What is required for the production
of sound is an impact of two solids against one another and against
the air. The latter condition is satisfied when the air impinged
upon does not retreat before the blow, i.e. is not dissipated by it.
That is why it must be struck with a sudden sharp blow, if it is
to sound-the movement of the whip must outrun the dispersion of the
air, just as one might get in a stroke at a heap or whirl of sand as
it was traveling rapidly past.
An echo occurs, when, a mass of air having been unified, bounded,
and prevented from dissipation by the containing walls of a vessel,
the air originally struck by the impinging body and set in movement by
it rebounds from this mass of air like a ball from a wall. It is
probable that in all generation of sound echo takes place, though it
is frequently only indistinctly heard. What happens here must be
analogous to what happens in the case of light; light is always
reflected-otherwise it would not be diffused and outside what was
directly illuminated by the sun there would be blank darkness; but
this reflected light is not always strong enough, as it is when it
is reflected from water, bronze, and other smooth bodies, to cast a
shadow, which is the distinguishing mark by which we recognize light.
It is rightly said that an empty space plays the chief part in the
production of hearing, for what people mean by 'the vacuum' is the
air, which is what causes hearing, when that air is set in movement as
one continuous mass; but owing to its friability it emits no sound,
being dissipated by impinging upon any surface which is not smooth.
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