Page:Popular Science Monthly Volume 23.djvu/483

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467
ON RADIATION.

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-thermometer being in both cases the colder of the two. On February 3d, observations 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 calculated 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; sky clear; light northeasterly air.

TIME Air. Wool. Difference.
a.m. Degrees. Degrees. Degrees.
8.10 29 16 13
8.15 29 12 17
8.20 27 12 15
8.30 26 11 15
8.40 26 10 16
8.45 27 11 16
8.50 29 11 18

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-thermometer. 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