Appendix 5. 



EEPORT 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. 



(5) At Mount Wilson, California, upon a leased plat of ground 

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

 ment observing structure, designed especially for solar-constant meas- 

 urements, and also a little frame cottage, 21 feet by 25 feet, for ob- 

 server's quarters. Upon the observing shelter at Mount Wilson there 

 is a tower 40 feet high above the 12-foot piers which had been pre- 

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

 equipped with a tower telescope for use when obser^dng (with the 

 spectrobolometer) 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 

 toihe 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- 

 meter. All three instruments are now in use and, so far as known, 

 with satisfaction. < 



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