APPENDIX. 599 



to ask how it happens that Prof. Birks, elaborately criticizing 

 my views step by step, deliberately ignores the passages in 

 which I have repudiated this doctrine? In the chapter on 

 " The Continuity of Motion," I have, at considerable length, 

 given reasons for regarding the conception of Potential Energy 

 as an illegitimate one; and have distinctly stated that I am 

 at issue with scientific friends on the matter. Devoting, as 

 Prof. Birks does, his chapter entitled " The Transformation 

 of Force and Motion," to the incongruities which result when 

 the doctrine of the Persistence of Force is joined with the doc- 

 trine of Potential Energy, as commonly received, it was doubt- 

 less convenient to assume, spite of the direct evidence to the 

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

 the consequences. But there can be but one opinion respect- 

 ing the honesty of making the assumption. Let me add that 

 my rejection of this doctrine is not without 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 physicist, has pub- 

 lished in the Philosophical Magazine for Oct., 1876, p. 241, 

 a paper in which he shows, I think conclusively, that the com- 

 monly accepted view of Potential Energy cannot 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 com- 

 ment, concerns my motive for suppressing a certain passage 

 in the chapter on " Ultimate Scientific Ideas," and substitut- 

 ing another passage. Before proceeding to state the reasons 

 for this substitution, and to disprove the inferences which 

 Prof. 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. Prof. 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 



