378 



LECTURE XXXII. 



ON THE SOURCES AND EFFECTS OF SOUND. 



J. HE examination of the origin of sound might naturally be deemed anterior 

 to the inquiry respecting its propagation ; but it will appear, that the pro- 

 perties of many of the most usual sources of sound depend immediately on 

 the velocity, with which an impulse of any kind is transmitted through an 

 elastic medium ; it was therefore necessary to consider this velocity, before 

 the production of sound in general could be discussed. 



The origin of a simple sound, without any alternation, requires very little 

 investigation : it appears that the only condition necessary for its production 

 is a sufficient degree of velocity in the motion or impulse which occasions it. 

 A very moderate velocity must be sufficient for producing an impression on 

 the ear; there is reason to believe, that, when the sound is continued, it may 

 remain audible with a velocity of no more than one hundredth of an inch in 

 a second, and perhaps even with a velocity much smaller than this : but, at its 

 origin, it is probable that the velocity of the motion, constituting a sound, 

 must always be considerably greater. 



A continued sound may be produced by a repetition of separate impulses 

 independent of each other, as when a wheel strikes in rapid succession the 

 teeth of a pinion, so as to force out a portion of air from between them; 

 when a pipe, through which air is passing, is alternately opened and shut, 

 either wholly or partially, by the revolution of a stopcock or valve; or when a 

 number of parallel surfaces is placed at equal distances in a line nearly per- 

 pendicular to them, and a noise of any kind is reflected from each of them 

 in succession; a circumstance which may often be observed when we are 

 walking near an iron railing, an acute sound being heard, which is com- 

 posed of such reflections from the surfaces of the palisades. 



