the  kidneys, and from them by fit channels, is carried  off,
holding in solution various other matters, which the system does not require.
The kidneys  are situated in the loins, one on each  side of the spine;  and
the constant  drain of liquid  from them passes down by two membranous
canals called ureters into the bladder, from which the liquid is again expelled
through  the urethra, at considerable  intervals,  according to the rapidity of
accumulation.
  The bladder is a curious  membranous and  muscular reservoir,  of which
the fibres can contract so as to expel the last drop, and yet can yield so as to
admit a quart or more.
  The passage of fluid downwards through the ureters from the kidneys to
the bladder resembles, in some respects, the  passage of blood in  the  veins.
Authors  have erroneously supposed  that the weight of the fluid suffices to
cause its descent: but the  bladder  and ureters are  enclosed in a common
cavity with the intestinal canal;  and while this is full of a semi-fluid mass of
greater specific  gravity than  the urine, the latter is  not  only supported by
the surrounding pressure, as water would be supported by water, but is forced
upwards  or resisted, as water would  be  in  honey or treacle:  in descending,
therefore, it obeys some other force than  gravity, namely, that of the secreting
vessels and heart.
  The ureters, bladder and urethra are the seats of some of the most distress-
ing diseases  to which the human frame  is liable.   Two  classes of these
being relievable  chiefly by mechanical means, require to  be shortly  consi-
dered  here.  They are, obstructions in the urethra; and concretions, or
stones, as they are called,  in the bladder.
   Obstructions or strictures in the urethra are generally consequences of an
inflammation, which has destroyed the dilatability of a part of the canal.  They
appear as if a thread or a bit of tape  were tied round the canal, so  as to nar-
row its calibre.  Constant irritation,  which destroys the general health, fits
of fever, broken rest, and even death from  total  suppression  of urine, have
been common consequences of stricture.
  Until within a recent period, the treatment of such obstructions  was pur-
sued  very generally according  to a blind routine.  The attempt was made
either to bore them open by wedges, called bougies, often of doubtful  and
tedious operation, or to destroy them by caustic passed down to them in the
end of a  bougie, which caustic often  hurt  the  part of the canal anterior to
them, or eat out false passages about the stricture, or opened blood-vessels so
as to cause dangerous haemorrhage.
  Struck by the defective state of this branch of the healing art, the author
of the present work, while  abroad, and  situated  where he had interesting
opportunities of observation,  had bestowed considerable  attention upon it,
and he then contrived and  tried several  new means of relief.  These were
afterwards brought more extensively  into use and improved, and others were
added, by his brother Dr. James Arnott, superintendent surgeon in the ser-
vice of the  Hon. East India  Company, who gave a minute account of them
in a treatise on urethral diseases, and a supplement, published in  the years
1818 and 1820.   They have become, perhaps,  still better known in France
than in England, through the  work of Dr. Ducamp, which described  them,
and which, having been submitted to the French Institute, and most favoura-
bly reported upon by  the appointed authorities, soon became a standard trea-
tise in the country;—in France, also, the philosophy of mechanics had been 
PELVIC APPARATUS.
471
obliged  to  JfT  ™orege»era«y than  in England.  It is  painfol to be
vS!„r r    '  thai Dr' DuCamP' as reSarded these instruments  and the
views of disease and treatment which had  suggested them, concealed the fact
ot his being only a translator.  The  imposition was  not discovered  at the
time ot his death, which happened two years afterwards, hastened apparently
by the fatigues of the  extensive  practice  which the report of the Academy
brought upon him.  The author has had  so much pleasing intercourse with
enlightened and  honourable Frenchmen, that it pains him to have this fact to
relate.
  The objects aimed at by the new means were,—to ascertain the exact con-
dition of the diseased canal—to facilitate the passing of instruments in cases
of difficulty—and to effect a permanent cure.   The following seven of these
means may here be particularized :
  1st. An examining sound ; being a bougie with the point formed of a softer
tenacious material;  in which fibres of cotton  or silk are mixed to  prevent
any portion from being broken off or detached  during use.  This  sound.
pressed against the obstruction, takes a correct impression of its anterior face,
and shows the magnitude and exact position of the still remaining opening.
  2d. An expanding or dilator sound, which  is a small tube  with a dilatable
button at its extremity.   The button consists of a little  bag, which is passed
through the stricture empty,  and  is filled  with fluid after  it has passed.   It
readily discovers any other strictures beyond the first, and, to a certain degree,
the state of each.
  3d. A conducting canula or tube, open at both ends.  It is passed down
to the stricture,  for the purpose  of supporting and  directing small bougies
seeking entrances through very narrow strictures, or of guarding the  caustic
bougie in its approach  to the place of its action.
  4th. In cases  where the attempt to open the passage  has failed by all com-
mon means, a conducting tube is first introduced, and through it six or more
small bougies are passed side by side, so as to  probe  the whole face of the
stricture at the  same  time.  It is thus scarcely possible that  the  opening
should not be found.
  5th.  Were even this means to fail, the  conducting tube may be filled with
water, under any degree of pressure,  which water will either open  the pass-
age for  the small  bougies, or will itself act as the sharpest and  most insinu-
ating  of all instruments.  The stricture, by which ever means opened, will
then allow the urine to escape.  As patients might fear that water forced to-
wards a bladder  already too full would only increase the evil, J. Arnott waited
for  more numerous proofs of the  utility  and  safety  of the  practice, before
strongly recommending it: Dr.  Amussat, of Paris, has since published a state-
ment  of numerous cases of retention thus  relieved.
  6th. A dilator for widening the stricture, after a small  instrument  can be
passed through it.  It  is intended as a substitute for the bougies and sounds
of former times.   The chief objections to these last are, the  painful friction,
the  danger of making false passages, the tediousness and imperfection of the
cure,  and that they cannot dilate any part  of the canal beyond the size of its
orifice, through which they have to pass, and which, during health, is the nar-
rowest part of the canal.                               ,*,,-,
  The dilator consists of a tube of thin membrane introduced while empty
into the stricture, on  a ball-pointed wire, and then filled with fluid by  a
syringe  so as to dilate the stricture, with  any  degree of force, from  the mere
fillinoVf the part to the strain of the hydrostatic press,  sufficient to  tear the
strongest texture that disease can form.  The dilating tube is about two inches 
472
FLUIDITY IN RELATION TO ANIMALS.
 long, and its end next to the operator is fixed to the point of a small catheter,
 through which the distending fluid is injected.  The tube is  formed of thin
 silk riband of various sizes, with the edges joined.   It is lined with prepared
 gut of the cat or dog, which is almost as thin as gold-beater's skin, although
 very strong  and water-tight.; and  it is  covered with the same to give  the
 smoothest and softest possible  external  surface.   When  complete and  en-
 closing its blunt wire, it is still much less bulky than the bougie which would
 be required for the same case.  Thus, it  passes easily; it cannot tear the canal
 or make false passages ; it can enter through a small orifice, and then dilate to
 any desired extent; and its  greatest  advantage is, that by swelling so as  to
 follow the yielding of the stricture, it can effect at one application, what only
 a succession of hard bougies, during  long treatment, could accomplish.  In
 one day it has often removed disease  which  had resisted other means  for
 months or even years.
   Some practitioners and critics, not understanding the law of fluid pressure,
 (explained at p. 128) objected at first to the dilator, that a little  water or  air
 pressed into it by a syringe,  would be unable  to overcome much resistance.
 Had they seen the instrument lifting so  readily as  it does, a heavy weight
 laid upon it, or snapping a strong ligature tied  round it, they would  not have
 had this prejudice.  It was objected,  also, that the instrument would do mis-
 chief by dilating the urethra  before and  behind the stricture  more than the
 stricture itself; now its dimensions being determined and  fixed by those of
 its silken tunic, it never can distend beyond the diameter chosen, and, there-
 fore, if of the proper size, it can only press on the  stricture itself.  It was
 also said, that this instrument requires, in the  operator, greater  manual dex-
 terity and acquaintance  with mechanical  philosophy than many surgeons
 possess; but this is merely saying that the arts  are progressive, and that the
 accomplished surgeon of the present  day is more dexterous  and intelligent
 than his predecessors of the last  century.   It is  not accounted  a reason why
 the delicate apparatus of  the  oculist should fall into disuse, that all surgeons
 are not able to apply it.
   Some attempts had been made before,  to construct a dilator of fluid pres-
 sure, but they produced nothing of value.   For urethral purposes, a simple
 gut or intestine is worse than useless, for, being yielding in its texture, the
 surgeon can never know  truly the size of his instrument, and  therefore may
 do much mischief by it.   Dr. Ducamp, in speaking of the dilator, allows that
 he did  not first invent it,  but then, from ignorance of what constitutes its true
 value, he takes praise to  himself for simplifying and improving it, by throw-
 ing away the silk, and using the gut only.—A variety of metallic dilators have
 been contrived and used by English surgeons since the publication of Arnotfs
 Treatise on Strictures, but although manageable with less  trouble than the
 fluid dilator, they want its chief  merits.
   The dilator is applicable to many other purposes  in  surgery besides that
 now mentioned,—as for removing stricture of  the gullet, and of the rectum,
 for checking haemorrhage in deep wounds, for dilating wounds as a tent, &c.
 And the operation of lithotomy was saved to a gentleman, whom Sir Astley
 Cooper and the author of this work were attending  together, by the dilator
 opening a fistula  in perineo, so that a large stone was extracted  without cut-
 ting.  The dilator has also served in removing stones from the female bladder.
   7th. Another improved means for the  treatment of stricture,  described in
the treatise, is a mode of applying caustic for its entire destruction,  but so as
not to touch any other part of the canal.   Formerly the caustic was applied
to the face or anterior part of the structure, and, therefore, had  almost always 
                             PELVIC  APPARATUS.                         473


fibre/^thl P.0?i0f °rf thelhealthy canal ^fore it could reach the narrowest
fibres 'r7hP n  fnV   s"ch Portlon depending on  the distance  from  these
fhem    T.  P    iWhere u16 Uning °f the canal be£an  ^ be drawn  inwards by
 J'™ 'A™  exPla'ns why not unfrequently a hundred applications of caustic
were made  in a  single case, and why, during such treatment, false passages
were often bored, and other mischiefs produced.  Now by applying the caustic
within the stricture at once,  a single application  generally suffices.   To  ac-
complish  this  a ring of caustic is placed (as  described in  the Treatise, and
in the  Cases,)  on  a bougie  of peculiar construction, about an inch from its
extremity;  and the bougie being then passed  down  to the stricture through
a tube or conductor,  and  the point being  passed beyond the stricture, the
caustic is  guided  to the very spot where it is  desired  to act.*


  * Dr. Ducamp incurred a singular risk in giving himself out as the first proposer of the
instruments and practice  described above; for he was already known as a  translator of
English medical books, and  the Treatise  on Strictures of J. Arnott had been held  up to
public attention two years before by the various medical reviews, in terms such as the fol-
lowing : " We have carefully.perused this little volume, and are of opinion that it is by far
the best systematic work on the subject in  the English language."—" It is a judicious com-
pilation, interwoven with  much original and  acute observation ; and it gives publicity to
instruments which promise to be of essential  benefit to operative surgery."—Medico-Chi-
rurgical Review, January, 1819.
  Perhaps Dr. Ducamp imagined  that the slight alterations  proposed  by him in the con-
struction of three of the new instruments, might be a shield to him when detected: but as the
chief merit was in the  analysis of the subject which suggested  such instruments and not in
the  mere mechanical fulfilment of intentions, even a  considerable  improvement in the in-
struments would not have been a sufficient excuse. His changes, however, were either trifling
or retrograde.  His metallic dilating sound is less perfect than metallic sounds contrived
by J. A., but  not described, because the fluid dilating sound  was found to be preferable.
Hisporte-caustique  is  defective in not distending the stricture at the moment of applying
the caustic ; and his mode of making a dilator without the silken tunic, renders it not only
a useless, but a dangerous instrument:—indeed, such as obliged him to use the caustic in
almost every  case.   His silence with respect to the liquid probe favours the conclusion that
he did not understand it, although Dr. Amussat of Paris  has since used it with much suc-
cess:—and the same remark applies to the double catheter (see Arnott's cases,) or sonde ii
double current, as it has been called by those who have since used it in Paris.
  The following are extracts from the report made by the commissioners of the French
 Institute, Doctors Deschamps and Percy, in May, 18ii2, on the subject of Ducacap's work
entitled Traiti des retentions (Purine.
  " This treatise concerning a most  important malady, because one of  the most common
and painful which affects humanity, has appeared  to us to merit more than ordinary atten-
tion.
  ." When, some years ago, your same commisioners had to express their opinion of ano-
ther work on this subject, they commended the zeal and industry of its estimable author
(Dr. Petit;) but they could not conceal that there were still imperfections in his  modes of
treatment;  and also that they were almost entirely either borrowed or imitated from the
English.
  " The work of Dr. Ducamp now leaves us, however, nothing  more to desire, and we have
no longer reason, as regards this subject, to envy our neighbours.  Although  a volume of
moderate size, it is incomparably more complete and full of matter than the bulky treatises
lately published in other countries.
  " * * Ducamp leaves all these authors far behind him, whether as to the soundness of his
doctrines, the superiority  of his trials, or the invention of instruments.
  " He takesa printor model "of the stricture by an instrument of his invention, called Sonae
Exploralrice.  (Arnott's examining sound, page 471.)
  " For introducing bougies  in difficult cases, he uses an elastic gum tube, which he calls
conducteur.   (Described above, page 471.)
  " Mr D  has invented, for measuring the length  of strictures, &c, an  instrument wnicn,
when introduced, enlarges beyond the stricture. ( The dilating sound, page 471.)
  " The nitrate of silver, or common caustic, is what he uses for destroying strictures, but
heemnlovs it in a new manner, which appears to us to give it  new  powers, and to deprive
it of all its former dangers.   *  *   He carries the caustic into the stricture by  means ol  his
parte caustique.   (See  above, page 473.  JYo. 7 of Mr. Arnott.)
  «     ....To enlarge  the canal at the morbid part of its true calibre, he u
ses an instrument
which"he"names a dilataleur.  {Dilator,page 471.)  He does not conceal that this instru- 
474
FLUIDITY IN RELATION TO ANIMALS.
  Stone in the bladder is another disease relievable chiefly by mechanical
                                 means.
  The urine, as secreted in the kidneys, contains dissolved in it, a variety of
substances which, under certain circumstances, separate and assume the solid
form,—as sugar separates in small crystals  from cooling syrup, or salt from
cooling brine :—and it is thus that those minute grains  are produced which
we  call urinary gravel.  A  single particle of gravel remaining by any acci-
dent in  the bladder, soon attracts to itself more matter of the same kind, and
becomes the nucleus or centre of an  increasing mass, which is what we call
the  stone in the bladder.
  In a second Tract by the author's brother, published in 1820,* the follow-
ing paragraph appears:
  " From the severe suffering of the patient labouring under stone  in  the
bladder, and the remedy being an operation so painful  and dangerous, that
many wear out  their lives  in certain misery, rather than submit to it, it has
arisen that no part of surgery has excited more attention, either in the medical
profession or out of it.t  No very important change in the treatment of this
disease has now been made  for  upwards of a century; and, indeed, it has
appeared to be the opinion of modern surgeons, that the manner of operating
practised by Cheselden, about a century ago, and which has  been called the
'glory of English surgery' was so nearly perfect, as to  leave little room for
improvement.   The hopes which the rapid  progress  of chemistry, and  the
grand  discoveries  relating to  stone of Scheele, Wollaston,  Fourcroy, and
others,  some time ago gave birth to, that we should  be able to dissolve stone
by  lithontriptics, and thus save  the horrors of lithotomy, had again  died
away,  and the researches of many ingenious men who have  been, and still
are  employed about  the question, have, for their end, more  to prevent  the
formation of stone by remedies and regimen, than to  improve the manner of
removing it when once  formed.  I trust, however, notwithstanding the sup-
posed exhausted nature of the subject, that the following essay will prove that
much was still possible in the improvement of this department of the healing
art."
  The  publication from which the above paragraph is taken, and the "Trea-
tise" which preceded it, in  both of which  new  instruments and new pro-
cesses  were described, and interesting facts were detailed, aroused the public
attention in England to the possibility of improving the treatment of stone;
and about the same time, a similar spirit awoke  with  more decided effects in
France.  The results have now become of great importance to humanity.
In  the  medical publications  since  that time, cases  soon began to  be re-
corded  in lithotomy superseded by  new means, and  lately such cases form
the majority.   We shall now briefly animadvert to  the principal  of these

ment had been imagined before him, but he has the merit of perfecting it, and of reducing
to practice what before had  only existed as  a project.
  "*••■•.......In rendering justice to the able men who have preceded Ducamp, we must still
say, that no one has displayed so much industry, dexterity and talent, and we think that he
has  high claims to the confidence of patients and the gratitude of the profession, and that his
work merits the eulogium of the Academy.
               (Signed)  "Deschamps,—Pehct, Reporters,
                        Cuyier,.........Secretary."
  *  Cases illustrative of the Treatment of Urethral Obstructions and of  Stone.  By James
Arnott.—Longman and Co., 1820.
  + The Catalogue of authors who have written upon stone occupies in Plocquet'a Litera-
tura Medica, no less than twenty-nine very closely printed quarto pages. 
PELVIC APPARATUS.
475
means, intending, however, only to interest the reader in a manner that may
lead him to the perusal of the original works, where more minute information
is to be found.  They shall be named in the order in which they have come
into use.
  The dilator, as applied to the treatment of stone, has already been spoken
of in the preceding pages.
  The  double catheter.   This  instrument, with its applications  to cases of
stone and other affections of the bladder, is described in ArnoWs  Cases.  It
has  two channels, by one of which a fluid may pass into the  bladder, while
by  the  other there  is a returning current mixed with urine.   It is equipped
with two pliant tubes, of which one leads, from  a  supplying reservoir, and
the other to a waste vessel.  It will soothe irritation of the bladder, whether
arising from stone  or not,  by keeping the acrid urine in a diluted state, or by
applying bland and  medicated liquids directly to the internal surface of the
bladder.   Not being larger than a common catheter, it may be worn  for any
period as the common catheter now is.  It need prevent no sedentary occu-
pation, and may be used  during  sleep.   It will act powerfully to  dilate a
contracted bladder, if the reservoirs be placed high, and the fluid be caused to
distend with the pressure of a lofty column.   It also affords by far the best
means of admitting to the  bladder any solvent of stone.   Even pure water is
 a weak solvent of  most  animal calculi, as is proved  by placing them in a run-
 ning stream ; but  the living bladder bears with impunity a diluted acid or
 alkali.
   The syphon catheter  (also first described in Arnott's  Cases) is merely a
 catheter of a length that will allow its external part to descend, so as to con-
 stitute  the long leg of a  syphon.  (See Pneumatics.)  Its outer extremity is
 turned up a little,  or has a portion of soft  animal gut tied upon  it to act as a
 valve, for  preventing the entrance of air.  The most useful application of this
 instrument is to keep the  bladder empty after operations, until the  healing
 process has made  a certain advance.  The diffusion of urine among the sur-
 rounding  parts after lithotomy, particularly after the high operation, is often
 a cause of death;  and the syphon catheter, by providing a channel by which
 the urine  must immediately  pass away as secreted, obviates  the  danger.
 This instrument is sometimes useful in very irritable bladders, by preventing
 the repeated distensions  of the bladder, with the consequent  excruciating
 contractions.  It has also  relieved in the deplorable case of the bladder torn
 or  opened by sloughing in parturition, as it can  keep the unhappy patient
 quite dry.
    Af
 there
 to it,  »v
 but was ao-ain foraotten until John Hunter's investigations led him to  a second
 invention of it.  Such an  instrument had for a considerable time passed under
 the appellation of Hunter's urethra or bladder forceps, answering for extract-
 inff small stones, and  therefore, if  used  in time, preventing occasionally the
 necessity of lithotomy.   Soon after the publication of Arnott s  Essay, it was
 modified  and much more extensively used by Sir Astley Cooper and other

