A NOTICE 
OF 
RECENT RESEARCHES IN SOUND 
BY 
WILLIAM B. TAYLOR. 
Reprinted from the American Journal of Science and Arts, for 
January and February. 1876. 
NEW HAVEN: 
TUTTLE, MOREHOUSE & TAYLOR, PRINTERS. 
1876. [Prom the Am. Journal of Science and Arts, January and Pebruary, 1876.] 
ON RECENT RESEARCHED IN SOUND. 
Hv WILLIAM 4^TAYLOR 
That two so eminent physicists as Professor Tyndall in 
England, and Professor Henry in our own country, should have 
been for some time past (and almost simultaneously) engaged in 
investigating the aberrant actions of Sound, is a noteworthy cir- 
cumstance. That these investigations should also have been 
undertaken in both cases with especial reference to securing 
increased efficiency to the national systems of Fog-signaling, is 
no less notable. In view of the many disastrous marine accidents 
resulting from fogs on either coast, such inquiries so conducted, 
have a no slight practical importance; and every thoughtful 
mind must regard with profound interest a series of researches 
requiring so much patient labor for the attainment of new and 
accurate information on the subject, and so high a degree of scien- 
tific sagacity and skill for its right interpretation. 
As somewhat different explanations have been offered by these 
two distinguished observers to account for certain abnormal phe- 
nomena of sound, a concise statement of the facts and views 
respectively announced, will interest the general reader. The 
records of these investigations are, on the one side, the Philosophi- 
cal Transactions of the Royal Society of London, for the year 1874, 
vol. clxiv, page 188, ‘‘On the Atmosphere as a Vehicle of Sound,” 
by John Tyndall, LL.D., F.R.S., a communication read February 
12, 1874; and on the other side, the annual Report of the Light 
House Board of the LTnited States for the year 1874; the Appendix 
to which is an account of the operations of the Board relative to 
Fog-Signals, by Joseph Henry, Chairman of the Light House 
Board. In addition to these principal sources of information, 
reference will be made to an interesting communication read 2 
Recent Researches in Sound. 
before the Royal Society, April 23, 1874, “On the Refraction of 
Sound,” by Professor Osborne Reynolds, and published in the 
Proceedings of the Royal Society for 1874. The salient points of 
the observations are selected, and are here arbitrarily designated 
by bracketed numbers, to facilitate comparisons. 
Ten years ago, or in 1865, Professor Henry commenced his 
investigations on the subject of Sound in connection with fog- 
signals, at the Light House station near New Haven, Connecticut. 
Omitting here his careful experiments in regard to the character 
of the various instruments employed, the principal results then 
obtained, were the following: 
[1.] The reflection of sound was observed to be very imperfect 
and inexact. A large concave reflector with a smoothly plastered 
surface of 64 square feet, produced a sensible increase of effect in 
the sound, within a distance of 500 yards in front of the signal: 
beyond this distance, the difference became imperceptible. It 
appeared that “ while feeble sounds at small distances are reflected 
as rays of light are, waves of powerful sound spread laterally, and 
even when projeoted from the mouth of a trumpet, at a great dis- 
tance tend to embrace the whole circle of the horizon.” (L. H. 
Rep., p. 88.) A trumpet, however, which could be heard six 
miles in front (in the direction of the axis) was heard only three 
miles in the rear. (p. 92.) 
[2.] “ For determining the relative power of the instruments, 
the use of two vessels had been obtained.” The instruments at 
the light-house station were a large bell, a steam-whistle 6 inches 
in diameter, a double whistle, “improperly called a steam gong,” 
12 inches in diameter, the cups being 20 and 14 inches deep, pro- 
ducing the harmonic interval of a tone and its fifth, and a Daboll 
trumpet operated by a hot air engine. The blow-off sound from 
the “ exhaust” of the air engine was also noted. “ The penetrating 
power of the trumpet was nearly double that of the whistle.” 
{Rep., p. 90.) The order of audible range on the first day was 
found to be 1st, trumpet, 2nd, exhaust, 3rd, bell, the whistle not 
being sounded. On the second day, 1st, trumpet and “gong,” 
2nd, whistle, 3rd, exhaust. In the rear the trumpet was heard no 
I. Recent Researches in Sound. 
3 
farther than the whistle. On the third day, the order was similar, 
—1st, trumpet, 2nd, whistle, 3rd, exhaust, 4th, bell. (p. 91.) 
The opportunity was unfavorable to the observation of these 
sounds when they were moving directly with the wind. 
[3.] Simultaneous observations from two vessels sailing in 
nearly opposite directions, showed that the sound did not extend 
against the wind so far as in the direction of the wind ; and on 
subsequent days, results obtained from sounds moving nearly 
against the wind, and at right-angles to it, indicated that an 
opposing wind, when light, obstructed sound less than when 
stronger, and that wind at right-angles to the sound, permitted it 
to be heard farther. (Rep., p. 92.) 
[4.] *• During this series of investigations an interesting fact 
was discovered, namely, a sound moving against the wind, inaudi- 
ble to the ear on the deck of the schooner, was heard by ascending 
to the mast-head.” (p. 92.) These results were obtained in 1865. 
[5.] An experiment subsequently made at Washington during 
a fog, with a small clock-work alarm bell, indicated that the fog 
did not absorb sound; though want of the opportunity of a com- 
parative observation prevented the result from being entirely 
satisfactory, (p. 93.) 
In 1867, the principal object of investigation was a comparison 
of different instruments, the character and value of the improve- 
ments made in them and especially an examination of a new fog- 
signal made under the direction of the Board by Mr. Brown, of 
New York,—the steam siren (p. 194), an instrument which has 
since played an important part in fog-signaling. Employing 1st a 
large Daboll trumpet, 17 feet long, (its steel tongue being 10 
inches long), and operated by a hot air engine, 2nd, a siren oper- 
ated by a tubular steam boiler, and 3rd, a steam whistle, 8 inches 
in diameter,—an elaborate series of experiments was made as to 
their penetrating power, as to the most efficient pitch or tone, 
(p. 95), the effect of varying steam pressure from 20 pounds per 
square inch to 100 pounds per square inch, (p. 97), the material 
and shape of the trumpets, &c. (p. 98.) 
[6.] During this series of experiments in 1867, attention was 
called by General Poe, of the Light House Board, to the circum- 
stance that the sound of the paddle-wheels of a steamer some four 4 
Recent Researches in Sound. 
and a half miles distant from the shore could be distinctly heard 
by bringing the ears near to the surface of the beach. This fact 
had previously been noticed on the northern lakes. The desira- 
bility of experimenting with large hearing trumpets placed near 
the surface of the water is suggested by Professor Henry, (p. 98.) 
