ng area many times as great as that afforded by a surface
of bared earth. Moreover, the ground itself can not well cool down to
the point where it will wring the moisture out of the air, while the
thin membranes of the plants readily become so cooled. Thus vegetation
by its own structure provides itself with means whereby it may be in a
measure independent of the accidental rainfall. We should also note
the fact that the dewfall is a concomitant of cloudless skies. The
quantity which is precipitated in a cloudy night is very small, and
this for the reason that when the heavens are covered the heat from
the earth can not readily fly off into space. Under these conditions
the temperature of the air rarely descends low enough to favour the
precipitation of dew.

Having noted the process by which in the rain circuit the water
leaves the sea and the conditions of distribution when it returns to
the earth, we may now trace in more detail the steps in this great
round. First, we should take note of the fact that the water after it
enters the air may come back to the surface of the earth in either of
two ways--directly in the manner of dewfall, or in a longer circuit
which leads it through the state of clouds. As yet we are not very
well informed as to the law of the cloud-making, but certain features
in this picturesque and most important process have been tolerably
well ascertained.

Rising upward from the sea, the vapour of water commonly remains
transparent and invisible until it attains a considerable height above
the surface, where the cooling tends to make it assume again the
visible state of cloud particles. The formation of these cloud
particles is now believed to depend on the fact that the air is full
of small dust motes, exceedingly small bits of matter derived from the
many actions which tend to bring comminuted solid matter into the air,
as, for instance, the combustion of meteoric stones, which are greatly
heated by friction in their swift course through the air, the
ejections of volcanoes, the smoke of forest and other fires, etc.
These tiny bits, floating in the air, because of their solid nature
radiate their heat, cool the air which lies against them, and thereby
precipitate the water in the manner of dew, exactly as do the leaves
and other structures on the surface of the earth. In fact, dew
formation is essentially like cloud formation, except that in the one
case the water is gathered on fixed bodies, an