NO. 4 THE BRIGHTNESS OF THE SKY — MOORE AND ABBOT I9 



Method of observing. — First the instrument was pointed directly 

 at the sun and one observation of the intensity made. The follow- 

 ing minute, a similar observation was made at the same altitude 

 but with the instrument revolved 60° in azimuth toward the east 

 from the position of the sun in the iirst observation. Similarly 

 the following minute 90°, and the next 120°, and the next 180°. 

 These are termed respectively I, II, III, IV and V. Several sets 

 were usually taken in this way, the azimuth and altitude being 

 measured from the position of the sun at the beginning of each set. 

 Then finally, the instrument was pointed vertically, and a few read- 

 ings taken of the radiation of the same angular area directly over- 

 head. The vertical readings are termed VI. Current compensation 

 measurements were taken on the half minutes between sky readings. 

 At first the set at 90° azimuth was not taken. 



As in the case of the horizontal zones, a sample set of observa- 

 tions will be included, then a table of the results obtained on each 

 day of such observations. 



The sample set will be the first one taken February 11, 1918. The 

 sky was quite clear and the air very dry for Hump Mountain, 

 (about 35 per cent relative humidity) and the sky cloudless. The 

 90° set, or III, was omitted in this case. 