[7.] Experiments on the divergence of acoustic beams, while 
indicating a considerable reduction of sound toward the rear of 
the trumpet, showed also very strikingly, the increasing tendency 
of sound to spread on either side of the axis of the trumpet, (p. 98.) 
This corresponds with the observations [1] on the employment of 
sound reflectors. 
An important suggestion is made, requiring experimental deter- 
mination, namely, that condensed air would probably give more 
efficient results to both the fog-whistle and the siren, than steam. 
From hypothetical considerations this would appear to be the 
case, since the intensity of sound depends on the density of the 
medium in which it is produced; and as the steam is considerably 
lighter than air, and as the cavities of all these instruments are 
largely filled with steam, the intensity of sound would on this 
account seem to be less.” (Rep., p. 99.) 
In the absence of Professor Henry in England in 1870, experi- 
ments were continued by General Duane, one of the Light House 
District engineers. These will presently be noticed. 
[8.] In 1872 Professor Henry observed from a steamer in the 
harbor of Portland, Maine, that while approaching an island from 
which a fog-signal was audible,—at the distance of two or three 
miles, the sound was lost for nearly a mile, and then slightly 
regained at nearer approach. This was partly in the rear of the 
signal; and from its position on the farther side of the island from 
the steamer, with a large house and rising ground interposed, Pro- 
fessor Henry infers that the region of inaudibility was covered by 
an acoustic shadow, encroached upon at a greater distance by the 
divergence of the rays of sound, which, bending, reached ultimately 
the surface of the water, (p. 107.) A similar phenomenon was 
observed in the same year on approaching Whitehead station 
near the coast of Maine. The fog-signal was heard from the dis- 
tance of six miles to about three miles, and then lost until within 
a quarter of a mile. (p. 107.) Again, at little Gull Island, in a Recent Researches in Sound. 
5 
vessel receding from the siren signal in the direction of its trumpet 
axis, the sound was lost at a distance of two miles, and then 
regained at a distance of four and a half miles, (p. 111.) These 
last cases are referred by Professor Henry to a flexure of the rays 
of sound resulting from differences of wind velocity in the upper 
and lower strata of air. 
[9. ] In 1872, it was observed that a fog-signal was heard from 
one station to another, while a simultaneous signal from the latter 
was inaudible in the opposite direction. On board a steamer 
approaching Whitehead station (a mile and a half from the coast 
of Maine), the signal, a steam-whistle, failed to be heard from the 
distance of about three miles to about a quarter of a mile from the 
station; while a smaller whistle on the steamer was distinctly 
heard by the keeper at the station during that time. The wind 
was slightly transverse to the direction from the steamer to the 
station, but approximately in that direction. The steamer after 
stopping at the station, on passing from it almost directly against 
a light wind, continued to hear the signal with variable distinct- 
ness for about fifteen miles, (p. 108.) In September, 1874, the 
keeper at Block Island, on the coast of Rhode Island, observed 
according to instructions, the times when the fog-signal from 
Point Judith at a distance of seventeen miles was audible, and in 
comparing the times when the Block Island signal (a powerful 
steam siren) was heard at Point Judith, it appeared that the two 
sounds had not been heard simultaneously by the two keepers. 
(p. 112.) 
[10.] In August, 1873, at Cape Elizabeth station in Maine, the 
phenomenon of ocean-echoes was distinctly noticed on board a 
steamer as it was passing directly outward from the signal; the 
sound after each whistle being returned from the unobstructed 
space beyond, (p. 109.) In September, 1874, at Black Rock 
Island also, shortly after each blast of the trumpet, a prolonged 
echo from the open ocean was distinctly heard. The echo was 
observed not to be loudest at the siren-house, but at a point several 
hundred yards to one side; the wind being in the direction of 
the primitive sound, and nearly opposite to the direction of the 
reflected echo. (p. 112.) This was supposed by Professor Henry 
to be caused by a reflection of the sound from the crests and 
slopes of the waves. 6 
Recent Researches in Sound. 
[11.] On September 23rd, 1874, three observations were made 
on board steamers moving in opposite directions about one and a 
half miles from Sandy Hook, New Jersey. First, before noon 
with the wind from the west, second, at noon with the wind lulled 
to a calm, and third, an hour and a half later, with the wind blow- 
ing from the east. These observations gave the unexpected result 
of the sound being heard in each case uniformly farthest from the 
west, irrespective of the wind. (p. 114.) On the next day, Sep- 
tember 24th, the observations were repeated farther out at sea, or 
six miles from the nearest land. Small balloons, sent off with 
each observation on the sound, showed that notwithstanding the 
change of surface wind as before, from morning to afternoon, the 
upper current of wind was steadily and continuously from the 
west. (p. 115.) Professor Henry supposes that in the first case 
“ the motion of the air being in the same direction both below 
and above, but probably more rapid above than below on account 
of resistance, the upper part of the sound-wave would move more 
rapidly than the lower, and the wave would be deflected down- 
ward, and therefore the sound as usual heard farther with the 
wind than against it.” In the third case with a local sea-breeze 
in the opposite direction, and the upper current remaining un- 
changed, “ the sound should be heard still farther in the same 
direction or against the wind at the surface, since in this case the 
sound-wave being more retarded near the surface, would be 
tipped over more above, and the sound thus thrown down.” 
(p. 115.) This explanation derived from a communication of 
Professor Stokes, at the Dublin Meeting of the British Association 
in 1857, {Rep. of B. A., 1856, p. 22 of Abstracts) would appear to 
be a very satisfactory solution of the apparent anomaly. 
In 1870, General Duane, the engineer in charge of the Light 
House District embracing the coast of Maine, New Hampshire, 
and Massachusetts, was assigned by the Light House Board, (as 
one “ who from his established reputation for ingenuity and prac- 
tical skill in mechanism, was well qualified for the work,”) to 
make experiments and observations on fog-signals. Accordingly 
during the year 1871, extensive investigations were made by him 
II. Recent Researches in Sound. 
at Portland, Maine. Passing over his valuable remarks on the 
qualities of fog-signals, the following are the principal facts ob- 
served bj him: 
[A.] The extremely variable range of sound. The steam fog- 
whistles on the coast of Maine could frequently be heard at a 
distance of twenty miles, and as frequently could not be heard 
two miles, with apparently the same state of the atmosphere. 
(L. H. Rep., p. 100.) 
[B.] The signal was often heard at a great distance in one direc- 
tion, while scarcely audible at a mile in another direction, and 
this quite irrespective of the wind. (p. 100.) 
