SOUND WAVES 



273 



T 



FIG. 171. The two hammocks swing differently. 



than one hung with short ropes, and that a stone suspended 

 by a long string swings more slowly than one suspended by a 

 short string. No two 

 rocking chairs vibrate 

 in the same way unless t'v 



they are exactly alike in \ : *$\ ^ ^ , , M , - r - Tffl *^ r /it*) 



shape, size, and material. 

 An object when dis- 

 turbed 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. 

 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 vibra- 

 tions 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 mo- 

 tion, however, is seen by the action of a pith 

 ball when brought in contact with the prongs 

 (see Section 250). 

 The disturbance created by a vibrating body is called a 

 wave. 



CL. GEN. SCI. 1 8 



FlG. 172. The 

 pitch given out 

 by a fork de- 

 pends upon its 

 shape. 



