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.

[Illustration:  FIG. 175.—­The hollow wooden box reenforces the sound.]

258.  Borrowed Sound.  Picture frames and ornaments sometimes buzz and give forth faint murmurs when a piano or organ is played.  The waves sent out by a sounding body fall upon all surrounding objects and by their repeated action tend to throw these bodies into vibration.  If the period of any one of the objects corresponds with the period of the sounding body, the gentle but frequent impulses affect the object, which responds by emitting a sound.  If, however, the periods do not correspond, the action of the sound waves is not sufficiently powerful to throw the object into vibration, and no sound is heard.  Bodies which respond in this way are said to be sympathetic and the response produced is called resonance.  Seashells when held to the ear seem to contain the roar of the sea; this is because the air within the shell is set into sympathetic vibrations by some external tone.  If the seashell were held to the ear in an absolutely quiet room, no sound would be heard, because there would be no external forces to set into vibration the air within the shell.

Tuning forks do not produce strong tones unless mounted on hollow wooden boxes (Fig. 175), whose size and shape are so adjusted that resonance occurs and strengthens the sound.  When a human being talks or sings, the air within the mouth cavity is thrown into sympathetic vibration and strengthens the otherwise feeble tone of the speaker.

259.  Echo.  If one shouts in a forest, the sound is sometimes heard a second time a second or two later.  This is because sound is reflected when it strikes a large obstructing surface.  If the sound waves resulting from the shout meet a cliff or a mountain, they are reflected back, and on reaching the ear produce a later sensation of sound.