KEPORT OF THE SECRETARY. 69 



ometry as now satisfactorily established, and with it the mean value of the 

 solar constant of radiation for the epoch 1905-1910 as fixed at 1.922 calories 

 per square centimeter per minute. 



Additional copies of the secondary silver-disk pyrheliometer shown in the 

 accompanying illustration (fig. 3) have been standardized and sent abroad by 

 the Institution as loans or purchases. There have now been sent copies to 

 Russia, Germany, France, Italy, England, Peru, Argentina, and several within 

 the United States, making in all 10 copies now in other hands than ours, be- 

 sides several now being made to order. The Institution has undertaken the 

 business relating to furnishing these pyrheliometers, which are standardized 

 at the Astrophysical Observatory, to promote exact knowledge of the sun and 

 its possible variability. 



SUMMARY. 



The year has been distinguished by a successful expedition to Mount Whituey. 

 The results obtained there confirm the view that determinations of the intensity 

 of the solar radiation outside the earth's atmosphere by the spectrobolometric 

 method of high and low sun observation are not dependent on the observer's 

 altitude above sea level, provided the conditions are otherwise good. The 

 Mount Whitney expedition furnished opportunities also for measurements of 

 the brightness of the sky by day and by night, the influence of water vapor on 

 the sun's spectrum, and the distribution of the sun's energy spectrum outside 

 the atmosphere. 



Solar-constant observations and closely related researches were continued 

 daily at Mount Wilson until November, 1910, and were taken up again in 

 June, 1911. 



Further research tends to confirm the conclusion that the sun's output of 

 radiation varies from day to day in a manner irregular in period and quantity, 

 but roughly running its courses within periods of 5 to 10 days in time and 3 to 

 10 per cent in amplitude. Assurance seems now complete that this result will 

 be tested in the next fiscal year by long-continued daily observations made 

 simultaneously at two widely separated stations. 



Many copies of the silver-disk secondary pyrheliometer have been standard- 

 ized and sent out to observers in this and foreign countries to promote exactly 

 comparable observations of the sun's radiation. 



Measurements of the transparency, for long-wave radiation, of columns of air 

 containing known quantities of water vapor have been continued, and promise 

 highly interesting results. 



Respectfully submitted. 



C. G. Abbot, Director, 



Dr. Charles D. Walcott, 



Secretary of the Smithsonian Institution. 



