ISAAC NEWTON. 439 



law. There is no event in this history more significant, 

 more epoch-making, than this. 



It will be remembered, that among the fundamental 

 principles of physics are the three laws of motion which 

 Newton formulated; the first stating the effect of force 

 upon a body left to itself; the second defining the relation 

 of the change of motion of the body to the force impressed, 

 and the third that perennial stumbling-block to all the 

 perpetual-motion seekers of the past and most u new 

 motor" contrivers of the present that action and reaction 

 are equal and in contrary directions. This last obviously 

 defines the effect of the action of two bodies one upon the 

 other that of the first upon the second being equaled by 

 the contrary reaction of the second upon the first; or, to 

 borrow Newton's own illustration, "If you press a stone 

 with your finger, the finger is also pressed by the stone. 

 If a horse draws a stone tied to a rope, the horse will be 

 equally drawn back toward the stone; for the distended 

 rope, by the same endeavor to relax or unbend itself, will 

 draw the horse as much toward the stone, as it does the 

 stone toward the horse, and will obstruct the progress of 

 the one as much as it advances that of the other." 1 



Under this law, Newton makes the first close linkage of 

 gravity, electricity and magnetism. If the sun draws a 

 planet, so that planet draws the sun ; if the amber draws 

 chaff, so that chaff draws the amber; if the lodestone draws 

 iron, so the iron draws the stone. The law is the same 

 for all. It is the law of stress. 2 



But the bond is closer than this. He mentions the com- 

 mon habit of referring the reacting forces to that body of 



1 Principia, Axioms or Laws of Motion. 



2 " Every force, in fact, is one of a pair of equal opposite ones one 

 component, that is of a stress either like the stress exerted by a piece 

 of stretched elastic, which pulls the two things to which it is attached 

 with equal force in opposite directions and which is called a tension; or 

 like the stress of compressed railway buffers, or of a piece of squeezed 

 india rubber, which exerts an equal push each way and is called a pres- 

 sure." (Lodge.) 



