HEARING. 415 



ments of a particular nerve called the acoustic^ 

 or auditory nerve, of which the structure is 

 adapted to receive these peculiar impressions, 

 and to communicate them to the brain, where 

 they produce changes, which are immediately 

 followed by the sensation of sound. Sound 

 cannot traverse a void space, as light does ; but 

 always requires a ponderable material vehicle 

 for its transmission ; and, accordingly, a bell 

 suspended in the vacuum of an air-pump, gives, 

 when struck, no audible sound, although its 

 parts are visibly thrown into the usual vibratory 

 motions. In proportion as air is admitted into 

 the receiver, the sound becomes more and more 

 distinct ; and if, on the other hand, the air be 

 condensed, the sound is louder than when the 

 bell is surrounded by air of the ordinary den- 

 sity.* 



The impulses given by the sounding body to 

 the contiguous particles of the elastic medium^ 

 are propagated in every direction, from particle 

 to particle ; each in its turn striking against the 

 next, and communicating to it the whole of 

 its own motion, which is destroyed by the re- 

 action of the particle against which it strikes. 

 Hence, after moving a certain definite distance, 

 (a distance, indeed, which is incalculably small,) 



* These facts were first ascertained by Dr. Hauksbee. See 

 Philosophical Transactions for 1705, vol. xxiv. p. 1902, 

 1904. 



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