Dr. C. K, Akin on the Conservation of Force. 205 



cialities that do not always go together. The main end and 

 purpose of these would be to obtain some idea as to the working 

 arrangements between the aether — that higher potential form of 

 matter in which the might of the Infinite resides — and ordinary 

 molecules, the agents of its development. If we confine our 

 attention to the planetary movements, nothing seems clearer 

 than that its density must be inappreciable. On the other hand, 

 were we to make legitimate inferences from the most obvious 

 phenomena of radiant heat, there is evidence that its density 

 may not differ much from that of water, and at least that it is 

 quite impossible that its non-resistance to the celestial motions 

 can be owing to its extreme rarity. 



Edinburgh, December 4, 1864. 



XXIX. On the Conservation of Force. By Dr. C. K. Akin*. 



ABSENCE, and another controversy of a very different 

 nature, prevented my noticing hitherto Professor Tait's 

 answer to my remarks published in the last December Num- 

 ber of this Magazine. Professor Tait begins by calling atten- 

 tion to the fact that, although omitting the words "in omni 

 instrumentorum usu " from the passage which he quoted from 

 Newton's scholium, he indicated the omission by dots. The readers 

 of this Magazine will have seen that in reproducing from Pro- 

 fessor Tait's paper the paragraph in question, I took care to 

 cause the dots also to be inserted, to which I made special refer- 

 ence in my remarks. On this point, therefore, there can be no 

 misunderstanding. But when Professor Tait says that " in ordi- 

 nary mechanics " is the " perfectly complete " free rendering of 

 the above Latin words, I can only partially agree with him. No 

 doubt the rendering is free, " not literal," and in some instances 

 it might also be correct ; but I contend that in the present case 

 it is not properly admissible. In the sentences preceding the 

 one cited both by Professor Tait and myself, Newton instances 

 expressly the cases of the "balance," "pulley," "clocks," 

 "screw," and "wedge"; and in my opinion, therefore, the free 

 English translation of "in omni instrumentorum usu," as appli- 

 cable to the case in hand, is not "in ordinary mechanics," but as 

 given by Motte, "in the use of all sorts of machines," or 

 something like it. 



Professor Tait allows that " in Newton's time, and long after- 

 wards, it was supposed that work was absolutely lost by friction " 

 — in other words, that Newton himself supposed it to be so ; but, 

 considering that it was known that friction excites heat, as well 

 * Communicated by the Author. 



