SOUND. 
699 
at all. That the air is capable of being agi- 
tated with great force, appears from the 
violent concussions produced by explosions 
of gunpowder, as well as from the power, 
which some persons are known to possess, of 
breaking drinking-glasses, by means of their 
voice, when sounded in unison with the note 
which the glass would have produced when 
struck. r l he tremulous motion excited in 
the air by sounding bodies, has been supposed 
analogous to the successive rings which are 
produced by disturbing the surface of the 
water. This hypothesis, however, was dis- 
proved by the observation that sounds, whe- 
ther weak or loud, always travel with the 
same velocity, which does not hold true with 
respect to the rings on the surface of water, 
since these move faster or slower according 
to the force of the cause which excited them. 
Every* sound is rendered stronger or weak- 
er, and may be heard at a greater or less dis- 
tance, according to the density or rarity of 
that elastic fluid by which it is propagated. 
According to Mr. Hauksbee, who has made 
deep researches into this branch of philoso- 
phy, when air has acquired twice its com- 
mon density it transmits sound twice as far 
as common air; whence he reasonably con- 
cludes, that sound increases, not only in -direct 
proportion to the density of the air, but in 
proportion to the square of this density. 
If sound was propagated in an elastic fklid 
more dense than the air, it would be canted 
proportion ably farther. I have proved this, 
says M. Brisson, by putting a sonorous body 
into carbonic acid gas or fixable air, the 
density of which is about one-third more than 
.that of atmospherical air; the consequence 
-was, that at that time, and in that situation, 
the sound was very considerably increased. 
Tor the same reason, the dryness of the air, 
which increases its density,* has a consider- 
able effect in rendering sound louder and 
more audible. Sound is also much increased 
by the reverberation of the pulses of the air 
from those surrounding bodies against which 
they strike, whence it happens that music is 
so much louder in a close apartment than in 
fhe open air. 
Elastic fluids are, however, not the only 
medium through which sound may be trans- 
mitted ; for it may be propagated by means 
of water and other liquors, which may be 
proved by immersing a sonorous body in 
water; but it must be observed, that in this 
.case the sound will be less perceptible, and 
will not extend to so great a distance; the 
.cause of this diminution is, because mediums 
for the transmission of sound should be elas- 
tic, and that is a property which water and 
.other liquors possess only in a very restricted 
degree. 
Sound is also transmitted by solid bodies, 
provided they possess a sufficient degree of 
elasticity to produce this effect. 
Light, we have already seen, is projected 
or reflected with incredible velocity ; but 
sound is transmitted much more slowly, and 
its progression is very perceptible to our 
senses. The flash from a cannon, or even a 
musket, may be seen some seconds before 
the sound reaches our ears. As the motion 
of light, therefore, is instantaneous with re- 
spect to any moderate distance, this has 
been the common means employed for as- 
certaining the progress of sound. Sir Isaac 
Newton observes that “ all sounding bodies 
propagate their motions on all sides by suc- 
cessive condensations and relaxations ; that 
is, by an alternate progression and return of 
the particles and these vibrations, when 
communicated to the air, are termed pulses of 
sound. 
All pulses move equally fast. This is 
proved by experiment ; and it is found that 
they pass about one thousand one hundred 
and forty-two feet in a second, whether the 
sound is loud or lows grave or acute. 
Some curious experiments were made, re- 
lative to the propagation of sound, by Mes- 
sieurs De Thury, Maraldi, and De la Caille, 
upon a line fourteen thousand six hundred 
and thirty-six fathoms in length, having the 
tower of mount Lheri at one end, and the 
pyramid of Montmartre at the other ex- 
tremity of that distance : their observatory 
was placed between those two objects. The 
results of their observations were these : 1 st. 
That sound moves one hundred and seventy- 
three fathoms, French, in a second, when 
the air is calm, 2d. That sound moves with 
the same degree of swiftness whether it is 
strong or weak; for these gentlemen ob- 
served, that the disch. rge of a box of half a 
pound of gunpowder exploded at Mont- 
martre was heard at mount Lheri in the 
same space of time as the report of a great 
gun charged with nearly six pounds of 
powder. 3d. That the motion of sound is 
uniform ; that its velocity neither accelerates 
nor diminishes through all the intervals of its 
progress, as is the case with almost every 
other species of motion, 4th. That the ve- 
locity of sound is the same, whether a cannon 
is placed towards the person who hears its re- 
port, or turned a contrary way; in other 
words, a great gun tired from the Tower of 
London eastward, would be heard at West- 
minster in the same interval of time as if it 
was discharged towards the latter place. And 
if the gun was discharged in a direction per- 
pendicular to the horizon, it would be heard 
as soon as if discharged in a right line to- 
wards the hearer. By other experiments, 
however, the progress of sound appears to 
be impeded by a strong wind, so that it tra- 
vels at the rate of about one mile slower in a 
minute against a strong wind than with it. 
