94 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



simultaneously the total sky radiation over a fixed small area imme- 

 diately surrounding the sun. It seems probable that as the bright- 

 ness of the sky depends on the prevailing humidity and dust, and as 

 the radiation of the sun is diminished by presence of humidity and 

 dust, a method of combination of the two measurements may be 

 found, adapted to give approximately the solar constant. When com- 

 putations are further advanced the matter will be tested. 



Restandardization of secondary pyrheliometers in 1916 against our 

 standard water-flow pyrheliometer indicated no change in their 

 constants. 



A vacuum bolometer was employed during a large part of the ob- 

 serving season. The sensitiveness was so much greater that consider- 

 able improvement in the work on the investigation of the distribution 

 of radiation over the sun's disk was possible. 



Redeterminations were made with great care on the form of dis- 

 tribution of the solar energy curve outside the atmosphere. New 

 mirrors of stellite, a very hard nontarnishing alloy, were substituted 

 for the silvered mirrors of the spectrobolometer. It is hoped that 

 the work of 1916 will indicate conclusively how the sun's variations 

 affect the distribution of energy in the solar spectrum. 



SUMMARY. 



Preparation of apparatus and equipment for a new solar-constant 

 station of the Smithsonian Institution, now located at Hump Moun- 

 tain, North Carolina, led to valuable improvements in the bolometer 

 and the pyranometer, and to the invention and construction of a new 

 instrument for avoiding computation in reduction of spectrobolo- 

 metric observations. 



A long research on the transmission of long- wave rays by atmos- 

 pheric columns of known hmnidity and carbon dioxide contents, 

 has been completed and prepared for publication by Mr. Fowle. In 

 expeditions to Mount Wilson the observation of the amount and 

 distribution of solar radiation has been continued. In cooperation 

 with the new station above mentioned it is hoped to obtain much 

 more complete records of the variation of the sun, now shown by 

 Clayton to be of great meteorological significance. 



Respectfully submitted. 



C. G. Abbot, 

 Director Astro physical Observatory. 



Dr. Charles D. Walcott, 



Sewetary of the Smithsonian Institution, 



