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TREATISE
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PNEUMATICS:
THE   PHYSICS   OF  GASES,
              INCLUDING VAPORS.
                      CONTAINING


A FULL DESCRIPTION OF THE DIFFERENT AIB, PUMPS, AND THE EXPERIMENTS WHICH MAY EE
   PERFORMED WITH THEM; ALSO THE DIFFERENT BAKOMETEES, PRESSURE GAUGES,
       HYGROMETERS, and other METEOROLOGICAL INSTRUMENTS,
          EXPLAINING THE PRINCIPLES ON WHICH THEY ACT, AND
                 THE MODES OF USING THEM.


          Illustrate!! Jj Numerous jFtiu (Utooli Jjtnjrabittijjs.
                        BY


           MARTIN" II. BOYE, M.D.,A.M.,
PROFESSOR OF NATURAL PHILOSOPHY AND CHEMISTRY IN THE CENTRAL HIGH SCHOOL OF PHILADELPHIA,
   FORMERLY ASSISTANT GEOLOGIST AND CHEMIST TO THE GEOLOGICAL SURVEY OF THE STATE OF
       PENNSYLVANIA, MEMBER OF THE AMERICAN PHILOSOPHICAL SOCIETY, ETC. ETC.
           PHILADELPHIA:
E. C. & J. BIDDLE, No. 8 MINOR STREET, _-.. .
     (Between Market and Chestnut, and Fifth and Sixtlrffts.)   \ f^33!!!c:l ^t^^V-
               1856.     ..i'^k*        "*A
'J*?
 
1951,
fc^ll-V
      Entered according to the Act of Congress, in the year 1855, by

                         M. H. BOYE,

in the Clerk's Office of the District Court of the United States for the Eastern
                    District of Pennsylvania.
STEREOTYPED BY L. JOHNSON & CO.
       PHILADELPHIA.
Pidated by T. K & 1' G CoVLins 
PREFACE.
  The frequent inquiries made in regard  to the principles, differ-
ent constructions, and modes of using  the  different meteorological
instruments,  which  come  within  the subject treated  of in this
little volume, and the general and increasing  interest felt in these
matters, induce the  author to believe that the present work will
supply a want which has been much felt.   While he has adhered
to a strict systematic arrangement,  and,  on  the part  of science,
sacrificed nothing to popularity, he hopes that  he has made  the
explanations  so  clear and full as to  be intelligible  to all.   Nor
has he spared any trouble or expense in illustrating the subject by
numerous appropriate wood-cuts made expressly for this work, and
many of them entirely original.  For the use of the different instru-
ments  a series  of Tables has been added, including  those of  the
Tensions of Vapor of Water, used with the Boiling-Point Barome-
ter and the different Hygrometers, which Tables have  been calcu-
lated for this work  from those of Regnault,  and are here given,
for the first time, complete  in English  measures and  Fahrenheit
degrees.
  Philadelphia, May \Qth, 1855.
3 
 
CONTENTS.
ON    INANIMATE   MATTER.
         GENERAL INTRODUCTION.
Paragr.                               Page
 1. Matter.  Sciences.  Physical Sciences.   9
 2. Forces.  Laws.   Object of Physical
      Sciences...................................   9
 3. Life.  Physics  of Animate  Matter
      or Physiology...........................   9
 4. Physics of Inanimate Matter, how di-
      vided.......................................   9
 5. Descriptive Sciences.....................  10
 6. Applied  or Practical Sciences.........  10
 7. Mixed Sciences............................  10


          DIVISION  I.
PHYSICS PROPER, OR NATURAL PHILOSOPHY.

              INTRODUCTION.

 8. Physics  proper defined.................  11
 9. Plan of distribution of Matter through
      Space.......................................  11
10. Ultimate  construction of  matter.
      Atoms......................................  11
11. Cohesion.....................................  12
12. Different Forms or States of Matter.
      Solid, liquid and gaseous states.....  12
14. Ether, or Imponderable Matter........  13
15. Adhesion....................................  13
1.6. Gravity......................................  13
17. Impenetrability............................  14
IS. Impact or Impulse........................  14
19. Inertia.......................................  14
20. Limit.  Form.   Numbers...............  14
21. Motion and Kest..........................  15
22. Their relation to matter.................  15
23. Physics proper,  how divided...........  15
      General Table of Divisions and Sub-
      divisions of Physical Sciences......  16

               PART L
    PHYSICS  OF PONDERABLE MATTER.

               SECTION I.
   PNEUMATICS, OR  PHYSICS OF  GASES.
     Properties depending on Cohesion.
24. Expansibility...............................  17
Paragr.                               P<ge
25. The Atmosphere..........................  17
26. Different gases of the Atmosphere... 18
27. Extent of Atmosphere................... 18
28. Exhausting Air-pumps.  Single-bar-
     relled stopcock-pump.................. 18
29. Mode of Action.  Injurious Space... 20
30. Single-barrelled Valve-pump or Sy-
     ringo....................................... 21
31. Wide-mouthed Receivers and Plate... 21
32. Double-barrelled  Exhausting Air-
     pump....................................... 21
33. Single-barrelled, double-acting........ 22
34. Improved  single-barrelled,   single-
     acting...................................... 24
35. Mode of calculating rarefaction....... 2fi
37. Suction by the mouth.................... 27
38. Other means of exhaustion.  Filling
     of Thermometer-bulbs................. 27
39. Compressibility  and  Elasticity  of
     Gases....................................... 27
40. Forcing or Condensing Air-pumps.... 28
41. Single-barrelled........................... 28
42. Receivers and Plate...................... 29
43. Single-barrelled, double-acting........ 30
44. Mariotte's Law............................. 30
45. Permanent  and Liquefiable  gases.
     Vapors..................................... 30
48. Diving Bell................................. 31
49. Air-gun....................................... 31
50. Other means  of compressing gases.
     Steam-boiler.  Fire-arms............ 32

    Properties depending on Adhesion.

52. Diffusibility of Gases..................... 32
53. Diffusion through Porous Bodies...... 33
54. Condensation of Gases  on   Solids.
     Hygroscopic Water. Platinum Ig-
     niter........................................ 33
55. Solution of Gases in Liquids........... dA
56. Diffusion of Gases in Solution.  Re-
     spiration. Confining Gases by Wa-
     ter and by Mercury.................... 35

     Properties depending on Gravity.
57. Weight of Gases........................... 35
58. Specific Gravity of Gases............... 36 
vi
CONTENTS.
Paragr.                               Pa«e
 59.  Pressure caused by Weight of Atmo-
        sphere....a............................  *7
 60.  Torricellian Tube......................;•  38
 61.  Pressure  of  Atmosphere, how esti-
        mated.  Used as Unit..............  38
 62.  Torricellian Vacuum.....................  39

              The Barometer.
 63.  Cup and Syphon Barometers.........  40
 64.  Source of Inaccuracy, how remedied.  41
 66.  Water Barometer........................  42
 68.  Diagonal or Inclined Plane Baro-
        meter....................................  43
 69.  Wheel Barometer........................  43
 70.  Huygen's Double-Barometer.........  44
 71.  Means  of increasing Accuracy in
        measuring..............................  44
 72.  Vernier, its Nature and Construction  44
 75.  Effects of Capillarity on Barometer..  47
        Table of Correction for same......  47
 76.  Friction and Adhesion of Mercury...  48
 77.  Expansion by Heat of Mercury, and
        of Scale.................................  48
 78.  Marine Barometer.......................  50
 79.  Gay-Lussac's Portable Syphon Baro-
        meter....................................  51
 80.  Preventing Air getting into the Va-
        cuum....................................  53
 81.  Accurate Levelling Barometer.......  53
 82.  Standard Barometers...................  55
 83.  Self-registering...........................  55
 84.  Objections to Mercurial Barometer.. 55
 85. Substitutes for Mercurial Barometer. 56
 86. Sympiesometer........................... 56
 87. Boiling-Point Barometer...............  57
 88. Aneroid Barometer...................... 58
 89. Metallic Barometer (Bourdon's)...... 61
 90. Nature of Barometer....................  62
 91. Manometer................................. 62

           Uses of the Barometer.
 92. As Weather-glass........................ 63
  93. Range and Variations of Barometer. 65
 94. For measuring Heights. Principle of. 65
        Mode of Calculating................ 67
  96. Example.................................... 69
  98. Rapid  Decrease  in Pressure and
        Density  of the Atmosphere....... 70
  99. Mode  of taking   Observations for
         Levelling.............................. 70
 100. Estimating the   true  Volume  of
         Gases, and from it, their Weight.. 71

              Mariotte's Law.
 101. Experiments  to  prove  Mariotte's
         Law..................................... 73
 103  Mariotte's Tube.......................... 75
 104. Exceptions to Mariotte's Law......... 75

               Pressure-gauges.
 105. Mercurial Exhaustion-gauge for Air-
         pumps.................................. 75
Paragr.                                Page
106.  Mercurial Pressure-gauges............ 76
107.  Condensed-Air Pressure-gauges...... 77
108.  For very high Pressures............... 78
109.  Of very small Dimensions............ 79
110.  Steam-gauges (Manometers). Safe-
        ty-valves............................... 79

    Experiments to illustrate Pressure of
               Atmosphere.
111.  Fountain in Vacuo...................... 80
112.  Mercurial Rain........................... 80
113.  Bursting of Bladder..................... 80
114.  Upward Pressure of Atmosphere.... 80
115.  Magdeburg Hemispheres.............. 81
116.  Pressure on Human Body............. 81

Experiments to illustrate Expansibility, Elas-
  ticity, and Compressibility of Atmospheric
  Air.
117.  Difference  between  Expansibility
        and Elasticity.  Tension.......... 82
118.  Inflation  of Bladder by  Expansi-
        bility.................................... 83
119.  Mechanism of Respiration..........-. 83
120.  Expulsion  of  Air from Water by
        Exhaustion............................ 84
121.  From the Pores of Charcoal.  Their
        filling with water.................... 84
122. Hero's Ball............................... 84
123.  Condensed-Air Chamber of Hydrau-
        lic Engines............................ 85

        Impact and Inertia of Gases.

124. Resistance of Air.  Windmill Ex-
        periment............................... 85
125. In a Vacuum all Bodies fall equally
        fast.  Feather and  Guinea Ex-
        periment............................... 86
126. Resistance of Air to Projectiles.
        Flight of Birds.......................  87
127. Winds.  Table of their  Velocities
        and Force.*............................  87
128. Anemometer............................... 88
129. Efflux of  Gases  into a Vacuum
        through Capillary Orifices (Effu-
        sion)....................................  88
 130. Through Capillary Tubes (Transpi-
        ration)..................................  89
 131. Efflux of Gases into the Atmosphere.  89
 132. Revolving Gas Jet......................  89
 133. Pneumatic Paradox.....................  90

                 VAPORS.

 134. Circumstances under which formed. 91
 135. Nature of Vapors........................ 91

     Formation of Vapors in a Vacuum.

 136. Has  a Limit.  Maximum Quantity
        and Tension; depends on  Tem-
        perature................................. 92 
CONTENTS.
vii
Paragr.                               Page.
137.  Quantity of Vapor estimated from its
        Tension.................................  93
138.  Table of Maximum Quantities and
        Tensions................................  93
139.  Tension or Elasticity of Steam at
        high Temperaturos.................  93
140.  Expansion of Vapors by Heat.......  94
141.  Vapors not filling space to Satura-  ,
        tion may be subjected to Pressure
        and Cold..............................  95
142.  Hlustration as regards Pressure.....  95
143.  As regards Cold. Dew-point. Proof
        of Saturation........................  96
144.  Boiling in a Vacuum, how produced.  96
145.. Culinary Paradox.......................  97
146.  Papin's Digestor........................  97
147.  Theoretical Stop  to Evaporation...  97
148.  Different Volatility of Substances..  98
149.  Modes of increasing Evaporation in
        a Vacuum.............................  98
150.  Applications in Chemistry...........  98

       Formation  of Vapors in a Gas.

151.  Has a Limit.  Maximum Quantities
        and Tensions the same  as in a
        Vacuum..............................  99
152. Difference between the Formation of
        Vapors in a Gas and in a Vacuum 100
153. Boiling in Open Air.  Simmering. 100
154. Boiling-Point  of  different  Sub-
        stances................................ 101
155. Boiling-Point altered by change in
        Pressure.............................. 101
156. Limit to Boiling in a Vacuum...... 101
158. Modes of increasing Evaporation in
        a Gas.................................. 101
159. Applications in Chemistry........... 102

     Vapor of Water in the Atmosphere.

160. Different  States in which  Water
        exists in the Atmosphere.  Dew.
        Fogs.  Clouds;  different Varie-
        ties of.   Rain; Rain-gauge or
        Ombrometer.  Hail.  Snow...... 102
161. How Vapors affect the Atmosphere. 103
162. Moisture or Humidity of the Atmo-
        sphere.   Relative Moisture or
        Humidity............................. 103
163. Amount of  Vapor, how estimated
        by Chemical Method............... 104

               Hygrometers.

164. Their  use.  Mode of  finding the
        Relative  Humidity  from the
        Dew-point and the Temperature
        of the Atmosphere................. 105
165. To find Tension of Vapor and Dew-
        point  from Relative  Humidity
        and Temperature of Atmosphere 106
166. To find per centage of Vapor by
        Volume................................ 106
Paragr.                               Page.
167.  Per centage of Vapor by Weight... 106
168.  Absolute Weight of Vapor in a cer-
        tain Volume......................... 107

     Hygrometers giving the Dew-point.
169.  Daniell's Hygrometer.................. 108
170.  Bache's Hygrometer.................. 109
171.  Regnault's Hygrometer............... 109
     August's Psychrometer, or the Wet-
        Bulb Hygrometer................... 110
     Formula for Tension of Vapor, and
        Dew-point, from Wet-Bulb Hy-
        grometer.............................. Ill
     Example.................................. 112
     Precautions  in  using  Wet-Bulb
        Hygrometer........■.................. 113
172.

173.
    Hygrometers acting by Absorption.
177. Their mode of Action.................. 113
178. Saussure's Hair Hygrometer......... 114
179. Table of Relative Humidities cor-
       responding to its Degrees........ 115
180. Objections to the Hair Hygrometer 115
181. Hygroscopes mado from Whale-
       bone,  Wood, Twisted Strings,
       Beard of Sensitive Oats, Blad-
       der, <fcc................................ 115

                 Tables.
Table I. Correction for  Temp, for Barome-
  ters mounted in Wood.
Table II. Correction for Temp, for Baro-
  meters with Brass Scale,  extending  the
  whole length.
Table III. For finding differences in Height
  between two Places from Barometric Ob-
  servations.
Table IV. Correction of same for Latitude.
Table V. Correction of same for Altitude.
Table VI. Conversion of French into Eng-
  lish, and  of English  into  French mea-
  sures, <fec.
Table VII. Maximum Tension, or Elastic
  Force of Vapor of Water for every 0.2 de-
  gree from 214° to 185.° For Boiling-Point
  Barometer.
Table VIII.  Maximum Tension of Vapor
  of Water for every degree from 185° to
  104°.
Table IX. Maximum  Tension, or Elastio
  Force of Vapor of Water,  for every 0.2
  degree from 204° to 0°, and for every de-
  gree from 0° to — 31°.  For (Dew-Point)
  Hygrometers. 
 
                        ON

INANIMATE    MATTER
                GENERAL INTRODUCTION.

  1. By Matter we understand all that acts on our senses.  Matter, there-
fore, constitutes the whole external or Material World,  the  Universe.
Our knowledge of matter systematically arranged constitutes the Sciences
of Matter, or the different Material  or  Natural or Physical Sciences, or
Physics in its widest sense;  in contradistinction to the sciences of the
mind, or the Mental and Moral  Sciences, treating of the internal or
immaterial world.
  2. The different phenomena and properties of matter we account for
by ascribing them to certain  causes inherent in matter itself, which we
call Forces.   These  forces are always  found  to act according  to certain
rules or laws. The main object of the physical sciences must, therefore,
be to discover these forces, and expose  the laws according to which they
act.
  3. But besides the general forces, which all matter obeys at  all times,
matter is also capable of being brought under a  peculiar influence, which
we call Life.  While under such influence it is called Animate  matter, in
contradistinction to which, when  not  under this influence, it is called
Inanimate matter.  We therefore  get  two main branches of the physical
sciences:  Physics of Animate matter, or Physiology (Special  Physics),
which treats of Life  and the  manner in which matter is influenced by it,
or of Animate matter; and Physics of Inanimate matter (General Physics),
which treats of Inanimate matter.
  4. Our knowledge of inanimate matter must refer either to its place, or
to its  nature; we  therefore get two divisions of  Physics of  Inanimate
                              9 
10
BOYE'S INANIMATE  MATTER.
matter, Physics proper, or Natural Philosophy, which treats of the place
of inanimate matter, and Chemistry, which treats of its nature.
   5.  The physical sciences have each their  descriptive part, describing
the different objects or bodies formed in nature by the forces or influences
of which they treat.  These descriptive parts  are often considered as sepa-
rate sciences,  and called  the  sciences of Objects, in  contradistinction to
which the others, of which they are only descriptive parts, are called the
sciences of Phenomena.   Thus the descriptive part of Physiology, or a
description of all  the  different forms of matter assumed under the influ-
ence of life, or animate  objects,  constitutes  Natural History, of  which
again Anatomy is  a subordinate  branch.   Uranography is a descriptive
part of Physics,  Mineralogy  of Chemistry,  and Geology, Meteorology
and Physical Geography, are descriptive parts of Physics and Chemistry.
  6. The above main branches and divisions of the natural sciences, when
applied to particular purposes, as the performance of certain  mechanical
operations, or the production of certain chemical  compounds,  required for
our necessities or  comforts or  other relations  of social life, constitute the
different applied,  or practical, or industrial sciences.   These are used in
the different trades, manufactures  and arts, and a systematic arrangement
of the greater  number  of them is often called Technology.  Agriculture,
Surgery, Medicine, &c, are instances of applied branches of Physiology,
in connection with Physics and Chemistry.
  7. The different  economical,  political and philological  sciences  are
combinations of mental and physical sciences,  pure or applied.
10 
                          DIVISION I.

PHYSICS PROPER, OR NATURAL PHILOSOPHY.
                         INTRODUCTION.

   8.  Physics proper, it has been said in a general way, treats of the place
of matter.   But as we account for  all phenomena connected with matter
by ascribing them to certain causes  inherent in matter, which we call
forces, which forces always act according to certain rules or laws, Physics
proper must treat of the forces, by which matter holds its place in space,
and expose the laws according to which they act.
   9.  That which first strikes us in regard to the place of matter on a
large  scale,  is, that we do not find it to be  equally distributed through
space, but collected in large  masses, constituting  the  heavenly bodies and
the earth, and the  space between them to be comparatively void.
   10. We have reason to  believe, that internally matter is constructed on
a similar plan, so  that any portion  of it does not consist of matter  uni-
formly diffused through the space which it  occupies,  but  that the matter
of which it consists is collected  in small particles called atoms,  with a
small  space between them, which is comparatively void.  These ultimate
particles or  molecules are  called atoms  (from a  privative and  re/vsti
(temno) I cut), meaning  what can  not  be cut or divided,  because they
are considered  to  be indivisible and  indestructible.  Though practically
they  may be considered  infinitely  small,  still  in reality  they have  a
certain definite size and form.  Their form is generally considered to be
that of small solid spheres or spheroids, inside perfectly uniform;  single
spheres  for  simple bodies and clusters of such spheres for  compound
bodies.*
  * The main arguments in favor of the existence of atoms with spaces between them
are; the general nature of chemical combination with the laws of definite and multiple
proportions, isomerism and allotropism, for  which see under Chemistry; cleavage and
crystallization, see  unde/  Stereotics;  expansibility of gases, see  Pneumatics;  the 
12
BOYE'S INANIMATE  MATTER.
  11. All bodies, by which we understand limited portions of matter, must
therefore consist of aggregations of such  atoms.   These atoms being  not
in contact are kept at certain extremely small but  definite  distances from
each other by  two forces, an  attractive  force, which tends to approach
them to each other, and a repulsive  force, which tends to  separate them.
The resulting effect of these two forces is  called cohesion,  and constitutes
the force with which each atom  is held  in  the  same relative position to
the other atoms of the same kind of matter.  Compressibility and Elas-
ticity are properties of matter depending on this same  force.
  12. According to the greater or less strength of the above attractive and
repulsive forces between the atoms, constituting  the  degree of cohesion,
matter presents itself in one or the other of the following three states or
forms.
  1st.  The  solid state.   Whenever the  attractive and repulsive forces
between the atoms  are great, the atoms are kept  firmly in their  relative
position, so that they offer  considerable resistance to  any force that tends
to move them  among themselves, or to  separate them from each other.
In  this case, therefore, the cohesion is said to be great, and  the matter
presents itself in the solid state.
  2d.  The liquid state.  In this state matter presents itself when the
attractive and  repulsive forces between  the atoms are but small.   The
atoms  are then held in their  relative  position with  but a slight force, so
that they can easily be moved among themselves, or  separated from  each
other.   In liquids,  therefore, the cohesion is small.
  3d.  The  gaseous state.   This  state  matter assumes when  the atomic
repulsive  force is greater  than  the attractive.  The atoms then  have a
tendency  to separate from  each  other  and spread  themselves  through
space, unless prevented by some other cause.  This  property in  gases is
called  Expansibility, and distinguishes them from liquids.   Cohesion in
this case is said to be negative.   They also offer little or  no resistance to
the  motion  of  their particles among themselves, in which point  they re-
semble liquids.   For this reason  liquids and gases are comprised together
under the common name of fluids in contradistinction  to solids.
   13.  One and the same kind of matter may often, under different circum-
stances, exist in  either of the above three  states.  Thus water when ex-
posed to cold becomes solid or ice, and by heat may be  converted  into gas
or steam.  But matter can only exist in  one state  at the  same time, and
under  the same circumstances it  nearly always assumes the same state.
expansion of all matter by heat, see Thermics; and the undulatory nature"of light, and
its passage through all forms of ponderable matter, see Photics.  For  the particulars
regarding the form, size and weight of atoms, see under Stereoties. 
            PHYSICS  PROPER,  OR NATURAL  PHILOSOPHY.         13

   14. The ethereal state.  The existence of a fourth state of matter is ren-
dered highly probable, filling the spaces  between the  atoms of the above
three states (the interatomic  spaces), and  the spaces between the planets
and between the  stars (the  interplanetary and interstellar  spaces).  This
state is  called the Ethereal, and the matter itself Ether.*  That Ether
must differ materially from other states  of matter, follows from  the fac';,
that it fills the spaces between their  atoms.  Either therefore it can not
be composed of similar ultimate atoms, or these atoms must at least be of
a  much smaller size.   As it has been found  to offer a sensible resistance
to the comets in their motion, it must, as it will afterwards be understood,
possess inertia and in this point resemble the  other kinds of matter.   If,
however, it is affected by gravity so as to possess weight, (see further  on),
this is so inconsiderable, that it cannot be ascertained by the same means
by which it is proved for other  matter, hence it is generally called  Im-
ponderable matter, in contradistinction to which the other states of matter
are called Ponderable matter.   It has not been ascertained whether other
states of matter may also exist in the Ethereal state, or the Ether itself
be condensed  or  converted  into the others.   On  the whole, though its
existence is well established, our  knowledge of its  nature is yet but very
imperfect.
   15. The same  attractive  and repulsive forces, which exist between the
atoms of matter  of the same kind,  we  also  find  between the atoms of
different kinds of matter, by which these are  held  in their relative posi-
tion at a small distance from each other.   The  resulting effect is in  this
case called Adhesion, because if after having brought the atoms of two
different bodies together, we again  attempt to separate them, particles of
the one often remain by this force attached, or adhere to the other.  Thus
if we dip a glass rod into water and  then again withdraw it, some of the
water will adhere to the glass in preference to cohering to  the other  par-
ticles of itself.   Capillary attraction and  solution are caused by the same
force.
   16.  The attractions and repulsions, of which we  have spoken (Cohesion
and Adhesion), do not  extend perceptibly beyond  a very small distance,
probably  not beyond the distance of proximate or  neighboring atoms.
We observe, however, another attractive force to exist between atoms of
the same or different kinds of matter,  and acting also at distances greater
than the distances of proximate atoms, only in a certain diminishing ratio.
  * The main argument for the existence of Ether in all these spaces and others, not
filled with ponderable matter, we have in the passage through them of light, which can be
proved to be formed by undulations, which therefore require the existence of an undula-
ting medium.                                                      ,, 
14
BOYE'S INANIMATE MATTER.
 As it  thus  acts on all the atoms, of which a body consists, and  at great
 distances, it becomes also an attraction between  masses of atoms, or bodies
 towards each other.   This attraction is called Gravity, and must there-
 fore be greater according to the number of atoms  in the different bodies.
 We thus find  that a very strong  attraction  exists  by gravity between the
 heavenly bodies and the earth, and between the earth and  all terrestrial
 bodies on or near its surface; but it is exceedingly small between the
 terrestrial bodies themselves, though it can  be proved also to exist be-
 tween  them according to  their  size.   Gravity has by some  been con-
 sidered as  the  result  of the attractions of  cohesion and adhesion,  but
 this is not  probable;  at all  events we are  not  acquainted with a corres-
 ponding repulsive force acting at a distance  like  Gravity.
   17.  It has  been stated  that the ultimate atoms are considered solid.
 They therefore  allow no other atoms of the same,  or any other  kind of
matter to enter or occupy the same space at  the same time.  This property
of matter is  called Impenetrability.   The space between the atoms may
be diminished  (Compressibility),  but the atoms  of the  same body  can
never be forced into  each other  even by the greatest  pressure, nor will
they allow the atoms  of any other body to be forced into their place.
One kind of matter may,  however, allow the atoms of another kind to
penetrate with  considerable  facility into  the spaces  between  its atoms,
while it will resist with great force the  further approach  of its  own
atoms.   This property is called Diffusibility, and depends on the attrac-
tion, which  has been spoken  of before as Adhesion, and  is particularly ob-
served  between the atoms of solids and liquids, and also between the atoms
 of different kinds of gases.
   18.  When matter has been influenced by a force to move, and in its way
 meets other matter, so that it can not continue its  motion without putting
 this matter also in motion, we find this  latter to take place,  and a portion
 of its own motion  to  be transferred to  it.   We thus find, that motion is
 transferable  by Impact or Impulse from one portion of matter to another.
   19.  Matter has also an inherent force to preserve its state of  rest or
 motion.  This force or property of matter is called Inertia,  and is gene-
 rally expressed thus,  that  matter when at  rest  cannot  by itself beo-in
 motion, nor when in motion  can it alter this so as to pass to rest  or to a
 slower or faster motion, or in a  different direction, unless influenced by
 some other cause.
   20.  With the idea  of matter and its existence  is necessarily given  th
 idea of space to exist in.  Where  one kind of matter ceases and an th
 begins^ there must be a limit, and all limited portions of matter or b A'
 must therefore have a  Form.  But the abstraction of space and forn f    '
                                  14 
           PHYSICS  PROPER, OR NATURAL PHILOSOPHY.         15

matter, and its separate consideration can  only be made in the mind, and
constitutes, therefore, a purely mental science, Geometry, which does  not
belong  to  the  natural sciences, while the application of its results to  the
forms of matter, as they actually occur in nature, is of the utmost import-
ance to them  (Crystallography,  &c).   The same is the  case  with  the
abstraction of the idea of repetition of separate but like portions of matter
or Numbers, and their  separate study, which constitutes Algebra, and in
its application is of equal  importance to the natural sciences.
   21.  Matter, while it by its own inherent forces  influences other matter
and itself to motion, is equally susceptible to the forces of all other matter,
and will move under their influence.  The influence of  a single force is to
move it in a straight line.  But as it is always acted on  at the same time
by a number  of  forces, and  has  to  move  according to  all of them,  its
motion is  always  more or less  complex.  If at the same time matter be
influenced by different forces to move equally in opposite directions, it will
retain  the  same place or be at rest.  Though  experience teaches us that
all matter  is in constant motion, no particle retaining the same  place for
any length of time, so that there  is no  absolute rest, still a body may be
influenced so as not to alter its position in regard to surrounding objects,
and we then generally say, that it is at rest, though it is only relative or
apparent rest.
   22.  As  the amount of matter in existence always remains the same, and
matter, therefore,  cannot be destroyed any more than created, the amount
of its inherent force to produce motion, and the effects produced by it at
any  moment must also remain the same.   Applying  this  to the forces
producing motion, it follows from  this and what has been said of Inertia,
that motion can no more be destroyed or created than matter itself; and as
all matter  is now in constant motion, motion must be coeval with matter.
  23.  The forces and properties of which we have spoken so far (Cohesion,
Adhesion,  Gravity, Impenetrability,  Impact  and Inertia), are  the main
causes due to ponderable matter itself, on which  depends its position in
space.   There are yet other attractions and repulsions between the atoms
and  masses of ponderable matter, such  as the expansion  by  heat,  the
attractions and repulsions by electricity, &c.; but these seem to be con-
nected with or imparted  to  it, by certain states or motions of the ether
between its atoms, and the causes of  which  are designated as light, heat,
magnetism and electricity.  They will, therefore, be treated of separately
in connection with the ether.  We thus  obtain two  parts of  Physics
proper,  Physics of Ponderable matter, or Mechanical Physics; and Physics
of Imponderable matter, or Ethereal Physics, which treats of the ether, and
the influences it exercises  on  ponderable  matter.  Physics of Ponderable
                                  15 
16
BOYE'S  INANIMATE MATTER.
matter  we  again subdivide into  three  sections;  Physics  of Solids,  or
Steoretics;  of  Liquids,  or Hydraulics;  and  of  Gases,  or Pneumatics.
Physics of Imponderable matter is sub-divided into four sections;  Physics
of Light, or Photics, or  Optics; Physics of Heat, or Thermics;  Physics
of Magnetism,  or Magnetics;  Physics of Electricity, or  Electrics.   The
following table will exhibit the respective divisions and subdivisions of the
Natural Sciences.
Physical ob
  Natural
  Sciences.
(Physics in its
  widest sense.)
'Physics of
 Inanimate
 Matter.
 (General
 Physics.)
 Physics of
 Animate
 Matter, or
 Physiology.
 (Special
L Physics.)
r Physics
 proper, or
 Natural
 Philosophy.
 Chemistry.
 (Atomic or
 Chemical
_ Physics.)
' Physics of
 Ponderable
 Matter.
 (Mechanical
 Physics.)
Physics of
Imponderable
Matter.
(Ethereal or
Imponderable
Physics.)
[Physics of
 Solids, or
 Stereotics.

 Physics of
 Liquids, or
 Hydraulics.

 Physics of
 Gases, or
 Pneumatics.
 Physics of
 Light, or
 Photics, or
 Optics.

 Physics of
 Heat, or
 Thermics.

 Physics of
 Magnetism, or
 Magnetics.

 Physics of
 Electricity, or
L Electrics.
16 
PART I.
             PHYSICS OF PONDERABLE MATTER.

  Though in a systematic  point of view it would be better to treat first
of solids,  still as it practically is more important, first to have a know-
ledge of the physical properties of gases, we  shall begin with these.

                             SECTION I.
            PNEUMATICS,  OR PHYSICS OF GASES.
  The word Pneumatics  is derived  from a  Greek word meufia (pneuma),
signifying air.
              Properties of gases depending on Cohesion.
  24.  We have seen that whenever the repulsive force between the atoms
preponderates over  the attractive,  matter  assumes the state called  the
gaseous or aeriform.  Gases, therefore, not only possess fluidity like liquids,
that is, they offer but a slight resistance to  the moving of their particles
among themselves, but their atoms have also a constant tendency to recede
from each other, and therefore to extend themselves over space, until limited
or confined by some  outer boundary, or restrained  by some counteracting
force.  This  property is  called Expansibility, and constitutes the main
difference  between gases and other states of matter.
  25.  Nature has  placed us in an ocean of gases called the Atmosphere,
which  forms  the uppermost portion of the whole  earth.   Thus  circum-
stanced, we are apt to feel less conscious of their material existence and to
overlook the fact, that they form the medium, through which we generally
receive the impressions on our senses from  other bodies.   Thus when we
hear a  sound  caused by the vibrations of a solid, it is not these latter that
act  on  our ears, but the vibrations of the  air produced by them.   And if,
in the same manner,  on account of the  extreme fluidity and tenuity of the
        B                         17 
18
BOYE'S INANIMATE  MATTER.
 atmospheric  gases, they under ordinary circumstances are not perceived,
 we may easily render air as  tangible as a solid or liquid by allowing it to
 impinge  against  any part of our body; for instance, by blowing on  it.
 And even to our eye-sight air is as visible as any other kind of transparent
 matter; we have colored  gases,  and a bubble  of a colorless gas  is  as
 visible in water, as a drop of water is in air.   Their effect on the senses of
 smell and taste is also familiar.   It will also be  shown, hereafter,  that we
 are capable of weighing gases like any other forms of ponderable matter.
   26.  Atmospheric air is, however, not one kind of gas, but a mechanical
 mixture of four different gases. Oxygen, about \ by vol., and Nitrogen, about
 \, or more accurately, in the relative  proportion to each  other of 20.8 ox.,
 to 79.2 nitr., form the main portions of it.  Besides  these it contains small
but varying quantities of Carbonic acid (about \  per mille), and Vapor of
water (\ to 2 per cent.).
   27.  On account of its expansibility it might be  supposed, that the at-
mosphere surrounding the earth  would extend  itself infinitely  far  into
space.  This is, however, not the case.   We can prove from the property
of refraction, which the atmosphere possesses, or that of bending the light from
its  straight  path, when  penetrating  in  an oblique  direction through its
strata of different densities, that it does not extend sensibly beyond  the
height of 45 miles.   It is therefore probable, that as the rarefaction of the
atmospheric gases increases with the distance from the earth, their expan-
sibility also becomes less, and  is at last  overcome by gravity,  drawing
them  toward the earth, so that  where these two  forces are equal, they
will assume a definite limit.  This  is  confirmed by the experiments  of
Faraday, according to which  the vapors of mercury enclosed in a  tall jar,
only rise to a certain height, presenting an upper  level surface.   If this
be correct, the different gases of whicn the atmosphere is formed, ou°-ht to
assume each a separate level at different heights from  the surface of the
earth, according to their different densities.  This might, however, be pre-
vented by the commotion caused by currents.
   28.  The expansibility of gases affords us the  means of removing them
 from any containing vessel, or of rarefying  them to  any extent.  Appa-
ratus  constructed for this purpose are called exhausting air-pumps.   In
the simplest form an  exhausting air-pump consists  of a single  hollow
 cylinder, generally of brass  (see a figs. 1 and 2),  called the barrel, and
 having the inside ground perfectly true, so  that a  short solid cylinder b
 called the piston, may be moved in it perfectly air-tight by the  aid of the
piston-rod  c, furnished  for this purpose  with a  handle  d.    At  the
 bottom of the barrel is an orifice, which forms the beginning of  a passage
 (see fig. 1 and efig. 2), which  at  its other extremity is furnished with a
                                  18 
PNEUMATICS.

