Appendix V. 



REPORT ON THE ASTROPHYSICAL OBSERVATORY. 



Sib : I have the honor to present the following report on the operations of the 

 Smithsonian Astrophysical Observatory for the year ending June 30, 1910 : 



EQUIPMENT. 



The equipment of the observatory is as follows : 



(a) At Washington, in an inclosure of about 16,000 square feet, are contained 

 five small frame buildings used for observing and computing purposes, three 

 movable frame shelters covering several out-of-door pieces of apparatus, and 

 also one small brick building containing a storage battery and electrical dis- 

 tribution apparatus. 



(b) At Mount Wilson, California, upon a leased plot of ground 100 feet square 

 in horizontal projection, are located a one-story cement observing structure, 

 designed especially for solar constant measurements, and also a little frame 

 cottage, 21 feet by 25 feet, built and furnished last September for observer's 

 quarters. It is highly satisfactory to note from the decrease in probable error 

 of the observations secured in 1909 on Mount Wilson, compared with those of 

 previous years, that the new cement observatory there, located as it is far from 

 the dust, smoke, and disturbances of the other parts of the mountain, is excel- 

 lently adapted for securing the most exact results. 



WORK OF THE YEAR. 



The present year's results are of uncommon interest, for they appear to fix 

 within narrow limits the value of the solar constant of radiation. When in 1902 

 the first attempts were made here to measure it, that first-rank constant of 

 nature, the intensity of the solar radiation at the earth's mean distance from the 

 sun, was unknown within the wide range between 1.75 and 4 calories per square 

 centimeter per minute. This range of values is given, with a preference for 

 Langley's A-alue (3 calories), by Hann in his standard work on meteorology, 

 published in 1905. 



It is improbable that this observatory would have continued since 1902 in 

 solar-constant work had it not been that the results of 1903 gave strong indica- 

 tions of considerable variability of the sun in short intervals and that later 

 work also strongly supported this presumption. The late director, Secretary 

 Langley, shared, with many others of the most competent judges on the subject, 

 the impression that to determine the solar constant of radiation with any con- 

 siderable degree of accuracy or certainty was, if not impossible, yet a thing 

 which would probably be long deferred and would involve spectro-bolometric 

 measurements at the highest possible altitudes at which men may exist. He 

 did not at all believe that our results of 1903 approximated to the true value 

 of the solar constant, but only that they might be so far independent of ordinary 

 atmospheric changes as to be used in determining the probability of solar vari- 

 ability. Hence, in 1905, he instructed the present writer to bear in mind, in 

 going to Mount Wilson for the first time, that it was not the solar constant but 



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