, the body of air that is saturated with this invisible moisture
becomes suddenly chilled, the moisture condenses into cloud or mist.

If we watch a passing railroad train we shall notice a mass of fleecy
white mist floating away from the smokestack, assuming the billowy forms
of some of the clouds in summer. This cloud is produced by the sudden
condensation of steam, which was transparent before it came in contact
with the cold, outside air, the effect being much more pronounced in
cold than in warm weather. We may liken these floating globules of mist
to the dust of the earth which floats in the air, and it has not been
inaptly called water-dust. Anyone who has seen an atomizer used or has
stood at the foot of a great waterfall, like Niagara, has seen the fluid
so finely divided that it will float in the air, instead of falling to
the ground. What takes place is that a number of these transparent atoms
of moisture that are released in the process of evaporation coalesce
into one small drop or particle of water, and they will continue to
float in the air as mist or cloud until a sufficient number have
combined into one solid mass to render that mass heavier than the air,
when it falls in the form of rain.

If we live in a region--and there are such on the face of the
earth--where there is very little evaporation and consequently very
little moisture in the air, there is rarely ever a cloud seen nor is
there any rainfall, for the reason that there is no material existing
out of which to form clouds, and the clouds precede the rain. Hence, all
the artificial attempts to produce rain in these arid regions have been
futile. If a body of warm air, when saturated with invisible moisture,
is suddenly chilled by coming in contact with a cold wave, it is
squeezed like a sponge, so to speak, and the invisible particles become
visible because a number of them have coalesced as one particle; the
particles gather in a large mass, and we have the phenomenon of cloud
formation.

Clouds more generally form in the upper regions of the atmosphere
because it is normally colder in the higher regions. In some cases
clouds float very high in the air and in others very low. This is due to
two causes:

If we should send up a balloon containing air rarefied to a certain
extent it would continue to ascend only until it reached a point where
the outside air and that contained in the balloon are of the same
density. If we should send up this