    Fig. 1.
19
Fig. 2.
 screw /, by which it  can be  attached  to  any vessel  or receiver h, from
 which it may be desirable  to exhaust  the air.  Across  this passage ef,
              as near as possible to the barrel, is inserted a  conical piece
              of  metal g figs. 1  and 2,  and represented  separately by
              fig.  3, called the  plug, fitting across  the  passage in  a
              corresponding conical  hollow, so as to be movable round its
              axis, which is at right angles to the passage; the  whole,
              the  passage with  its  conical hollow and the plug,  con-
              stituting a  stop-cock.   The  plug of a stop-cock has always
              one perforation  through it, which in one  position  forms  a
 continuation of  the  passage;  but when the  plug is  turned 90  degrees
 round its axis, so as to have the perforation at  right angles to the passage,
 this is interrupted.  The  stop-cock used in this case  is what is termed
 a  two-ways stop-cock,  having  two perforations, see fig. 3, the usual  one
 i to close and interrupt  the  passage e / between the  barrel  and  the
 receiver  h,  to  which  the  air-pump is attached, see fig.  1, and  a  second
 one  h fig.  3, which when the first perforation  is at  right angles  to
 the passage, see fig. 2, forms at first  a continuation of it, but then turns
 so as to  run parallel with the  axis of the  plug, and terminates  outward
 into the  atmosphere, thus establishing a  communication between the barrel
 and the outer air, when the  communication with the receiver is  shut off,
 as seen in fig. 2, which, however, is interrupted when the communication
 with the  receiver is open, as  seen in fig.  1.
   29. If now after having attached the air-pump to any vessel or receiver
h, from  which  we  intend to  exhaust the  air, and having  turned   the
                                  19 
20
BOYE'S INANIMATE  MATTER.
stop-cock g, so as to establish a communication  between it and the barrel,
see fig. 1,  we  draw out  the piston  as  represented  in  fig. 2,  the  air
in the  receiver will expand and fill both the receiver and the barrel.   The
stop-cock is then turned so as to shut off the communication between the
receiver and the barrel, and to open it  between the  barrel and  the  outer
atmosphere,  as  represented in fig. 2, and the piston  pushed in to  the
bottom of  the barrel, by which the air in the  barrel is expelled into  the
atmosphere.  If then again by turning the stop-cock, the  communication
be interrupted between the. barrel and the atmosphere, and opened between
the barrel and the receiver, and the piston drawn out, and the same process
repeated, a portion of air  will by every outward stroke of the piston enter
from the receiver into the  barrel, and  by the next inward stroke be  ex-
pelled  into the atmosphere.  This might  thus be continued as long as  the
remaining  air retains  its expansibility,  though a  last portion, however
small,  would always  remain behind.   Practically, however, it is not pos-
sible to carry the  exhaustion this far; for, however near the plug of the
stop-cock be placed to the barrel, a small space will always remain between
it and  the bottom of the latter, called the Injurious  Space, into which the
piston  cannot enter.   After the piston has  been pushed to the  bottom to
expel the air in the barrel into the atmosphere, this space will always re-
main filled with air of the same density as the atmosphere.  If this air
which  thus remains in the injurious  space, by  expanding over the barrel
when the piston is again drawn out, be yet of the  same density as  the
remaining  air in the receiver, none of the latter can  enter  into the barrel,
when the communication between them is established, and thus all further
exhaustion becomes impossible.  Besides this, such apparatus are often apt,
from imperfect make, to admit small portions of air by leakage.
   30.  Stop-cock pumps have  the  inconvenience, that the stop-cock must
be turned at every stroke.   This may be performed  by mechanical contri-
vances connecting it with the motion of the piston-rod, and they then con-
                                Fig. 4.
A
 
PNEUMATICS.
21
stitute very  superior pumps.   It  is,  however, more convenient and less
expensive to  substitute pneumatic valves, which  are  self-acting.   Such
valves are generally constructed of a strip  of oil-silk, see v and v1 fig. 4,
and v fig. 5,  fastened  by its two  extremities, so as to lay close over the
                 orifice by which the passage terminates, or when the  valve
                 is placed in the passage itself, the latter is made to ter-
                 minate by an orifice s in a projection, over which the oil-
                 silk v is tied or otherwise fastened, as shown  by fig. 5,
                 which represents  separately the valve-piece  screwed into
       '^'        the piston  of fig. 4.   Such valves will then allow the
air to pass  in the one  direction between it and the  orifice, but as soon
as the air presses in the opposite direction, the  oil-silk is forced  close
against the  orifice and prevents  the  air from passing in that direction.
Instead of the two-ways  stop-cock, two  such valves are substituted,  see v
and v1 fig. 4.  One v is placed in  the  bottom of the barrel over the orifice
of the passage leading  to it  from  the receiver, so as to  allow the  air to
pass  from the receiver into  the barrel  but  not  back again.   The  other
valve v1 is placed in a  passage  through the piston, permitting  the  air to
pass out through the piston from the barrel into the atmosphere, but not
back  again.   It will thus be evident, that every time the piston is drawn
out,  the air  in the receiver is allowed to pass through  the valve v into the
barrel, the valve  v1 in the piston remaining  closed.  When, on the con-
trary, the piston is  pushed  in, the valve v  between  the barrel and the
receiver closes, and the air  in  the barrel is expelled  through the valve
v1 in  the  piston.
   31. As it  is often desirable to  place  in the exhausted vessel different
objects or apparatus,  it becomes  necessary  to have pneumatic receivers
with  large mouths or openings.   They are then generally made bell-shaped
or cylindrical, closed at the top, see  hfig. 4, but  open at the other ex-
tremity, the edge of which is ground true, so as  to  fit  air-tight  on a brass
or glass plate p,  also ground perfectly plane, and  having an opening in
its centre  leading to a passage furnished  at its other extremity with a stop-
cock  and  a screw, to which  the air-pump may be  attached.  Any object
may then  be placed on  the plate, after which the bell jar, having had its
ed"-es greased, is inverted  over it and  pressed with  the edges against the
plate, so as to form a perfectly air-tight joint.
   32. A single barrelled  air-pump, or Syringe, as called when small and
worked by  hand,  always acts  unequally, requiring,  on account of the
atmospheric  pressure on  the piston (see  further on), much  more force to
move the  latter out than back again.  To avoid this and  also to expedite
the exhaustion, which is a tedious  process when the capacity of the barrel
                                  21 
22                  BOYE'S  INANIMATE MATTER.

is small in proportion to that of the  receiver,  double-barrelled air-pumps
are  constructed, see fig. 6.   These consist of  two complete air-pumps,
each barrel a and b having its piston and  two valves, one  in the  piston

                                Fig. 6.
and the other at the bottom of the barrel in the passage to the receiver.
But  these  two passages unite  into one leading to the  receiver h, ter-
minating at the plate.  The piston-rods  are  furnished with teeth, so as to
form  racks c c, which are moved by a small  cog-wheel or pinion d, to
the axis of which  is attached a two  armed  lever e,  with  handles  f.
By moving the  lever and consequently turning the pinion in  alternate
directions, one piston is always  moved up, while the other is moved down,
and thus, while the one barrel is exhausting the receiver, the other is dis-
charging air into the atmosphere.
  33. Instead of a double-barrelled air-pump, a single-barrelled but double-
acting may be used, as represented in fig. 7.   In this case the cover of the
barrel must be air-tight, and the piston-rod made  to slide air-tight through
it by means of a stuffing box or packing screw.   This consists  of a hollow
cylinder s fig. 7, made in the cover round  the piston-rod  c,  where it
passes through it.   Into this stuffing box, the bottom of which has a per-
foration, merely sufficient to let the piston-rod  pass through it without
friction, the stuffing or packing is introduced, consisting of oiled hemp or tow,
or circular pieces of leather (washers or collars), with perforations through their 
PNEUMATICS.
23
Fig. 7.
middle, barely sufficient to allow the piston-rod to be pushed through them.
A screw stopper I, also perforated through its middle, but so as to allow
the rod to pass  easily through it, is then  screwed down into the stuffing
box, so as to force the hemp or leather washers  against the piston-rod, so
that the latter may slide air-tight through it.  The barrel has four valves,
u and u±—v and vx  which in this case, as always when  the pumps are
large and  subject to  constant wear, are made of metal, and have  then
generally a conical  shape, fitting air-tight  in  a  corresponding conical
aperture called the valve-seat.  By any pressure from the one side, these
valves are forced from their seat, while  pressure from the  other side will
force them  back again.    To restrain their motion and secure their  easy
return into their seat, they are,  in  most  cases, furnished with a stem,
which slides in a cross-piece or guide.   As the air when rarified would soon
become incapable of opening, by its expansibility, such valves, they must,
for exhaustion, as in the present case, be moved by some mechanical con-
trivance.   Of the above four valves, two,  v and  vt open  inward to admit
the air from the receiver h into the barrel,  and are  worked by a valve-rod
o sliding air-tight through the piston b.   The two others, u and u± open
outward to let the air out from  the  barrel into the  atmosphere, and are
held in their places by spiral springs.  In  order to secure their opening to
expel the air, they have a short stem projecting into the barrel, against
which the piston strikes, when it arrives near  either end.  Leading from
the valves,  v and  i\  which  open inward, are  two  passages,  tt  and  tlt 
24
BOYE'S INANIMATE  MATTER.

            Fig. 8,
uniting into one  I  leading  into  the receiver h  through the  plate p.
Being made of  lead, and therefore flexible, the tube t may easily be con-
nected or disconnected with the plate by a knob and gallows-screw joint m.
It will easily be seen that by each stroke the piston must, on the one side,
draw air in from the receiver, while on its other side it  expels the air from
the barrel into  the atmosphere.  Fig. 8  gives a full  view of a pump of
this kind,  constructed by Dr. Hare, and  used  by him for many years in
his  Laboratory.  It has two additional passages leading from the valves u
and ut uniting  also into one, open to the atmosphere  at n.  These, how-
ever, are not necessary when used only for exhaustion.
  34.  Fig. 9 exhibits another efficient  single-barrelled but single-acting
exhausting air-pump,  of Boston  manufacture, often met with, and known
as an  Improved 'Leslie' Air-pump.   The piston-rod c  passes  air-tight
through a stuffing-box s, in the  top of the barrel a, its end sliding  in a
cross-piece or guide d, to keep it perpendicular during its motion; t—t is
                                  24 
PNEUMATICS.                          25
the tube  forming the passage from the barrel  to  the plate p, into the
receiver h.  The pump has two  valves, one in the piston, opening  from
the receiver towards the top of the barrel, the other in the top of the
barrel at v, opening from this into the atmosphere.   These valves are made
of circular pieces of thin calf-skin seaked in oil and lard, laying close over
the orifices,  that at v being fastened on one side  by the cap-piece screwed
down over it.   From  this latter valve the  tube u, which is removable,
leads  into a cistern f, open to  the  atmosphere and  intended as a recep-
tacle for the oil, as  also for ether, or other volatile liquids, which often
have to be removed  as vapors  from  the receiver by exhaustion and  may
condense  in  the barrel or the tube, and  thus be forced  out through it. 
26
BOYE'S INANIMATE MATTER.
 When the piston is pushed in, the valve at v prevents the air from entering
 into the barrel, and a vacuum is formed in the barrel  above  the  piston,
 into which the air enters, by its expansibility, from the receiver and barrel
 below the piston through the valve in the latter;  when the piston is raised,
 the  air  above  the piston cannot return through the valve in it, and is
 forced out through the valve at v in the top of the barrel, while the barrel
 below the  piston is again filled with air from the receiver, following, by
 its expansibility, the  piston as it moves out.   This portion of air in the
 barrel below the piston, will  then again pass through  the valve in the
 piston to above it, when this is again pushed in, and by the next outward
 stroke will be forced out as before.  This  pump has the advantage over
 other single-barrelled, single-acting air-pumps, that after the first outward
 stroke has been performed, all the subsequent ones are, as in the double-acting,
 performed  through the greater part of their motion, not  against the atmo-
 sphere, but against a partial vacuum, until the piston arrives near the top,
 when the air becomes condensed  to the same  density as the  outer atmo-
 sphere, and of course the last effort to expel it through the valve at v, must
 be against the whole atmospheric  pressure.  To carry the exhaustion to
 the furthest  possible limit,  the tube u, may be removed and a small ex-
 hausting syringe  screwed  on, by which  a  vacuum may be produced
 above the valve at v, by which the  injurious space  below the same valve
 will  remain filled with air  of much less density  than  the atmosphere,
 and thus have less effect  when  expanding  in  the  barrel by  the  inward
 stroke of the piston, by which  the  exhaustion may  be  carried  much
 further.
   35.  The amount of air remaining in the receiver at any moment during
 the process of exhaustion, or the  degree of rarefaction, may be calculated,
 assuming that no leakage  takes place, by knowing the relative capacities
 of**he receiver and the barrel.  For calling the former R and the latter B,
 and the ordinary density of  the air D, we have  after the first stroke, that
 the air in the receiver fills both the receiver and the barrel, and its density
 after the first stroke Dj must therefore be to its former density D, inversely

 as the spaces occupied, or that ~D1  : D : : ^5-7-^ : ^; hence Dj = — —— D.
                                      -tv-(-x>  R              R-{-B
 After the  second  stroke  we get in  the same  manner the  density
 D3 = _5_ D, =  R   X —— D = (—-Y D,  and at the nth
  2     R+B   1   K+B    R+B      VR+B/   '
              /  R  \n
stroke D„ —  ( _----) D.  Thus if the barrel have I the capacity of the

receiver, we have R = 9, B  = 1, and-----=  J_, and the density  or
                                    R+B     10                J
                                  26 
PNEUMATICS.
27
quantity remaining in the receiver at the 3d stroke, = (~\ D = T7D2090

of the original density or quantity.
   36. The  rarefaction at any time  is, however, generally estimated by a
barometer guage connected with the  receiver, see mfig. 6 and g, fig. 9,
the principle of which will be  explained hereafter under pressure-guages.
   37. Suction by the mouth depends on the same principle as exhaustion
by an air-pump.   The vessel is first connected by the lips with  the mouth,
and the air then expelled from the mouth by pressing  its walls close to-
gether.  A vacuum is then produced in the mouth by withdrawing  the
tongue from the roof of the mouth without admitting any air, which  con-
stitutes the effort of sucking.  The air then passes, by its expansibility,
from the vessel into the  mouth, as  in the barrel of the air-pump.  The
communication between the vessel and the mouth is then  closed by using
the tongue as a valve, and the same again repeated.
   38. Besides the above means  of exhaustion by air-pumps, a  partial
vacuum may be produced by the increased expansibility of gases by heat.
Thus, the suction of an  ordinary plain cupping-glass is produced by ex-
pelling a portion  of the air by heat, by holding it with the mouth down-
ward over a spirit lamp  or a piece of burning paper, and then  quickly
placing it on the  skin.  Another means of removing atmospheric air from
a vessel and thus producing a vacuum, is, by the introduction of a volatile
liquid and  the application of heat  to it, by which it is  converted  into
vapor, which will  expel  the  atmospheric air.   By then closing the vessel
and  allowing  the vapors  to  condense, a vacuum is produced, which is
entirely free from atmospheric air, but always contains more or less vapor.
Thus, thermometer bulbs, and other vessels with very narrow mouths,  are
filled with mercury or any other liquid, by first  expelling a portion of the
atmospheric air by heating  them  over a  spirit-lamp, and then inverting
them with  the mouth into the liquid.   When the air then contracts, a
partial vacuum is produced,  by which a  portion of the liquid is forced
up into  it  by the  atmospheric pressure  (59).   They are then again
heated till the liquid  inside boils, and its vapour has expelled  all the re-
maining atmospheric air, when  they are  again  inverted with  the mouth
into the liquid, by which they  become entirely  filled with the liquid as
soon  as the vapors condense.   The vacuum in the cylinder  below  the
piston of the  early or 'atmospheric' steam-engine of Newcomen, wTas pro-
duced by the  expulsion of the air by steam from a boiler, and its subse-
quent condensation.
   39. Compressibility and Elasticity ofigases.   From the nature  of gases
it mi°-ht be  inferred, that  the atoms are not so close together as in liquids
                                  27 
28                    BOYE'S  INANIMATE  MATTER.
and  solids.   Indeed, we  find  that  the spaces  between  their  atoms are
capable of being considerably  reduced by mechanical pressure  and their
volume in consequence diminished.   This  property is called Compressi-
bility.   The property of offering to the compression  a constantly increasing
resistance, and when the pressure ceases, of again  resuming their former
volume, is called Elasticity.  Gases  thus possess the properties of Com-
pressibility and Elasticity to a  much  greater extent than either solids or
liquids.
  40.  This  is also the reason why we are capable of forcing a considerable
quantity of  gas into a comparatively small  space.   Contrivances for  this
purpose are called  Forcing or  Condensing  Air-pumps.   In its simplest
form the Condensing air-pump  is identical with the Exhausting Syringe,
see figs. 1 and 2,  consisting of  a barrel with  a solid  piston, and furnished
with a two-ways  stop-cock,  by which  it is attached to the  receiver,  into
which  the air is to be condensed, only that in using it, the order of turn-
ing the stop-cock is reversed.   For if the piston be pushed in, while the
barrel communicates with the  receiver, it is easily  seen  that the air  con-
tained  in the  barrel  must be forced into the receiver.   If, now,  the
stop-cock be turned so as to shut off  communication  with  the receiver, but
to establish it between  the barrel  and  the outer atmospheric air, the
latter will enter and fill the barrel when the piston is again drawn  out.
By repeating the  same process, a fresh portion of air  is  by every  inward
stroke  introduced into  the  receiver,  the limit being  dependent  on the
strength of the  apparatus and  the  size of the  injurious  space (29).
For it  will easily be seen, that  as  soon as the air admitted into the barrel
may be condensed into the injurious space, without  acquiring greater  den-
sity than the air in the receiver, no more can be forced into it.
  41.  Instead of the two-ways stop-cock we may,  as in the exhausting
air-pump, substitute two self-acting valves  of oil-silk, see fig. 10, one v
                                Fig. 10.
at the bottom  of the barrel  in the passage leading to the receiver, and
another vx  in a  passage  through the  piston, both, however,  opening
inward as represented in fig. 10.  The valve  in  the piston may  be dis-
pensed with, and the latter remain  solid, if the barrel be furnished with a
                                  28 
PNEUMATICS.
29
small perforation on its side, at a distance from the cover just sufficient to
be cleared by the piston when drawn out, for the admission of atmospheric
air.   On pushing the solid piston in, the air thus admitted into the barrel
is confined as soon as the piston has passed the orifice, and forced into the
receiver, and so on.
  42.  Where larger objects are to be  placed in the  receiver, the latter
must be furnished  with a wide mouth,  see fig. 12, the edge of which is
                 Fig. n.
ground true and fitted on a plate as for exhaustion, but generally with the
interposition of a ring or washer of oiled leather.  An additional contri-
vance also becomes  necessary, to keep the receiver  against the plate, con-
sisting of two uprights, I and I, and a cross-piece m, which can be screwed
down on it, as otherwise the  inner pressure of the air would force  them
apart.   Such  receivers  should also be  made as much  as  possible  of a
spherical  form,  and, if of glass, very thick, as much greater strength is
                                   29 
30                 BOYE'S INANIMATE  MATTER.

required to withstand a pressure from the inside than from  the  outside,
and by bursting accidents are likely to occur.
   43. Where considerable  quantities  of air are to be condensed, the pump
may be made double-acting and its size increased; in which case it be-
comes necessary to work it by machinery.   When high degrees  of con-
densation  are required, it  also becomes necessary to substitute metallic
valves  instead of those of oil-silk.   The pump  fig. 7, described  in 33,
answers admirably for condensing, if furnished with two additional passages
leading  from the valves u and ulf as represented by fig. 11, which  two
passages unite  into one, terminating  in  a  knob n, so that, being  of lead,
and  therefore flexible, it may be connected by a gallows-screw joint m
with the receiver^. 12, into which  the air is to be condensed.  In  this
use of the pump the other  forked tube t, fixed over the valves, opening in-
ward, must of course be left open, so as to  allow the atmospheric  air free
access through  these valves into the barrel.  When the piston is moved,
atmospheric air is drawn in through the tube t on the one side of the piston,
while the air on the other side of it is forced into the receiver through the
tube n.   Such pump will also answer for transferring  and condensing any
gas different from atmospheric air.  For this purpose the receiver  fig. 12,
into which the gas is to be transferred or condensed, is first exhausted by
being connected with the pump by the tube t.  It is then to be connected
with the pump by the tube n, after the tube t has been connected with the
receiver containing the gas to be transferred, and one stroke been performed
to expel the atmospheric air from the barrel.
   44.  It has been ascertained by accurate experiments, which will after-
wards be detailed, that the volumes which a  gas  occupies under different
pressures, but otherwise similar circumstances, are inversely  proportional
to the  pressures, and  the  densities of the  gas, therefore, directly propor-
tional to them.  This law is called, from its discoverer, Mariotte's  law.
   45.  Liquefaction of gases.   In regard to their conduct under increased
pressures, gases differ  materially.   Some  of  them  obey Mariotte's law
under any pressure which  has yet been applied  to them, and  are  there-
fore called permanent gases.   Of these we have six;  Oxygen,  Hydrogen,
Nitrogen, Bin-oxide of Nitrogen, Carbonic Oxide and Light Carburetted
Hydrogen.   Others conduct themselves  in  a similar manner,  obeying
Mariotte's law, only until the pressure has been increased to a certain point,
when  they suddenly  yield and are  converted into  liquids.   These are
called liquefiable, sometimes compressible, or condensable gases, the latter,
referring mainly to the fact, that this same effect  is assisted by the simul-
taneous exposure to  cold, or may even in some  cases be  produced by it
alone.   Of the liquefiable gases a certain number are formed from  sub-
                                  30 
PNEUMATICS.
31
stances existing,  under ordinary circumstances,  as liquids or solids, and
when  filling the  space to their fullest extent, will stand no increase what-
ever  in pressure or cold,  without becoming wholly  or in part liquid.
Such gases are called Vapors.  As instances of liquefiable  gases  may be
mentioned Sulphurous acid,  liquefiable at a pressure of about 5 atmo-
spheres (1 atm. = 151bs. to sq. in.),  and by strong cold alone, and Car-
bonic  acid, requiring 38 atmospheres at 32°.   Of vapors may be  men-
tioned vapor of water or Steam.
   46.  It is  supposed that  all gases by sufficient  pressure would  become
liquid, but even should this not be the case, it is evident that no pressure,
however great, could reduce their volume to nothing, which constitutes
their property  of Impenetrability.
   47.  To  illustrate the  compressibility and elasticity of the atmospheric
air,  fix  a  burning taper on a cork  floating  on  water.   Invert  a  large
tumbler  or jar  over it, and depress  this below the surface of the water.  As
the depth to which it is immersed increases, the  compressibility of the air
will allow the water to  ascend to a greater height  into  the jar, but its
elasticity will offer a constantly increasing resistance, so that much the
greater portion of the jar will still  remain  filled with the  air and allow the
candle to continue to burn.
   48.  On  this depends the  action of the diving-bell, which consists of an
open inverted box filled with air, generally made of cast-iron, and heavily
loaded, so  as to  sink when  let  down into the  water by a rope, and fur-
nished with thick glass to admit light.   The operator is supported on cross
benches  near the bottom.   As the bell is  lowered to a greater depth, the
pressure of the water becomes greater, and the air in consequence  more and
more compressed, so  that the water ascends higher into it. To prevent the
diver becoming thereby partly immersed in water, and  to replace the air,
which  becomes vitiated by the  respiration and the burning of  the  light
sometimes  employed, it is furnished with a valve and hose, through which
fresh air is  forced in, from a boat  above, by a forcing  pump.  By  this
means  it soon becomes again entirely filled with air, while the vitiated air
is allowed to escape.
  49.  As an application of  the condensation of air by the condensing air-
pump,  may  be mentioned  the air-gun, of which  the  essential  part is a
strong  metallic receiver,  into which atmospheric  air is  compressed to a
considerable degree by a condensing syringe, which may be attached to it.
Between this receiver and the barrel containing the ball, is a  valve, which
by pulling the  trigger is struck  open, thereby letting out a portion of the
confined  air, which propels the ball.   In the ordinary air-gun the stock
forms  the  receiver,  and in  the cane air-gun the  receiver is formed out 
32
BOYE'S INANIMATE  MATTER.
of the  hollow space  between  the  barrel and the outer  tube  forming
the cane.
   50. Besides the  condensing air-pump, other means are  sometimes re-
sorted to for the  compression of gases.   Thus, vapors are often  obtained
in a compressed state by the introduction  of a volatile liquid  into a con-
fined space, and its conversion  into  vapors by heat.  The  steam-boiler is
an illustration of this.  The high-pressure steam-engine may be considered
as a single-barrelled, double-acting air pump attached to it, the barrel being
called the cylinder,  but the piston of which, instead of condensing the gas
by its motion, is itself moved by the elasticity of the gas, the  vapor of
water, already in the compressed state and let in alternately above and be-
low the  piston.
   51. Another way of obtaining gases in a highly compressed state, is by
generating them by chemical action in large  quantities in  a small  space.
Fire-arms may be considered as an  application of  this, the mixture em-
ployed in them for this purpose being the gunpowder.   Many gases, such
as carbonic acid, are most conveniently liquefied by the pressure produced
by their own generation in an appropriate apparatus  (see Chemistry under
Carbonic acid).
                   Properties  depending on Adhesion.
   52. The repulsive action  between the  atoms of  the same gas,  which
causes the property of Expansibility, we do not find to exist between the
atoms of different gases.  On  the  contrary, the atoms of one  gas  will
allow the atoms of other gases to push themselves between  them, and seem
even to  assist this action  by an attractive force toward them (Adhesion).
Thus, if two  vessels, h and c fig. 13, separated  by a partition p, be filled,
the upper h with  a  light  gas as hydrogen, and  the lower c by a heavy gas
as carbonic acid, and the partition between them be withdrawn, the hydro-
               gen will not  remain on  top, but expand and spread  down-
               ward through the carbonic acid; and in the same manner
               will  the carbonic acid rise up, spreading through the hydro-
               gen, till they both are evenly diffused through the whole
             ^ space.   This property is called Biffusibility.   In virtue of
               this  property one gas seems hardly to offer  any resistance
               to  the expansibility of  another, and gases  are therefore
               not  capable of  limiting  each other, or of maintaining a
               distinct boundary between  themselves (like  oil and water
               among liquids).
    Fig. 13.       53.  Diffusibility of gases suffers a peculiar modification,
when they communicate with each other through extremely small openings,
 
PNEUMATICS.
33
as through a  crack  in a glass, or through a porous partition, as when
formed of  plaster of Paris, unglazed earthenware, common wood, particu-
larly  when cut across  the grain, and animal membrane, as bladder, skin,
&c.   In all such  cases the lighter gas will be found to pass through such
into the heavier, faster than the heavier passes in the opposite direction into
the lighter.  Thus, if in fig. 13, the  upper vessel h be filled with  hydro-
gen, and the lower c with carbonic acid, and the partition p be a plate of
plaster of  Paris, it will be found that  the  hydrogen will pass  faster into
c, than the carbonic acid into h,  and thus a partial vacuum is produced in
the vessel  h, occupied by the hydrogen, and a condensation in c.  But after
some time, when the gases become thoroughly diffused through each other,
equilibrium is  again restored on  both sides  of the partition.  This may be
                                      illustrated by the diffusion tube b
                                      fig. 14, which is a glass tube open
                                      at the lower end and closed at the
                                      upper by a plug a, of perfectly  dry
                                      plaster of Paris.  If this be filled
                                      with hydrogen  by displacement of
                                      the  atmospheric air (see    ), so
                                      as to avoid  wetting the plaster of
                                      Paris,   and  then  quickly  placed
                                      with its open  end  in  a shallow
                                      vessel  d, containing water,  diffu-
                                      sion will take  place  through  the
                                      Paris plaster, between the  hydro-
                                      gen  in the tube  and  the  atmo-
                                      spheric air outside, and  the  hydro-
gen  passing out quicker  than the  atmospheric air  passes in,  a partial
vacuum will be formed, by which the water will be forced up  in the tube
to c by the atmospheric pressure (see  59),  several  inches above the level
outside.   But  after some time  it again falls  to its former level.   This
kind of diffusion, particularly when taking  place through animal or vege-
table  membranes, is often called by the  name of Endosmosis  and  Exos-
mosis.   The velocities with which different gases diffuse  themselves, have
been  found to be, under otherwise similar  circumstances, inversely pro-
portional to the square roots of their densities or specific gravities.
   54.  The adhesion of gases  toward Solids is  quite considerable, so that
in many cases it causes them  to be condensed in greater or less quantities
on their surface.   Thus, it is  found that ordinary  glass, even when per-
fectly dry to the touch, always contains a thin  film of vapor of  water con-
densed  on  its surface.  This  becomes  more perceptible when its surface
      C                           33
Fig. 14. 
34
BOYE'S INANIMATE  MATTER.
is increased by pulverizing it, when the quantity of vapor condensed by it
may be so great as to amount to more  than $ per cent, of its weight.
The same is the case with most  other pulverulent or porous bodies, such
as clay,  and  particularly  animal  and  vegetable substances,  as paper,
wood, hair, membranes.  Such water is called hygroscopic  moisture and
is  found to vary in quantity according to the state of humidity of the
atmosphere ( 177 ), and  interferes materially  in  many  experiments with
the accurate  determination  of their weight.  Recently  ignited  charcoal
will absorb many times its own volume of  different gases, such as oxygen,
and  particularly sulphuretted hydrogen and other similar gases or vapors,
which are the cause of offensive odors.   On this depends its preserving and
deodorizing properties.   The most extraordinary instance of such condensa-
tion of gases is presented by platinum towards hydrogen and oxygen, when in
porous and finely divided states, in which it is  called  platinum sponge and
platinum black, the latter of which has been  found  to absorb more than
250 times its own volume of oxygen.   By this condensation a subsequent
chemical action is often induced.   Thus, oxygen when absorbed by ehar-
coal combines after some  time with it, forming carbonic  acid in its pores;
and  hydrogen and  oxygen when absorbed together  by  platinum sponge
unite to form vapor of  water, so that platinum sponge when held before a
jet of hydrogen, where it mixes with the  oxygen of the atmosphere, will
become heated by the union of the two gases, and ignite the, jet of hydro-
                        gen.   On this depends the Platinum Igniter,
                        (fig. 15),  which  is an  apparatus  for obtaining
                        fire, consisting of a self-regulating generator of
                        hydrogen  (see     ), which by turning up the
                        box h, opens  a stop-cock and  causes the hydro-
                        gen to issue from the  jet  e, on  the  platinum
                        sponge h± and thereby to become ignited.
                           55. Towards Liquids also,  a positive attrac-
                        tion or adhesion  is very manifest,  by which the
                        atoms of gases are drawn in between the atoms
                        of liquids, which constitutes what is  called ab-
                        sorption or solution of gases in liquids.   Thus
         Fiv-15>         all the atmospheric  gases  dissolve in  water in
small quantities, and on the oxygen  thus dissolved  (about T J^ vol. in 1
vol.  of the water), depend all  gill-breathing animals  for their respiration.
Some gases dissolve in considerable quantities in water, as carbonic acid
(1 vol;, and sulphurous acid (50 vols).  It is. however,  often difficult to
draw the line between mere solution or  absorption and chemical  combina-
tion.  Thus, chlorohydric acid dissolves in water to the amount of 418 vols.
 
PNEUMATICS.
35
and  ammoniacal gas to  the  amount of 500 vols.; but  in  these cases a
chemical combination with the water takes place at the same time.
   5(5.  When a gas is dissolved in a liquid, and the free surface of this solu-
tion be exposed to, or brought in contact with another gas, or be separated by
a porous partition from it or from a solution of it in a liquid, diffusion will,
in all such  cases, take place between them.   It js by such diffusion that
by respiration  an exchange takes place, through the membrane of  the
lung, between  the oxygen of the air and the  carbonic  acid dissolved in
the blood :  and that  in  gill-breathing  animals an exchange  is effected,
through the membrane of the gill,  between  the oxygen  dissolved  in  the
water and the carbonic acid dissolved in  the blood.  This is also  the cause
why, when  gases are separated by liquids in  which they are more or  less
soluble, an  exchange of them always takes place by diffusion through the
liquid.   This i;3  not only the case when a gas is confined by a  very thin
film  of liquid, for instance, when  enclosed  in a  soap-bubble;  but even
when gases  are kept in jars, placed with their mouth in water, it is found,
that in  the course  of time  more  or less of  an  exchange takes place
through the water with the atmospheric air outside.  Thus, if the gas be
hydrogen,   in the course of some weeks, some of it will  have escaped
through the water, while a perceptible  quantity of atmospheric air  will
have found its way through the water  into the hydrogen.  As gases are
utterly insoluble in mercury, this liquid is often employed  for  confining
them more  perfectly, and answers well when the surfaces of the  glass  and
the mercury are perfectly clean.   But if a film of dust cover the glass or
be on  top  of the mercury, when immersing the mouth of the vessel  into
it, so as to  prevent perfect contact  between  the  glass and  the  mercury,
diffusion will take place through  this film.
                    Properties depending on Gravity.

