160 THE WORLD-ENERGY 



be the quantity of that change. In other words, the 

 greater will be the quantity of motion. 



It is evident,, then, that while in every respect the 

 actual force impressed upon each other by two gravi- 

 tating bodies, must be the same for the one as for the 

 other, the quantity of resultant motion of each must be 

 directly dependent upon, and precisely proportional to 

 the mass of the body. The greater the mass, the greater 

 the quantity of motion; the less the mass, the less the 

 quantity of motion. 



Nevertheless, as we have but just seen, the second law 

 of motion declares, what proved on investigation to be 

 necessarily true, that change of motion is precisely pro- 

 portional to the impressed force. Evidently, then, the 

 mass of a body is not the sole factor of motion. 



Indeed, it has just been shown that, since decrease of 

 distance between gravitating bodies intensifies the force 

 impressed by each upon the other, and since the motion 

 is always proportional to the impressed force, the quan- 

 tity of motion necessarily varies with the distance. But 

 the variation here is a variation in the rate of approach. 

 Each of the bodies must approach the other with a 

 regularly increasing velocity. And yet, it has been 

 shown that the force impressed by each body upon the 

 other is precisely the same in quantity as that impressed 

 upon it by the other. In short the "impressed force" is a 

 relation in which each acts and is acted upon in precisely 

 the same degree. 



The same quantity of force, then, is, in the one case, 

 impressed upon a greater mass, in the other case upon 

 a less mass. But, where there is less mass, there is 

 also less resistance to change of motion. In this respect, 



