to vibrate in its natural period. If a sounding body is near a body
which has the same period as itself, the pulses of air produced by the
sounding body will, although very small, set the second body into
motion and cause it to make a faint sound. When a piano is being
played, we are often startled to find that a window pane or an
ornament responds to some note of the piano. If two tuning forks of
exactly identical periods (that is, of the same frequency) are placed
on a table as in Figure 174, and one is struck so as to give forth a
clear sound, the second fork will likewise vibrate, even though the
two forks may be separated by several feet of air. We can readily see
that the second fork is in motion, although it has not been struck,
because it will set in motion a pith ball suspended beside it; at
first the pith ball does not move, then it moves slightly, and finally
bounces rapidly back and forth. If the periods of the two forks are
not identical, but differ in the slightest degree, the second fork
will not respond to the first fork, no matter how long or how loud the
sound of the first fork. If we suppose that the fork vibrates 256
times each second, then 256 gentle pulses of air are produced each
second, and these, traveling outward through the air, reach the silent
fork and tend to set it in motion. A single pulse of air could not
move the solid, heavy prongs, but the accumulated action of 256
vibrations per second soon makes itself felt, and the second fork
begins to vibrate, at first gently, then gradually stronger, and
finally an audible tone is given forth.

[Illustration: FIG. 174.--When the first fork vibrates, the second
responds.]

The cumulative power of feeble forces acting frequently at definite
intervals is seen in many ways in everyday life. A small boy can
easily swing a much larger boy, provided he gives the swing a gentle
push in the right direction every time it passes him. But he must be
careful to push at the proper instant, since otherwise his effort does
not count for much; if he pushes forward when the swing is moving
backward, he really hinders the motion; if he waits until the swing
has moved considerably forward, his push counts for little. He must
push at the proper instant; that is, the way in which his hand moves
in giving the push must correspond exactly with the way in which the
swing would naturally vibrate. A very striking experiment can be made
by suspending from the ceiling a h