same balloon on different days with
the same rarefaction of internal air we should find that on some days it
would float higher than others, because the density of the air is
constantly fluctuating, as is indicated by the rise and fall of the
barometer. Now let us consider the balloon as a globule of moisture of a
definite weight, and this globule only one of an aggregation of globules
sufficient to form a cloud. We can readily see from what has gone before
that a cloud thus formed, having a definite density and weight, would
float higher some days than others.

Assuming again that the density of the air remains the same from day to
day, the clouds will still float high or low in the atmosphere from
another cause. Let us go back to our illustration of the balloon. If we
have a fixed condition of atmosphere, external to the balloon, and vary
the conditions internally, which means varying its weight, the balloon
will float higher or lower as the internal conditions are varied. Now
apply this principle to the moisture globules of which a cloud is formed
and we can understand why a cloud will float high or low from the two
causes that we have described. Clouds are of different color and
density, and this is due to the differences of the make-up of the
moisture globules of which the clouds are formed. If these globules are
in an advanced stage of condensation the cloud is darker and more
opaque. In earlier conditions of condensation the cloud will have a
bright look, which shows that it reflects most of the light, whereas in
the case of the dark cloud the light is largely absorbed.

There is a sort of notion prevailing that clouds come up from the
horizon, and in many cases they do, but they may form directly over our
heads. There always has to be a beginning, and that occurs wherever the
conditions are most favorable for condensation of vapor. If the earth is
wet and the sun is hot the evaporation may be very rapid as well as the
ascent of the invisible moisture, which carries with it the air, which
in turn expands the higher it rises, thus producing cold. This, taken
with the normal cold that exists in the higher regions, may be
sufficient to produce a sudden condensation of this ascending vapor,
which is all that is necessary to form a cloud.

The inquiry may arise, Why is the moisture condensed, almost always, in
the upper regions of the air, where it is rare? Because the more rare
and therefore expanded it is, the mor