Prof. TyndalPs Notes on Scientific History. 33 



to his compressed steam, are not thermal units at all, and that there 

 is no determination whatever of the mechanical equivalent of heat 

 in the above Table. 



19. The number 363 has been found for M. Seguin, not by 

 him : he never made the division which results in this quotient. 

 In 1847, for the first time, and without giving any description of 

 his method *, M. Seguin gives his results " reduced to the type 

 of 1 gramme elevated 1 metre, and corrected with reference to 

 the specific heat of water and vapour." His equivalent there 

 given varies from 395 to 529 kilogrammetres (Comptes Rendus, 

 vol. xxv. p. 420). The data, moreover, on which M. Seguin 

 founded this last calculation were subsequently declared erro- 

 neous by himself : the experiments of M. Regnault, he states, 

 "defeated the calculations" (Cosmos, vol. vi. p. 684) ; and Mr. 

 Grove has shown that when the correct specific heat of steam, as 

 determined by Regnault, is introduced into the calculations, M. 

 Sdguin's equivalent becomes 1666 kilogrammetres instead of 363 

 (Proceedings of the Royal Institution, vol. ii. p. 155). We have 

 already seen that in Mayer's case, when the correct specific heat 

 of air is employed, his result is almost identical with that derived 

 from the mean of all the best experiments of Mr. Joule. The 

 one is 426, the other is 425 f. 



III. 



20. After going formally through the calculation of the me- 

 chanical equivalent of heat, Mayer proceeds to determine the 

 useful effect in steam-engines, and finds it to be about 5 per 

 cent, of the consumed fuel. He then determines the useful 



* Such a description would be a desirable addition to our knowledge. 



t I have already drawn attention to these facts (Phil. Mag. vol. xxv. 

 p. 385), but have been met, not by explanation, but by iteration. This, I 

 trust, will now cease. It is no compliment to the scientific public to think 

 that mere hardihood of assertion can decide this question. 



To illustrate the difficulty of satisfying rival claimants, I may remark 

 that in 1862 I withdrew the name of Dr. Mayer from the list of candidates 

 for the Copley medal, out of deference to an eminent man — not Mr. Joule — 

 who thought his own claims prior to those of Mayer. What I have had 

 to endure at the hands of two northern critics for my supposed depre- 

 ciation and suppression of the claims of Mr. Joule is at least partially 

 known to the scientific public. Again, a writer in M. Seguin's periodical, 

 he Cosmos, while declaring that Mr. Joule must be entirely put aside, the 

 question of priority resting solely between Seguin and Mayer, charges me 

 with having manifested a wholly insufficient appreciation of M. Seguin. 

 I should be a mere intellectual quicksand if I allowed myself to be swayed 

 by such criticisms. I have, judging from the facts, steered through these 

 rival claims with the best light that I possess, and not one of my censors 

 appears to have gone to one-tenth of the trouble that I have incurred to 

 inform myself of the rights of the question. 



Phil Mag. S. 4. Vol. 28. No. 186. July 1864. D 



