Lesson xvin.] SOUND. 101 



The kind of motion which produces sound must 

 be understood to be that which is brought about 

 by means of the elastic particles of which sonorous 

 bodies consist; for without such an elastic dispo- 

 sition of parts they could not by any means be ren- 

 dered sonorous, or capabje of emitting sounds ; be- 

 cause the stroke being made externally, affects the 

 particles of such a body but with one single act: 

 the particles of the body could therefore, in such a 

 case, be moved only through a certain small space, 

 and would there be made to stop by the resistance 

 of, the parts beyond : thus without an elastic force, 

 the particles would remain at rest, after the percu- 

 tient body was removed ; and therefore, from a 

 single stroke the parts of unelastic bodies could 

 emit but a kind of blunt, short sound : in such, 

 cases, it is common to say we hear the stroke, as 

 when we strike with a hammer on a piece of lead. 



But when we consider the stroke impressed on 

 bodies whose parts are in any considerable degree 

 elastic, they not only yield to the stroke and go 

 forward through a small space, but, after the strik- 

 ing body is removed, those elastic parts, by their 

 renilent force, return again with a velocity equal 

 to that by which they were displaced ; and thus a 

 vibratory motion being produced, will continue a 

 perceptible time, and produce successive impulses 

 on the contiguous air; hence, the air being thus 

 agitated by the elastic particles of the body, trans- 

 mits its impulses successively to ^he ear, and there 

 produces a sensation of sound of some duration. 

 Thus amongst sonorous bodies, we notice a wire 

 F 3 when 