 ^ButTnew and  intense interest has now been  excited with  respect to the
 forceos  as a means of  removing stone, by the discovery—also an old disco-
 verrevived—thatarfraifffa tube may be passed to the bladder, as a conductor,
     iad of the bent tubes or catheters commonly used.   A door is thus, as it
     re, opened directly into the bladder, through which a stone might even be
very
ins
were 
476
FLUIDITY IN RELATION TO ANIMALS.
seen, if light were directed upon it, and through which it easily may be caught
and broken to pieces, and brought away without doing injury to the living
parts.  Dr. Civiale, of Paris, had  the  merit first of contriving  good instru-
ments for this operation, and of himself operating with complete success in
many cases.   But the praise of carrying the operation of Lithotrity (stone-
wearing-down,) as it is now named, to its present state of perfection, is shared
by various other ingenious surgeons, as  Gruithuisen (who first used  the
straight sound,)  Amussat, Leroy,  Heurteloup, (who proposed the mode  of
percussion,) &c.   The operator introduces a strong forceps, which  seizes
and holds fast the stone, and  he then weakens the stone by boring it in various
directions with a simple drill, which passes through the handle of the forceps,
and is turned rapidly by a drill-bow acting on its external end, or with a drill
of which the point can be bent to one  side, so as to excavate to any desired
extent: after which weakening, the stone is crushed, either by the forceps
which first held it, or by another instrument called brisecoque, made on pur-
pose:—or without boring at  all, Heurteloup and others at once break the stone
to pieces by blows of a small hammer acting on a sliding limb of the forceps.
  Dr. Darwin, in his Zoonomia, published  in 1790, proposed to seize stones
by forceps passed into the bladder, and then to break them down or destroy
them mechanically;  but  the supposed necessity of working through a long
bent tube prevented trials from  being made.   The author of this work also
showed some  years  ago, before any  of the above-described improvements
were made (see Cases, page 93,) that it was possible to pass a bag into the
living bladder, and to enclose a stone there,  so that any solvent might  be
injected into the bag, and again  withdrawn without coining into contact with
the bladder.  This was shown  rather to excite attention to the possibility of
operating within the  living bladder with great  precision, than to recommend
that precise means of destroying stone.
  To all the ingenious instruments above spoken of for breaking down the
stone, there is still this objection, that it is broken into such fragments, that
many of them require to be afterwards treated as  distinct stones, and thus the
painful operation has to be repeated again and again, and whole months may
pass before the operation be completed.—The  author deems it possible to
make a forceps of several claws or ribs, which should surround the stone so
loosely as to leave it  freedom of motion within the claws, like a loose kernel
in a shell, and so  that on making the forceps itself whirl backwards and for-
wards, like the drill in Civiale's apparatus, the stone might be quickly rubbed
to dust by the friction or  file action of  the  roughened interior of  the claws.
The bladder would be filled  during the operation, with water, or even air, to
secure  plenty of room for the turning instrument:—or a slender  external for-
ceps might be used as a guard, to prevent contact of the bladder with the
moving instrument.   Out of the body, a stone harder than urinary calculus,
placed in such a cage with  rough  interior, and  subjected  to the  action de-
scribed, is soon reduced to dust.  There are various ways of making a forceps
or cage for this operation, which will readily suggest themselves  to persons
knowing what has already been achieved in this^department  of practice, and
having the ingenuity  likely to engage them  in such a pursuit.
  The high operation of lithotomy possesses over the common lateral ope-
ration such advantages as the following:—thinness of the parts cut through
—distance of the knife from important arteries—stones of very large size may
be more easily extracted—the prostate  gland is not wounded.  But the high
operation  has not become general,—because—there was difficulty in avoid-
ing the peritoneum while making the opening  into the bladder—there was 
PELVIC APPARATUS.
477
danger of effusion of urine among the cut parts, after the operation—and where
the bladder was contracted, the incision had to be very deep.   Now these
objections  are obviated by, 1st, the double catheter, which  will dilate the
contracted  bladder; 2d, by the syphon catheter, which will prevent the effu-
sion of urine; and 3d, by the jointed sliding sound, (see Cases, page 104,)
which will ensure the accurate cutting in the desired place. Had we possessed,
then, for the removal of stone, no less hazardous  means  than  cutting, the
high operations with the new securities might have been best.
   When a catheter has to be retained in the bladder after any operation, in
cases where, if it slipped out, it might with difficulty be replaced, something
should be passed through it like a small  spring forceps to expand and be-
come an internal button preventing its escape.  (See Cases, page  94.)
                          UTERINE PHENOMENA.

   Although so many of the uterine phenomena are mechanical, there are few
of them which could be treated of with advantage, except in connection with
particulars, of  which the  consideration does not belong to  a work like this.
We shall,  however, cite the following particulars as examples.
   The protection given to the tender  fcetus by the liquor amnii in which it
floats, is such,  that a blow from without is expended on the surrounding water,
and cannot reach the foetus.
   The head  of the foetus, because ossification  begins in it first, becomes of
greater specific gravity than the other parts of the body, and therefore gene-
rally lies at the bottom of its liquid bed.  It is thus ready to  appear first in
parturition, according to the safest course of delivery.
   The membranes distended by the liquor amnii descend before  the head, as
a soft but  powerful wedge preparing the way, according to the principle ex-
plained in  a previous page.
   We have spoken, at page 168, under the name of pneumatic  tractor, of  a
circular piece of leather or similar soft substance, kept extended by included
solid rings or radii, as being adapted to some  purposes of surgery.   Now it
seems peculiarly adapted to a  purpose of obstetric surgery, viz., as a substi-
tute for the steel forceps, in the hands of  men who are deficient in manual
dexterity, whether from inexperience or natural inaptitude.   The forceps, to
be well and safely used, requires address, which even the naturally dexterous
man cannot possess without a certain degree of continued practical familiarity
with it, and except in large towns, a man  must be  very unfortunate in his
practice who often requires it: hence the really small number of persons who
use  it well.  The consideration of the tractor as a substitute  for it belongs
properly to the present section: but as the true mode of action of the tractor
is not very readily conceived by  persons who either have never  been in-
structed in the  general  laws of physics, or who  have ceased  to be  familiar
with them, such persons are advised to read  this paragraph in continuation
of that at page  168, and to weigh well the following remarks.   A tractor of
three inches in  diameter, would act upon any body, to lift or draw it, with a
force of about a hundred pounds—with more, therefore, than is ever required
or allowable in  obstetric practice.   Tn lifting a stone, the tractor  does not act
as if it were glued or nailed to the  stone, but  merely bears or takes off the
atmospheric pressue from  one  part, and allows the pressure on the opposite
side not then counterbalanced, to push the stone  in the direction of the trac-
tor .l_s0 when  placed upon the head, it would not pull by the skin, in the 
478
FLUIDITY IN RELATION TO ANIMALS.
manner of  a very strong adhesive  plaster  applied there, as  uninformed per-
sons would be apt to suppose, but by taking off a certain atmospheric pres-
sure from the part of the head on which it  rested, it would allow the pressure
on the other side or  behind to urge the head forward on  its way.  Of course
the forwarding pressure in such a case would not operate on the head directly,
but through the intervening parietes  and contents of  the maternal  abdomen.
It would  be much belter to have a gentle and  diffused  action of the  tractor
over a large surface, than an  intense  action on  a small surface, and therefore
a tractor  for the purpose now contemplated  should   not be very small, and
should  have a little  air  underneath it in a slight depression or cavity at its
centre.—The  forceps must be  more effective  than the  tractor for rectifying
malposition of the  head, and  diminishing its  transverse  diameter ;  but the
tractor  will answer both these purposes in a degree greater than many would
expect.*    The author proposes to  publish on this matter,  and on some other
strictly professional  subjects  which are lightly touched  upon in the present
general work, such a practical detail, as for the dilator, syphon catheter, &c,
is found  in his brother's "Treatise" and "Cases."


                                  Conclusion.

  It  is almost superfluous  to remark  here,  that, for the  practice of  general
and obstetric surgery, learning and judgment are of little avail unless  accom-
panied  by manual dexterity :  and it is one of the improvements yet to be made
in our system of education for various  professions, to cultivate  more metho-
dically the use of the hand3.    Children and young  people, in obtaining prac-
tical  familiarity with  ingenious toys, tools of carpentry,  games of address,
musical instruments, Sic, are often fitting themselves for  the important busi-
ness of their future  life.


  * We have been already compelled on  one or two occasions to differ from the able author
of this work, in relation to the practical application of some  of his principles, and we must
be again permitted to record our dissent  from his opinion that the  pneumatic tractor, under
certain circumstances, is peculiarly adapted as a substitute for the obstetric forceps.  Our
author cannot be a practical accoucheur  or he would at once perceive that the  various ma-
noeuvres by  which labour is assisted with the forceps, cannot be accomplished  with the
tractor.  That  address and knowledge are requisite to apply the forceps properly, is  no
objection to  their use; it only shows the necessity  of the operator's acquiring this dexterity
and knowledge before attempting to apply  the instruments ; and these acquirements are not
so difficult as our author seems to think, nor  do we believe that  the number  who possess
them is so very small.  It is not contended even by the author that the tractor is superior to
the forceps ; he only recommends  it as being less dangerous in the hands of the unskilful.
Now it might be supposed from this that the tractor is  readily applied and cannot effect in-
jury, both  of which are erroneous.  Every instrument is dangerous in the hands of igno-
rance.  If a person deficient in dexterity could succeed in applying the tractor, (of which
we have strong doubts, believing it would require, in most instances, even Dr. Arnott's skill
 and knowledge) it is quite as probable that he would produce injury as benefit. In certain
 states of labour, the tractor may be applied to the neck of the uterus instead  of  the head
 of the child, or to both, drawing out the uterus thus as well as the child ; it may be applied
 before the uterus is sufficiently dilated, or the force  may be  applied in the wrong direc-
 tion ; indeed, there are but  few cases in which force could be applied  in the proper direc-
 tion with  the tractor, Sec.  These accidents cannot happen to the well instructed ; but in the
 hands of such, the forceps are more effectual and equally safe.   The tractor, then, requiring
 skill for its  proper application, and being a less efficient instrument than the forceps, ought
 not, independent of many other reasons, to be recommended.  It is  not to those who devise
 imperfect substitutes for valuable instruments, or temporary palliatives for important ope-
 rations, in order that the awkward and ignorant may  imperfectly perform what the skilful
 or instructed should only attempt, or are capable of accomplishing, that praise is to be
 awarded.  It is the just meed of those who furnish proper instructions for the use  of instru-
 ments and for performing operations, and present the means  of gaining  information and
 skill.  Am. Ed. 
PELVIC APPARATUS.
479
  While the author directs the attention of the  profession to the important
physical considerations set forth in the  preceding pages, he deems it  neces-
sary most pointedly to remark, that in the living  body mechanical principles
are generally associated in their operation with the more  recondite principles
of chemistry and of life; and that the man who allows his mind to dwell
too exclusively oh any one of the three classes, must be a very bad reasoner
in questions either of health or disease.  It is within a very recent period,
however, that just views on  this subject  have begun to prevail, and that the
titles of the peculiarly mechanical physician, or  chemical physician, or phy-
sician  attending only to the influence of nerves  or life, are likely to be no
longer justly applicable.  The light of true philosophy is at last breaking in
upon the very complex and difficult subjects of medical inquiry ; and where
formerly keen penetration beheld only confusion, even common minds now
begin to see clear divisions and beautiful  arrangement. 
 
THE
ANALYTICAL   TABLE
Introduction.
  Progress of man and stationary condition of inferior animals.
  The progress more rapid at present than ever.
  The divisions of human knowledge.
  Natural philosophy particularly considered.
Synopsis.
  The fundamental truths of natural philosophy explained under the terms
    atom, attraction, repulsion, and inertia—the divisions of this  work,
    page 19.
PART I.—The FOUR FUNDAMENTAL TRUTHS minutely examined
   and used to explain the general constitution of material substances, and of
   the motions going on among them, page 22.

      Sect.  I.—Constitution of Material Masses, 22.

         Atoms—minute—indestructible—occupying space, 22.
         Attraction  of atoms is mutual, 25.
           Gravitation, 26—Cohesion, 27—Capillary attraction, 28—Che-
           mical attraction, 28—Definite proportions, 29.
         Repulsion—The influence of heat on  masses, 30.
           Solid—liquid—air, 31.
           Repulsion of surfaces, 32.                      .
           Modification of Masses: 33-Crystal, 33-Poros.ty, 34-Den-
             sitv,  35—Hardness,  36—Elasticity,  37—Brittleness, 38—
             Malleability, 38—Ductility,  38—Pliancy, 39-Tenacity, 39.
         Appendix, Properties of Matter, 41.

      Sect.  II.—Motions among  Bodies, 41.

           Motion and Best;
         Tnertia of matter, 43.                               ,  ,   .„
           Motion is  naturally permanent, 46—uniform, 47—straight, 47.
           Centripetal and centrifugal forces, 49.
           Quantity of motion and  force, 53—momentum, 54.
           Direction of forces and composition of motion, 55.
           The two forces of nature are Attraction and Repulsion,  58. 
482
ANALYTICAL TARLE.
  Accelerated motion, 58.
  Retarded motion, 60.
  Pendulum and balance wheel, 60.
  Projectiles, 64.
  Tides, winds, currents, Sic, obey attraction, 66.
  Explosion, steam, &c, obey repulsion, 66.
  All great velocities are results of continued action, and are destroyed
     by continued action, 67.
  Action and reaction equal and contrary, 70.
Appenoix,—Definitions, 74—Laws of motion, 75—Impulsive force
  and rectilinear motion, 75—Constant force  and constantly accele-
  rated motion, 76—Of gravity, 77—Motion produced by joint forces,
  77—Equilibrium, 78—Joint action of an impulsive and constant
  force, 78—Laws of central forces, 80—Joint effect of active and
  and inactive forces, 80—Pendulum, 82—Impact of bodies, 83.
PART II.—DOCTRINES OF SOLIDS, or MECHANICS, 84.

         Centres of inertia and gravity, 85.
           In animal bodies, 90—Sea sickness, 91.
           Centres of percussion and oscillation, 93.
         Solids moving round a centre, or so that different parts may have
             different speed, 94.
         Simple machines, 95.
         Lever, 95—Wheel and Axle, 103—Inclined plane, 105—Wedge,
           106—Screw, 107—Pulley, 108—Engine of oblique action, 109.  •
         Fly wheels, 111.
         Complex machines, 113.
           Friction, 114.
           Wheel carriages, 115—Rail-ways, 116.
         Strength of materials, 119.
           Influence of form—Arches, &c, 122.
PART  III.—DOCTRINES  OF  FLUIDS,  or  HYDRODYNAMICS,
         126.

       Sect. I.—Hydrostatics, or fluids in repose, 126.

         Pressure in a fluid extends equally through the whole, 127.
           Hydrostatic press, &c, 129.
         Pressure in a fluid increases with the. depth, 129.
           Compressibility of water,  &c, 131.
         Not influenced by shape of vessel, 132.
         Level surface of fluids, 133.
           Spirit level, 133—Canals, 134—Running streams, 135—gradual
             change of the earth's  surface produced by running water, 136.
           Same level in communicating vessels, 139.
             City water  works, 140.
             Springs and wells, 141. 
analytical table.
483
     Support of bodies floating in fluids, 143.
       Specific gravities, 144.
       Floating bodies, 148.
       Swimming of man and inferior animals, 149.
       Ballast, 152.
       Fluids of different density, 153.

Sect. II.—Pneumatics, or  phenomena of air, 156.

  Lightness, 158.
  Elasticity, 158.
     Air-pumps, 159—Diving-bell,  162—Water-balloon, 163—Hiero's
       fountain, 164.
  Pressure in all directions, 165.
  Pressure as depth, 165.
     Weight of the atmosphere, 167.
  Atmospheric pressure on  solids, 167.
     Magdeburgh hemispheres, 168.
     Pneumatic tractor, 168.
  Atmospheric pressure on liquids, 169.
     Pumps, 171—Syphon, 172—Intermitting fountains, 173—Bird-
       glass, 174—Vent-plugs, 174.
  Atmospheric pressure on  animal body, 175.
     Cupping, &c. 176.
     The barometer, 176.
  Atmospheric pressure determines the  liquid or aeriform state of aeri-
       form substances, 182.
     Boiling, 182.
       —at different heights,  183.
       —in vacuo and distilling, 184.
     Elastic force of steam,  186.
       Steam-engines, 187.
       Explosions, 190, 193.
  Atmospheric pressure affecting combinations of bodies,  194.
     Effervescence—sparkling liquids, 194.
  Atmospheric pressure affecting the density and temperature  of the
       air, 195.
     Climate depending on  elevation,  196.
  Atmospheric pressure affecting the  humidity of the air,  196.
     Rain, mist, snow, hail, dew, 197—Hygrometer, 198.
     Rain and clouds among mountains, 199.
     Fluid support or floating in air, 200.
     Balloons, 200.
     Ascent of flame and smoke, 202.
     Chimnies, 203.
     Warming and ventilating of houses, 205.
       Apartments for consumptive patients, 207.
     Winds, 208.—Trade-winds, 208.
       Land and sea breezes, 209.
       Monsoons, 210.
     Pneumatic trough, 210.
     Gasometer, 211.
     Pneumatic chemistry,  212. 
484
ANALYTICAL TABLE.
Sect. III.—Hydraulics, or fluids in motion, 213.

  Fluids moving in channels or issuing from vessels, 213.
    Aqueducts, 216.
    Fountains and jets, 216.
  Waves, 216.
  Momentum and resistance of fluids, 220.
  Resistance to  ships, Sic, increases much  more  rapidly than  the
       velocity, 220.
    Steam-boats, 221.
    Paddle-wheels,  221.
  Resistance to bodies in air, 222.
  Fluid resistance limits many velocities, 222.
  --------------is influenced by shape of solid, 223.
    Water-wheels, 224.
  Fluid resistance proportioned to surface of contact, and not to quantity
    of matter, 224.
  Projectiles, 225—levigating, 225—Winnowing, 226—Washing gold
    dust, 2261
  Oblique action of fluids, 226.
    Navigation—Sails, 226—Rudder, 227.
    Windmills, 229—Feathered  arrrows,  229—Paper kites, 230.
  Lifting fluids, 231.
    Buckets—Pumps—Wheels—Water-screw, 231—Water-ram, 232.

