76 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1910. 



many interruptions due to the difficulty of the research. The work has been 

 carried to wave-length beyond 15/i in the infra-red, and for columns of air 800 

 feet long. It is not yet possible to summarize the results. 



Messrs. Fowle and Aldrich and Miss Graves have made rapid progress with 

 the reduction of solar-constant work of 1909. 



Experiments have been begun for the purpose of devising economical means 

 of utilizing solar energy for domestic purposes. 



PERSONNEL. 



Dr. L. R. Ingersoll served as temporary bolometric assistant on Mount Wilson 

 to September 6, 1909. 



Mr. L. B. Aldrich was given a temporary appointment as bolometric assistant 

 at Washington beginning September 1, 1909. He passed a competitive exami- 

 nation and was reappointed provisionally on January 10, 1910. His appoint- 

 ment was made permanent, to begin July 1, 1910. 



SUMMARY. 



The work of the year is notable for the determination of the absolute scale 

 of pyrheliometry and for the success of spectrobolometric observations of the 

 solar constant of radiation on Mount Whitney. These agree with simultaneous 

 observations of the same kind on Mount Wilson. Reducing these and other 

 results to the absolute scale of pyrheliometry, we may fix the average value of 

 the solar constant of radiation at 1.925 calories per square centimeter per minute 

 for the epoch 1905-1909. Making allowance for the higher values which must 

 prevail at sun-spot minimum, the solar constant may be estimated at 1.95 cal- 

 ories as an average value for a sun-spot cycle. No reason has been found 

 for departing from the view heretofore held that short-interval variations 

 of 5 per cent or more from this value occur. The energy distribution in 

 the solar spectrum outside the atmosphere has been determined with the 

 bolometer on Mount Whitney between wave lengths 0.29/* in the ultra violet 

 and 3.0/i in the infra red. This region appears to contain full 99 per cent of all 

 the solar energy outside the atmosphere. The apparent temperature of the sun 

 as computed by three different methods comes out 6430°, 5S40°, and 6200° of 

 the absolute scale. Researches on the transmission of moist columns of air for 

 long-wave rays, such as the earth emits, have been continued to wave lengths 

 beyond 15/*, and for columns of air S00 feet in length. Secondary pyrhelio- 

 meters, standardized to the absolute scale, have been sent to Russia, France, 

 and Italy, and also furnished to the United States Weather Bureau and Depart- 

 ment of Agriculture. 



Respectfully submitted. 



C. G. Abbot, Director. 



Dr. Charles D. Walcott, 



Secretary of the Smithsonian Institution. 



