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CHAPTER III. 

 PROBLEM OF THE PROPAGATION OF SOUND. 



WE have seen that the ancient philosophers, for 

 the most part, held that sound was transmit- 

 ted, as well as produced, by some motion of the air, 

 without defining what kind of motion ; that some, 

 however, applied to it a very happy similitude, the 

 expansive motion of the circular waves produced by 

 throwing a stone into still water; but that notwith- 

 standing, some rejected this mode of conception, as, 

 for instance, Bacon, who ascribed the transmission 

 of sound to certain " spiritual species." 



Though it was an obvious thought to ascribe the 

 motion of sound to some motion of air ; to conceive 

 what kind of motion could and did produce this 

 effect, must have been a matter of grave perplexity 

 at the time of which we are speaking ; and is far 

 from easy to most persons even now. We may 

 judge of the difficulty of forming this conception, 

 when we recollect that John Bernoulli the younger 1 

 declared, that he could not understand Newton's 

 proposition on this subject. The difficulty consists 

 in this, that the movement of the parts of air, in 

 which sound consists, travels along, but that the 

 parts of air themselves do not so travel. Accord- 



1 Prize Dis. on Light, 1736. 



