Mr. J. P. Joule on the Dynamical Theory of Heat, 145 



the principle has been finally established. [On this point I may 

 remark that Joule's early papers contain stores of most import- 

 ant truths, and constitute a whole province of Thermo-dynamics, 

 as yet known to very few. Some of the simplest of the results 

 which they contain have gained great credit for many able natu- 

 ralists who have recently rediscovered them.] 



(h) That Joule was the first (in print) to apply the principle 

 to organic processes and to celestial dynamics. [I may mention 

 here that Prof. J. Thomson remarked the effect of tidal friction 

 on the earth's rotation as early as 1840, and stated it to the 

 President of the Mathematical and Physical Section of the 

 British Association on the occasion of its meeting in Glasgow in 

 that year; and that his brother (Prof. W. Thomson) has taught 

 it, both privately and publicly, in the Universities of Cambridge 

 and Glasgow for at least twenty years.] 



In our article in ' Good Words ' there are a few trifling slips, 

 easily explained by our absence from books when it was written ; 

 but none of those that we are aware of at all impair the accuracy 

 of the whole. One of these has been commented on by Prof. 

 Tyndall. It consists in saying " series of experiments" instead 

 of "experiment," in describing Joule's important numerical 

 result of 1843. The former expression happens to be correct in 

 fact, but the latter would more justly describe Joule's published 

 account which was not before us when we wrote. 



Having mentioned no latitude, or locality, we gave the number 

 772 (instead of 770 for the latitude of Manchester, or 772'5 for 

 equatorial gravity), and we said that this agreed, within - J— th of 

 difference, with the more precise result cf Joule's later researches. 

 Our effort to explain the principle to an unscientific reader would 

 not have been promoted by our entering on such perplexing 

 minutiae. 



That we referred to Joule's experiments with the perforated 

 piston, in giving the date of 1843, must be evident to every 

 candid reader of the article, j am ^ Gentlemen, 



6 Greenhill Gardens, Edinburgh, &c - _ & c '> 



July 21, 1863. P. GUTHRIE TaIT. 



XX. On the Dynamical Theory of Heat. 



By James P. Joule, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Douglas, Isle of Man, 

 Gentlemen, July 16, 1863. 



I AM unwilling to do anything that might seem to prolong a 

 controversy which must already have well nigh exhausted 

 the patience of your readers, but I must trouble you with a few 

 Phil. May. S. 4, Vol. 26. No. 173. Aug. 1863. L 



