NATURE OF SOUND. 



269 



of water waves, for example, the horizontal distance from 

 one crest to the next crest would be a wave length. 



419. Amplitude. Amplitude means the dis- 

 tance between the extreme positions of the vibrating 

 particle, or the length of its journey. As in the case of 

 the pendulum, amplitude and period are independent of 

 each other. Amplitude is also independent of wave length. 



420. Relation of Period, Wave Length and 

 Velocity. During one period there will be one com- 

 plete vibration, and the wave will advance one wave length. 

 The velocity of the wave may be found by multiplying the 

 wave length by the number of vibrations per second. 

 Conversely, the wave length may be found by divide 

 ing the velocity by the number of vibrations. 



421. Cause of Sound. All sound may be traced 

 to the vibrations of some material body. When a 



bell is struck, the edges of the 

 bell are set in rapid vibration, 

 as may be seen by holding a 

 card or finger nail lightly upon 

 the edge. The particles of the 

 bell strike the adjacent parti- 

 cles of air, these pass the 

 motion thus received on to the 

 air particles next beyond, and 

 these to those beyond. 



(a.) That sound is due to vibra- 

 tory motion may be shown by nu- 

 merous experiments. Holding one 

 end of a straight spring, as a hick- 

 FIG. 211. ory stick, in a vise, pull the free 



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