NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 



27 



RADIATION FROM HILL-SIDE 



On Februaiy 23, 191 8, an observation was made of a definite area 

 of a hillside (approximately at right angles to the line of sight) 

 covered with dry grass and leaves, and on which the sun was shining 

 from an altitude of about 43°, directly back of the instrument. The 

 vertical zone arrangement was used, but without the sun shade in 

 place. An average of six- very concordant values of the radiation 

 from the hillside gives an intensity of .0546 calorie. 



Immediately the instrument was pointed vertically, and a similar 

 area of clear sky observed. The value in this case was .0169 calorie. 

 This shows that the radiation outward from dry grass, etc., on which 

 the sun is- shining nearly vertically, at mid-day, is about three or 

 four times that radiated by an equal area of clear zenith sky. 



