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THE POPULAR SCIENCE MONTHLY. 



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 obser- 

 vations made during the afternoon and evening of a day of extraor- 

 dinary calmness and serenity. The visible condition of the atmosphere 

 at the time was that which has hitherto been considered most favora- 

 ble 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 com- 

 panion 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 ob- 

 served 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 ther- 

 mometers on the evening here referred to varied from 4 to 9, the 

 latter being the maximum. 



