NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 



15 



A sample set of observations will be given, followed by a table- 

 giving the results of all the zone observations taken. Usually the 

 observations were begun with a full sky (%), then with the upper 

 60° (b), then the upper 30° (c), and finally another set with full 

 sky (as). Since the sky radiation is continually changing, even with 

 a clear sky, an average was taken of % and a^ when comparing with 

 b and c. Several readings were taken at each position, these read- 

 ings usually being one minute apart with a current compensation 

 measurement on each of the half minutes. The glass hemisphere 

 was used in all zone work. 



The following set was taken on November 17, 1917, at Hump 

 Mountain on a very clear sky but with some haze in the valley and 

 along the horizon. 



