VOL. LXX.] PHILOSOPHICAL TRANSAC TIONS. 74) 



F. R. s. he presented to the Society the following account of some thermometrical 

 experiments and observations, the greatest part of which he says were made so 

 long since as the year 1776- Having read in a volume of the Philos. Trans, the 

 account of an experiment made with a thermometer, whose bulb was painted 

 black, and was exposed to the rays of the sun, in which case it had been found, 

 that the thermometer showed a much greater degree of heat than when not 

 blackened, he was desirous of trying the ultimate limits of this difference. For 

 which purpose he constructed two thermometers, the scales of which, being made 

 by trial, coincided so perfectly well together, that when the thermometers were 

 put in equal circumstances, no difference could be perceived between the de- 

 grees of heat shown by them. The length of a degree on the scale of those 

 thermometers was a little more than -^V of an inch, and though those scales 

 were divided into degrees only, yet by inspection a person a little versed in these 

 observations could easily distinguish the height of the quicksilver within a quarter 

 of a degree. 



These thermometers were both fixed on the same frame, at the distance of 

 about an inch from each other, having the balls quite detached from the frame, 

 and in this manner they were exposed to the sun, or to the light of a lamp. 



When these thermometers were exposed to the sun, or kept in the shade, they 

 showed the same degree precisely. The difference between the degree shown by 

 these thermometers when exposed to the sun, and when kept in the shade at 

 about the same time of the day, was very trifling. When the ball of one of 

 those thermometers, which we shall call a, was painted black with Indian ink, 

 or with the smoke of a candle, and that of the other thermometer b was left 

 clean, on being exposed to the sun they showed different degrees of temperature; 

 the quicksilver in the tube of a was much above the quicksilver in the tube of b. 

 This difference sometimes amounted to about 10° but it was never constant, vary- 

 ing according to the clearness of the sun's light as well as of the air, and also 

 according to the different degrees of temperature of the atmosphere. 



Keeping the frame with those thermometers, one of which had the ball painted 

 black, hung on the side of a window, Mr. C. observed a remarkable fact, viz. 

 that these thermometers showed unequal degrees of heat, not only when pre- 

 sented to the sun, but also when exposed to the strong day-light. He cleaned 

 the bulb of the thermometer a, and blackened that of b, but the effect was 

 constant, viz. the quicksilver in the tube of the thermometer, whose bulb was 

 painted black, was constantly higher than the other, whenever they were ex- 

 posed to the strong day-light. This difference was commonly about -f of a 

 degree, but sometimes it amounted to -3- and even to a whole degree. The situa- 

 tion in which those thermometers were usually placed, was such that the light of 



