1921] 



on Cloudland Studies 



417 



practically to the whole sky from zenith to horizon. The strips 

 were in pairs, blackened and polished respectively, and the difference 

 of temperature between the black absorbent strips and the polished 

 reflecting ones when exposed to a source at a different temperature 

 from that of the surrounding air (Fig. 3), was compensated by 

 electrical energy which was carefully measured. From these results 

 and the dimensions of the strips the radiation exchange was com- 

 puted in terms of the energy that had to be supplied to equilibrate 



Fig. 



the absorbent strips, as compared with that required when black 

 body radiation from a source at the earth temperature was employed. 

 For night sky observations the greater this ratio the greater was the 

 loss by radiation to the cold sky. The maximum ratio recorded was 

 H'±§1 on March 12, 1915, with the surface air just above the 

 freezing point. Conversely, by subtracting the experimental ratio 

 from unity, the proportionate radiation received from the sky as 

 compared with that from the earth black body was obtained, and 



