er
portion becomes warmed, and if in contact with water takes up a
quantity of its vapor proportioned to the temperature, or in ordinary
circumstances somewhat less than this proportion. It then tends to
ascend, and as it rises and becomes mixed with colder air it gradually
loses its power of sustaining moisture, and at a height proportioned
to the diminution of temperature and the quantity of vapor originally
contained in the air, it begins to part with water, which becomes
condensed in the form of mist or cloud; and the surface at which this
precipitation takes place is often still more distinctly marked when
two masses or layers of air at different temperatures become
intermixed; in which case, on the principle already stated, the mean
temperature produced is unable to sustain the vapor proper to the two
extremes, and moisture is precipitated. It thus happens that layers
of cloud accumulate in the atmosphere, while between them and the
surface there is a stratum of clear air. Fogs and mists are in the
present state of nature exceptional appearances, depending generally
on local causes, and showing what the world might be but for that
balancing of temperature and the elastic force of vapor which
constitutes the atmospheric firmament.[68]

The quantity of water thus suspended over the earth is enormous. "When
we see a cloud resolve itself into rain, and pour out thousands of
gallons of water, we can not comprehend how it can float in the
atmosphere."[69] The explanation is--1st, the extreme levity of the
minute globules, which causes them to fall very slowly; 2d, they are
supported by currents of air, especially by the ascending currents
developed both in still air and in storms; 3dly, clouds are often
dissolving on one side and forming on another. A cloud gradually
descending may be dissolving away by evaporation at the base as fast
as new matter is being added above. On the other hand, an ascending
warm current of air may be constantly depositing moisture at the base
of the cloud, and this may be evaporating under the solar rays above.
In this case a cloud is "merely the visible form of an aerial space,
in which certain processes are at the moment in equilibrium, and all
the particles in a state of upward movement."[70] But so soon as
condensation markedly exceeds evaporation, rain falls, and the
atmosphere discharges its vast load of water--how vast we may gather
from the fact that the waters of all the rivers are