To say that on a day of average humidity in
England, the atmospheric vapour exerts 100 times the action of the air
itself, would certainly be an understatement of the fact. Comparing a
single molecule of aqueous vapour with an atom of either of the main
constituents 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 vapour
seems vast, because that of the air with which it is compared is
infinitesimal. Absolutely considered, however, this substance,
notwithstanding 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 feet 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 imparting it to the surrounding aether; this
motion would be carried rapidly away, and lost for ever to our planet,
if the waves of aether had nothing but the air to contend with in
their outward course. But the aqueous vapour takes up the motion, and
becomes hereby heated, thus wrapping the earth like a warm garment,
and protecting its surface from the deadly chill which it would
otherwise sustain. Various philosophers have speculated on the
influence of an atmospheric envelope. De Saussure, Fourier,
M. Pouillet, and Mr. Hopkins have, one and all, enriched scientific
literature with contributions on this subject, but the considerations
which these eminent men have applied to atmospheric air, have, if my
experiments be correct, to be transferred to the aqueous vapour.

The observations of meteorologists furnish important, though hitherto
unconscious evidence of the influence of this agent. Wherever the air
is dry we are liable to daily extremes of temperature. By day, such
places, the sun's heat reaches the earth unimpeded, and renders the
maximum high; by night, on the other hand, the earth's heat escapes
unhindered to space, and renders the minimum low. Hence the
difference between the maximum and minimum is greatest where the air
is driest. In the plains of India, the heights of the Himalaya, in
central Asia, in Australia--wherever drought reigns, we have the heat
of day forcibly contrasted with the chill o