Vol. 8, 1922 ASTRONOMY: ABBOT, FOWLE AND ALDRICH 
177 
THE LARGER RESULTS OF 20 YEARS OF SOLAR 
RADIA TION OBSER VA TIONS 
By C. G. Abbot, F. E. FowIvK and L. B. Aldrich 
ASTROPHYSICAL OBSERVATORY, SMITHSONIAN INSTITUTION, WaSHINTON 
Read before the Academy, April 24, 1922 
In Volume 4 of the Annals of the Astro physical Observatory are given 
the results obtained from 1912 to 1920, but the discussions go back to the 
beginning of the investigations of solar radiation at the Smithsonian 
Institution in 1902. Where so many details are taken up it is a little hard 
to keep in mind the broad outlines, and we propose to give a general view 
of the work in what follows. 
We measure the solar heat received by the earth; determine the limits 
of its variability; study the distribution of solar radiation in the spectrum, 
and its intensity over the solar disk; and we investigate the properties of 
the terrestrial atmosphere in relation to the passage of solar rays to the 
earth's surface and the escape of earth rays to space. 
When the work was begun in 1902, great diversity of results prevailed. 
The black bulb in vacuo thermometer, Pouillet's pyrheliometer, Tyndall's, 
Violle's and Crova's actinometers, and the older form of Angstrom's com- 
pensation pyrheliometer, differed through a range of at least 50 per cent 
in their indications. Values of the "solar constant" or intensity of solar 
radiation outside our atmosphere at mean solar distance ranged from 1.76 
to 4.0 calories as given in the best text-books. Nothing was known as to 
the limits of the solar variability. 
1. Standardization and perfecting of methods and apparatus. — While 
the methods we have used are Langley's they have been reduced to a 
finished system of every day procedure. The theory, practice, sources of 
error and details needful to accurate results in solar constant observing 
have been worked out and published in Volumes 2, 3, and 4 of the Annals 
of the Astrophysical Observatory. 
Two forms of standard pyrheliometer have been devised, perfected and 
used to establish the scale of solar radiation measurement. Both types of 
instrument have hollow chamber or "black body" receivers of radiation, 
and in both test quantities of electrically introduced heat have been meas- 
ured with close agreement. 
A secondary instrument, called the silver-disk pyrheliometer, has also 
been perfected. About 30 standardized copies have been purchased by 
observers in many countries of the world. Thus the standard scale of 
radiation has been widely diffused. Our measurements are approximately 
2 per cent higher than those of the system of Angstrom with its latest 
corrections. 