[C.] Falling snow was observed not to obstruct sound sensibly, 
as the steam-whistle on Cape Elizabeth can be “distinctly heard 
in Portland, a distance of nine miles, during a heavy northeast 
snow-storm, the wind blowing a gale directly from Portland 
toward the whistle.” (p. 100.) 
[D.] The signal station frequently “appears to be surrounded 
by a belt varying in radius from one to one and a half miles, from 
which the sound appears to be entirely absent.” Receding from 
the signal, its sound may be audible for the distance of a mile, 
then lost for the distance of a second mile, and then audible again 
for a much farther distance. This abnormal phenomenon has 
been observed at various stations, and at one where the signal is 
on a bare rock in mid-ocean, twenty miles away from land, and 
with no surrounding objects to affect the sound, (p. 100.) 
No observations have been made to show that this occasional 
sound-chasm is really a “belt” entirely surrounding the signal; 
a supposition which appears to be antecedently improbable, and 
one which would require a large number of radiating observations 
made simultaneously, to establish it. The curious and exceptional 
fact, however, is confirmed by the observations of Henry [8] made 
subsequently. 
[E.] Confirmatory of Henry [1], General Duane found that a 
whistle in the focus of a large parabolic reflector, though giving 
a notably louder sound in front near the reflector, yet at the dis- 
tance of a few hundred yards, had its beam of sound so spread 
that the acoustic shadow behind the mirror vanished, and no per- 
ceptible difference appeared. A wooden trumpet or square pyra- 8 
Recent Researches in Sound. 
midal box 20 feet long, in a horizontal position with the whistle 
in the smaller end, gave, however, more successful results, the 
increase of sound in the open axis being perceptible at the distance 
of a mile. (Rep., p. 103.) This corresponds also with fiend’s 
observation [7]. 
[F.] In repetition and explanation of observation [A] General 
Duane remarks: “It frequently occurs that a signal which under 
ordinary circumstances would be audible at the distance of fifteen 
miles, cannot be heard from a vessel at the distance of a single 
mile. This is probably due to the reflection mentioned by Hum- 
boldt.” (p. 104.) This great traveler and scientific observer, in 
his graphic narrative of exploration in the northern part of South 
America, published at the beginning of the century, ascribes the 
diminished audibility during the day, of the noise from the cata- 
racts of the Orinoco, at a place on the Atures, to the unequal 
heating of the air and the reflection and dispersion of the sound 
from the surfaces of the striae of differing density. 
[G.] It was further noticed by General Duane that “when the 
sound is thus impeded in the direction of the sea, it has been 
observed to be much stronger inland;” tending to confirm his 
idea that the sound in passing from a warmer to a cooler region 
of air “undergoes reflection at their surface of contact.” (p. 104.) 
Professor Henry dissents from this opinion that the extinction 
of powerful sounds is due to unequal density of the atmosphere. 
Admitting that “a slight degree of obstruction of sounds may be 
observed” from such a condition, he thinks it “entirely too minute, 
to produce the results noted.” (p. 104.) He believes that the 
“ true and sufficient cause” is the difference between the upper 
and lower currents of air, which tends to bend the sound rays 
either upward or downward, as suggested by Professor Stokes in 
1857. He adds, “In the comments we have made on the Report 
of General Duane the intention wTas not in the least to disparage 
the value of his results which can scarcely be too highly appreci- 
ated.” (Rep., p. 106.) 
[H.] A difficulty occasionally observed with vessels in a fog, is 
an apparently false direction of the audible signal; which General 
Duane regards as “due to the refraction of sound in passing 
through media of different density.” (p. 104.) Recent Researches in Sound. 
9 
[I.] While thus adopting “ the conclusion that these anomalies 
in the penetration and direction of sound from fog-signals, are to 
be attributed mainly to the want of uniformity in the surrounding 
atmosphere,” General Duane was also led from observation and 
experiments to believe “ that snow, rain, fog, and the force and 
direction of the wind, have much less influence than has generally 
been supposed.” (p. 104.) This is in confirmation of his previous 
observation [Cj. 
III. 
Professor Tyndall commenced his investigations on fog-signals 
on the 19th of May, 1873, “ at the instance of and in conjunction 
with the elder brethren of the Trinity House,” as the scientific 
adviser of the Corporation. 
[l.J On May 20, 1873, observations showed the relative pene- 
trating power of different instruments to be variable. At six 
miles the fog-horn was inaudible, while an eighteen pound gun 
with three pound charge was heard for ten miles. On many sub- 
sequent occasions the horn was found to be superior to the gun. 
{Trans. R. S., p. 188.) Occasionally the whistles were superior to 
the trumpet, though not generally so. (p. 189.) Later experi- 
ments in October showed that the pitch of the sound had variable 
penetration on different days and even at different times on the 
same day. The siren (an American instrument lent by the United 
States Lighthouse Board, and put in use October 8, 1873) was 
generally decidedly triumphant, but not always so. (7Vans., pp. 
220, 221.) 
[2.] The defect of sound in the acoustic shadow of an interven- 
ing obstacle (a chalk cliff) was very strikingly manifested. In 
June the same sharpness of shadow line was observed ; and even 
with the instruments in view, at the distance of a mile, their sound 
entirely failed near the shadow line at one side. {Trans., p. 190.) 
[3.] Although “ the wind exerts an acknowledged power over 
sound,” yet, on the 25th of June, “ when the range was only six 
and a half miles, the wind was favorable; on the 26th when the 
range exceeded nine and a quarter miles, it was opposed to the 
sound.” (p. 194.) On October 11, the sound was observed to be 
much affected by an adverse wind. It was also noticed on this as 10 
Recent Researches in Sound. 
well as on subsequent occasions, that “ an opposing wind affects 
the gun-sound far more seriously than that of the siren.” With a 
favoring wind, sounds were heard twice as far as with an adverse 
wind, even at a point “ more deeply immersed in the sound- 
shadow.” (p. 224.) 
[4.] July 1, at a distance of five and a quarter miles from a 
rotating horn it was observed that the sound was sensibly stronger 
in front than at the rear of the trumpet, the reduction being esti- 
mated as seven to ten. (p. 192.) 
[5.] July 1. “ In a thick haze, the sound reached a distance of 
twelve and three-quarter miles, while on May 20, in a calm and 
hazeless atmosphere, the maximum range was only from five to 
six miles.” (p. 193.) And subsequent observations made in 
London, December 10 and 11, showed that a thick fog offered no 
sensible obstruction to the passage of sound, (pp. 209, 210.) 
