f night. In the Sahara
itself, when the sun's rays cease to impinge on the burning soil, the
temperature runs rapidly down to freezing, because there is no vapour
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 vapour of the air is condensed to water on the surface of the
earth; and, as only the superficial portions radiate, the act of
condensation makes water the radiating body. Now experiment proves
that to the rays emitted by water, aqueous vapour is especially
opaque. Hence the very act of condensation, consequent on terrestrial
cooling, becomes 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 selfsame covering which protects the earth from
chill must also shut out the solar radiation. 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 thickness, the sun's rays
are transmitted with comparative freedom; 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 freedom through the aqueous vapour of the air: the
absorbing power of this substance being mainly exerted upon the
invisible heat that endeavours 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.

********************

13. Liquids and their Vapours in relation to Radiant Heat.

The deportment here assigned to atmospheric vapour has been
established by direct experiments on it taken from the streets and
parks of London, from the downs of Epsom, from the hills and sea-beach
of the Isle of Wight, and also by experiments on air in the first
instance dried, and afterwards rendered artificially humid by pure
distilled water. It has also en established in the following way: Ten
volatile quids were taken at random and the power of these quids, at a
c