r would travel
thousands of miles without falling, before a wind having any
considerable velocity.

Suppose the sun to be shining with intense heat upon a certain area of
the earth's surface and the conditions to be right for very rapid
evaporation of moisture. The air which is heated close to the ground,
being expanded, will rise, together with the invisible particles of
moisture, and there will be a column of moisture-laden air continually
ascending until it reaches a point in the upper atmosphere where it is
condensed into a cloud that takes on the billowy form which in summer
time we call a thunder cloud, but which in the science of meteorology is
called cumulus, or heap-cloud. If there were no air currents this
billowy cloud would stand as the capping of an invisible pillar of
ascending vapor, but as it is never the case that air is not moving at
some velocity in the upper regions, it floats away as rapidly as it is
formed. This peculiar kind of cloud is formed in the mid-regions of the
atmosphere, and it is a summer cloud as well as a land cloud. Of course,
it may float off over the ocean and maintain its peculiar shape for a
certain distance, but it is rare that such a cloud would ever be seen in
mid-ocean or in midwinter. As the warm season advances in summer, and
evaporation from the earth is less than the rainfall, there is less and
less moisture in the air, when, of course, the conditions for cloud
formation, especially inland, are not so favorable as in the early
spring or summer. Frequently there comes a time when we have a long
season of dry, settled weather. Probably during most of the days clouds
will form and we think it is going to rain, but before night they have
vanished, and the same thing is repeated the next day and the next,
perhaps for weeks at a time.

The explanation is this: We have already said that so long as the air
remains in a uniform condition as to temperature it will absorb moisture
in a transparent state until it is filled to the measure of its capacity
at a given temperature. If there were no change of temperature, it would
not condense into cloud. Clouds may be absorbed into the atmosphere--or
evaporated--and become invisible; and this process is going on to a
greater or less degree continually. If we watch the steam as it escapes
from a steam boiler, the first effect is condensation into cloud, but as
it floats away it gradually melts and is absorbed into the atmosphere as
invis