Sect. IV.—Acoustics, or doctrines of sound, 234.

  Nature of simple sound, 235.
  Continued sound or  tone, 235—Grave and sharp sounds, 238,
Musical sounds, 238.
  Musical scale, 239.
  Melody—Harmony—Accompaniment—Time,  241.
  Tuning-forks, 242.
  Musical instruments, 242.
  Musical ear, 245.
Spreading of sound—in solid and  fluid, 246—Stethoscope, 247.
  Velocity of sound, 248—Many examples, 248.
Reflection of sound,  248.
  Echo—whispering galleries—Ear-trumpets—Speaking-trumpets,
       249.
  Animal ear, 252.
PART IV.—HEAT, 256.

    Sect. I.—Heat, phenomena which it produces, 257.

       Properties 258—Means of measuring  quantity in  bodies, 259—
         Diffuses itself equally, 259.
       Cold, 260.
       Conducting power of bodies, 260—Natural covering of  animals,
         262—Clothing, 262. 
                      ANALYTICAL TABLE.                     _jq-

    Mode in which heat spreads, 266.
      By the motion of the particles in fluids, 266.
       By radiation, 269.
    Expansion of bodies by heat, 274.
    Capacity of bodies for heat, 275—Influence of bulk on this, 277—
      Of density, 280.
    Each substance expands in a degree peculiar to itself, 281—Expan-
      sion of solids, 282—Expansion of fluids,284-Expansion of gases,

    Latent heat, 290.
    Boiling, 295.
    Distillation, 295.
    Evaporation, 295.
    Table of temperature, 300—Thermometer, 300.
    Absolute quantity of heat, 304.
    Influence of heat on chemical combinations, 305.
    Influence of heat on vegetables and animals, 307.
    Sources of heat, 309—The sun, 309—Electricity, 311—Combustion,
      311—Fuel, 317—Condensation and friction,  322—Functions of
      animal life, 323.

Sect. II.—Light, 325—Sources of, 327.

    Becomes less intense as it spreads, 327—Falling on bodies renders
      them  visible, 328.
    Shadows, 329,  331.
    Velocity of light, 329.
    Direction of light, 330.
    Transmission of bodies, 332.
    Refraction, 333.
    Lenses,  339.
    Camera obscura, solar microscope, magic lantern, 341.
    Eye, 346.
    Vision. 348, 352.
    Distinct vision,  350.
    Short-sightedness, 350.
    Long-sighledness, 351.
    Visual angle, 355.
    Apparent size of objects, 357.
    Foreshortening, 360.
    Perspective, 362, 373.
    Intensity of light, shade and colour, 369.
    Divergence of rays of light, 374.
    Convergence of axes of the eyes, 375.
    Cosmorama, 377.
    Painting representing motion, 378.
    Art  of painting, 379.
    Telescope^ 381.
    Mirrors, straight, 386—Curved, 390.
    Comparison of light and sound, 392.
    Perfection of the eye, 395.
32 
4S6
ANALYTICAL TABLE.
PART V.—ANIMAL AND MEDICAL PHYSICS.

  Sect. I.—Animal Mechanics, 399.
       Skull, &c, 399.
       Spine and its distortions, 400.
       Limbs and mechanical surgery, 401.
       Living force, 409—Treadmill,  410.
       Surgical instruments, 410.

  Sect. II.—Animal Hydrostatics and Hydraulics, or Fluidity in
       relation to animals, 414.
  1. Circulation of blood :
         In  arteries, 416.
         In  capillaries, 421.
         In  veins, 423.
       Force of the heart, 430.
       Hydrostatic or floating bed, 431.
       Velocity of blood, 435.
       The pulse, 436.
       Circulation in the head, 440.
       Effects of position on the circulation, 441.
       Fainting from diminished arterial tension, 442.
       Diffused pressure, 444.
         Mercurial bath, 445.
       Transfusion of blood, 445.
  2. Respiration and voice, 445.
       Action of chest, 446.
       Wounds of chest, 447.
       Hemoptysis, 448.
       Coughing, 448—Sneezing—Hiccup, &c, 449.
       Suffocation, 449.
         Humane Society's apparatus, 450.
         Artificial respiration, 450.
       Speech, 451.
         Modifications of voice,  452.
       Table of articulations, 455.
       Stuttering, 456.
       Ventriloquism, 461.
  3. Digestion, 463.
       Mechanism of the organs, 463.
       Effects of abdominal pressure, 465.
       Vomiting, 466,
       Stomach pump, &c, 466.
       Enema funnel, 468.
  4. Secretion of the kidneys, 469.
       The apparatus.
       Obstructions in  urethra, 470.
         New instruments and means for treatment, 471.
       Stone in the bladder, 474.
         New instruments and means, 475.
  5. Uterine phenomena.
       Protection of fcetus by the liquor  amnii, 477.
       Position of ditto, 477.
       Importance of pliysical knowledge and manual dexterilv, 478. 
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man wiilci^-  *                    .   ,   ,     i  •  -t    i *^ •  i   j
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3
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Walpole's New Letters to Sir Horace Mann, 2 v. Svo.
Walpole's Memoirs of George the Third, 2 vtiJs. 8vo.
White's Universal History, a new and improved work
  for  Schools, Colleges. &c„  with Questions by Pro-
  fessor Hart, in 1 vol. large 12mo, extra cloth, or half
  bound.
Wheaton's  International Law, 1 vol. large 8vo, law
  sheep or extra c^lh, 3d edition, much improved.
Wheaton on the Right of Search, in 1 vol.  Svo.
Mrs. Washington Potts, by Miss Leslie, 25 cents.
Wraxall's Posthumous Memoirs, 1 vol  8vo.
Wraxall's Historical Memoirs, 1 vol. Svo.
Youatton the  Horse, &c., 1 vol. Svo.
Youatt on the  Dog, with plates, preparing. 
5
                     TO   THE  MEDICAL   PROFESSION.

  The following list presents nearly all the works  on Medical  Science published,  at  press and
preparing by the Subscribers.  They will  be found to be  executed in a superior style, and  sold
at prices as low as can be afforded consistent with correct editions.  These works can be had
in all the principal bookstores throughout  the Union;  from which, or from  the subscribers, all in-
formation relative to prices, &c, can be obtained, on application, free of postage.

                                             L.EJ1   K BIsJlJYCHJlRB,  Philadelphia,
  Anatomical Atlas, by Smith & Horner, imp. 8vo, 650 figs.
  Arnott's Elements of Physics, new ed.l vol. Svo, 4S4pp.
  American Medical Journal, quarterly at $5 a year.
  Abercrombie on the Stomach, 1 vol. Svo, 320 pages.
  Abercrombie on the Brain, new ed., 1 vol. Svo, 3:24 pp.
  Alison's Outlines of Pathology, in 1 vol. Svo, 420 pp.
  Ashwell on Females, one vol. 8vo, 520 pages.
  Andral on the Blood, 120 pages, Svo.
  Bird on Urinary Deposits,  1 vol. Svo, 228 pp.
  Bird's Natural Philosophy, preparing.
  BurrowB on Cerebral Circulation, preparing.
  Budd on the Liver, 1 vol. Svo, 392 pp.
  Buckland's Geology and Mineralogy, 2 vols. 8vo, with
    numerous plates and maps.
  Berzelius on the Kidneys and Urine, 1 vol. 8vo, ISO pp.
  Bridgewater Treatises, with numerous illustrations, 7
    vols. 8vo. 3287 pp.
  Bartlett on Fevers, &c, 1 vol. 8vo., 394 pages.
  Bartlett's Philosophy of Medicine, 1 vol. Svo, 312 pp.
  Brigham on Mental Excitement,  1 vol. 12mo. 204 pp.
  Billing's Principles of Medicine, 1 vol. 8vo, 304 pp.
  Brodie on Urinary Organs, 1 vol. Svo, 214 pp.
  Brodie on the Joints, 1 vol. Svo, 216 pp.
  Brodie's Surgical Lectures, 1 vol. Svo, 350 pp.
  Benedict's Compendium of Chapman's Lectures, 1 vol.
    8vo, 258 pp.
  Chapman on Viscera, 1 vol. 8vo, 384 pages.
  Chapman on Fevers, Gout, &c, 1 vol. 8vo, 450 pp.
  Chelius' Surgery, by South and Norris. at press.
  Chitty's Medical Jurisprudence, Svo, 510 pp.
  Clater and Skinner's Farrier, 12mo, cloth, 220 pp.
  Carpenter's Principles of Human Physiology,  1 vol.
    8vo, 644 pages, with cuts, second edition.
  Carpenter's General and  Comparative Physiology, 1
    vol. 8vo, preparing, many plates.
  Carpenter's Vegetable  Physiology,  1 vol.  12mo, with
    cuts, 300 pp.                         . ,
  Carpenter's Manual, «r Elements of Physiology, 1 vol.
    8vo, 566 pages, many  cuts.
  Carpenter's Cyclopediaof Natural Science, preparing.
   Cooper, Sir Astley on Hernia, imp. 8vo, plates. 42S  pp.
   Cooper on Dislocations, 1  vol. 8vo, with cuts, 500 pp.
   Cooper on ihe Testis and  Thymus Gland, 1 vol. impe-
    rial Svo, many plates.
L  Cooperonthe Anatomy and Diseasesof the Breast, &c.
■   &c , 1 vol. imperial 8vo.  splendid lithographic plates.
   Ifiondie on Diseases of Children, 1 vol. Svo, 652 pages.
  i^nrchill on Females, 3d edition, 1 vol. 8vo, 572 pp.
  !-Clrarchill's Midwifery, 1 vol.  Svo, 520 pp., many cuts.
   Cyclopaedia  of Practical  Medicine,  by Forbes, &c.
    Edited  by Dunglison, in 4 large super-royal vols ,
WZ  3154 double columned pages.
   Carson's Medical Formulary, in  preparation.
   Dewees' Midwifery, with  plates, 10th edit, 660 pp.
   Dewees on Children, 8th edition, 548 pp.
   Dewees on Females, with plates, 8th edition, 532 pp.
   Durlacher on Corns, Bunions, &c, 1 vol. 12mOQ, 134 pp.
   Dunglison's  Physiology, 6th edition, 2 vols. 8vo, 1350
    nao-es, with 370 cuts.
   Dunllison's Therapeutics and Materia Medica, a new
  •   edition with cuts, 2 vols. 8vo,  986 pages.
   Dnndison^Medical Dictionary, 6th ed.t.on, 1 vol. Svo,
     804 verv large pages, double columns
   Dunglison on Newfeemed.es, 4th ed.t.on, 616 pp.

   S!^«ce^^^^dUS^ols.


   gSS^^S^K^^cuts.

   I  ES=C^Sv^^'nearlyre"dy-

   |S^^SertrV?i^royaH2mo,4e0

   Fe^'GSal and Special, edited by Clymer, 1  vol
     8vo, 600 pages
Fergusson's Practical Surgery, 1 vol. 8vo,2d ed , 640pp.
Graham's Chemistry, wiih^uts, 1 vol. Svo, 750 pp.
Griffith's Chemistry of the Four Seasons, 1 vol. 12mo.
  with cuts.
Griffith's Medical Botany, many illustrations, at press.
Guthrie on  the Bladder and Urethra, 1 vol. Svo, 150 pp
Hoblyn's Dictionary of Medical Terms, by Hays, 1 vol.
  large 12mo, 402 pages.
Harris on the Maxillary Sinus, 1 vol. 8vo, 166 pages.
Horner's Special Anatomy, 2 vols. 8vo, 7th ed., cuts.
Horner's Mew Dissector, lvol. 12mo, with many cuts.
Hasse's Pathological Anatomy, nearly ready.
Hope on the Heart, 1 vol 8vo, with plates, a new ed.
Harrison on the Nervous System, 1 vol. Svo, 292 pp.
Hughes on  the Lungs and Heart, 1 vol. 12mo, 270 pp.
Jones' (T.  Wharton) Manual of Ophthalmic Surgery,
  with many cms,  at press.
Jones and Todd on the Ear, 1 vol., preparing.
Kirby on Animals, many plates, 1 vol. Svo, 520 pp.
Kirby and  Spence's Entomology, 1 vol. Svo, 600 pages,
  plates, plain or colored.
Lawrence  on the Eye, a new ed., many cuts, 1 vol. Svo.
Lawrence  on Ruptures, 1 vol. 8vo, 480 pp.
Liston's Surgery, by Mutter, 1 vol.  8vo, 566 pp., many-
  cuts.
Miller's Principles of Surgery, 1 vol. 8vo, 526 pp.
Miller's Practice of Surgery, 1 vol. Svo, 496 pp.
Maury's Dental Surgery, with plates, 1 vol. 8vo, 2S6pp.
Muller'a Physiology, 1 vol. 8vo, 886 pages.
Medical News and Library, published monthly.
Meigs' Translation of Colombat de l'Isere on the Dis-
  eases of  Females, 1 vol. 8vo, 720 pp.
Metcalf on Caloric, in one large vol., Svo, (at press.)
Prout on the Stomach. 1 vol. 8vo, col'd plates, 466 pp.
Popular Medicine, by Coates, 1 vol. 8vo. 614 pages.
 Philip on Protracted Indigestion, 1 vol., 240 pages.
Phillips on Scrofula, 1 vol Svo, 350 pages.
Pereira's Materia  Medica, 2 vols Svo. 1560 very laTge
  and closely printed pages.  Second Edition.
Roy'le's Materia Medica. with wood-cuts, nearly ready.
Roget's Animal and Vegetable Physiology, with many
  cuts. 2 vols. 8vo,  S72 pages.
 Roget's Outlines of Physiology, 1 vol. 8vo. 516 pages.
 Rigby's System of Midwifery, 1 vol. Svo. 492 pages.
 Rieord on  Venereal, new edition, 1 vol. Svo. 256 pages.
 Ricord's large work on Venereal Diseases, with nume-
   rous plates  preparing
 Ramsbotham on  Parturition, with  many plates, 1 vol
   imperial Svo. a new and improved edmon, 520 pp.
 Robertson on the Teeth, 1 vol.  Svo, 230 pages.
 Simon's Chemistry of Man. 1 vol. Svo, 730pages.
 Tay lor's new work on Medical Jurisprudence, by Grif-
   fith, 1 vol Svo. 540 pages.
 Traill's Medical Jurisprudence, 1 vol^8vo, 234 pages.
 Trimmer's Geology and Mineralogy, 1 vol. 8vo, 528 pp.
 Todd & Bowman's Physiology, publishing in the Medi-
   cal News and Library, many cuts.
 Thomson on the Sick Room, 1 vol. 12mo, 360 pp., cuts.
 Vogel's Pathological Anatomy, with plates preparing.
 Walshe on the Diseases of the Lungs, 1 vol. 12mo, 310

 Wauon's  Principles and Practice of Physic, by Con-
   die, 1 vol. 8vo, 1060 large pages.
 Wilson's Human Anatomy, with outs,l vol. 8vo, a new
   and improved edition, 608 pages        .    .
 Wilson's Dissector, or Practical and Surgical Anato-
   my, by Goddard, with cuts, 1 vol. 12mo, 444 pages.
 Wilgon on the Skin, 1 vol. 8vo, 370 pag«s-
 Williams' Pathology, or Principles of Medicine, 1 vol.

 Williams*' Lecw'res on the Stomach, &c , rearing.
 Williams  on the Respiratory Organs, by Clymer, 1 vol.

 Yo8ua«5o°n  Morse, by Skinner,cuts 448pp.1jol.8^
 Youatt and  Clater's Cattle Doctor, 1 vol. 12mo, wi*
   cuts, 2S2 pages
*«* They have other works in pteparation not included in this lilt 
          LEA & BLANCHARD'S  PUBLICATIONS.

                       Now  Ready,

CARPENTER'S   NEW   WORK.
                                A MANUAL,  OR


   ELEMENTS    OF    PHYSIOLOGY,

              INCLUDING PHYSIOLOGICAL  ANATOMY.

                 FOR THE  USE OF THE MEDICAL STUDENT.

             BY WILLIAM B.  CARPENTER, M. D., F. R. S.,
           Fullerian Professor of Physiology in the Royal Institution of Great Britain, &c.

         With   One  Hundred and  Eighty  Illustrations,

                     IN ONE OCTAVO VOLUME OF 566 PAGES.

             Elegantly printed to match his " Principles of Human Physiology."

  The sheets of this volume, in their passage through the press have been carefully examined by
Dr. Meredith Clymer, the editor of Dr. Carpenter's Principles of Physiology.  The mannerin which
the work is brought up to the day, and its perfect adaptation to its purposes as an elementary text-
book for students have rendered  unnecessary any alterations or additions.  The efforts of the pub-
lishers have therefore been directed to obtaining a correct reprint ofthe London edition.
  "The  author has shown singular skill in preserving so  marked a line of distinction between the present
Manual and the ' Principles of Physiology' previously published by him. They are both on precisely the same
subject; but the one  is neither a copy, nor an abstract, nor an abridgment of the other.  In one thing, how-
ever, they are exactly alike—in their general  excellence, and  in their perfect adaptation to their respective
purposes.  The reputation of Dr. Carpenter as a physiologist is too well established throughout the whole medi-
cal world to admit of increase from any commendation of  ours ; but we should be doing injustice to our own
feelings if we did not  here express our admiralion of  his great intellectual powers, of his extensive learning,
ofthe comprehensiveness of his views, ofthe quickness with which he seizes the important points and bearings
of each  subject, of the logical order in which he arranges his fads, and of the clearness and precision, with
whieh he explains, and exposes his doctrines. Dr. Carpenter's various treatises are in fact models in their
respective departments.  It is their great and varied excellence which accounts for their unrivalled popularity.
We can pay no higher compliment to the work before  us, than to say, that it is equal in merit to the former
productions ofthe author.  This is  equivalent to saying that it is, without question, ihe best manual or short
treatise on physiology extant. Although designed for the student, and framed expressly to meet his wants; it is
a work, we will venture to say, that may be consulted with advantage by most physicians and surgeons, how-
ever learned."—The British and Foreign Medical Review.
                CARPENTER'S HUMAN  PHYSIOLOGY.



            PRINCIPLES  OF  HUMAN  PHYSIOLOGY.
                          WITH THEIR CHIEF APPLICATIONS TO

    PATHOLOGY,  HYGIENE,  AND FORENSIC MEDICINE.

               BY  WILLIAM B.  CARPENTER, M.D.,  F.R.S.,  &c.

      Second American, from  a New and Revised London Edition,
                             WITH NOTES AND ADDITIONS,

                      BY MEREDITH  CLYMER, M. D  , &c.

