RHYTHM 



Strictly speaking, no such case can be realized ; 

 since all the lost momentum is not expended in 

 neutralizing gravity, but part of it is employed 

 in communicating motion to the environing 

 atmosphere, and part of it is transformed into 

 heat. But if all the molar momentum thus dis- 

 sipated could be retained, the rhythmic motion 

 of the pendulum would continue forever. But 

 why ? Simply because the momentum acquired 

 during the descending rhythm cannot cease to 

 manifest itself, save as it is neutralized during 

 the ascending rhythm. And to adduce this rea- 

 son is to appeal directly to the persistence of 

 force. 



The case of undulatory motions propagated 

 among the molecules of matter is precisely 

 similar. The passage of an undulation implies 

 at each instant a momentary local rarefaction, 

 followed by a momentary local condensation. 

 At a given instant certain molecules are removed 

 farther from each other, while at the next suc- 

 ceeding instant they approach each other, and 

 the molecules immediately adjacent are removed 

 from each other. Why is rarefaction thus suc- 

 ceeded by condensation ? What is it that deter- 

 mines the rebound of the disturbed molecule 

 towards its original position ? Obviously the 

 progress of a pair of molecules toward positions 

 farther and farther from each other is opposed 

 by the inertia of adjacent molecules, which these 

 167 



