the Foundations of Dynamics. 11 



so slight as to remove all difficulty from what otherwise would 

 seem, and indeed, strange to say, to many still does seem, an 

 exceedingly tough morsel to swallow. 



[I mean that one constantly finds examination candidates, 

 and even Engineers, when catechized about a horse pulling a 

 cart, though they may, some of them, politely admit that 

 approximately the pull of the horse and the pull of the cart 

 are equal (constituting the stress in the trace whose inertia 

 we may agree to neglect), yet nevertheless assert that in 

 reality the pull of the horse must be the least little bit bigger 

 than the pull back of the trace, else the thing could not start. 

 The fact is that the universal truth of the third law is not 

 axiomatic, or at least is not obvious"*, and hence its deduction 

 from the other two laws is really a useful deduction^ 



I do not see any point in Dr. MaoGregor's second objection 

 that the proof is inapplicable to a body without parts. For 

 if such a body anywhere exists, plainly its parts cannot act 

 on each other, and so there are no actions or reactions in such 

 a body worth troubling about. 



The concluding portion of Prof. MacGregor's address has 

 to do with the Conservation of Energy and the question of 

 how far it can be deduced from the third law. But all these 

 questions respecting energy we must fight out at greater 

 length. Dr. MacGregor well knows that from the third law 

 and the denial of action at distance together I claim to have 

 deduced a law of conservation simpler and more precise than 

 the ordinary law ; but his objection to thisf is that though such 

 a law may come to be accepted as sufficient in the future, 

 when the universality of contact-action is fully recognized, 

 it is inadequate for the present, when action at a distance 

 still holds a portion of the field ; by which I suppose he really 

 only means that many mathematical methods of treatment are 

 based at present on action-at-a-distance modes of expression. 

 I have no fault to find with any convenient mode of attacking 

 specific problems ; it is permissible to everyone to use the 

 language of distance-action for practical purposes ; but when 

 it comes to formulating fundamental laws I have no ambition 

 to legislate for such cases until they can be shown actually to 

 occur. 1 am open to experimental proof of their existence, but 

 to none other. It is premature to legislate for them. If any 

 action other than contact-action exists, we had better know 

 more about it before formulating its laws. If the will-power 

 of a " medium " for instance can really move a chair without 



* See for instance ' Nature/ vol. xxxvii. p. 558 ; and see all recent 

 volumes of 'The Engineer ' passim, especially about 1885 and 1891. 

 t Expressed elsewhere, viz. Phil. Mag. February 1893. 