         With Two  Hundred and Sixteen Wood-cut and other Illuatrattons.
          In one octavo volume, of about 650 closely and beautifully printed pages.
  The very rapid sale of a large impression ofthe first edition is an evidence of the merits of this valuable work
and that it has been duly appreciated by the profession of this country. The publishers hope that the present
edition will be found still more worthy of approbation, not only from the additions of the author and editor but
also from its superior execution, and the abundance of its illustrations. No less than eighty-five wood-cuts'and
another lithographic plate will be found to have been added, affording ihe most material assistance to the student
  " We have much satisfaction in declaring our opinion that this work is the best systematic treatise on physi-
ology in our own language, and the best adapted for the student existing in any language. >—Medico-Chirurei-
cal Review.
  " The work as it now stands is the only Treatise on Physiology in the English language which exhibits a clear
and connected, and comprehensive view of the present condition of that  science ''—London and Edinburgh
Monthly Journal.                                                                    s


                  CARPENTER'S POPULAR TREATISE

     ON   VEGETABLE   PHYSIOLOGY.

            In one neat duodecimo volume, extra cloth, with numerous illustrations.

  This woik forms a part of Carpenter's Popular  Cyclopedia of Natural Science, the remainder of
which will be published with all the beautiful illustrations.
CARPENTER'S PRINCIPLES OF GENERAL AND  COMPARATIVE
         PHYSIOLOGY, AND HIS  WORK ON ANIMAL PHYSIOLOGY,
                 WITH ALL THE BEAUTIFUL ILLUSTRATIONS, ARE NOW PREPARING.
38 
LEA & BLANCHARD'S PUBLICATIONS.
7
        LISTON  AND  MUTTER'S  SURGICAL  LECTURES,

     A  BEAUTIFUL  VOLUME,  PROFUSELY  ILLUSTRATED.



      LECTURES  ON  THE   OPERATIONS  OF   SURGERY.

                                      AND ON

   DISEASES  AND  ACCIDENTS  REQUIRING   OPERATIONS

               DELIVERED AT UNIVERSITY COLLEGE, LONDON,

                   BY ROBERT LISTON, ESQ., F.R.S., &c

          EDITED,  WITH NUMEROUS ALTERATIONS AND ADDITIONS,

                        BY  T. D.  MUTTER, M. D.,
           PROFESSOR OF SURGERY IN THE JEFFERSON MEDICAL COLLEGE, PHILADELPHIA.

        IN ONE LARGE AND BEAUTIFULLY PRINTED  OCTAVO VOLUME.

           WITH TWO HUNDRED AND SIXTEEN ILLUSTRATIONS ON WOOD.

     This work contains much  original matter of Professor Mutter's, amounting to

   about two hundred and fifty pages, embodying the results of his great experience,

   and adapting the whole to the wants of the American Profession.  The Lectures

   are those which  have attracted so much attention as published in the  Lancet

   They are  here reproduced  entire, omitting none of the original wood engravings,

   and  introducing  many new and valuable ones, rendering this altogether one of

   the most  completely illustrated works of the  kind that has for some time  been

   presented to the medical public.   Among the additions of Professor Mutter will

   be found full and elaborate Treatises on Staphyloraphy, the different Plastic  Ope-

   rations, Club-Foot, Affections of the Eye,  Deformities from  Burns, and many

   other important subjects, not to be met with in  so enlarged  a form in, perhaps,

   any other work on Surgery.   The chapters containing them  are fully illustrated

   with numerous original and highly curious engravings.

    "This is a very attractive book, both in respect to mechanical execution and  contents. Printed  on fine
   white paper, with a fair new type, and  embellished with more than 200 wellexecuted wood-cuts, it presents
   altogether as beautiful an appearance as a work of art as its master is interesting and important to the prac-
   tical surgeon.  The lecture on operations on the eye is a very lucid and instructive oiie, but we can only refer
   to it in the most cursory manner. The editor has fully supplied everything that was wanting to render the
   chapter complete, and we would gladly quote his remarks on the operations of Strabismus and Staphyloma,
   did our limits permit.  The chapter on Cataract, by Dr. Miitter, extending over thirty pages, we look upon as
   one of the most valuable essays on the subject in the language.  It is clear and methqdical, sufficiently full, yet
   precise, and abounding in practical precepts of the highest interest. The remarks on deformities of the nose
   are evidently the result of much experience and observation.  The operations for hare-lip and cleft palate are
V  treated at considerable length, and made exceedingly clear by means of the accompanying illustrations: we
»  can recommend these sections alone as worth far more to the surgeon than many times the price of the whole
____»&, .work. The same remark will apply to the next lecture, on the Diseases of ihe Antrum, Carious Teeth, Erected
H*Tamours of the Face, and Solid and Encysted Tumors. We know of no systematic work on surgery in which
■ "die anaplastic operations for removal or remedying deformities of the  face are so fully and satisfactorily de>
P 'scribed as in this. But we have already exceeded our limits, and can only say in conclusion that Liston's Lec-
   tures, with Mutter's additions, should be in every surgeon's library, and in every student's hands, who  wishes
   to post up his surgical knowledge to the present moment."—The New York Journal of Medicine.
T    " These lectures will be found  eminently practical, a point of no small moment in a wort
   on Surgery.  Indeed, we have no hesitation in affirming,  that it is a compendium of the
   modern practice of Surgery as complete and accurate as any treatise of similar dimensions
   in the English language.—The Western Lancet.



           Lately Published, a New and Much Improved Edition of

                    DRUITT'S   SURGERY.
THE  PRINCIPLES AND  PRACTICE  OF MODERN SURGERY,

                   By  ROBERT DRUITT, Surgeon.

             FROM  THE  THIRD  LONDON  EDITION.
     ILLUSTRATED BY ONE HUNDRED AND FIFTY-THREE WOOD ENGRAVINGS.
                      WITH NOTES AND COMMENTS,

                  By  JOSHUA B.  FLINT, M. M., S. S.

                         In One Volume, Octavo.
  "An unsurpassable compendium not only of surgical but of medicaj practice."—London

Medical Gazette. 
8
LEA & BLANCHARD'S PUBLICATIONS.
BRODIE'S   SURGICAL  WORKS.
                             NOW READY,


 CLINICAL  LECTURES  ON  SURGERY,
                   DELIVERED AT ST. GEORGE'S HOSPITAL

            By SIR BENJAMIN BRODIE, Bart., V. P. R. S.,
                     SERJEANT SURGEON TO THE QUEEN, ETC. ETC.

                IN ONE  NEAT OCTAVO VOLUME.

  These Lectures, in passing through the columns of "The Medical News," during the
last year, have received the unanimous approbation of the profession in this country, and
will no doubt be eagerly sought for in their complete state.
  " Sir Benjamin Brodie has long been distinguished as a surgeon, even among those in the front
rank, and may now be regarded as occupying, the first  place in Great Britain, if not in the world.
It is not as a mere operator that he is distinguished—that, it must be conceded by all right thinking
minds, is but a very humble part of the qualifications of a surgeon—but as a profound pathologist
and therapeutist.  We may be prejudiced or mistaken, but in these higher and really intellectual
qualifications, we know of no living surgeon  that approaches him.  His well considered opinions
and practical instructions are conveyed, too, in language so appropriate, so simple, clear and chaste,
that one scarcely knows whether most to admire the excellence of the matter or the beauty of the
style in which it  is clothed. Theory and practice go hand in hand throughout. Rarely is a pre-
cept given without being illustrated by some apposite case, selected from his vast experience, and
always in the fewest and most  expressive words. Nothing more than is necessary to enforce the
point is said, and nothing that is necessary is left untold."—The Medical Examiner.
  " It would not be easy to find in the same compass more useful matter than is embraced in each
of these discourses, or indeed in this volume. We the less regret the limited extracts we have it
in our power to make from it, because we feel sure that it will, in a short time find its way into all
the medical libraries in the country.'—The Western Journal of Medicine and Surgery.
  " Sir B. Brodie has rendered his clinical lectures—what all clinical lectures should be—clear,
impressive and practical expositions of curative measures, in which all the most important features
of the subject are plainly set  forth, without being  unnecessarily encumbered with details. We
attach a  very high value to these Lectures: their style is clear, demonstrative  and  unaffected, de-
cided and energetic; but altogether free  from dogmatism or  over-confidence.  They are strictly
practical; and much of the information which they contain will assuredly do  the  reader and  his
patients  good service in time of need.'*—Medical Gazette.



               BRODIE  ON  URINARY ORGANS,
                         LECTURES

     ON THE DISEASES  OF  THE  URINARY  ORGANS,
                  FROM THE THIRD LONDON  EDITION.
               WITH ALTERATIONS  AND ADDITIONS.
                     In One Small Octavo Volume, Cloth
  This work has been entirely revised throughout, some of the author's views have been
modified, and a considerable proportion of new matter has .been added, among which is
a lecture on the Operation of Lithotomy.


                BRODIE  ON   TftE   JOINTS.
         PATHOLOGICAL AND SURGICAL  OBSERVATIONS

      ON  THE  DISEASES  OF  THE  JOINTS.
                 FR'iVI  'IHE FOIJKi'H LONDON  EDITION.
          OTCth the authors alterations art* Sftttftfons.
                     In One Small Octavo Volume, Cloth.
r "To both the practical physician and  the student, then, this little volume will be one of much
service, inasmuch as we have  here a condensed view of these complicated subjects  thoroughly
investigated by the aid of the light afforded by modern Pathological Surgery."—N. Y. Journal of
Medicine. 
                 LEA  & BLANCHARD'S PUBLICATIONS.                9

                           •TV O W RE Jl U Y.     ~"


THE  SIXTH EDITION  OF  DUNGLISON'S  MEDICAL  DICTIONARY.

                                To 1846.



              medicaiTlexicon,


                         A  DICTIONARY

                                    OF

        MEDICAL    SCIENCE.

                                CONTAINING
A CONCISE ACCOUNT OF  THE VARIOUS SUBJECTS AND TERMS ,  WITH
    THE FRENCH AND OTHER  SYNONYMES ; NOTICES OF CLIMATES
        AND  OF CELEBRATED MINERAL  WATERS, FORMULAE
           FOR  VARIOUS OFFICINAL AND EMPIRICAL PREPA-
                              RATIONS, &c.

                  BY  ROBLEY DUNGLISON,  M. D.,
        Professor ofthe Institutes of Medicine, &c. in Jefferson Medical College, Philadelphia.
            SIXTH EDITION, REVISED AND GREATLY ENLARGED.

      In one Royal Octavo Volume of over 800 very large pages, double columns.

               Strongly bound in the best leather, raised bands.
 It is not necessary for the author to say more, as a preface to this sixth edition of his dictionary, than that he
has bestowed the same care on its revision and improvement, which he did on the others.  Inproof of this, he
may remark, that the present edition comprises nearly two thousand five hundred subjects and terms not con-
tained in the last.  Many of these have been introduced into medical terminology in consequence of the pro-
gress ofthe science ; and others had escaped him in previous revisions.
 It is the author's anxious wish to render the work a satisfactory and desirable—if not indispensable—Lexicon.
in which the student may search without disappointment for every term that has been legitimated in the nomen-
clature ofthe science. This desideratum he is enabled to attempt in successive editions, by reason ofthe work
not being stereotyped; and the present edition certainly offers stronger claims to the attention of the practi-
tioner and student than any of its predecessors.—Preface to Sixth Edition, May 1846.
                  A NEW EDITION OF

THE  MEDICAL   STUDENT,

     OR  AIDS TO THE STUDY  OF MEDICINE.
          A  REVISED AND MODIFIED EDITION.

               BY ROBLEY DUNGLISON, M. D.,
                    In One neat 12mo. volume.
            HUMAN   HEALTH;

'• OR, THE  INFLUENCE  OF ATMOSPHERE AND LOCALITY, CHANGE OF AIR
f    AND CLIMATE, SEASONS, FOOD, CLOTHING, BATHING AND MINE-
         RAL SPRINGS,  EXERCISE, SLEEP, CORPOREAL AND INTEL-
J            LECTUAL  PURSUITS, &c &c, ON HEALTHY MAN:
                              CONSTITUTING
                    ELEMENTS OF HYGIENE.

         '     BY  ROBLEY  DU N G L IS 0 N, M. D.

         A  New Edition with many Modifications and Additions.  In One Volume, 8vo.


            DUNGLISON  ON  NEW,REMEDIES.



                  NEW   RE~MEDIES,

   PHARMACEUTICALLY AND THERAPEUTICALLY CONSIDERED.

                           FOURTH  EDITION.

                WITH EXTENSIVE MODIFICATIONS AND ADDITIONS,
                      BY ROBLEY DUNGLISON, M.D., &c.
   „,    „„rmiR valuable therapeutical agents which have of late years been introduced into the Materia
   The nurae/°"?t-difficult matter for the practitioner to keep up with the advancement of the science, espe-
  Medica, «»*« "cVpS 0T them are difficult of access, being scattered so widely through transact.ons of
  cially as thes  de-cr p    o  "    h &c &c  To obviate this difficulty. and to place within reach ofthe
  learned socieii e=,.'  am lntorm|tjon j„ a compendious form, is the object ofthe present volume, and the num-
  E   /'Silinnsthrough which it has passed show that its utility has not been underrated.
  beJ^ apothecaries and Physicians may rely upon having a late edition of this useful work. 
10
LEA & BLANCHARu•»  rubl,iuA'ilUJNS.
       PROFESSOR  DUNGLISON'S  WORKS.

              NOW READY.—A NEW EDITION OF


       DUNGLISON'S    PHYSIOLOGY,

                       Brought up to 1846.

HUMAN    PHYSIOLOGY,
                                WITH
          THREE HUNDRED  AND SEVENTY ILLUSTRATIONS.
                BY ROBLEY DUNGLISON, M. D.,
 PROFESSOR OF THE INSTITUTES OF MEDICINE IN THE JEFFERSON MEDICAL COLLEGE, PHILADELPHIA,
                               &C. &.C.
                     SIXTH  EDITION.
                         GREATLY IMPROVED.
             In  two large octavo volumes, containing nearly 1350 pages.
  " It is but necesary for the Author to say that all the cares that were bestowed
on the preparation of the fifth edition have been extended to the sixth, and even
to a greater amount.   Nothing  of importance that  has been recorded since its
publication, has, he believes, escaped his attention.   Upwards of seventy illustra-
tions have been added; and many of the former cuts have been replaced by others.
The work, he trusts will be found entirely on a level with the existing advanced
state of physiological science."
  In mechanical and artistical execution, this edition is far in advance of any former one.  The
illustrations have been subjected to a thorough revision, many have been rejected and their places
supplied with superior ones, and numerous new wood-cuts have been added wherever perspicuity or
novelty seemed to require them.
      THE PRACTICE OF MEDICINE,

                         OR A TREATISE ON

SPECIAL  PATHOLOGY  AND   THERAPEUTICS.
                     BY ROBLEY DUNGLISON, M.D.

                             CONTAINING
THE DISEASES OF THE  ALIMENTARY CANAL, THE DISEASES OF  THE
    CIRCULATORY  APPARATUS, DISEASES  OF THE  GLANDULAR OR-
     GANS, DISEASES OF THE ORGANS OF THE  SENSES, DISEASES
       OF THE RESPIRATORY ORGANS, DISEASES OF THE GLAN-
         DIFORM GANGLIONS, DISEASES OF THE NERVOUS SYS-
          TEM, DISEASES  OF  THE  ORGANS OF REPRODUC-
            TION,  DISEASES INVOLVING VARIOUS  ORGANS,
                               &c. &c.
                      In Two Volumes, Octavo.
  This work has been introduced as a text-book in many of the Medical Colleges,
and the general favor with which it has been received, is a guarantee of its  value
to the practitioner and student.
  " In the volumes before us, Dr. Dunglison has proved that his acquaintance with the present facts
and  doctrines, wheresoever originating, is most extensive and intimate, and the judgment, skill,
and impartiality with which the materials ofthe wbrk have been collected, weighed, arranged, and
exposed, are strikingly manifested in every chapter. Great care is everywhere taken to indicate
the source of information, and under the head of treatment, formula? ofthe most appropriate reme-
dies are everywhere introduced. In conclusion, we congratulate the students and junior  practi-
tioners of America on possessing in the present volumes a work of standard merit, to which they
may confidently refer in their doubts and difficulties."—Brit, and  For. Med. Rev. for July, 1842.
  " Since the foregoing observations were written, we have received a second edition of Dungli-
son's work, a sufficient  indication of the high character it has  already attained in America, and
justly attained."—Ibid, for October, 1844. 
LEA & BLANCHARD'S PUBLICATIONS.
11
                                       NOW READY

           BUNGLISON'S   THERAPEUTICS.
                         A NEW EDITION, MUCH IMPROVED, TO 1846.



     GENERAL  THERAPEUTICS  AND   MATERIA MEDICA
             WITH ONE  HUNDRED  AND 'TWENTY IXLUS^rItION^^^'

                        ADAPTED FOR A  MEDICAL TEXT-BOOK

                  BY  ROBLEY  DUNGLISON,  MD



            Third Edition, Revised and Improved, in Two Octavo Voluvies, well bound.

 it to * tW^T mUCb imPro7«nent wi» °e found over the former ones.  The author has subjected

 iS^^±Z^^^^r7^ t°1Sr°dif\the W°rk 3S t0 -ake'Hamore compete
 The favor wfth whLh the Cf   ^v °    kn0wled?e °n jhe important subjects of which it treats.
 dered still  more worthv nfT   t     wf e/eceiv,ed'. dema"ded that the present should be ren-
 onlv inthe m   P Zr t fl L    Pat™»a8« »f the profesnon, and  this alteration will  be found not
 only in tne matter of the volumes, but also in the numerous illustrations introduced  and the cene-
 ral improvement in  the appearance of the work.                       miroaucea, ana tue gene-

                             LIST OF  ILLUSTRATIONS.—Vol. I.

