AND ITS SELF-CONSERVATION. 137 



constantly varying velocity of the falling body in its asso- 

 ciation with the orbital motion of the earth, and the 

 problem becomes highly complex, while, with the 

 inclusion of the unknown velocity of the solar system 

 through space, the problem of the velocity of the falling 

 body, of course, becomes altogether insoluble. 



But more and more it comes to light that motion, 

 whether in respect of direction or of velocity, is a result 

 that can arise only from the mutual action of forces 

 upon each other. A force can really act, or become an 

 " impressed force/' on no other condition than that of 

 overcoming resistance. This we have seen to be involved 

 in the very nature of force. And when it is declared, 

 in the law of gravity, that every body attracts every 

 other body, it is declared, in effect, that between every 

 two bodies there is a mutual attraction. Or, since every 

 center of force lays hold on every other center of force, 

 it may be otherwise said that every force-center attracts 

 and is attracted by, repels and is repelled by, every 

 other force-center in the entire range of the extended 

 world. 



Each force-center, then, to repeat once more, is a 

 veritable center of the physical universe, and as such 

 acts upon arid is in turn reacted upon by every other 

 force-center. So that, in the cases of falling bodies 

 just named, it is evident that there is one factor which we 

 have wholly overlooked, and that is the fact that while 

 any given body is falling toward the earth, the earth is 

 also falling toward that body. And if the variations 

 thus introduced into its movements are too minute for 

 even the most refined infinitesimal calculus to seize and 

 measure, that does not render them any the less real. 



