of the prismatic Colours to heat and illuminate Objects. 261 



2 degrees for the violet rays ; and therefore we have the rising 

 of the quicksilver in red to that in violet, as 55 to 16. 



A sufficient proof of the accuracy of this determination we 

 have, in the result of the four last experiments. The rising for 

 red rays in the 5th, is qj- ; and in the 6th, 4 degrees : a mean 

 of both is 3|. In the 7th experiment, we have ii, and in the 

 8th, 2 degrees, for the rising in green : a mean of these is 1^. 

 Therefore, we have the proportion of the rising in red to that 

 in green, as 27 to 11, or as 55 to 22,4. 



We may take a mean of the result of both thermometers, 

 which will be 55 to 24,2, or more than 2^ to 1, in red to green ; 

 and about 3^ to 1, in red to violet. 



It appears remarkable, that the most sensible thermometer 

 should give the least alteration, from the exposure to the coloured 

 rays. But since, in these circumstances, there are two causes 

 constantly acting different ways ; the one to raise the thermo- 

 meter, the other to bring it down to the temperature of the 

 room, I suppose, that on account of the smallness of the ball in ' 

 Dr. Wilson's No. 1, which is but little more than -f of an inch, 

 the cooling causes must have a stronger effect on the mercury 

 it contains than they can have on mine, the ball of which is 

 half an inch. 



More accuracy may hereafter be obtained, by attending to 

 the circumstances of blacking the balls of the thermometers, 

 and their exposure to a more steady and powerful light of the 

 sun, at greater altitudes than it can be had at present; but 

 the experiments which have been related, are quite sufficient 

 for my present purpose; which only goes to prove, that the 

 heating power of the prismatic colours, is very far from being 



