PROPAGATION OF SOUND. 341 



ingly Otto Guericke 2 , the inventor of the air-pump, 

 asks, "How can sound be conveyed by the motion 

 of the air ? when we find that it is better conveyed 

 through air that is still, than when there is a wind." 

 We may observe, however, that he was partly mis- 

 led by finding, as he thought, that a bell could be 

 heard in the vacuum of his air-pump; a result 

 which arose, probably, from some imperfection in 

 his apparatus. 



Attempts were made to determine, by experi- 

 ment, the circumstances of the motion of sound ; 

 and especially its velocity. Gassendi 3 was one of 

 the first who did this. He employed fire-arms for 

 the purpose, and thus found the velocity to be 1473 

 Paris feet in a second. Roberval found a velocity 

 so small (560 feet) that it threw uncertainty upon 

 the rest, and affected Newton's reasonings subse- 

 quently 4 . Cassini, Huyghens, Picard, Romer, found 

 a velocity of 1172 Paris feet, which is more accurate 

 than the former. Gassendi had been surprized to 

 find that the velocity with which sounds travel, is 

 the same whether they are loud or gentle. 



The explanation of this constant velocity of 

 sound, and of its amount, was one of the problems 

 of which a solution was given in the Great Charter 

 of modern science, Newton's Principia (1687). 

 There, for the first time, were explained the real 

 nature of the motions and mutual action of the 



2 De Vac. Spat. p. 138. 3 Fischer, Gesch. d. Physik. vol. i. 171 . 

 4 Newt. Prin. B. ii. P. f>0, Schol. 