   57.  Gases are subject  to the action of gravity, and they are,  therefore,
like  all other ponderable  matter,  attracted by the  earth towards its centre,
which constitutes their loeight.   To prove this,  attach a spherical receiver
furnished with a stop-cock, to an  exhausting air-pump, and having removed
the air, counterpoise it on a balance, see fig. 16,  so as to produce equili-
brium.   Allow then the atmospheiic air to fill the receiver by opening the
stop-cock.   It  will  be found that the receiver now weighs more.   This
gain is due to the weight  of the gases which  now fill the  receiver.   By
forcing more air into  the receiver by the condensing  air-pump, we shall
find  that its weight is still  further increased.   By accurate experiments it
has  been found, that  100 cubic inches of atmospheric air, freed from its 
36
BOYE'S INANIMATE  MATTER.
  carbonic acid and vapor of water, at 30 inches barometric pressure and 60°
  Fahrenheit, weigh exactly 30.82926 grains, (or at 32° Fah. 32.58685 grs).
ji,;  58. Different gases have different weights for the same volume.   Thus,
  100 cubic inches of oxygen weigh 34.19 grains, of hydrogen 2.14 grains, of
  carbonic acid 47.14 grains.   By the density ox specific gravity of a gas we
  understand the number which expresses, how many times a gas is heavier
                                 Fig. 16.
 than the same volume of atmospheric air, which is, therefore, the standard
 of comparison and its specific gravity = 1.   To obtain the specific gravity
 of a gas, we first fill a suitable spherical glass receiver, as above, with atmo-
 spheric  air, freed from its carbonic acid and vapor of  water by passing it
 through  a  tube filled with unslacked lime, and  ascertain accurately the
 weight  of  the atmospheric air in it.  We  then  again  exhaust the atmo-
 spheric air  and fill it with the gas (see    ), at the same temperature and
 at the same pressure, and ascertain  its weight.   The  weight of the gas
 divided by  the weight of the atmospheric air will then give us its specific
 gravity.  The following are the specific gravities of some of the different
 gases;
    Atmospheric air.......   1.0000     Nitrogen...............  0.97137
    Oxygen.................   1.1056     Carbonic  acid.........  1.529
    Hydrogen...............   0.06926    Vapor of Water......  0.622
 To avoid fractions the specific gravity of atmospheric air is often called
 1000  instead of 1, that of oxygen then becomes 1105, hydrogen 69, &c.
/ 59. As gases  possess weight, it follows that  the surface of the  earth 
PNEUMATICS.
37
must sustain a considerable  pressure from  the weight  of the surrounding
atmosphere  resting on  it.   To prove  this, place an open  glass tube with
one of its extremities in water, see fig.  17,  and remove  the air which it con-
tains by suction with the mouth, or by an air-pump attached to the other
end a.  We shall find that as the air is removed, the pressure of the atmo-
sphere  on  the water outside the  tube will  force it up into it.  On re-
admitting the air into the tube the water will again fall to its former level.
For the same purpose expel the air from a tube closed at one end, by filling it
             with water; invert it, keeping  the finger on the open end to
       F     prevent the water from escaping,  and introduce this end into
       |      a vessel with water.   On removing the finger the  water does
        |     not run down, but the tube remains filled with the water  to
             the top,  caused  by the  pressure  of the atmosphere on the
        |     water  outside of it.   As soon as the air  be again in any way
        if     admitted into  the tube, the  water will fall as before.   If we
     ^# 4n^  perform the same experiments  with mercury instead of water,
  =^mHiS   an<* use a tu^e l°nger tnan  30 inches, we shall find, that on
  fjjjlilll   removing the air from the inside,  the  pressure of the  atmo-
              sphere on the outside is not capable of forcing the mercury
     Fig'17'    up to the top of the tube; or of retaining it there, if closed at one
  end and filled and inverted as before, but only at the  perpendicular height
                       Fig. 18.                         Fig. 19.
37
4 
38
BOYE'S INANIMATE MATTER.
of about 30 inches above the level of the mercury outside, see a fig. 18, and
at which level, therefore, the  mercury will  remain, whatever inclination
we  give the tube, as represented at a± a1± aiU fig. 18.  That it still is the
pressure of the atmospheric air outside, which sustains the mercury in the
tube, may be further proved by placing the whole under  an appropriate
pneumatic receiver, see fig. 19, and  exhausting the air, when the mercury
in the tube will be found to fall as the air is withdrawn from outside  of it;
and if it were possible to remove the air perfectly, the level inside and out-
side would be  the same in this case, as when the atmosphere is both in-
side and outside.   As water is 13.6 times lighter than mercury,  the atmo-
spheric pressure is capable of  forcing it up to a height 13.6 times greater
than that of the mercury, or to about 34 feet.
   60. The pressure of the atmosphere was discovered by the circumstance,
that some Italian pump-makers  had in vain endeavored  to raise water by
a suction-pump to a greater height than 34 feet, and applied to Galileo for
the reason.   Previously, the cause  of water rising  in a tube under  such
circumstances  had been  ascribed to what was called the abhorrence of
nature  to a vacuum, by which nature always  endeavored to  fill it up.
Galileo referred the subject to his  pupil Torricelli, who at once suspected
the real cause to be the pressure of the atmosphere consequent to its weight,
and to convince himself of the correctness of the above facts in regard  to
water, performed (about  1643 A. D.), the experiment of filling a tube longer
than 30 inches with mercury  and inverting it in a cup of mercury.   Such
apparatus is yet called  after  him  a   Torricellian tube.   The real  proof,
however, of the mercury in the tube being supported by pressure from the
atmosphere, was  obtained by Pascal having it carried up a high mountain,
by which the air underneath became incapable of pressing on the mercury,
and this therefore gradually fell as the height became greater.
    61. The  Torricellian tube furnishes us with the means of estimating the
pressure of the  atmosphere on  the  surface of the  earth,  which  for the
greater part, though not entirely,  depends on  the weight  of  the  atmo-
 sphere.  For this  purpose" it' is only necessary to measure accurately the
 perpendicular height of the mercurial column,—this being the only part
 of it which is  sustained  by the atmosphere, the rest, when inclined  being
 supported by the sides of the tube.   This height will be found, as  before
 stated,  to be about 30  inches.  The pressure of the atmosphere on the
 surface of the  earth is therefore equal to a layer of mercury all over it 30
 inches  in  height.   We  therefore  only need  calculate  the weight of a
 column of this height and of a certain base, in order to obtain the  pressure
 of the  atmosphere  on an area equal to this base.  We thus find,  that a
 column of mercury, which has the  height of 30 inches and rests on  a base
                                   38 
PNEUMATICS.
39
of one square  inch, contains 30 cubic inches of mercury and will weigh
14| pounds, which is therefore the amount of the pressure of the atmo-
sphere on  every square inch  of surface.   The  mercurial column in  the
Torricellian tube does not, however, always remain the same, but is found
to vary in the same place at different times about 3 inches.  The pressure
of the atmosphere is, consequently, not uniform, but varies to the  amount
of ly pound on the square inch.  In  most calculations it is considered as
being equal to  15 pounds to  the  square inch, and in the estimation of
pressures this is considered  as a unit under the name  of one Atmosphere,
so that for instance  a pressure of 3 atmospheres  means a pressure of 45
pounds to the square inch.
   62. If the Torricellian tube be prepared with care so as to expel all  the
atmospheric air and moisture, which adhere to the tube, and which is done
by boiling the mercury in  it  before  inversion, it will easily be seen that
the vacuum  produced above  the mercury by the subsequent inversion,
must be entirely free from any of the gases of the atmosphere.  Hence,
this space is called  the Torricellian vacuum, in  contradistinction to  the
vacuum  which may be produced  by an  air-pump.   At  the  temperature
between  60° and 80° Fah., it begins, however, to  contain a  perceptible
trace of vapor  of mercury.
                        THE  BAROMETER.

  63.  As the pressure of the atmosphere varies, it becomes important to
estimate at any time its  amount with accuracy.   Instruments constructed
for this purpose are called  Barometers, from fiapos (baros) a Greek word
signifying  weight, and  /xerpov  (metron) measure, meaning literally mea-
surer of the weight of the air (see 90 ).  In the ordinary form it consists
of a carefully prepared Torricellian tube (60), inverted in a very small cup
or cistern containing mercury,  and furnished with an accurate scale, by
which  we  are able to read off at any time  the height of the mercurial
column above the level of  the mercury in the cup.*   This  is called  the
cup or cistern barometer, see figs.  20 and 32.  In order to fix  the tube to

  * In the making of accurate barometers certain Vecautions are necessary in the filling
of the tube.  By keeping, more or less  dust always  finds its way into open tubes.  Ba-
rometer tubes should therefore, if practicable, be sealed at both extremities immediately
after they have been drawn at the glasshouse, and be kept in this state till  ready for use,
when one end is cut off.  Where this cannot be done, it may become necessary to wipe
them clean inside by a thin copper wire, wrapped over with dry thread.  Should it be
found indispensable to clean them with water, this is best removed by rinsing with strong
                                  39 
40                   BOYE'S  INANIMATE MATTER.

the cup, the latter  may be furnished with a cover of  wood, cut across the
grain, by which it is sufficiently porous  to  let  the atmospheric  pressure
through it, without allowing the mercury to be spilled out of the cup, and
through which  cover the tube may then be fixed (fig. 32), or the whole
cistern may be  made of wood, as in fig. 20,  the top being  in one piece
with  it, and the  bottom  screwed on  before inverting  it.   Instead  of
                                             having  a cup  attached to the
                                             tube, the tube may be bent at
                                             the lower extremity so  as to
                                             have the  open end turned up-
                                             ward, see fig. 21, in which case
                                             this open end acts as the eup,
                                             and  it  is then called  a plain
                                             syphon  barometer,  or if  the
                                             open end  be blown into a bulb
                                             or cup, as in fig. 22, it is called
                                             a syphon cup-barometer.   The
                                             whole apparatus is then  fast-
                                             ened to a board, figs.  21 and
                                             22,  or enclosed in a case  of
                                             wood or brass, figs. 28 and 32,
                                             on which the  scale is fixed.
                                             The  whole  scale  is, however.,
                                             rarely affixed to the barometer,
                                            but only so much of its upper
                                            portion, as is necessary for the
                                            intended use; on ordinary baro-
alcohol, after which the tube is dried by heating it on the outside at a short distance from
one of the open ends, and drawing dry air through it by suction from the other end.  As
it is almost impossible to remove any moisture in the tube after it has been sealed at one
end, the greatest care should be taken to avoid  introducing any by the breath, or by the
flame of the blowpipe lamp.  The sealing should therefore he done by drawing the tube
out at such a distance from  the end as to prevent this.  Before filling,  both the tube and
the mercury should be strongly heated, and in  some cases it may even be necessary to
heat the mercury to boiling  after its introduction into the tube.  The mercury employed
should be purified.  This is generally done by forcing it through skin and by digesting it in
a lukewarm place with muriatic or diluted sulphuric acid.  For  standard barometers it
should be distilled.  Distilled mercury is apt to become covered with a black film at the
open end, but  this is prevented by subsequent  digestion with strong muriatic acid and
thorough washing with water to  remove the acid.  Syphon barometers are filled with the
mercury as high up  as practicable before bending  them, after which  the filling is com-
pleted through the open end by suitable manipulations. Barometer tubes contracted at any
point to capillary dimensions must be filled in the same manner, as thermometer bulbs (38)
                                    40
Fig. 20.         Fig. 21.         Fig. 22. 
PNEUMATICS.
41
meters  seldom more than 4  or 5  inches.  In  all cases, whether  cup or
syphon barometer, the height of the mercurial column is measured by theper-
pendicular distance from the level of the mercury in the open part to the top
of the mercury in the closed end of the tube.
   04. All barometers have the inconvenience, that when  the  mercury in
the upper closed end of the tube  rises or falls by a variation in the pres-
sure of the atmosphere, a portion of the mercury is either abstracted from,
or added to the mercury in the open part, by which the level of this latter,
which forms the beginning of the scale, is altered.  In  the cup-barometer
this error may be diminished  sufficiently for ordinary purposes, by making
the upper part of the cup, where the mercury rises and falls, see g fig. 20,
of a considerably larger diameter  than that of the tube at the upper level
of the mercury.   Thus, if the diameter of the cup be  10 times greater than
that  of the  tube, their relative contents,  which are proportional to the
squares of their diameters, will be  as 100  is to 1, and therefore a fall of
one inch in  the tube will only raise the level in  the cup T -J-^ of an inch.
Where, however, the utmost  accuracy is required, it  becomes necessary to
avoid  this error altogether, which is done, either by  making the scale
movable and adjusting its lower  end to  the  level of the  mercury in  the
cup, or by furnishing the  cup with a movable bottom of skin, which may be
raised by a screw, see h fig. 32, by which the mercury may always be ad-
justed  to the same level.   This level is sometimes indicated by a float in
the mercury, the  stem  of which passes through the  cover, but more  fre-
quently, and with greater reliance,  by a  point  of ivory projecting down
from the cover of the cup,  see fig. 32 at p, the cover being made of wood cut
across the grain, so as to allow the air free  ingress through its pores, and
the sides of the cistern  of glass, so that the point is visible  through it.  To
adjust the level of the mercury in such cistern  before making an observa-
tion,  the mercury in  it  is raised by the screw at the  bottom, till the ivory
point, by dipping into the mercury, forms a small cavity in its surface; it
is then lowered  till this cavity just disappears.
   65.  In the plain  syphon  barometer, fig. 23, the  above inconvenience
may be avoided by having the bore of the two limbs of the tube of exactly
the same diameter or calibre.   It will then be seen, that when the mercury
in the closed end rises,  for instance, $ inch, it must fall exactly the same
amount, or \ inch, in the open end; and thus the difference between  the
two levels will be one inch.   In the same manner  all changes of the  ba-
rometer will always be double that indicated in  the closed  end, so that if
the barometer be correct at 30 inches, it is only necessary to double  the
value of the other divisions  of the  scale,  that is, half an inch above is
marked 31 inches, and  half  an inch below, 29  inches,  and so on.  As,
                                  41 
42
BOYE'S  INANIMATE MATTER.
 however, it is  extremely difficult to obtain the bore of the two  limbs
  Fig. 23.    Fig. 24.      of exactly the same  diameter,  any uncertainty
                        arising from a variation in their calibre, may be
                        avoided by drawing an arbitrary horizontal line,
                        see a fig. 24, between the upper and lower level
                   s    of the mercury, and furnishing each limb with a
                        separate scale,  which two scales, s and s, measure,
                        the one the distance from this horizontal line to
                        the level of the mercury above  it  in the closed
                        limb, the other the  distance from this same line
                        to  the level of  the  mercury below  it in the open
                   ■a    limb, which two measures added  together will
                        give  the true height of the whole column.
                          66. A great object in a good  barometer  is to
                        be  able to measure  with accuracy small changes
                        in  the  pressure of the atmosphere.   But  on
                        account of the high specific gravity of the mer-
                        cury, being nearly 11000 times  heavier  than
                        atmospheric air, these changes are only indicated
                        by  extremely small  changes  in the mercurial
                        column.   To remedy this inconvenience, so as to
                        increase the actual motion or show of the  barome-
ter, different means have been proposed.   As the first of these, may be men-
tioned the substitution of a specifically lighter liquid instead of the mercury,
But in the same proportion as the specific gravity of the liquid becomes less,
the barometer becomes longer and less  portable.   In the Royal  Society
of London, there is  a  barometer which was  constructed  by Daniell with
Water, instead of mercury, the column of which was therefore 34 ft. hi»h,
and varied by the  changes in the atmosphere  about 3 ft.,  so as to be
almost constantly in a state of motion.  But besides the  above named in-
convenience from its size, which would not be an objection for stationary
observatories, all such liquids  are  liable, if volatile, as water, to evaporate
from the open end, and for the same reason to form a vapor in the vacuum
at the  closed  end, which varies  with the temperature, and of which  an
account must be kept;  or, if not volatile, as oil, to change by contact with
the air or the sides of the tube.
  67. The mercury being thus the only liquid, which can be employed with
advantage in the construction of barometers, it has been attempted to pro-
duce the same effect of increasing its show by attaching certain mechanical
contrivances to the mercurial barometer.
42 
PNEUMATICS.
43
  68. Thus, in the Diagonal or Inclined Plane Barometer, the upper closed
portion of the tube, in which the mercury rises and falls, instead of being
perpendicular, is inclined so as to form a considerable angle with the per-
pendicular.  As the changes of the barometer are measured by the perpen-
dicular height, it is  evident, that the mercury in order to arrive at  the
same  perpendicular  height, must travel through a longer distance along
the inclined part of the  tube, and  thus  the motion of the barometer is
increased in the proportion of the hypothenuse of a right angled triangle,
to its perpendicular  side, or as  the diagonal of a rectangle, to  the same.
But as only the perpendicular part of the mercury on the inclined portion
is supported by the atmospheric pressure, the rest being supported by the
inclination of the tube, the friction of the mercury against the sides of the
tube is much greater, and will prevent small changes in the pressure of the
atmosphere from moving the mercury until they become  larger, when they
will appear in the above increased proportion. Thus the small changes, which
are the most difficult to observe, are not indicated at all in this barometer.
   69. Another barometer constructed with a view to the same advantage,
is the Wheel Barometer (Hooke's), see fig. 25, which consists of a syphon
     Fig. 25.       barometer, having in  the mercury  of its open  limb,
                   an iron or glass float, to which is attached a string, that
                   passes over a small wheel or pulley and is kept extended
                   by a small weight attached to the other end. The axis of
                   the wheel is furnished  with an index, which traverses
                   a circular scale.  It will easily be seen that when the level
                    of the mercury changes in  the open  end, the float will
                   follow it and by the string move the wheel, and its index
                   will  thus pass over the  circular scale, the  length  of
                   which must be in proportion to the length of the index.
                   The  graduations on the scale are made to indicate the
                   corresponding rise and fall of the mercurial  column in
                   inches.   Though as  regards  very small  changes, this
                   barometer is  liable to the same objections as the  for-
                   mer,  that these are  not  indicated on account of  the
                   friction  of the weight and the pulley, and the rigidity
                   of the cord;  still for ordinary meteorological purposes
                   it  forms both a cheap and an handsome instrument,
                   and is therefore often met with in parlors and studies,
                   as a 'weather glass.'   As regards  accuracy they are,
                   however,  often made very indifferently, and in such
cases are not reliable for barometrical observations.
                                   43
 
44
BOYE'S INANIMATE MATTER.
V
J
   70.  A  third barometer of  this kind is Huyghen's  Double-Barometer,
     Fig. 26.  fig. 26.  It is a syphon-barometer, the two ends of which are
             widened where the  mercury rises  and falls.   The  open end
             terminates in a long open capillary tube.  The mercury of
             the barometer fills half of the wide portion of  the open  end
             to a, but the other half of it and  part of the capillary tube,
             are  filled with colored spirits of wine.  It is evident, that any
             change in the level of the mercury by the pressure of the atmos-
             phere, will cause a certain quantity of the spirits to be  forced
             into, or withdrawn from, the capillary tube, and thus produce
             a change in  the level of the spirits in the  latter so much
             greater, as its relative capacity is less,  which change may be
             magnified to any desired extent by diminishing the diameter
             of the capillary tube.   It  has,  however, been found that the
             spirits is apt, by its greater adhesion to the glass, to work its
             way between the mercury and the tube  into the vacuum at
             the closed end, and thus render it liable  to get out of  order.
               71. All these  contrivances for increasing the actual motion
             or show  of  the mercurial barometer have  therefore  been
             abandoned for very accurate scientific  purposes, and, instead
             of them, increased power and accuracy  of observing  and
             measuring have been substituted.  For this purpose the scale
of the barometer is furnished  with a sight,  or  horizontal  line, which the
observer may slide along  the tube until, by looking over it,  he may bring
the top of the mercury on  the same horizontal level with it, and thus trans-
fer the level of the mercury to the exact point on the  scale, which corres-
ponds to it.  On account of the  difficulty to the eye to count small divi-
sions, the  scale is rarely divided into smaller parts than tenths of an inch,
or at most, the tenths are again divided into halves, or T^ths.  As on this
account the  point transferred will rarely coincide with a  division of the
scale, a vernier is attached to the sight, in order to measure  the exact dis-
tance of the point from the nearest division of the scale.
   72.  The Vernier see v vxfig. 27, is a short scale sliding on the main scale,
the use of which therefore is, when a point does not coincide with a division
of the main scale,  to measure its distance from this division.    To obtain this
distance, one of the extremities of the vernier, either its zero or its highest
number, is placed at  the point in question, and the vernier  then gives its
distance from the last counted division on the main  scale  by a fraction
which has for  its numerator  the number of that division of the vernier
whieh coincides with a division on the main  scale, and for  its denominator
the whole number of divisions of the vernier, multiplied by the  denomina-
                                  44 
PNEUMATICS.
45
tor of the value of the smallest divisions of the main scale.  The vernier
is always fixed in such manner to the sight, that when the latter is brought
on a level with the  top of the mercury, the nearest  extremity of the ver-
nier (either its zero or  its highest number) is  made to indicate the exact
point on the main scale, which corresponds to the top of the mercury.   If
this then coincide exactly with a division on the main scale, this  division
is counted and the vernier is not used.   But if  the extremity of the ver-
nier do not  coincide with a  division on  the main  scale, we first count or
read off the height to the nearest lower division on the main scale, and add
to this the distance from it to the extremity of the vernier, which  distance
is obtained,  as stated before, by looking along the vernier,  to find the divi-
sion on it, which coincides with  a division  on  the main scale.  Thus, let
11 fig. 27 represent a section of a portion of the tube of a mercurial baro-
               Fig- 27.                meter, with its scale s s divided into
                                     inches and  tenths of inches, a the
                                     top  of  the  mercury in  the closed
                                     limb, and v p the sight transferring
                                     its level to the scale s s at p, being
                                     also  the zero-extremity of the  ver-
                                     nier v vv   It is evident that the
                                     nearest  lower division on the main
                                     scale is 30.1 inch,  and the height
                                     to the point p, therefore, 30.1 inch
                                     + the distance  from 30.1 to p.
                                     This distance is then given by the
                                     vernier  to be T^  of an  inch,  7
                                     being the number  of  the  division
                                     on the vernier, which coincides with
                                     a division on the  main scale, taking
                                     this number as the numerator, while
                                     the denominator 100 is obtained by
                                     taking the whole number of divisions
                                     of the vernier, 10, and multiplying
                                     it by 10, the number which is the de-
                                     nominator of the value of the small-
                                     est division of the main scale (JDth
                               inch  of an inch.)  The  distance from
                                     30.1 inch top thus being y^ =
                                     0.07 inch, the whole height of the
mercurial column must of course be 30.1+0.07=30.17 inch;  so that hav-
                                  45
I
w
----OUinch.
20 
46
BOYE'S INANIMATE  MATTER.
vOtncli.
 ing read off on the main scale the number of inches and  tenths, we only
 have to add the number on the vernier at the coincidence as hundredths.
                                        73. To understand this, it must
                                      be stated,  that  the vernier always
                                      subdivides  the  smallest divisions
                                      of the main scale in as  many parts
                                      as it has itself  divisions.   This is
                                      effected by taking the number, into
                                      which it is to subdivide the smallest
                                      divisions of the main scale, +  or
                                      — 1, as its length, and dividing this
                                      length into the  former  number  of
                                      equal parts.   Thus in fig.  21 the
                                      smallest divisions of the main scale
                                      are tenths of inches, which the ver-
                                     nier again  subdivides into tenths
                                     and  thereby  gives  hundredths  of
                                     inches.   It is therefore constructed
                                     by taking eleven (10+1) divisions
                                      of the main scale ({-Jths inch)  as
                                     its own length,  and dividing this
                                     into ten equal parts.  We thus have
                                     that ten divisions of the vernier are
                                     equal to eleven of the scale, or 10
                                     v = 11 s, therefore, 1 v = l7\s,
                                     or that each division of the vernier
                                     is  TLth larger  than  the smallest
                                     division of the main scale,  and  as
this is  itself one-tenth  inch,  each  division  of  the  vernier  must  be
one hundredth  of an inch longer than the smallest division of the scale.
If therefore (see fig. 27) the 7th division of the  vernier  coincides with
29.4 inch  on the main  scale,  the  6th division of the vernier must be
one hundredth of an inch above 29.5 (the next higher division on the main
scale); the 5th  be  two hundredths above 29.6; the 4th be three hundredths
above 29.7; the 3d be four hundredths above 29.8; the 2d  be five hun-
dredths above 29.9;  the  1st be six hundredths above 30.0; and 0 or the
zero point be seven hundredths of an inch (7 being the number at the coin-
cidence) above 30.1 inch  on the main scale.   The distance from 30.1 inch
to the pointy is thus indicated by the vernier to be 0.07 inch,  as  stated
above, and the whole height of the mercurial  column 30.17 inches.   It  is
                                 46
I'll Hi			J	?ig. 27	
1 I					s
LI					
1	'ir			A	
	\	0 1 2 3 % 5 6	—		
HIBI					
HI					
i				---	
|fii||		7 8 9			
I					
1					
1					
! HI		10			
HI		V.			*)
ill					s
-20 inch 
PNEUMATICS.
47
also evident from the same fig. 27, that the vernier may equally well be
fixed in such manner, that its  lower  extremity v± (or  10 of the vernier)
transfers the top of the mercury to the scale, as  the same number at the
coincidence 7 will indicate the distance of its lower extremity from 29.0
inch on the main scale, and this point  therefore be 29.07 inches; only that
in this case in  looking from the last counted division on the scale along
the vernier,  its numbers  appear  reversed  in order,  beginning with the
highest.
   74.  When the vernier  (so named after its  inventor) is  so  constructed,
that its ten divisions are equal to nine of the main scale, it is often called a
Nonius, meaning the  ninth.   Each division of this vernier is J^th shorter
than the smallest division of the main scale.   The principle and the mode
of reading it off are exactly the same  as those  of the last described vernier,
(fig. 27), only that when both are fixed in  the  same manner, their numbers
always run in  the reversed order of each other.  Two  similar  verniers
constructed by making their 20 divisions equal  to 19 of the main scale,
are seen in fig. 33, which represents the upper  portion of the Levelling
Barometer, fig. 32, and will be further explained under it (81).
   75.  Effect of Capillarity.  A source of inaccuracy in obtaining the true
height of the mercurial column is caused by the capillary action of the tube
on the mercury, by which the latter is prevented from rising to its proper
height, and thus instead of a higher and level surface, presents one that is
lower  and  convex.  This  error is, however,  constant for the  same baro-
meter, and  may be avoided altogether by fixing the scale, as is usual with
all excepting standard barometers, not by the actual height of the mercu-
rial  column, but by comparison with a standard.  Should  this  not  have
been done, this  error may be estimated from  the diamater of the bore of
the tube, as given by the following table.
Table of Corrections for Capillarity.
Diameter of Tube.	Correction for Capillarity.		Diameter of Tube.	Correction for Capillarity-	
	Mercury boiled.	Mercury not boiled.		Mercury boiled.	Mercury not boiled. i
Inch.	Inch.	Inch.	Inch.	Inch.	Inch.
0.60 0.50 0.45 0.40 0.35	0.002 0.003 0.005 0.007 0.010	0.004 0.007 0.010 0.014 0.020	0.30 0.25 0.20 0.15 0.10	0.014 0.020 0.029 0.044 0.070	0.028 0.040 0.060 0.088 0.142
  This correction is always to be added to the observed height.   The first
column is to be used, where all the air and vapor adhering to the tube (54)
47 
48
BOYE'S INANIMATE  MATTER.
have been expelled by the boiling of the mercury (62 and 63);  the second
column where this is  not the case, and is also applied  to the level in the
open part of syphon barometers.
   76.  Effect of Friction and Adhesion.   Another source of error, more diffi-
cult to guard against, is caused by the friction of the mercury against, and
its adhesion to the tube, by which, instead of moving freely up and down
by any change in the pressure of the atmosphere, it remains attached to the
sides, so that when it is falling, it at first exhibits  a less convex and after-
wards  even  a concave surface, and may at last be prevented from falling
any further  by the mercury on the sides not following it; when, on the
contrary, it is rising, it exhibits a more convex surface than it ought, and is
also prevented by the adhesion of the mercury to the sides, from rising  to
its proper height.  To avoid this error the mercury should always, before
taking an observation, be put in motion by gently tapping with the finger on
the outside of the  tube;  or, if  the barometer be suspended so as to swing
freely, a moderate  motion may be imparted to the whole instrument.
   77.  Effects  of Temperature.  As mercury expands  by heat and thus
diminishes its density or specific gravity, it will require a proportionally
greater height to counterbalance the same pressure, and it therefore becomes
necessary to refer all observations to a standard temperature.  This stand-
ard temperature is generally assumed at 32°  Fahrenheit,  or 0° Centi-
grade.   The temperature of the mercury must, therefore, be ascertained,
at the same  time  that its  height  is observed, by a separate  thermome-
ter, with which  all accurate barometers  are  furnished, see I fig. 28  and
g fig. 32; and if the mercury be  not of this standard temperature, the
observed height  must be reduced  to the true height, that  is, the height
it would  have,  if it had the  standard temperature, by  applying  a
correction to  it.  As mercury expands  for  every degree  Fahrenheit
0.0001001 of  its volume, we obtain this correction by  multiplying  this
fraction, first, by the number of degrees above or below 32°, and then by
the observed height,  which correction is deducted if the temperature be
above 32°,  and  added if the temperature be  below 32°,  or  calling the
observed temperature  t, and the observed height ht the  correction for tem-
peratures above 32° will be =  — 0.0001001 (t—32°) hv and  for tempe-
ratures  below 32°  =  +  0.0001001 (32°—t) h±.   But the scale  also ex-
pands by heat and contracts by  cold and  is therefore  only correct at a cer-
tain  normal or standard temperature, which for English  measures is 62°.
Above this  temperature we therefore measure the height by  too long a
scale and obtain  the height too small, and we must  therefore  add  to the
observed height  the  expansion of the scale.   Below this temperature we
                                 48 
PNEUMATICS.
49
 measure  it with too short a scale and obtain the height too large, and we
 must  therefore deduct  from the observed  height  this  contraction.   The
 expansion of brass being for every degree Fahrenheit 0.0000104, we obtain
 this correction for a scale entirely of brass, and extending the whole length
 from  the lower to the  upper level of the barometer, by multiplying this
 expansion, for one degree, first, by the number of degrees above or below
 62°, and then by the observed height, which correction is to be added, if
 the temperature be above 62°, and  deducted if below, being therefore for
 temperatures above 62° = +0.0000104 (t—62°)^, and for temperatures
 below 62° = — 0.0000104  (62°—*.)^. These two corrections for the ex-
 pansion of the mercury and the  scale will be found to  counteract  each
 other  at 29°, which is therefore the only temperature at which  no correc-
 tion is necessary.
   If the barometer be  French, and therefore have a scale of millimeters
 and a  Centigrade thermometer, we have, that the expansion of the mercury
 for each  degree Centigrade  is 0.0001802, and for  brass 0.0000188.   But
 as  the standard  temperature for French measures is  0° Centigrade, the
 same as for  the mercury, we may deduct the expansion of the brass from
 that of the mercury, leaving only one correction for both, being for  each
 degree Centigrade 0.0001614, which  fraction we  multiply,  first, by the
 number of Centigrade degrees, and then by the observed height, being for
 temperatures above  0°  =  — (0.0001614x0 ht and for  temperatures
 below  0° = + (0.0001614X0 hv*
   Where many such corrections are to be made, they are  most conveniently
 performed by the aid of a table, for which purpose  see Table II, at the end
 of Pneumatics page   .   For a more complete table the reader is referred
 to Meteorological Tables prepared  by Guyot and published by the Smith-
 sonian Institution.
   If the scale be engraved on  the  glass tube, and therefore  of glass, the
 expansion of glass must be substituted in the above formulas for  that of
 brass, being for 1° Fahrenheit 0.0000045 and for 1°  Centigrade 0.0000081.
 Where the scale  is a brass plate fastened on wood, no accurate  correction

  * Tho above expansions ought properly to he referred to the true height (32°) instead of
the observed height.  The accurate and complete formula for  this correction for tempera-
 ture is-----—.----;---rr— h, ; in which ht — the observed height; m = cubical expansion
          \-\-m (t—1)
of the mercury for 1 degree, I = the linear expansion of the material of the scale for 1 de-
gree; t  = the observed temperature of the mercury; T = the standard temperature, to
which the observed mercurial height is to be reduced; 5 the normal  or standard tempera-
ture, to  which the scale must  be reduced in order to be correct,  and which for English mea-
sures of brass is at 02°.
      D                            49                  5 
50
BOYE'S INANIMATE  MATTER.
29.
can be made for 4he temperature.  As wood is also  influenced by mois-
ture, and its expansion by heat very small (about one-half that of glass),
it is common to apply in such cases only the correction for the expansion
of the mercury, which will  be found in Table  I, at the end of Pneuma-
t'cs.   By deducting from these corrections ^th of their amount for wood,
and T/0th for glass, they  will be sufficiently accurate for all purposes.
  We will now describe a few mercurial barometers as intended for special
purposes* and point out their peculiarities.
                                     78. On  board  vessels a barometer
       Fig. 28.                      is required of at least moderate accu-
                                   racy, but of simple construction, so as
                                   not to be liable  to get out of order.
                                   It  is also  desirable to avoid metallic
                                   scales or cases, as they are liable  to
                                   rust by the moist air and salt  water.
                                   Fig.  29 represents the inner arrange-
                                   ment of such  a  Marine  Barometer,
                           )7»       and fig. 28 shows it fixed in its  case
                                   and suspended.   The  cistern is made
                                   of wood, in one piece with the cover,
                                   into which the tube is cemented.  The
                                   bottom, being in  part of skin,  see
                                   c fig. 29, is screwed on before  invert-
                                   ing it.   The cistern at g, where the
                                   mercury rises  and falls,  is widened,
                                   to  diminish the  error  arising  from
                                   a change in its level (64.)  The greater
                                   portion  of  the tube, as  far  as d, is
                                   contracted to a very small diameter to
                                   avoid violent oscillations.   The  tube
                                   with the cistern  is  introduced  into
                                   the case by unscrewing the lower part
                                   of h fig. 28, which is of brass.  The
                                   rest  of the case  b  bx  is of wood,
                                   widening  at the top  into  a box with
             *"                     glass front and  containing the scale.
 The latter is of ivory and is divided into tenths of inches, with  a vernier
 v fig.  29, also of ivory and worked by a rack and pinion, the head of which
 p fig. 28, is on the outside.   The inches  and tenths of inches are read off
 on the main scale, and the number on the vernier at the coincidence added
 as hundredths,  as explained in  72  by fig. 27.  It  is suspended  from a
                                   50 
PNEUMATICS.
51
bracket i by a universal joint k, so as to remain perpendicular during the
motions of the ship.
   79.  For accurate barometric observations through long and rough jour-
neys over mountainous regions,  particularly with  a view to estimate the
relative elevations of the country, GayLussac's Portable Syphon Barometer
is generally employed.  It consists of a plain syphon barometer, enclosed
in a brass tube with longitudinal openings for viewing the mercury, on the
edges  of which the scales  are engraved,  each
limb being furnished with a separate scale, see
s  and  s fig. 30, to measure the distance  from      Fig. 30.   Fig. 31.
an arbitrary horizontal line  a between them,
which two  measures  added together, give the
height of  the  mercurial  column, as explained
in 65.  As this and other  barometers are fre-
quently imported  from France, its  scales are
generally in French Millimeters.*   Each  scale
has its  separate  sight,  consisting  of  a  ring
moving on the outside of the brass tube, with a
vernier attached giving  tenths of millimetres.
The glass tube, as represented in fig. 30, has the
lower  extremity of the long limb and the  bend
c  contracted  to a capillary  diameter, so that
any violent motion of the mercury is checked,
and by inverting  it, in  which position trans-
portable  barometers are  always  carried, the
mercury remains in the  bend c as represented
in fig. 31,  and  to  prevent  the mercury in the
rest of the short  limb from  escaping from it,
this is closed at its extremity, which receives
the mercury, but  a  small capillary orifice n,
with the edges turned inward, is left at a  short
distance from  it to admit  the air.  Near the extremities  of both limbs
at I and I, the tube  is  contracted to prevent  the  mercury from stri-
king  forcibly  against the ends  by inversion   or  by  violent  jolting,
by which barometers  are  liable  to be broken.    To  prevent the  same
when  carrying  ordinary  barometers which  cannot be  inverted,  they
  * For the conversion of millimeters into English inches, and of English inches into milli-
meters, a table will be found at the end of Pneumatics.
51 
52                 BOYE'S INANIMATE MATTER.

