VOL. XC.] PHILOSOPHICAL TRANSACTIONS. fyf 



4th Exper. I now exposed my thermometer to the violet rays, N* l. N°2. 



and compared it with N° 2. Here we have a rising of 2°, in 10 44 

 minutes, for the violet rays. 44| 43£ 



45 43 



5 th Exper. I now exposed Dr. Wilson's thermometer N° 2 to *J 2 - N ° 3# 



the red rays, and compared its progress with N° 3. Here the 46' 44 



thermometer, exposed to red, rose in 5 minutes 24 degrees. *?§ 43 l 



' r 4 & 46J 43| 



N° 2. N° 3. 



44 44 



6th Exper. In red rays again. And here the thermometer, 46 44 



exposed to red, rose in 5 minutes 4 degrees. *$| 43j 



47 43 



7*A Exper. In green rays. This made the thermometer rise, N ° f N ° 3 - 



in the green rays, 14 degree. 44 j 43 | 



44| 43 



8* h Exper. Again in green rays. Here the rising, by the green N ° 2 * N ° 3 * 



- 4o 43 



rays, was 2 degrees. 44^ 42 g 



44| 42| 



From these experiments, we are authorised to draw the following results. In 

 the red rays, my thermometer gave 6^ degrees in the 1st, and 7 degrees in the 2d, 

 for the rising of the quicksilver : a mean of both is 64-. In the 3d experiment, 

 we had 3-A- degrees, for the rising occasioned by the green rays ; from which we 

 obtain the proportion of 55 to 26, for the power of heating in red to that in 

 green. The 4th experiment gave 2° for the violet rays ; and therefore we have the 

 rising of the quicksilver in red to that in violet, as 55 to ]6. A sufficient proof of the 

 accuracy of this determination we have in the result of the last 4 experiments. The 

 rising for red rays in the 5th, is 24/ ; and in the 6th, 4°: a mean of both is 3-f-. 

 In the 7th experiment, we have 14;, and in the 8th, 2° for the rising in green : a 

 mean of these is 14. Therefore, we have the proportion of the rising in red to 

 that in green, as 2/ 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 34/ 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 circum- 

 stances, there are 2 causes constantly acting different ways ; the one to raise the 

 thermometer, the other to bring it down to the temperature of the room, I sup- 

 pose, that on account of the smallness of the ball in Dr. Wilson's N° 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 circum- 

 stances 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 j but the experimen ts which have been related, are quite sufficient for 



