RADIATION. 43 
amounts to. 372. It would be idle to speculate on the 
quantities of matter concerned in these actions. 
12. Aqueous Vapor in Relation to the Terrestrial Tem- 
peratures. 
We are now fully prepared for a result which, without 
such preparation, might appear incredible. Water is, to 
some extent, a volatile body, and our atmosphere, resting 
as it does upon the surface of the ocean, receives from it a 
continual supply of aqueous vapor. It would be an error 
to confound clouds or fog or any visible mist with the 
vapor of water, which is a perfectly impalpable gas, dif- 
fused, even on the clearest days, throughout the atmos- 
phere. Compared with the great body of the air, the aque- 
ous vapor it contains is of almost infinitesimal amount, 99 
out of every 100 parts of the atmosphere being composed 
of oxygen and nitrogen. In the absence of experiment, 
we should never think of ascribing to this scant and varying 
constituent any important influence on terrestrial radia- 
tion; and yet its influence is far more potent than that of 
the great body of the air. To say that on a day of average 
humidity in England, the atmospheric vapor exerts 100 
times the action of the air itself, would certainly be an 
understatement of the fact. Comparing a single molecule 
of aqueous vapor with an atom of either of the main con- 
stituents of our atmosphere, I am not prepared to say how 
many thousand times the action of the former exceeds that 
of the latter. 
But it must be borne in mind that these large numbers 
depend, in part, on the extreme feebleness of the air; the 
power of aqueous vapor seems vast, because that of the 
air with which it is compared is infinitesimal. Abso- 
lutely considered, however, this substance, notwithstand- 
ing its small specific gravity, exercises a very potent action. 
Probably from 10 to 15 per cent, of the heat radiated from 
the earth is absorbed within 10 or 20 teet of the earth's 
surface. This must evidently be of the utmost consequence 
to the life of the world. Imagine the superficial molecules 
of the earth agitated with the motion of heat, and impart- 
ing it to the surrounding ether; this motion would be carried 
rapidly away, and lost forever to our planet, if the waves of 
ether had nothing but the air to contend with in their out- 
ward course. But the aqueous vapor takes up the motion, 