[6.] On July 8, at 2.15 P. M. “ with a calm clear air and smooth 
sea,” at three miles from the signal station “neither horn nor 
whistle was heard. The guns were again signaled for; five of 
them were fired in succession, but not one of them was heard.” 
(pp. 194, 195.) As a hot sun was pouring its beams on the sea, 
Professor Tyndall supposed that the copious evaporation resulting, 
would most probably act very irregularly, producing streams or 
wreaths of vapor, and thus render the air flocculent with these in- 
visible cloudlets, whose surfaces would occasion a large amount of 
repeated reflection and dispersion of the sound waves. As the sun 
afterward became clouded at 3.15 P. m., the sounds of the signal 
were heard at three miles, and very faintly at four and a quarter 
miles; and later at six miles, and seven and three-quarter miles. 
Toward the close of the day the signals were heard at twelve and 
three-quarter miles, (pp. 196, 197.) 
[7.] On the same day at one o’clock, the echoes from the direc- 
tion of the open sea were very distinct at the signal station. 
“ The instruments hidden from view, were on the summit of a cliff 
235 feet above us, the sea was smooth and clear of ships, the atmos- 
phere was without a cloud, and there was no object in sight which 
could possibly produce the observed effect. From the perfectly 
transparent air, the echoes came, at first with a strength apparently 
but little less than that of the direct sound, and then dying grad- Recent Researches in Sound. 
11 
ually and continuously away/’ (p. 198.) These remarkable 
echoes are supposed by Professor Tyndall to be returned from the 
invisible surfaces of the vaporous striae, which thus render the air 
opaque to the sonorous waves. Subsequently, on the 8th of Octo- 
ber, the American siren being just received and set up, its loud 
echoes were observed to be “far more powerful than those of the 
horn,” and to last eleven seconds, while those of the horn had eight 
seconds duration, (p. 199.) On the 15th of October, the direc- 
tion of the echoes was found to correspond with the principal axis 
of the direct or primitive sound; the direction of the return sound 
changing with the rotation of the horn. (p. 200.) 
[8.] On October 8th rain and hail were found not to obstruct 
sound. While in the morning (after a thunder storm) from Dover 
and the South Foreland across the English channel “ for a time 
the optical clearness of the atmosphere was extraordinary, the coast 
of France, the Grisnez lighthouse, and the Monument and Cathe- 
dral of Boulogne being clearly visible in positions from which they 
were generally quite hidden; the atmosphere at the same time 
was acoustically opaque ; ” and the horn was feebly heard at six 
miles, (p. 205.) But in the afternoon a storm arose, and although 
the rain was falling heavily all the way between the signal station 
at Foreland and the point of observation on the steamer, “the 
sound instead of being deadened, rose perceptibly in power. Hail 
was now added to the rain, and the shower reached a tropical vio- 
lence.” “ In the midst of this furious squall both the horn and 
the siren were distinctly heard,” and as the shower lightened, 
diminishing the local pattering of the deck, they were heard “at a 
distance of seven and a half miles distinctly louder than they had 
been heard through the rainless atmosphere at five miles.” (p. 
206.) On the 23d of October, a similar experience was noticed on 
land, and contrary to the usual impression, snow was also observed 
to offer no serious obstacle to sound, (p. 207.) 
It must be borne in mind that the investigations by Professor 
Tyndal were concluded before the publication of the United States 
Lighthouse Report. And it is noticeable that these two series of 
original observations thus independently made on the opposite 
sides of the Atlantic, in the main quite strikingly confirm each 
other. 12 
Recent Researches in Sound. 
Tyndall’s notice [1] of the inconstant relative range of different 
instruments corresponds with Henry (2), though indicating a much 
more marked variability. 
Tyndall's notice [2] of the sound shadow, corresponds generally 
with Henry [7], and [8], and Duane [E], but assigns a sharper 
definition to its limit; probably in consequence of the intervention 
of a larger obstacle (a cliff), and an observation within a shorter 
distance. 
Tyndall [3] confirms Henry [3], and [11]. 
Tyndall [4] corresponds with Henry [7], and Duane [E]. 
Tyndall [5] confirms by a series of careful observations, the 
opinion of Henry [5] and Duane [I], 
Tyndall [6] confirms Duane [A], and [F], and in like manner 
adopts and extends the suggestion of Humboldt as to the cause of 
acoustic opacity. Professor Tyndall’s admirable skill in experi- 
mental physics enabled him to illustrate and fortify his hypothesis 
by exhibiting in a popular lecture an apparatus for producing in 
an elongated box or tunnel, aerial laminae of unequal density, 
through which the sound from a small alarm box failed to excite 
a sensitive flame. That this mottled condition of the air is there- 
fore a true cause of acoustic obstruction is no longer doubtful. 
To what extent a similar condition of the atmosphere actually 
prevails, in view of the law of the diffusion of gases, and how far 
such usual or unusual inequalities of density in the air are capable 
of entirely dispersing the powerful sound of a steam trumpet or 
siren, at the distance of a quarter of a mile, are not so positively 
determined. With a continuous wind any such condition of aerial 
“ flocculence ” might be expected to be very speedily dissipated. 
This theory, however, fails entirely to explain the interesting 
observations of Henry [4], [8]. and [9]. It is scarcely credible 
that a local screen of aerial flocculence could obliterate on the deck 
of a schooner, a fog-signal audible at the mast-head. Atmos- 
pheric refraction on the other hand, completely satisfies the ob- 
served condition; an opposing wind blowing at the time. Still 
less successful is the theory, in dealing with the abnormal phe- 
nomenon of simultaneous audibility at long range, with the 
intermediate “ belt ” of acoustic opacity, first observed by Duane 
[D]. And lastly, the assumption of the simultaneous transmission Recent Researches in Sound. 
13 
of sound through a flocculent air-screen in one direction and its 
absorption or dissipation by the screen in the opposite direction, 
(acoustic “ non-reversibility,”) is obviously inadmissible. Nor is 
the supposition of acoustic “ diffraction” around the defined edge 
of a vapor cloud, more available. 
Professor Tyndall in his recent Preface to the last edition of 
“Sound” remarks upon this observation of Henry [9]—“a suffi- 
cient reason for the observed non-reciprocity is to be found in the 
recorded fact that the wind was blowing against the shore-signal, 
and in favor of the ship-signal.” (Preface, p. xxi.) But he offers 
no suggestion how this “sufficient reason” is supposed to apply. 
