1883.] on ThougJds on Badiation, Tlieoreiical and Practical. 263 



logical observations do not clearly indicate its influence on terrestrial 

 radiation. 



With a view to this examination, I will choose a series of observa- 

 tions 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 favourable to 

 the outflow of terrestrial heat, and therefore best calculated to establish 

 a large difference between the air and wool thermometers. The 16 th 

 of last January was a day of this kind, when the observations recorded 

 in the annexed table were made. 



JaQuary 16th. — Extremely serene. Air almost a dead calm. Sky without a 



cloud. Light south- westerly air. 



During these observations there was no visible impediment to 

 terrestrial radiation. The sky was extremely pure, the moou 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 observation, my ncjte- 

 book contains the remark, " Atmosphere exquisitely clear ; from zenith 

 to horizon cloudless all round." 



A moment's attention bestowed on the column of ditferences in 

 the foregoing table will repay us. Why should the diiference 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 6 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 



