ON RADIATION. 



467 



Such observations might be multiplied, but, with a view to saving 

 space, I will limit the record. On the evening of January 30th the 

 atmosphere was very serene ; there was no moon, but the firmament 

 was powdered with stars. At 7.15 p. m. the difference between the two 

 thermometers was 6; while at 9.30 p. m. it was 4, the wool-thermome- 

 ter being in both cases the colder of the two. On February 3d, obser- 

 vations were made under similar conditions of weather, and with a 

 similar result. At 7.15 p. m. the difference between the thermometers 

 was 6; while at 8.25 p. m. it was 4. On both these evenings the sky 

 was cloudless, the stars were bright, while the movement of the air was 

 light, from the southwest. 



In all these cases the air passing over the plateau of Hind Head 

 had previously grazed the comparatively warm surface of the Atlantic 

 Ocean, where it had charged itself with aqueous vapor to a degree 

 corresponding to its temperature. Let us contrast its action with that 

 of air coming to Hind Head from a quarter less competent to charge 

 it with aqueous vapor. We were visited by such air on the 10th of 

 last December, when the movement of the wind was light from the 

 northeast, the temperature at the time being very low, and hence cal- 

 culated to lessen the quantity of atmospheric vapor. Snow a foot 

 deep covered the heather. At 8.5 a. m. the two thermometers were 

 taken from the hut, having a common temperature of 35. The one 

 was rapidly suspended in the air, and the other laid upon the wool. I 

 was not prepared for the result. A single minute's exposure sufficed 

 to establish a difference of 5 between the thermometers ; an exposure 

 of five minutes produced a difference of 13; while after ten minutes' 

 exposure the difference was found to be no less than 17. Here follow 

 some of the observations : 



December 10th. Deep snow; low temperature ; slcy clear ; light northeasterly air. 



During these observations, a dense bank of cloud on the opposite 

 ridge of Blackdown virtually retarded the rising of the sun. It had, 

 however, cleared the bank during the last two observations, and, 

 touching the air-thermometer with its warmth, raised the temperature 

 from 26 to 27 and 29. The very large difference of 18 is in part 

 to be ascribed to this raising of the temperature of the air-thermome- 

 ter. I will limit myself to citing one other case of a similar kind. On 

 the evening of the 31st of March, though the surface temperature was 