  9 Krn!lar.e^IpenaCUanha-17- LeSume and leaflet of C. 37. Cetraria Tslandica.    57. Lycopus Viro-inicus
  % feVf CaCUa"- r00t-, o   °bovata-             38. Fucus vesiculosus.    58. StrycnSosNSi Vomica
  a Striated lpecacuan. root IS. Tinnevelly Senna.     39 Inhaler.              59 RuTa emveobL
     Tnha roof   P6CaCU" 20 °a"ia Matilandica.    40. Cantharides.         % Sf coTnutum.
     anharoot.           20. Podophyllum.         41. Leontodon Tarnxacum.Cl Cinnamomum  Zevlani-
  4. Ton.dium Ipecac, root  21. Hebradendroncambogi-42. Erigeron  Ph.Jadelphi-   ^namomura  ^eylanl-
  5. Gillenia stipulacea.         oi'des.                  cum               no Cardnmnm
  e.Lobehainnata.        22. Momordica Elatermm.  43 Arbutus Uva ursi.     63. CariophX^aromaticus
  8 AZn™?;^6"5" §?• tPOCyr? c*»nabinurn44. Eupa.oriumperfoliatnm.64. FcenTcukm vulgare
     fnuJ™   Androsffim.- 24. Convolvulus panduralus45. Asclepias tuberosa.   65. Monarda coccinea.
  q f£.Wninm  i          Chenopodmm Anthel-  46. Arum triphyllum.     66. Hedeoma pulegioides.
     2£romum  Amenca-     m.m.cum            47. Carthamus tinctorius.  67. Myristica moschata.
 m r-  ?" u-      „      26. Spigelia Manlandica.   48. Warm-bath.         68. Nutmeg in ihe shell-ur-
 10. Euphorbia corollata.   27. Nephrodium  Filix max.49. Hip-bath.                roSd by'he mace
 11. *icus Carica.         23. Punica granatum.      50 Foot-bath.            69. Gaultheria nrocumbens
 12. R.cinis communis.     29, 30 Inhaling Bottles.     51. Hyoscyamus Niger.   70. Juniperus communis.  '
 13. Kheum palmatum.     31. Balsamadend'n Myrrha. 52. Datura Stramonium.   71. Citrus Aurantium
 14. Rheum eompaclum.    32. Acacia Arabica.       53. Conium maeulatum.   72. Laurus camphora.
 lo. Aloe bocotorma.      33. Olea Europeea.        54. Humulus Lupulus     73 Drymis Winteri
 16. Legume  and  leaflet of 34. Saccharum officinarum.55. Dried lupulinic  grain 74 Acorus Calamus
     Acute leaved Alexan- 35. Linum usitatissimum.     with its hilum magnifi'd 75. Piper nigrum
     dnan Senna.        36. Astragalus verus.     56. Cannabis sativa.       76. Electrical Apparatus for
                                                                          Medical purposes.
                                           VOL. n.
  1. Coceulus palmatus.    13. Hepatica Americana.  26. Abies excelsa.        35. Panic's of Potato starch
     (Male plant.)        14. Indigo.               27. Ranunculus acris.        seen by the microscope.
  2. trentiana Catesbaei.    15. Cornus Florida.  "     23. Aralia nudicaulis.     36. Janipha Manihot.
  3. Frasera Walteri.      16. Liriodendron  tulipifera 29. Solanum dulcamara.   37. Particles of Tapioca as
  4 ftabbatia  ahgularis.    17. Dyospyros Virginiana. 30. Tacca pinnatifida.        seen by the microscope.
  5. Coptis trifolia.         18. Heuchera acerifolia.   31. Particles of Tahiti Ar-39. Sagus Rumphii.
  6. Aletris farinosa.       19. Spiraea tomentosa.         row-root.           39 Particles of Sago-meal.
  7. Anstolochiaserpentaria20. Statice Caroliniana.    32. Particles of white East 40. Particles of Potato sago.
  8. Asarum Canadense.    21. Colchicum autumnale.      India Arrow-root.    41. Cycas revoluta or the
 9. Anthemis Cotula.      22. Veratrum Album.  Var. 33. Particles of West India    Japan Sago-tree.
 10. Magnolia glauca.          Albiflorum.              Arrow-root.         42. Avena Sativa.
 11. Magnolia macrophylla. 23. Cinvcifuga racemosa.  34. Particles of Tous-les- 43. Para's of Wheat Starch
 12. Geum Virginianum.    24,25. Shower-bath.           mois.
  "Our junior brethren in America will find in these volumes of Professor Dunglison, a 'Thesaubus Medica-
 Minum,' more valuable than a large purse of goW."—Medico-Chirurgical Review for Jan. 1845.


                    ELLIS'S   MEDICAL  FCXUVZULARIT,

                NEW AND IMPROVED  EDITION,  TO JULY,  1846.


         THE     MEDICAL   FORMULARY:
  BEING A COLLECTION OF PRESCRIPTIONS, DERIVED FROM THE WRITINGS AND PRACTICE  OF MANY OF
                    THE MOST EMINENT  PHYSICIANS OF AMERICA AND EUROPE.

                          TO WHICH IS ADDED AN APPENDIX,
          CONTAINING THE USUAL DIETETIC PREPARATIONS AND ANTIDOTES FOR POISONS.
  THE  ■WHOLE ACCOMPANIED WITH A FEW BltlEF PHARMACEUTIC AND MEDICAL OBSERVATIONS.

                          BY  BENJAMIN  ELLIS, M. D.,
        Late Professor of Materia Medica and Pharmacy in the Philadelphia College of Pharmacy.
           EIGHTH  EDITION, WITH EXTENSIVE ALTERATIONS AND ADDITIONS.
                          BY SAMUEL GEORGE MORTON, M. D.

                                  In One neat Octavo Volume.

  t-V« nomilar work has been too extensively and favorably known to the profession in the United States to
     e any remarks in introducing a new ediiion. except to state that the improvtments in it will be found to
reqUl merous and important.  Great care has been taken in its passage through the press to insure the utmost
be na»       .^ ^g co„g(jenliy presented as worthy the increased confidence of physicians and apothecaries.
accural) •* 
12                LEA  & BLANCHARD'S PUBLICATIONS.
        HASSE'S  PATHOLOGICAL ANATOMY.

                AN ANATOMICaT DESCRIPTION
                        OF THE DISEASES OF THE

   ORGANS   OF  CIRCULATION  AND   RESPIRATION,
                  BY CHARLES EWALD HASSE,
       Professor of Pathology and Clinical Medicine in the University of Zurich, &c.
                      TRANSLATED AND EDITED BY
                   W. E.  SWAINE,  M.D.3 &c.
                         In one octavo volume.
             A New Work, Just  Ready.-Jnly 1846.

             HOPE ON THE HEART,—New Edition.

                   A  TREATISE ON THE  DISEASES

OF   THE   HEART   AND   GREAT  VESSELS,

  AND ON THE AFFECTIONS  WHICH MAY  BE MISTAKEN
                           FOR THEM.
   COMPRISING THE AUTHOR'S VIEW OF THE PHYSIOLOGY OF THE HEART'S
      ACTION AND SOUNDS AS  DEMONSTRATED BY HIS EXPERIMENTS
            ON THE MOTIONS AND SOUNDS IN 1830, AND ON THE
                           SOUNDS IN 1834-5.
                BY J. HOPE, M. D., F. R. S., &c. &c,
               Second American  from the Third London Edition.
            WITH NOTES AND A  DETAIL OF RECENT EXPERIMENTS.
                    BY C.  W. PENNOCK,  M.D., &c.
           In one octavo volume of nearly 600 pages, with lithographic plates.

      HUGHES  ON  THE LUNGS AND HEART.

  CLINICAL INTRODUCTION TO  THE PRACTICE OF AUSCULTATION,
             AND OTHER MODES OF PHYSICAL DIAGNOSIS.
INTENDED TO  SIMPLIFY THE STUDY OF THE DISEASES  OF  THE HEART
                             AND LUNGS.
              BY H.  M.  HUGHES,  M.D.,  &c.
                     In One Duodecimo  Volume, with a plate.
          A NEW WORK.—PHILLIPS  ON SCROFULA.
                      JUST UEJIDW—July 1846.


                SCR  Ol^  U  L  A,
 ITS NATURE, ITS PREVALENCE, ITS CAUSES, AND THE PRINCLPLES OF
                          ITS TREATMENT.
                 BY BENJAMIN PHILLIPS, M. D., F. R. S., &c.
                     In one neat octavo volume, with a plate.
 "There can be no doubt that there was ample room for a fresh review of scrofula, and that a work on the
subject, from a writer of learning, industry, and talent, must be acceptable; and such, after a careful perusal,
the work before us appears to be.  Very uncommon pains seem to have been taken in collecting information
from men as well as books.  Our impression of this work is, that it is a very important accession to our profes-
sional literature, and does the greatest credit to the industry, research and talent of the author. Its utility is,
we think, likely to extend far beyond the limits of professional society."—The London Medical Gazette.
 This work i6 just published. The author has been for years engaged in the collection of materials and sta-
tistics from all parts of the world.

             SMALL BOOKS ON  GREAT SUBJECTS.
                           jvow r jemn *•,

     "The Connection between Physiology and Intellectual Science."
Forming No. 2 of a series of small works now issuing at Twenty-five Cents each.
  To be followed shortly by "Philosophical Theories and Philosophical Experience,"—On Man's
Power over Himself to Prevent or Control Insanity,"—" An Introduction to Practical Organic Che-
mistry,"—"An Introduction to Vegetable Physiology, with References to the Works of De Can-
dolle^ Lindley, &c," and several other new and valuable works.  Each one to form a very neat
and portable volume.
  5)5= These works have acquired great popularity in England, and the publishers take pleasure
in introducing the series neatly printed, and at so low a price. 
                 LEA  <k,  BlaNCHaRD'S  PUBLICATIONS.                13


      compeot)i^^

                   A COMPENDIUM OF LECTURES

                                   ON THE

   THEORY  AND  PRACTICE   OF   MEDICINE.

DELIVERED BY PROFESSOR  CHAPMAN IN THE  UNIVERSITY  OF  PENN-
                                 SYLVANIA.
 PREPARED, WITH PERMISSION, FROM DR. CHAPMAN'S MANUSCRIPTS,
                AND PUBLISHED WITH HIS APPROBATION,

                     BY  N. D. BENEDICT, M>D.
              IN  ONE  VERY  NEAT  OCTAVO  VOLUME.
                                  CONTENTS.
Remarks on the Classification of Diseases—Fever in General—Intermittent Fever—Remittent Fever
  —Continued Fever, (Mild, Intermediate, and Extreme  Forms)—Yellow Fever—Endemic Pneu-
  monic, or Spotted Fever—Diseases ofthe Heart and Blood-vessels, (Inflammatory, Organic, and
  Nervous)—Acute Carditis, Pericarditis, and Endocarditis—Chronic Carditis, Pericarditis, and En-
  docarditis—Hypertrophy ofthe Heart—Dilatation of the Heart—Atrophy ofthe Heart—Rupture
  of the Heart—Affections of the Valves of the Heart—Palpitations—Acute Arteritis—Degenera-
  rations of Arteries—Aneurism of Arteries—Phlebitis—Acute Inflammation ofthe Throat—Chronic
  Inflammation ofthe Throat—Dysphagia—Parotitis—Dysentery, (Inflammatory)—Dysentery, (Con-
  gestive)—Diarrhoea—Cholera Morbus—Cholera Infantum—Flatulent Colic—Bilious Colic—Colica
  Pictonum—Acute Peritonitis—Chronic Peritonitis—Acute Catarrh—Catarrhus iEstivus—Chronic
  Catarrh—Acute Bronchitis—Chronic Bronchitis—Catarrhus Senilis—Acute Infantile Bronchitis—
  Chronic Infantile Bronchitis—Croup—Acute Infantile Asthma—Whooping-Cough—Acute Laryn-
  gitis—Chronic Laryngitis—Pleuropneumonia—Congestive  Pneumonia—Chronic  Pleurisy  and
  Pneumonia—Apoplexy—Palsy—Epilepsy—Hysteria—Chorea—Neuralgia—Diabetes.
  The subjects treated of in  this volume are entirely distinct from those considered in Dr.
Chapman's two works on  "Thoracic  and Abdominal Viscera," and on "Eruptive  Fevers,"
&c.  These works are all printed and bound to match, and form three very neat octavo
volumes.
        LECTURES ON THE MORE IMPORTANT DISEASES
                                    OF THE

THORACIC   AND   ABDOMINAL  VISCERA
          DELIVERED IN THE UNIVERSITY OF PENNSYLVANIA.
                          BY N. CHAPMAN,  M. D.
               PROFESSOR OF THE THEORY AND PRACTICE OF MEDICINE, ETC.
                             In One Volume, Octavo.
           CHAPMAN  ON   FEVERS,  &c.

                  LECTURES ON THE MORE IMPORTANT


ERUPTIVE  FEVERS,   HEMORRHAGES  AND  DROPSIES,

                AND ON GOUT AND  RHEUMATISM,

          DELIVERED IN THE UNIVERSITY OF  PENNSYLVANIA.

                          By N. CHAPMAN, M.D.,
             PROFESSOR OF THE THEORY AND PRACTICE OF MEDICINE, ETC. ETC.
                           In One Neat Octavo Volume.
  "The name of Chapman stands deservedly high in the annals of American medical science.  A teacher and
a Wtnrer for nearly forty years, in the oldest and, we believe, the first medical school on this side ofthe At-
lantir the intimate friend and companion of Rush, Kuhn, Phyeick, Wistar, Woodhouse, Dewees, and a host
rfnth«™ scarcely less renowned, Professor Chapman reflects upon the profession of this generation something
!f,h» Junius and wisdom of that which has passed; he stands out the able and eloquent champion ofthe doc-
trines and principles of other times, when Cullen's « first lines" formed the rule of faith for all the Doctors in
MedTrine throughout Christendom.  In  him is embodied the experience pf threescore and ten, strengthened
f^in"and enlightened by a familiar intercourse with many of the ablest medical men in the New and
?J/wnrld  In conclusion, we must declare oar belief that the name of Chapman will survive when that of
     f his cotemporaries shall have been forgotten; when other generations shall tread the great theatre of
rnany 01 ni   ^ ^^ ^^ djeCoveries yet undisclosed, shall shed a brighter light upon the path of medi-
human an   .^^ various lectureB which he has been publishing, containing, as they do, the doctrines that
t he solong and so eloquently taught to large and admiring classes, we doubt not will be welcomed with
dl eM by his numerous pupils throughout the Union."—New Orleans Medical Journal. 
14
LEA & BLANCHARD'S PUBLICATIONS.
HORNEK'S    ANATOMY,
     NEW EDITION—To be Ready by October.
     SPECIAL  ANATOMY
                          AND

             HISTOLOGY.

                          BY
           WILLIAM E. HORNER,  M. D.,
  PROFESSOR OF ANATOMY IN THE UNIVERSITY OF PENNSYLVANIA, &c &c
                   SEVENTH EDITION,
          WITH MANY IMPROVEMENTS AND ADDITIONS.
       In tioo Octavo  Volumes, with Illustrations on Wood.
  This standard work has been so long before the profession, and has been
so extensively used, that, in announcing the new edition, it is only neces-
sary to state that it will undergo a most careful revision ; the author will
introduce many illustrations relating to Microscopical Anatomy, and will
add  a large  amount of text on these various points of investigation  that
are rapidly advancing and attracting so much attention.  This new edi-
tion  will be arranged to refer conveniently to the illustrations inSmith
and  Horner's Anatomical Atlas, and will be ready for the Fall Lectures.


          HORNER'S JDISSECTOR.


   THE UNITED STATES  DISSECTOR,

BEING A NEW EDITION, WITH EXTENSIVE MODIFICATIONS, AND
                 ALMOST REWRITTEN,
                          OF

    "HORNER'S  PRACTICAL ANATOMY."
                IN  ONE VERY NEAT VOLUME,
                     ROYAL 12mo.
              With many Illustrations on Wood.
  The  numerous alterations and additions which this work has under-
gone, the improvements which have been made in it, and the numerous
wood-cuts which have been introduced, render it almost a new work.
  It  is the standard work for the Students in the University of Pennsyl-
vania.
                BUDD ON THE LIVER.

    ON  DISEASES OF THE LIVER.

                BY GEORGE BUDD, M.D., F.R.S.,
             Professor of Medicine in King'a College, London, &c. &c.
       With colored plates, and numerous wood-cuts.  In one neat octavo volume.
 " We cannot too strongly recommend the diligent study of this volume.  The work cannot fail to rank the
name of its author among the most enlightened pathologists and soundest practitioners of the day."— Medico-
Chirurgical Review. 
              A  SflLaGlJnTICEIWT  AND CH2BA?  WOKE.



SMITH   &  -HORNER'S   ANATOMICAL    ATLAS,

                     Just  Published, Price Five Dollars in  Parts.



                                              AN

                     ANATOMICAL   ATLAS


       ILLUSTRATIVE OF  THE  STRUCTURE  OF THE  HUMAN  BODY.

                         BY  HENRY  H.  SMITH, M.D.,

                               Fellow ofthe College of Physicians, Sfc

                                  UNDER THE SUPERVISION OF

                         WILLIAM  E. HORNER,  M. D.,

                    Professor of Anatomy in the University of Pennsylvania.

                         In One large  Volume, Imperial Octavo.

  This work is but just completed, having been delayed  over the time intended by the gTeat difficulty in giving
lo the illustrations the desired finish and perfection.  It consists of five parts, whose contents are as follows:
  Part  I. The Bones and Ligaments, with one hundred and thirty engravings.
  Part  II. The Muscular ana Dermoid Systems,  with ninety-one engravings.
  ,Part III. The Organs of Digestion and Generation, with one hundred and ninety-one engravings.
  Part IV. The Organs of Respiration and Circulation, with ninety-eight engravings.
  Part  V. The Nervous System and the Senses,  with one hundred and twenty-six engravings.
Forming altogether a complete System of Anatomical Plates, of nearly
                          SIX  HUNDRED AND  FIFTY  FIGURES,
executed in the best style of art, and making one large imperial octavo volume.  Those who do not want it in
parts can have the work bound in extra cloth or sheep at an extra cost.
  This work possesses novelty both in the design  and the execution. It is the first attempt to apply engraving
on wood, on a large scale, to the illustration of human anatomy, and the beauty ofthe parts issued induces the
publishers to flatter themselves with  the hope of the perfect success of their undertaking. The plan of the
work is at once novel and convenient.  Each page is perfect in itself, the references being immediately under
the figures, so that the eye takes in the whole at a glance, and obviates the  necessity of continual reference
backwards and forwards.  The cuts are selected from the best and most accurate sources; and, where neces-
sary, original drawings have been made from the  admirable Anatomical Collection ofthe University of Penn-
sylvania.  It embraces all the late beautiful discoveries arising from the use of the microscope in the investi-
gation of the minute structure of the tissues.
  In the getting up of this very complete work, the publishers have spared neither pains nor expense, and they
now present it to the profession, with the full confidence that it will be deemed all that is wanted in a scientific
Said artistical point of view, while, at the same time, its very low  price places it within the reach of all.
  It is particularly adapted to supply tlie place of skeletons or subjects, as the profession will see by examining the list
qf plates
  " These figures are well selected, and present a complete and accurate representation of that wonderful fabrie,
the human  body.  The plan of this Atlas, which renders it so peculiarly convenient for the student, and its
superb  artistical execution, have been already pointed out.  We must congratulate the student upon the
completion of this atlas, as it is the most convenient work of the kind that has yet appeared; and, we  must
add the very  beautiful manner  in which it is ' got up' is so creditable to the country as to be flattering to our

national pride."—American Medical Journal.                                        .,•,..,
  "This is an exquisite volume, and a beautiful specimen of art.  We have numerous Anatomical Atlases.
but we  will venture to say that none  equal it in cheapness, and none surpass it in faithfulness and spirit.  We
Mronelv recommend to our friends, both urban and suburban, the purchase of this excellent work, for which
both editor and publisher deserve the thanks of the profession "-Medical Examiner.
  «Wp would stronglv recommend it, not only to the student, but  also to the working practitioner,  who,
  ., vve,_ _"_.:___.stv in the toils  of his harness still has the desire, and often the necessity, of refreshing his
                fundamental part of the science of medicine."—New York Journal of Medicine and Surg.
although grown rusty
knowledge: in this fundamental part of the science ot medicine.' —ive«i lor* journal uj lucu.cn* „„«, o«,s^
  "Therflanofthis Atlas is admirable, and its execution superior to any thing of the kind before published in
(his country  It is a real labour-saving affair, and we regard its publication as the greatest boon that could be
this country. 11ls      , anatomv. "n will be equally valuable to the practitioner, by affording him an easy
^^ZmJ^^?ZS* in the dissecting room, and which are soon forgotten."-^*™™ Medl
  ufiZT) eautiful as well as particularly useful design, which should be extensively patronized by physicians,
       fo.„i mprfipal students."—Boston Med. and Surg. Journal.
^Ir has beenThe aimof the author of the Atlas to comprise in it the valuable points of all previous works, to
  v   t thflateVt microscopical observations on the anatomy of the tissues, and by placing it at a modera e
embrace the  ate i n"""^ u who may need its assistance in  the dissecting or operating room, or other field
price toename *jg urnalofMed .and Surgery.                       .        .
of practice."— ^^«^o"     J   ■    f hi  beautiful work, which fully merits the praise bestowed upon the
  *These n™^e» ^P^Ki  the engravings  as possessing an accuracy only equalled by their beauty,
Md corSaU6 recommendthe work to all engaged in the study of anatomy.»-iVeu> York Journal of Medicine

and Surgery^ ^ ^^ th&n ^ one before us cou]d not easl]y be placed by a physician upon the table of


tot^^^^&^MJ&ZnA Part gratifies us still more, both as regards the attract-
  "We were ™ch plea*edw "■*£   ^ and Muscular Systems,) and the beautiful artistical execution of the
ive nature °f ^"^Kv1" delineated the most accurate microseopic views of some of the tissues, as, tor
illustrations.  Wehave "eredj  se tissues, the  epidermis, rete mucosjim and cutis  vera, the sebaceous and
instance  the ceHular a„d  adipose »-   >     V    g ^ ^ ^ ^       h    anc     ls.  Th

perspiratory °T»*".8°">«-f1"' £  ^Jes, and, lastly, their  separate delineations. We would.recommend
follows the^^'^rXUa" to our readers in the very  strongest terms."—*™ York Journal of Medicine and Sur-
gery. 
16                 LEA & BLANCHARD'S PUBLICATIONS.


