378 Royal Society : — 



on the galvanometer-scale for star on a. and star on j3, 1 remove the pile 

 from the telescope, leaving all its galvanic connexions untouched, 

 and mount the pile so that, of the two halves of the face of a Leslie's 

 cube containing boiling water, each radiates heat upon one face, a or 

 (3 of the pile, placed at a known distance of about twenty inches 

 from the cube. After some time the deflection of the needle will 

 fall nearly to zero, and become steady enough for observation. A 

 piece of glass, G, is then placed to intercept from /3 a portion of the 

 heat radiating from one half of the face of the cube, and when the 

 needle has taken up its position, the reading is taken. Next the glass 

 G is placed to intercept a portion of the heat from the face a, and 

 the galvanometer-reading taken, as before, as soon as the needle has 

 assumed its position of rest. 



If, then, d is the mean difference of readings for star on face a and 

 face /3, <p the mean difference for glass before (3 and a, C the heating- 

 power of each half of the cube at its distance from the faces of the 

 pile, and p the measure of the absorption of the piece of glass G, then 

 the heating-power of star 



=-XCXp. 

 </> 



The quantity p has been determined by merely comparing the 

 readings of the galvanometer, obtained by cutting off the whole heat 

 from one-half of the cube, with that obtained by intercepting a portion 

 of this heat by the glass G. A considerable number of accordant 

 results gave p = 0'725. 



To determine the quantity C, I have proceeded as follows : — 



1st. I have placed two very delicate thermometers, one in contact 

 with each face a and (3 of the pile, along the lines of junction of 

 the plates. The thermometers were separated from each other, and 

 the direct radiation of one on the other prevented by the interposition 

 of a piece of blackened card. The two thermometers, with the faces 

 of the pile in contact, were then exposed to the radiation of the halves 

 of the face of the cube containing the boiling water. A third delicate 

 thermometer was read for registration of any change in the tempe- 

 rature of the surrounding air. This thermometer was protected from 

 the direct radiation from the cube. The pile, with thermometers 

 in contact, was then placed at different distances from the cube and 

 the thermometer-readings taken. I have usually taken readings at 

 three distances — one at about 23*5 inches, another at 11*9 inches, 

 another at 2*5 inches. From a comparison of these readings with 

 those taken before the heat from the cube fell upon the thermo- 

 meters, I infer the heating-power of each half of the cube upon the 

 thermometers, with the faces of the pile in contact. Calling this 

 quantity for one inch of distance H', I find for my cube in its present 

 state, with slightly lacquered face, H' = 130° F. 



2nd. If H denote the corresponding heating-power of each half of 

 the cube upon the faces of the pile a and j3, I have found the ratio 

 H : H' as follows : — 



The thermometers being placed in contact with the faces of the 

 pile, and the galvanic connexions made, we may be certain that the 



