242 



ABBOT— SOLAR CONSTANT OF RADIATION. [April 2., 



It was thought doubtful by Langley, and others, if correct esti- 

 mates of the atmospheric transmission can be made, even by the 

 spectro-bolometric method of high and low sun observations. Lang- 

 ley, indeed, gave an argument tending to show that the values of the 

 solar constant thus obtained fall far below the true intensity of the 

 solar radiation outside the atmosphere. This argument, however, 

 seems to be unsound. ^^ In order to test the accuracy of the method 

 I made spectro-bolometric measurements on Mount Whitney (4420 

 meters elevation) in 1909 and 1910 simultaneously with similar ob- 

 servations made by Messrs. Ingersoll and Fowle, respectively, on 

 Mount Wilson (1800 meters elevation). In 1905 and 1906 solar- 

 constant measurements were made nearly simultaneously at Mount 

 Wilson and at Washington (10 meters elevation). It does not ap- 

 pear from these observations that there are any differences in the 

 solar-constant values depending on the altitude of the observer, and 

 not due to accidental errors of observations.^* 



In illustration of this conclusion I give the results obtained simul- 

 taneously at Mount Wilson and Mount Whitney : 



Date. 



1909, Sept. 3. 



Mount Wilson... 

 Mount Whitney. 



1-943 

 1-959 



1910, Aug. 12. 



1-943 

 1.979 



1910, Aug. 13. 



1910, Aug. 14. 



1.924 

 1-933 



1.904 

 1-956 



The very slight excess of the Mt. Whitney values is not large 

 enough to be significant. 



We conclude that the solar-constant values computed from the 

 method of high and low sun observations do not depend on the 

 altitude of the observing station up to altitudes of 4,420 meters, 

 provided the sky conditions are satisfactorily clear and uniform. 



Reducing values published in Vol. II. of the Annals to standard 

 calories at 15° centigrade, and including the mean values obtained 

 in later years, ^"' we have : 



"See Aiiiuils. Vol. II., pp. 119-121. 



'*As regards the Washington and Mount Wilson comparisons, see 

 Annals, Vol. II., pp. 99 and 102. Note that the provisional scale of these 

 Annals values is 5 per cent, too high. 



'° Many of the values of 1910 are not yet reduced. 