               THERAPEUTICAL  LIBRARY.
                                     _________                             »


   PEREIRA'S   MATERIA  MEDICA.

        WITH NEARLY THREE HUNDRED  ENGRAVINGS ON WOOD.

                  A NEW  EDITION, LATELY PUBLISHED.


                       THE  ELEMENTS  OF


        MATERIA  MEDICA  AND   THERAPEUTICS,

COMPREHENDING  THE NATURAL HISTORY, PREPARATION, PROPERTIES, COMPO-
                  SITION, EFFECTS AND USES OF MEDICINES.

               BY JONATHAN PEREIRA, M.D., F.R.S. and L.S.,
Member of the Society of Pharmacy of Paris; Examiner in Materia Medica and Pharmacy of the University
             of London; Lecturer on Materia Medica at the London Hospital, &c. &c.

          Second American, from the last London Edition, enlarged and improved.

       WITH  NOTES AND  ADDITIONS BY  JOSEPH  CARSON, M.D.

In Two Volumes, Octavo, containing Fifteen Hundred very large  Pages, illustrated by
                    Two Hundred and Seventy-five Wood-cuts.

  Part I. contains the General Action and Classification of Medicines and the Mineral Materia
Medica.  Part II., the Vegetable and Animal Kingdoms, including diagrams explanatory of the
Processes of the Pharmacopoeias, a tabular view of the History of the Materia Medica, from the
earliest times to the present day, with the Introduction of the Processes of the New Edinburgh
Pharmacopoeia, and a very copious index.  It also contains additional articles on Mental Remedies,
Light, Heat, Cold, Electricity, Magnetism, Exercise, Dietetics  and Climate, and  many additional
Wood-cuts, illustrative of Pharmaceutical Operations, Crystallography, Shape and Organization of
the Feculas of Commerce, and the Natural History of the Materia Medica.

  In passing through the press the second edition of this standard work, the opportunity has been
taken by the editor to correct any mistakes or inadvertencies that may have escaped him or the
author, in the first edition.   It may now be considered as entirely worthy of the confidence of the
physician and pharmaceutist, as an accurate edition ofthe most complete work extant on the .subject.
  "An Encyclopcedia of knowledge in that department of medical science—by the common consent ofthe pro-
fession the most elaborate and scientific Treatise on Materia Medica in our language."— Western Journal of
Medicine and Surgery.
THE  STUDENT'S TEXT-BOOK OF MATERIA MEDICA.
                         WOW  AT  PRESS,

                           A  MANUAL  OF

     MATERIA  MEDICA  AND  THERAPEUTICS.

                      By J.  FORBES ROYLE, M. D.,
                 PHOFESSOK  IN KING'S  COLLEGE,  LONDON.

                                 EDITED BY

                            J.  CARSON,  M.D.,
     Professor of Materia Medica and Pharmacy in the Philadelphia College of Pharmacy, etc. etc.

           In One Octavo Volume, with Numerous Splendid Illustrations.

  This work will contain all the most recent information  and investigations in the various
branches connected with  the Materia Medica, and  under the supervision of its able editor,
will receive whatever alterations  and additions  may be necessary to adapt it to the United
States Pharmacopoeia, and to the practice of this country.  The high character of the author
will attract attention to the work as a text-book  for the next session of the various colleges,
if ready.  The numerous and beautiful illustrations will far surpass anything that has as
yet been attempted in this way.  This volume will be brought out in a style to match Fer-
guson's Surgery, Wilson's Anatomy, &c, and will be sold at a low. price. 
LEA & BLANCHARD'S PUBLICATIONS.             17
 THE  GREAT  MEDICAL LIBRARY.

   THE  CYCLOP/EDIA  OF  PRACTICAL MEDICINE;
                COMPRISING TREATISES ON THE
        NATURE AND TREATMENT OF DISEASES,

   MATERIA MEDICA &  THERAPEUTICS,

      DISEASES  OE WOMEN AND CHILDREN,

         MEDICAL JURISPRUDENCE, &c. &c.
                         EDITED BY
              JOHN  FORBES, M. D., F. R. S.,
     ALEXANDER  T W E E D I E, M.D.,  F.R.S.,
                            AND
               JOHN  CONOLLY,  M.D.

                   REVISED, WITH ADDITIONS,
          By ROBLEY DUNGLISON, M.D.

                THIS WORK IS NOW COMPLETE, AND TORMS
       FOUR LARGE SUPER-ROYAL,  OCTAVO VOLUMES,
   CONTAINING THIRTY-TWO HUNDRED AND FIFTY-FOUR
         UNUSUALLY LARGE PAGES IN DOUBLE COLUMNS,
        PRINTED ON GOOD PAPER, WITH A NEW AND CLEAR TYPE.
              THE WHOLE WELL AND STRONGLY BOUND,
           WITH RAISED BANDS AND  DOUBLE TITLES.
     Or, to be had in twenty-four parts, at Fifty Cents each.
 For a list of Articles and Authors, together with opinions of the press, see Supplement to the No-
vember number of the Medical News and Library.
  This work having been completed and placed before the profession, has
been  steadily advancing in favor with all classes of physicians.   The nu-
merous advantages which it combines, beyond those of any other work ;
the weight which each article carries with it, as being the production of
some physician of acknowledged reputation who has  devoted himself
especially to the subject confided to  him, the great diversity  of topics
treated of; the compendiousness with which everything of importance is
digested into a comparatively small space; the manner  in which it has
been brought up to the day, everything necessary to the American prac-
titioner having been added by Dr. Dunglison ; the neatness of its mecha-
nical  execution,  and the extremely low price at  which it is  afforded,
combine to render it one of the most attractive works now before the pro-
fession   As a book for constant and reliable reference, it presents advan-
tages which are shared by no other work ofthe kind.  To country prac-
titioners  especially, it  is  absolutely invaluable, comprising in  a mode-
rate space, and trifling cost, the matter for which they would have  to
accumulate libraries/when removed from public collections. The steady
and increasing demand with which it has been favored since its completion,
shows that its merits have been appreciated, and that it is now universally
considered as the
   LIBRARY  FDR  CONSULTATION  AND REFERENCE. 
18
LEA & BLANCHARD'S PUBLICATIONS.
WORKS BY PROFESSORS  CHURCHILL, MEIGS, &c.
CHURCHILL'S   MIDWIFERY.
           A New Edition, Just Published.
 ON  THE  THEORY  AND   PRACTICE   OF  MIDWIFERY,
           BY FLEETWOOD CHURCHILL, M.D.. M.R.I. A.,
Licentiate of the College of Physicians in Ireland; Physician to the Western Lying-in Hospital; Lecturer on
               Midwifery, &c., in the Richmond Hospital Medical School, &c. &c
                    WITH  NOTES  AND  ADDITIONS
                       BY ROBERT M. HUSTON,  M.D.,
Professor of Materia Medica and General Therapeutics, and formerly of Obstetrics and the Diseases of Wo-
    men and Children iu the Jefferson Medical College of Philadelphia; President of the Philadelphia
                              Medical Society, &c. &c.
                       SECOND AMERICAN EDITION.
       WITH ONE HUNDRED AND TWENTY-EIGHT ILLUSTRATIONS,
   ENGRAVED  BY GILBERT  FROM  DRAWINGS  BY  BAGG AND  OTHERS.

                         In One beautiful Octavo Volume.
  The call for a second edition of Dr. Churchill's Midwifery, within so short a time  after the ap-
pearance ofthe first, is satisfactory evidence that the profession in this country appreciate the high
value of the work.  Both as  a text-book for the student and as a manual for the practitioner, it has
a deservedly great reputation, especially for the fulness and  clearness with which the  physiological
details are wrought out and brought to illustrate the practical part.  To render the present edition
worthy of a continuance ofthe favor and confidence so signally manifested towards its predecessor,
the editor has carefully added all the  new facts and observations which have transpired since the
publication of the last edition, or such at least as appeared to him deservipg of being recorded.
These relate to some of the most important points in physiology and obstetrical practice.  Various
new illustrations have been introduced, and the whole brought up, as far as possible,  to the day of
publication.
A NEW  EDITION OF CHURCHILL  ON FEMALES.
   THE   DISEASES    OF    FEMALES,
                          INCLUDING THOSE OF

          PREGNANCY  AND  CHILDBED.

                 BY FLEETWOOD CHURCHILL, M.D.,
              Author of " Theory and Practice of Midwifery," &c. &c.
THIRD AMERICAN, FROM THE SECOND LONDON EDITION, WITH ILLUSTRATIONS-
                        EDITED,  WITH  NOTES,
                   BY  ROBERT M. HUSTON, M. D.; &c. &c.
                             In One Volume, 8vo.
  " In complying with the demand of the profession in this country for a third edition, the Editor
has much pleasure in the opportunity thus afforded of presenting the work in its more perfect form.
All the additional references and illustrations contained in the English copy are retained in this."


 A  TREATISE  ON THE  DISEASES OF  FEMALES,


         AND ON THE  SPECIAL HYGIENE OF THEIR SEX.

                      WITH NUMEROUS  WOOD-CUTS.

                 BY COLO MB AT DE  LTSERE, M. D.,
Chevalier of the Legion of Honor; late Surgeon to the Hospital of the Rue da Valois, devoted to the Diseases
                               of Females, &c. &c.
             TRANSLATED, WITH MANY NOTES AND ADDITIONS,
                          By C. D. MEIGS, M.D.,
   Professor of Obstetrics and Diseases of Women and Children in the Jefferson Medical College, &c. &c.
                          In One Large Volume, 8vo.
  " We are satisfied it is destined to take the  front rank in this department of medical science;  it is
beyond all comparison, the most learned Treatise on the Diseases of Females that has ever been
written, there being more than one thousand distinct authorities quoted and collected by the inde-
fatigable author.  It is in fact a complete exposition of the opinions and practical methods of all
the celebrated practitioners of ancient and modern times. The Editor and Translator has per-
formed his part in a manner hardly to be surpassed.   The translation is faithful to the original, and
yet elegant. More than one hundred pages of original matter have been incorporated in the text,
constituting a seventh part of the whole volume."—New York Journal of Medicine. 
LEA & BLANCHARD'S PUBLICATIONS.
19
WORKS BY PROFESSOR W.  P.  DEWEES.
                     NEW EDITIONS.
               DEWEES'S  MIDWIFERY,


     A COMPREHENSIVE SYSTEM  OF  MIDWIFERY.
CHIEFLY DESIGNED  TO FACILITATE THE INQUIRIES OF THOSE WHO MAY BE PUR-
                       SUING THIS BRANCH OF STUDY.
   ILLUSTRATED  BY OCCASIONAL CASES AND MANY ENGRAVINGS.

           Tenth Edition, with the Author's last Improvements and Corrections.

                   BY WILLIAM P. DEWEES, M. D.,
        LATE PROFESSOR OF MIDWIFERY IN THE UNIVERSITY OF PENNSYLVANIA, ETC.
                             In one volume, octavo.
  That this work, notwithstanding the length of time it has been before the profession, and the
numerous treatises that have appeared since it was written, should have still maintained its ground,
and passed to edition after edition, is sufficient proof that in it the great practical talents of the
author were fully placed before the profession.  Of the book itself it would be superfluous to
speak, having been so  long and so favorably known throughout the country as to  have become
identified with American Obstetrical Science.
               DEWEES  ON  FEMALES.


A  TREATISE   ON  THE  DISEASES  OE  FEMALES,
             BY WILLIAM P. DEWEES, M.D.,  &c.
      LATE PROFESSOR OF MIDWIFERY IN THE UNIVERSITY OF PENNSYLVANIA, ETC.
                         EIGHTH  EDITION,
        With the Author's last Improvements and Corrections.
                     In one octavo volume, with plates.
            DEWEES   ON  CHILDREN.

                          A TREATISE ON THE

PHYSICAL  AND MEDICAL TREATMENT OF  CHILDREN,

                   BY WILLIAM P. DEWEES, M.D.,
       LATE PROFESSOR OF MIDWIFERY IN THE  UNIVERSITY OF PENNSYLVANIA, ETC. ETC.
                          EIGHTH  EDITION.
                            In one volume, octavo.
 This edition embodies the notes and additions prepared by Dr. Dewees before his death, and will be found
much improved.
 The objects of this work are, 1st, to teach those who have the charge of children, eitheras parent or guardian,
the most approved methods of securing and improving their physical powers. This is attempted by pointing
out the duties which the parent or the guardian owes for this purpose, to this interesting but helpless class of
beings, and the manner by which their duties shall be fulfilled.  And 2d, to render available a long experi-
ence'to those objects of our affection when they become diseased.  In attempting this, the author has avoided
as much as possible, "technicality," and has given, if he does not flatter himself too much, to each disease of
which he treats, its appropriate and designating characters, wilh a fidelity that will prevent any two being con-
founded together, with the best mode of treating them, that either his own experience or that of others has
sugsested.
 Physicians cannot too strongly recommend the use of this book in all families.

   ASHWELL  ON  THE  DISEASES  OF  FEMALES.

                    A PRACTICAL TREATISE ON THE


       DISEASES  PECULIAR  TO  WOMEN.
                        ILLUSTRATED BY CASES
           DERIVED FROM HOSPITAL AND PRIVATE PRACTICE.
                     By SAMUEL ASHWELL,  M. D.,
   Member ofthe Royal College of Physicians; Obstetric Physician and Lecturer to Guy's Hospital, &c.
               Edited by PAUL BECK GODDARD, M. D.
                    The whole complete in one large octavo volume.
  a The most able, and certainly the most standard and practical work on female diseases that we
have yet seen."—Medico-Chirurgical Review. 
20
LEA & BLANCHARD'S PUBLICATIONS.
               LATELY PUBLISHED,

                 A NEW EDITION OF

WILSON'S  HUMAN  ANATOMY,
                MUCH  IMPROVED.
   A   SYSTEM   OF   HUMAN   ANATOMY,

            GENERAL   AND   SPECIAL.

                   BY ERASMUS  WILSON,  M.D.,
                       LECTURER ON ANATOMY, LONDON.

               SECOND AMERICAN EDITION, EDITED  BY

                   PAUL B. GODDARD,  A.M.,M.D.,
        Professor of Anatomy and Histology in ihe Franklin Medical College, Philadelphia.

             WITH OVER TWO HUNDRED ILLUSTRATIONS.

 Beautifully Printed from the Second London Edition, in One very neat Octavo Volume.
  "Mr. Wilson, before the publication of this work, was very favorably known to the profession by his trea-
tise on Practical and Surgical Anatomy; and, as this is the Second American Edition, from the second London
Edition, since 1840, any special commendation of the high value of the present work, on our part, would be
supererogatory.  Besides, the work has been translated at Berlin, and overtures were repeatedly made to the
London publisher for its reproduction in France. The work  is, undoubtedly, a complete system of human
anatomy, brought up to the present day.  The illustrations are certainly very beautiful, the originals having
been expressly designed and executed for this work by the celebrated Bagg of London; and, in the American
edition they have been copied in a masterly and spirited manner. As a text-book in the various colleges we
would commend it in the highest terms."—New York Journal of Medicine.
                 WILSON'S  DISSECTOR.

                      THE  DISSECTOR;

     OK, PRACTICAL  AND SURGICAL  ANATOMY.
                      BY ERASMUS WILSON,
                  Author of "A System of Human Anatomy, &c.
            WITH ONE HUNDRED  AND  SIX ILLUSTRATIONS.
                MODIFIED AND  RE-ARRANGED  BY
                    PAUL B. GODDARD,  M.D.,
     Professor of Anatomy and Histology in  the Franklin Medical College, Philadelphia.
                In One Large Royal Duodecimo Volume, Sheep.
  " It strikes us as being all that a "Dissector" should be.  The wood-cuts are numerous and will
afford the student the most essential aid in the  dissecting room."—West. Journ. of Med. and Surg.
       WILSON  ON   THE  SKIN.

A  PRACTICAL  AND  THEORETICAL  TREATISE
ON THE
   DIAGNOSIS, PATHOLOGY AND TREATMENT

               OF  DISEASES OF THE  SKIN;

 ARRANGED ACCORDING TO A NATURAL SYSTEM OF CLASSIFICATION,
                            AND PRECEDED BY
   AN OUTLINE  OF THE ANATOMY AND PHYSIOLOGY OF THE SKIN.

