224 Royal Society : — Messrs. Balfour Stewart and Tait on 



more is given than its equation in its simplest form, and those of its 

 tangent and normal, together with a few properties that follow directly 

 from those equations. The object of the work is to put the merest 

 elements of the subject in a very simple form, with a view to en- 

 abling a beginner to obtain some knowledge of it before proceeding 

 to more elaborate works. The simplification is mainly effected by 

 inserting steps of algebraical processes which are commonly sup- 

 pressed or curtailed. The author tells us, in his preface, that in 

 writing the book he has had an eye to what, from long experience in 

 tuition, he has found needful. We therefore infer that there are 

 mathematical students who find 'An Easy Introduction to Conic 

 Sections ' necessary. To ourselves, unenlightened by the author's 

 experience, it would have seemed" better that a student who found 

 insuperable difficulties in such a book as Mr. Todhunter's " masterly 

 work " would do well to spend a little more time over the lower 

 branches of mathematics before proceeding to the higher. 



XXXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 73.] 



Dec. 6, 1866. — Lieutenant- General Sabine, President, in theChair. 



THE following communication was read :— 

 " On the Heating of a Disk by rapid Rotation in vacuo" By 

 Balfour Stewart, M.A., F.R.S., and P. G. Tait, M.A. (Continued.) 



16. The apparatus and certain preliminary experiments having 

 been described in the previous paper *, the authors now proceed to 

 relate what further experiments have been made. 



In the preliminary experiments it was conclusively shown (art. 8) 

 that the effect on the pile caused by rotation of the disk was due 

 to radiant heat, and also (art. 9) that this effect was not due to 

 the heating of the rock-salt which, in most of the experiments, was 

 placed before the mouth of the cone. 



It was also rendered probable that the effect was not due to ra- 

 diation from heated air, by the two following considerations : — 



(1) Because in order that nearly dry air of such a tenuity might 

 give such a radiation it would require to be heated enormously. 

 , (2) Because when the lampblack was removed from the aluminium 

 disk, leaving it a rough metallic surface, the indication afforded by 

 the galvanometer was reduced to about one-fourth of the amount 

 with the blackened disk. 



The following observations tend to strengthen this proof:— 



(3) The heating effect is the same in hydrogen or in coal-gas as 

 in air, although there is no question that the absorptive, and there- 

 fore the radiative power of coal-gas is much greater than that of air. 

 This is shown by the following sets of experiments, which were 

 made with the blackened aluminium disk insulated with ebonite, and 

 with rock-salt in the cone. 



* Phil. Mag. vol. xxx. p. 314. 



