the Foundations of Dynamics. 15 



merely, and leaves untouched the question of what happens 

 during simple storage. 



That is true, and I do not think that I had noticed the 

 objection before. My deduction proves that in all cases of 

 activity energy passes on without loss ; it does not prove that 

 without any activity energy may not leak away in some silent 

 unobtrusive fashion, leaving no trace, but simply vanishing. 



To meet this objection, an appeal may be made to the 

 definition ; it may be called on to assert that anything that 

 can thus disappear without force or motion is not energy, 

 according to its terms; but I doubt whether such an appeal 

 is perfectly cogent, though it has some, perhaps much, plausi- 

 bility. I think that the non-disappearance of energy in this 

 occult manner had perhaps better be regarded as an axiom, a 

 statement which may be believed until some clear experi- 

 mental disproof is forthcoming : no such disproof being 

 meanwhile in the least expected. 



Briefly summarised, the matter stands thus : — 



(1) If energy can only be got rid of by activity, and (2) if 

 that activity never affects its quantity, the law of conservation 

 is completely stated. My proof covers (2), but not (1). I 

 may therefore agree with what I suppose Prof. MacGregor to 

 mean, that the portion (1) must be left as an axiom based on 

 experience. 



Prof. MacGregor goes on to say (p. 137) that my law of 

 conservation during mutual action has no deeper meaning 

 than the conservation of their joint momentum, and is quite 

 consistent with the non-conservation of their working- power. 



To this I reply, first, that I mistrust the vague term 

 " working-power," it is apt to mean whatever may be con- 

 venient ; from one point of view a given amount of energy 

 may have an infinite " working-power," since it can do work 

 at every transfer without itself diminishing; while from 

 another point of view it is rather bold to maintain the conser- 

 vation of working-power in face of the doctrine of the dissi- 

 pation of energy. And secondly I reply, that the conservation 

 of momentum rests on the equality of the forces exerted by 

 two mutually operative bodies, combined with the obvious 

 equality of the durations or times of action of these forces. 

 Whereas the conservation of energy rests on the equality of the 

 forces, combined with the altogether less obvious fact of the 

 equality of velocities, or distances traversed, by each of two oper- 

 ative bodies. Anyone in his senses who believes in action at 

 a distance would deny that the velocities of two directly 

 acting bodies are necessarily equal (no other bodies being 

 assumed present, so as to simplify the problem) ; and in so 

 doing he practically denies the conservation of energy. 



