742 rHILOSOPHICAl. TRANSACTIONS. [aNNO 1780. 



the sun could not be reflected on them by any object standing before ; but the 

 experiment answered even when the san was hidden by clouds. 



This observation seemed to show that perhaps t\ cry degree of light is attended 

 with a proportionate degree of heat ; and induced Mr. C. to try, in a similar 

 manner, whether, by directing the concentrated light of the moon on the 

 blackened ball of one of these thermometers, he could render sensible the 

 effect of that light.* But though he attempted it some time ago with a large 

 lens several times, and had lately tried it again with a burning mirror of 18 

 inches diameter, yet sometimes for want of proper means of observing the 

 height of the mercury in the tubes of the thermometers, sometimes for want of 

 a continued clear light of the moon, and in short from one unfavourable circum- 

 stance or other, he had not yet been able to make a fair and decisive trial of this 

 experiment. 



The light of the sun being very inconstant on account of clouds and of its 

 diurnal motion, Mr. C. thought to make some experiments with the above- 

 mentioned two thermometers, by exposing them to the light of a lamp, and he 

 found that this light had a considerable effect upon them. The ball of one of 

 the thermometers being blackened, and both being set at 2 inches distance 

 from the flame of a lamp, they both rose from 58°, at which the mercury stood 

 before the lighting of the lamp, to 65%, and the blackened thermometer to 

 67°i. Another time, being set at the same distance from the lamp, the un- 

 coloured thermometer came up to 67°i , and the blackened one to 68%. In 

 short, by various repeated trials it appeared, that the difference generally 

 amounted to about 1°. When the thermometers were put farther than 2 inches 

 from the lamp, this difi^erence decreased, and at about 14 or 13 inches it quite 

 vanished. 



It is mathematically true, that emanations which proceed from a centre, and 

 expand in a sphere, must continually become more and more rare in proportion 

 to the squares of the distances from the centre. Thus it is said, that the in- 

 tensity of light proceeding from a luminous body at the double, treble, quadru- 

 ple, &c. of a given distance from that body, must be respectively 4,Q, l6 times 

 less dense. The same thing may be said of heat. Being willing to ascertain 

 this truth by actual experiment, Mr. C. placed several thermometers, whose balls 

 were not painted, at different distances from the flame of the lamp, and expected 

 to find, when the thermometer at 4 inches distance was 1° above that placed at 8 

 inches distance, the thermometer placed at 2 inches distance should be 4" higher. 



* The concentrated light of tlie moon has often been tlirown upon thermometers without any 

 effect ; but it does not appear that any blackened thermometer was ever used before for this purpose. 

 — Orig. 



