316 Royal Society : — Messrs. Stewart and Tait on the 



then attached to its spindle again, being in this position exposed to 

 the pile, and having a temperature higher than that of the pile by 

 a known amount. The deflection produced by this exposure being 

 divided by the number of degrees by which the disk was hotter than 

 the pile, we have at once the value in terms of the galvanometric scale 

 of a heating of the disk equal to 1° Fahr. 



7. The following sets of experiments were made with blackened alu- 

 minium disk and rock-salt in the cone. 



>. of 



Heat Tension 



indication. of air in 



° Fahr. inches, 

 0-85 0-3 



0-87 0*3 



0-81 0-3 



0-75 0-65 



8. A piece of wood precisely similar to the rock-salt plate'was next 

 inserted into the fitting of the latter, and after rotation there was no 

 indication whatever. Hence the above effect (art. 7) is due to radiant 

 heat, and not to currents of heated air reaching the pile. Again re- 

 taining the rock-salt, the interior of the cone was covered by black 

 paper, and the effect upon the pile was very much diminished : this 

 also goes to prove that the effect (art. 7) is due to radiant heat ; and 

 it now remains to discover whether this radiant heat comes from the 

 rock-salt or from heated air, or from the surface of the disk. 



9. The following sets of experiments were made with blackened 

 aluminium disk, but without rock-salt. 





No. of ob- 





No. of 





servations 





turns of 



No. of 



in each 



Time at 



handle at 



set. 



set. 



full speed. 



full speed. 



I. 



3 



30 



20 



II. 



4 



30 



20-5 



III. 



4 



30 



20 



IV. 



3 



30 



20 





No. of ob- 





No. of 









servations 





turns of 





Tension 



No. of 



in each 



Time at 



handle at 



Heat 



of air in 



set. 



set. 



full speed. 



full speed. 



indication. 



inches. 



V. 



3 



30 



20 



0-92 



0-37 



VI. 



3 



30 



20 



0-93 



0-60 



And when a black paper cover was introduced into the cone, other 

 things remaining as before, the indications of the galvanometer were 

 greatly diminished. The effect produced without rock-salt is there- 

 fore also a radiant heat effect ; and as the indications (in terms of tem- 

 perature) are as large as when rock-salt was used, we may conclude 

 that the effect of art. 7 was to no perceptible extent due to heating 

 of the rock-salt, otherwise it would have been diminished when the 

 plate of rock-salt was taken away. Besides, as rock-salt is a bad ra- 

 diator and a good absorber of its own heat, the plate would have had 

 to be heated perhaps as much as 15° or 20°, in order to furnish a 

 radiation equal to o, 8 from the disk. On both these accounts it is 

 impossible to believe that the effect was due to heating of the rock- 

 salt. 



10. Nor is it probable that the heating effect is due to radiation 

 from heated air, since in order that nearly dry air of such a tenuity 

 might give such a radiation, it would require to be heated enor- 

 mously. But another proof that the effect is not due to air is afforded 



