286 Mr. J. P. Harrison on Radiation and Vapour. 



On comparing these observations with the mean results ob- 

 tained under similar circumstances at higher elevations, the dif- 

 ferences are found slightly to increase with height. Still it 

 would appear that the excessive readings of solar thermometers 

 in dry regions, mountainous, or otherwise, must be in a great 

 measure due to reflexion and secondary radiation, and not (at 

 least to the extent supposed) to increased force in the solar rays 

 in consequence of the absence of vapour, or their passage through 

 a less depth of atmosphere*. 



The conclusion arrived at by Mr. Croll, that the moon, if sur- 

 rounded by sether, would be colder than the earth, is neither in 

 accordance with the laws of radiant heat nor, if correctly under- 

 stood, to be gathered from the remarks on the subject by Pro- 

 fessor Tyndall. Indeed I was not long ago informed by this 

 eminent physicist and experimenter, that the question had been 

 set at rest by the exhaustive researches of Dulong and Petit. 

 The law, as laid down by them, is very simple and easy to be 

 understood. " When a body cools in a vacuum its heat is en- 

 tirely dissipated by radiation ; when it is placed in air, or in 

 any other fluid, its cooling becomes more rapid, the heat carried 

 off by the fluid being in that case added to that which is dissipated 

 by radiation " f. 



Count Rumford also subjected the question to experiment, 

 and found that a thermometer half an inch in diameter in the 

 centre of a hollow globe of glass void of air and hermetically 

 sealed, after having been transferred from a bath of boiling 

 water to one of water mixed with pounded ice, on the mean of 

 two experiments, cooled down from 80° to 10° (R.) in 16 m 10 s , 

 whilst the same thermometer under similar treatment in a globe 

 filled with air, cooled down from 80° to 10° in 9 m 45 s J. 



It appeared well that the fact should be made more widely 

 known that, whilst the sun's rays may pass through dry air 

 as freely as through a vacuum, heat from a secondary source, 

 or from a surface heated by the sun, passes with more difficulty 

 through a vacuum than through dry air§. 



* Under a vertical sun and clear atmosphere no doubt maximum insola- 

 tion is obtained independently of radiation from cloud or other matter. 



t Encyclop. Met. art. "Heat" (172). See also Balfour Stewart's work 

 ' On Heat,' 1866, p. 260. 



X I obtained similar results last summer in the receiver of an air-pump, 

 with a thermometer heated by the sun's rays admitted through a hole in a 

 shutter, and directed on the black bulb of the instrument by means of a 

 lens. The mercury cooled down to the temperature of the room more ra- 

 pidly in air than in vacuo. 



§ Leslie ascertained that the greatest intensity of the sun's rays at noon 

 in the winter solstice, at Edinburgh, was 4° - 5 F. In the height of sum- 

 mer, when the sun was 3° above the horizon, the intensity was 1° F. 



