290 Messrs. Balfour Stewart and Tait on the Heating [Dec. 6, 



manner, together with curves of the separate effects of the Solar Parallax 

 and Solar Declination. 



A sheet from the Tide-Gauge Cylinder has also been sent, as a specimen 

 of the regularity of the registered Curves in tranquil weather. 



II. 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. (Con- 

 tinuation of a Paper read before the Royal Society on June 15, 

 1865, and published in the 'Proceedings.^) Received October 

 30, 1866. 



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 radiation 

 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 galvano- 

 meter 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 therefore the radia- 

 tive 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. 



No. of set. 



No. of observa- 

 tions in each 

 set. 



Time at full 

 speed, in se- 

 conds. 



No. of turns of 

 handle at full 

 speed. 



Heat indication. 



Nature 

 of gas. 



Tension 

 of gas, in 

 inches. 



[e purit y 



(abso- 

 pure gas 



Divi- 

 sions. 



Falir. 



Probab] 

 of gas 

 lutely 

 — 1 00' 



YII. 



2 



30 



22 



22-51 





hydrogen 



0-6 



95 



VIII. 



3 



30 



22 



23-3/ 





air 



0-7 





IX. 



2 



30 



20 





0°*95 



hydrogen 



0-5 



97 



X. 



2 



30 



20 





0°-87 



air 



M 





XI. 



3 



30 



20 





0°-85 



hydrogen 



0-25 



98-5 



Xll. 



3 



30 



20 





0°-86 



coal-gas 



0-25 



95 



(4) It may be objected to (2) that the greater heating effect from a 



