The motion of the successive turns of the coil is very
similar to what can be shewn theoretically to be the motion of
successive layers of air over which a sound wave is passing.
76. General description of the propagation of
Air-Waves. Having thus obtained a general idea of the
nature of longitudinal wave motion, we pass to the case of
waves in air.
B




D E F

Fig. 32

Imagine a circular cylinder AB containing near one end a
piston C, which fits the cylinder air-tight, but can move easily
within it. Suppose the piston is moved rapidly a little to the
right, and is held at rest in its new position. A thin layer
of air CD close to the piston will be momentarily compressed,
whilst the rest of the air in the cylinder will be momentarily
unaifected, for the air has inertia, and the motion of the layer
near G cannot set the next layer in motion instantaneously.
The layer CD will therefore be compress; d, and its pressure
will be greater than that of DE ; but this state of affairs
cannot remain, and CD will at once begin to expand towards
the right, communicating its compression to DE, which in
turn will communicate it to EF, and so on, a pulse of
compression travelling onwards towards the right. We will
for the moment postpone consideration of what happens when
the pulse reaches B.
