nning to
the end of its vibration; or as long as it can be distinguished by
the ear; and why the pitch remains still unvaried whether the sound
is loud or soft, and all this because the vibrations of the same
pendulum whether they are longer or shorter, when compared among
themselves, are found to be all performed in equal times till the
pendulum be at rest, the difference of the space, which is moved
over, compensating for the slowness of the motion till its decay.

To illustrate this subject still further, suppose we have a piece of
catgut stretched between two pins; I lay hold of it in the middle and
pull it sideways; I let it go, and you will observe that it first
straightens itself or returns to its original position. This depends
on the elasticity of its particles, which tend to reunite when they
have been separated by an external force, just in the same way that
the particles of a piece of caoutchouc or Indian rubber attract each
other when pulled asunder; and this force not only enables the string
to restore itself to its former situation, but will carry it nearly
to an equal distance on the other side, just in the same manner as a
ball falling down an inclined plane will rise nearly to the same
height up another, or a pendulum will rise nearly to the height from
which it fell.

In this way will a string move backwards and forwards, till friction
and the resistance of the air have destroyed the velocity which it
acquired by the force of elasticity.

It is obvious that when a string is thus let fly from the finger,
whatever be its own motion, such will also be the motion of the
particles of the air which fly before it: the air will be driven
forwards, and by that means condensed. When this condensed air
expands itself, it will expand not only towards the string, but as
its elasticity acts in all directions, it will also expand itself
forwards and condense the air that is beyond it, this last condensed
air, by its expansion, will produce the same effect on the air that
lies still further forwards, and thus the motion produced in the air,
by the vibration of the elastic string, is constantly carried
forwards and conveyed to the ear.

It will be proper however to observe, that these pulses are sometimes
produced without any such vibration of the sounding body, as we find
it in musical strings and bells. In these cases we have to discover
by what cause these condensations or pulses may be produced without
any a