254 Prof. J. G. MacGregor on the 



of the fiction* of contact-action. But it may be expressed 

 without any reference to such fictions. Moreover, it may be 

 deduced from the second law of motion and the impossibility 

 of the perpetual motion, neither of which axioms involves 

 such fictions. It is thus quite general, involving no assumption 

 as to the distance at which bodies can act on one another, and 

 applying to all cases of action, whether at distance zero, at 

 constant distance, or at variable distance. 



It will thus be evident why " the true law " does not lend 

 itself to fictions, and why the ordinary law does. "The true 

 law " does not, because it already embodies a fiction. The 

 ordinary law does, because it embodies none, and is equally 

 applicable, whatever fiction we may find it convenient in the 

 meantime to assume or may ultimately find apparently 

 coincident with fact. It is no discredit to " the true law " 

 not to lend itself to fictions, provided the fiction it embodies 

 assists us in coordinating the whole range of dynamical phe- 

 nomena f. But discredit must attach to it so long as there 

 are groups of phenomena to which the ordinary law can, 

 while "the true law " cannot, be applied. 



It will also be evident that, since the ordinary law involves 

 no fictions, there need be no fear lest the employment of it 

 should lead to the confounding of fictions with realities. 



Not only does the ordinary law make no assumption as to 

 the distance at which action may occur, it also assumes 

 nothing as to the mechanism of action, and holds whether 

 bodies be regarded as acting on one another through a 

 medium or not, and if they are, whatever the medium may 

 be through which their action is supposed to be conveyed. 

 To speak of the law as on this account incomplete seems to 

 me to be incorrect. Until we find some hypothesis as to 

 acting mechanism which will enable us to coordinate dyna- 

 mical phenomena, the science of dynamics must of course be 

 incomplete ; and doubtless as soon as possible some such 

 hypothesis should be framed. But no such axiom has yet 

 been suggested which is capable of general application. We 

 cannot therefore help ourselves. The foundations of dy- 

 namics must in the meantime remain incomplete, though 

 they are none the less firm on that account. Even, however, 

 when the time of omissions and slurrings shall have passed, 

 the law of the conservation of energy will be no more com- 



* As Prof. Riicker has pointed out in ' Nature,' vol. xlviii. p. ] 26, 

 contact-action is as inconceivable as action at a distance. Both are thus 

 equally fictions. 



t Prof. Lodge holds, somewhat inconsistently, that " in a fundameDtal 

 or theoretical treatment convenient fictions are better avoided " (p. 17). 