As it is well known that an ordinary wind cannot increase the 
range of sound more than two or three per cent (an amount quite 
inappreciable), this circumstance alone is wholly inadequate to 
account for the complete suppression of the shore-signal (a ten- 
inch steam-whistle) from the distance of three miles to a quarter of 
a mile, while the feebler sound of the ship-signal (a six-inch steam 
whistle) was making itself distinctly heard throughout the three 
miles. Something more therefore than the direct or convective 
action of the wind must be invoked to explain the facts. 
Tyndall’s observation [7] on the aerial or ocean echoes, corre- 
sponds with Henry [10] excepting as to the direction of the prin- 
cipal echo. This difference is doubtless due to the special arrange- 
ment of the surfaces or points of reflection in the respective cases 
observed. Professor Tyndall connects this phenomenon with that 
of acoustic opacity [6] ; and here again his fine experimental skill 
is brought into requisition to demonstrate the reality of artificial 
“aerial echoes.” By so simple a device as the employment of the 
flat side of a “ bat-wing” gas-jet, the sound beam from a reed 
instrument was shown to be entirely deflected from one sensitive 
flame, and reflected back toward another. 
This view of a relation between the acoustic opacity outward or 
seaward, and the reinforcement or reflection of sound inward, is in 
striking accord with Duane [G], who, however, in referring to the 
“reflection” of sound, does not specifically allude to the ocean 
“ echo.” On the refraction theory also, a necessary result is that 
a deflection of the sound-beam upward in one direction, must be 
attended with a downward deflection and consequent increase of 
sound in the opposite direction. 14 
Recent Researches in Sound. 
Professor Henry had referred these mystic echoes to the crests 
and slopes of distant waves; (in conjunction probably with a 
curvature of the sound-beams, constituting a kind of acoustic 
“mirage.”) To this suggestion, Professor Tyndall opposes the 
observation that “the echoes have often manifested an astonishing 
strength, when the sea was of glossy smoothness.” (/Sound, Pref., 
p. xxiii.) 
That this very interesting subject presents features requiring 
still further and more relined investigation is sufficiently obvious 
from the single consideration that aerial opacity and echo have 
not been shown to bear that direct relation which the vapor 
theory requires. Professor Tyndall has recorded that, on the 17th 
of October (1873), “ It is worth remarking that this was our day 
of longest echoes, and it was also our day of greatest acoustic 
transparency, the association suggesting that the duration of the 
echo is a measure of the atmospheric depths from which it comes. 
On no day, it is to be remembered, was the atmosphere free from 
invisible acoustic clouds; and on this day when their presence 
did not prevent the direct sound from reaching to a distance of 15 
or 16 nautical miles, they were able to send us echoes of 15 seconds 
duration.” (Trans., p. 202.) If these echoes were not “folded,” 
this would represent an extreme limit of about a mile and a half. 
Our most powerful sounds cannot afford to waste much of their 
energy on echoes, if under the inexorable law of increasing atten- 
uation as the square of the distance they are to be audible through 
a range of 16 miles: less than the 400th of the intensity at one 
nautical mile, that is heard at the distance of 100 yards from the 
source; and one 256th of this at the distance of 16 nautical miles, 
or less than the hundred thousandth of the intensity at 100 yards. 
And the inference is strong that in such a case, accompanying 
echoes must be derived from sound beams in a somewhat different 
direction. 
Further observations are needed also to ascertain whether these 
aerial screens of unequal density and acoustic opacity are capable 
of returning echoes on opposite sides, as is to be expected if we 
may accept the analogy of catoptrics: and whether the echoes are 
as frequently heard from steamers in mid-ocean, or whether they 
mainly attach themselves to coast lines. As Professor Henry has Recent Researches in Sound. 
15 
well stated: “Much farther investigation is required to enable us 
to fully understand the effects of winds on the obstruction of 
sound, and to determine the measure of the effect of variations of 
density in the air due to inequality of heat and moisture.” (L. H. 
Rep., p. 117.) 
As the last of the series here selected, Tyndall’s observation [8] 
agrees well with the observation of Duane [I]. 
IV. 
The communication of Professor Reynolds “On the Refraction 
of Sound by the Atmosphere,” is in two parts: the first of which 
considers “The effect of Wind upon Sound,” and the second part 
“The effect of variations of Temperature.” The experiments 
were all made in “a flat meadow of considerable extent;” and 
the apparatus employed “consisted of anelectrical bell mounted 
on a case containing a battery. The bell was placed horizontally 
on the top of the case, so that it could be heard equally well in 
all directions; and when standing on the ground, the bell was 
one foot above the surface.” An anemometer was also used to 
determine the velocity of the wind. (.Proceedings of the Royal 
Society; republished in the L. E. D. Phil Mag., for July, 1875, 
vol. 1, p. 67.) 
The experiments were made on four different days, the 6th, 9th, 
10th, and 11th of March, 1874; and on the last two days the 
ground was covered with snow, which furnished an opportunity 
of comparing the effect of different surfaces on the range of Sound. 
Additional experiments were made on the 14th of March. 
[1.] “On all occasions the effect of wind seems to be rather 
against distance than against distinctness. Sounds heard to wind- 
ward [that is against the wind] are for the most part heard with 
their full distinctness; and there is only a comparatively small 
margin between that point at which the sound is perceptibly 
diminished, and that at which it ceases to be audible.” (Phil. 
Mag., p. 63.) 
[2.] The sound of the alarm-bell was always heard “farther 
with the wind than at right-angles to its direction; [contrary to 
the old observation of De La Roche in 1816,—which was obvi- 
ously an exceptional one;] and when the wind was at all strong, 16 
Recent Researches in Sound. 
the range with the wind was more than double that at right 
angles .... With the wind, over the grass the sound could be 
heard 140 yards, and over the snow 360 yards, either with the 
head lifted or on the ground; whereas at right angles to the wind, 
on all occasions the range was extended by raising either the 
observer or the bell.’' (p. 68.) 
[3.] When the wind was light the sound beyond the distance 
of 20 yards, was much less audible at the ground than a few feet 
above it; and when inaudible in every direction at standing 
height, the sound could be distinctly recovered by mounting a 
tree. The same result was obtained by raising the alarm-bell 
upon a post four feet high ; which while materially increasing the 
audible range of the sound—even in the direction of the slight 
wind, in all other directions doubled the range. This is explained 
by Professor Reynolds, by the continual waste and destruction of 
the sound waves which pass along the rough surface of the ground 
or grass, causing the waves immediately above to diverge contin- 
ually downward, to be in like manner absorbed; the effect of 
which is to gradually weaken the sound more and more, as the 
waves proceed; so that even “ when there is no wind, the distant 
sounds which pass above us are more intense than those we hear.” 
(p. 68.) 
