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NEW EVIDENCE ON THE INTENSITY OF SOLAR 

 RADIATION OUTSIDE THE ATMOSPHERE 



By C. G. ABBOT, F. E. FOWLE, and L. B. ALDRICH 



The following- investigations were suggested by several criticisms 

 of the work of the Astrophysical Observatory on the " Solar Con- 

 stant of Radiation." We shall show: (i) That on fine days at Mt. 

 Wilson there is no observable systematic change of atmospheric 

 transparency from the moment of sunrise to about 10 o'clock, and 

 (2) That the intensity of solar radiation even at 24 kilometers (15 

 miles) altitude, at less than one twenty-fifth atmospheric pressure, 

 falls below 1.9 calories per square centimeter per minute. 



It will be useful to preface the paper by a brief account of our 

 earlier work. We shall draw attention also to various facts tending 

 to support the result heretofore obtained, namely : The mean value 

 of the "solar constant" is 1.93 calories per square centimeter per 

 minute. 



SUMMARY OF EARLIER WORK 



In Vol. Ill of the Annals of the Astrophysical Observatory of 

 the Smithsonian Institution, we published the methods employed, the 

 apparatus used, and results obtained in determinations of the mean 

 intensity of solar radiation outside the atmosphere during the years 

 1902 to 1912. The method employed was that of Langley. 1 It 

 requires measuring the intensity of the total radiation of the sun 

 with the pyrheliometer and also the measurement of the intensity of 

 the rays of the different wave lengths with the spectro-bolometer. 

 Measurements of both kinds are made repeatedly' during a clear 

 forenoon or afternoon from the time when the sun is low until it 

 becomes high or vice versa. In this way we determine how rapidly 



1 " Report on the Mount Whitney Expedition," Professional Papers, Signal 

 Service, No. 15, pp. 135 to 142, and table 120, values 1 to 5: 



Smithsonian Miscellaneous Collections, Vol. 65, No. 4 



