REPORT OF THE SECRETARY. 77 



OBSERVATIONS AT WASHINGTON. 



Measurements for the determination of the " solar constant " of radiation 

 were made whenever the atmospheric conditions permitted. These occasions 

 are too infrequent to permit us to make at Washington a full record of the condi- 

 tion of the sun, but in connection with and supplementary to the Mount Wilson 

 Work, the Washington results are of very great value. 



Measurements have been made frequently of the distribution of brightness 

 over the solar disk, and the results of these measurements indicate, though 

 perhaps not conclusively, that when the contrast in brightness between the 

 center and edge of the solar disk is greater than usual, the intensity of solar 

 radiation available to warm the earth is less than usual, and vice versa. This 

 relation was suspected in former years. 



PREPARATION OF VOLUME II OF THE OBSERVATORY ANNALS. 



The reduction and preparation for publication of the results of the thousands 

 of holographic records made in the research on solar radiation has involved 

 measurements and computations requiring the recording of upward of 2,000,000 

 separate figures. The reductions have been chiefly in the care of Mr. Fowle, 

 and he has been ably seconded by Miss Graves. Very useful and painstaking 

 assistance has been rendered by Mr. Dwyer and by the temporary computers 

 employed. The text and illustrations have been prepared under the care of the 

 writer, in consultation with Mr. Fowle. The whole work was nearly completed 

 at the conclusion of the fiscal year, and at this writing (September 15) is now 

 ready for the press. As the story of the year's work is chiefly the story of this 

 volume, it will not be out of place to give here a summary of its principal 

 contents. 



SUMMARY OF . THE FORTHCOMING VOLUME OF ANNALS. 



The present volume is an account of the work of the Astrophysical Observa- 

 tory from 1000 to 1907, with details of the investigations made, the apparatus 

 and methods used, and the results obtained. 



Speaking broadly, the investigation relates to the intensity of the rays of the 

 sun, and the dependence of the earth's temperature thereon. The subject is here 

 treated in three parts : First, the amount of the solar radiation as it would be 

 found if measured outside the earth's atmosphere, at mean solar distance, or, as 

 it is often termed, " The solar constant of radiation." Second, the dependence 

 of the earth's temperature on the amount of solar radiation. Third, the differ- 

 ence in brightness between the center and edge of the sun's disk and its relation 

 to the quantity of solar radiation received by the earth. 



The work is not limited to a determination of constants of nature, for the 

 possibility was early recognized that the radiation of the sun might be far 

 from uniform, so that the " solar constant of radiation " might prove to be 

 a mean value about which the intensity of the solar beam would be found to 

 fluctuate very perceptibly from time to time. A principal aim of the work has 

 therefore been to prove whether such fluctuations of the quantity of solar 

 rays do exist, and, if so, what may be the magnitude of the changes, their 

 effects on climate, and their causes. For these purposes the measurement of 

 the intensity of solar radiation and of the distribution of brightness over the 

 disk of the sun have been made as often as possible for several years, and a 

 study of the variation of temperature for the last thirty years at about fifty 

 stations scattered as widely as possible over the inland areas of the world has 

 also been made. 



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