276 SOUND 



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 back- 

 ward, 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. 



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 ob- 

 jects 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, 

 FIG. 175. The which responds by emitting a sound. If, how- 

 hollow wooden ever, the periods do not correspond, the action 



box reenforces 



the sound. 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 res- 

 onance. 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 



