278 Prof. Challis on the Action of the Cup-shaped 



than in the other two. These, however, were closely accordant 

 with each other, and would lead to 431*5 as the value of the 

 mechanical equivalent." 



In view of this comment, the above values cannot be 

 regarded as beyond admissible limits. We may conclude, 

 then, that the values 1*40 for ratio of specific heats, and 331*2 

 metres or 1087 feet for the velocity of sound, are a fair repre- 

 sentation of experimental results — and that they give rise to 

 no discordances in their connexion with other data and theories, 

 which are of an amount to call for rectification on the evidence 

 at present before us. 



P.S. — Since the above was written, and after my forth- 

 coming little work, in which the matter is similarly treated, 

 has gone to the printers, I read in the number of PoggendorfF 

 just received (1877, No. 1, p. 175) a short note from Boltz- 

 mann, being an abstract from a paper in the Vienna Berichte, 

 in which he shortly announces the above result and his adhe- 

 sion to it. It is stated that the experimental values of sound 

 are discussed in the original paper; so that the substance of 

 my communication is anticipated. I have, however, thought 

 that it may still not be entirely without interest. 



XXXVII. A Theory of the Action of the Cup-shaped Radio- 

 meter with both sides bright. By Professor Challis, M.A., 

 F.R.S., F.R.A.S.* 



OUT of the many experiments relating to the radiometers 

 described by Mr. Crookes in his communication con- 

 tained in No. 175 of the c Proceedings of the Royal Society,' 

 I have selected for theoretical consideration No. 1035, in 

 p. 313, because this experiment exhibits a novel and signifi- 

 cant phase of the action of the instrument. It is evident that 

 in this instance the radiometer is caused to rotate simply be- 

 cause the vanes are convex on one side and concave on the 

 other, the conditions as to the incidence of light or heat being 

 exactly the same for both ; and accordingly a theory of the 

 motion must explain how the effect is produced solely by the 

 difference of the two sides as to form. Such an explanation 

 will, I think, be found to be given by the following theory, 

 which rests on the same principles as those applied in the ex- 

 planation of the phenomena of the radiometer in two prece- 



* Communicated by the Author. 



