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RADIATION. 197

Australia—wherever drought reigns, we have the heat of
day forcibly contrasted with the chill of night. In the Sa-
hara itself, when the sun’s rays cease to impinge on the
burning soil, the temperature runs rapidly down to freezing,
because there is no vapor overhead to check the calorific
drain. And here another instance might be added to the
numbers already known, in which Nature tends as it were
to check her own excess. By nocturnal refrigeration, the
aqueous vapor of the air is condensed to water on the sur-
face of the earth, and as only the superficial portions ra-
diate, the act of condensation makes water the radiating
body. Now experiment proves that to the rays emitted by
water, aqueous vapor is especially opaque. Hence the very
act of condensation, consequent on terrestrial cooling, be
comes a safeguard to the earth, imparting to its radiation
that particular character Which renders it most liable to be
prevented from escaping into space.

It might however be urged that, inasmuch as we derive
all our heat from the sun, the self-same covering which pro-
tects the earth frOm chill must also shut out the solar ra-
diation. This is partially true, but only partially; the
sun’s rays are different in quality from the earth’s rays, and
it does not at all follow that the substance which absorbs
the one must necessarily absorb the other. Through a
layer of water, for example, one-tenth of an inch in thick-
ness, the sun’s rays are transmitted with comparative free-
dom ; but through a layer half this thickness, as Melloni has
proved, no single ray from the warmed earth could pass.
In like manner, the sun’s rays pass with comparative free-
dom through the aqueous vapor of the air; the absorbing
power of this substance being mainly exerted upon the heat
that endeavors to escape from the earth. In consequence
of this differential action upon solar and terrestrial heat,
the mean temperature of our planet is higher than is due to
its distance from the sun.

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