                               Fig. 32.
should first  be inclined gently till  the mercury fills  the  whole empty
space at the top of the  closed limb, and then be  carried in this position.
                                 62 
PNEUMATICS.
53
  80.  As by rough  travelling, in spite of all precautions, a bubble of air
will sometimes get into the tube, to prevent it in such cases from getting
into the  vacuum at  the closed  end and  thereby spoiling  the  instrument,
when its use  may be  important, and when it could  not  be replaced, the
above  and other similar barometers have  sometimes the  tube drawn out
at some part into a capillary extremity, see d fig. 30, and this set and sealed
into  another  tube,  which  thus  forms  the  continuation of  the former.
Any small  air-bubble, that should find  its way into  the  tube,  will then
be intercepted at this place, between the capillary termination and the tube
surrounding it, where  it  may remain, doing  no  harm, until it can be re-
moved by inversion or other suitable or necessary treatment.  It, however,
increases the liability of the tube to break at this place.
  81.  Where portability is required in connection with the greatest scien-
tific accuracy, as for very accurate levelling purposes,  the form represented
in fig. 32,  is used.   This  Levelling Barometer  is  a cistern barometer.
The  sides  of  the cistern are formed of a short glass  cylinder c, the edges
of which are ground true, the upper being pressed firmly against a wooden
(box-wood) top, through the  centre of which the barometer-tube passes
loosely, being secured to it by an annular piece of kid, the  inner edge of
which is tied around  the  tube, its outer edge round the projecting edge
of the opening  in  the  cover.   The lower  edge of the glass  cistern is
pressed tightly  against a wooden ring  furnished  below with  an external
screw, on  which may be  screwed  another  wooden  ring,  around which
a piece  of  kid  is  secured bag-fashion,  forming  the bottom of  the  cis-
tern for containing the  mercury.  The above wooden  top  and  bottom
pieces are secured  against  the glass cistern by two  annular  brass pieces
forced together  by three bolts as seen at c.   The lower brass  piece is fur-
nished with a screw, on which  may be screwed the brass cylindrical case h,
having through its bottom a screw, by which the mercury in the cistern
may be raised or lowered  to the beginning  of the scale, indicated by its
just touching an ivory point, seen inside the cistern near p,  projecting down-
wards from the top of the cistern as  described in 64.   To the upper brass
piece of the cistern, the long upright brass tube k is fixed, enclosing  the
barometer  tube, see  t t  fig. 33, containing  the mercurial column.    To
observe this the brass  tube, see k fig. 32, and k k fig.  33, is furnished from
its middle  upwards with  two longitudinal openings on its opposite sides,
along the edges  of which the scale is engraved, which on the one side may
be French Centimetres, on the other English Inches.  The arrangement
of the sight  and vernier  or nouius is  seen of natural  size  in  fig.  33.
The sight consists of a ring, the two lower edges of which z and the cor-
responding  one  on  the  other side, form  the sight-line, which is to be
                                 53 
54
BOYE'S INANIMATE MATTER.
Fig. 33.
brought on a level with the top of the mercurial column.   For this pur-
pose this ring is attached by the screw r to the piece m n, of which n slides
on the outside of the brass tube k k, and is  moved  up and down by the
hand, until the sight z is near the top  of the mercury.   We then turn
                          the piece m, which is movable round the piece
                          n,  with  which  it  is  connected,  and  by
                          the fine screw r works on the sight z, so that
                          this latter is moved  still further towards the
                          top of the mercury, till at last the light seen
                          between them through the tube just disappears
                          in  the middle, by their apparently touching
                          each other, when the sight-line z is exactly
                          on a level with the top of the mercury.  The
                          highest division,  or 20, of both verniers v and
                          v then indicates the point on  their respective
                          scales, which corresponds  to the top of the
                          mercury.   To read this off on the right hand
                          side, the  numbers on the  scale  are French
                          centimetres,  divided into tenths.   As the ver-
                          nier has 20 divisions, it subdivides the tenths
                          of  centimetres again into twentieths and thus
                          (10x20=200) gives two-hundredths of cen-
                          timetres.   We  therefore obtain  the  height
                          of the mercurial column by reading off on the
                          main scale the centimetres  and tenths  to the
                          nearest lower division,  which  is  77.4,  and
                          adding to this  the  distance from  it to the
                          extremity of the vernier, by taking the number
                          of that division of the vernier, which coincides
                          with a division on the scale, being 3 two-hun-
                          dredths (^oo)?  which by  dividing  by 2 are
                          converted  into  one-hundredths =  1.5  hun-
                          dredths=0.015, which  added  to the  above
                          77.4  gives  77.415  centimetres, or  774.15
millimetres, as the whole height.
  To read off the left hand side, it will be seen that the  scale is divided
into inches, tenths, and half of tenths, that is twentieths (3^ = 0.05).   As
the vernier has 20 divisions, it  subdivides the twentieths of inches into 20
parts and thus gives  (20x20=400) four-hundredths of an inch.   On the
main scale we therefore read off the nearest lower division, which is  30.45,
                                 64
\\
■:■■
 
PNEUMATICS.
55
to which we add the distance to the extremity of the vernier, by taking the
number of that division of the vernier, which coincides with a division of
the  scale,  being  13  four-hundredths  (= 4^), which divided by  4
to convert them into hundredths gives 3.25 hundredths = 0.0325, which
added to 30.45 gives 30.4825 inches, as the whole height of the mercurial
column.   It would have been more convenient, if this vernier had contained
25 divisions, so as to subdivide the twentieths  of the scale into 25 parts
instead of 20, as then the  divisions of the vernier would have indicated
500ths so that  multiplied  by 2 they would be converted into lOOOths of an
inch.
  The enclosing  brass  tube of this  barometer, k fig. 32, contains the
bulb of  a thermometer g,  to ascertain the temperature  of  the mercury
in the tube, and which  should be read  off immediately after adjusting
the sight z, as  the proximity of the observer  is apt to increase  the  tem-
perature,  while reading  off the  vernier.  This brass tube  is  also fur-
nished with two  small transverse  axes,  of which one is at its middle,
by which it is  suspended when in use by a universal motion in the top
w of a three-legged mahogany stand, q q q.  When required to be fixed
for transportation the barometer  is lowered,  so  as  to be suspended  in
the stand at w  by the other or upper axis a.  The screw at h is then turned
till the mercury is nearly  up to the top of the cistern, the stay-wires i i i,
are raised,  and the legs of the  stand which  move  on hinges  are folded
together,  so as to form  an enclosing case around  the barometer.  The
whole is then cautiously turned upside down, and having secured the legs
of the stand together by a brass ring, slipped into a leather case.
   82.  Standard barometers have a similar construction to the Levelling
barometer, fig. 32, only as  they  remain  permanently fixed in one place,
they are generally made of a much larger diameter, so as to avoid altogether
the capillary action of the tube on  the mercury.  Instead of the ordinary
sight, they are  sometimes furnished with  a sliding telescope, moved  by a
rack and pinion.   The vernier which is attached to it, is read off by the aid
of a magnifier and is so constructed as to  indicate thousandths of an inch.
   83.  For stationary observatories  the mercurial barometer may be made
self-registering, the mode  and principle  of which  will  be described  ir
another place (see Thermics under Thermometers).
   S4.  Although the changes in the mercurial  barometer are small, still
with proper precautions,  they are always  given in  the right proportion.
The mercurial barometer with the previously described  improved means
of observing and measuring, constitutes therefore tne most accurate instru-
ment which we have for  measuring the  pressure of the atmosphere, the
only objections to it being its weight and liability to break by transportation.
                                  55 
56
BOYE'S INANIMATE  MATTER.
Fig. 34.
Jldies'
I*
But  even in this latter point it has the advantage, that it rarely deceives.
To obviate, however, these objections, several  substitutes have been in-
vented, which we will now describe.

                Substitutes for the Mercurial Barometer.

  85. In the mercurial  barometer we measure the  atmospheric  pressure
by the weight of the mercury,  due to the action of Gravity.  Instead of
gravity may be substituted the  Elasticity of bodies, such as that of perma-
nent gases, of vapors, or  of metals.
                   86.  In  Adie's  Sympiesometer (from aoix-KieZw (sym-
                 piezo), I compress, and perpov (metron) measure), the elas-
                 ticity of a confined gas is used to estimate the pressure of the
                 atmosphere.  It is  made of glass, see fig. 34, and has the
                 general appearance of a syphon barometer, but is much
                 shorter, and  the closed end  a considerably enlarged to
                 contain the  gas, which  should be  one that does  not
                 act on  oil, such as  Hydrogen or Nitrogen, generally the
                 former.  The bend e?with part of the open end c is filled
                 with oil, which thus  confines the gas by separating it
                 from the atmosphere.  It will thus  be seen, that when
                 the pressure  of the atmosphere on the oil in the  open
                 end at c varies, the volume of the gas in a is diminished
                 or increased  according to Mariotte's  law, and the  level
                 of the  oil at b thereby altered.  But the  volume  of
                 the gas is  also altered by the temperature.  To correct
                 it for this  influence, without  the aid of calculation, the
                 scale e, which is to indicate from the volume of the gas
 the  pressure of the atmosphere in inches, corresponding to the mercurial
 barometer, is made movable,  its index % sliding over  another but  fixed
 scale s, which contains numbers  corresponding  to  the  different  tem-
 peratures of the gas, as indicated  by a  delicate thermometer  I, which
 is inverted in  order  to have its  bulb near  the  gas.  Before reading
 off  the  pressure,  the temperature  of the  gas is  observed  with  great
 accuracy by the thermometer  I, and the index i of the pressure  scale e
 is next placed on the exact number  of the fixed scale s,  which  corresponds
 to the temperature, and the pressure is then read off, as indicated by the
 level of the  oil at b, without  any further correction.   The Sympiesome-
 ter is said to indicate the changes in the pressure of the atmosphere much
 sooner than the mercurial barometer, and is therefore mainly used on board
 vesseis,  for prognosticating  the sudden and dangerous  squalls experienced
                                   56 
PNEUMATICS.
57
VU-s
in the tropical regions.  To prevent the effect of the motion of the vessel,
its diameter is often contracted near the bend at d.   The Sympiesometer
in its present form is due  to Mr. Adie, of Glasgow, and  only those made
by him and bearing his name are considered as good and  reliable.   It has,
however, the inconvenience, that the oil is apt to become thick by the action
of the air at the open end, by which the working of  it is impaired.  To
prevent the oil from spilling, by transportation, from the open end, this  is
generally furnished with a stopper, which can be drawn down  on it by a
wire, projecting from the lower edge of the instrument and furnished with
a nut to tighten it.
  87.  Another substitute, is the Boiling Point Barometer, generally known
urider the name of the Boiling Point or Hypsometric Thermometer, which
acts on the principle of estimating the atmospheric pressure from the elas-
ticity or tension of the vapor, generated from pure water,  boiling in an
open vessel, which tension is the same as the atmospheric pressure.   Fig.
35  represents  the most  approved form, that of Regnault,       F-  35
(Ann  de Chimie, 3d ser. Vol. XIV., p. 202.)  The  in-        ,"
strument is made of  sheet brass, and consists of a small       "|j
cylindrical vessel I k  i,  into  the lower closed end  of
which pure distilled water w is introduced.   This part is
fixed into the  top of another cylindrical vessel g h m n,
the bottom of which is  formed of a small spirit lamp a b,
which fits  into it by a  catch, and by which the water is
made to boil.  The necessary draft of air enters through the
lower  orifices  o o, and  passes out at the upper ones ot ov
m n is a sliding ring  extending  down on  one side, by
which,  in case of windy weather, the lower openings o o
may be diminished  or closed on the  side towards  the
wind.   The upper part r  t s of the vessel Iki containing
the water, is  formed of short  sections of tubes, sliding
inside of each other, as those of a telescope.    The upper-
most has an opening ou large enough for the free escape
of the steam, and its top is closed by a stopper v, through
which  the stem of  the thermometer u u slides easily,  but
safely.  The scale of this thermometer is marked on the glass, and includes only
about 25° next below the ordinary boiling point, each degree being very large,
and divided into tenths and even smaller fractions.  Having introduced 2 or
3 cubic inches of distilled water, the alcohol lamp is lighted, so as to cause
the water to  boil,  while the  thermometer is  constantly adjusted  by
moving the stem down through the stopper, so that the top of the mercu-
                                  57
'lift
 
58
BOYE'S INANIMATE  MATTER.
rial column is barely visible above it at v, while by sliding the tubes sir
up and down, inside each other, the bulb is kept at the distance of  about
one inch above  the water.   When the  mercury becomes stationary,  while
the water is all the time boiling, the  exact temperature is read off to the
smallest  possible fraction of a degree.  The elasticity  or  tension of the
vapor corresponding to this temperature,  is  then ascertained from  Table
VII at  the end of Pneumatics.   A  well-constructed instrument of this
kind, will, with all due precaution, give results not varying from those of
the Barometer more than from T-^ to y^  of  an inch, the main source of
inaccuracy being the difficulty of graduating  a thermometer correctly into
so small parts of a degree, and the liability of these to alter by the subse-
quent irregular contraction of the glass  of the bulb and even of the  stem.
This instrument, being only 14 inches in  length when drawn out, is more
portable and  much easier packed without danger of derangement or break-
age, and is  therefore often used as a substitute for the barometer on rough
travels,  to estimate the height of the different elevations, see 94, &c.; but
for accurate levellings of small heights it is not suitable.  It will  be seen
from Table VII,  that, at 30 inches, barometric pressure, a diminution  of y1^
degree in the boiling  point corresponds  to a difference in the atmospheric
pressure of about 0.059 inch, and will therefore, when the temperature of
atmosphere is 32°, indicate a difference in height of 51.3 feet.
  88. In the Aneroid Barometer (the name said to be formed by the inven-
tor from a privative and peto (reo), I flow,  intended to mean, without fluid)
the atmospheric pressure is measured  by the elasticity of a metallic spring.
Its general form and size, see fig. 36, is that of an ordinary chronometer.
            Fig. 36.                              Fig. 37.
Its interior is shown by fig. 37.   The main part of it is a metallic vacuum
                                  58 
PNEUMATICS.
59
vessel b d, having the form of a very short cylindrical box of about two and
a half  inches  diameter, from which the air has been almost, though not
entirely, exhausted through the opening at/, which opening, after effecting
the exhaustion, is soldered up.   The  two ends (top and bottom) of this
box  are made of thin corrugated sheet copper  or brass, strengthened  at
their middle, one being fastened to the supporting plate of the instrument,
while the  other d, by the upright rod a, is connected at w with a one-armed
lever m n v,  resting by its  two fulcra at n  and n, on the ends of two
uprights B B.   It will thus be seen, that the atmospheric pressure on the
two ends of the vacuum box d, will have a tendency to force them together,
and  thereby  to depress the end v of the lever.  To  prevent this and
to counteract the pressure  of the atmosphere on the vacuum box, this end
of the lever is  supported by  a  spiral spring s, the other end  of which is
fastened to a small plate, which rests on the supporting  plate of the instru-
ment, but can be raised or lowered by the screw A.   It must be evident,
that as the pressure of the atmosphere  on the vacuum-box increases, it will
compress  the  spring  s, and depress the end v; when,  on the contrary, it
decreases, the elasticity of the  spring will again  raise  it.  To  show this
motion the end v is connected by the rod v r with the  arm r, which again
is connected with the axis  u by  a curved spring and the screw z, by which
screw the length of its leverage on the  axis u may be altered.  To the
axis u is  again attached the other arm x, the two arms r  and x, together
with the axis u, thus forming an angular lever.   From the end of the arm
x, a slender rod h terminating in a chain c passes  to and round a cylinder,
the axis of which at one end is connected with  a flat spiral hair-spring y,
and  at the other end passes through the face of the instrument and  carries
its index or hand  i, which, see fig. 36,  traverses a graduated circle  on
the  face,  which  circle is  divided into parts  marked  as inches and cor-
responding  to the  height of the mercurial column in  an ordinary baro-
meter.  Disregarding the peculiar form a«d exact relative position of the
different  parts, they may be  represented, and  their mode of action better
understood by a reference to fig. 38.                Fi9- 38-
When the atmospheric pressure on the
vacuum box b increases,  it forces the
top d of the latter further in;  by this the
rod a  depresses the lever n v,  thereby
compressing  the spring s.  The end  v
of the lever n v, then acts by the rod v r
on the angular  lever r  u x,  which
again draws the rod h and the chain c,
and  thereby turns the cylinder, and the hand i, which  latter thus moves
                                  59
 
60
BOYE'S INANIMATE  MATTER.
over the face from left to right, whereby at the same time the spring y is
slightly coiled.   When, on the contrary, the atmospheric pressure on the
vacuum-box diminishes, the  spiral spring s raises the  lever n v, which,
through the angular lever rux slackens the rod h and chain c, and  thus
allows the  spiral spring  y to turn the  cylinder back  again, whereby the
index is moved in the opposite direction, from right to left.
   To set the hand to correspond with a  standard mercurial barometer, it is
moved by turning the small screw A, jiff. 37, the head of which will  be
found on the back of the instrument.  If then the  space, moved over  by
the changes of  the atmospheric pressure (its rate of motion), does not cor-
respond with the mercurial barometer, it is adjusted inside by the screw z,
see fig. 37, which alters the length of the arm r.   The face of this instru-
ment is generally furnished with  a thermometer, see fig. 36, the  bulb  of
which  is inside, and thus  indicates  the temperature of  the  instrument.
The inventor of the construction of this barometer, Mr. Vidi of Paris,
claims however to have rendered  it  independent of the influences of the
temperature, by leaving a certain,  very small, portion of gas in the vacuum-
box.  The preponderating  effect of heat on this instrument, he asserts to
be to weaken the elasticity of the vacuum-box  and of the spring s, and
thereby to increase the compressing effect of the atmosphere on the vacuum-
box, and that this effect  therefore may be  counteracted by leaving in it a
very small portion of air, just enough to counterbalance,  by its increased
elasticity by heat, the increased compression of the vacuum-box, in conse-
quence of the  diminished elasticity of the spring s.  To test the instru-
ment for the completeness of  this  compensation for temperature,  it is only
necessary, while the atmospheric pressure is found by a mercurial barome-
ter to be stationary,  to expose it to two different temperatures,  and ascer-
tain its variation, which variation, divided by the number of degrees pro-
ducing this change, will give  the correction for each degree.
   The use of the register-hand t  is,  as in the Wheel-barometer, merely to
note the exact place of the hand  * of the instrument at the last observa-
tion, which is done  by moving it by the small knob in the centre of the
glass covering the face, till it is exactly  over it.  Inspection at  any subse-
quent time will then easily  tell,  how much the  barometer has risen  or
fallen.
   The complicated construction of this instrument must always  render  it
for very accurate scientific purposes,  inferior to the mercurial barometer.
Its extreme portability, being only of the size of an ordinary chronometer
must however prove it to be a useful,  and, for many purposes, even a highly
valuable instrument.  The great  objection to it is its liability  to get out
of order without any previous warning  to the observer.   When used for
                                 60 
PNEUMATICS.
61
Fig. 39.
important observations, it should therefore constantly be compared with a
good mercurial barometer.
   89.  The " Metallic Barometer" (Bourdon's) acts on a similar principle.
                                      Fig. 39 represents its interior ar-
                                      rangement,  the  face  having been
                                      removed  and  the hands replaced.
                                      It consists of a flat, hollow, metallic
                                      vacuum vessel v v, made of very
                                      thin sheet brass, of a doubly-arched
                                      or lenticular cross-section, as seen
                                      at the end e, and curved as part of
                                      a hoop, so that the two ends e and
                                      et are only  a  short distance from
                                      each other.  The outer side of the
                                      vacuum-vessel having a greater ex-
                                      tent of surface than the inner, on
                                      account of the longer radivite of its
                                      curvature,  the atmospheric  pres-
                                      sure on it, which acts at every point
                                      in the direction of the radius of its
                                      curvature, so as to force it inward,
                                      will be greater than  the pressure
                                      on its inner side, which also acts
                                      at every point in the direction of
the radius of its curvature, but so as to force it outward.   The atmospheric
pressure has  therefore a constant tendency to increase the curvature of the
vacuum-vessel, so as to cause its two ends e and e± to approach each other,
which effect is counteracted by the resistance offered by the elasticity of the
vessel itself, so that when the atmospheric pressure  increases, it will cause
the ends to move  still nearer to each other; when, on the contrary, the
atmospheric  pressure becomes  less, the elasticity of  the vessel will cause
them again to recede from each other.   To increase this motion, the ends
e and e1  are made  to act by the  rods r and rx  on the ends of the two-
armed  lever s s,  to the axis of which another lever  n n is  attached, the
end of which carries  a section  of a cog-wheel a a.   This cog-wheel  acts
on a pinion i, the axis of which passes through the face of the instrument,
and carries the hand y,  which is thus made to pass over  a graduated circle
on the face, the parts of which indicate  the corresponding changes in the
mercurial barometer in inches.  When the atmospheric pressure is increased,
the hand y is  thus made  to  move from left  to right;  but  when it is
                                   61                           6
 
62                  BOYE'S  INANIMATE  MATTER.

decreased, the elasticity of the vacuum-vessel will  move  it in the opposite
direction, from right to left, to assist in which, the small weighty is attached
to a short arm, which acts on the axis of the lever s s, thus assisting in
forcing the ends e e, apart,  p is the register-hand, which by being placed
over the  hand y, will indicate, what change  has taken place since the last
observation.
                        Nature of the Barometer.
  90. The Barometer measures the pressure of the  atmosphere.  This pres-
sure depends mainly, though not altogether, on the weight of the atmosphere,
because in many cases the atmosphere is not allowed to press with its whole
weight on account of the lateral or upward currents, which take place in it
and constitute what we call winds, while if these currents should meet each
other or  have a descending direction, it would increase the pressure beyond
what is due to its weight.   If, in a similar manner, the air near the earth
should, from some cause, become  suddenly heated, so as to have its elas-
ticity increased, it would require some time  to put the  surrounding air  in
motion";  this would meanwhile increase the  pressure beyond what  is due
to its weight alone.   In this latter case it will also  be seen, that the specific
gravity or density of the air would not be increased.  From these considera-
tions, it  will be  evident, that the barometer cannot correctly be said  to
indicate  the weight or the density of the atmosphere, but only its pressure.
   91. The Manometer.  To indicate  the changes  in the specific gravity or
density of the atmosphere, a separate instrument was proposed by Otto Gue-
ricke, called the Manometer, from pavoq (manos), rare, and perpov (metron),
measure, meaning, measurer of the density of the air.*  It consists of two balls
of nearly the same weight, but of very different diameters, the one being hol-
low, the other solid, both  suspended to a balance-beam, so as to counter-
 poise each other in the air.  As  bodies suspended in a fluid  lose  as
 much of their  weight, as  the volume of the  fluid which  they displace
 weighs,  it will be  seen  that  the larger ball, displacing a larger volume of
 air, has, under the above circumstances, lost more of  its absolute weight,
 than the smaller; and that any change in  the density of the air will affect
 it  more than the  smaller, detracting more from its weight, when  the
 density  becomes  greater, and restoring to it more of  the  weight, already
 lost, when the density becomes less.   In either case, therefore, the equili-
 brium of  the balance will be destroyed, the large  ball rising when the
  * The word Manometer is  sometimes, though incorrectly, applied to pressure-guages,
see (105), for measuring the tension or elasticity of gases, this latter being considered pro-
portional to their density: but in the confined state, the elasticity is altered by the tempe-
rature, while the density is not affected, unless they be vapors in contact with a liquid.
                                  62 
PNEUMATICS.
63
density of the air is increased, and falling when it is diminished.  For the
same reason, very light and bulky substances, such as feathers, have a per-
ceptibly greater weight than that ascertained in the ordinary way in air.
Hence as a puzzle it may be said, that a pound of feathers is heavier than
a pound of lead.

                         Uses of the Barometer.
  92.  As a weather-glass.   The most popular use made of the barometer is
for prognosticating the weather.   The weather is said to be bad when either
windy, or  rainy, or  both.  Winds are masses of air in  motion.  Their
direction being mostly lateral  and upward, their most frequent effect on
the barometer is to prevent the air from pressing with its whole weight on
it, and thus in most  cases to cause the barometer to fall, but not necessarily
so, since if the wind  have a downward tendency, it will cause the barometer
to rise.  Bain is most frequently caused by a hot and moist current of air
mixing with a cold current or passing over a cold country, or by the ascent of
a heated column of air saturated with moisture, in which case the moisture con-
denses by the cold produced by the expansion of the air, on account of the
less pressure as it ascends.  Bain thus depends more or less on currents of
air, either near the surface of the earth or higher up, which, as we have just
seen, will affect the barometer;  and thus rain, like wind, generally causes the
barometer to fall, but not necessarily so, as it may even have a contrary effect.
The condensation of the vapor and its consequent withdrawal  from the
atmosphere,  causes also a certain, but comparatively  very small, depression
of the barometer.  As thus changes in the barometer nearly always accom-
pany changes in  the weather and frequently precede them by more or less
time, this instrument serves as an  excellent guide to account for present,
and even to anticipate coming changes in the weather.
   The Wfceel and other cheap barometers constitute, therefore, the common
and most popular Weather-Glass, and for this purpose instrument-makers
have affixed to different parts of the scale certain inscriptions, indicative of
the weather, viz.  " Fair," at the average stand of 30 inches; " Change," at
29.5;  "Bain," at 29;  "Much  Bain," at  28.5, and "Stormy," at 28;
while above 30 inches we find " Set Fair," at 30.5, and " Very Dry," at 31
inches.  These inscriptions are very fallacious, and have done much harm
by bringing the barometer in disrepute  and calling the attention away from
the real scale of the instrument, which indicates the height of the mercu-
rial  column.   For although the most  severe  gales  are  generally accom-
panied by rain and  cause the  lowest  stand of  the  barometer, much rain
and  bad weather may also occur  at  a high stand, where therefore  the
inscriptions indicate fair weather.  Even for such one-sided use of th^
                                  63 
64
BOYE'S INANIMATE  MATTER.
barometer, it would be better, instead of having or looking at the inscrip-
tions, to note whether the barometer is in the act of falling or rising, since
even at a high stand a considerable fall  is most  likely to bring  about a
change in the weather, although the fall  might not reach  the  ominous
inscriptions on the " Weather-glass," while, on the other hand, immediately
after a storm, fine weather often appears before announced by the glass.   To
find whether a rise or fall has taken  place since the last observation, most
" glasses"  are  furnished  with a register-hand, which is placed by  the
observer over the hand of the instrument, but which register-hand,  in many
cases, is much larger and more conspicuous  than the  index-hand itself, so
as to attract the sole notice of casual observers,  and, by being  mistaken for
the hand of the instrument, gives them the idea of a stand still in the instru-
ment, when they most  expected a change.   It  is given as a rule, that a
change in the weather,  accompanied by a gradual and slow change in  the
barometer, is likely to last longer than those changes,  which are indicated
by a sudden change in the barometer.
  From  the  nature  of  the barometer, as  explained in 90, an intelligent
observer will, therefore, no more expect bad  weather to follow invariably a
fall or low stand of  the barometer, or good weather to accompany invari-
ably a rise or high  stand, than  he would expect  one kind of weather to
follow invariably a wind from  the east, and the opposite kind, one from the
west.   He will know that changes may occur,  which not at all or but slightly
affect the  barometer, and in  such  cases he  will rely on other indications.
On the other hand, while from a change in the barometer he will not expect
invariably a change in  the weather, still  he  will know, that in such cases
causes are active, which may bring about such  changes.  He will consult
the Hygrometer (164), to ascertain whether the amount of moisture is at
the same time increasing or decreasing; the Thermometer, to observe any
simultaneous change in the temperature; and,  above  all, by constant and
attentive observation and by careful study  of  previous records,  he will
become familiar with the habits of different winds, at the different seasons
of the year, and in the particular localities.   Thus he may find that in the
fall of the year, after a  long and continued  spell of  dry and  fine
weather, a change is often anteceded by a rise in the barometer instead of
a fall; that certain  directions of winds are more apt than others to bring
about a fall or rise without a corresponding  change in the weather;  that
certain storms, which by careful attention he will be  able to distinguish,
are preceded  by a fall, as many of the most violent  and sudden  tropical
gales, others by a rise, and others  again will at first cause a fall, but then
a subsequent rise will indicate their increased violence, &c. &c.  In fact, to
unprejudiced minds, the barometer has the advantage over all other meteoro-
                                  64 
PNEUMATICS.
65
logical instruments, that  it indicates changes  in  the  equilibrium of the
atmosphere, while they are often yet far  distant from the place of the
observer;  and thus not only puts him on his guard against them, but also,
more than any other instrument, guides him in finding and studying their
causes and their progress.
  93.  As a general rule the barometer is much less variable in the Tropi-
cal  and Torrid zones than in the more northern latitudes.   Thus, in Peru
the range  of its variations is only about i  inch, while in London it is about
2£  inches, and in  St. Petersburg over 3 inches.  The average stand of
the barometer at  the level of the sea at 45° latitude is 30 inches (or more
correctly 29.922 inches), which is considered as the standard pressure for
measuring gases, and to which, therefore, their volume is always referred
(100).  It is somewhat higher near the Tropics, from which latitude, there-
fore, the average stand of the barometer decreases both  towards the equator
and towards the poles.   The average stand of the barometer at any particular
place inland depends mainly on  its  elevation above the level  of the sea,
but is also influenced to some  extent by the latitude, and by the particular
conformation of the whole continent, or that part of it where it  is situated.
The average stand of the barometer at Philadelphia is 29.95 inches.   The
barometer is  subject to monthly variations, the  greatest  monthly  mean
pressures  being those for June and January; the lowest, those for Novem-
ber and March.   At moderate latitudes, the  average difference between
the means of June and  November  amounts  to  about 0.11  inch.   The
barometer also  exhibits a regular diurnal variation, standing  highest  at
nine o'clock A. M.  and  P. M., and lowest at three o'clock P. M. and A. M.,
being  unaffected  by it at noon.  The  hours of nine and three, or at twelve,
are therefore  recommended  as most suitable for regular meteorological
observations of the barometer.   The average of this daily variation, which
is ascribed  to the heat  of the sun, amounts in moderate latitudes to  about
0.03 inch, but increases towards the  equator  to 0.1 inch;  in  higher lati-
tudes  it is lost in the irregular changes.
   94.   For Measuring Heights  or  Levelling (Hypsometry,  from  b<poq
(hupsos),  height,  and  perpuv  (metron),  measure).    If   the   atmo-
sphere were of uniform density throughout  its whole extent,  the height
of  the mercurial  column in  the barometer  would  afford us  an  easy
means  of  calculating  the  perpendicular  height of  the whole   atmo-
sphere, or of  any part  of it,  from the  known laws of Hydrostatics, that
the heights of columns  of  different liquids, equilibrating  each  othei
in  communicating  tubes,  are inversely as their specific gravities.   Hence
it would  only be necessary  to multiply 30 inches  by 11000, which is
the number expressing  how  many  times mercury is heavier than  air,
      E                          65 
66
BOYE'S INANIMATE  MATTER.
in order to obtain the height of the  whole atmosphere  in  inches, which
would  make it about 5.12 miles; and in the same  manner the  perpen-
dicular height of any intermediate part of  the  atmosphere, between two
places  not situated on  the same level, would be  obtained by multiplying
the difference in the stand of the barometer at  these two places by the
same  number, 11000.   But  this  is not  the case.  It has  already been
stated  in 27, that from other experience it is known that the atmosphere
extends much farther;  and both reason and experience tell us, that as we
ascend into the atmosphere, the strata below are  not capable of exercising
any pressure on those above, and that the upper strata, therefore, are  sub-
ject to less pressure and consequently have  also less density.   In  this
manner both the pressure and the density of the atmosphere must decrease, as
we ascend from the level of the sea to greater elevations: still, knowing the
exact ratio  between the different heights to which we  ascend into the
atmosphere, and the decrease  in the corresponding pressures, the barome-
ter will yet afford us one of the most valuable means to ascertain the differ-
ences in level of different places.
  95.  To understand the principle on which this is ascertained, it  may be
stated, that  while the different perpendicular heights above the surface of
the earth, if counted from the upper sensible limit of the atmosphere down
to its lower  limit at  the level of the sea,  form an increasing arithmetical
progression  (1 ft., 2 ft., 3 ft, 4 ft., &c, from the top of  the atmosphere),
the corresponding pressures on the barometer form an increasing  geome-
trical progression.   Between any two such series there is  a similar  relation
as between the ordinary logarithms and their corresponding numbers, the
logarithms forming an arithmetical series, and therefore  corresponding to
the distances from the top of the atmosphere; while the numbers, to which
they are the logarithms, form a  geometrical progression,  and therefore
correspond to the barometric pressures.   If, therefore, at  the same  time or
moment, we ascertain in two different places, situated at different  heights
or on different levels, the true barometric  pressures, that is, the heights of
the mercurial columns, corrected for the influence of the temperature (77),
and then from  an  ordinary table of logarithms  take  the logarithms cor-
responding to these two pressures (it matters not whether the pressures be
expressed in English inches or in French  millimeters), these two loga-
rithms will indicate the relative distances of those two places from the
upper  limit  of the atmosphere, and may, therefore, by multiplying them by
a constant number,  be made  to give these distances in English feet or any
other  measure.  These distances, deducted from each  other, will then, of
course, give the  difference in their level, or the height of the one above the
other.   To  avoid the double multiplication of the two logarithms by the
                                  66 
PNEUMATICS.                            67
constant, the logarithms may first be deducted from each other, and their dif-
ference multiplied by it, which will then give the difference in their level.
   To obtain these distances from what may, icith sufficient accuracy for pre-
sent purposes, be considered the upper sensible limit of the atmosphere, in
English feet, the constant number by  which we multiply the logarithms
of the true pressures, is 60158.5, the temperature of the atmosphere being
supposed to be 32° Fahrenheit, and the difference  between the logarithms of
the true barometric pressures, multiplied  by this  number, will therefore at
once give the corresponding difference in level in English feet, the  tempe-
rature of the intermediate column of atmospheric air being 32°.
   To facilitate these calculations, tables have been constructed, which give
the different distances from the above assumed upper limit of the sensible
atmosphere, calculated in this manner for all the  different barometric pres-
sures.  These distances  for pressures from  28 to 31 inches will  be found
in Table III at the end of Pneumatics, page
   But the above distances are  only correct for the  standard  temperature
of the atmosphere of 32°.   As air expands by heat, and thus, with the for-
mation of an additional quantity of vapor of water, diminishes its density or
specific gravity for  every degree of Fahrenheit by 0.00222 of its density
at 32°, the same mercurial column will, at higher temperatures, counter-
balance a  proportionally higher column of air.  The  temperature of the
atmosphere must, therefore, always be ascertained  at the same time that we
observe the pressure, by an accurate thermometer, which has been sufficiently
long exposed to  it in a suitable place.  Should the  temperatures at both
places not  be the  same, their average is taken as the  temperature of the
column of air between them.   If then this average temperature be not 32°,
a correction  must be applied  to the above difference in level or height,
which correction is obtained by multiplying the above  given expansion of
the atmosphere  for  1° Fahrenheit, 0.00222, first, by  the  number  of
degrees which the  average temperature of  the air is above or below 32°,
and then by the above-obtained height for 32°, which correction is to  be
added, if the average temperature of the air be above  32°, and deducted,
if below; or, calling the above height, corresponding  to  32°,  hiv  and
the temperatures of the air at the two places or stations, T and Tv the
   Cor. for temp, of the air above 32° = + 0.00222 (T+Tl — 32°) h11}

