82 
ASTRONOMY: C. G. ABBOT 
Proc. N. a. S. 
THE LARGER OPPORTUNITIES FOR RESEARCH ON THE 
RELATIONS OF SOLAR AND TERRESTRIAL RADIATION^ 
By C. G. Abbot 
Smithsonian Astrophysical Observatory 
Communicated by the National Research Council, December 8, 1919 
The earth is maintained at its present temperature by a balance be- 
tween the solar radiation received and the terrestrial radiation emitted. 
The mean intensity of the solar radiation as it is at the earth's mean 
distance outside the atmosphere is believed to be well known. However, 
it appears to be variable from year to year and also irregular over short 
intervals of days, weeks or months. These variations, which have been 
discovered and to a certain extent followed for about fifteen years by the 
Smithsonian Astrophysical Observatory at its station on Mt. Wilson, 
have lately been compared with terrestrial temperatures and atmospheric 
pressures at different stations of the world by several meteorologists, 
notably by Clayton, and it appears that the effects of solar variations are 
important as governing changes of terrestrial temperature. 
This indicates two highly desirable researches: First, to make daily 
observations of the sun competent to determine its variations. Second, 
to determine by further statistical inquiries the exact influence of the 
variations of the sun on the terrestrial phenomena. 
In order to carry out the first research thus outlined, it requires not 
less than four special solar radiation observing stations in the four most 
cloudless regions of the earth. One such is now in existence at Calama, 
Chile, where it is being maintained by the private funds of the Smith- 
sonian Institution. If there was available the necessary income of thirty 
or forty thousand dollars a year, the Institution would establish the re- 
quired observing stations and would arrange for the daily wireless trans- 
mission of the results obtained on solar radiation so that they might be 
available for meteorologists in all parts of the world to use for forecasting 
purposes. The same could be done by international cooperation if due 
regard were given to making the measurements homogeneous. It is not 
probable that any steps of this kind will be taken within a year or two. 
By that time the series of measurements being made at Calama, Chile, 
will enable meteorologists to determine to what extent the results will 
be valuable for forecasting purposes. 
It would promote the proposed solar investigation if there could be 
devised autographic recording instruments for determining the exact 
value of the cloudiness at all times of the day when the sun is above the 
horizon. Hitherto measurements of cloudiness have been made at com- 
paratively few stations, and as they depend upon personal estimates of 
observers who are apt to overlook wisps of cirrus clouds especially dan- 
gerous for solar radiation work, and as they are usually taken only two 
