ACOUSTICS. 
sounds at all. Let the air be exhausted 
from a receiver, and a bell will emit no 
sound, for, as the air continues to grow 
less dense, the sound dies away in propor- 
tion, so that at last its strongest vibrations 
are almost totally silent. Thus air is a 
vehicle for sound. However, we must 
not, with some philosophers, assert, that it is 
the only vehicle ,• that, if there were no 
air, we should have no sounds whatsoever : 
for it is found by experiment, that sounds 
are conveyed through water with the same 
facility with which they move through air. 
A bell rung in water returns a tone as dis- 
tinct, as if rung in air. This was observed 
by Dr. Derham, who also remarked, that 
the tone came a quarter deeper. It ap- 
pears from the experiments of naturalists, 
that fishes have a strong perception of 
sounds, even at the bottom of deep rivers. 
From hence it would seem not to be 
very material in the propagation of sounds, 
whether the fluid which conveys them be 
elastic or otherwise. Water, which, of all 
substances that we know, has the least 
elasticity, yet serves to carry them for- 
ward; and if we make allowance for 
the difference of its density, perhaps the 
sounds move in it with a proportional ra- 
pidity to what they are found to do in the 
elastic fluid of air. But though air and 
water are both vehicles of sound, yet nei- 
ther of them, according to some philoso- 
phers, seems to be so by itself,, but only as 
it contains an exceedingly subtle fluid, capa- 
ble of penetrating the most solid bodies. 
One thing, however, is certain, that what- 
ever sound we hear is produced by a stroke, 
which the sounding body makes against the 
fluid, whether air or water. The fluid 
being struck upon, carries the impression 
forward to the ear, and there produces its 
sensation. Philosophers are so far agreed, 
that they all allow that sound is nothing 
more than the impression made by an elas- 
tic body upon the air or water, and, this 
impression carried along by either fluid to 
the organ of hearing. But the manner in 
which this conveyance is made, is still dis- 
puted : whether the sound is diffused into 
the air, in circle beyond circle, like the 
waves of water when we disturb the smooth- 
ness of its surface by dropping in a stone ; 
or whether it travels along, like rays dif- 
fused from a centre, somewhat in the swift 
manner that electricity runs along a rod of 
iron ; these are the questions which have 
divided' the learned. Newton was of the 
first opinion. He has explained the pro- 
gression of sound by an undulatory, or rather 
a vermicular motion in the parts of the air. 
If we have an exact idea of the crawling of 
some insects, we shall have a tolerable "no- 
tion of the progression of sound upon this 
hypothesis. The insect, for instance, in its 
motion, first carries its contractions from 
the hinder part, in order to throw its fore- 
part to the proper distance, then it carries 
its contractions from the fore part to the 
hinder to bring that forward. Something 
similar to this is the motion of the air when 
struck upon by a sounding body. All who 
have remarked the tone of a bell, while its 
sounds are decaying away, must have an 
idea of the pulses of sound, which, accord- 
ing to Newton, are formed by the air’s al- 
ternate progression and recession. And it 
must be observed, that as each of these 
pulses is formed by a single vibration of 
the string, they must be equal to each 
other ; for the vibrations of the string are 
known to be so. Again, as to the velocity 
with which sounds travel, this Newton de- 
tei mines, by the most difficult calculation 
that can be imagined, to be in proportion 
to the thickness of the parts of the air, and 
the distance of these parts from each other. 
From hence he goes on to prove, that each 
little part moves backward and forward 
like a pendulum ; and from thence he pro- 
ceeds to demonstrate, that if the atmos- 
phere were of the same density every 
where as at the surface of the earth, in 
such a case, a pendulum, that reached from 
its highest surface down to the surface of 
the earth, would by its vibrations discover 
to us the proportion of the velocity with 
which sounds travel. The velocity with 
which each pulse would move, he shows, 
would be as much greater than the velocity 
of such a pendulum swinging with one com- 
plete vibration, as the circumference of a 
circle is greater than the diameter. From 
hence he calculates, that the motion of 
sound will be 979 feet in one second. But 
this not being consonant to experience, he 
takes in another consideration, which de- 
stroys entirely the rigour of his former de- 
monstration, namely, vapours in the air, 
and then finds the motion of sound to be 
1142 feet in one second, or near 13 miles in 
a minute, a proportion' which experience 
had established nearly before. Many other 
theories on this subject have been advanced 
by ingenious men, but our limits do not al- 
low to enter farther into them. 
Since by experiments it has been proved, 
that sound travels at about the rate of 1 140 