   Cor. for temp, of the air below 32 = — 0.00222 ( 32° — T+Tl \ hlv

It will be  seen that this correction is quite considerable.  Thus at a
barometric  stand of 30 inches, a fall of ^  inch  corresponds  at  32°  to a
difference in level of 87.2 feet,  but  at 80°, this correction for temperature
                                 67 
68
BOYE'S INANIMATE  MATTER.
of the air being = + 0.00222 X (80°— 32°) X 87.2 feet = 9.3 feet, it
will correspond to a difference in level of 96.5  feet.
  Two other, but comparatively small, corrections are yet to be applied to
the thus corrected height, on account of the decrease of  gravity: 1. from
the poles toward the equator, 2. from the level of the sea upward into the
atmosphere, by which the weight of the mercury becomes less, and  the
same column of mercury will thus counterbalance a smaller column of  air.
These  corrections for  latitudes  near  45°, and  for small heights, are often
for  ordinary amateur  purposes  entirely disregarded.   They will be given
by Tables IV and V at the end of Pneumatics.   The first of them depends,
as stated, on the latitude;  and taking gravity and the consequent weight
of the mercury at 45° latitude as standard or unit, we obtain  this correc-
tion by multiplying 0.0028371,  first by the cosine of the double latitude, and
then by the last-obtained height, which correction, as indicated by the sign
of the cosine, is to be deducted for latitudes greater  than 45°,  and  added
for those less than 45°; or, calling the obtained  height, corrected for tem-
perature of air, ht and the latitude L, this
     Cor. for Lat. = 0.0028371 cos.  2LJ.J £dd  foJ\Lat- s+maller,tha* 45°"
                                     1 (_ Deduct for Lat. greater than 45°.
If the two places have a  sensible difference of latitude, the average  lati-
tude is used.  The second correction for gravity depends on the height or
altitude itself, and is obtained  by first adding to the obtained  height the
number 52252, then dividing by the mean radius of the earth, 20886861 ft.,
and then again multiplying by the height, which correction is always to
be added, as on account of the less weight of the mercury, the  upper por-
tion  of the  atmosphere  has given too great  a column  of mercury and
thereby caused too small a difference  in pressure.  Calling the height cor-
rected for temperature of the air and  for  the latitude, h0, the
                 Cor. for altitude == + h° +  52252 h
                                   ^  20886861  *<>•
  Calling the true height or difference in level of two places, h, the barometric
pressures at those places corrected  for temperature of the mercury and of the
scale, B and b,  the following formula will give all the different operations:
                                                     -T+T±
h = (Log. B — log. b) X 60158.5 ft. X
1
+ 0.00222 (-4—1— 32°  )
or
  — 0.00222  (32°— ^t^-1)

1+0.0028371 cos. 2 L
14_A+52252
 "*"  20886861
68 
                             PNEUMATICS.                             69

  96.  To illustrate the  above by an example, we  may select the calcula-
tion of the height attained by Gay Lussac in his famous balloon ascension
from Paris in 1804, being the greatest height ever attained in this manner.

Observed height of Barometer        Temp, of Merc.    Temp, of Air      Lat.

In Balloon = 12.945 inch = \   14°.90 = t1   14°.90=r,  AQORnr    T
At  Paris  =30.145inch = Bt  87°.44=t   87°.44=!T  48  50 =L

  Applying the corrections for temp, of the mercury and  of the scale (77), we
obtain the
                                          True height of Barometer
In Ball. = 6, + 0.0001001 (32° —14°.90)6.1     100/!1 .  .
              — 0.0000104 (62° —14».90) 6, j"  = 12'%1 mch  = b
At Par. = BX — 0.0001001 (87°.44 — 32°)5, \    9QQIM.  .    „
              + 0.0000104(87°.44 — Q2°)Blf = zy-yb6 mcfl = B

Log. B = Log. 29.983 = 1.4768747
Log. b = Log. 12.961 = 1.1126365
       (Log. B—Log. b)= 0.3642382 = Difference of Logs.
Difference of Logs, multiplied by 60158.5
                 = 0.3642382 X 60158.5 =                21912.03 feet = hu

Average Temp, of Air =    ■" \ = 51°.17

Cor. for Temp, of Air = + 0.00222 Ay+yi _  S2°\hu

  = + 0.00222 X 19°.17 X 21912.03 feet =              + 932.52  «
                   Height of Balloon, cor. for Temp, of Air = 22844.55 feet = A,

Cor. for Lat. = 0.0028371 cos. 2 L. hl
     = _ 0.0028371 cos. 97°40' X 22844.55 feet.
     = — 0.0028371 X 0.1334097 X 22844.55/ee< =         — 8.65  "
             Heightof Ball. cor. for Temp, of Air and for Lat.= 22835.90/eei = A0


Cor. for Altitude = + h° ~*~ 52252 h.
    J             ^  20886861   °

            22835.90+52252
       = +---2088686T~~X 22835.90/«* =            + 82.09  "
              Height of Balloon above Barometer at Paris = 22917.99 feet = h

Add height of Barometer at Paris, above level of sea,              159.78
                   Height of Ball, above the level of the sea, = 23077.7 7 feet,
                                                           or 4.37 miles.

  By  the aid of Logarithms these calculations are considerably facilitated.
                                    69 
70
BOYE'S INANIMATE MATTER.
  97. To obtain the same differences in level in French metres, the constant
number for multiplying the difference of the logs, of the two true barome-
tric pressures (being in this case generally obtained in millimetres) is 18336.
The expansion of the air by heat and by the addition of vapors, being for
every degree  Centigrade 0.004, the correction for  temperature is 0.004
t_i_t       2 (T+T ")
~-^ htl=   10q0     hiv to be added for temperatures above, and de-
ducted for temperatures below 0°, as  indicated by the sign of (T + TJ ,
the correction for latitude is of course the same,  and that for altitude
     A.+15926           ,  „    , . .
is + ~~fiQfiflOf)n ^0) *ne whole formula being
h = (log B — log b) X 18336 metres X
                                                  2 (T+TJ
                                              1"+"  1000
                                              1 + 0.0028371 cos. 2 L
                                                   A+15926
                                              1+  6366200
  To obtain these different heights in metres almost entirely by the aid of
Tables, the reader is again  referred to Meteorol. Tables by Guyot, published
by Smithsonian Inst.   To facilitate the conversion of French  metres into
English feet, and of English feet into French metres, Table VI will be found
at the end of Pneumatics.
  98. By calculating  in the above manner the height corresponding to a
barometric pressure of 15 inches, we  obtain the height of about 18000 feet
or 3.4 miles as  that, at which  the density of the atmosphere is only one-
half of its density at the level of the  sea; and as the densities  increase in
the same geometrical progression as the  pressures, it follows that if  we
leave out of consideration  the effect of the rapid  diminution of the tempe-
rature of the atmosphere as we ascend higher, both  the  pressure and the
density of the atmosphere ought to  become  one-half less for  every addi-
tional 3.4 miles.
  99.  For the estimation of the difference in level of two places from the
barometric pressures, only the  most accurate instruments, such  as  the
Levelling Barometer described in 81, figs. 32 and 33, should be used.   As
the barometric pressure of  the atmosphere is constantly changing, it is neces-
sary to observe the pressures at the same moment in both places, for which
purpose, therefore, two instruments are  required, the moments for observ-
ing being indicated by signals or  by chronometers.   Where  this cannot
be done, and the two places are at  no very great  distance  from each other,
the observer  may travel with  his  instrument from the  one place  to the
other, and then immediately back again to  the first station,  and if  any
change has occurred,  take for  this station the average of the two observa-
                                 70 
PNEUMATICS.
71
tions.  If the  two places are very distant from  each other, the average
stand of  the  barometer, derived from  observations for a length of time,
also affords data from which the difference in their level is often estimated.
As the ordinary variations  of the barometer,  leaving  out the  extremes,
which occur only at considerable intervals, rarely exceed even in moderate
latitudes 1£ inch, and become much less as we  approach the equator (93),
observations with  the barometer,  performed on  a single journey over a
mountainous  country, where therefore the differences in the elevations and
the consequent differences in the  barometric pressures are very great, will
afford data sufficiently accurate for an  approximate  estimation of these
elevations; and  the  barometer  is therefore   the  instrument commonly
employed for this purpose, the form  combining the greatest  portability
with sufficient accuracy being  that of Gay-Lussac's, described in 79.   The
Boiling-Point Barometer described  in 87, though  less  accurate,  has
been found to give available results.   The Aneroid and Metallic Barome-
ters, being the most portable of all, have not yet been  sufficiently tested for
such purposes.
   100.  For estimating the true volume of gases, and from it, their weight.
Another use of the barometer, for  which it is constantly required in a che-
mical laboratory, is in estimating the weight of a gas from its volume.   As
the volume of a gas varies with the pressure on it, it becomes necessary,
when its volume is observed for the purpose of estimating its quantity or
weight, to note  the pressure  by which  it is confined, and then  to reduce
the observed volume to what it would be  at a standard pressure, which is
assumed at  29.9218  inches of mercury (760  milimetres), this  being the
average stand of the barometer at the  level of the sea  at 45° latitude (93),
and which number is used for all important estimations, serving as a basis
for other calculations, such as the exact weight of 100  cub.  inch, of air
(57), but for most  ordinary  purposes  30 inches  is  taken as  sufficiently
accurate.  Suppose, thus, that the volume of a gas, confined in a gradu-
ated glass tube by mercury or water contained in a pneumatic cistern (    ),
be found by  the graduation of the tube to be  24 cubic inches, when the
barometer stands at 29 inches, the level of the confining  liquid being the
same inside the tube as outside.  We then have by Mariotte's law (44), that
                                                       1    1
        24 cubic in. (vol. at 29 in.) : x (vol. at 30 in.)::  ^q : inr
                                       29
       therefore:  x  = 24  cubic in.  X on = 23. 2 cub. inches
which is the volume the gas would occupy at the standard pressure of 30
inches.   If the level of the confining liquid should not be the same inside
the tube as outside, but for  instance higher, this column, being supported
                                  71 
72
BOYE'S INANIMATE  MATTER.
by the atmospheric pressure, must of course be deducted from its pressure
on the gas.  Thus, suppose the confining liquid to be water, the volume of
the gas, as before, 24 cubic inches, and the barometer 29 inches, but  the
water inside the  tube  2.9 inches higher than  outside.  By dividing  the
latter by 13.6 (the specific gravity of  mercury), we find this column of
water to be equivalent to 0.21 inch of mercury, which,  deducted from
the observed atmospheric pressure of 29 inches, leaves 28.79 inches of mer-
cury, as the pressure o,n  the gas; 24 cubic inches, at 28.79 inches' pres-
sure,  are then reduced to the standard pressure of 30 inches as above, by
multiplying by the former (the observed pressure), and dividing by  the
                                          28.79
latter (the  standard), = 24 cubic inches X  ~Wn~ =23.032 cubic inches.
In the latter case, however, where a gas is measured over water as confining
liquid, the thus obtained volume includes the portion of vapor of water,
which is always formed by evaporation and adds its volume, which depends
on the temperature,  to that  of  the gas.  To avoid  this error, it  is only
necessary, in reducing the observed volume to the  standard pressure, to
deduct from the atmospheric pressure also that portion  of it, which  is sus-
tained by the tension of the vapor, and which is obtained by taking from
Table IX, the  maximum tension of vapor of water  corresponding to  the
observed temperature of the gas.   Thus, suppose in the above case,  the
temperature of the gas to be 79° Fah., we then find from  Table IX that
the maximum tension of vapor of water corresponding to this temperature,
is 0.99 inch.   From the whole  pressure of the atmosphere, 29 inches,  we
then  deduct, not  only as before, the portion sustained by the column of
water above the level outside, equivalent to 0.21 inch of mercury, but also
that, sustained by the tension of the vapor,  0.99  inch, which thus leaves
only 29 — 0.21 — 0.99 = 27.80 inches as the real pressure on the gas.
The volume of this, without the  vapor of water, at 30 inches, will there-
                       27.80
fore be = 24 cubic in. X   m  =22.24 cub. inches.
  This volume must then also be reduced to the standard temperature (see
Thermics, under Expansion of Gases), which is assumed in England at 60°,
but in  most other countries at 32°.  This is done by multiplying  the
volume  of the gas by  1 + 0.002178 X Stand.  Temp.,  and dividing it
by 1 + 0.002178  X Obs.  Temp*  Thus, for the  above 22.24 cub. in. of
  * If 32° Fah. be adopted as the standard Temp., the reduction to this from any higher
degree t is more conveniently performed by dividing the volume by 1 + 0.00203611 (« —
32°); the coefficient of expansion for 1° Fah. referred to the volume at 32° as unit being

0.00203611.  Thus, in this case: 1+0.002^11 (V^-32°) == 20-298 cub- inc^s,
                                 72 
PNEUMATICS.
73
<jt.
79° temperature, we have its volume at the standard temperature of 32°
   m,(    ,  .      1 + 0.002178x32°
= 22.24  cub. in. X i+q.002178 X 79° = 20-298 cub" m"
For the true volume V, of a gas, we thus have the following formula,
                              b_   1+0 002178x7*
                    V—V1XBX i+o.002178 X *
V1 being the  observed volume; 6 = the  true (77) barometric pressure,
with deduction, if necessary, for any inequality in the level of the confining
liquid and for admixture of vapor of  water;  B = the standard  barometric
pressure  to which it is to be reduced; t = the temperature of the gas;
T= the  standard temperature to which it is to be reduced;  and 0.00217802
<= the expansion for 1° Fah. referred to the volume at 0° as  unit.
   Having thus reduced the volume of the gas to the         Fig. 40.
standard  pressure  and temperature, its weight is then
easily obtained, if it be atmospheric air, by multiplying
the number of cubic inches  thus  found, by the weight
of 1  cubic inch of atmospheric air of the same stand-
ard pressure and  temperature, and which has been
given in  57.   If the gas be any other than atmospheric
air, we obtain its weight by multiplying the weight,
thus found for  atmospheric  air, by the specific gravity
of the gas, referred to atmospheric air  as a unit, see
58.  Thus, if the above  20.298 cub. in. be Nitrogen,
we have  its weight:
      =  20.298 cub. in.  X  0.325868 grs. X 0.97137
= 6.425 grains.

        Experiments to prove Mariotte's Law.
   101. Having now become familiar with the pressure
of the atmosphere and the means of estimating it, we
may again revert to the  compressibility and elasticity
of gases, and  describe the experiments, by which  the
law already stated in  44 was  established by its dis-
coverer, Mariotte,  after whom it has been called Mari-
otte's Law.   He  enclosed a quantity of air in a tube
bent as the letter J, or as it is technically termed, in
the shape of an inverted syphon, see fig. 40, the short
limb of which was sealed and graduated into volumes,
but the long one  left open and furnished  with  a scale measuring inches.
Mercury  was then poured into the open end, so as to fill the  bend to 1,
                                 73                           7
30- 
74
BOYE'S  INANIMATE MATTER.
thereby enclosing a certain volume of air in the short limb, without its stand-
ing with a higher level in the open limb.  Under these circumstances, the
enclosed air, the volume of which we will call 1, is only under the ordi-
nary atmospheric pressure, say 30 inches of mercury.   More mercury was
then poured gradually into the open limb, by which the air in the closed
limb became more and more  compressed.  The height of the mercury in
the open  limb, above its level  in  the closed  limb, was then carefully
observed, and compared with the corresponding volume of the air in the
closed  limb itself.   It was thus found, that when the air was reduced to
% of its original volume, the  height  of the mercury in the  open  limb
above its level in the short limb, from a to a, measured 10 inches, to which
must be added the ordinary atmospheric pressure of 30 inches, in order
to obtain the whole pressure on the gas, making it equal to 40 inches of mer-
                  Fig. 42.    cury or i j _-_| Atmosphere's pressure (61).
                            More mercury was then poured into the open
                            end, till the volume of  the air was reduced
                            to  I,  when the  height  of the mercurial
                            column, from b to bv producing this effect,
                            was found to  be  30 inches, or  1 Atmo-
                            sphere, which, added to the pressure of the
                            atmosphere itself, made the pressure on the
                            enclosed air 2  Atmospheres.  In the  same
                            manner, the column c'clf when  the  volume
                            was reduced to }, was found to be 90 inches,
                            which  being 3 Atmospheres, added to the
                            pressure of the atmosphere itself, made the
                            pressure on the gas 4  Atmospheres.   The
                            different volumes of the air were thus found
                            to be as 1: |: J: \, while the pressures corres-
                            ponding to them, were as 1 Atmos. : * : 2 : 4,
                           that is, the volumes occupied by the air were
                           inversely proportional to the pressures on it.
                              102.  To prove the same  law for smaller
                           pressures than one Atmosphere, a graduated
                           straight tube, see fig. 41, open at its lower
                           extremity, and furnished with a screw-stop-
                           per at its  upper extremity, is immersed with
                           its open end into a deep glass jar containing
                           mercury, until only a certain known volume
ol air is left at its  upper end.  This volume we will call 1.  The tube
                                 74 
PNEUMATICS.
75
being yet open, and the mercury having the same level inside and outside,
this volume of air must of course be under the same  pressure as the  rest
of  the atmosphere, that  is, under 1 Atmosphere's pressure.  The tube is
then closed and raised out of the mercury, until the volume of the enclosed
air is increased to double its  former  volume, see fig. 42.  The mercury
will then be  found to stand much higher inside the tube than  the level
a outside it in the jar.  This height, from o to 2, is then measured,  and
will be found  to be 15 inches, which, being supported by the atmosphere,
must of course  be deducted from the ordinary atmospheric pressure of
30  inches, in order to obtain the pressure on  the gas in the tube, which,
therefore, will be 30 — 15 = 15  inches of mercury, = J Atmosphere.
The tube may then be raised still higher out of the mercury,  until the
enclosed air acquires 4 times its original volume, when the height of  the
mercurial column, raised above the level outside, will be found to  be  22 £
inches, which deducted from the atmospheric pressure of 30 inches, leaves
of this only 7£ inches or  i Atmosphere, as the pressure on the gas.   We
thus find in these experiments, the volumes of the enclosed air to be as 1  :
2 : 4, while the pressures are as  1 Atmosphere :  $ : J, or, as before, the
volumes  are  inversely proportional to the pressures.
   103.  A tube similar  to any of the above,  closed  at one end, and con-
taining a portion of air  confined by mercury, is often designated by the
name of a Mariotte's tube.
  104.  The above experiments have since been extended with atmospheric
air  from  3^ Atmosphere's pressure to that of 27 Atmospheres (139)  and
more, and Mariotte's law confirmed to this extent.  But it has  also been
found, that this law strictly applies only to permanent gases, and to such lique-
fiable gases as are remote from their point of liquefaction, but that as soon
as they approach the latter, their volume  will diminish by increased pres-
sures in a somewhat greater ratio.  This has been found to be the case
with Sulphurous acid and several others.   In the same manner, even Car-
bonic acid, if cooled to 32°, has been found to expand by diminished pres-
sures more than it ought according to Mariotte's law, or more than atmo-
spheric air does.   This is probably also the reason,  why most compound
liquefiable  gases and vapors are found, by experiments, to have  a some-
what greater specific gravity than that calculated from the volumes of their
component ingredients.

                           Pressu re- Gauges.
  105. Instruments on  the principle of the Barometer or Mariotte's tube,
are often used for measuring the tension and elasticity of gases, or the pres-
sure which they exercise  when  confined (see 117).    Such instruments
                                  75 
76
BOYE'S INANIMATE MATTER.
Fig. 43.
are called Pressure-Gauges, sometimes Manometers (see note to 91).  Fig.
43  shows,  on an  enlarged  scale, the Mercurial  Exhaustion-Gauge,  m,
              attached to the double-barrelled Exhausting Air  Pump,
              fig. 6, to indicate the quantity of air remaining at any time
              during the exhaustion, by the tension or pressure which it
              exercises, and to which its quantity is proportional.   It will
              be seen, that it is  an abridged or shortened syphon baro-
              meter, which is enclosed  in a small separate  receiver, con-
              nected with the passage  leading from the barrels a and 6
              fig. 6 to the large receiver h.   From an inspection of fig.
              43, it will easily be seen,  that the closed limb, being only
              12  inches long, will  exhibit no  Torricellian vacuum, but
              remain filled with mercury, to the top, until the  tension or
              pressure, which the air in the receiver is capable of exer-
              cising on the mercury in  the open limb, is  reduced to  12
              inches of mercury, and therefore  the density of  the air is
              only | § or | of its original density, J having been removed;
              after which all further rarefaction will be indicated by it,
              the amount of air remaining at any time, being  given as a
              fraction, which has for its numerator the mercurial column
              sustained by it in the gauge, and which is measured by the
              perpendicular height between the levels of the mercury in
              the two limbs as stated in 65, and for the denominator the
              whole atmospheric pressure as indicated  by the  barometer
              at the time, and which may be assumed at 30 inches.  Thus,
              when the gauge indicates 10 inches as in the figure,  the  re-
                                   of its original amount, and when the
1		i_
1___;		3_
l"|i		2
1		1
— l/_ 1 12. I r-U_ Is. = Ik 1 11—5	-I	
maining air is -£-g
3 Off*
gauge indicates  ^ inch, the remaining air is on
  106. For measuring pressures larger than  the ordinary atmospheric
pressure, Mercurial Pressure-Gauges receive the forms represented in figs.
44 and 45.   Fig. 44 has the general form of a cistern barometer, but the
cistern c containing the mercury is closed air-tight at  the  top  and made
to communicate with the vessel,  in which the  gas  is confined, by a small
tube, passing from the top, or as a fig. 44, through the bottom  to above
the level of the mercury.  The tube b is open at the upper end, and the
pressure, therefore, estimated by the  height of the column of mercury,
which  is forced  up in it, for which purpose it is  furnished with a scale
measuring inches, 2 inches being equivalent to 1 pound on the square inch.
Fig. 45 exhibits another pressure-gauge, which is easily constructed out of
                                 . 76 
PNEUMATICS.                         77
Fig. 45.
■I5vw
fr^\
a glass tube by bending it twice.  The pressure is measured by the difference
  Fig. 44.             between the two levels of the mercury in  the two
                     limbs  (65).   For measuring very small pressures,
                     such as that  under which ordinary lighting gas is
                     forced through the burners from the pipes, it is made
                     to contain water instead of mercury, in  which case
                     for great accuracy the tube should be \ inch in diam.
                     and  each limb furnished with  a vernier.  As  the tube
                     of this kind of pressure-gauges is open  towards the
                     atmosphere, and  the mercurial column in it, there-
                     fore, subject to the  atmospheric pressure, it is neces-
                     sary, in order to obtain the whole tension or elasticity
                     of the confined gas, to add to the above pressures indi-
                     cated  by the  mercurial column in the gauges, the
                     ordinary  atmospheric  pressure,  but this is  often
                     omitted, and  the pressure only given as being over
                     and above the outer atmospheric pressure.
                        107. The above mercurial gauges, in which the
pressure is measured by the height of the  column of mercury, which it can
sustain, are the most reliable of all, but they have the serious inconvenience,
that when the pressure becomes large, for instance in high-pressure steam-
boilers, where it often exceeds 60 pounds to the square inch, or 4 Atmospheres,
the tube must be more than 4 X30 in. =10 feet long (see 139).  For such
  Fig. 46.    high pressures Condensed Air or Mariotte's Tube  Gauges are
            often substituted, acting on the principle of estimating the pres-
            sure from the  volume of a confined portion of air.  Any of the
            above gauges^s. 44 and 45 may be converted into such by closing
            the upper end of the tube, so as to confine the portion of atmo-
            spheric air which is  contained  in  it,  which volume is  then
            divided into fractions.   Fig. 46  exhibits a gauge of this kind,
            such as is used by gas-fitters to prove by high pressure the
            tightness of gas pipes.  For  small pressures the tube is left
            open at the top,  and it then acts as  one of the above-described
            mercurial gauges.  When  used, it  is  screwed  on  the end of
            one of the pipes, into which  air is forced  by a forcing pump.
            Any leakage is indicated by the gradual diminution of the pres-
       For convenience  in the making of  it,  the cistern c is made  of
       but as this is corroded  by mercury,  the latter is contained in  an
iron cup i, placed inside.  The cover into which the tube b is cemented,
is made to screw on air-tight.  The compressed air, the elasticity of which
                                 77
-13dm
. / /en-
sure
brass; 
78
BOYE'S  INANIMATE MATTER.
 we want to measure, finds its way between the cup i and the inside of the
 cistern c, so as to press on the top of the  mercury, which, being forced up
 into the  tube  b closed at the upper end, will compress the atmospheric air
 which it  contains, from  the volume  of which the pressure is ascertained.
 Thus, when its volume is reduced to \, the pressure on it is 2 Atmospheres,
 or 1 Atmosphere over the ordinary atmospheric pressure;  when compressed
 to  \, the pressure is 3  additional Atmospheres  over the  ordinary atmo-
 spheric pressure;  when  compressed  to \, 7 additional Atmospheres.   To
 these pressures must, however, be added,  in order to find the  pressure or
 elasticity  of the confined gas, which we want to measure,  the  column of
 mercury inside the tube above the level in the cup i.   Thus, if the height
 of this be 6 inches, when the volume is $,  the elasticity of the confined gas
 is -36a = I Atmosphere more than indicated by the volume of  the  air in
 the tube,  or altogether 1+' Atmosphere,  = 36 inches of mercury, or  18
 pounds to the  square inch;  if 9 inches, when the air is  compressed.to J,
                the whole pressure is 3+g9^ Atmospheres; if 10£ inches,
                                          10J
                when compressed to |,  7+y7p = 7 §£ Atmospheres, &c.
                It is a matter of course, that if the  temperature be  not
                constant, its effect on the confined air  in the tube must also
                be taken into consideration, by first reducing its volume to
                the  same temp., as in 100.
                  108.  Condensed  air  pressure-gauges,  besides  being
                considerably affected by  the temperature, have  also the
                great objection, that  as the pressure increases, and it  in
                many cases  becomes important to estimate it with increased
                accuracy, the divisions of the scale,  corresponding to the
                same increase in pressure, diminish  very rapidly in size,
                and thus become less accurate.  This  latter  may, how
                ever, be partly remedied by furnishing the gauge with two
                tubes, see fig. 47, as first  contrived-by Br. J. K. Mitchell
                in his experiments on the liquefaction  of  carbonic acid.
                The second tube b is enlarged at  the end which dips into
                the mercury, by being cemented  into a  short iron tube d
                of larger diameter, which forms its lower extremity and the
                capacity of  which is such, that the  mercury only enters
                the glass tube ate, when the pressure approaches that which
                we particularly want to measure.   Thus, suppose that the
               mercury in  d only reaches to e, when the ah" in a is com-
pressed to  | its  original volume, and that then the mercurial column in it is 36
t-
 
PNEUMATICS.                          79
inohes above the level at c.   The  pressure  measured  will then  be 9?
Atmospheres.   Deducting from this the column, from c to e, the pressure
On the air in the tube b will be exactly 9 Atmospheres.   If this volume of
the tube above e be divided into fractions, it is evident that when  the
enclosed air is  reduced to £ of this volume, the pressure on  it will be 18
Atmospheres, and when reduced to £, 36 Atmospheres;  to which, of course,
in order to obtain the pressure we want to measure, must be added the mer-
curial column beyond e.
   109.  For experiments on a  small scale, as for the compression of gases
in glass tubes, a capillary tube  of the proper length, see a b fig. 48, is
           Fig. 48.            employed  as  a  gauge,  having no cistern.
                %    % k-tck. Being  closed  at one end  at a, a  small
 & c                        column  of mercury  c is  introduced  into
the other open  end b,  by expelling from it, by heat, the smallest possible
quantity of air, and then  dipping  the open end into mercury,  till on
cooling a small quantity of this  is  drawn into  it (38), which  then con-
fines the air remaining in the tube.   The space ac occupied by this air is
then  divided as before into fractions of its own volume.  When using it,
the open end b is either cemented into a metallic socket, which is screwed
on to the end of the tube in which the gases are compressed, or in some cases
the whole gauge-tube may be  slipped  into the compression-tube, in which
case no strength is required of its  sides, and these may therefore be of any
thinness, and the whole gauge, therefore, of miniature dimensions.   If this
gauge be in a horizontal position, no allowance whatever, need be made for
the weight of the mercurial column c; and the volume of the confined air,
therefore, indicates the whole  tension of the gas which we want to measure.
   110. Gauges for measuring high pressures are particularly required for
hi"h-pressure steam-boilers, to indicate at any time with accuracy the ten-
sion or elasticity  of the  steam,  and thereby to warn  against  accidents.
Such gauges are  called Steam-Gauges, sometimes also Manometers, see
foot-note  to 91.   Besides the before  described  pressure-gauges,  many
others have  been  constructed for steam-gauges on  different principles.
Thus, the principle of Bourdon's Metallic Barometer (89), was first employed
for a steam-gauge, by admitting the steam into its hollow hoop-like vessel.
In the same manner an accurate thermometer will indicate from  the temp.
of the steam, its pressure, see 138 &c.   A number of steam-gauges act on
the principle of letting the steam  act on a metallic valve, so as to compress a
spring (Spring-Gauges), or raise a known weight.   These are, however, not
so much for the purpose of measuring the pressure of the steam as for afford-
ing  escape and safety from it, when  its elasticity should exceed a  certain
limit, and they are therefore called Escape or Safety Valves, see \±§fig. 70. 
80
BOYE'S INANIMATE  MATTER.
         Experiments to illustrate the pressure of the Atmosphere.
   111.  The  atmospheric pressure on the surface of liquids, may be illus-
trated by the Fountain or Jet in Vacuo,  see fig.  49,  which  consists of
       Fig. 49.                    a closed receiver, which is furnished
                                 at its lower extremity with  a stop-cock
                                 c, from which a jet projects into  the
                                 receiver, terminating outside by a screw
                                 s, by which it  may be attached to
                                 the  air-pump.   Having exhausted  the
                                 receiver, it  is  detached from the  air-
                                 pump, and the mouth of the stop-cock
                                 immersed into a vessel containing water.
                                 On  opening  the stop-cock  the atmo-
                                 spheric  pressure will force  the water
                                 in a jet into the exhausted receiver.
                                   112.  The  Mercurial  Bain, see fig.
                                 50,   is used  to  illustrate the  same in
                                 connection with the porosity of certain
                                 substances such as wood, leather,  &c.
                                 It consists of a receiver having inserted
                                 in  the  top  a cup c, which is  closed
                                 at the bottom by a stopper of wood  cut
                                 across the grain, or by a piece of buck-
                                 skin, and which contains mercury.   On
                                 exhausting the receiver, the atmospheric
                                 pressure will force the mercury through
                                 the  pores in small globules as  a rain,
   113.  If a piece of thin bladder be  tied over the top of a  small wide-
mouthed  receiver,  the Bladder  Glass,  see fig.  51, and the air then
    Fig. 51.     quickly exhausted, the atmospheric pressure will burst  the
               bladder inward with a  loud report as from an explosion.
                  114. The pressure on any part of an elastic  fluid being
               equally communicated  to all parts of it, it  is evident that
               the pressure which it  in return exercises on all the con-
               fining  limits  must be  uniform, and must, therefore, also
               extend to the whole surface of any  object immersed in it.
               The direction of  its pressure at any point of all such sur-
faces, is always in the perpendicular to them at that point.   The Upward
Pressure of the atmosphere on an under surface,  may be illustrated by
                                 80
 
PNEUMATICS.
81
a syringe with a solid piston, see fig. 52.   Having drawn the piston out and
     FvJ- 52-     attached a weight  to the piston-rod,  suspend it, and con-
      L^S^^T nect the upper extremity of the barrel by the tube a with
                an exhausting air-pump.   When the air is exhausted from
                it, the atmospheric pressure, acting perpendicularly upward
                on the lower surface of the piston, will force it up, thereby
                raising the weight attached to the piston-rod.
                   115.  The same may be illustrated by the Magdeburg
                Hemispheres, which are two hollow hemispheres, see a and
                b fig. 53, having their edges ground true, so as to fit air-
                tight together, thus forming a hollow sphere.  One of them
                is furnished with a handle, and the other with a stop-cock
                and a screw c, by which it may be attached either to an
                air-pump, or to a haudle  d.  If the  two hemispheres be
                put together, and  the air inside exhausted, the pressure
                of the atmosphere outside will force  them  together, so
                that if they be removed from the air-pump, and the handle
                attached, it will  require a considerable force to separate
                them.   To calculate the exact force with which they are
                held together, it  must be remembered, that though the
                whole atmospheric pressure on them is equal to 15 pounds
                on each square inch of the whole outer surface, it is only
                that portion of it which acts at right angles to the plane
                of the joint, which holds them together.  Thus, if the
                radius of the sphere be 2 inches, the plane surface of the
              s circular joint (r2 tt) will  be = 22 X  <H  = 12|  square
                inches,  and the  pressure  on it, therefore, 12|  X 15 lbs.
             = 1881 lbs.   To pull them apart, this force must,  therefore,
             be applied  from each  side.  They have received their name
             from  the  fact, that they  were first  contrived  by  Otto von
             Guericke,  Burgomaster of Magdeburg, a town in  Germany,
             who in 1650 had  invented the Air-Pump.   To  illustrate
             the Atmospheric pressure, he exhibited, in 1654 at Regens-
             burg, to the Emperor Charles V, in presence of the Imperial
             Diet,  a pair of these of about two feet in diameter, to which
             twenty-four horses were attached, without their being able to
             separate them.
                116.  The external  surface of the human body being about
             2000  square inches, it is evident  that  it must be exposed tr«
a pressure from the atmosphere, of about 30.000 lbs., or nearly 14 tons.  That
      F                           81
Fig. 53.
 
