ACOUSTICS. 
feet in a second, and that no obstacles hinder 
its progress, a contrary wind only a small 
matter diminishing its velocity ; the method 
of calculating its progress is easily made 
known. When a gun is discharged at a 
distance, we see the fire long before we 
hear the sound. If then we know the dis- 
tance of the place, and know the time of 
the interval between our first seeing the 
fire and hearing the report, this will shew 
us exactly the time that the sound has been 
travelling to us. For instance, if the gun 
is discharged a mile off, the moment the 
flash is seen, you take a watch and count 
the seconds till you hear the sound ; the 
number of seconds is the time the sound has 
been travelling a mile. We are also ena- 
bled to find the distance between objects 
that would be otherwise immeasurable. 
For example, suppose you see the flash of a 
gun in the night at sea, and tell seven se- 
conds before you hear the report, it follows 
therefore that the distance is seven times 
1142 feet. In like manner, if you observe 
the number of seconds between the light- 
ning and the report of the thunder, you 
know the distance of the cloud from whence 
it proceeds. But according to another phi- 
losopher, Dr. Thomas Young, the velocity 
of sound is not quite so great. “ It has been 
demonstrated,'’ he observes, “ by M. De 
La Grange and others, that any impression 
whatever communicated to one particle of 
an elastic fluid, will be transmitted through 
that fluid with an uniform velocity, de- 
pending on the constitution of the fluid, 
without reference to any supposed laws of 
the continuation of that impression. Their 
theorem for ascertaining this velocity is the 
same as N ewton has deduced from the hy- 
pothesis of a particular law of continua- 
tion : but it must be confessed, that the re- 
sult differs somewhat too widely from expe- 
riment, to give us full confidence in the 
perfection of the theory. Corrected by 
the experiments of various observers, the 
velocity of any impression transmitted by 
the common air, may, at an average, be 
reckoned 1130 feet in a second.” Phil. 
Trans, vol. XC, 
Dr. Derham has proved by experiment, 
that all sounds whatever travel at the same 
rate. The sound of a gun, and the striking 
of a hammer, are equally swift in their mo- 
tions ; the softest whisper flies as swiftly, as 
far as it goes, as the loudest thunder. To 
these we may add, that smooth and clear 
sounds proceed from bodies that are homo- 
geneous, and of an uniform figure; and 
harsh or obtuse sounds, ftom such as are 
of a mixed matter and irregular figure. The 
velocity of sounds is to that of a brisk wind 
as fifty to one. The strength of sounds is 
greatest in cold and dense air, and least in 
that which is warm and rarefied. Every 
point against which the pulses of sound 
strike, becomes a centre from which anew 
series of pulses are propagated in every di- 
rection. Sound describes equal spaces in 
equal times. 
There is probably no substance which is 
not in some measure a conductor of sound ; 
but sound is much enfeebled by passing 
from one medium to another. If a man 
stopping one of his ears with his finger, stops 
the other also by pressing it against the end 
of a long stick, and a watch be applied to 
the opposite end of the stick, or of a piece 
of timber, be it ever so long, the beating 
of the watch will be distinctly heard ; 
whereas, in the usual way, it can scarcely 
bte heard at the distance of 15 or 18 feet. 
The same effect will take place if he stops 
both his ears with his hands, and rests 
his teeth, his temple, or the cartilaginous 
part of one of his ears against the end of 
the stick. Instead of a watch, a gentle 
scratch may be made at one end of a pole 
or rod, and the person who keeps the ear 
in close contact with the other end of the 
pole, will hear it very plainly. Thus, per- 
sons who are dull of hearing, may, by ap- 
plying their teeth to some part of an harp- 
sichord, or other sounding body, hear the 
sound much better than otherwise. 
If a person tie a poker or any other 
piece of metal on to the middle of a strip 
of flannel about a yard long, then press 
with his thumbs or fingers the ends of the 
flannel into his ears, while he swings the 
poker against any obstacle, as an iron or 
steel fender, he will hear a sound very like 
that of a large church bell. 
Sound, like light, after it has been re- 
flected from several places, may be collected 
in one point, as into a focus ; and it will be 
there more audible than in any other part, 
even than at the place from whence it pro- 
ceeded. On this principle it is that a whis- 
pering gallery is constructed. The form of 
a whispering gallery must be that of a con- 
cave hemisphere, as ABC, plate Acoustics, 
fig. 2. ; and if a low sound or whisper be 
uttered at A, the vibrations expanding them- 
selves every way will impinge on tire points 
D, D, D, &c. and from thence be reflected to 
E, E, E, and from thence to the points F and 
G, till at last they all meet in C, where the 
