and let it separate the waters from the waters. And God made
the expanse, and separated the waters which are under the
expanse from the waters which are over the expanse: and it
was so. And God called the expanse Heaven. And the evening
and the morning were the second day."--Genesis i. 6-8.

At the opening of the period to which we are now introduced the earth
was covered by the waters, and these were in such a condition that
there was no distinction between the seas and the clouds. No
atmosphere separated them, or, in other words, dense fogs and mists
everywhere rested on the surface of the primeval ocean. To understand
as far as possible the precise condition of the earth's surface at
this period, it will be necessary to notice the present constitution
of the atmosphere, especially in its relations to aqueous vapor.

The regular and constant constituents of the atmosphere are the
elements oxygen and nitrogen, which, at the temperature and pressure
existing on the surface of our globe, are permanently aeriform or
gaseous. Beside these gases, the air always contains a quantity of the
vapor of water in a perfectly aeriform and transparent condition. This
vapor is not, however, permanently gaseous. At all temperatures below
212 degrees it tends to the liquid state; and its elastic force, which
preserves its particles in the separated state of vapor, increases or
diminishes at a more rapid rate than the increase or diminution of
temperature. Hence the quantity of vapor that can be suspended in
clear air depends on the temperature of the air itself. As the
temperature of the air rises, its power of sustaining vapor increases
more rapidly than its temperature; and as the temperature of the air
falls, the elastic force of its contained vapor diminishes in a
greater ratio, until it can exist as an invisible vapor no longer, but
becomes condensed into minute bubbles or globules, forming cloud,
mist, or rain. Two other circumstances operate along with these
properties of air and vapor. The heat radiated from the earth's
surface causes the lower strata of air to be, in ordinary
circumstances, warmer than the higher; and, on the other hand, warm
air, being lighter than that which is colder, the warm layer of air at
the surface continually tends to rise through and above the colder
currents immediately over it. Let us consider the operation of the
causes thus roughly sketched in a column of calm air. The low