the Foundations of Dynamics, 19 



is no discredit to the true law not to lend itself to fictions ; but 

 I would also say, and freely adrnit ; that it is often permissible 

 for practical purposes to work in a fictitious or incomplete 

 manner (as, for instance, when dealing with the diurnal 

 motion of the sun or the libration of the moon), ignoring 

 communicating mechanism in cases where one is ignorant of 

 or need not attend to its nature, and in fact proceeding as the 

 great physicists have constantly proceeded when engaged on 

 a practical quest. 



Never have I imagined that the practice of men of genius 

 is other than the practice most suited to their immediate 

 end. But when it comes to the philosophy and essence of the 

 matter, I do think that there is some fear lest a constant 

 habit of ignoring unknown essentials for practical and tem- 

 porary convenience should lead these great mathematicians 

 into forgetting that they have ignored anything, should in 

 the long run delude them into treating their fictions as if they 

 corresponded to the reality of things ; and it is permissible 

 for me or others, as lookers-on seeing the structure from 

 another point of view, unobtrusively to point out to the 

 master builders, in their moments of leisure, the fact that 

 while they have been engaged in the upper stories the founda- 

 tions have almost automatically expanded, that the available 

 basis of axiomatic reality has become broader and simpler than 

 could be perceived when they started on their work, and that 

 now some portion of their temporary scaffolding and under- 

 pinning may without danger, and with advantage to the 

 general appreciation of the fabric, be discreetly and quietly 

 removed. 



Part IV. 



The Dissipation of Energy , the Nature of Potential Energy, 

 and the Second Law of Thermodynamics. 



Passing on to Prof. MacGregor's fourth objection (p. 140) 

 that i have not proved that transformation and transference 

 necessarily go together, I hope to be able to meet him more 

 nearly than has been possible to me on most of the other 

 counts of his indictment. 



My proposition was that the change of form is always from 

 kinetic to potential or vice versa. But it is not necessary to 

 attach the same importance to this law as an ultimate fact, 

 because it is so extremely probable that most or all cases of 

 potential energy will ultimately be resolved into essential 

 motion. Nevertheless it is for the present a convenient 

 statement of truth, having the same sort of validity that the 

 second law of Thermodynamics possesses. 



C 2 



