CHAPTER XV. 



MOMENTUM. 



WE have next to trace out the necessary implications 

 or corollaries of the law of gravity. And, first, 

 let it be remembered that the attraction is between the 

 ultimate force-centers severally, and not between aggre- 

 gated masses, as such. 



At the same time, the mass itself is nothing else than 

 an aggregation of force-centers, and the attractive force 

 exerted by a given body must therefore be directly and 

 exactly proportional to its mass. 



Between any two bodies, or aggregations of force- 

 centers, then, there will be a constant strain, tending 

 to bring the bodies nearer to each other with a force 

 proportional to the product of the masses of the two. 

 That is, there will be a constant pull between each 

 force-center in the one body, and every force-center in 

 the other. 



It is to such strain, indeed, that we have already 

 traced the primary aggregation of force-centers into sin- 

 gle masses. And it is evident that the continuance of 

 the strain between the force-centers of any one of these 

 already formed groups, and the force-centers of any other 

 group, is but the continuation of the same tendency. 

 The motion due to the strain, therefore, can but result 

 in further aggregation through the meeting and coales- 

 cence of such groups into larger ones. 



157 



