HEARING. .371 



space, as light does ; but always requires a ponder- 

 able 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 density.* 



The impulses given by the sounding body to the 

 contiguous particles of the elastic medium, are pro- 

 pagated in every direction, from particle to particle; 

 each in its turn striking against the next, and com- 

 municating to it the whole of its own motion, which 

 is destroyed by the reaction of the particle against 

 which it strikes. Hence, after moving a certain 

 definite distance, (a distance, indeed, which is in- 

 calculably small,) each particle returns back to its 

 former situation, and is again ready to receive a 

 second impulse. Each particle, being elastic within 

 a certain range, f suffers a momentary compression, 

 and immediately afterwards resumes its former 

 shape: the next particle is, in the mean time, 

 impelled, and undergoes the same succession of 

 changes; and so on, throughout the whole series of 

 particles. Thus the sonorous undulations have an 

 analogy to waves, which spread in circles on the 



* These facts were first ascertained by Dr. Hauksbee, See Phi- 

 losophical Transactions for 1705, vol. xxiv. p. 1902, 1904. 



t The particles of water are as elastic, within a limited distance, 

 as those of the most solid body; although, in consequence of their 

 imperfect cohesion, or rather their perfect mobility in all directions, 

 this property cannot be so easily recognised in masses of fluids, as it 

 is in solids. 



