394 THE POPULAR SCIENCE MONTHLY. 



friends on the matter. Devoting, as Professor Birks does, his chapter 

 entitled " The Transformation of Force and Motion," to the incon- 

 gruities which result when the doctrine of the persistence of force is 

 joined with the doctrine of potential energy, as commonly received, it 

 was doubtless convenient to assume, spite of the direct evidence to the 

 contrary, that I accept this doctrine, and am implicated in all the con- 

 sequences. But there can be but one opinion resj^ecting the honesty 

 of making the assumi^tion. Let me add that my rejection of this doc- 

 trine is not Avithout other warrant than my own. Since the issue of 

 the last edition of this work, containing the passages I have referred 

 to, Mr. James Croll, no mean authority as a mathematician and physi- 

 cist, has published in the " Philosophical Magazine " for October, 1876, 

 page 241, a paper in which he shows, I think conclusively, that the 

 commonly accepted view of potential energy can not be sustained, but 

 that energy invariably remains actual. I learn from him that he had 

 in 1867 indicated briefly this same view. 



The remaining case, above adverted to as calling for comment, 

 concerns my motive for suppressing a certain passage in the chapter 

 on " Ultimate Scientific Ideas," and substituting another passage. Be- 

 fore proceeding to state the reasons for this substitution, and to dis- 

 prove the inferences which Professor Birks draws from it, I may 

 remark that it is usual in literary criticism to judge an author by the 

 latest expression of his views. It is commonly thought nothing but 

 fair that if he has made an error (I say this hypothetically, for in this 

 case I have no error to acknowledge), he should be allowed the benefit 

 of any correction he makes. Professor Birks, however, apparently 

 thinks that, moved by the high motive of " doing God service," he is 

 warranted in taking the opposite course perhaps thinks, indeed, that 

 he would fail of his duty did any regard for generous dealing prevent 

 him from making a point against an opponent of his creed. 



But now, saying no more about the ethics of criticism, I pass to the 

 substantial question. In the first place, I have to point out that in 

 the passage suppressed I have not said that which Professor Birks 

 alleges. He represents me as asserting that " gravitation is a necessary 

 result of the laws of space " (p. 227). I have asserted no such thing. 

 He says, "There can be no a priori necessity that every particle 

 should act on every other at all at every distance" (p. 222). I have 

 nowhere said, or even hinted, that there is any such a priori neces- 

 sity. The notion that " gravitation results by a fatal necessity from 

 the laws of space," which he ascribes to me (p. 229), is one which I 

 should repudiate as utterly absurd, and one which is not in the remotest 

 way implied by anything I have said. What I have said is that 

 " light, heat, gravitation, and all central forces, vary inversely as the 

 squares of the distances," and that " this law is not simjjly an empirical 

 one, but one deducible mathematically from the relations of space." 

 Now, what is here said to be " deducible mathematically from the rela- 



