PHYSICS: C. G. ABBOT 
105 
ties of water vapor up to the equivalent of 0.3 cm. of precipitable water. 
He also observed the solar spectrum to 1 7^6 through paths of atmosphere con- 
taining water vapor up to the equivalent of 3.0 cm. of precipitable water. 
Since rock salt ceases to be sufficiently transparent beyond 17ju Fowle's spec- 
trum work stopped there. But Aldrich, on Mount Wilson, by experiments 
not yet entirely finished, seems to have shown that neither incoming sun- 
rays nor outgoing earth-rays non-transmissible to rock-salt (that is over. 17/* 
in wave-length) can traverse the atmosphere. Assuming that this result 
will be confirmed we have the following results from Fowle's and Aldrich' s 
investigations representing the output and atmospheric transmission of rays 
from a perfect radiator at earth temperature. 
Per cent of atmospheric transmission for stated cm. ppt. H 2 0 
WAVE-LENGTH 
INTENSITY 
cm. 0.003 
cm. 0.03 
cm. 0.3 
cm. 3.0 
4- 5 
50 
15 
45 
70 
95 
5- 6 
142 
16 
43 
66 
95 
6- 7 
242 
45 
85 
95 
100 
7- 8 
315 
13 
42 
85 
100 
8- 9 
360 
0 
2 
40 
50 
9-10 
380 
0 
0 
0 
15 
10-11 
370 
0 
2 
5 
40 
11-12 
350 
0 
0 
4 
10 
12-13 
320 
0 
0 
13 
20 
13-16 
810 
100 
100 
100 
100 
16-20 
510 
90 
100 
100 
100 
> 20 
1,450 
100 
100 
100 
100 
4— CO 
5,300 
49 
57 
66 
75 
From these results Fowle has computed that in clear weather, when pre- 
cipitable water in the atmosphere is 1 cm., the atmosphere transmits 28% 
to space of the radiation emitted by the earth's surface. In the tropics where 
a load of atmospheric humidity equal to precipitable water of 3 cm. or more 
is common, the transmission would not exceed 20% on clear days. A. Ang- 
strom has shown 2 that on cloudy nights practically all radiation from the 
earth's surface to space is cut off. Hence (as it is cloudy half the time on the 
average of the earth's surface) out of 0.50 calories per square centimeter per 
minute emitted, the average escape to space, taking both clear weather and 
cloudy, is only 0.06 calories. As 0.29 calories per cm 2 per minute on the 
average must leave the planet earth, and as the earth's surface contributes, 
only 0.06 calories, it follows that the atmosphere is the main radiating source, 
furnishing three-fourths of the output of radiation of the earth as a planet. 
Principal sources of the atmospheric radiation in order of their importance 
are: (1) The clouds; (2) water-vapor; (3) ozone; (4) carbon-dioxide. Ther 