[4.] Whatever therefore tends to gradually bend downward the 
sound rays will increase their sensible range. Professor Reynolds 
found by observations with the anemometer that the velocity of 
the wind increased from the ground upward; (pp. 63, 64) and 
hence it must give greater rapidity to the upper portion of the 
sound waves in the direction in which it is blowing and cause 
their impulses to continually tip downward. “ This was observed 
to be the case on all occasions. In the direction of the wind 
when it was strong, the sound could be heard as well with the 
head on the ground as when raised, even when in a hollow with 
the bell hidden from view by the slope of the ground; and no 
advantage whatever was gained either by ascending to an eleva- 
tion, or raising the bell.” (p. 68.) 
[5.] “ Elevation was found to affect the range of sound against 
the wind in a much more marked manner than at right angles. 
Over the grass no sound could be heard with the head on the Recent Researches in Sound. 
17 
ground at 20 yards from the bell, and at 30 yards it was lost with 
the head three feet from the ground, and its full intensity was lost 
when standing erect at 30 yards. At 70 yards when standing 
erect the sound was lost at long intervals, and was only faintly 
heard even then; but it became continuous again when the ear 
was raised nine feet from the ground, and it reached its full inten- 
sity at an elevation of 12 feet.” (p. 69.) The same results were 
obtained with snow on the ground, excepting that the sound was 
heard somewhat lower, being less dissipated or absorbed by the 
surface contact. At 160 yards the bell was inaudible—even at 
an elevation of 25 feet, and the sound was supposed to be hope- 
lessly lost; but at a further elevation of 33 feet from the ground, 
it was again heard ; while at five feet lower it was lost. At the 
proper elevation the sound appeared to be as well heard against 
the wind as with it, at the same distance. These last two obser- 
vations very strikingly correspond with and confirm the observa- 
tions of Henry [3], and [4]. 
[6.] “ The least raising of the bell was followed by a consider- 
able intensifying of the sound;” and while it could be heard only 
70 yards when resting on the ground (i. e., one foot high), when 
set on a post five feet high, it could be heard 160 yards, or more 
than twice the distance,—the sound-beams evidently rising faster 
at or near the ground, than they do higher up. (p. 69.) “ The 
intensity of the sound invariably seemed to waver, and as one 
approached the bell from the windward side, the sound did not 
intensify uniformly or gradually, but by fits or jerks.” This is 
supposed to be the result of the more or less curved sound rays 
crossing each other at a small angle and producing an “ inter- 
ference.” (p. 70.) 
A subsequent experiment was made on the 14th of March, 
during a strong west wind, its velocity at an elevation of 12 feet 
being 37 feet per second, at eight feet, 33 feet per second, and at one 
foot from the ground (there being no snow on the grass) 17 feet per 
second. While the results as to varying range fully confirmed 
the previous experiments, the raising of the bell caused the sound 
to be heard even better against the wind than in the direction of 
the wind. (p. 71.) This curious circumstance is explained by 
Professor Reynolds as “ due to the fact that the variation in the 18 
Recent Researches in Sound. 
velocity of the air is much greater near the ground, than at a few 
feet above it;” and “when the bell is raised the rays of sound 
which proceed horizontally will be much less bent or turned up 
than those which go down to the ground; and consequently after 
proceeding some distance these rays will meet or cross, and if the 
head be at this point they will both fall on the ear together, 
causing a sound of double intensity. It is this crossing of the 
rays also which for the most part causes the interference” just 
mentioned, (p. 71.) 
Professor Reynolds concludes that “these experiments establish 
three things with regard to the transmission of sound: 1. That 
when there is no wind, sound proceeding over a rough surface is 
more intense above than below. 2. That as long as the velocity 
of the wind is greater above than below, sound is lifted up to 
windward and is not destroyed. 3. That under the same circum- 
stances it is brought down to leeward, and hence its range 
extended at the surface of the ground. These experiments also 
show that there is less variation in the velocity of the wind over a 
smooth surface than over a rough one. It seems to me that these 
facts fully confirm the hypothesis propounded by Professor Stokes; 
that they place the action of wind beyond question; and that they 
afford explanations of many of the anomalous cases that have 
been observed.” (p. 71.) 
[7.] In regard to the second part of the communication, treating 
of the effect of Temperature differences in refracting sound, Pro- 
fessor Reynolds shows that as “ every degree of temperature 
between 32° and 70° adds approximately one foot per second to 
the velocity of sound,” there must necessarily be an upward 
flexure of the rays, whenever by reason of any considerable 
increase of temperature in the lower strata of the air, the lower 
portion of the sound waves is projected in advance of the upper 
portion, (p. 71.) Atmospheric vapor also, though exercising but 
little direct influence on the velocity of sound, “ nevertheless plays 
an important part in the phenomena under consideration; for it 
gives to the air a much greater power of radiating and absorbing 
heat, and thus renders it much more susceptible of changes in the 
action of the sun. ... It is a well-known fact that the tempera- 
ture of the air diminishes as we proceed upward, and that it also Recent Researches in Sound. 
19 
contains less vapor Hence it follows that, as a rule, the waves of 
sound must travel faster below than they do above, and thus be 
refracted or turned upward.” (p. 72.) 
The variation of temperature will be greatest in a quiet atmos- 
phere when the sun is shining. The report of Mr. Glaisher ‘‘On 
eight Balloon Ascents in 1862” showed that “The decline of 
temperature [upward] near the earth with a partially clear sky is 
nearly double that with a cloudy sky.”* “During the night the 
variations are less than during the day. This reasoning at once 
suggested an explanation of the well-known fact that sounds are 
less intense during the day than at night. This is a matter of 
common observation, and has been the subject of scientific 
enquiry.” (p. 78.) The opinion must here be hazarded that this 
familiar phenomenon (as for example, the great distance at which 
water-falls can be heard at night,) has first received its true and 
satisfactory explanation from Professor Reynolds. 
Assuming that for a few hundred feet upward, the diminution 
of temperature on a clear summer day is 1° for each hundred feet, 
a horizontal sound-ray would be bent up in an arc having a radius 
of about 20 miles. From a cliff 235 feet high, a sound should be 
audible from to 2 miles on the sea, and the ray should then 
begin to rise above the observer’s head. This is shown to accord 
very closely with the observation of Tyndall [6]. Professor Rey- 
nolds after quoting the observation at length, remarks : “ Here we 
see that the conditions which actually diminished the range of the 
sound were precisely those which would cause the greatest lifting 
of the waves. And it may be noticed that these facts were 
observed and recorded by Professor Tyndall with his mind alto- 
gether unbiased with any thought of establishing this hypothesis. 
He was looking for an explanation in quite another direction. 
