Appendix V. 

 REPORT ON THE ASTROPHYSICAL OBSERVATORY. 



Sir: During the past year the temporary wooden shelters used for observing 

 on Mount Wilson, California, in 1905, 1906, and 1908 have been torn down and 

 replaced by a structure of cement blocks. This was erected at a cost of $2,200 

 on a plot of ground 100 feet square in horizontal projection, leased for a term 

 of ninety-five years by the Smithsonian Institution from the Mount Wilson 

 Solar Observatory. The new observing shelter is L shaped in plan, 36 feet long, 

 27 feet wide, and with the two branches of the L14 and 10 feet wide, respec- 

 tively. Four tall piers are provided for the future erection of a tower over the 

 south end of the structure. The proposed tower is intended for use as a vertical 

 telescope in solar observations and also as a suitable station for making meas- 

 urements of the brightness of the sky and clouds. Within the new building is 

 a chamber of constant temperature in which is the spectro-bolometric outfit, 

 and also a dark room for photographic work and a small office room. The site 

 leased is on the edge of a preciptous ridge overlooking canyons about 1,000 feet 

 deep on the east, south, and west. It is thoroughly isolated from disturbances 

 caused by electric service, gas engines, or traffic, and seems to be peculiarly 

 well adapted for the work in hand. 



The personnel of the observatory has continued principally unchanged. Mr. 

 L. B. Aldrich completed his temporary service as bolometric assistant on Sep- 

 tember 20, 1908. Dr. L. R. Ingersoll was engaged temporarily as bolometric 

 assistant on Mount Wilson beginning June 21, 1909. 



WORK OF THE TEAR. 



1. Work at Washington. 



Mr. Fowle has continued bolometric observations of the brightness of dif- 

 ferent parts of the sun's image whenever conditions favored. No measure- 

 ments of the solar constant of radiation were attempted at Washington, as that 

 branch of the work can seldom be done there successfully, on account of smoke 

 and clouds. 



A most interesting piece of experimental work on the transparency of air 

 for the long-wave rays, such as the earth radiates, had been begun by Mr. Fowle 

 early in 190S. Results have been obtained by him for the transmission of all 

 rays between wave-lengths In and 10/i, through a column of air 400 feet long, 

 containing various known amounts of water vapor. Computation of the re- 

 sults from these experiments is so far advanced as to show their satisfactory 

 quality. Many additional experiments with still longer columns of air and other 

 amounts of water vapor, and extending as far down in the spectrum as wave- 

 length 17/u, are in preparation. 



The late Secretary Langley stated, as a result of his Mount Whitney ob- 

 servations : " I consider that the temperature of the earth under direct sun- 

 shine, even though our atmosphere were present as now, would probably fall 



a Report of the Mount Whitney Expedition, p. 123. 

 64 



