530 Dr. Lodge on Action at a Distance, 



conservation of energy is not identical with that given in the 

 text-books ; but he goes off it again. I assure him that I 

 really do not assume that conservation is true of potential 

 energy only, and forget to take account of kinetic : I should 

 be rather mad if I did. 



A case is then put " in mathematical form;" and by the 

 ingenious, but rather cheap, device of writing vdv in the shape 



cud_rcu\ ht 



dt dt \dtJ 

 and then mixing together the energy-equations for two distinct 

 particles into one equation, an expression intended to be 

 alarming is manufactured. But, unfortunately, Mr. Browne 

 starts by saying that I " suppose two particles moving with 

 different velocities v and v/, and acting on each other with equal 

 and opposite moving forces F." Now, with all due deference, 

 this is exactly what I do not suppose. For as long as two 

 particles are moving with different velocities they cannot be 

 in contact ; and my case is, that two bodies not in contact 

 are ipso facto incompetent to act directly on each other ; it is 

 only when they touch and during the period of contact (that 

 is, while they are both moving at the same rate in the same 

 direction) that any immediate action can take place between 

 them. 



But now that we have come so near to the real peculiarity 

 in my argument, perhaps we had better have it out, not for 

 the sake of prolonging the present controversy, nor, indeed, 

 with any reference to it, but because I do undoubtedly attach 

 importance to the matter, and distinctly believe that it proves 

 direct action at a distance to be impossible. Moreover, I am 

 of the opinion that this is not a metaphysical or trivial ques- 

 tion, but an essentially physical one, and one that is by no 

 means unimportant just now, when opposition theories of 

 electricity — the medium-theory of Maxwell and Faraday, and 

 the action- at-a-distance theories of "Weber, Gauss, Neumann, 

 and others — are in the field against each other. 



Theories of physical phenomena worked out on the hypo- 

 thesis of direct forces across intervening space are, at certain 

 stages of our knowledge, an immense help and of great im- 

 portance — notably the gravitation theory of Newton and the 

 electrical theories of Coulomb and Ampere ; but if it can 

 once be distinctly proved and clearly recognized that they 

 can only be provisional, and must necessarily be replaced by 

 medium theories as the science progresses, a useful step will 

 have been made ; and one result will be that there can be no 

 further question as to which of the rival electrical theories 



