THE RHYTHM OF MOTION. 279 



time to time revives extinct types of dress, these always 

 re-appear with decided modifications. 



88. The universality of this principle suggests a ques- 

 tion like that raised in foregoing cases. Rhythm being 

 manifested in all forms of movement, we have reason to sus- 

 pect that it is determined by some primordial condition to 

 action in general. The tacit implication is that it is de- 

 ducible from the. persistence of force. This we shall find to 

 be the fact. 



When the prong of a tuning-fork is pulled on one side 

 by the finger, a certain extra tension is produced among its 

 cohering particles; which resist any force that draws them 

 out of their state of equilibrium. As much force as the fin- 

 ger exerts in pulling the prong aside, so much opposing force 

 is brought into play among the cohering particles. Hence, 

 when the prong is liberated, it is urged back by a force equal 

 to that used in deflecting it. When, therefore, the prong 

 reaches its original position, the force impressed on it during 

 its recoil, has generated in it a corresponding amount of mo- 

 mentum an amount of momentum nearly equivalent, that 

 is, to the force originally impressed (nearly, we must say, 

 because a certain portion has gone in communicating mo- 

 tion to the air, and a certain other portion has been trans- 

 formed into heat). This momentum carries the prong be- 

 yond the position of rest, nearly as far as it was originally 

 drawn in the reverse direction; until at length, being grad- 

 ually used up in producing an opposing tension among the 

 particles, it is all lost. The opposing tension into which the 

 expended momentum has been transformed, then generates 

 a second recoil; and so on continually the vibration even- 

 tually ceasing only because at each movement a certain 

 amount of force goes in creating atmospheric and etherial 

 undulations. Now it needs but to contemplate this repeated 

 action and reaction, to see that it is, like every action and re- 

 action, a consequence of the persistence of force. The force 



