when disturbed vibrates in a manner peculiar
to itself, the vibration being slow, as in the case of the long-roped
swing, or quick, as in the case of the short-roped swing. The time
required for a single swing or vibration is called the _period_ of the
body, and everything that can vibrate has a characteristic period.
Size and shape determine to a large degree the period of a body; for
example, a short, thick tuning fork vibrates more rapidly than a tall
slender fork.

[Illustration: FIG. 171.--The two hammocks swing differently.]

Some tuning forks when struck vibrate so rapidly that the prongs move
back and forth more than 5000 times per second, while other tuning
forks vibrate so slowly that the vibrations do not exceed 50 per
second. In either case the distance through which the prongs move is
very small and the period is very short, so that the eye can seldom
detect the movement itself. That the prongs are in motion, however, is
seen by the action of a pith ball when brought in contact with the
prongs (see Section 250).

[Illustration: FIG. 172.--The pitch given out by a fork depends upon
its shape.]

The disturbance created by a vibrating body is called a wave.

256. Waves. While the disturbance which travels out from a sounding
body is commonly called a wave, it is by no means like the type of
wave best known to us, namely, the water wave.

If a closely coiled heavy wire is suspended as in Figure 173 and the
weight is drawn down and then released, the coil will assume the
appearance shown; there is clearly an overcrowding or condensation in
some places, and a spreading out or rarefaction in other places. The
pulse of condensation and rarefaction which travels the length of the
wire is called a wave, although it bears little or no resemblance to
the familiar water wave. Sound waves are similar to the waves formed
in the stretched coil.

[Illustration: FIG. 173.--Waves in a coiled wire.]

Sound waves may be said to consist of a series of condensations and
rarefactions, and the distance between two consecutive condensations
and rarefactions may be defined as the wave length.

257. How One Sounding Body produces Sound in Another Body. In
Section 255 we saw that any object when disturbed vibrates in a manner
peculiar to itself,--its natural period,--a long-roped hammock
vibrating slowly and a short-roped hammock vibrating rapidly. From
observation we learn that it requires but little force to cause a body