                       BY ERASMUS WILSON,
       Lecturer on Anatomy and Physiology in the Middlesex Hospital Medical School, &c. &c.
                      In One Neat Octavo Volume, Cloth.
 " It is a sound book of practice. As a practical guide to the classification, diagnosis and treatment ofthe dis
eases of the skin, the book is complete. We know nothing, considered in this aspect, better in oir lan-ua»e ■
it is a safe authority in all the matters which, in this range of diseases, engage  the practitioner's attention and
possesses the high quality, unknown, we believe, to every older manual, of being on a level with Science's'hieh
water mark.—Medical Times.                                                   °
31 
LEA & BLANCHARD'S PUBLICATIONS.
21
        A NEW AND COMPLETE WORK ON FEVERS.



                  FETKES;

THEIR DIAGNOSIS, PATHOLOGY & TREATMENT.
          PREPARED AND EDITED WITH LARGE ADDITIONS,

                     FROM THE ESSAYS ON FEVER IN

        TWEEDIE'S LIBRARY OF PRACTICAL MEDICINE,

                                BY
               MEREDITH  CLYMER,  M. D.,

 Professor of the Principles and Practice of Medicine in Franklin Medical College, Philadelphia ;
          Consulting Physician to the Philadelphia Hospital; Fellow of the Col-
                         lege of Physicians, fyc. SfC.

                   In one octavo volume of 600 pages.
  The want of a distinct treatise on Fevers, embodying the received doctrines of their pathology and
 treatment has long been felt and generally acknowledged.  To supply this deficiencj- ii>' ™d™J
 literature is the object of the present volume.  It has been prepared from the Essays on Fever
 contributed by Drs. Christison, Shapter, Burrows, Gregory and Locock, to Dr. Tweedie's  library
 of Practical Medicine," and will be found to embrace the whole class of

   Idiopathic Fevers,—Continued,  Periodical, Eruptive,  and Puerperal,
  The additions ofthe Editor, amounting to about one-half of the volume, have bee nchiefly ™de
 with reference to the Fevers of this country.  It has been his aim to render the work as complete a3
 possible, and to adapt it particularly to the necessities of the American Practitioner.

           WILLIAMS'   PATHOLOGY.


     PRINCIPLES   OF   MEDICINE,
                              COMPRISING               mT««
          GENERAL PATHOLOGY AND THERAPEUTICS,
                          AND A GENERAL VIEW OF           T,„rkr>T\Tr\«T«
    ETIOLOGY  NOSOLOGY, SEMEIOLOGY, DIAGNOSIS AND PROGNOSIS.
           BY CHARLES J.  B.  WILLIAMS, M.D., F.R.S.,
                   Fellow ofthe Royal College of Physicians, &c.
                       WITH NOTF.S AND  ADDITIONS,
                 BY MEREDITH CLYMER, M.D.,&c.
                            In one volume, 8vo.


         WILLIAMS AND CLYMER  ON THE CHEST, &c

                         A TREATISE  ON  THE

 DISEASES  OF   THE  RESPIRATORY  ORGANS,
                               INCLUDING
        THE TRACHEA, LARYNX,  LUNGS, AND PLEURA.
                   BY  CHARLES  J. B. WILLIAMS, M.D.,
    Consulting Physician to the Hospital for Consumption and Diseases ofthe Chest j Author of
    Umsumng r j        "Principles of Medicine," &c. &c.
                 WITH NUMEROUS ADDITIONS AND NOTES,
                    BY MEREDITH CLYMER, M.D., &c.
                       In one neat octavo volume, with cuts.


  detaueu »v,               ,.,-.■  e .1 „ „i,00, nrp admirably executed, and the whole 
22                  LEA & BLA  CHARD'S  PUBLICATIONS.
KIRBY & SPENCE'S  ENTOMOLOGY, FOR POPULAR USE,



          AN  INTRODUCTION  TO  ENTOMOLOGY;

 OR, ELEMENTS  OF  THE  NATURAL  HISTORY  OF  INSECTS :  COMPRISING

      AN ACCOUNT  OF  NOXIOUS  AND  USEFUL INSECTS, OF THEIR

          METAMORPHOSES,  FOOD, STRATAGEMS, HABITATIONS,

               SOCIETIES,  MOTIONS,  NOISES, HYBERNATION,

                               INSTINCT, &c, &c.


                        With Plates.  Plain or Colored.


  By WILLIAM KIRBY,  M.A., F.R.S.  And WILLIAM SPENCE, Esq., F.R.S.

                       FROM THE SIXTH LONDON EDITION.


             Which was Corrected and  Considerably Enlarged,


                     In One Large  Octavo Volume, extra cloth.

  This work, as it at present stands, is acknowledged to be the best extant as a popular introduction to (he
science, containing an immense amount of singular and  interesting information, conveyed in a simple and
agreeable manner.  In preparing the last edition, from which this is primed, the authors have omitted the two
last volumes, as being too scientific for popular use, and arranged it as it now is, forming a complete exposi
lion of the principles ofthe study, unincumbered with Anatomical or scientific details.
  "We are well aware that the physician engaged in an engrossing practice, whether in town or country,
has not much leisure for the perusal of book? unconnected with his profession; but we know just as well, that
while the few are thus immersed in business, the many have the command of more time than they are disposed
to give to professional reading.  How many are the hours wasted by nearly every young physician, waiting
for practice—anxious, dreary hours, because unoccupied !  Why not spend these hours  in the study of such
works as that of Kirby & Spence. wherein the physiologist, farmer, horticulturist, philosopher, and moralist,
may find matter to instruct him.—The Western Journal of Medicine and Surgery.
  "The republication of this work, which has for many years enjoyed a very high and constantly increasing
popularity in Great Britain, confers a decided benefit upon natural science in this country. It is a free, careful
and authentic  exposition ofthe very extensive  department of study, of which  it treats, and has done much
abroad to attract attention to a branch which, until its publication, has found comparatively little favor with the
scientific students of Great Britain.  The treatise of Messrs. Kirby  & Spence is well adapted, not only by its
intrinsic merit, but by its attractive style, to introduce the subject to popular favor. It is thrown into the form
of letters, and although abounding to some extent in scientific terms, it is divested of technicality so far as pos-
sible, and by a very agreeable intermixture of anecdotes. &c, is made a pleasant and entertaining, as well as
very instructive and important work. It is now reprinted from the sixth London edition, which has been  U
revised and corrected, and forms a very handsome octavo volume of about 600 pages.  Those who have n^ver
given any attention to the subject of which it treats, will find embodied in it an immense amount of very inte-
resting and useful information, set forth in an agreeable and attractive style."—N. Y. Courier and Enquirer.
                             THE  CHEMISTRY
                                      OF THE

              FOUR    SEASONS,

                     BY  THOMAS GRIFFITH,
               Lecturer on Chemistry at St. Bartholomew's Hospital, &c. &c.

       IN  ONE  VERY  NEAT  DUODECIMO VOLUME.

                      With Numerous  Wood-cuts.  A New Work.
The object of this little book is to show in  a popular and agreeable manner the chemical agency
exerted in the various phenomena of nature.  It forms a neat volume for the Centre Table.
                               A  TEXT BOOK OF


    PRACTICAL  GEOLOGY  AND   MINERALOGY.

     WITH INSTRUCTIONS FOR THE QUALITATIVE ANALYSIS OF MINERALS,

                         BY JOSHUA TRIMMER, F. G. S.

             WITH TWO  HUNDRED AND TWELVE WOOD-CUTS.

                    A handsome octavo volume, bound in embossed cloth.

 This is a systematic introduction to Mineralogy, and Geology, admirably calculated to instruct the student in
those sciences.  The organic remains ofthe various formations are well illustrated by numerous figures, which
are drawn with great accuracy. 
LEA & BLANCHARD'S PUBLICATIONS.                 23
            GRAHAM'S   CHEMISTRY.


THE ELEMENTSOF  CHEMISTRY.
     INCLUDING THE  APPLICATION OF THE SCIENCE TO THE ARTS.

                      With Numerous Illustrations.
             BY  THOMAS GRAHAM,  F. R. S. L. and E. D.
                  Professor of Chemistry in University College, London, &c. &c.
                         WITH NOTES AND ADDITIONS,
                  BY ROBERT BRIDGES, M.D., &c. &c,
                               In One Volume Octavo.
  The great advancement recently made in all branches of chemical investigation, renders neces-
sary an enlarged work which shall  clearly elucidate the numerous discoveries, especially in the
department connected with  organic Chemistry and Physiology, in which such gigantic  strides
have been made during the last few years.  The present treatise is considered by eminent judges
to fulfil these indications, and to  be peculiarly adapted to the necessities ofthe advanced medical
student and practitioner.  In adapting it to the wants of the American profession, the editor has
endeavored to render his portion  of the work worthy the exalted reputation of the first chemist ot
England.  It is already introduced in many of the Colleges, and has universal approbation.

       FOWNES'S CHEMISTRY FOR  STUDENTS.
ELEMENTARY   CHEMISTRY
         THEORETICAL AND  PRACTICAL.
                       By GEORGE FOWNES,  Ph. D.,
             Chemical Lecturer in the Middlesex Hospital Medical School, &c. &c.

       With Numerous  Illustrations.  Edited, with Additions,

                       By ROBERT BRIDGES,  M. D.,
 Professor of General and Pharmaceutical Chemistry in the Philadelphia College of Pharmacy, &c. &c.
                    In one large duodecimo volume, sheep or extra cloth.
   The character of this work is such as to recommend it to all colleges in want of an elementary
;' text-book, and to all practitioners who wish to place a compendious manual in the hands of their
' students.  It is fully  brought up to the day, containing all the late views and discoveries that have
 so entirely changed the face of the science, and it is completely illustrated with very numerous
 wood engravings explanatory of all the different processes and forms of apparatus.  Though strictly
 scientific, it is written with great clearness and simplicity of style, rendering it easy to be mastered
 by those commencing the study.   The low price at which it is sold, places it within the reach of all.
   Though this \#>rk  has been so  recently published, it has  already been adopted as a text-book by
 many ofthe Medical Institutions throughout the country. As a work for the first class student, and
 as an introduction to the larger systems of Chemistry, such as Graham's, there  has been  but one
 opinion expressed concerning it, and it may now be considered as
                 THE  TEXT-BOOK FOR THE CHEMICAL STUDENT.
             SIMON'S  CHEMJSTRY  OF  MAN.


    ANIMAL    CHEMISTRY.

WITH REFERENCE TO THE PHYSIOLOGY & PATHOLOGY OF MAN.

                        BY DR. J. FRANZ SIMON.
                     TRANSLATED  AND EDITED BY
                  GEORGE  E. DAY, M. A. & L. M. Cantab., &c.
  With plates.  In one octavo volume, of over seven hundred pages, sheep, or in two parts, boards.
  This important work is now complete and may be had in one large octavo volume.  Those who
obtained the first part can procure the second separate.
  " No treatise on physiological chemistry approaches, in fulness and accuracy of detail, the work which
Brands at the head of this article. It is the production of a man of true German assiduity, who has added to his
nwn researches the results ofthe labors of nearly every other inquirer in this interesting branch of science.—
Jj/hi ,ieath of such a laborer, which is mentioned in the preface to the work as having occurred prematurely in
icio is indeed a calamity to science.  He had hardly reached the middle term of life, and yet had made himself
v nwn all over Europe, and in our country, where his name has been familiar for several years as among  the
  «t successful of the cultivators of the Chemistry of Man.....It is a vast repository of facts, to which  the
macher aud student may refer with equal satisfaction."— The Western Journal of Medicine and Surgery. 
24
LEA & BLANCHARD'S PUBLICATIONS.
WATSON'S   PRACTICE    OF   PHYSIC,
                  New Edition by Condie.
                           LECTURES ON THE

  PRINCIPLES AND  PRACTICE  OF  PHYSIC.

                  DELIVERED AT KING'S COLLEGE, LONDON.

               BY  THOMAS WATSON,  M. D., &c.  &c.

          Second American,  from the Second London  Edition.

                        REVISED,' WITH ADDITIONS,
                      BY  D. FRANCIS CONDIE, M. D.,
                 Author of a work on the "Diseases of Children," &c

                          In one Octavo Volume,
      Of nearly ELEVEN HUNDRED Large Pages, strongly bound with raised bands.
  u We know of no work better calculated for being placed in the hands of the student, and for a
text-book, and as such we are sure it will be very extensively adopted.   On every important point
the author seems to have posted up his knowledge to the day."—American Medical Journal.
  The rapid sale of the first edition of this work is an evidence  of its merits, and of its  general
favor with the American practitioner. To commend it still more strongly to the profession, the
publishers have gone to a great expense in preparing this edition with larger type, finer paper, and
stronger binding, with raised bands. It is edited with reference particularly to American practice^
by Dr. Condie ; and with these numerous improvements, the price  is still kept so low as to be within
the reach of all, and to render it among the cheapest works offered to the profession. It has been
received with the utmost favor by the medical press, both of this  country and of England, a few of
the notices of which, together with a letter from Professor Chapman, will be found in the  Supple-
ment to The Medical News and Library, for November, 1845.                           *
                     A NEW AND IMPROVED EDITION OF

   RAMSBOTHAM'S STANDARD WORK ON PARTURITION.

                   THE PRINCIPLES AND PRACTICE OF


     OBSTETRIC  MEDICINE  AND  SURGERY,
                             IN REFERENCE TO

            THE  PROCESS OF PARTURITION.
                                ILLUSTRATED BY
     One hundred and forty-eight Large Figures on 55  lAtlwgraphic Plates,

            BY FRANCIS H. RAMSBOTHAM,  M. D., &c.t
    A NEW EDITION, FROM THE ENLARGED AND REVISED LONDON EDITION.
                    In one large imperial octavo volume, well bound.
  The present edition of this standard work will be found to  contain numerous and important improvements
over the last. Besides much additional matter, there are several more plates and wood-cuts, and those which
were before used have been re-drawn.  This book has long been known to the profession, by whom it has
been most flatteringly received. A. more extended advertisement, with a recommendatory letter from Professor
Hodge of the University of Pennsylvania, may be seen in the November Supplement to The Medical News.


                     CONDIE  ON_CHILDREN.

                        A PRACTICAL TREATISE ON

   THE'DISEASES   OF    CHILDREN,
                     BY D. FRANCIS CONDIE, M. D.,
     Fellow of the College of Physicians; Member ofthe American Philosophical Society, &c. &c
                         IN ONE VOLUME,  OCTAVO.
   OJ" The Publisher/ would particularly call the attention ofthe Profession to an examination of this work.
  "Dr. Condie, from the very great labor which he has evidently bestowed upon this book, is entitled to our
respect as an indefatigable and conscientious student; but if we consider the results of his labor, we cannot
but admit his claim to a place in the very first rank of eminent writers on the practice of medicine.  Regard-
ing his treatise as a whole, it is more complete and accurate in its descriptions, while it is more copious^and
more judicious in its therapeutical precepts than any of its predecessors, and we feel persuaded that the Ame
ric&n medical profession will very soon regard it, not only as a very good, but as^Ae very best ' Practica"
Treatise on the Diseases of Children.' "—Am. Med. Journal. 
                 LEA  & BLANCHARD'S PUBLICATIONS.                25


  THE SURGICAL WORKS  OF  SIR  ASTLEY  COOPER.
 LEA & BLANCHARD have now completed the last volume of the illustrated works of Sir Asiley Cooper.
They form an elegant series; the works on Hernia, the Testis, the Thymus Gland and the Breast, being printed,
illustrated and bound to match, in imperial octavo with numerous LITHOGRAPHIC PLATES, while the
Treatise on Dislocations is in a neat medium octavo form, with NUMEROUS WOODCUTS similar to the
last London ediiion.
               SIR ASTLEY  COOPER ON  HERNIA,
            With One Hundred and Thirty Figures in Lithography.

               THE ANATOMY AND SURGICAL TREATMENT OF


 ABDOMINAL    HERNIA.

                   By  Sir ASTLEY COOPER, Bart.

        Edited by C. Aston Key, Surgeon to Guy's Hospital, &c.
 This important work of Sir Astley is printed from the authorized second edition, published in London, in large
super-royal folio, and edited by his nephew, Professor Key.  It contains all the Plates and all the Letterpress—
there are no omissions, interpolations, or modifications—it s the complete work in
                   One Large Imperial Octavo Volume.
        With over 130 Figures on 26 Plates, and over 400 Large  Pages of Letterpress.
 , The correctness ofthe Plates is guaranteed by a revision and close examination under the eye of a  distin-
guished Surgeon of this city.

         ANOTHER VOLUME OF THE SERIES CONTAINS  HIS TREATISE

   ON THE  STRUCTURE AND  DISEASES OF  THE  TESTIS.
         Illustrated by i tO figures.  From the Second London Edition.
                      BY BKAJNSBY B  COOPER, ESQ.

      AND ALSO ON THE ANATOMY OF THE THYMUS GLAND.
                            Illustrated by 57 Figures.
 The two works together in one beautiful imperial octavo volume, illustrated with twenty-nine plates in the
                  best style of lithography, and printed and bound to match.

                     The Series is concluded by
                             COOPEK UN  THE


 ANATOMY   AND  DISEASES   OF   THE BREAST,  &>,,



           ILLUSTRATIONS OF THE DISEASES OF HIE  BREAST;
And Twenty-five Miscellaneous Surgical Papers, now first published in a collected  form.

           By Sir  ASTLEY COOPER, Bart., F.R.S.,  &c.
   The whole in one large imperial octav , volume ,ll„s ra„d with . w . hundred and fiftytwo figures on
                  thirty six Lithographic Plat-.; well and smugly bound.

  COOPER ON FRACTURES  AND  DISLOCATIONS,
                       WITH NUMEROUS WOOD-CUTS.

 A TREATISE ON DISLOCATIONS  AND  FRACTURES  OF  THE JOINTS.
       By Sir ASTLEY COOPER, Bakt., F. R. S., Sergeant Surgeon to the King, &c.
                         A NEW EDITION MUCH ENLARGED _
           Edited by BRANSBY COOPER, F. R. S., Surgeon to Guy's Hospital.
                       WITH ADDITIONAL OBSERVATIONS FROM
                     Professor JOHN C. WARREN, of Boston.
 With numerous engravings on wood, after designs byBagga memoir and a splendid portrait of Sir
 vvu"'          °           Astley.  In one octavo volume.






 life0f,PhvJohnC Warren MD , the Professor of Anatmny and Surgery in  Harvard University.
 nj8hed ^^f^-fZIrofcMion it would be absurd in us to eulogize Sir Astley Cooper's Work on Fractures
  7ScaUons°f lITa"ffi one, and will probably subsist as long us English Surgery --Medico-Chi-
 wrgkal Review. 
26
LEA  & BLANCHARD'S PUBLICATIONS.
              A  NEW  MEDICAL DICTIONARY,

        In  one Volume, large  12mo., now ready, at a low price.