A knowledge of the progression of sound is 
not an article of mere sterile curiosity, but in 
several instances useful ; for by this we are 
enabled to determine the distance of ships or 
other moving bodies. Suppose, for example, 
a vessel fires a gun, the sound of which is 
heard live seconds after the flash is seen; as 
sound moves 1 142 English feet in one second, 
this number multiplied by 5, gives the dis- 
tance of 5710 feet. The same principle has 
been applied to storms of lightning and 
thunder, as to calculating the distance of it 
from us. See Electricity. 
The waves or pulses of sound being re" 
flexible in their course when they qieet with 
an extended solid body of a regular surface, 
an ear placed in the passage of these reflected 
waves will perceive a sound similar to the 
original sound, but which will seem to pro- 
ceed from a body situated in a similar po- 
sition and distance behind the plane of re- 
flection, as the real sounding body is before 
it. This reflected sound is commonly called 
an echo, which, however, cannot take place 
at less than fifty-five feet ; because it is neces- 
sary that the distance should be such, anfi 
the reverberated or reflected sound so long 
in arriving, that the ear may distinguish 
clearly between that and the original sound. 
Reflected sound may be magnified by 
much the same contrivances as are used in 
optics respecting light : hence it follows, that 
sounds uttered in one focus of an elliptical 
cavity are heard much magnified in the other 
focus. The whispering-gallery at St. Paul’s 
cathedral in London, is of tins description ; 
a whisper uttered at one side of the dome is 
reflected to the other, and may be very dis- 
tinctly heard. The speaking and ear trum- 
pets are constructed on this principle. The 
best form for these instruments is a hollow- 
parabolic conoid, with a small orifice at the 
top or apex, to which the mouth is applied 
when the sound is to be magnified, or tiie ear 
when the hearing is to be facilitated. 
The structure of the ear is one of the most 
complicated and difficult subjects of physi- 
ology ; and the reader is, therefore, referred 
to that article for what concerns this branch 
of acoustics. 
Sound, musical. Sounds of such qualities 
and dispositions as to produce that agreeable 
and appreciable effect upon the ear which we 
call melody, or harmony. We shall at 
present confine our observations to that affec- 
tion of sound by which it becomes distin- 
guished into acute and grave 
This difference has hitherto appeared t© 
have no other causes than the different velo- 
cities of the vibrations of the sounding bodies, 
in fact, the tone or pitch of a sound seems to 
have been discovered, by an abundance of 
experiments, to depend on the nature of 
those vibrations, whose difference we can con- 
ceive no otherwise than as having different 
velocities; and since it is proved, that all the 
vibrations of the same chord are performed 
in equal time ; and that the tone of a sound, 
which continues for some time after the 
stroke, is the same from first to last; it fol- 
lows, that the tone is necessarily connected 
with a certain quantity of time in making 
each vibration : and it is from this principle 
that all the phenomena of tune are deduced. 
If the vibrations are isochronous, the sound 
is called musical ; and is said to be acuter, or 
higher, than any other sound whose vibra- 
tions are slower and graver, or lower than any 
other sound whose vibrations are quicker. 
From the same principle arise what we call 
concords, &c. which are resolvable into the 
frequent unions and coincidences of the vi- 
brations of two sonorous bodies, and conse- 
quently of the undulations of the air which 
they occasion. On the contrary, the result 
of less frequent coincidences of those vibra- 
tions is what we call discord. 
Another considerable distinction of musical 
sounds is, that by which they are denomi- 
nated long and short ; not with regard to the 
sonorous body’s retaining a motion, once re- 
ceived, a longer or lesser time, but to the 
continuation of the impulse of the efficient 
cause on the sonorous body for a longer or 
shorter time ; as in the notes of a violin, &c. 
which are made longer or shorter by strokes 
ot different length or quic kness. 
This continuity is, properly, a succession 
of several sounds, or the effect of several dis- 
tinct strokes, or repeated impulses, on the 
sonorous body, so quick that we may judge 