82
BOYE'S INANIMATE  MATTER.
this  pressure does not force in the Abdominal and Thoracic cavities of the
body, is prevented by the access of the atmosphere to them, by which the
external and internal pressures are counteracted.  The solid walls of the body
forming them, are, however, subject to it; these and the internal organs
are, however, prevented from being crushed by it on  account of the uni-
formity of the pressure, by which the particles, being pressed equally on all
sides, have no tendency to change  their relative position, crushing being
merely produced by an unequal pressure.  This is also the reason why we
are not conscious of its existence.   It may, however, easily be made mani-
fest by removing the pressure from any part of the body, for instance, by
          Fig. 54.          placing the hand over the mouth of a small
                          receiver, see fig. 54, and  exhausting  the air
                          from within it;  the pressure on  the opposite
                          side of the hand will then force it against  the
                          edge of the receiver and cause those parts, from
                          which  the  pressure is removed, to bulge  into
                          it.   The operation of cupping depends on  this
                          same, for if small cuts be previously made
                          through the skin, the  blood will be forced out
                          through  them by the pressure on  the  rest of
the body.   In such places, however, of the  body,  where the parts are not
soft or permeable to fluids, this  pressure is  used by nature to sustain  and
keep  together its different parts, without  calling into  requisition for  this
purpose the power of the muscles.   Thus  all the movable joints of  the
body are kept together by the articulating surfaces of the bones being sur-
rounded by an air-tight ligament,  so that they may slide freely over each
other, but cannot  be separated without producing a vacuum,  and are thus
forced together by the atmospheric pressure, amounting, for instance, on
the knee-joint to upwards of 100 lbs.   By actual  experiment, by  dissect-
ing away from the hip-joint every  thing excepting the capsular  ligament,
and suspending  it under  a pneumatic  receiver with a  weight attached to
the thigh-bone, this has been found to drop  out of the socket, on exhaust-
ing the air from the receiver, but  to return again into it, on re-admittino-
the air  The excessive fatigue experienced in ascending high mountains,
has been ascribed to the diminution of the atmospheric pressure, by which
the weight  of the limbs  has  to be in part supported by the muscles,
instead of by the atmospheric pressure alone.

Experiments to  illustrate the Expansibility, Elasticity and Compressi-
                      bility of Atmospheric Air.
   117. Expansibility and Elasticity of gases both depend on  the  same
                                  82
 
PNEUMATICS.
83
repulsive  action between the atoms, which we have  called negative cohe-
sion (12), and which  causes them to have a constant tendency to extend
their volume and  thereby to exercise a certain pressure on  the  confining
limits, which these must return, in order to restrain them; and as soon as
this restraining pressure is diminished or ceases, it causes them actually
to extend their volume.   They are therefore in fact the same property, but
the word elasticity is only applied to their expansive force after a previous
diminution of their volume, by an increase of the pressure of the confining
limits, while  for their expansive force  under  the ordinary atmospheric
pressure, or after its diminution,  the word  tension is generally used.
Fig. 55.
Fig. 56.
                                          118. The Expansibility of atmo-
                                       spheric air is illustrated by forcing
                                       the greater portion of  the air out
                                       of a sound bladder or small gum-
                                       bag by compression,  and then
                                       closing the  orifice by  tying  it
                                       firmly with  a  string.    Place  it
                                       under a receiver as in jig. 55.  As
                                       soon as the  air is exhausted from
                                       the receiver outside the bladder,
                                       the  small  quantity of  air con-
                                       tained  inside it will expand, and
                         swell the bladder out, as seen in fig. 56.   When
                         the air is again admitted into the receiver, the
                         bladder  will collapse  to its former dimensions.
                         The  same  experiment will often succeed with
                         dried and shrivelled fruit, as raisins, which, if the
                         skin be sound, will, in a similar manner,  be blown
                         out to their original fullness by the small quantity
                         of air  which they contain.
                           119. Mechanism  of Respiration.   It is by a
                         similar contrivance that air is made to enter into
                         the lungs by respiration.  The lungs may be/Hon-
                         sidered as two membranous bags, only divided into
                         a number of smaller compartments or cells, but
                         all communicating with each other by the bron-
                         chial ramifications, through which  the  air may
                         enter  into them by way of the mouth  and  the
                         windpipe;  the whole apparatus being  suspended
in the cavity of the chest, as may be represented by the bladder a fig. 57,
                                  83
 
84
BOYE'S INANIMATE  MATTER.
 attached to the pipe b and fixed in the receiver h.  The expiration is effected
 by diminishing the cavity of the chest by the contraction of the ribs and
 the raising of the diaphragm, by which the air, in consequence of its elasticity,
 is forced out through the windpipe by compression.   This may be imitated
 by blowing the bladder out through the  pipe b and closing  this with  the
 finger, until the  mouth of the receiver be immersed into a vessel e e, with
 water.  On removing  the finger and  depressing the receiver further,  the
 air will be forced out through the pipe b, as represented in fig-. 57.   The
 inspiration, on  the  contrary, is  effected  by enlarging  the  cavity of  the
 chest by expanding the ribs and  flattening  the diaphragm, by which a
 vacuous space is produced between the inside of the chest and  the mem-
 brane of the lungs, by which the  air, in virtue of its  expansibility, will
 enter and inflate them.  This  operation may be imitated with  the above
 apparatus  by gradually drawing the receiver h again out of  the water,
 thereby enlarging  its capacity and producing a partial vacuum.  The  air
          Fig. 58.         then enters by its expansibility through the pipe
                         b and inflates the bladder as in fig.  58.
                            120. Common water  freshly  drawn  always
                         contains more   or  less  air  in  solution (55).
                         When the pressure is removed from  its surface
                         by  placing  it in a tumbler  under a receiver and
                         exhausting the  air, the expansibility  of the dis-
                         solved air will overcome  the adhesion, by which
                         it is kept in solution, and most of it will appear
                         as  small bubbles on the  sides of the vessel and
                         escape through the water.
                            121. Place a piece  of charcoal  or any other
                         porous body in a tumbler filled with  water, and
                         this under a receiver, and  exhaust  the air from
                         the latter.   The air contained  in the pores of the
                         charcoal will expand and escape in bubbles
                         through the water.   On  readmitting the air, the
                         atmospheric  pressure will  force the  water into
                         the pores, which thus will become  filled  with
                         water  instead  of  air.   This  method  is  often
 employed  to fill the pores of other porous bodies with water.   If the pores
 of wood be filled in this manner with water  instead of air, it will become
 water-logged and incapable of floating.
y" 122.  Hero's Ball is  an apparatus used  to illustrate the compressibility,
 elasticity and expansibility of atmospheric air.  It  consists  of a strong
                                  84
 
PNEUMATICS.
85
    Fig. 59.        generally spherical vessel, a fig. 59,  having a tube in-
                  serted at the top,  reaching nearly, though not quite, to
                  the bottom, and furnished outside with a stop-cock and
                  screw for attaching a jet i.  A quantity of water suffi-
                  cient to close the end c of this tube,  is introduced into
                  the vessel, either by unscrewing the tube, or by remov-
                  ing a portion of the air from  it by suction,  and then,
                  after having inverted  it  and immersed the jet  of the
                  tube into water, opening the stop-cock, when  the atmo-
                  spheric pressure will force in a sufficient quantity of the
 latter (111).   Bemove then the jet and attach to it a condensing syringe
 (41).   By every stroke of  the piston, the air forced into  the vessel will
 be seen  to  bubble  through the water.  Having closed  the  stop-cock,
 remove the condenser and replace the jet.   On turning the stop-cock, the
 elasticity of the compressed air will force the water out in a jet.  Having
 replenished, if necessary, the vessel  with water, place  it under  a receiver
 and exhaust the air, see fig. 60.   By thus  removing the pressure  of the
         Fig. 60.         atmosphere  on the water inside the jet, the  expan-
                        sibility of the atmospheric air enclosed in the ves-
                        sel will force the water out in a jet.  Hero's ball
                        is  so called from its  inventor,  who lived in Alex-
                        andria, and  described this  apparatus about one
                        hundred and twenty years before  the  Christian
                        era.
                           123. Contrivances acting on  the same principle
                        as  Hero's  ball,  and  called  Air-Chambers, are
                        attached to most hydraulic engines, such  as the
                        Fire Engine and the Hydraulic Ram, in order to
                        convert the  intermitting jet of these into  a con-
                        tinuous.   The  air-chamber consists of a strong,
                        more or less spherical vessel of metal, at the bot-
tom of which the water is forced in through a valve faster than it issues from
the jet, which may either  pass from near the bottom  through the  top,  as
in Hero's ball, or, as is  more common, from the side near the bottom.  The
air enclosed in the chamber is thus compressed and, by its elasticity, forces
the water out in a constant stream.

                     Imjmct and Inertia of Gases.
   124.  Gases, like all other matter, possess Inertia ; hence the atmosphere
offers a resistance  to all bodies moving in it, because these have to impart
                                  85                           8
 
86
BOYE'S INANIMATE  MATTER.
to it by impact some of their motion, in order to move it out of their way.
For  this reason, bodies which present a large surface,  lose their  motion
sooner than those which present a smaller,  or  one of a more favourable
shape.  In the same manner, specifically light  bodies  lose their  motion
sooner than heavy bodies,  which within the same space contain  more
moving matter and, therefore, more motion.   This may be illustrated by
the Windmill experiment, which  is performed by an apparatus, see fig. 61,
           Fig. 61.          having two axes i i, perfectly alike, and fur-
                           nished with small pinions, that are worked by
                           two perfectly similar racks r r, attached to
                           the same weight w, so that the latter by its
                           descent imparts exactly the same velocity to
                           them both.  At right angles to each of the axes
                           i i, are attached four perfectly similar wings,
                           which may be turned so as to present either
                            their broad surface or their edge to the air,
                            when the axes revolve.  Place first the wings
                            of  both axes, so as to present the broad sur-
                            face  to the air, when revolving.  Let then
                            the weight drop so as to impart to them both
                            the same velocity.  They will both  stop at
                            the same time, and soon,  on account of the
                            resistance of the air.   Turn then the wings
                            of  one axis so as to present the edges to the
                            air, and start them again by the descent of the
                            weight.   The  one  with  the wings turned
                            edgeways will then continue  its motion much
                            longer than the other.  But if the apparatus
                            be placed under  an exhausted receiver, and
                            the weight again  made to  descend by the
rod g g, passing through a stuffing-box s at the top  of  the receiver,  both
axes will  be found to continue their  motion equally long in the  vacuum,
although the wings of the one are turned differently from those of the other.
   125. The resistance of the air by its inertia, is the cause why specifically
lighter bodies fall in the atmosphere slower, than heavier.  In a  vacuum
all bodies fall  equally fast.  This may be illustrated by the Feather and
Guinea experiment, fig. 62, which is performed by a tall receiver h,  contain-
ing several drop-stages dd d, on  one of which is placed a gold-piece, and on
another a feather.  The air being exhausted, these  are allowed  to begin
their descent  at the same time,  by allowing the stages  to drop simultane-
                                  8G
 
PNEUMATICS.
87
ously by the rod g, passing through  the stuffing-box s.   If the air is well
                        exhausted, they will both reach the bottom at the
         FiU- 62-         same time, showing that it is the resistance of the
                        air, which causes the feather and specifically lighter
                        bodies in general to fall slower than heavier ones.
                           126. The resistance on a  sphere  of  5 inches
                        diameter, falling through the  air, has been esti-
                        mated to be 1.211 oz., when it acquires  the velo-
                        city of 30 feet per second.  But this resistance
                        is increased in  a much greater ratio than  the
                        velocity of the moving body, it being proportional
                        to the squares of the velocities  (See under Stereo-
                        Dynamics).   For  this  reason  rain-drops,  hail-
                        stones, and all kinds of projectiles, such as musket
                        and cannon balls, have all a maximum velocity in
                        the air, which they cannot exceed.  But the larger
                        their size, or the greater the specific gravity of the
                        material of which they  are made, the greater is
                        the  velocity that can be  given to them.   Thus, a
                        bullet of lead is capable  of a greater velocity than
                         one of iron.  The flight of birds depends on this
                         same increase in the resistance of the air, the mo-
                         tion of their wings being performed,  in one direc-
 tion, both with greater surface and with greater velocity, than in the other.
   127.  Air which  thus receives  motion by impact or  otherwise, will by
 the same  property of inertia continue  its motion, on which the operations
 of fanning and  blowing depend, until it in  its  turn is  checked by some
 other cause;  for instance, by  striking against other  air, or against im-
 movable objects on the earth.  The  performance of  windmills  and the
 sailing of ships depend on motion received by  impact from  moving masses
 of air, which constitute winds.  The power of winds increases in the same
 augmented ratio of the squares of their velocities, which are stated to be as
 follows:
	Vel. in
	miles
	per hour.
Gentle breeze.	. 3.25
Pleasant breeze.	6.5
High wind.	. 16.25
Storm or gale.	32.5
Great storm.	. 56.29
Hurricane.	79.61
Tremendous hurricane. .	. 97.5
Vel. in	Inch, of	Pressure on
ft. per second.	water supported.	a square ft. in lbs. Avoir d.p.
4.77	0.01	0.83 oz.
9.53	0.04	3.33 "
23.83	0.25	1 lb. 5 oz.
47.66	1.	5 " 3 "
82.56	3.	15 " 9 "
116.76	6.	31 " 3 "
143.00	9.	46 " 12 "
87		
 
88
BOYE'S INANIMATE  MATTER.
  128.  The direction of the wind is generally ascertained by the vane,
but when  feeble, by a suspended  silk ribbon, or an ascending column of
smoke;  and sometimes  also by the cold  experienced  on the finger, when
moistened and  held up  to the air.   The force of winds is estimated  by
instruments called Anemometers, the best of which are constructed on the
principle of the pressure-gauge (106) jig. 45, being made of large diame-
ter and  containing water instead of mercury, having also the limb, acted
on,  horizontal, so  as to turn it against  the  wind.  But those  generally
adopted as the most convenient in  meteorological observatories,  are made
on the  principle of spring-gauges, exposing a  surface of a  known area to
the  action of the wind, the pressure on it being estimated by the compres-
sion of  springs.   Such has been  made self-registering by Osier (L. & E.
Phil. Mag. vol. xi. p. 476), so that, being connected with a vane, it will note
by a pencil both the direction and the force of the wind for  every moment.
  129.  When a gas is allowed to escape from a confining  vessel through
a small  or capillary orifice in a thin plate into a vacuum, the velocity with
which it issues remains the same;  for, as the density and consequent elas-
ticity or propelling force of the gas decreases, its specific gravity, and con-
sequently  also the propelled quantity, decreases in the same ratio, so that
in the same time the same volume of gas always  passes out, but of course
of constantly diminishing density.
  If a gas be allowed to flow through a similar  small orifice, but from a
vessel in which it is kept under a constant pressure (see gasometers     ),
it will be  found that the velocity with which it flows out increases rapidly,
as the space  into which it flows is rendered more and more vacuous, until
the tension of the remaining air  is only  about J  Atm. (10 inches of mer-
cury), after which further exhaustion will not be  found to increase  the
velocity in the same proportion, and when  the state of rarefaction reaches
-3^th (1 inch of mercury, see 105), all further exhaustion seems scarcely
to affect the velocity, if the pressure on the gas be 1 Atmosphere.  In this
manner, in 1000 seconds, 60 cub. inches (15148 fluid-grain  measures) of
dry atmospheric air, have been found to  flow into such a vacuum through
an orifice  in a platinum foil of -g^th of  an inch  in diameter.  The times
which  the same volumes of different gases require for their passage into
such a vacuum,  have been found to vary so as to be  proportional to  the
square roots of their specific gravities, and their velocities, therefore, under
the same circumstances, to be inversely proportional to these  numbers.
Mixtures  of gases ought to have a mean rate of their constituent gases;
from which rule,  however, some, as hydrogen and carburetted  hydrogen,
have been found  to make a remarkable  exception, their rate being under
such  circumstances diminished considerably  beyond  what  it  should be.
                                  88 
PNEUMATICS.
89
Thus, only 1£ per cent, of air or of oxygen, added to hydrogen, was found
by Graham to retard its passage very perceptibly, and at least 3 times more
than it ought, by calculation.
/ 130.  If, however, instead of  a capillary orifice in a thin plate, a capil-
lary tube of the same  diameter  be substituted, a very great change  takes
place in  the above rates, the velocities decreasing rapidly, as the orifice is
elongated into a tube,  with  the first additions, but becoming gradually less
affected,  and after a  certain length, they remain constant for  any further
increase  in  the length of the  tube.   By a comparison of  these ultimate
velocities for different gases, it  is found that the ratios between them re-
main the same for a considerable range of pressures (from  1  to j^th Atm.),
but that these ratios are very different from those between  their velocities
through  capillary orifices.   In  some cases they approach to the ratios of
their different densities, but not uniformly so.   Hydrogen and carburetted
hydrogen suffer also in this case, by admixture of other gases, a considerable
retardation over the mean of their mixture.   And even for the same gas,
the velocity is found to change, becoming greater as the density of the gas
is increased, so that the higher  the barometric pressure  on it, in the less
time  will the  same  volume of  gas  escape.   Graham  considers  this a
proof that the effect  cannot be  ascribed  to friction, and he therefore dis-
tinguishes the flow of gases into a vacuum through  capillary tubes, from
their flow into the same  through capillary orifices, designating the  latter
by the name of Effusion, while their flow through capillary tubes he calls
 Transpiration.   When the space into which the gases escape, instead of
being kept vacuous, is allowed to become  filled  with the gas, the velocities
decrease slowly, while the tension of the gas  increases from  1 to 10 inches,
after which, however, the decrease is very rapid (Graham's Chemistry, p. 86).
   131.  When gases issue under pressure into the Atmosphere, they seem
also  to obey the same law that, for different gases, their  relative velocities
under the same pressure are inversely as  the square roots of their specific
gravities.  For the same gas, its velocities under different pressures are as
the  square  roots  of  these  pressures.   Thus, according  to Fyfe (Edinb.
New Phil. Journ., 1848, vol. xlv.), in 1  hour,  0.927  cub. foot of common
lighting  gas (carburetted hydrogen), of  spec.  gr. 0.6026  (ref. to 60° as
stand.),  will pass out through a  jet formed of a circular orifice of ^ inch
in diam. under a pressure  of {-J-J inch of water (burning  with a flame 5
inch. high).  Of a gas of  0.500 sp. gr.,  1.118 cub.  foot will pass out of
the same jet in the same time under a pressure of i-J-g inch of water.    __
   132.  When a gas is allowed to escape under pressure from an orifice in
one  side of a vessel, no pressure can  of course  be exercised  by the  gas
on this orifice,  to counteract its  corresponding pressure on an equal surface
                                  89 
90
BOYE'S INANIMATE  MATTER.
on the opposite side of the vessel, hence this pressure must produce a ten-
dency in the vessel to move in the opposite direction of that in which the
gas flows out.   This may be seen illustrated in the  revolving gas-lights,
seen in shop-windows  in cities.   In these the  gas is made to enter into
two lateral branches, see a and ax fig. 63, which are capable of revolving,
         Fig. 63.         their revolving motion  being  produced by the
                         gas escaping near the  end on one side,  while
                         no corresponding orifice  or jet  exists on  the
                         opposite side, as seen in the horizontal section
                         at n and nv   Instead of an orifice on the side
                         of the lateral  branch  near its  end,  the  same
                         effect is produced by bending  sideways the end
                         itself, this forming the jet.
                            133. If a thin plate of metal or pasteboard,
                         b fig. 64, be perforated at its middle, and fas-
                         tened by  sealing-wax  or otherwise  at  right
                         angles to the end of a glass tube a, so that the
aperture of the  plate is directly over the bore of the tube, and another
card or piece of stiff paper c be laid over the opening, having a  pin d
stuck through it, so as to prevent its sliding off,  it will be impossible to
forcexit off by blowing through the tube.   On the contrary, if the apparatus
be inverted, so that the paper is lowermost, blowing through the tube will
prevent it from falling down, and the  greater the blast, the greater will be
the force by which it is held up.  This experiment is called the Pneumatic
Paradox.  The cause of this is, that as  the air from  the tube spreads out
when escaping between the  plate and the paper, it can only separate them
to a certain  distance (about T^ inch), since pushing them apart beyond
                         this, would cause its  density to become less than
                         that of the atmospheric air on  the other side of
                         the paper, and  thus  produce a  partial vacuum
                         between  them.    That  it  is  the atmospheric
                         pressure,  which  prevents  the  plate  and  the
                         paper from being separated, can be proved by
                         furnishing the other end of the tube with a screw
                         s,  and attaching it to  the air-pump plate, placing
                         over it a receiver with a  stuffing-box and sliding
                         rod, by which the paper  may be held up by a
                         loop fastened  to it, till the air  is exhausted, and
                         then let  down  on the  plate.   On readmitting
the  air  suddenly through  the tube, the  paper  is  blown off.  Fig. 65
                                  90
lM 
PNEUMATICS.
91
 exhibits another modification of this experiment, the tube terminating in a
 bowl c.  By blowing through the tube g, a ball h of cork or any other light
 material, will remain suspended, instead of falling or being blown out.
    We will now give a separate consideration  to the class  of gases (45)
 which are called

                               VAPORS.

j^  134. Many liquids and solids, when their limit is towards a vacuum
 or towards a gas, are capable of passing wholly or in part into the gaseous
 form, and of  spreading in this state over the vacuum or through the gas.
 Such liquids and  solids are said  to be volatile, while those which are not
 capable of assuming  the gaseous state (as oils), or owing to  other circum-
 stances, cannot be  made  to assume it (as  platinum), are said  to be fixed.
 The gases thus formed are  called vapors.  This  conversion into vapors
 (vaporization) may take place either only from the free surface, which limits
 them towards the vacuum or the gas, in which case it is called Evaporation,
 or if the substance be a liquid, the conversion  into vapors may  also take
 place below the free  surface, the vapors  escaping as bubbles through the
 liquid and agitating it, in which case it is called Ebullition or Boiling.
    135. All vapors, being true gases, are, therefore, perfectly transparent;
 and,  when colorless, as invisible while vapors, as all other gases,  until
 they again assume the liquid or solid state, and at that moment again  cease
 to be  vapors.  It  is, therefore, a popular error to apply the word steam,
 by which we understand vapor of water, to the  smoke or cloud formed  by
 the  particles  of liquid water, into which the steam again condenses at a
 short distance from  a steam-pipe, when escaping  into  the atmosphere.
 Near the pipe, where it is yet real steam or vapor,  it is as invisible as the
 rest of the atmosphere.  Even vapors of perfectly opaque bodies  are trans-
 parent and in many cases, such as that of mercury, also perfectly colorless.
 In other cases, although always transparent, they may possess  color.  Thus,
 vapor of sulphur is yellow, and vapor of Iodine is of a beautiful violet color.

                   Formations of vapors in  a vacuum.
   136. To  illustrate  the formation  of vapors from volatile  substances,
 when limited towards a vacuum, we may employ a Torricellian Tube (60),
 inverted in a large cup of mercury, see 1 fig.  66, and furnished  with  an
 accurate scale to  measure  the height of the  mercurial column.   This
 column, which is  supported  by the  atmospheric pressure,  we  will  sup-
 pose to be exactly 30 inches.   If we now introduce, through the mercury
 in the cup d, the  smallest possible quantity of water into the tube 1, it
                                  91 
92                  BOYE'S  INANIMATE MATTER.
will rise to the top of the mercury at 30, and thus present an upper or
free surface towards the  Torricellian Vacuum.   It will then in a short
                  Fig 66.                   time be found to disappear as
                                          water,  being converted into
                                          vapor, the presence of which
                                          as a gas in  the  vacuum is in-
                                          dicated by its property of ex-
                                        • pansibility, that is, its spread-
                                          ing over the vacuum with  a
                                          certain force, until it is resisted
                                          by the limits of the latter, viz.
                                          the sides of  the tube and the
                                          top of the  mercury  at  30,
                                          thus causing a certain uniform
                                          pressure on  them  all, called
                                          its tension, and  by which the
                                          mercury becomes slightly de-
                                          pressed below its former level
                                          at 30.  By introducing addi-
                                          tional small quantities  of
                                          water, we shall find that the
                                          same continues,  the water dis-
                                          appearing as liquid, and  the
mercury becoming more depressed, until at last no more water is found to
disappear, and  no more depression occurs, however  much water we  may
introduce, provided  the temperature remains the  same.  Thus,  if the
experiment be performed,  when the stand of the mercury is 30 inches and
the temperature 59° Fah., this depression will stop at J inch  at b, see tube
1 jig. 66, or when the  mercury has  a height of 29? inches.   If on the
contrary, the temperature be raised, more liquid will again disappear, more
vapor be formed, and the depression of the mercury become greater, till at
last, when it has reached a certain point, it again becomes stationary.  If
the temperature be raised to  79°, this will occur when the depression
becomes 1 inch, or when the height of the mercurial column is 29 inches, after
which the depression does not increase any further, as long as the tempe-
rature remains the same;  and so on.  We conclude from this, that  the
formation of vapors from volatile  liquids  in  a vacuum has a limit, which
depends on the temperature, so that for every temperature, there is a cer-
tain greatest or maximum quantity of vapor which can be taken up, with
a corresponding maximum tension, beyond which no more can be taken up.
                                   -92
 
PNEUMATICS.
93
   107. An otherwise vacuous  space  may therefore, at a certain tempera-
ture, coutaiu less than this maximum  quantity, if there  be no more liquid
present to form more vapor, but it can never contain more.   The quantity
which  is present, whether it be the maximum or less, is always,  for the
same  temperature,  proportional  to  its tension, or the pressure which it
causes  on  the  mercury.   Should  the temperature  not be the  same,  a
deduction must  first be made from  its tension at the higher temperature
of so much, as  is due to the expansion of the vapor by heat by the diffe-
rence in temperature (see  140 and  100).  If, on the other hand,  we can
prove,  that a space  contains the maximum  quantity, or, as it is termed, is
filled to saturation with vapor, which may be known, for instance, by its
having been sufficiently long in  contact with an abundance of  the liquid,
then we may, from  the temperature, estimate the quantity of vapor in the
space, and its tension, since these will be the maximum quantity and tension,
which  correspond to the temperature.
   188. By experiments, the following temperatures  have been found to
correspond to the annexed maximum tensions and quantities of vapor of
water:
Temp. Fahren.	Max. in inch, of	tens. mercury.	Max. quan. iu 1 cub. foot, in grains.	Temp. Fahren.	Max. tens. in Atmos.		Max quan. in 1 cub. foot, in grains.
18°.3	A	inch.	1.204	149°.65	i Atmos.		70.640
23°.3	*	a	1.490	179°.08	i	"	134.766
32°	1 s	(t	2.120	212°	1	"	256.317
40°.25 59° 79°.3 101°.4	i i 1 2	u it «	2.878 5.548 10.677 20.512	250°.52 293°.72 341°.78 398°.48	2 4 8 16		484.791 913.951 1718.225 3209.251
126°.6	4	tt	39.260				
   139.  From this table it will be seen, that at 212° the maximum tension
of the vapor of water is equal  to the atmospheric pressure, and  that it
therefore at that temp, will cause a depression of the mercury inside the
Torricellian tube to the same level as outside.   The  tension or  elasticity
for higher  temperatures than   212° cannot, therefore,  be conveniently
estimated in the same apparatus as described above, but we may then sub-
stitute for  it  the apparatus represented in fig. 67, consisting of a small
boiler b l, furnished with a mercurial pressure-gauge c g, (106), the cistern
of which, c, communicates by an opening with the vapor inside the boiler,
so that  by  it we estimate the tension, while the temperature is indicated
by the thermometer t.  The boiler is also  furnished with a stop-cock i.
The boiler  having been partly filled with water, the latter is made to boil
by the application of heat.  As soon as the escaping  steam has expelled
completely  the atmospheric air, the stop-cock i is closed.   The tension of
                                 93 * 
94
BOYE'S INANIMATE MATTER.
the vapor will  then be found  to  increase rapidly, being indicated by the
           Fig. 67.           height of the mercurial column in the gauge,
                            while the corresponding temperatures are in-
                            dicated  by the  thermometer.   In the experi-
                            ments performed for the French Academy in
                            1829, by Arago  and Dulong, for  estimating
                            the elasticity of steam at higher temperatures,
                            the highest tension  measured was 24 Atmo-
                            spheres.   The  tensions were  estimated by a
                            condensed  air-gauge (107),  which had  pre-
                            viously  been tested  by a  mercurial  gauge
                            (106) to the  extent  of  27 Atmospheres.
                            The  tube of this latter was therefore over 68
                            feet high, having been ingeniously constructed
                            and arranged in an old church-tower.   Mar-
                            iotte's law was thus found to be  correct to
                            the above  extent (Annal.  de  Chim.  et de
                            Phys.,  2d  series, vol. xliii).  The  tensions
                            below 212° have been  estimated with great
                            accuracy by Regnault (Ann. de Chim. et de
                            Phys., 3d ser.,  vols,  xi, xiv and xv).
                              A complete set of tables of the tensions of
vapor of water in English  inches, and  the  temps, in  Fahr.  degrees, has
been computed for this work from the tables furnished by these  authors,
and will be found at the end of Pneumatics,  in Tables VII, VIII  and IX.
  140.  It  is evident from  these  and the above-given table (138) of the
maximum tensions and quantities  of vapor, that these increase with extra-
ordinary rapidity and in a much greater ratio than the temperatures, when
in contact with the liquid.   This is  due  to the additional vapors formed
from it.*   If, on the contrary, at  any time,  there be no liquid present, the
increase  in tension will only be that  which follows from  the expansion of
the vapor by heat, which is the same as that  of any other gas  under the
  * As regards the tensions of vapors at very high temperatures, it would seem from some
interesting experiments of Cagniard de la Tour, that they do not continue to increase in the
same augmented ratio.  By enclosing volatile liquids in sealed glass tubes, and exposing
these to heat, he found that ether passed at 320° entirely into the state of vapor in a space
scarcely double its own volume, and without exerting a pressure of more than 38 Atmos.
Alcohol passed into the gaseous state at 404J°, in a space of 3 times its own volume,
thereby exercising a pressure of only 139 Atmos., and water (to which a small quantity of
Carbonate of Soda had been added to prevent the breaking of the tube), in a space 4 times
»ts own volume, at about 648°.
                                   91 
PNEUMATICS.
95
same  circumstances, or for every degree  Fahrenheit 0.00203611 of its
volume at 32°, or 0.00217802 of its volume at 0°.
  141. Conversely, if vapors do not fill the space to saturation, as in the last->
mentioned case, when heated to a higher temperature without contact with
the liquid, or when allowed to spread through a vacuum in a less quantity
than  to fill it to saturation at the existing temperature, such vapor may
again, without  becoming liquid, be subjected to so much pressure or cold,
as will again reduce it to the state of saturation.   But as soon as the pres-
sure becomes greater than its maximum tension at the existing temperature,
it will all be reconverted into liquid; and  if the temperature becomes less
than that, at which its tension is the maximum, a portion of it will condense.
   142. Thus, as  an illustration of this in regard to pressure,  suppose that
at the temperature of 79°.3 and  30 inches barometric stand, the Torricel-
lian vacuum bac fig. 68  tube 1, contains vapor  of  only $ inch tension,
                   Fig 68.                   that is only \ the  maximum
                                           tension and quantity,  which
                                           belong to  that  temperature.
                                           The level of the  mercury will
                                           then  of   course  be at  29 £
                                           inches, or at  b.  The vapor
                                           being thus only £ the quantity
                                           that can  exist in  the space, it
                                           may be subjected to an addi-
                                           tional  pressure of  $ inch, or
                                           till its volume is compressed
                                           to i of its former volume, or
                                           into c a,  without any conden-
                                           sation taking place.   This in-
                                           crease in pressure is produced
                                           by inclining the tube, as tube 2
                                           in  the fig.,  which has  the
                                           effect of diminishing the Tor-
                                           ricellian  vacuum above  the
                                           mercury, by which  the vapor
                                           becomes more compressed, and
its  density and tension thereby greater, so that it  depresses   the mercury
more, say to 29£  inch at b±.  The compression of the vapor may thus be in-
creased by still farther inclining the tube, without  any condensation occur-
ring,  until the depression in the perpendicular height of the mercury is 1
inch, or the perpendicular  height of the mercurial column 29 inches,  see
                                  9o
 
