at the subject is beset with difficulties--the
discussion of which does not belong to this little volume; but the
following notice of Dr. Hutton's theory may not be out of place.

It has been already stated, that the air supplies itself with moisture
from the surface of the waters of the earth. This it continues to do at
all temperatures, until it is so charged with vapour that it cannot
contain any more. The air is then said to be _saturated_. Now, the
quantity of moisture which a given bulk of air can contain, depends
entirely upon the temperature of the air for the time being. The higher
the temperature of the air the greater will be the quantity of vapour
contained in it; and, although it may be perfectly invisible to the eye,
on account of the elasticity which the heat imparts to it, yet it can
easily be made visible by subtracting a portion of the heat. If, for
example, a glass of cold water be suddenly brought into a warm room,
moisture from the air will be condensed upon the outside of the glass in
the form of dew. A similar change is supposed to take place when two
currents of air having different temperatures, but both saturated with
vapour, are mingled together; an excess of vapour is set free, which
forms a cloud or falls down as rain. If the currents continue to mingle
uniformly, "the clouds soon spread in all directions, so as to occupy the
whole horizon; while the additional moisture, incessantly brought by the
warmer current, keeps up a constant supply for condensation, and produces
a great and continued deposition of moisture in the form of rain. By
degrees, the currents completely intermingle, and acquire a uniform
temperature; condensation then ceases; the clouds are re-dissolved; and
the whole face of nature, after being cooled and refreshed by the
necessary rain, is again enlivened by the sunshine, thus rendered still
more agreeable by its contrast with the previous gloom."

If the cloud, produced by the mingling of two differently heated currents
of moist air, happen to form in the upper regions of the sky, it may be
heavier than its own bulk of air, and will consequently begin to sink.
Should the atmosphere near the earth be less dense than the cloud, the
latter will continue to descend till it touches the ground, where it
forms a mist. If the vapour has been condensed rapidly and abundantly,
the watery particles will form rain, hail, or snow, according to the
temperature of the air throu