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IX. Remarks on Professor Tait's last Letter to Sir David Brewster. 

 By John Tyndall, F.R.S., $&.* 



THE scientific public has already formed its own estimate of 

 the charge made against me in the October Number of 

 ' Good Words/ and it is therefore not necessary that I should 

 refer to it further. But in the last paragraph of Prof. TanVs 

 last communication to the Philosophical Magazine a new point 

 is raised against me, which I now beg permission to examine. 



In the article which generated this discussion Profs. Thomson 

 and Tait write as fohWs : — "In 1843 he (Mr. Joule) published 

 the results of a well-planned and executed series of experiments, 

 by which he ascertained that a pound of water is raised 1° Fahr. 

 in temperature by 772 foot-pounds of mechanical work done upon 

 it." And in reference to this passage, I state that "it was in 

 1849, and not in 1843, that Mr. Joule proved the mechanical 

 equivalent of heat to be 772 foot-pounds. His determinations 

 in 1843 varied from 1040 to 587 foot-pounds." To men of 

 science throughout the world I leave it to decide whether this 

 statement is not strictly true. What, then, is the "remark- 

 able exception " referred to by Prof. Tait ? If the reader have 

 patience to follow me he shall learn. In an appendix which fol- 

 lows my letter to Prof. Thomson, I quote from my book on Heat a 

 resume of the results obtained by Mr. Joule in 1843. Mr. Joule 

 describes each series of his experiments, gives the numbers 

 obtained, and deduces from these numbers the mechanical 

 equivalent of heat, which he places in each case at the end of a 

 paragraph. When I wrote my book I went carefully over 

 Mr. Joule's paper, — first looking at his numerical data, and then 

 taking up his result. This paper was dated from " Pendlebury, 

 near Manchester, July 1843." 



A postscript followed the paper, commencing with words 

 which indicated a reference to the results previously recorded. 

 I cast my eye over the postscript ; I saw no numerical data, 

 and no mechanical equivalent at the end of any one of its three 

 paragraphs, and hence I concluded that the results ended with 

 the paper itself. Prof. Tait, however, has discovered that in 

 the middle of the first paragraph of the postscript a determination 

 is alluded to. Here is Mr. Joule's whole statement regarding 

 it : — " I have myself proved that heat is evolved by the passage of 

 water through narrow tubes. My apparatus consisted of a piston, 

 perforated by a number of holes, working in a cylinder glass jar, 

 and containing about 7 lbs. of water. I thus obtained one 

 degree of heat per pound of water from a mechanical force 

 capable of raising about 770 lbs. one foot high." This, then, is 

 * Communicated by the Author. 



Phil. Mag. S. 4. Vol. 26. No. 172. July 1863. F 



