AND ITS SELF-CONSERVATION. 139 



If, again, the body be acted upon by any number of 

 forces, the resultant of any two may be found, then this 

 resultant may be compounded with a third, and this 

 resultant with a fourth, and so on until the resultant 

 of all the forces has been ascertained. 



If, finally, the forces are infinitely multiple, as must 

 be the case in the total round of force-centers in the 

 physical universe, then the forces acting from all direc- 

 tions upon the body must balance each other. 



And this will be the more readily admitted if we 

 remember what has been more than once repeated, that 

 force really acts or can act only in overcoming opposi- 

 tion it being now necessary to add the explanatory 

 clause "or in balancing opposite phases of force." 



Such must be the conclusion from the second law 

 of motion. And it is really to this conclusion that 

 Newton gave utterance in his statement of the 



THIED LAW OF MOTION. 



In this law it is declared that "to every action there 

 is always an equal and contrary reaction ; or, the mutual 

 actions of any two bodies are always equal and oppositely 

 directed." 



Of course, the most elementary case to which this law 

 would apply would be that of the action and reaction, or, 

 as the second part of the law significantly expresses it, 

 the mutual actions between two bodies. This second 

 part is, indeed, manifestly offered, not as an addition to, 

 but rather as an interpretation of, the first part. 



But the full significance of this third law is to be 

 apprehended only when it is regarded in connection with 

 the second law in its widest range of meaning. We have 



