Prof. Tait on the Conservation of Energy. 431 



principles) could never have given even an approximation, was 

 furnished by Joule in 1851 and 1852, by means of his own value 

 of the mechanical equivalent as determined from the friction of 

 liquids, and by actual experiments on the specific heat of air 

 made for the purpose of verifying this deduction. 



In the application of energy to organic processes, Joule pre* 

 ceded Mayer (at all events as far as publication is concerned) by 

 two years; and in its application to shooting-stars and some 

 other points of celestial dynamics he had at least one year's 

 priority. 



Mayer's later papers are extremely remarkable and excessively 

 interesting, and certainly deserve high credit ; but, as we have 

 just seen, they are, though greatly superior in development to 

 the earliest cosmical speculations of Joule, certainly subsequent 

 to them in publication. 



My remark about Seguin was already virtually made by Joule 

 in his published letter of August last. He there gives Seguin's 

 equivalent as 363 kilogrammetres, and Mayer's as 365 ; and 

 quotes from Seguin the following sentence which embodies the 

 assumption of Mayer (but without his false analogy) : — " La 

 force mecanique qui apparait pendant l'abaissement de tempera- 

 ture d'un gaz comme de tout autre corps qui se dilate, est la 

 mesure et la representation de cette diminution de chaleur." I 

 am not aware that Prof. Tyndall has pointed out any inaccura- 

 cies in that letter. 



That Joule gave in 1843 the value 772 foot-pounds (or 770, 

 which, as Prof. Thomson and I remarked in our article in ' Good 

 Words/ is within about ^th of difference) is denied by Prof. 

 Tyndall. Prof. Tyndall strangely enough quotes all Joule's 

 published values for that year with this remarkable exception] 

 and this is the more strange that in a foot-note to his last 

 paper he quotes Joule's very next sentence. In ' Good Words ' 

 we distinctly said " the actual method which he [Joule] first em- 

 ployed was to force water through small tubes." The determi- 

 nations which Prof. Tyndall has quoted were made by means of 

 the magneto-electric machine, and, considering the excessive ex- 

 perimental difficulties which this process involves, it is wonderful 

 that they were not much more widely discordant. 



With many apologies for thus trespassing on your valuable 

 space, believe me, 



My dear Sir David Brewster, 



Yours very truly, 



P. Guthrie Tait. 



6 Greenhill Gardens, Edinburgh, 

 May 8, 1863. 



2 G2 



