90 ANNUAL KEPOBT SMITHSONIAN INSTITUTION, 1914. 



emitted by this lamp, first directly, and then, after the rays have 

 traversed twice or four times a tube 200 feet long, containing air of 

 measured humidity. During the past year Mr. Fowle has been 

 dealing principally with rays of the very longest wave lengths of the 

 terrestrial energy spectrum which moist air transmits. He has 

 reached a wave length of about eighteen microns, which is about 

 twenty-five times the longest wave length visible to the eye, and 

 about three and one-half times the wave length of the solar rays 

 investigated by this observatory in the years 1890 and 1900. 



A great number of difficulties are met with. In the first place, 

 great sensitiveness of the bolometer is required, owing to the feeble- 

 ness of these rays. Attempts to use a vacuum bolometer have con- 

 sumed much time, but not yet with entire success. Full success in 

 this seems now probable. In the second place there is great difficulty 

 in determining the amount of radiation lost in the optical train re- 

 quired to reflect the beam to and fro through the long tube. A prin- 

 cipal difficulty in this matter arises from the fact that the lamp and 

 its surroundings are unequally hot at different parts, for this has led 

 to different degrees of loss at different wave lengths. This last source 

 of error is so obscure that it escaped our attention for a long time 

 and has required the observations to be repeated after results worthy 

 of publication had, as it was thought, been reached. These and a host 

 of other difficulties have delayed the research, but great hope is now 

 felt that satisfactory results will be ready for publication in another 

 year. 



Computations. — The reductions of Mount Wilson and Washington 

 observations take a large part of the time of Mr. Fowle and Mr. Aid- 

 rich, as well as the entire time of Miss Graves and a portion of that 

 of Mr. Carrington. This work is nearly up to date. 



Mr. Fowle has continued the study of the effect of terrestrial water 

 vapor on the Mount Wilson solar observations and has published 

 several valuable papers upon it. An interesting result is, that after 

 determining and correcting for the effect of atmospheric water vapor 

 on the transmission of solar rays the coefficients of atmospheric trans- 

 parency determined at Mount Wilson when combined with the baro- 

 metric pressure after the manner indicated by Lord Rayleigh's theory 

 of gaseous scattering of light, yield the value 2.70 billion billion as 

 the number of molecules at standard pressure and temperature in a 

 cubic centimeter of gas. Prof. Millikan, by a wholly independent 

 kind of reasoning, has derived from electrical experiments the value 

 2.705 billion billion. The close agreement found is a strong confirma- 

 tion of the accuracy of our determinations of atmospheric transpar- 

 ency, and accordingly tends to increase confidence in our determina- 

 tion of the solar constant of radiation. 



