352 Mr. J. D. Chorlton. Examination of some of the [Mar. 19, 



Joule's value subject to the above correction assumes the electro- 

 chemical equivalent of silver to be 0*001118; if the correction be 

 neglected, his value is obtained from purely electro-magnetic measure- 

 ments, and is 773*3, closely agreeing with his valae obtained from 

 frictional experiments. The values of Griffiths and Schuster and 

 Gannon also depend on silver electrolysis. 



The last Apparatus for Determining the Mechanical Equivalent of Heat 

 ('Philosophical Transactions,' 1878. Part II; also * Collected 

 Papers,' vol. 1, pp. 632—657). 



Joule gives the following account of the circumstances which 

 induced him to undertake the construction of this apparatus : — 



" The Committee of the British Association on Standards of Elec- 

 trical Resistances, having judged it desirable that a fresh determina- 

 tion of the mechanical equivalent of heat should be made, by 

 observing the thermal effects due to the transmission of electrical 

 currents through resistances measured by the unit they had issued, 

 I undertook experiments with that view, resulting in a larger figure, 

 782'5, than that which I had obtained from the friction of fluids, 

 772'6. The only way to account for the discrepancy was to admit 

 the existence of error either in my thermal experiments or in the 

 unit of resistance. A Committee consisting of Sir ¥m. Thomson, 

 Professor P. G. Tait, Professor Clark Maxwell, Professor B. Stewart, 

 and myself were appointed at the meeting of the British Association 

 in 1870, and with the funds thus placed at my disposal I was charged 

 with the present investigation for the purpose of giving greater 

 accuracy to the results of the direct method." (' Collected Papers,' 

 vol. L p. 632). 



The method adopted was to revolve a paddle in a suspended vessel 

 of water, and to find the heat produced, the work being measured by 

 the force required to hold the vessel from turning, and the distance 

 run as referred to the point at which the force was applied. 



The result of the paper was to confirm the earlier experiments, 

 and to show that the B.A. ohm was in all probability wrong to the 

 extent of 1*3 per cent., the final value of the equivalent by this 

 method being 772'55. 



The apparatus is intact in all essential points, and it is, I believe, 

 the intention of Mr. Joule to send it shortly to the South Kensington 

 Museum. 



An Electro-magnetic Engine (' Collected Papers,' vol. 1, pp. 1 — 3). 



This little instrument is of interest because it was the occasion of 

 the first paper Joule ever published which appeared in 'Sturgeon's 



