94 



SOUND 



rapidly moving to and fro, causes many condensations 

 and rarefactions to follow each other away from the 

 prong. These pulses of air are sound waves. In Fig. 71 

 the condensations (A) and rarefactions (B) are supposed 

 to be moving away from a vibrating bell. 



106. Vibrating Bodies. It will perhaps be hard to 

 grasp at once the idea of sound as merely a sensation, 



FIG. 71 



which reports to the brain the vibrations in matter about 

 us. One difficulty is that whereas there is no end to 

 the sounds we hear, it is not often we can discover any 

 vibration in the body which caused a sound. Tuning 

 forks, violin strings, piano wires, bells, and a few 

 others show vibration plainly, but in many cases there 

 is none to be seen. Sometimes these vibrations are 



