Vol. 6, 1920 
ASTRONOMY: C. G. ABBOT 
89 
other obstructing materials, yet it is highly important to have measure- 
ments of the radiation of the sun made at the earth's surface in many 
parts of the world. A series of measurements of this kind is being con- 
ducted by the U. S. Weather Bureau at its stations in Washington, Madison, 
Lincoln, and Santa Fe. This work, in charge of Professor Kimball, has 
gone on now for about ten years, and is being published in excellent form 
in the Monthly Weather Review of the U. S. Weather Bureau. Observa- 
tions were also undertaken in Egypt and a report was published several 
years ago by Dr. Shaw. Observations were carried on for a number of 
years by the Harvard College Observatory at Arequipa, in Peru, but about 
a year ago the instrument was accidentally broken. The Arequipa series 
was then discontinued, partly because the Smithsonian Institution has 
its spectrobolometric and pyrheliometric station in Chile not far from 
Arequipa. Some measurements are being made also in Argentina, Brazil 
and elsewhere. There are also pyrheliometric instruments at the Island 
of Teneriffe which were in use before the war, but whether the measure- 
ments are being conducted there regularly now I am not aware. Dr. 
Dorno, of Davos, Switzerland, has made such measurements with many 
others of related subjects for a long time, and has just published a great 
volume of valuable results. 
It would be very desirable if work of this kind could be carried on 
regularly at favorable stations employing strictly comparable instru- 
ments. There are about thirty copies of the Smithsonian silver disc 
pyrheliometer distributed to a number of different parts of the world 
but as stated above it is not probable that during the period of war they 
have been in regular use. Many of them are in very cloudy localities. 
4. Distribution of Radiation over the Solar Disc. — As stated above, it 
has been found at the Smithsonian observing station on Mt. Wilson, 
in California, that the distribution of radiation along the diameter of the 
solar disc is subject to variability of two kinds — one of a long period of 
years, and the other of a short period of days, weeks, or months, and 
that these two kinds of variation are fairly closely correlated with the 
variations of the total radiation of the sun. The work of observing the 
radiation along the solar diameter requires the use of a long focus telescope 
and a spectrobolometer, and may very readily be carried on nearly 
simultaneously with measurements of the solar constant of radiation. 
Such investigation is going on regularly at Mt. Wilson during the summer 
season when the Smithsonian observers are working there. Perhaps it 
would not be necessary to set up other stations since the main object of 
this work was accomplished when it was shown that it confirmed the 
existence of the solar variability shown by the measurements of the solar 
radiation itself. 
The method of observing the distribution of brightness along the solar 
diameter which we have adopted is to stop the clock-work of the coelostat 
