When, however, the supply of heat from the sun is interrupted dur-
ing long nights, the radiation which goes on into space causes the pre-
cipitation of a great part of the watery vapor from the air, and the
earth, thus deprived of this protecting shield, becomes more and more
rapidly cooled. If now we could suppose the atmosphere to be min-
gled with some permanent gas, which should possess an absorptive
power like that of the vapor of water, this cooling process would be in
a great measure arrested, and an effect would b^ ^T^d»si^^^\\ssJ«at\fe

18* 'iQ^



210 ANNUAL OP BCIENTIPIC DIBCOVERY.
that of a screen of glass ; which keeps up the temperature beneath it,
directly, by preventing the escape of radiant heat, and indirectly by
hindenng the condensation of the aqueous vapor in the air confined
beneath.
