THE RADIATION OF THE SUN—ABBOT. 149 
phere could be absorbed completely in a layer of water 1 centimeter 
(about three-eighths of an inch) thick, exposed at right angles to the 
solar beam, this layer of water would be warmed 1.93° C. during each 
minute of time. Expressed in another way, the sun’s radiation 
outside the atmosphere would be able to melt a layer of ice 105 feet 
thick each year. . 
An extremely interesting feature of the measurements has been 
that they show a variation of the sun. This conclusion has been 
tested in every way, not only by making measurements at different 
altitudes but by comparing results obtained on the same days at 
Mount Wilson, in California, and at Bassour, Algeria. As these sta- 
tions are separated by about one-third the circumference of the earth, 
it seems not possible that they could be generally influenced by local 
conditions in a way to disturb the measurements in the same direction 
at the same time. Nevertheless, the results of about 50 days of simul- 
taneous observing at the two stations agree in showing that when 
the radiation of the sun is found above the normal at the one station 
it is found also above the normal at the other station, and vice versa. 
The fluctuations of the intensity of the sun’s radiation outside the 
atmosphere thus indicated range over about 10 per cent. Often 
within a single week or 10 days a fluctuation of radiation as great as 
5 per cent is shown. The variation of the sun in these short periods 
appears to be irregular, both as regards the magnitude of the varia- 
tion and as regards the period of it. 
The measurements made at Mount Wilson, which extend over 
the years 1905 to 1913, indicate also a fluctuation of the intensity 
of solar radiation, attending the changes of the number of sun spots. 
There appears to be about 3 per cent increase of the solar radiation 
outside the atmosphere for an increase of 100 in the Wolf sun-spot 
numbers. It is a very curious thing that the solar radiation increases 
with increasing numbers of sun spots, whereas the temperature, 
which directly depends upon the solar radiation, falls with increasing 
numbers of sunspots. It appears that there is attending sun spots a 
direct and an indirect influence on terrestrial temperature. The direct 
influence is due to the increased solar radiation. The indirect influ- 
ence is perhaps due to a change in cloudiness, but as yet is not certainly 
understood. These two influences are of almost equal magnitude in 
general, but with the indirect influence, which tends to lower tempera- 
tures, slightly predominating. It will be a research of great interest 
and value to determine the cause of the indirect influence.! 
In connection with these researches on the solar radiation the 
transparency of the air for light of all colors and for invisible rays 
has been determined. This is a matter of great interest to those 
1The author is informed that researches by the meteorological service of India indicate that not all 
stations of the world are cooler at sun-spot maximum. 
