KEPORT OF THE SECRETARY. 87 



observations made at different levels general formulae for the aver- 

 age humidity of the atmosphere have been derived. Mr. Fowle finds, 

 however, that these formula?, while representing average conditions, 

 are often widely astray on individual days. He is preparing further 

 data from Washington, Mount Wilson, Mount Whitney, and Bassour 

 spectrobolometric work, to promote a more complete study of atmos- 

 pheric humidity. 



This investigation has yielded a valuable application for solar 

 constant work, for Mr. Fowle has found a way to very greatly 

 shorten the work of correcting for water vapor absorption in reduc- 

 ing the bolographic observations. This will diminish by about one- 

 fifth the labor of reducing the solar constant work, and at the same 

 time will yield results of slightly greater accuracy than before. 



Atmospheric water vapor absorption work has been confined to 

 the upper infra-red spectrum bands this year. A vacuum bolometer 

 is in preparation, by means of which a considerable gain in sensi- 

 tiveness of the apparatus is hoped for. This will greatly promote 

 the value of the work at very great wave lengths, and accordingly 

 this part of the work has been allowed to await the introduction of 

 the vacuum bolometer. 



PERSONNEL. 



Prof. F. P. Brackett served as temporary bolometric assistant to 

 the Algerian expedition of 1911. 



Mr. Anders Knutson Angstrom served as temporary bolometric 

 assistant to the Algerian expedition of 1912. 



Miss F. E. Frisby was appointed temporary computer, February 

 12, 1912. 



Minor Clerk M. Segal resigned March 1, 1912. 



F. E. Carrington was appointed messenger boy on March 25, 1912. 



SUMMARY. 



The year has been notable for expeditions to Algeria and Cali- 

 fornia to test the supposed variability of the sun by making simul- 

 taneously at these two widely separated stations spectrobolometric 

 determinations of the solar constant of radiation. The measure- 

 ments in Algeria agree with earlier ones at Washington and Mount 

 Whitney and indicate that Mount Wilson values are systematically 

 a little low. Apart from this systematic error the average acci- 

 dental differences between Algerian and Mount Wilson determina- 

 tions were only 1.2 per cent, indicating an average accidental error of 

 a single solar constant determination at one station of only 0.9 per cent. 

 So^ far as yet reduced, high solar constant values obtained in Algeria 

 coincide with high values at Mount Wilson, and vice versa. A solar 