96
BOYE'S INANIMATE MATTER.
tube 3, when, in consequence, the atmospheric pressure on the vapor will be
30 — 29 inches, = 1 inch of mercury.   At the same time the vapor will
also be compressed to the volume ct av that is J its former volume, and its
tension in consequence doubled or  equal to 1  inch.   The pressure on  the
vapor being thus equal to its maximum  tension at that temperature, any
farther inclination of the tube will not cause the mercury to become more
depressed, but  merely  diminish  the Torricellian space,  by which as  the
space become diminished, the vapor in it will be compressed to liquid watery
till at last, when the top of the tube reaches the level of 29 inches, see tube
4, no vapor will remain, all having been converted into liquid, which will
appear as a drop at  the very top of the tube.
   143. In  the  same manner, as regards temperature, if the tube or any
other vessel containing vapor, not filling it to saturation, be subjected to
cold, the temperature  may be  lowered  without  any condensation taking
place, until  it reaches  that degree  at which the vapor forms a maximum,
after which a portion of it will be reconverted into liquid, only leaving so
much  vapor, as will be the maximum at the temperature to which it is
cooled.  Thus,  as in the above case, if the temperature be 79°.3, and the
tube contain vapors  of  only \ inch tension, which is only \ the maximum
tension and  quantity corresponding to this temperature, it may be cooled
without any condensation taking place, to the temperature of 59°, this
being the temperature at which its tension will be the maximum.   But if
then the temperature  be  still farther lowered to 40°, so much of it will
condense, that what  remains  has only a tension  of \ inch, which is  the
maximum at that temperature.  As the condensation of a portion of  the
vapor gives the  appearance of a dew on the sides  of the vessel, the tempe-
rature at which this  begins to take place, is called the Dew Point.  The
condensation of a portion of the vapor or its appearance as a dew, by the
slightest increase in  cold or pressure, is  the  surest proof that  the space is
filled with vapor to  a maximum or to saturation.
   144. The formation  of vapors by boiling, will take place whenever  the
temperature of the  liquid becomes so high,  that the maximum tension,
which corresponds to its temperature, is  equal to, or greater than, the ten-
sion or pressure of the vapor on  its  free surface.    By this the liquid
will be capable of  forming  vapors below the free surface, whieh vapors
generally  appear  as small bubbles on   the  surface of  the containing
vessel, where the liquid is  in contact with  it, and  which bubbles force
their  way through  the liquid,  and agitate  it.   In  a  close  vessel,  like
that of jig. 67, the temperature of the  water may,  therefore, by a very
gradual heating be raised, without producing boiling, to  any degree, the
maximum tension of which  the vessel  will  bear without bursting, since
                                 96 
PNEUMATICS.
97
by such  gradual heating the formation of vapor by evaporation from the
free surface, will keep pace with the maximum tension, which corresponds
to the  temperature of the liquid.  If, however, the vessel be heated very
suddenly from below, so as to raise the temperature very rapidly, boiling-
may be  produced for a short  time,  till  the tension  of  the  vapor above
becomes the maximum for the temperature of the liquid.  Another much
easier  way of producing boiling on  the  same  principle, is  by suddenly
diminishing the tension of the vapor on the free  surface.    This  may be
done, where the tension is greater than the atmospheric pressure, as in the
apparatus fig. 67,  by letting the vapors escape into the air by opening the
stop-cock i, by which  a violent ebullition will take place, until the temp.
of the  liquid is again lowered to 212°, which is  the temperature which cor-
responds to the diminished pressure  of the vapor on its surface (1 Atm).
   145. Another mode of diminishing the  tension of the vapors, particularly
if less  than the atmospheric pressure,  is by their condensation, absorption,
or exhaustion.  Thus, the production of boiling by  condensation of the
vapors, by applying cold to that portion of the  vessel  where they are con-
      Fig. 69.      tained,  may  be illustrated by an  experiment,  known
                  under  the  name  of the Culinary Paradox (so called
                  because it produces boiling by cold), which consists in
                  boiling water  in'  a globular glass vessel  with a long
                  neck (bolt-head), till all the atmospheric air is expelled.
                  It is then quickly closed up  by a cork, while removing
                  it from the  fire, and inverted as in fig. 69.  By apply-
                  ing carefully, so as to prevent  its breaking, a piece of
                  ice or a sponge moistened with cold water to the top at
                  c, where the vapors are contained,  these are  condensed,
                  and the water will then begin to boil violently.      __
                     146. Strong  vessels for heating liquids  to  a high
                  temperature,  furnished with a safety-valve to regulate
                  the highest  temperature of the liquid, and consequent
pressure of the vapor, affording the latter an escape, if exceeding a certain
   Fig. 70.      limit, are known under the name of Papin's  Digestor, see
               fig. 70.   Such  have been applied to different purposes by
               the greater solvent power, acquired by liquids at tempera-
               tures higher  than their  boiling  point in open air; for
               instance for the extraction of gelatine from bones by water,
               or the solution  of resinous substances  for varnishes by alco-
               hol or oil of turpentine.
                  147. From the  table given in 138 it will  be seen, that
               *                   97                           9
 
98
BOYE'S INANIMATE  MATTER.
water continues to emit vapors many degrees below its freezing point, and
that,  therefore, even  ice  is volatile.   The  question  therefore arises: do
volatile  substances  continue to emit vapors at all temperatures, however
low, although of course in a continually diminishing ratio, so that for those
substances which  are  volatile to a perceptible  degree  only at  higher tem-
peratures, their evaporation becomes at last inappreciable, and,  therefore,
imperceptible at lower temperatures ? or do they exhibit at a certain tem-
perature a theoretical  or absolute  stop to the further formation of vapors?
According  to the experiments of Faraday, mercury has been found to
begin to emit a very small but perceptible quantity of vapor in summer
between  60° and 80°;  but in winter  the formation  of not even a trace
could be detected by the most  delicate tests.   It seems therefore probable,
that volatile substances cease all at once to emit vapors, and that  this point
will be  arrived at,  when  their expansive or  evaporative  power becomes
so small, that it is counteracted or overcome by the forces of cohesion and
gravity (compare  also 27).
   148.  The maximum  quantities and  corresponding  maximum tensions
of other volatile  substances for the same temperatures, are  different from
those  of water, being greater for the same temperature, the more volatile they
are.  An idea of their  relative volatility may be obtained  by referring to
their boiling-point in air  (see 154), which indicates the temperature at which
their  maximum tension is  the same as that of water at 212°.   The lower
their  boiling point,  of course the greater is their volatility.   But the ratio
of the increase of the  tension of their vapor, to the  increase in the tempera-
ture, is somewhat different for  the different substances.  Thus the boiling-
point of mercury  is  662°,  and  the tension  of its vapor at that temperature,
therefore, 30 inches, or 1 Atm.  For lower temperatures Regnault obtained
the following maximum tensions of  its vapor in a vacuum :
Temperature   212°.2     144°. 93       120°.47        77°.7
Tension         0.160 in.  0.0072 in.     0.0034 in.    0.0013 in.
/ 149.  It will be evident  from the  foregoing, that the conversion of vola-
tile substances  into vapors in  a vacuum is facilitated:  1st, by an increase
in  the temperature, and,  2d,  by the removal  of  the  vapor  as  fast as it
is formed.   The  latter  may be  effected either by condensation,  by  the
external application of cold to  a different part of the vacuum at  a distance
from  the liquid; by absorption, by  placing  in  a different  part  of  the
vacuum a substance, that  by its adhesion  or chemical affinity will  attract
and thereby remove  the vapors;  or in. some  cases by exhaustion  of  the
vapor by an air-pump.
   150.  In the same  manner  as  the removal of the atmospheric pressure
                                  98 
PNEUMATICS.
99
will cause  the expansibility of gases to overcome their adhesion to solids
(121) or liquids (120), so the placing of volatile liquids in a vacuum will
have the same effect, causing their expansive or evaporative power to over-
come their adhesion or even feeble chemical affinities.  Hence in chemistry,
desiccation or drying, evaporation and boiling, and the expulsion of che-
mically combined water, are often effected or assisted by placing such sub-
stances  with suitable arrangements in a vacuum.

                     Formation of vapors in a gas.
   151.  To illustrate   the formation of vapors from liquids,  when their
limit is towards a gas,  we  may  use several  receivers,  see  d  and h
fig. 71, filled with different gases, such as atmospheric air and hydrogen,
           Fig- 71.          and  placed in  a pneumatic cistern g,  con-
                          taining  mercury, one  side of  which  should
                          be of glass, so to enable us to observe the mer-
                          curial  levels  inside.    The  receivers  having
                          been adjusted  so that  the mercury has  the
                           same level outside and  inside,  the gases exer-
                          cise themselves the same tension on the mer-
                          cury, and  are of course under  the same pres-
                          sure, as the air outside, that is, one  Atmo-
                          sphere.   If we  now introduce  into those two
                          receivers, as before into the Torricellian vacuum
(136), a small  quantity of water, we shall find that it  in  the  same man-
ner disappears as liquid, being converted into vapor, spreading through the
gas as  such, and indicating its presence there by  its tension, which  causes
the  mercury to be depressed lower inside  than outside,  and thus, like any
other gas, adding its volume and tension to the gas to which it mixed.   By
introducing additional quantities of water,  the same will be repeated until
the  depression of  the mercury has reached a certain point, when  it  will
increase  no more,  the water remaining liquid and no  more vapor being
formed, however much water be introduced, provided  the temperature remain
the same.  If, however, this be raised, more vapor will be formed, and the
depression  increased, till it again becomes stationary.  This proves that the
formation of vapor  in a gas from a liquid, has a limit as in a vacuum, beyond
which no more can be taken up, so that for a certain  temperature there
may be less, but there cannot be  more,  than this maximum quantity  with a
corresponding  maximum tension.   As the vapor in every case  adds its
tension  to that of the  gas, its quantity must therefore always be, making
allowance for  differences  in  temperature,  proportional to  the  additional
                                99
r^\
r\
I v\ 
100                BOYE'S INANIMATE MATTER.
tension acquired by the gas.   What, however, is very extraordinary is, that
the maximum quantities for the same temperatures, which can exist in the
different gases, are the same for them all, and exactly the  same  as in a
vacuum.
   152. Dr. Dalton of England, who first discovered this law, connecting it
with the fact that gases are  not capable of resisting each other's expansi-
bility,  or of limiting  each other  (52), expressed it in  this manner, that
gases are  to each  other as vacua.  There is,  however, this  difference
between the formation of vapor in a vacuum and in gases, that while in a
vacuum it takes place very rapidly, and the  maximum quantity is attained
soon, it is  much slower  in gases, requiring much  longer time to  attain
the maximum,  and  the times varying for different gases, being shorter for
those  the  specific gravities of which are less.  The relative  times  for
obtaining the  maximum quantity of  vapor in  different  gases, have been
found,  under otherwise similar circumstances, to be inversely proportional
to the  square roots of their specific  gravities, which is the same  law as for
diffusion.  This seems to indicate that the formation of vapor in  a gas
depends on the same cause as the penetration of gases through each other
by diffusion, and therefore depends not only on  their own  expansibility,
but also on the attraction of the atoms of the gases toward  each other, or
adhesion.   Begnault  has also found that the  tension of vapor of water
in atmospheric air  is two  or three per cent, less  than  in  a vacuum at
the same temperature, and  that  its density also has  a slight deviation,
but it  is uncertain whether this apparent deviation  may not  be ascribed
to other causes.
   153.  Boiling depends here, as in a vacuum,  on the same  principle, and
will occur whenever the  maximum tension corresponding to the tempera-
ture of the liquid is greater than that of  the pressure of the gas and the
vapor on its surface.   It  will thus be seen that  the boiling of water in the
atmosphere must occur at 212°, since at this temperature the maximum
tension is equal to the pressure of the atmospheric air on its surface, and its
vapors, therefore, are capable of sustaining  themselves against this pressure,
so that by forcing their way  as bubbles through the water, they cause the
agitation, which we call boiling in open air.  The singing or hissing noise,
generally called simmering,  which is heard  just before  boiling, is caused
by the  water above not having yet acquired the full temperature of 212°,
by which the vapors formed at this temperature below, in contact with the
vessel,  are again condensed by contact with the water.
   154. The  more volatile substances  are, the greater  is the tension or
elastic  force of their vapor at the same temperature, and the lower is there-
fore their boiling-point in open  air.  The following table  exhibits the boil-
                                  100 
PNEUMATICS.
101
mg-point  in open air of different substances  at the mean barometric pres-
sure of the atmosphere of 29.918 inches :
                                                       Boiling-Point.
Chlorohydric or Muriatic Ether	52°
Ether (a liquid, frequently called Sulphuric Ether)	96°
Alcohol (Sp. Gr. 0.798).....	173°
Water .......	212°
Oil of Turpentine .....	314°
Oil of Vitriol (Sp. Gr. 1.845) ....	620°
Mercury .......	662°
   155.  If, however, the atmospheric pressure on  the surface of the water
or other volatile liquids be increased, it will  require a higher temperature
to produce boiling;  and if it, on the contrary, be decreased, boiling will
take place at a lower temperature.   If, therefore, water of  less  tempera-
ture than 212°, or  even  of  ordinary high temperatures (70° to  80°) be
placed under a receiver, and the air quickly exhausted, it will begin to boil, r__
   156.  As water emits vapors of a certain tension at all temperatures, it V_^
might be supposed that by removing  all pressure from its surface, it could
be made to boil at any temperature.   This is, however, not the case, as it
cannot be made to boil, even in a perfect vacuum, below the temperature
of 67°.   The reason of this is, that  although at this temperature it is yet
capable  of furnishing vapor of a tension of more than \ inch of mercury,
this tension is not sufficient to overcome the pressure caused by the weight
of the layer of liquid above it, or to break the cohesion of its particles.
Other volatile liquids have a similar limit or lowest  temperature,  below
which they cannot be made to boil in a vacuum, being approximately the
same  number, or 145° below their boiling-point in open air.
   157.  The  principle,  that  the temperature at  which pure water boils
depends on,  and varies with  the atmospheric pressure, being always that
at which the maximum tension of its vapor is equal  to the atmospheric
pressure on  its  surface, is used in the construction of  the  Boiling-Point
Barometer, described in 87.                                          >.
   158.  From the foregoing it will be evident, that the conversion of vola-^
tile liquids  into vapors in a gas in  a close  vessel, or in the open  atmo-
spheric  air,  is facilitated: 1st,  by heat, and, 2d,  by the removal of  the
vapor as fast as it is formed.   This latter may be effected by condensation,
by applying externally cold to another part of the close vessel at a  distance
from the liquid (Distillation); by absorption, by placing in a different part
of the close vessel substances, which, by their adhesion or chemical affinity,
will attract and thus remove  the vapor;  or  by displacement of the satu-
                                 101 
102                   BOYE'S INANIMATE MATTER,

rated  air  over the liquid by less  saturated or perfectly dry, and, in some
cases,  even heated air.
   159.  These  principles  are often applied in chemistry  for effecting or
accelerating the  drying of vessels or  substances  containing water.   Thus,
the drying of narrow-mouthed vessels, such as bottles, which even by heat-
ing requires considerable time, is  effected in  a few moments by removing
the saturated air by suction through a tube, the other end of which is intro-
ducad to the bottom of the vessel.


                    Vapor of Water in the Atmosphere.

   160.  The atmosphere always contains Vapors  of Water (26), which are
formed  by evaporation from  the sea and  the  moist earth.   From  various
causes (92), these again  condense to liquid water either on  the surface of
the  earth as dew, or in  the  atmosphere itself as small  hollow spheres or
vesicles, filled with air, which constitute fogs and clouds.

  These vesicles may be observed by a lens of 1 inch focus against a dark ground.  Saus-
sure found those forming the mist on high mountains to have a diameter of js1^  to  yVsiJ
inch, but occasionally to be as large as a pea. A fog is a cloud resting on the earth.  On
the other hand, by ascending into the clouds, these appear as fogs.   According to Howard
the different varieties of clouds are named as follows:
   Cirrus, Curl- or Feather-Cloud, composed of delicate  feathery streaks or filaments, more
or less straight, curly, or confused.  After a spell of fine weather they  are generally the
first to change the blue color of the sky, and they are often the last remaining, when the
weather becomes fine.  They are  the highest of all clouds, and have, in some cases, been
estimated to have an elevation of 20,000 feet.
   Cumulus, Accumulated or Heap-Cloud, forming large hemispherical masses, with a more
or less horizontal base.  They are often piled on each other, and when lighted by the sun,
appear as mountains of  snow.  In hot weather they frequently appear as the heat of the
day increases, and disappear again toward evening.
   Cirro-Cumulus, is the name given to those small, white, generally  rounded clouds,
arranged in rows, mostly with the blue sky visible between them.  After rainy  weather,
the clouds often break into these, and they give to the sky a mottled appearance (Mackerel-
back sky).
   Stratus, Layer-Cloud, forms a misty layer  of clouds near the  earth.   It often forms at
sunset, and again disappears after sunrise.  It sometimes resolves itself into a heavy dew,
at other times it rises as cumulus.
   Cirro-Stratus, forms streaks or bands, but heavier than the cirrus, which often passes
into it.  When in the horizon it causes the beautiful colors of the sunset; but when  heavy
gives it the dark-red appearance, which by  many is considered as the precursor of rain.
When high up, it often appears as attenuated clouds, covering the  sky as with a veil, but
at other times it assumes a darker and more threatening aspect.
   Cumulo-Stratus,  consists of dense masses and layers.  It is generally formed by the
increase of the  cumulus, extending irregularly at the  top, and losing its straight base by
 the addition of irregular appendages hanging down from it.  It is then apt to  pass into
 the next.
   Nimbus, or real rain-cloud, characterized by its uniform grey or dark appearance, with
                                      1Q2 
PNEUMATICS.
103
  fringed or indistinct edges, not allowing the different clouds of which it is composed to be
  well distinguished.
     The word Scud, is often  applied  to the loose and low masses of clouds, which during a
  storm are seen to move with great rapidity below the other clouds, and often in a different
J direction from them.
 »    AVhen  the vesicles of the clouds break and unite into solid drops, they form rain.  As,
  in the rule, the atmosphere near the  earth must always become saturated with vapors,
  before rain can fall, the rain-drops  increase in their descent by the condensation of addi-
  tional vapors on their surface, and their size therefore depends on the height of the clouds.
  This increase is very perceptible by measuring the quantity of rain falling at different
  heights in the same place.   Thus, an increase in the annual amount of rain of over one-half,
  has  been observed in a fall of  240 feet.  The amount of rain which falls is estimated in
  inches, indicating the depth of the layer of water  which  it would form, if allowed to
  remain standing on the earth.  The instrument used for this purpose is called the Bain-
  gauge or Ombrometer, and consists  of a funnel, the mouth of which has a known area, and
  which discharges  the water into a large bottle or other suitable vessel of sufficient capacity,
   in or from which it is measured in  cubic inches.  The number of cubic inches, divided by
   the  number of square inches constituting the area of the mouth of the funnel, gives the
   height or  depth  of the water fallen. Thus, if the mouth  of the gauge be  circular
   and 7.98 inch, in  diameter, each cubic inch of water will correspond to 0.02 inch of rain.
   Rain-gauges may also be made self-registering (Osier's).   The annual amount  of rain
   increases from higher latitudes toward the  equator, varying from  13  to 126 inches.  In
   Philadelphia (Penn. Hospital) it is U inches.  But the number of rainy days, over which
   the  fall of the rain is distributed, varies in the reverse order.               •
     Hailstones are frozen rain-drops, their size increasing by a prolonged suspension in the
   atmosphere by powerful upward currents or by electricity.   Snow is formed by the freezing
   of vapor or of the vesicles.  Snow-flakes often exhibit the most beautiful starlike appear-
   ances, varying much in the form of their rays, but are always of the same form in the same
   snow-fall.   Their  form is  produced by the different small  crystals of which the flake is
   composed, arranging themselves in different manners, although always at the same angles.
      161.  The two most important forms  in which water exists in the atmo-
   sphere, are, therefore, in the liquid  state as vesicles, and in  the gaseous
   state as vapor.   Both states  constitute what is commonly (see 162) under-
   stood by the  dampness or moisture of  the atmosphere.   When, however,
   the atmosphere is perfectly transparent, the water may be  considered as
   existing entirely  in  the  state  of  vapor.   But  even in this state,  when
   approaching the point of saturation, it imparts to  the atmosphere a decided
   dampness;  and by depressing  the perspiration of the  skin, which cannot
   pass off  as vapor, when the  air is saturated, it causes such air, if cold, to
   feel chilly and harsh or raw,  and, when hot, sultry and oppressive.  In the
   same de-ree also, as the air approaches the state of saturation, the tendency
   of  the vtpor  to precipitate in  the liquid state, increases, and  it therefore
   becomes important  to estimate at  any  time the vapor  in the atmosphere,
   and its approach to  saturation.                         #                #
      162   By moisture or   humidity,  or absolute  moisture  or humidity
   in  the' meteorological sense,  is understood the quantity of  water,  which
   exists  in the  atmosphere  in the state  of vapor,  while by  relative  mois-
                                         103 
104
BOYE'S INANIMATE  MATTER.
Fig. 72.
ture or  humidity, is understood the fraction which  this constitutes of the
maximum quantity or of saturation for the existing temperature.  Thus,
a relative humidity of 0.31 means, that the atmosphere contains T30VUS °^
the quantity of vapor, which at the temperature in question, whatever this
may be, would  constitute  saturation.   Instead of referring  the relative
humidity to saturation as 1, it is often  referred to it as 100, in which case
the  above relative humidity will be 31.   It is therefore on the relative
moisture, and not on the absolute quantity of vapor,  that what is commonly
called the dryness of the air  depends, for if the quantity of vapor  only
forms a small portion of the quantity which constitutes saturation, the air
will  yet freely take up more vapor, and therefore  appear dry.   Thus  the
same air that in winter is called damp, will in summer, when the  tempera-
ture  is higher, appear dry.
                                 163. The  most  accurate way of  esti-
                               mating the quantity of vapor in the atmo-
                               sphere is  by the chemical method, see fig.
                               72, which consists in passing  a known
                               volume of air though  a U-shaped tube e,
                               filled  with pieces  of  pumice-stone,  pre>
                               viously moistened with  oil of  vitriol,
                               which absorbs all the vapor from it.   The
                               air is  drawn very gradually through  this
                               tube by connecting it with the aspirator #
                              filled with water, which latter is allowed to
                              run out very slowly through the stop-cock
                              s,  and thereby draws the  air through the
                              tube e, to replace it.   The  tube  c is  also
                              filled with pumice,  moistened with oil of
                              vitriol, but is  permanently attached to
the aspirator, to prevent any vapor passing from it  into the  tube e.   The
tube d is similarly filled, but  serves only as a check to ascertain whether
all  the  vapor has been absorbed by  the  tube  e, and may be dispensed
with.  The  tube e is weighed  accurately before and after  the experiment,
and  its  increase in weight  is  the  amount of  vapor in the volume of  air
drawn through it by the aspirator #.   This volume is estimated by measuring
the quantity of water which it holds.   A  strict account must be kept of
the temperature of the air during the experiment, by placing a thermometer
at /, where  it enters the tube.  The  aspirator  is  also furnished with a
thermometer b u, and should its temperature at the end of the experiment
differ from the average temperature of the air which  entered, its volume
                                 104
 
PNEUMATICS.
105
must be reduced to the same, making also a deduction for the quantity of
vapor in it, and for  any variation in the barometric pressure during the
experiment (100).   Should the state of moisture of the room in which the
experiment is performed be different from that of the atmosphere, the air
must  be drawn in  from the outside by a  longer tube.  Having thus
obtained by weight the absolute  quantity of vapor in a certain volume of
the atmosphere, the relative humidity is easily obtained by dividing this
obtained quantity  by the maximum  quantity for  the same volume (168),
corresponding to the  observed temperature of the air; or the tension of the
vapor may be calculated from the obtained weight (168) and divided by
the maximum tension for the temperature of the air (164).   This method
allows us also to estimate the quantity of vesicular water existing in the
atmosphere, since in such case the air must  be saturated with vapor, and
its  quantity,  therefore, equal to  the difference  between  the  quantity
obtained by the  experiment, and  the maximum quantity for the  tempera-
ture.  It has, however, the inconvenience, that it requires longer  time,
considerable  skill  in the operator, and  expensive apparatus, particularly
for weighing the tube with sufficient accuracy.  Other methods and instru-
ments have therefore been contrived, which will now be described.

                            HYGROMETERS.
   164.  By hygrometers (from vypos (hugros) moist, and ptrpov (metron),
measure), we  understand instruments for estimating the moisture of the
atmosphere.   The best of these act on the principle of finding the  Dew-
Point, that is, the temperature at which the vapor existing in the atmo-
sphere would  be the maximum  quantity or fill it  to saturation.   This  is
done  by cooling a  portion of it till the vapors  condense as a dew (143),
and then observing  the exact temperature at which this begins to take
place, which  temperature constitutes the dew-point.  As the vapor in the
atmosphere is not confined, but free to contract or  expand, the maximum
tension  corresponding to its dew-point must  be the same as its tension in
the atmosphere at the existing temperature, and will therefore bear  the same
ratio  to  the  maximum  tension corresponding to the  temperature of the
atmosphere, as its quantity bears  to the  maximum quantity for this  same
temperature.   We therefore obtain the relative humidity  of the atmosphere
by dividing the maximum  tension, corresponding to the temperature of the
Dew-Point, by the maximum tension corresponding to the temperature of
the atmosphere.  For this purpose the maximum tension for every 0.2 degree
Fah. from 104° to 0° will be found in Table IX, at the end of Pneum.
  Thus, suppose that the
               Dew-Point = 60°      Temp, of Atm. = 85°;
                                 105 
106
BOYE'S INANIMATE MATTER.
we then have from Table IX,
                    Max. Tension for 60° = 0.518 inch
                      «     "   « 85° = 1.203  "
therefore: Relative Humidity =       = 0.431;

that is, the atmosphere contains T4oV<jtns of tne quantity of vaPori wnicQ li
is capable of taking up, and which would constitute saturation at its tem-
perature  of  85°.  Instead of referring  to saturation  as  1, the relative
humidity is often referred to it as 100.   In the above  case it would then
be 43.1.
  165. Conversely to find from the relative humidity and the temp, of the
atmos., the tension of the vapor in it and the dew-point, we multiply the
max. tens, for the temp, of  the  atmos., taken from Table IX, by the rel.
humidity referred to saturation as 1, which gives us the tension of the vapor
in the atmos.,  and as this is also the  max. tension for the dew-point, the
temp, which in Table IX corresponds to this tension is the dew-point.
  166. To obtain the  quantity  of vapor in the atmosphere,  either  by
volume or by weight, referring it to the atmospheric air itself as 1  (which
if referred to it as 100, constitutes the per centage by volume or by weight),
we may consider vapor of water as obeying Mariotte's law, both  as regards
its volume  and its density in the atmosphere.   To estimate, therefore, its
volume, it must be kept in mind, that while occupying the whole volume
of the atmosphere through which  it  is  diffused (the observed volume), it
only sustains so much of the atmospheric pressure as is  equal  to its own
tension /, and  that to obtain the  true volume V, which  it would  occupy
under the whole atmospheric pressure B (see 100), we have that:
                        V: Vol. of Atmos. : : _ : _
                                         B  f
therefore, calling the volume of the atmosphere 1, we have

                                   B
/being = the tension of the vapor, which  is the same as the  maximum
tension for the dew-point, and B — the stand of the Bar.  Thus, suppose
the dew-point = 60°, and the stand of the Bar. = 29 inch., we then have
from Table IX, that the max. tension for 60° = 0.518  inch, and therefore:
                         V = °'518 =0.01786;
                                29
that is, the volume of the vapor constitutes 0.01786 of that of the atmo-
sphere,  or it is 1.786 per cent, by volume.
   167.  To obtain the weight of the vapor in the atmosphere, referred to
that of the atmosphere itself as 1, we multiply the tension of the vapor by
0.622 (Sp. grav. of Vap. Water), and  divide this product by itself after
                                  106 
PNEUMATICS.
107
having added to it the difference  between the stand of the Barometer and
the tension of the vapor.  Or, calling the weight of the vapor W, we have:
                                  0.622 /
                         W= (B—f) -f- 0.622 /
/ being = the tension of the vapor, which is the same as the maximum
tension for the dew-point, and B = the stand of  the  Barometer.   Thus,
suppose, as in the former  case, the stand of the Barometer = 29 inch, and
the dew-point 60°, we then  have as before, from Table IX,  the maximum
tension for 60° = 0.518, therefore:
                    W=         °622 X 0-518      = 0.01119;
                         (29 —0.518) +0.622 X 0.518
that is, the weight of the vapor is 0.01119  of that  of the atmosphere, or it
is 1.119 per cent, by weight.
   168.  To obtain  the absolute weight of the vapor in  a given volume, for
instance, in 1 cubic foot of the atmosphere, or what is  the same, since this
quantity is the same as if the space contained no air,  the absolute  weight
of 1 cubic foot of Vapor of Water, we have by Mariotte's law, as stated
in 166, that the densities of the vapor in the air at ordinary temperatures
may be considered  proportional  to the pressures on it, the  pressure on it
at  any time being the same as its tension.  We know also that its expan-
sion by heat is the same as  that of other gases (140).   To find, therefore,
the weight of vapor in 1 cubic foot of the atmosphere, or 1 cubic foot of
vapor of the tension and temperature in which it exists in the atmosphere,
we proceed as directed in 100, by first reducing 1 cubic foot (considering
this as the observed volume of vapor) to the standard pressure (29.918
inch.) and temperature (32°), and then multiply the thus-reduced volume,
first, by the weight of 1 cubic foot of atmospheric  air of the  same standard
pressure and temperature (= 563.1007 grains), and  then by the specific
gravity of  Vapor of* Water  (= 0.622), so  that  calling the  weight  of the
vapor in 1 cubic foot W, we have:

      W= 1  X 29:918 X 1+0.00203161 (<-32°) X 563-1007 *"• X 0-622
                                         /
                  = 11.7055 grs. X i_j_ 0.0020361 (t—32°)'
/ being = the tension of the vapor, which is the same as  the maximum
tension for the dew-point, and t = the temperature of the atmosphere, or
of the vapor.   Thus, suppose the dew-point =  60°, and  the  temperature
= 85°, we then have from Table IX the maximum  tension for 60° = 0.518
in., and therefore the weight of vapor in 1 cubic foot,  W:
                                         0.518_________
                11'= 11.7055 £«. X i+o.00203"61  (85° —32°)
                             = 5.473 grains.
                                  107 
1Q8                BOYE'S  INANIMATE MATTER.

