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XIX. On the Conservation of 'Energy. 

 By P. G. Tait, M.A. \c. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I REQUEST the favour of your inserting this additional com- 

 munication. It is rendered necessary by the course which 

 Prof. Tyndall has followed, and by my not having, like Prof. 

 Thomson, declined to take further part in personal controversy 

 with him. But henceforth I must do so, as the pages of the Philo- 

 sophical Magazine have been already far too much occupied with 

 personal matters, totally irrelevant to the question originally at 

 issue — such as Prof. Tyndall's unwarrantable and utterly erro- 

 neous assumption that what he pleases to consider an elaborate 

 attack on himself (in l Good Words') was due to Prof. Thomson, 

 and his insinuation of "reasons too long to state." 



That question I understood to be the respective claims of Joule 

 and Mayer, and to that I have kept closely. I am happy to 

 find that Prof. Tyndall (by silence) intimates a tardy acquies- 

 cence in the statements which Prof. Thomson and I have made, 

 and which I shall condense here. 



(a) That Newton enunciated, in a complete form, the Conser- 

 vation of Energy in abstract dynamics. 



(b) That Davy, having experimentally proved that heat is 

 motion and that its laws of communication are the same as those 

 of the communication of motion, extended Newton's principle to 

 one class of molecular motions. 



(c) That Carnot supplied, among other valuable developments, 

 that most important branch of the theory, Reversible Cycles in 

 Thermo-dynamics, from which has been evolved the grand consi- 

 deration of the dissipation of energy ; but that some of his results 

 require modification, as he assumed the materiality of heat. 



(d) That Seguin in 1839, and Mayer in 1842, from unwar- 

 rantable assumptions combined with Davy's discovery, attempted 

 to find the mechanical equivalent of heat, but obtained very erro- 

 neous, although coincident, results. 



(e) That Joule experimentally proved that their method should 

 have given an approximately accurate result, and supplied, from 

 his own researches, the requisite data. 



(/) But that before Joule did so, he gave in 18i3 the almost 

 exact value 770 foot-pounds. [To Prof. Tyndall's remarks on 

 this point I shall presently advert.] 



(g) That, in 1840 and subsequent years, Joule made tremen- 

 dous strides in the experimental proof of the generality of the 

 Conservation of Energy, and that it is by these experiments that 



