[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 heavy weight and striking this weight
gently at regular, properly timed intervals with a small cork hammer.
Soon the pendulum, or weight, will be set swinging.
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