Chap. 10.] Velocity of Sound. 44 <j 



Sound is alfo tranfmitted by folid bodies, provided 

 they pofTefs a fufficient degree of elafticity to produce 

 this effea. 



Light, we have already fecn, is projected or re- 

 flected with incredible velocity; but found is tranf- 

 mitted much more flowly, and its progrefiion is very 

 perceptible to our fenfes. The fiafh from a cannon, 

 or even a mufket, may be feen fome feconds before 

 the found reaches our ears. * As the motion of light, 

 therefore, is inftantaneous with refpedl to any mode- 

 rate diftance, this has been the .common means em- 

 ployed for afcertaining the progrefs of found. Sir 

 Ifaac Newton obferves < that " all founding bodies 

 propagate their motions on all fides by fucceflive con- 

 denfations and relaxations} that is, by an alternate 

 progrefiion and return of the particles;" and thefe 

 vibrations, when communicated to the air* are termed 

 pulfes of found. 



All pulfes move equally faft. This is proved by 

 experiment; and it is found that they pafs about 

 one thoufand one hundred and forty-two feet in a 

 fecond, whether the found is loud or low, grave or 

 acute *. 



Some 



* That we labour under a deception with regard to tones, and 

 that they become higher as they come from a greater diftance, may 

 be inferred from mufical compofition. The greateft matters in this 

 art, when they would imitate a diftant echo, generally take the 

 founds an odlave higher. A few years ago, a fellow exhibited in 

 Weftminfter the art of imitating founds at any diftance whatever. 

 I remarked, that whenever he defigned to imitate a voice coming 

 from a great diftance, he not pnly made the found more low and 

 indiftindt, but raifed the tone feveral pitches higher than that ufed 

 in his nearer imitations. A few obfervations fmce made upon 

 founds, indues me to believe, that they become higher as they 

 come from a diftance more remote ; while, op the contrary, that 

 they deepen the more the vibrations approach the labyrinth of 



the 



