100 SOUND. [Lesson XVIIT. 



manner, moves a third, and so on, successively: 

 and by these means the motion is propagated in 

 the several particles, through a certain space 

 in a direction forward : but, on the other hand, 

 when the force which was first impre>t upon the 

 elastic particles of air ceases to act, those particles 

 return again, by the action of the air's elasticity, 

 through the same space. It is evident, that if the 

 producing force be continued on these elastic par- 

 ticles, there must necessarily be produced in them 

 a mutual vibratory motion of each particle, so long 

 as the repercussive force continues to act : and this 

 motion of the aerial particles being continued 

 until it reaches the ear, the different parts, nerves, 

 &c. belonging to and contiguous to the organ of 

 hearing, being exquisitely adapted to the purpose, 

 convey the sensation to the Irain, and so produce 

 the idea or perception of sound. 



The waves in water, and the pulses in aif which 

 produce sound, though brought about by two dif- 

 ferent causes, gravity and elasticity, are yet some- 

 what similar : if a stone be dropped into the water, 

 it will cause waves to be propagated in a circular 

 direction upon the water's surface all round the 

 point where the stone was thrown in ; in like 

 manner, the motion of a sounding body is propa- 

 gated around it in regular gradations : but the 

 aqueous waves differ from the aerial pulses in this, 

 that the former are circular, being generated on a 

 plane surface ; but the latter are of a spherical 

 form, because they are produced in the lody of an 

 elastic fluid. 



The 



