PROPAGATION OF SOUND. 343 



and is of the nature of undulations in general. The 

 parts of the fluid stir for a short time and for a 

 small distance, so as to accumulate themselves on a 

 neighbouring part, and then retire to their former 

 place; and this movement affect the parts in the 

 order of their places. Perhaps if the reader looks 

 at a field of standing corn when the gusts are 

 sweeping over it in visible waves, he will have his 

 conception of this matter aided; for he will see that 

 here, where each ear of grain is anchored by its 

 stalk, there can be no permanent local motion of the 

 substance, but only a successive stooping and rising 

 of the separate straws, producing hollows and waves, 

 closer and laxer strips of the crowded ears. 



Newton had, moreover, to consider the mecha- 

 nical consequences which such condensations and 

 rarefactions of the elastic medium, air, would pro- 

 duce in the parts of the fluid itself. Employing 

 known laws of the elasticity of air, he showed, in a 

 very remarkable proposition 6 , the law according to 

 which the particles of air might vibrate. We may 

 observe, that in this solution, as in that of the 

 vibrating string already mentioned, a rule was ex- 

 hibited according to which the particles might oscil- 

 late, but not the law to which they must conform. 

 It was proved that, by taking the motion of each 

 particle to be perfectly similar to that of a pendu- 

 lum, the forces, developed by contraction and ex- 

 pansion, were precisely such as the motion required; 



fi Princ. B. ii. Prop. 48. 



