PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 1 JUNE 15. 1915 Number 6 
CONFIRMATORY EXPERIMENTS ON THE VALUE OF THE 
SOLAR CONSTANT OF RADIATION 
By C. G. Abbot, F. E. Fowle, and L. B. Aldrich 
SMITHSONIAN INSTITUTION. WASHINGTON. D. C. 
Presented to the Academy. April 27.1915 
We have made hitherto nearly 1000 determinations of the intensity 
of solar radiation outside the atmosphere at mean solar distance, termed 
the solar constant of radiation. The mean value found is 1.93 cal- 
ories per square centimeter per minute. Langley's spectro-bolometric 
method was employed. This consists in determining the distribution 
of the energy in the solar spectrum at different solar zenith distances, 
and thereby computing coefficients of atmospheric transmission suitable 
to determine the energy curve outside the atmosphere. The bolo- 
metric measurements are reduced in terms of standard 15° calories per 
square centimeter per minute by the aid of comparisons made each 
day of observation with standardized pyrheliometers. Observations 
have been made at Washington (sea level); Bassour, Algeria (1160 
meters); Mount Wilson, California (1730 meters); and Mount Whitney, 
California (4420) meters. They have continued during all the years 
1903 to 1914. Great changes from day to day and from place to place in 
temperature, in barometric pressure, in humidity, in haziness, v^rhile of 
course greatly affecting measurements of intensity at the stations, and 
of atmospheric transparency computed, nevertheless have not produced 
differences of the solar constant values. This seems to us to be strorg 
evidence of the soundness of the method. 
In the second place it has been shown by Fowle that the atmospheric 
transmission coefficients obtained at Mount Wilson fit well with Lord 
Rayleigh's theory of atmospheric scattering, except for those regions of 
spectrum where numerous atmospheric hues and bands of true absorp- 
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