CHAPTER SIX 



SOUND 



SECTION 28. What sound is. 



What makes a dictaphone or a phonograph repeat your 

 words ? 



What makes the wind howl when it blows through the 

 branches of trees? 



Why can you hear an approaching train better if you put 

 your ear to the rail ? 



If you were to land on the moon tonight, and had 

 with you a tank containing a supply of air which you 

 could breathe (for there is no air to speak of on the 

 moon), you might try to speak. But you would find 

 that you had lost your voice completely. You could 

 not say a word. You would open and close your mouth 

 and not a sound would come. 



Then you might try to make a noise by clapping your 

 hands ; but that would not work. You could not make 

 a sound. " Am I deaf and dumb? " you might wonder. 



The whole trouble would lie in the fact that the moon 

 has practically no air. And sound is usually a kind of 

 motion of the air, hundreds of quick, sharp puffs 

 in a second, so close together that we cannot feel them 

 with anything less sensitive than the tiny nerves in our 

 ears. 



If you can once realize the fact that sound is a series 

 of quick, sharp puffs of air, or to use a more scientific 

 term, vibrations of air, it will be easy for you to under- 

 stand most of the laws of sound. 



A phonograph seems almost miraculous. Yet it 

 works on an exceedingly simple principle. When you 

 talk, the breath passing out of your throat makes the 



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