Appendix 5. 



REPORT OF THE ASTROPHYSICAL OBSERVATORY. 



Sir : I have the honor to present the following report on the opera- 

 tions of the Smithsonian Astrophysical Observatory for the year 

 ending June 30, 1917. 



EQUIPMENT. 



The equipment of the observatory is as follows : 



(a) At Washington there is an inclosure of about 16,000 square 

 feet, containing 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 distribution apparatus. 



(b) At Mount Wilson, Cal., upon a leased plat of ground 100 

 feet square, in horizontal projection, are located a one-story cement 

 observing structure, designed especially for solar-constant measure- 

 ments, and also a little frame cottage, 21 feet by 25 feet, for observer's 

 quarters. Upon the observing shelter at Mount Wilson there is a 

 tower 40 feet high above the 12-foot piers which had been prepared 

 in the original construction of the building. This tower is equipped 

 with a tower telescope for use when observing (with the spectrobolo- 

 meter) the distribution of radiation over the sun's disk. 



During the year apparatus for research has been purchased or 

 constructed at the observatory shop. The value of these additions 

 to the instrumental equipment is estimated at $1,000. 



WORK OF THE YEAR. 

 1. AT WASHINGTON. 



Three copies of the pyranometer, our new instrument for measur- 

 ing sky radiation, have been prepared by the Institution, respectively, 

 for the United States Weather Bureau, the University of Wisconsin, 

 and for the proposed expedition to South America mentioned in my 

 report for 1916. These instruments were finished and standardized 

 by Mr. Aldrich. The tests made led to long investigations and im- 

 provements, which greatly increased the sensitiveness of the pyrano- 

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