NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 



29 



Calama, Chile, December 10, 1918 



Sky cloudless, somewhat hazy, heavy wind at intervals. 



Sun disappearing 6*" : 42'" : 



Sun disappeared 44"" 



True sunset (0° altitude) 40" 



00^ H.A. 

 18'. 



2o^ 



After the last observation in both places, although the rays had 

 less than i/ioo the intensity of the average total mid-day sky radia- 

 tion at Hump. Mountain, or 1/200 at Calama ; or less than i/iooo 

 of the strength of the solar rays reaching the earth, the ordinary 

 small print of a newspaper was easily legible at arms length. 



RADIATION UNDER TREES 



During the autumn of 191 7 some observations were taken at 

 Hump Mountain in the shade of the trees on the mountainside. The 

 object was to find out how much light passed the overgrowth and 



