Mr. P. G. Tait on the History of Thermo -dynamics. 291 



Pintermediaire de la vapeur ou de tout autre agent que Pon pour- 

 rait y substituer." 



Compare this with Dr. TyndalPs quotation (Phil. Mag. Sept. 

 1862) . " The law," says Mayer, " < Heat = Mechanical Effect ' 

 is independent of the nature of an elastic fluid, which only serves 

 as the apparatus, by means of which the one force is converted 

 into the other." 



Again, Seguin says, " La force mecanique qui apparait pendant 

 Pabaissement de temperature d ; un gaz comme de tout autre corps 

 qui se dilate, est la mesure et la representation de cette diminu- 

 tion de chaleur;" and further, in speaking of steam escaping 

 into the air, " 1/ effort qu'elle exerce en recul contre les appareils 

 qui la laissent echapper, ou la vitesse quelle communique a Pair 

 ambiant, forme un equivalent de la perte de chaleur qu'elle 

 eprouve." Yet, according to Dr. Tyndall, it was Mayer " who 

 first used the term f equivalent ' in the precise sense in which 

 you" (Joule, to whom the letter is addressed) "have ap- 

 plied it." 



But there is more than this in SeguiiPs very able work. He 

 points out distinctly that steam which has done work in an 

 engine ought not to heat the water in the condenser so much as 

 if it had been led directly into it. He had made, he says, nu- 

 merous experiments to test this, without however obtaining 

 sufficiently decisive results. Again, he points out how very 

 small a proportion of the heat of the steam is really employed 

 in doing work. He says that the work obtained from a steam- 

 engine, as ordinarily used, is represented by "un abaissement 

 de temperature d'environ 20°, qui equivaut au trentieme environ 

 du calorique employe pour reduire en vapeur Veau necessaire a sa 

 formation.' 3 That is, only g^th of the heat used, disappears as 

 heat, and is given out as work. Thus we see that his 20° repre- 

 sent ? J Q th of the latent heat of steam, at 100° C. and at the 

 atmospheric pressure : and his Table gives for the corresponding- 

 work done by a cubic metre of steam (in round numbers) about 

 7000 kilogrammetres. Taking 540 as the latent heat, and 0*6 

 kilogramme as the weight of a cubic metre, of steam, we have 



for the mechanical equivalent „ ' =-rx =650, about 50 per 

 ^ 0-6x540 r 



cent, too great. 



As to the Conservation of Energy. Dr. Tyndall, in a note to 

 his last paper, ascribes the term, or terms, to llankinc. I cannot 

 ascertain precisely when the term "Conservation" was intro- 

 duced, but it must have been suggested at once to an English 

 writer by the old term " Conservation of Vis Viva," of which the 

 Conservation of Energy is only an extension. At all events Helm- 



