THE ASTROPHYSICAL OBSERVATORY—ABBOT 
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of Mount Whitney in 1909 and 1910, 
and to Bassour, Algeria, in 1911 and 
1912. Solar-constant measurements 
were made at these stations on the 
same days that they were being made 
on Mount Wilson. In later years, as 
I shall tell, we have amplified such 
tests by occupying mountain summits 
in other parts of the world. 
Criticism of our solar-constant work, 
based on the supposed impossibility 
of correctly estimating atmospheric 
transparency, led us to special observ- 
ations in 1913. On a very perfect 
September day we observed the sun 
with pyrheliometer and bolometer 
from the instant it rose until midday. 
From these observations, extending 
from air mass 20 to air mass 1.2, we 
computed a series of solar-constant 
values, instead of the usual one de- 
pending only on air mass change 
from 4 to 1.2. They agreed excel- 
lently. Also, at A. K. Angstrom’s 
suggestion, I designed an automatic 
recording pyrheliometer, to be carried 
by sounding balloons very high in 
the atmosphere. This was flown by 
L. B. Aldrich, in cooperation with the 
U. S. Weather Bureau, from Omaha, 
Nebr., and recovered in Iowa. It 
gave excellent records at 15 miles 
elevation, where the air pressure was 
only one twenty-fifth as great as at sea 
level. These records gave a value of 
solar radiation about 1 percent lower 
than those derived from solar-constant 
observing at Mount Wilson. 
Both on Mount Wilson and on 
Mount Whitney I made many meas- 
urements of the intensity of sky light 
at all parts of the sky, both by day 
and by night. We also confirmed 
Lord Rayleigh’s theoretical and obser- 
vational studies of the blue of the sky. 
Applying Rayleigh’s theory to our 
observations, F. E. Fowle found very 
close agreement with R. A. Millikan’s 
determination of the number of mole- 
cules per square centimeter in air. 
He also made classical researches 
which gave a method for measuring 
precipitable water in the atmosphere. 
Another thing which attracted wide 
attention and afforded amusement to 
ourselves and our visitors was the 
solar cooker I devised and installed 
close beside our cottage on Mount 
Wilson. It has a concave cylindric 
mirror which follows the sun by rotat- 
ing about a pipe at the focus of the 
mirror, mounted parallel to the earth’s 
axis. Cylinder oil in the pipe when 
heated by solar radiation circulates 
about ovens in a reservoir some 10 
feet above the mirror. Mrs. Abbot 
used to cook food for ourselves and 
our visitors there for many years. 
She baked bread, cooked meat and 
vegetables, made jam, and, in short, 
did all kinds of cooking by solar heat 
except frying and roasting. The ovens 
remained hot enough to bake with, 
by night as well as by day, for months 
each year. 
Several times in later years I 
attempted to measure the distribution 
of radiation in the spectra of the 
brighter stars, using a spectroscope at 
the coudé focus of the 100-inch 
reflector and a radiometer as my 
measuring instrument. I made in- 
teresting spectral-energy curves for 
yellow and red stars, but hope still 
