January 16th.—Extremely serene; air almost a dead calm; shy without a cloud; light southwesterly air.
With a view to this examination, I will choose a series of observations made during the afternoon and evening of a day of extraordinary calmness and serenity. The visible condition of the atmosphere at the time was that which has hitherto been considered most favorable to the outflow of terrestrial heat, and therefore best calculated to establish a large difference between the air and wool-thermometers. The 16th of last January was a day of this kind, when the observations recorded in the annexed table were made.
During these observations there was no visible impediment to terrestrial radiation. The sky was extremely pure; the moon was shining; Orion, the Pleiades, Charles's Wain, including the small companion star at the bend of the shaft, the North Star, and numbers of others, were clearly visible. After the last observations, my note-book contains the remark: "Atmosphere exquisitely clear; from zenith to horizon cloudless all around."
A moment's attention bestowed on the column of differences in the foregoing table will repay us. Why should the difference at 6 p. m. be fully 5° less than at 5 p. m.; and again 5° less than at 8 and at 8.30 respectively? There was absolutely nothing in the aspect of the atmosphere to account for the approach of the two thermometers at six o'clock—nothing to account for their preceding and subsequent divergence from each other. Anomalies of this kind have been observed by the hundred, but they have never been accounted for, and they did not admit of explanation until it had been proved that the intrusion of a perfectly invisible vapor was competent to check the radiation, while its passing away reopened a doorway into space.
It is well to bear in mind that the difference between the two thermometers on the evening here referred to varied from 4° to 9°, the latter being the maximum.