Had it not been so he would probably have ascended the mast 
and thus found whether or not the sound was all the time passing 
* Mr. Glaisher remarks: “ From these results we may conclude that in a cloudy 
state of the sky, the decline of temperature is nearly uniform up to the clouds; that 
with a clear sky the greatest change is near the earth, being a decline of 1 ° in less 
than 100 feet, gradually decreasing as in the general law indicated in the preceding 
section, till it requires BOO feet at the height of 5,000 feet, for a change of 1° of tem- 
perature.” {Rep. Brit. Assoc., 1862, p. 462.) 20 
Recent Researches in Sound. 
over his head. On the worst day an ascent of 30 feet should have 
extended the range nearly one quarter of a mile.” (Phil. Mag., p. 
76.) 
V. 
The instructive result, brought into veiw by the foregoing 
summaries, is that the differences noticed are essentially those of 
interpretation, and not to any important extent, of observation : an 
illustration if any were needed, of the high and rare order of imag- 
inative insight requisite to the successful investigation of the more 
recondite operations of natural law. The differing actions of 
acoustic reflection and acoustic refraction suggested by the ingen- 
ious hypotheses of Humboldt and of Stokes, and espoused respec- 
tively by Tyndall and Henry, are probably both operative but 
their relative importance has yet to be established. It is certain, 
as already indicated, that some of the phenomena observed lie 
quite beyond the reach of the acoustic cloud hypothesis. 
A particularly interesting case which is claimed with equal 
confidence for either theory, is the remarkable observation of Gen- 
eral Duane, that at Portland, Maine, the steam whistle on Cape 
Elizabeth, nine miles distant, “ can always be distinctly heard ” 
with “ the wind blowing a gale directly toward the whistle ” or 
against the sound. (L. H. Rep., p. 100.) At Portland Head, 
about midway between this fog-whistle and the point of observa- 
tion is another signal,—a Daboll trumpet. While both these 
signals are better heard with an adverse wind (“ a heavy north- 
east snow storm ”) than at other times, yet “ as the wind increases 
in force, the sound of the nearer instrument—the trumpet—dimin- 
ishes, but the whistle becomes more distinct." (Rep. p. 92.) The 
abnormal influence of the wind in reversing the order of these two 
signals is not the least surprising feature of the general phenom- 
enon. 
Professor Tyndall believes that this curious observation only 
“ proves the snow-laden air from the northeast to be a highly 
homogeneous medium (Sound, Preface, p. xix,) the intervening 
air at other times being acoustically less transparent. 
Professor Henry supposes “ that during the continuance of the 
storm, while the wind was blowing from the northeast at the sur- Recent Researches in Sound. 
21 
face, there was a current of equal or greater intensity blowing in 
an opposite direction above, by which the sound was carried in 
direct opposition to the direction of the surface current(Rep., p. 
92 ; )—somewhat in the nature of a vertical cyclone. He adds: 
“ The existence of such an upper current is in accordance with the 
hypothesis of the character of a northeast storm, which sometimes 
rages for several days at a given point on the coast without being 
felt more than a few miles in the interior, the air continuouslv 
flowing in below and going out above. Indeed in such cases a 
break in the lower clouds reveals the fact of the existence above 
of a rapid current in the opposite direction.” (p. 92.) 
Professor Henry’s attention had been directed to this point as 
early as 1865, by discovering that a signal was audible against the 
wind at the mast-head of a vessel, after ceasing to be audible on 
deck : Obs. [4]. “ This remarkable fact at first suggested the idea 
that sound was more readily conveyed by the upper current of air 
than the lower, and this appeared to be in accordance with the 
following statement of Captain Keeney, who is commander of one 
of the light-house vessels, and has been for a long time on the 
banks of Newfoundland in the occupation of fishing: ‘When the 
fishermen in the morning hear the sound of the surf to the lee- 
ward, or from a point toward which the wind is blowing, they 
take this as an indefallible indication that in the course of from 
one to five hours the wind will change to the opposite direction 
from which it is blowing at the time.’ The same statement was 
made to me by the intelligent keeper of the fog-signal at Block 
Island. In these cases it would appear that the wind had alreadv 
changed direction above, and was thus transmitting the sound in 
an opposite direotion to that of the wind at the surface of the 
earth.” (Rep., p. 92.) The full significance of this idea however 
was not apprehended until the hypothesis of Professor Stokes 
(already alluded to) was taken up and considered. This appeared 
to furnish a satisfactory explanation of the observed effect of an 
upper current,—not on the actual range, but on the direction of the 
sound waves. 
Professor Tyndall thus comments on the rival hypothesis of 
Professor Henry: “In the higher regions of the atmosphere he 
places an ideal wind, blowing in a direction opposed to the real 22 
Recent Researches in Sound. 
one, which always accompanies the latter, and which more than 
neutralizes its action. In speculating thus he bases himself on the 
reasoning of Professor Stokes, according to which a sound-wave 
moving against the wind is tilted upward. The upper and oppos- 
ing wind is invented for the purpose of tilting again the already 
lifted sound-wave downward.” (Pref. to /Sound., pp. xix, xx.) 
The word “ invented ” is scarcely the most appropriate term for 
an hypothesis derived from such patient research and careful 
induction. While in the case considered, the reversed upper wind 
of a local circulation is rendered so probable by the circumstances 
presented, it is proper to remark that this condition is not at all 
essential to the refraction doctrine. The hypothesis of Professor 
Stokes by no means assumes that “ a sound-wave moving 
against the wind is tilted upward.” (Rej?. Brit. Assoc., 1857, pp. 
22, 23, of Abstracts.) An opposing wind exercises no sensible 
influence on either the velocity or the actual range of sound, nor 
(if uniform) on the direction of sound. Ordinarily indeed, a wind 
(which may be likened to an aerial river) is retarded at the earth 
precisely as the current of a stream is, over its bed.* When, 
however, the mouth of the aerial chimney of ascent is low, it may 
very well happen that the lower current of air (excepting imme- 
diately at the surface of the earth) is considerably swifter than the 
successive layers of the wind above it; and in such a case the 
effect of the opposing wind will be not to tilt upward the sound- 
beam, but to tilt it downward. In like manner a “favoring” 
wind, if more sluggish above, will tilt the sound-beam upward, 
and thus prove unfavorable to its aubibility. In short, the postu- 
late required for acoustic refraction is simply that there shall be a 
difference of amount between the upper and lower currents of wind. 
And as this condition is certainly not an unusual one, we have 
here apparently a true and satisfactory account of the seeming 
anomalies of sound with reference to the influence of the wind. 
