no. 9 



AREQUIPA PYRHELIOMETRY ABBOT 



23 



mean monthly vapor pressures for these months for several years as 

 given in Table 2. Such solar constant values are given in parentheses. 

 Finally the 29 days with solar constant values available for fav- 

 orable comparison between Mount Wilson and Arequipa have been 

 divided into two groups of high and low values respectively, as in- 

 dicated by Mount Wilson work. The mean values are as follows : 



The days selected are these: 



("1913. Aug. 5, 12, 18; Sept. 2, 3, 9, 17, 18, 22. 

 Group l.^jQj^ j une jg^ 23, 24; July 17, 23, 28. 



f 1913. Aug. 4, 6, 15 ; Sept. 4, 8, 10, 26, 27, 29, 30; 

 Group II A Oct. 1, 6, 31. 



[1914. June 21. 



This comparison, so far as it has weight, evidently tends to con- 

 firm the existence of short-period irregular solar variations, dis- 

 covered by other investigations. 



Summary. — Observations with the silver disk pyrheliometer and 

 nearly simultaneous measurements of atmospheric humidity have 

 been made since August, 191 2, at Arequipa, Peru, at the station of 

 the Harvard College Observatory. 



From these observations have been determined values of the 

 solar radiation at Arequipa corresponding with secant Z equal to 

 1.0, 1.2, and 2.0; values of pressure of aqueous vapor, and values of 

 the diminution of radiation attending the passage of the sun from 

 the zenith distance whose secant is 1.0 to that whose secant is 2.0. 



Owing to other occupations the observers have generally made 

 these observations when the sun was within 6o° of the zenith. On 

 this account determinations of atmospheric transparency are not 

 always possible, and are of less weight than other data given. 



The results are collected to give monthly mean values. These 

 show a remarkably close connection between radiation and vapor 

 pressure. Advantage is taken of this close correlation to determine 

 by empirical formube values of the solar constant of radiation. 

 These empirical values agree quite as well as could be expected with 

 values obtained at Mount Wilson, California, by complete spectro- 

 bolometric and pyrheliometric measurements combined. The Are- 



