PNEUMATICS. 287 



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Propagation of Vibrations through the Air. 



385. The vibrations of sonorous bodies are com- 

 municated to the air, and by the impression thus 

 made on the ear, excite the sensation of sound. 



That air is necessary to the production of sound, is 

 evident from including a bell in a receiver, exhaust- 

 ing the air, and making the clapper strike on the 

 bell : the sound is hardly audible. 



386. It is not every kind of vibratory motion 

 produced in the air that is the cause of sound ; a 

 musical string may vibrate, but if it is touched by 

 a bit of cloth, or any soft body, no sound is heard. 

 The vibrations in the air that produce sound must 

 be communicated by some elastic substance. 



Sound is produced by the explosion of gunpowder, 

 that is, by the sudden extrication of a fluid mass, 

 dense, and highly elastic. It is produced also by 

 the sudden rushing in of air to supply a vacuum. 

 The crack of a whip appears to be an example of 

 this last. 



387. Though the vibrations of the air which 

 produce the sensation of sound, are no doubt al- 

 ternate 