  By actual experiments, Begnault found that the quantities thus calculated
on the  above-stated supposition, that vapor of water, when diffused through
air, obeys Mariotte's  law, and that its tensions and densities are the same
as in the vacuum, were only about 1 per cent, greater than  those obtained
by actual weighing of the vapor (compare 152).
                 Hygrometers giving the Dew-Point.
       Fig. 73.         169.  Daniell's Hygrometer.—It consists of a mode-
     a     __/     rately wide glass tube,  see ah fig. 73, blown out at its
                   two extremities into bulbs, and  bent twice at right
                   angles.   One bulb is  partly filled with liquid ether,
                   "while the rest  of the  apparatus  is  freed from atmo-
                   spheric  air,  but contains, of course, vapor of ether.
                   The bulb d containing the  ether, has a thermometer
                   inside;  while the other bulb c  is covered with  thin
                   muslin.  To use it, we first  pour ether, drop by drop,
                   on  the  bulb c, which  ether, by its  evaporation, pro-
duces cold (see Latent Heat under  Thermics), and thereby condenses the
vapor inside.  By this,  the tension  or pressure of the vapor on the liquid
ether  in  the  other bulb  d is  removed,  and  the  ether  in  it  thereby
begins  to boil, or evaporate very rapidly (145).   The temperature of the
remaining ether in the bulb  is thus  lowered, and thereby that of the  bulb
itself and the atmosphere surrounding the bulb on the outside;  till at last
the vapor of the atmos. forms a maximum, and then begins to condense on
the outside of the bulb as a dew.  At this moment the temperature of the
bulb is observed by the thermometer inside, and  this  gives the temperature
of the  dew-point of the atmosphere.   As this is apt to have been observed
too low, the thermometer should also be read off, when the dew again dis-
appears,  and the average between the two observations, taken, as  the true
dew-point.  Generally, the stand g  on which this instrument is supported,
is furnished with a thermometer, by which  the temperature of the atmo-
sphere at the  same time, is ascertained.
   Daniell was the first to furnish us with a practical hygrometer on a true
scientific principle—that of finding the dew-point.   It has, however, this
inconvenience, that, as the cooling of the ether takes place from the upper
surface and is not readily communicated  to the  layers below, the thermo-
meter  is apt not to indicate  accurately the  temperature at which  the  dew
deposits.   When the dew-point  is very low, it is also difficult to  manage,
and if not observed at the moment  when the first dew appears, which may
easily  escape notice, it  gives the dew-point too low, and the experiment
must be repeated.  To facilitate the observation of the first  dew,  the bulb
                                  108
 
PNEUMATICS.
109
is  often  made  of dark glass, or  it is furnished with  a gilt  band or zone
around it.
   170.  Bache's Hygrometer, see fig. 74, consists  of  a horizontal bar or
          Fig.  74.           tube a c, of steel or brass, kept bright on the
                            outside,  the one end of which is inserted in a
                            box  b, containing ice, or ice and  salt, by which
                            its temperature is made to decrease gradually
                            from the free  end a, which has the tempera-
                            ture of the atmosphere, to the end c inserted
in the box.   At the point, which has the temperature of the dew-point of
the atmosphere, the moisture will begin to precipitate and form a very dis-
tinct  limit,  from which  its amount increases more and more  toward the
cooled end.  To ascertain accurately the  temperature of the bar, where
the moisture begins to precipitate, and which indicates the dew-point, that
portion of it which is outside the box is hollow,  being  varnished  inside, if
of brass, and filled with mercury, in which  the bulb  of a small delicate
thermometer t slides, the stem  of which passes through a longitudinal
Fig. 75.
Fig. 76.
L
                               opening on  the upper side  of the bar as
                               seen in the figure.   This thermometer is
                               moved to the exact place> where the mois-
                               ture begins  to condense, and its tempera-
                               ture then indicates the dew-point.'  For
                               stationary observatories, where ice is easily
                               had, this  hygrometer is very convenient,
                               being easily observed.
                                  171.  Begnault's  Hygrometer (hygro-
                               metre  condenseur) is  a  modification of
                               Daniell's, but so contrived as to be easily
                               managed and to give results of the utmost
                               accuracy.   Fig.  75 represents it iu  sec-
                               tion.   It  consists of a glass tube h of
                               0.8 inch, diameter, having on the side near
                               the top a small horizontal tubulure t.   Its
                               lower end is closed  by being inserted  into
                               an  extremely thin  and highly polished
                               silver cup or thimble b of the same diame-
                               ter, and about If inch, high, but with a
                               round  bottom.  This and portion of the
glass tube up to m is filled with ether, or, as  a substitute, with alcohol.   The
        id of  the instrument is  closed by a  cork  a, through  which  is
                                  109                         10

uppe 
110
BOYE'S INANIMATE  MATTER.
inserted a narrow open glass tube g, reaching nearly to the bottom of the
silver cup, and a very accurate thermometer p, the bulb of which is in the
middle of the ether.
  The horizontal tube t is connected with an aspirator similar to g, fig. 72,
but of smaller size, by which air may be  drawn with any desired rapidity
through the tube g, so as to bubble through the ether.  By this contrivance,
the evaporation of the ether is under perfect control.  When the cooling
which it causes reaches the dew-point, the vapors of the atmosphere appear
on the outside of the silver cup,  and the thermometer is observed.  The
aspiration is  then stopped, and  the dew allowed to  disappear, and  the
temperature  when this happens,  again observed.   The true dew-point will
then be the mean between these  two temperatures.  Should it be desired
to estimate it with more accuracy, the aspiration is immediately  started
again, but much  slower, and the same experiments repeated.   By this con-
trivance the dew-point may be estimated to -J<j of a  degree.  To be better
able to observe the slightest dew by comparison with another similar appa-
ratus, Begnault  fixes  two such together by the  tube c d, which connects
them both with the aspirator, as shown in fig. 76, but the second of which,
hx is  not used at the same time, and therefore contains  no ether, and has
the tube g closed up.   The thermometer of this may be used for indicating
the temperature  of the atmosphere.
  172.  August's Psychrometer (from (['ugpoq (psuchros), cold, and perpov
(metron), measure), also called the Thermo-Hygrometer, but better known
      Fig. 77.      under  the .name of the  Wet-Bulb Hygrometer, gives
                  the dew-point or the tension of the vapor  in the atmo-
                  sphere indirectly, from the greater rapidity with which
                  water evaporates, the further the quantity of vapor in
                  the atmosphere is from the quantity which would con-
                  stitute saturation.   It consists of two perfectly similar
                  thermometers, see b and  a fig. 77, the bulbs of which
                  should be perfectly free in the air.  As they are only to
                  indicate the temperatures of the atmosphere, the degrees
                  may  be  made large and subdivided  into  fractions.
                  The bulb of the  one a is  covered with muslin.  The
                  instrument  having  been placed in  the  atmosphere,
                  where exposed to a free change of air, but  not  to a
                  decided wind, the covered bulb is thoroughly moistened
                  by dipping it to above the bulb in pure water, which
                  should previously have acquired  the temperature of the
                  ' air.  The water will then evaporate from  the moist-
                                 110
 
PNEUMATICS.                        HI
ened bulb  and thereby lower  its temperature (see Latent  Heat under
Thermics), so  that the quicker  the evaporation, the lower its  temperature
will fall.  As  soon as the full effect is produced, which generally occurs in
from 5 to 10 minutes after its  moistening, and is known by the tempera-
ture  becoming stationary, the thermometer is  read off; the other thermo-
meter is also read off at the same time, and gives the temperature of the
air.  Sometimes, instead of dipping  the  thermometer into water before
observing it, it is kept constantly moistened by a wick dipping into a small
vessel c with water.
  This  instrument  is  also employed,  when the temperature of the atmo-
sphere is freezing, and it then acts by the cold produced by the evaporation
of the  layer of ice,  which  is formed  round the bulb.   In this  case  the
thermometer should not be read off before the layer of ice  which forms
after the moistening of the bulb has  become perfectly dry, which will
require from 15  to 30 minutes.  It is then best to keep the bulb covered
permanently with a layer of ice, which should be neither too thin nor too
thick, since in both of these cases the temperature of the bulb will not fall
to the proper  point.   It requires much  care and  practice to  regulate the
thickness of the ice.
  173.  To obtain the tension of the vapor in the atmosphere in English
inches,  from the indications of this instrument, we have the following for-
mula calling this  tension f:
                               0.480 (T— TJ
                       f=Fi     1130— Tv   B'
in which T= the temperature of the atmosphere in Fah. degrees given by
the dry thermometer;  2^= the  temperature in Fah. degrees of the wet-
bulb thermometer;  F± = the maximum tension in English inches for the
temperature T± of the wet-bulb thermometer, and  which is found  in Table
IX;  and i? = the stand of the  Barometer in English inches.
  If the observations are taken below 32°, when the wet-bulb therefore is
covered with  ice, wre  must substitute in  the above formula, instead of
1130—Tx  which represents the latent heat of the vapor, 1272.2 — T1 the
formula then becoming:
                            0.480 (T—TJ
                   f=Fi —   127272__T[
  Having  thus  obtained the tension of the vapor in the atmosphere, the
relative humidity is easily calculated (164) by dividing this tension by the
maximum tension for the temperature T of the atmosphere, given  by the
dry thermometer, and which tension is found  in Table  IX.   If the dew-
point be required, it is easily obtained  by taking from Table  IX the tempe-
rature which corresponds to the tension  /, obtained by the above formula.
                                 Ill 
112                  BOYE'S INANIMATE MATTER.
  174. To illustrate this by an example, suppose that  the
         Dry Therm. = 68° = T         Wet Bulb Therm.  = 59° = Tx
                         Barom. = 29.922 inch. = B
We then have:
                                     T— Tx = 9°
                and from Table IX,     Ft = 0.500 inch.
Therefore, by the first formula :
                           0.480x9
               /=0.500— 1130_59  X29.922=0.379 inch;

which is therefore the tension of the vapor in the Atmosphere;  and 51°3
which in Table IX corresponds  to this tension, is the Dew-Point.   From
Table IX we then obtain :
                     Max. Tension for  68° = 0.685 inch.
Therefore:
                                        0.379
                    Relative Humidity  = q -.or = .0.553;

                   or = 55.3,  if referred to saturation as 100.

  To  avoid  these calculations, Tables have  been constructed, which give
from  the temperature Tt of the wet-bulb thermometer, and the temperature
 T of  the atmosphere or the  difference  T— Tx between the dry and wet-
bulb  thermometers, both the tension of the vapor in the atmosphere, and
the relative humidity,  which at the temperature  T of the atmosphere cor-
responds to this tension, supposing the Barometer to remain at the same ave-
rage  stand.   If it should be required to make a correction for the different
stands of the barometer, a table may also be constructed for this purpose.
  [The formula given by August,  of Berlin, the inventor of this instrument,  and  which
                               0.568 (t — t')
is yet used extensively, is:  x=f'— „ .„----f---  h; in which x = the tension of vapor

in atmosphere in millimetres; t and t' = temperatures of dry and wet bulb thermometers
in centigrade degrees; /' = maximum tension of vapors at temperature if, in millimetres ;
and h  = stand of Barometer also in millimetres.   By correction of some of the numerical
                                              0.429 (t—t')
data, Regnault has since altered this into: x =/'— —oin —,— h, which he found to
give correct results, whenever the relative  humidity is less than 0.40 (which results differ
not much from  those obtained  by August's formula, using August's Table of Maximum
Tensions, but when taken for a wider range are not so near the truth).  But whenever the
relative humidity is over 0.40, Regnault has found, that in order to obtain perfectly correct
results, it is necessary to substitute the coefficient 0.480 for 0.429, the formula then becoming:
      ,,   0.480 (t — t') ,    .F   ,     ,  ,   .,  .        .  .     „   0.4S0 (t—t')
x = /' —-----2------ h, and for temp, below the freezing point: x —f —-----1----' h
     J     610 — *'               l                SF        J        6S9 — t'  '
 which are the formulae given above, only with  the proper  substitutions for using English
inches and Fahr. degrees.   With the  same substitutions August's formula becomes:
         0.568 (T— r.)                                        0.429  (T— T,)
f—Fi—  .',o, _ y,—  B, and  as corrected by Regnault: f—Fi—-.  __,.----.]

                                     112 
PNEUMATICS.
113
  175.  For low temperatures  this instrument gives less accurate results.
on  account of the  small differences between the  temperatures of the
dry and wet-bulb thermometers; and when the temperature of the atmo-
sphere is near the freezing point,  its results are very unsatisfactory, on
account  of the uncertainty in the freezing of the water.   Begnault also
found that in order to obtain good results, a free change of air is  abso-
lutely necessary, so that in a  close room  its indications are  less correct,
the wet-bulb thermometer not descending to its proper point, and therefore
giving the relative humidity too high.   For  observations, it is therefore
generally placed in an open window, or fixed permanently outside of it.
But even when thus placed, the  air,  if very still,  should be agitated about
the bulb by fanning.  On the other hand, too strong currents of air will
affect the results in  the opposite  direction, so that  if the existing wind
have a greater velocity than from 15  to 18 feet per second, the instrument
should  be screened from  it.    Otherwise, Begnault found that within the
ordinary limits given to this instrument, it is not influenced  perceptibly
by  the size or shape of the thermometer-bulb,  nor by the thickness of the
covering muslin,  nor by the manner of moistening it either by immersion
of the bulb, or by supplying it by a wick from a  small vessel;  nor in the
latter case, by the length of the wick, or  the quantity of water by which
it is moistened, provided this be  sufficient  for complete  moistening and
full evaporation, so that if supplied from the wick in larger quantity than
this, it  may even without injury cause a drop to fall occasionally from the
bulb, but in no case should  it exceed this quantity.  The water used for
moistening, should  be  pure,  as otherwise  by its  evaporation it  causes
too  great a deposit of earthy ingredients on  the  bulb.   Bain-water is,
therefore, preferable.   To remove impurities  which collect on  the bulb,
it should be cleaned, and the covering renewed at least every two or three
months.
   176. The Psychrometer, from its  simplicity and the facility with which
it is observed and transported, is almost universally employed for meteoro-
logical  observations, both  by travellers and  at  stationary observatories.
The above-given precautions and  some practice  in  its  use, are, however,
necessary in order to obtain reliable  results.

              Hygrometers acting by absorption of the vapor.

   177. Many organic substances have  the  property of attracting, by
the force of  adhesion,  vapor from  the  atmosphere,  and of condensing
it on their surface  and in their pores  (see  54), by which they increase
their volume or swell.   The quantity of vapor which they thus attract or
      H                        113 
114                BOYE'S INANIMATE  MATTER.
absorb, varies with the greater or less proportion, which the quantity of
vapor in the atmosphere forms of the quantity that would constitute satu-
ration, or  in other  words,  with the  relative humidity (164), so  that
the latter to a certain extent may  be measured by the increase  or de-
crease of their volume.  Of Hygrometers, acting on this principle, only
one deserves a special mention, as giving results which  approach to scien-
tific accuracy, viz.
  178.  Saussure's Hair Hygrometer. It consists of a human hair deprived
of its natural grease by boiling in a feeble solution of carbonate of soda in
water.   This hair is suspended in a  metallic frame c fig. 78, the one end
of the  hair being  attached  to a bracket a, which may be adjusted by a
screw;  the other end is attached  to the circumference of a small wheel or
pulley n.   The circumference of this  wheel  has also another  groove,  in
    Fig. 78.   which is fastened and slightly wound around it in the opposite
             direction, a thin silk  thread, to which is  attached a small
             weight w, which therefore  constantly keeps the  hair tense.
             It  will  easily be seen,  that when the  hair by increased
             moisture of the atmosphere absorbs more vapor and thereby
             swells, this will be perceptible by its  elongation, by which
             it allows the weight to turn the wheel.  When, on the con-
             trary, the humidity of  the air diminishes, the hair loses some
             of  the condensed vapor, it contracts and turns the wheel in
             the opposite  direction.  The axis of the wheel carries a
             light index i, which is thus made  to traverse a graduated
             scale s.
  179.  To construct the  scale of this instrument, it is first placed in a
close receiver, the bottom of which  is covered with water and the sides
moistened, by which the air becomes saturated with vapor.  The  length
of the hair having  been so adjusted, that the index will then be  near
the one end of the scale, the point  where it then stands, as  soon as it
becomes  stationary, is  marked 100°, and  corresponds  to saturation  or a
relative humidity of  100.   It is then placed, after the removal  of the
water, in the same close receiver over oil of vitriol, which deprives  the air
of all the vapor;  and the point on the scale where the index then stands,
after it has become almost stationary, is marked 0°, which point corre-
sponds to a relative humidity of 0.   The distance between 0° and 100° is
divided into 100  equal parts, each  of which  is called  1 degree.  These
degrees do, however,  not correspond to the  same numbers  of relative
humidity.  The following table  has been  given  as indicating the differ-
ent relative  humidities  corresponding to the  different degrees of this
hygrometer:
                                 114
.. ew 
PNEUMATICS.
115
Tahlt		of Beta		ice Hum		dities co		/•respond		ng to the		degrees of Saussure's Hygrometer.							
*x		= a g a	"a	"&	~E	Bfipn				•Z£			— 3	a 5)	!iijfi		If	!!!l	
0°	0	10°	5	20°	12	30°, 19		40°	27	50°	35	60°	44	70°	56B80° 69			(.m°i 83	
1°	0	11°	6	21°	12	31°j 20		41°	27	51°	36	61°	45	71°	57 581°		70	91° 85	
2°	1	12°	6	22°	13	32°! 21		42°	28	52°	37	62°	46	72°	58	82°	72	92°i 87	
3°	1	13°	7	23°	14	33° 22		43°	28	53°	37	63°	47	73°	59	S3°	73	93° 88	
4°	2	14°	8	24°	15	34°	23	44°	29	54°	38	64°	49	74°	61	84°	75	94°	90
5°	3	15°	8	25°	16	35°	24	45°	30	55°	39	65°	50	75°	62	85°	77 95°		91
6°	3	16°	9	20°	17	36°	24	46°	31	56°	40	66° 51		76°	63	86°	78	96°; 93	
7°	4	17°	10	27°	18	37°t 25		47°	32	57°	41	i7°	52	77°	65	S7° 79		97°! 95	
8°	4	18°	11	28°	18	38°i 26		48°	33	58°	42	68°	53	78°	66	88° 81		98° 97	
9oi 5		19°	11	29°	19	39°| 26		49°	34	59°	43	69°	55	79°	68	89°l 82		99°' 98	
   180.  But this  instrument is not so uniform,  that the  above comparison
can be relied on.

  Saussure's directions (Essais surl'Hygrometrie, par B. II. de Saussure, Neufchatel, 1783)
are: to select fine, soft, not curly, nor splitting hair, cut from the head of a living and sane
person.  A bunch of these of the thickness of a quill is then sewed up between linen, sepa-
rating them as much as possible.  They are  then boiled for  30 minutes  in a solution of
154 grains of Crystallized Carbonate of  Soda in 32 oz. (Troy) of Water, which should be
performed in a flask  with a long neck, to prevent the evaporation of the water.  The  bag
is then twice boiled for a few minutes in pure water, cut open, and the hairs again washed
and separated by moving them to and  fro in a large  vessel with cold water, after which
they are dried in the open air.  The hairs should appear clean, soft, polished and trans-
parent, separating easily from each other.  If they are rough  and adhere,  they have either
been boiled too long, or the solution has become too strong by the evaporation of the water.
The length of the hair in the frame should be about 9£ inch.; the diameter of the pulley
on which it acts 0.2 inch.  The index should be light, and with the pulley perfectly balanced
by itself.  The extending weight should not exceed 3 grains; if increased  to only 9 grains,
the instrument will, after some time, work irregularly.  Saussure has also studied the influ-
ence of the temperature  on  it, and gives a table for reducing its indications  to the same
temperature.  He asserts that if his directions are strictly adhered  to, the instrument will
never vary more than 2  to 3 degrees.  By later experiments, Regnault found no  greater
difference  with the same kind of hair, if prepared in  one and the   same  operation;  with
different kinds of hair, also  prepared in the same operation, the difference amounted to
nearly 5°; about the same  difference (5°) was  produced  with the same kind of hair, and
prepared in the same operation, but with small differences in  the weights by which the hairs
were extended.  But with different kinds of hair, and prepared in different operations, and
having been in use for different lengths of time, the differences amounted in some cases to  15°,
even after the extreme points of the scale had been fixed correctly.   Regnault concludes from
this, that it is  necessary to construct a table for every instrument, by comparing its degrees
with known relative humidities of the  air, which  he produces by placing it in  a  close
receiver with different mixtures of oil of vitriol and water, for which mixtures he has given
an  elaborate table of tensions;  and also to test the instrument from  time to time when
in use.   He also proposes  to remove  the natural grease by placing the hairs for 24
hours in  ether, by which they retain their strength and  solidity, and  acquire almost the
same sensibility.   As by placing the instrument in perfectly dry air  in order to fix the 0*,
it requires  several days  to become moderately stationary, and the hair continues  to con-
                                        115 
116
BOYE'S INANIMATE  MATTER.
tract, though much less, even for several months, he considers the state to which it is thus
reduced as unnatural, and therefore permanently injuring its hygrometrical properties.
As the air also never reaches this degree, he proposes, therefore, to drop the present 0°
altogether, and to begin the scale from a point, which corresponds to a relative  humidity
of 20°, and which is produced in a close receiver at the temperature of 42?83 Fah. by the
mixture of oil of vitriol and water, which has the chemical composition of 1 atom of sul-
phuric acid, and 5 atoms of water, being represented by the formula, SO3 -f- 5 HO.
   181.  Hygroscopes.   Many other instruments have been constructed from
other organic substances,  acting on the same principle as the hair hygrome-
ter ; but all these have no scientific value whatever, as none of them can be
relied upon for indicating the relative humidity, even only approximately.
They are therefore not hygrometers, but hygroscopes (from bypos (hugros),
and ffxo-ea> (skopeo), I observe), and as such they may be used with advan-
tage to indicate a mere increase or decrease in the moisture of the  atmo-
sphere.   Of such may be mentioned,  strips or  bars of whalebone or wood,
cut across the grain.   The former may be reduced to a thin thread or band,
and may be  made to act on a wheel with an index in a similar manner as
the hair.  All  twisted strings  made of vegetable fibres, as hempen cords,
or of animal membranes, as cat-gut or violin strings, will swell by moisture
and thus by  the increase in their  diameter untwist themselves, or, if pre-
vented from this, become shorter by the increased twist.   A piece of violin
string, if properly prepared, may thus, by its untwisting, be made to turn
back  the hood or  cowl from the head of a figure in dry weather  and to
replace it in  damp weather; or to raise  its arm  and unfurl  an umbrella;
or to turn a lever so as to show alternately through a window or before the
door of a toy-house, two different figures, representing rainy and fine weather.
The beard of the husk around the seed of Sensitive Oats (Avena sensitiva),
is  naturally twisted or coiled  as  a double spiral, so that if one  end be
fastened  in  the centre of a graduated circle, and a light index of straw
attached  by sealing-wax to the other, the latter will  traverse  the  circular
scale by the coiling or uncoiling of the beard by the moisture in  the  air.
The bladder of a rat or squirrel, may also be converted into  a hygroscope,
by tying  its mouth over the end of an open glass-tube and filling the bladder
and part of the tube with mercury.  By the contraction or swelling of the
bladder by the  change in moisture, the mercury will rise or fall in the tube.
Whalebone, reduced to the  thinness  of fine paper, goldbeater's s7cin,  and
thin sheets  of  gelatine or glue, will show such sensitiveness to moisture,
that if cut into figures, as fishes, etc., and placed in the palm of the hand,
the natural  moisture of the latter will cause the side  next to it to swell,
and the figure to curl itself up.
116 
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Table V. Correction for Altitude, on account of Decrease of Gravity from level of sea upward
  into the Atmos.  To be applied to Height obtained from Barom. Observ., see par. 95, p. 68.
Obtained	Oorrec. to	Obtained	Correc. t >-OlitaindCorrec. to Obtain'd				Correc. to	Obtain'd Correc. 1'	
Hei-ht	lie added.	Height	be added.	Height	be added.	Height	be added.	Height	be add.-.;.
in Feet.	Feet.	in Feet.	Feet.	in Feet.	Feet.	in Feet.	Feet.	in Feet.	Feet.
200	0.502	5200	14.303	10200	30.492	15200	49.087	20200	69.876
400	1.008	5400	14.905	10400	31.196	15400	49.880	20400	70.959
600	1.518	5600	15.511	10600	31.897	15600	50.677	20600	71.851
800	2.032	5800	16.120	10800	32.602	15800	51.478	20800	72.748
1000	2.550	6000	16.734	11000	33.312	16000	52.282	21000	73.649
1200	3.071	6200	17.351	11200	34.024	16200	53.092	21200	74.553
1400	3.596	6400	17.972	11400	34.741	16400	53.904	21400	75.461
1600	4.125	6600	18.597	11600	35.462	16600	54.721	21600	76.374
1800	4.658	6800	19.225	11800	36.186	16800	55.541	21800	77.289
2000	5.195	7000	19.858	12000	36.914	17000	56.365	22000	78.209
2200	5.735	7200	20.494	12200	37.646	17200	57.193	22200	79.133
2400	6.280	7400	21.134	12400	38.382	17400	58.024	22400	80.060
2600	6.828	7600	21.778	12600	39.122	17600	58.860	22600	80.991
2800	7.380	7800	22.426	12800	39.866	17800	59.699	22800	81.926
3000	7.936	8000	23.733	13000	40.613	18000	60.542	23000	82.865
3200	8.496	8200	24.165	13200	41.364	18200	61.389	23200	83.808
3400	9.059	8400	24.392	13400	42.119	18400	62.240	23400	84.755
3600	9.627	8600	Iffl Ol	13600	42.878	18600	63.095	23600	85.705
3800	10.198	8800	25.722	13800	43.641	18800	63.953	23800	86.659
4000	10.773	9000	26.393	14000	44.407	19000	64.815	24000	87.617
4200	11.352	9200	27.068	14200	45.177	19200	65.681	24200	88.579
4400	11.934	9400	27.746	14400	45.952	19400	66.565	24400	89.545
4600	12.521	9600	28.428	14600	46.730	19600	67.425	24600	90.514
1 4800	13.111	9800	29.115	14800	47.512	19800	68.303	24800	91.488
| 5000	13.705	10000	29.804	15000	48.297	20000	69.184	25000	92.465
TABLE VI.  For the conversion of French into English, and English into French measures.
French Millime-tres.	English Inches.	English Inches.	French Millimetres.
1 2 3 4 5 6 7 8 9 720 730 740 750 760	0.03937079 0.07874158 0.11811237 0.15748316 0.19685895 0.23622474 0.27559553 0.31496632 0.35433711 28.84697 28.74068 29.13438 29.52809 29.92180	1 2 3 4 5 6 7 8 9 27 28 29 30 31	25.39954 50.79908 76.19802 101.59816 126.99770 152.39724 177.79678 203.19632 228.59586 685.78758 711.18712 736.58666 761.98620 787.38574
French Metres.	English Feet.	English Feet.	French Metres.
1 2 3 4 5 6 7 8 9	3.2808992 6.5617984 9.8426976 13.1235968 16.4044960 19.6853952 22.9662944 26.2471936 29.5280928	1 2 3 4 5 6 7 8 9	0.30479449 0.60958898 0.91438347 1.21917796 1.52397245 1.82876694 2.13356143 2.43835592 2.74315041
1 French Gramme Weight — 15.133 Engl, grains.
1 Fr. Kilogramme = 2.2047 Engl, pounds Av.d.p.
1 Paris or old French Foot r= 1.065765 English Foot.
1  «      "    «    Inch = 1.065765   "    Inch.
1  "      «    "    Line = 0.088S14   "      "
1 French Litre = 61.0275 English cubic Inches.
1 Engl. Wine Gallon = 231.044 Engl, cubic Inches.
1 Engl, cubic Inch = 0.00432818 Engl. Wine Gal.
1 Eng. cub. In.=2o2.4."iS Eng. grains of Water of 62°.
1C00 Ens. gr's Water of 62° = 3.961054 Eng. cub. In. 
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TABtE IX (Continued).  Giving the Maximum Tension or Elastic Force of Vapor of Water
'letup.  .Max. Tens
 Fah.   Inch. Merc
92°.0
91°.8
91°.6
91°.4
91°.2
91°.0
90°.8
90°.6
90°.4
90°.2
90°.0
89°.8
89°.6
89°. 4
89°.2
89°.0
88°.6
88°. 4
88°.2
88°.0
87°.8
87°.6
87°.4
87°.2
87°.0
86°. 8
86°.6
86°. 4
86°.2
86°.0
85°. 8
85°. 6
85°.4
85°.2
85°.0
84°. 8
84°.6
84°.4
84°.2
84°.0
83°.8
83°.6
83°. 4
83°.2
83°,0
82°.8
82°. 6
82°.4
82°.2
82°.0
81°.8
81°.6
81°. 4
81°.2
81°.0
1.5006
1.4913
1.4821
1.4729
1.4637
1.4545
1.4454
1.4364
1.4274
1.4185
1.4096
1.4008
1.3921
1.3834
1.3747
1.3661
1.3575
1.3489
1.3404
1.3319
1.3235
1.3152
1.3069
1.2986
1.2904
1.2822
1.2741
1.2660
1.2580
1.2500
1.2421
1.2342
1.2263
1.2185
1.2107
1.2030
1.1953
1.1877
1.1801
1.1726
1.1651
1.1576
1.1502
1.1428
1.1354
1.1281
1.1208
1.1136
1.1064
1.0993
1.0922
1.0851
1.0781
1.0711
1.0641
1.0572
biller-
ences.
.0093
.0092
.0092
.0092
.0092
.0091
.0090
.0090
.0089
.0089
.0088
.0087
.0087
.0087
.0086
.0086
.0086
.0085
.0085
.0084
.0083
.0083
.0083
.0082
.0082
.0081
.0081
.0080
.0080
.0079
.0079
.0079
.0078
.0078
.0077
.0077
.0076
.0076
.0075
.0075
.0075
.0074
.0074
.0074
.0073
.0073
.0072
.0072
.0071
.0071
.0071
.0070
.0070
.0070
.0069
JeUip.
 1'ah.
81°.0
80°.8
80°.6
80°.4
80°.2
80°.0
79°.8
79°.6
79°.4
79°.2
79°,0
78°.8
78°.6
78°.4
78°.2
78°.0
77°.8
77°.6
77°.4
77°.2
77°.0
76°.8
76°.6
76°.4
76°.2
76°.0
75°.8
75°.6
75°.4
75°.2
75°.0
74°.8
74°.6
74°. 4
74°.2
74°.0
73°.8
73°.6
73°.4
73°.2
73°.0
72°.8
72°.6
72°.4
72°.2
72°.0
71°.8
7P.6
71°.4
71°.2
71°.0
70°. 8
70°.6
70°.4
70°.2
70°.0
Max. TeD
Inch. Merc,
 L.U572
 1.0503
 1.0435
 1.0367
 1.0300
 1.0233
 1.0166
 1.0100
 1.0034
 0.9968
 0.9903
 0.9838
 0.9774
 0.9710
 0.9646
 0.9583
 0.9520
 0.9457
 0.9395
 0.9333
0.9272
0.9211
0.9150
 0.9089
0.9028
0.8968
0.8909
0.8850
0.8792
0.8734
0.8676
0.8618
0.8560
0.8503
0.8446
0.8390
0.8334
0.8279
0.8224
0.8169
0.8114
0.8060
0.8006
0.7952
0.7898
0.7845
0.7792
 0.7740
 0.7688
 0.7636
 0.7585
 0.7534
 0.7483
 0.7432
 0.7381
 0.7331
  10 '""
Hitler- I  Temp.
entes.    Fah.
.0069
.0068
.0068
.0067
.0067
.00(57
.0066
.0066
.0066
.0065
.0065
.0064
.0064
.0064
.0063
.0063
.0063
.0062
.0062
.0061
.0061
.0061
.0061
.0061
.0060
.0059
.0059
.0058
.0058
.0058
.0058
.0058
.0057
.0057
.0056
.0056
.0055
.0055
.0055
.0055
.0054
.0054
.0054
.0054
.0053
.0053
.0052
.0052
.0052
.0051
.0051
.0051
.0051
.0051
.0050
70°.0
69°.8
69°.6
69°.4
69°.2
69°.0
68°.8
68°.6
68°.4
68°.2
68°.0
67°.8
67°.6
67°.4
67°.2
67°.0
66°. 8
66°.6
66°.4
66°.2
66°.0
65°.8
65°. 6
65°.4
65°.2
65°.0
64°. 8
64°. 6
64°.4
64°.2
64°.0
63°,8
63°.6
63°.4
63°.2
63°.0
62°. 8
62°.6
62°.4
62°.2
62.°0
61°.8
61°.6
61°.4
61°.2
61°.0
60°.8
60°.6
60°. 4
60°.2
60°.0
59°.8
59°.6
59°.4
59°.2
59°.0
Max. Tens
Inch. Merc
0.7331
0.7281
0.7232
0.7183
0.7134
0.7085
0.7036
0.6988
0.6941
0.6894
0.6847
0.6800
0.6754
0.6708
0.6662
0.6616
0.6570
0.6525
0.6480
0.6435
0.6391
0.6347
0.6303
0.6260
0.6217
0.6174
0.6131
0.6088
0.6046
0.6004
0.5962
0.5921
0.5880
0.5839
0.5798
0.5758
0.5718
0 5678
0.5638
0.5599
0.5560
0.5521
0.5482
0.5443
0.5405
0.5367
0.5329
0.5291
0.5254
0.5217
0.5180
0.5143
0.5107
0.5071
0.5035
0.4999
 Differ-
 ences.

.0050
.0049
.0049
.0049
.0049
.0049
.0048
.0047
.0047
.0047
.0047
.0046
.0046
.0046
.0046
.0046
.0045
.0045
.0045
.0044
.0044
•0044
.0043
.0043
.0043
.0043
.0043
.0042
.0042
.0042
.0041
.0041
.0041
.0041
.0040
.0040
.0040
.0040
.0039
.0039
.0039
.0039
.0039
.0038
.0038
..0038
.0038
.0037
.0037
.0037
.0037
.0036
.0036
.0036
.0036 
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