676 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



centres. In the pasteboard, I cut an opening, mn, a little larger than the ball of a 

 thermometer, and of a sufficient length to let the whole extent of one of the 

 prismatic colours pass through. I then placed 3 thermometers on small inclined 

 planes, ef : their balls being blacked with japan ink. That of No. 1 was rather 

 too large for great sensibility. No. 2 and 3 were 2 excellent thermometers, which 

 my highly esteemed friend Dr. Wilson, late Professor of Astronomy at Glasgow, 

 had lent me for the purpose : their balls being very small, made them of exquisite 

 sensibility. The scales of all were properly disengaged from the balls. I now 

 placed the stand, with the framed pasteboard and the thermometers, on a small 

 plain board, gh ; that I might be at liberty to move the whole apparatus together, 

 without deranging the relative situation of the different parts. This done, I set a 

 prism, moveable on its axis, into the upper part of an open window, at right angles 

 to the solar ray, and turned it about till its refracted coloured spectrum became 

 stationary, on a table placed at a proper distance from the window. 



The board containing the apparatus was now put on the table, and set in such 

 a manner as to let the rays of one colour pass through the opening in the paste- 

 board. The moveable frame was then adjusted to be perpendicular to the rays 

 coming from the prism ; and the inclined planes carrying the 3 thermometers, 

 with their balls arranged in a line, were set so near the opening, that any one of 

 them might easily be advanced far enough to receive the irradiation of the colour 

 which passed through the opening, while the rest remained close by, under the 

 shade of the pasteboard. By repeated trials, I found that Dr. Wilson's No. 2 and 

 mine, always agreed in showing the temperature of the place where I examined 

 them, when the change was not very sudden : but that mine would require 10 

 minutes to take a change, which the other would show in 5. No. 3 never differed 

 much from No. 2. 



1st Exper, Having arranged the 3 thermometers in the place prepared for the 

 experiment, I waited till they were stationary. Then, advancing No. 1 to the red 

 rays, and leaving the other 2 close by, in the shade, I marked No 1 No< 2 No> 3 

 down what they showed at different times, as annexed. This, 43£ 43| 43| 

 in about 8 or 10 minutes, gave G§. degrees, for the rising ^ * 3 | *3| 

 produced in my thermometer, by the red rays, compared to 49| 43£ 43| 

 the 2 standard thermometers. 50 43 i l 3 * 



2d Exper. As soon as my thermometer was restored to the temperature of the 

 room, which I hastened, by applying it to a large piece of metal No } No 

 that had been kept in the same place, I exposed it again to the 45 45 



red rays, and registered its march, along with No. 2 as a stan- 

 dard, which was as annexed. Hence, in 10 minutes, the red rays 51 44| 

 made the thermometer rise 7 degrees. M ** 



3d Exper. Proceeding in the same manner as before, in the ^ 3 * 43 2, 



green rays I had as annexed. Therefore in 10 minutes, the 45J 43 



green rays occasioned a rise of 3^ degrees. * g ^ 



46 42£ 



