VOIy. 6, 1920 
ASTRONOMY: C. G. ABBOT 
87 
that a single observatory can obtain daily measurements of the intensity 
of solar radiation as it is outside the earth's atmosphere. The spectro- 
bolometric investigation of the solar constant of radiation requires that 
the observations shall extend over several hours from the time when the 
sun's altitude is small to the time when it is large, so as to determine for 
many wave-lengths of the spectrum what increase of intensity corresponds 
to the diminution of the length of the path of the solar rays within the 
earth's atmosphere. In order to prevent error in the determination of 
the transparency of the earth's atmosphere, the sky must be uniformly 
clear during the interval just mentioned. 
This is a condition very hard to fulfil. All stations in the world fulfil 
it well enough for certain days of the year, some stations for very many 
days of the year, but none for all. The experience of the Smithsonian 
Astrophysical Observatory in these matters indicates that probably there 
is no region in the world where more than 250 satisfactory days in the year 
can be confidently expected. Accordingly, it is highly desirable if these 
solar radiation changes have notable effects upon the earth's temperature 
and other meteorological and terrestrial phenomena, that several observa- 
tories shall join in making the measurements of the solar constant of radia- 
tion from day to day. On every day at least two values should be ob- 
tained strictly comparable with one another in their method of observa- 
tion and with a probable accuracy as far as experimental work is concerned 
to the order of one per cent. The mean of two, or still better, of three or 
four such values, would give a probable accuracy of the final result to 
better than one per cent, and if a series of such observations extending over 
every day in the year for a course of several years were available, it would 
probably be possible to determine very well the effect of the changes of 
solar radiation on the terrestrial climate. 
In order to carry on the work in this way, a considerable income is re- 
quired. The experience of the Smithsonian Institution in its several ex- 
peditions to Africa and to South America, and to the western part of 
North America, indicates that an income of between five and ten thousand 
dollars, preferably as much as eight thousand dollars per year, for each 
station would be requisite. If four stations should be considered compe- 
tent to carry out the problem, it would, therefore, require a yearly income 
of from thirty to forty thousand dollars for the whole series. It would be 
preferable that the work should be under a central direction so that the 
various measurements might be strictly comparable and homogeneous. 
If the Smithsonian Institution, with its long experience in these matters, 
had available for this purpose an income of the amount specified, that 
would offer a good way of dealing with the problem. 
On the other hand, it may be that support for the project would be 
more easily obtained if the several governments of the regions where the 
observatories for this purpose might be favorably located should be ap- 