                             A DICTIONARY OF


     THE  TERMS  USED  IN   MEDICINE
                                      AND

              THE  COLLATERAL  SCIENCES;

           By  RICHARD  D.  HOBLYN,  A.M.,  Oxon.
        FIRST AMERICAN, FROM THE SECOND LONDON EDITION.
                         REVISED, WITH NUMEROUS ADDITIONS,

                   By  ISAAC  HAYS,  M.D.,
                Editor of the American Journal of the Medical Sciences.
  Believing that a work of this kind would be useful to the profession in this country, the publishers
have issued an edition in a neat form for the office table, at a low price.  Its object is to serve as
an introduction to the larger and more elaborate Dictionaries, and to assist the student commencing
the study of Medicine, by presenting in a concise form an explanation of the terms most used in
Medicine and the Collateral Sciences, by giving the etymology and definition in a manner as simple
and clear as possible, without going into details ;  and bringing up the work to the present time by
including the  numerous terms lately introduced.  This design the author has so ably executed'as
to elicit the highest encomiums ofthe medical press.
  It has been  edited with especial reference to the wants of the American practitioner, the native
medicinal plants being introduced, with the formula? for the various officinal preparations : and the
whole being made to conform to  the Pharmacopoeia of the United States.  It is now ready in one
neat royal duodecimo volume of four hundred pages in double columns.
              TAYLOR'S MEDICAL  JURISPRUDENCE.



      MEDICAL   JURISPRUDENCE.

                        By ALFRED  S. TAYLOR,
             Lecturer on Medical Jurisprudence and Chemistry at Guy's Hospital, &c.

      With Numerous Notes and Additions, and references to American Practice and Law.

                           Br R.  E. GRIFFITH, M.D.
                               In one volume, 8vo.

  " We recommend Mr. Taylor's work as the ablest, most comprehensive, and, above all, the most practically
useful book which exists on the subject of legal medicine. Any man of sound judgment, who has mastered  ^
the contents of Taylor's ' Medical Jurisprudence,' may go into a court of law with the most perfect confidence  '^
of being able to acquit himself creditably."—Medico-Chirurgical Review.
  " As we expected, it has become truly the manual of both the medical and legal professions,
and is regarded by all as the standard authority on the subject; the author, also, as we find from
the public prints, is the person consulted, almost as a matter of course, in the more difficult medico-
legal  cases.—The British and Foreign Medical Review.
        LAWRENCE ON THE EYE.  New Edition—Now Ready.



    A  TREATISE   ON   THE   DISEASES   OF  THE  EYE,

                         BY W. LAWRENCE, F.R. S.,
         Surgeon Extraordinary to the Queen, Surgeon to St. Bartholomew's Hospital, &c. &c.
            SECOND AMERICAN, FROM  THE LAST LONDON EDITION,
  With many Modifications and Additions, and the Introduction of over one hundred Illustrations
                           BY ISAAC  HAYS, M.D.,
         Surgeon to Will's Hospital, Physician to the Philadelphia Orphan Asylum, &c. &c.
                           IN ONE LARGE  OCTAVO VOLUME.
 The character of this work is too well known to require a word of commendation. It is justly considered th«
best we possess on the subject. In this edition will be found many important alterations and improvement
bringing the work  up to the level ofthe present state of knowledge on the subjects of Ophthalmic Sureervami
Practice.  The chapters on the Anatomy and Physiology of the Organ have received especial auirmentatioiLa!
and many new cuts have been introduced, rendering the whole clear and comprehensible 
LEA  &  BLANCHARD'S  PUBLICATIONS.
27
                  MILLER'S  SURGICAL WORKS.



       THE  PRINCIPLES  OF  SURGERY.
               BY  JAMES MILLER, F.R. S.E.,  F.R.C.S.E.,
                     Professor of Surgery in the University of Edinburgh, &c.
            In one neat octavo volume, to match the Author's volume on " Practice."

 Bl2 °De Can Perfe th}S W?Tk With0Ut the conviction that he has been addressed by an accom-
 R!i  g-n°n' end"weduWlth nomean Iiterai7 skill or doubtful good sense, and who knows how
 to grace or illumine his subjects with the later lights  of our rapidly advancing physiology.  The
 book deserves a ^rong recommendation, and must secure itself a general peraBal.»-M«K<£/ Times.
     We feel no hesitation in expressing our opinion that it presents the philosophy of the science
 ?T? £ yJ^  ,e~ y *an any °ther work in the lanSuage  with which we are  acquainted.»-PAi-
 ladelphia Medical Examiner.                                           n
   "To the student who. wishes to acquire a useful practical knowledge of the pathology of sur-
 pcal diseases, it is impossible to recommend a better guide than the present treatise by Mr. Mil-
 ler."—Edinburgh Medical and Surgical Journal.
   " An admirable epitome of the surgical science of the day.  Being written by a sound practical
 surgeon accustomed to the public  teaching of his science,  it has  the clearness of diction and ar-
 rangement which renders it an  excellent manual for the students as well as that amount of scien-
 tific and practical information which makes it a safe and valuable guide to the  practitioner."— The
 Lancet.                                                           r


                        JUST  PUBLISHED.

     THE   PRACTICE   OF  SURGERY.

                             BY  JAMES  MILLER.
                     Professor of Surgery in the University of Edinburgh.
                               In one neat octavo volume.
   This work is printed and bound to match the " Principles of Surgery," by Professor Miller,lately
  issued by L. & B.  Either volume may be had separately.
   " This work, with the preceding one, forms a complete text-book of surgery, and has been under-
  taken by the author at the request of his pupils.  Although, as we are modestly informed in the
  preface, it is not put forth in rivalry ofthe excellent works on practical surgery which already exist,
  we think we may take upon ourselves to say, that it will  form  a very successful and formidable
  rival to most of them.  While it does not offer the same attractive illustrations, with which some of
  our recent text-books have been embellished, and while it will  not, as  indeed  is not its design,  set
^aside the more complete and elaborate works of reference which the profession is in possession of,
'ijare  have no  hesitation in stating that the two volumes form, together, a more complete text-book
  of surgery than any one that has been heretofore offered to  the student."—The Northern  Journal
  of Medicine.
   " Mr. Miller is already known in his profession as an able writer and  a well informed surgeon,
  and the book before us is calculated to maintain his reputation.  We recommend it to  those who
i  want a sound guide, or wish to refresh  their recollections.  The  characteristics which  especially
 (distinguish the  work are, its plain good sense, or the selection ofthe important from the unimport-
|f ant announcements in surgery; the sound indications of the judgment to be exercised in the treat-
f ment of surgical diseases."—The Lancet.

                        A NEW AND IMPROVED EDITION OF

          FERGUSSON'S  OPERATIVE   SURGERY.



      A  SYSTEM   OF   PRACTICAL  SURGERY,

                   BY  WILLIAM FERGUS SON,  F. R.  S. E.
               SECOND AMERICAN EDITION, REVISED  AND IMPROVED.
   With Two Hundred and Fifty-two Illustrations from Drawings by Bagg, Engraved by Gilbert,
                         With Notes and Additional Illustrations,
                     BY  GEORGE  W. NORRIS, M. D., &c.
                 In one beautiful octavo volume of six hundred and forty large pages.
   "ifwc were to say that this volume by Mr. Fergusson, is one excellently adapted to  the student, and the yet
P^iHatp the descriptions in the text. Dr. Norris has, as usual, acquitted himself judiciously in his office of
mc',a,'r  His additions are strictly practical and to the point."— Bulletin of Medical Science. 
28
LEA &  BLANCHARD « tubl.il;ATIONS.
LIBRARY    OF   SURGERY.
         CHELIUS'S    SYSTEM     OF    SURGERY,


                  A   SYSTEM   OE   SUKGEBY.

                                  By  J. M.  CHELIUS,
    Doctor in Medicine and Surgery, Public Professor o( General and Ophthalmic Surgery, etc. etc. in the
                                    University of Heidelberg.

                         TRANSLATED FROM  THE  GERMAN,
        AND ACCOMPANIED WITH ADDITIONAL  NOTES AND OBSERVATIONS,

                                 By  JOHN  F.  SOUTH,
                              SURGEON TO ST. THOMAS'S HOSPITAL.

              EDITED, WITH REFERENCE  TO AMERICAN  AUTHORITIES,
                             By GEORGE  W. NORRIS,  M.D.

                    Publishing  In Numbers, at Fifty Cents each.
Nine Numbers are now ready: and the whole is expected to  be  complete by September next, forming
                                 Three Large Octavo Volumes.
  That this work should have passed to six editions in Germany, and have been translated into no less than
seven languages, is sufficient proof of its value.  It contains what is, perhaps, embraced to an equal  extent
by no other work on the subject now before the public—a complete System of Surgery, both in its  principles
and practice. The additions of the translator, Mr. South, are very numerous, bringing the work up to the very
day of publication, and embodying whatever may have been omitted by the author respecting English Sus-
gery; while Dr. Norris will.take equal care in representing the state of the Science in America.
  "Judging from a single number only of this work, we have no hesitation in saying that, if the remaining por-
tions correspond at all with the first, it will be by far the most complete and scientific System of Surgery in the
English language. We have, indeed, seen no work which so nearly comes up to our idea of what such a pro-
duction should be, both as a practical guide and as a work of reference, as this; and the fact that it has  passed
through six editions in Germany, and been translated into seven languages, is sufficiently convincing proof of
its value. It is methodical and concise, clear and accurate; omitting all minor details and fruitless speculations,
it gives us all the information we want in the shortest and simplest form "— The New York Journal of Medicine.
  "The scope of Professor Chelius's Manual is indicated by its  title: it professes to treat, systematically, of tin
science and  art of Surgery, but within such compass as to  render the work an appropriate introduction and
companion to his lectures.  The care, however, which has been bestowed upon its construction, and the labor
Which its research evinces, would be ill repaid were  it confined to this sphere;  and we  may conscientiously
say, that we  know of no Manual of Surgery, on the whole, more deserving of public confidence, or more valu-
able as a guide and refresher to the young practitioner.  It is not our intention at present  critically  to analyze
Mr. South's  labors; but we should be guilty of an injustice  to him  and to our readers if we did not cordially
recommend his work as having fair promise of forming, what it  is the translator's ambition it should be, a sound
and comprehensive system of Practical  Surgery.  The notes and text are so intermingled as to render it con-
tinuously readable, without presenting those abrupt transitions which are so disagreeable in many works simi-
larly arranged.  The faults of omission. &c, at which we have hinted in our comments on the  first chapter of.
our author's work, (viz. that on 'Inflammation') have been amply compensated by the copious and excellentjj
digest of his translator and annotator who is justly proud of availing himself of  the labors of our own coun- •
trymen in this department of pathology, while lie gives their due meed of notice and respect to the contributions
of our continental brethren.  The references which are given to original works have evidently been carefully^
collated, and will be found ef great value to the student and practitioner who may wish  for more copious ir*-_»
formation on any particular branch of Surgery; and  the practical remarks and illustrations with which the*
work abounds,  are a good guarantee  of the translator's ability to do justice to his task, at the same time that
they prove that Mr. South has not failed to avail himself industriously of the large  opportunities which his Hos-
pital appointment has afforded him."—The British and  Foreign Medical Revieiv.
  " We will, therefore, content ourselves for the present with directing the attention of the profession to it, as
being the most complete system of Surgery in any language, and one that is of equal utility as a practical guide
and as a work of reference.  The fact of its having reached six editions in Germany, and its having been trans-
lated into seven languages,.are more convincing proofs of its value than anything that we can say.  Mr. South
has performed his task with much judgment, and has certainly  made a most useful addition to the medical lite-
rature of this country by rendering Chelius's work into English "—The Lancet.
  "This work has long been the chief text-book on  Surgery in the principal schools of Germany, and the pub-
lication of five editions of it in the original and of translations into no less than eight foreign languages, show
the high estimation in which it is held.  As a systematic work on Surgery it has  merits of a high order. It is
methodical and concise—and on the whole clear and accurate.  The most necessary information is conveyed
in the shortest and simplest form. Minor details and  fruitless speculations are avoided.   It is, in  fact, essen-
tially a practical book.  This work was first published nearly twenty years ago  and  its solid and permanent
reputation has no doubt led Mr. South to undertake the present translation of the latest edition of it, which, w«
are informed, is still passing through  the press in Germany.  We  should have felt at a loss to select any oiw
better qualified for the  task than the translator of Otto's Compendium of Human and Comparative Pathological
Anatomy—a surgeon to a large hospital, whose industry and opportunities have enabled him to keep pace with
the improvements of his time."—The Medico-Chirurgical Review.
  "Although Great Britain can boast of some of the most skilful surgeons, both among her past and her presenr
professors of that branch of medical science, no work professing to be a complete system of Surgery has beeft i
published in the British dominions since that of Benjamin Bell, now more than half a century old.
  "This omission in English medical literature is fully and saiisfactorily supplied by the translation of Professor
Chelius's System of Surgery by a gentleman excellently fitted for the task, both by his extensive reading and
the opportunities of.practical experience which he has enjoyed for years as surgeon to one of our largest me-
tropolitan hospitals.  The fact of Professor Chelius's work having been translated into seven languages is suf-
ficient proof of the estimation in which  it is held by our continental brethren, and the English edition now in
eourse of publication,  loses none of the value of the original from the treatment received  at the hands of its
translator.  The notes and  additions of Professor South are  numerous, and contain the opinions resulting from
his vast experience, and from that of his colleague.
  " We are free to confess, prejudiced though perhaps we are, in favor of the English practice of surgery tlwrt
this work is one of great value, and one which every practitioner and advanced student should posses*."— The
Medical Times. 
LEA & BLANCHARD'S PUBLICATIONS.             31
                         (See Page 33.)

    THE  MEDICO-CHIRURGICAL  REVIEW,

 EDINBURGH  MEDICAL AND SURGICAL JOURNAL,
                     AND NUMEROUS OTHERS.
                        WHILE FROM FRANCE
THE GAZETTE MEDIC ALE DE PARIS—L'EXPERIENCE—REVUE MEDICALE
     —JOURNAL DE MEDECINE—JOURNAL DES CONNAISSANCES
                    MEDICO-CHIRURGICALES,
                    AND VARIOUS OTHERS, WITH THE
         ZEITSCHRIFT FUR DIE GESAMMTE MEDICIN,
                  WITH SEVERAL OTHERS FROM GERMANY,
         AND THE BIBLIOTHEK FOR L^EGER, OF DENMARK,
                          TOGETHER WITH
             ALL TEE AMERICAN JOURNALS,
                     ARE PUT IN REQUISITION.
           It will thus be seen that the material for a full Summary of all
        NEW MATTERS AND LMPORTANT DISCOVERIES
 is full and ample, while the exertions of the Editor and the time of publication insure a fulness
                     and newness to this department.
                      ALL THE LATE AND IMPORTANT

            AMERICAN  INTELLIGENCE,
                       is fully recorded—while

                 THE MONTHLY  TCEWS
      furnishes the lighter and floating information, and embraces important Books for

         THE  LIBRARY  DEPARTMENT.
      Among ilwse works already published in the Monthly Library and News, may be mentioned
        WATSON'S LECTURES ON THE PRACTICE OF  PHYSIC,
                             AS ALSO
  BRODIE'S  LECTURES  ON  SURGERY,
              CONCLUDED IN MARCH OF THIS YEAR, (1846).
                 The work for  the year 1846 is a new one,
                     TODD & BOWMAN'S

PHYSIOLOGICAL  ANATOMY  & PHYSIOLOGY OF MAN.
                 NOW PUBLISHING IN ENGLAND.
  This work has  already earned for its authors a  high reputation in Europe and this
country. It will be reproduced entire, with all the  numerous and beautiful wood-cuts.
  Each Work in the Library is regularly paged so as to be bound separately.

                      THE TBBSSfXS ABE
For the Medical Journal and the Medical News, if paid for by
  the first of February of each Year, and the  amount remitted
  free of cost to the Publishers,  -    -   -    -  .  -  Flve Dollars.
For the Journal  only, when ordered without funds, or paid for
  after the first  of February of each year,     -     -    - Five Dollars.
For the Medical News only, to be  paid for always in advance,
  and free of postage,......"   0ne Dollar.
   (jc$p In no case  can the News  be sent without pay in  advance, cfjg 
32     TWO MEDICAL PERIODICALS FOR FIVE DOLLARS  A YEAR.

                                rr tl P


AMERICAN  JOURNAL   OF  THE  MEDICAL  SCIENCES,

                   Edited by ISAAC  HAYS, M. D.
                      SURGEON TO WILLS' HOSPITAL, ETC.
Published Quarterly on the first of January, April, July-and October; each Number having at least
                      264 large and closely printed pages.

                  WHEN NECESSARY, CASES ARE
    Fully Illustrated with LITHOGRAPHIC MATES and WOOD-CUTS.

                               ALSO,

       ©I)e  JtUMcal  News  anb  Ctbrarg,

   Of 32 Large Pages, Published Monthly, is given Gratis to Subscribers
to The Journal who paj£, by the first of February of each year, Five Dollars free of expense to the
                              Publishers.
  Under the new law the postage on the Journal is reduced to about 13£ cents per number, while
the News and Library is sent through the mail as a Newspaper.
  Persons wishing  to subscribe, to commence with the January number for 1847, should advise the
publishers at once, as the whole quantity for the present and two past years was taken up early
in the year.

  The publishers do not deem it necessary to refer to the past course of the Journal.  It is suffi-
cient that for the last TWENTY-SEVEN YEARS it has received the approbation of the profession
at home  and abroad; but they would call attention to the extended and  liberal arrangement exist-
ing and to be pursued that shall embody the latest intelligence, from all quarters.
                  ITS PAGES WILL BE DEVOTED FIRST TO.
          ORIGINAL   COMMUNICATIONS
                    FROM ALL SECTIONS OF THE UNION, WITH

                 REVIEWS OF ALL NEW  WORKS
                            OF INTEREST, AND

       HQ iLQ® Q R &f>U  0 © AL  !Nl©T[l©Etg
         While its QUARTERLY SUMMARY will embrace a full and extended

            RETROSPECT  AND  ABSTRACT
                           FROM THE VARIOUS

              FOREIGN AND DOMESTIC JOURNALS.
With reference to this department, the arrangements of the publishers are very extensive and em-
            brace for the gleanings of the editor the various Journals from

     GREAT BRITAIN, FRANCE, GERMANY,

                     DENMARK,  ITALY,

             JUYI> OTHER SECTIONS OF THE WORLD.
                  INCLUDING AS PROMINENT AMONG THE ENGLISH,

       B RAITH  WASTE'S  RETROSPECT,

              RANKING'S HALF-YEARLY ABSTRACT,



                THE  LONDON MEDICAL TIMES,

      THE  LONDON  MEDICAL  GAZETTE,

FORBES7  BRITISH  AND  FOREIGN  QUARTERLY.
                        (Continued on Page 31.)


  JKr This paper may be delivered to any physician if declined by the person to whom it is ad-
dressed,  or if they have removed—and Postmasters and others will particularly oblige the publish-
ers by furnishing a list ofthe Physicians and Lawyers of their county or neighborhood.  In addition
to the business it may bring to the office, a copy of" The Complete Florist," or such other volume,
will be sent by mail gratis for any ten or  more names furnished free of cost.
   Philadelphia, July, 1846. 
AUG 14 '47