18 KIMBALL: NORMAL ATMOSPHERIC TRANSPARENCY 



The following observations are available for a study of the grad- 

 ual precipitation of dust from the atmosphere during the past 

 year: 



1. Measurements of the intensity of insolation, or the in- 

 coming solar radiation. 



2. Measurements of the percentage of polarization of skylight. 



3. Measurements of the solar and the anti-solar distances of 

 the neutral points of Babinet and Arago, respectively. 



4. Observations of the intensity and duration of twilight colors. 



Solar radiation intensity measurements have been made at fre- 

 quent intervals thruout the day at Mount Weather since the 

 beginning of 1908. But few observations were obtained in 1910, 

 and in the winter months previous to 1912-1913. 



For comparative purposes I have summarized in Table 1 the 

 observations made since May 1, 1912, with the sun at zenith 

 distance 60°, which is the highest point reached by the sun in De- 

 cember at the latitude of Mount Weather. The monthly means 

 of measurements made in the successive years are not strictly 

 comparable, for the reason that since May, 1911, the attempt has 

 been made to obtain measurements whenever the sun was unob- 

 scured by clouds, while previous to that time measurements were 

 made on the best days only, and when there was a prospect of 

 obtaining a series extending over at least two hours. 



The ratios in column 6, Table 1, are the means of the quotients 

 obtained by dividing the monthly means, given in columns 2 and 

 3, by the corresponding averages for the respective months, de- 

 rived from observations obtained previous to June, 1912. ^ Simi- 

 larly, the ratios in column 7, Table 1, are the means of the quo- 

 tients obtained by dividing the monthly maxima, given in columns 

 4 and 5, by the corresponding average maxima for the respective 

 months, derived from observations obtained previous to June, 

 1912. 



After what has been said relative to the monthly means for 

 consecutive years we might expect that the ratios given in column 

 7, which are derived from maximum radiation values, would be 



2 These observations will be found tabulated in Bull. Mount Weather Obs., 5, 

 303-310, 1913. 



