Royal Society. 369 



the theory founded upon the conditions of stability of a liquid 

 surface gave him 21*13 millims. as the exact value of the limit- 

 ing diameter. 



I conclude by pointing out that all my Series together, count- 

 ing from the second inclusively, establish the Experimental and 

 Theoretical Statics of Liquids acted on solely by Molecular Forces. 



The present Series is followed by an analytical index to the 

 contents of the eleven Series. 



XLVII. Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from p. 308.] 

 June 16, 1870. — General Sir Edward Sabine, K.C.B., President, in 



the Chair. 

 r PHE following communications were read : — 

 -*• " Note on the Construction of Thermopiles." By the Earl of 

 Rosse, F.R.S. 



Although in the measurement of small quantities of radiant heat 

 by means of the thermopile much may be done towards increasing 

 the sensibility of the apparatus by carefully adjusting the galvano- 

 meter and rendering the needle as nearly astatic as possible, there 

 must necessarily be some limit to this ; and it therefore appears de- 

 sirable that the principles on which thermopiles of great sensibility 

 can be constructed should also be carefully attended to. 



With the view of obtaining a pair of thermopiles of greater sen- 

 sibility and of more equal power than I had been able to procure 

 ready made, I made a few experiments with various forms of that 

 instrument ; and I was led to the conclusion (one which might have 

 been foreseen) that the sensibility of the thermopile is much in- 

 creased by reduction of its mass, and more especially by a diminution 

 of the cross section of the elements. 



To obtain a clear idea of the problem before us, which is how to 

 construct the thermopile so that, with a given amount of radiant heat 

 falling on its face, the greatest current may be sent through the 

 galvanometer, let us consider the thermopile under two different con- 

 ditions : — 



1 . With the circuit open. 



2. With the circuit complete. 

 In the first case, when radiant heat falls on the face of the pile, 



the whole mass of metal rises in temperature, the rise being greatest 

 at the anterior face, and less and less as you approach the other end. 

 This rise of temperature will increase till the heat radiated from the 

 anterior face, together with that which traverses the depth of the 

 pile and is radiated from the posterior face, is just equal to that 

 radiated to the anterior face at that moment, or when 





sc 

 k(t + t') = Jct +j (t—t')= Q, 



where (t, t') are respectively the temperatures of the anterior and 



