From the Philosophical Magazine for March 18G7.

ON THE REASON

WHY

THE DIFFERENCE OF READING BETWEEN ATHERMOMETER EXPOSED TO DIRECTSUNSHINE AND ONE SHADEDDIMINISHES AS WE ASCEND IN THE ATMOSPHERE.

BY

JAMES CROLL .

T HE remarkable fact was observed by Mr. Glaisher , that thedifference of reading between a black-bulb thermometerexposed to the direct rays of the sun and one shaded diminishesas we ascend in the atmosphere. On viewing the matter underthe light of Professor Tyndall’s important discovery regardingthe influence of aqueous vapour on radiant heat, the fact statedby Mr. Glaisher appears to be in perfect harmony with theory.The following considerations will perhaps make this plain.

The shaded thermometer marks the temperature of the sur-rounding air ; but the exposed thermometer marks not the tem-perature of the air, but that of the bulb heated by the directrays of the sun. The temperature of the bulb depends upontwo elements: (1) the rate at which it receives heat by directradiation from the sun above the earth beneath and all sur-rounding objects, and by contact with the air; (2) the rate atwhich it loses heat by radiation and by contact with the air. Asregards the heat gained and lost by contact with the surroundingair, both thermometers are under the same conditions, or nearly so.We therefore require only to consider the element of radiation.We begin by comparing the two thermometers at the earth’s