AND ITS SELF-CONSERVATION. 161 



then, the less mass will yield more readily to the mutual 

 attraction than will the greater mass. The less mass 

 will, therefore, acquire the greater velocity. And its 

 velocity at any given moment must be precisely as much 

 greater than that of the greater body as its mass is less 

 than that of the greater. 



It is now evident, that while the greater body has pre- 

 cisely as much greater quantity of motion than has the 

 less body, as its mass exceeds the mass of the less, when 

 we regard the quantity of motion from the side of the 

 number of force-centers moved; it has, when we con- 

 sider the quantity of motion from the side of velocity, 

 as much less motion than the smaller body, as its mass 

 exceeds the mass of the smaller body. 



It is clear, then, that the total quantity of motion is 

 compounded of the quantity of motion dependent upon 

 the number of force-centers moved, on the one hand, with 

 the quantity of motion dependent upon velocity, on the 

 other hand. 



We thus arrive at the definition of momentum, as 

 equal to the product of the mass into the velocity. In 

 other words, momentum is merely the technical term 

 expressive of quantity of motion in this compound 

 sense. And this the usual works on physics declare it 

 to be. 



Having, then, traced the conditions determining the 

 quantity of motion in any body, we have next to follow 

 the actual motion and ascertain its essential phases. And 

 since, at the outset, any change in the motion of any 

 given body must take place in the direction of the line 

 in which the impressed force acts, the motion of any two 

 bodies attracting each other, must be in the direction 



