114 Mr. R. Moon on Helniholtz's Memoir on 



by Bumford and Davy, there were published a great many specu- 

 lations as to the nature of Heat and its relation to work. Several 

 of these speculations, especially those of Mayer and Scguin, have 

 been discussed, and at least in part reprinted, in the Philosophical 

 Magaziue. It is right, therefore, th:it the same journal should recall 

 attention to the above paper, which was recently pointed out to me 

 by Professor Crum Brown, and contains what are in some respects 

 the most remarkable of all these speculations. 



Singularly enough, it is not even referred to by Mayer, though 

 his much belauded earliest paper appeared only live years later and 

 in the very same journal. It contains, in a considerably superior 

 form, almost all that is correct in Mayer's paper ; and, though it 

 contains many mistakes, it avoids some of the worst of those made 

 by Mayer, especially his false analogy and his a priori reasoning. 



Polarization of Heat is ascribed to Melloni instead of Forbes ; 

 the calculation from the compressibility and expansibility of water 

 is meaningless ; and the confusion between the two perfectly di- 

 stinct meanings of the word Kraft is nearly as great as that which 

 some modern British authors are attempting to introduce into their 

 own language by ascribing a second and quite indefensible meaning 

 to the word Force. 



On the other hand, several of the necessary consequences of the 

 establishment of the Undulatory nature of Radiant Heat are well 

 stated ; and the very process (for determining the mechanical equi- 

 valent of heat by the two specific heats of air) for which Mayer 

 has received in some quarters such extraordinary praise— though it 

 is in principle, albeit not in practice, utterly erroneous — is here 

 stated much more clearly than it was stated five years later by 

 Mayer.— P. Q. Tait.] 



XV. Some Further Remarks on Helmholtz's Memoir on the 

 Conservation of Force; and on the more Modern Mode of 

 Presenting his Theory, By Hobekt Moon, M.A., Honorary 

 Fellow of Queen s College, Cambridge*. 



I. TN my former paper f I drew attention to Dr. Helmholtz's 

 -■- tacit assumption that, when waves traverse a medium, 

 the term in the equation of vis viva depending on the " ten- 

 sions " vanishes, and to his inference, from a crude and im- 

 perfect treatment of a particular case of interference, that the 

 sum of the vires vivo3 in such cases is always constant. 



I showed that, although when a wave is propagated un- 

 alterably through a cylindrical tube filled with air, for ex- 

 ample, " the sum of the tensions vanishes, and the sum of the 

 vires vivce is necessarily constant, this will not be the case 

 when two such waves, in other respects similar, interfere with 

 each other. 



* Communicated by the Author. 



t See Phil. Mag. for May 1875. 



