Vol. 6, 1920 
ASTRONOMY: C. G. ABBOT 
that no greater degree of accuracy than to one per cent will be possible 
even with them. 
The spectrobolometric investigations of this subject by the Astro- 
physical Observatory of the Smithsonian Institution which have been 
continued for the last seventeen years have yielded the general indication 
that the value of the solar constant of radiation is within one per cent 
of 1.93 calories per square centimeter per minute. This result is con- 
firmed by experiments made with sounding balloons by the same observa- 
tory in cooperation with the U. S. Weather Bureau in 1914. Automatic 
recording pyrheliometers were sent to a height of about 25 kilometers. 
It was found that the intensity of the radiation there, at a point where 
24/25 of the atmosphere was beneath the observing instrument, fell within 
the range of solar constant values as obtained by spectrobolometric work 
at the surface of the earth. The actual value obtained by the balloon 
work was 1.84 calories per square centimeter per minute. Allowing for 
the obstruction of the sun rays by the still superincumbent atmosphere 
a reasonable extrapolation would give 1.88 calories. This differs from 
1.93 calories less than the error of the observations and is well within the 
range of solar constant values obtained at different days from surface 
observatories. 
Apparently the close agreement of these widely different independent 
methods of obtaining the intensity of solar radiation outside our atmos- 
phere permits us to say authoritatively that the mean value of the solar 
constant is at any rate between 1.9 and 2.0 calories per square centimeter 
per minute and most probably not greatly different from 1.93. A few 
men cling to the view that much higher values should be set for it— values 
between 3 and 4 calories per square centimeter per minute, but their argu- 
ments carry little weight, as it seems to me, and those best qualified to 
know are agreed that the values given above have a strong foundation. 
The methods of observing the solar constant of radiation by means of 
the spectrobolometer have been carefully worked out and the whole sub- 
ject has been published with satisfactory fullness and is in shape to be 
transmitted to the scientific men of the future in order to enable them 
to reproduce the measurements or to compare their own measurements 
with the results now obtained. ' ^..x ,.t . ^ . ^ , ' ^ r^-, 
2. The Sun's Variability. — Passing now to the second branch of the 
subject, the investigations of the Smithsonian Astrophysical Observatory 
conducted at Washington, Mt. Wilson, Mt. Whitney, Bassour (Algeria), 
and now the investigations supported by the Smithsonian Institution 
from its private funds in North Carolina and Chile have all united in giving 
the impression that the solar radiation is not constant, but varies from 
day to day through a range of certainly five and possibly at times ten 
per cent. The conclusion that the sun is a variable star is confirmed 
in several ways but most notably by the results of measurements made by 
