Manner in which Sound is produced. 205 



The facts which I have stated appear sufficient to esta- 

 blish that, in all cases in which sound is produced, a vacuum 

 is formed and filled. This, indeed, must be the case where- 

 ever there is motion, such as to cause a body to change its 

 place, when surrounded by an elastic medium, such as air. 

 I, therefore, now proceed to consider in what manner sound 

 is made known to us. 



Sound is said to travel through the air at the rate of 1130 

 feet in a second of time ; and that whether it goes to a long 

 or a short distance. Now, if, according to Sir John Her- 

 schel, sound consists of motion, we may say that, a motion 

 (but not such a motion as one that causes advance), being 

 communicated to the air, it is somehow propagated at the 

 above rate. But rate is proportional to impulse, and that of 

 sound does not vary with that of impulse. Hence it may be 

 argued that sound, when evolved, moves independently, as 

 light does ; and that, though it has motion, it is something 

 different from motion. A mass of air set in motion produces 

 no sound until it meets resistance ; not the most violent hur- 

 ricane. The motion of the air which produces sound, is not 

 the advancing motion of a mass, but one given to it, and as- 

 sumed, and somehow propagated, as Sir J. Herschel says, in 

 virtue of its elasticity. This peculiar motion among its parts, 

 is evidently the cause of the conveyance or propagation of 

 sound. But how its rate should be uniform, while the im- 

 pulses that cause it vary, is a question unanswered hitherto ; 

 unless sound be analogous to light in respect to its uniform 

 rate, being something sui generis. It is unnecessary to re- 

 peat how the motion exciting the perception of sound, is pro- 

 pagated through the air, as it is described in every treatise 

 on acoustics. The rate of ordinary motion through various 

 substances, has not yet been ascertained ; but, probably, it 

 depends on degrees of density and elasticity, both in the body 

 giving and in that receiving impulse. Whatever be the im- 

 pulse or degree of force producing the peculiar motion by 

 which sound is made known, the rate through the air, as be- 

 fore noticed, is uniform ; for sound, whether loud or soft, 

 though it does not go to a uniform distance, travels at a uni- 



