PKOBLEM OF THE PKOPAGATION OF SOUND. 33 



of air themselves do not so travel. Accordingly 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, how- 

 ever, that he was partly misled 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 experiment, the circum- 

 stances of the motion of sound ; and especially its velocity. Gassendi' 

 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 surprised 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 parts of the air through which sound is transmitted. It 

 was shown 5 that a body vibrating in an elastic medium, will propagate 

 pulses through the medium ; that is, the parts of the medium will move 

 forwards and backwards, and this motion will affect successively those 

 parts which are at a greater and greater distance from the origin of 

 motion. The parts, in going forwards, produce condensation ; in 

 returning to their first places, they allow extension ; and the play of 

 the elasticities developed by these expansions and contractions, supplies 

 the forces which continue to propagate the motion. 



The idea of such a motion as this, is, as we have said, far from easy 

 to apprehend distinctly : but a distinct apprehension of it is a step 

 essential to the physical part of the sciences now under notice ; for it 

 is by means of such pulses, or undulations, that not only sound, but 

 light, and probably heat, are propagated. We constantly meet with 

 evidence of the difficulty which men have in conceiving this undulatory 

 motion, and in separating it from a local motion of the medium as a 



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



4 Newt. Prin. B. ii. P. 50, Schol. * Newt. Prin. B. ii. P. 43. 

 VOL. II. 3. 