* Professor Henry determined by experiment in 1865, when the velocity of the 
wind was not more than six miles per hour, that the speed of the clouds as indicated 
by their moving shadows, was several times this rate. (L. H. Rep., p. 93.) And 
Professor Reynolds in 1874, by observations with the anemometer, ascertained that 
near the ground the retardation of the wind rapidly increased; so that the lower 
sound rays move more nearly in the arc of a parabola, than of a circle. (Phil. Mag., 
pp. 64 and 70.) Recent Researches in Round. 
23 
But if the natural tendency of a mere diminution of velocity in 
the upper strata of an adverse wind is thus to bend an advancing 
sound downward, “ a precisely similar effect” as Professor Henry 
has well remarked, ‘‘will be the result but perhaps in a consider- 
ably greater degree, in case an upper current is moving in an 
opposite direction to the lower, when the latter is adverse to the 
sound.” {Rep., p. 107.) In September, 1874, when a signal near 
Sandy Hook, N. J., was observed to be audible at a greater dis- 
tance against the afternoon sea-breeze than with it, Professor Henry 
ascertained by the employment of toy balloons, that the upper 
current was opposed to the lower one, and in the direction of the 
maximum sound range: Obs. [11.] He was enabled thus to 
demonstrate experimentally the reality of the “ideal wind” which 
had been so confidently accepted before, from other conspiring 
intimations. 
The critical commentary above cited, which postulates for 
this doctrine of acoustic refraction the super-position of “an ideal 
wind blowing in a direction opposite to the real one,” as a condi- 
tion “which more than neutralizes its action,” quite fails to appre- 
hend its true import, and seems to take in view merely a convec- 
tive effect. No action analogous to “ neutralization ” is assumed 
by the doctrine. Generally speaking, there is no solution of 
continuity between opposing currents; but every gradation of 
movement in each successive intermediate stratum. And as it is 
wholly improbable that the sound-beam which reaches the observer s 
ear, ever passes high enough to approach the upper “ideal wind,” 
nothing is neutralized. Obedient to the law of instantaneous 
resultants, the beam of acoustic impulse presses on ever at right 
agles to the wave-surface which is conditioned in form by com- 
pounded factors. 
As wide of the mark is the supposition that the upper and 
opposing “ ideal wind” is “ for the purpose of tilting again the 
already lifted sound-wave, downward.” As has just been con- 
tended, the one wind is as incapable of depressing the sound-wave, 
as the other is of lifting it. 
The misconception culminates in the objection that “ Professor 
Henry does not explain how the sound-wave re-crosses the hostile 
lower current, nor does he give any definite notion of the condi- 24 
Recent Researches in Sound. 
tions under which it can be shown that it will reach the observer.’ 
(.Loc. cit., p. xx.) There is no “hostile lower current,” since as 
above pointed out, an opposite wind may be just as favorable to 
the propagation of sound, as a concurrent one. 
To give, however, a more definite notion of the conditions 
under which it can be shown that the sound-wave will reach the 
observer without crossing currents, the accompanying diagrams 
are submitted. 
Fig. 1 exhibits the more ordinary effect of a favorable wind in 
depressing the beam of sound: s being the signal station, and o 
the point of observation ; the wind blowing from w to E. As the 
spheroidal wave-faces become more pressed forward above by the 
freer wind (assuming it to be retarded at the surface by friction), 
and as the direction of the acoustic beam is constantly normal to 
the successive aerial surfaces of impact, it follows that very 
minute differences of concentricity in the successive waves will, 
by constant accumulation gradually bend the line of dynamic 
effect downward, as shown in the sketch on a very exaggerated 
scale. Of the sound rays below the line represented, some will 
by reflection from the sea, reach the observer’s ear and thus 
increase the sound. 
Fig. 1.—Favoring Wind. 
Fig. 2.—Adverse Wind. 
Fig. 2 represents the ordinary effect of an opposing wind here 
blowing from e to w. The wave faces being more resisted above 
by the freer contrary winds (assuming as before a surface retarda- 
tion), the sound-beams are curved upward, and the lowest ray 
that can reach the distance of the observer at o, is that which 
touching the surface of the sea is gradually so tilted upward that Recent Researches in Sound. 
25 
it passes above the ear of the listener, leaving him practically in 
an acoustic shadow; very much as an observer on the deck of a 
vessel when losing the sight of the hull of another vessel ten miles 
off, by reason of the interposed convexity of the ocean, stands in 
the optical shadow of the earth. In both cases if the conditions 
favor, the boundary of the shadow may be re-crossed by ascending 
from the deck to the mast-head, and the sight or the sound-beam 
thus regained. 
Fig. 3 represents the disturbing effect of a lower contrary wind 
with a favoring wind above. In this case the principal result will 
be a depression of the sound-beam as in fig. 1, but more strongly 
marked, as the differences of motion as we ascend will be more 
rapid. Attending this action, however, there will probably be 
some lagging of the lower stratum of the adverse wind by reason 
of the surface friction; the tendency of which will be to slightly 
distort the lower sound radiations, by giving them a reverse or 
serpentine curvature. 
One result of this condition of the locus of the normals in the 
lower portion of the deformed wave-fronts, would be to make the 
sound less audible (or possibly sometimes inaudible) at a point (as 
at t) midway between the two stations. This hypothetical case of 
compound refraction would appear to offer a plausible explanation, 
not only of the paradox of a nearer trumpet-sound being diminished 
in power by the adverse wind which increased the effect of a more 
distant whistle, bat also of the puzzling “belt'7 of inaudibility pre- 
viously noticed. Duane [D], and Henry [8]. 
Numerous other cases might be represented by diagrams, as of 
a sound being hindered or tilted upward by a concurrent wind of 
unequal velocity, or downward by an opposing wind of similar 
character, and of the various permutations of differing currents in 
oblique directions; to which might be added various resultants of 
unequal motion producing lateral refraction, but this is unnecessary. 
Fig. 3.—Compound Wind. Recent Researches in Sound. 
Enough has been said, it is hoped, to clear from popular misappre- 
hension, the admirable hypothesis of Professor Stokes, raised by 
the equally admirable investigations of Professor Henry, to the 
rank of a “ theoryand to show that it has a real and demon- 
strated basis, or in other words that it is a vera causa. The ques- 
tion of its sufficiency lies entirely within the grasp of mathematical 
discussion ; but a long series of accurate and comprehensive obser- 
vations will yet be required to discover its full compass of practical 
result, and to determine its precise limit of capacity in subjugating 
the “abnormal phenomena'’ of sound. Sn|ei‘i(ikr| of $dier|de kt)d 
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