This irregular bed shifts from month to month, from
day to day, and even from hour to hour; and the lines that are drawn
on the maps are only averages for the year or the season.
In the midst of these irregular, but continuous agencies, the rain
introduces a peculiar discontinuity, and turns irregularity into
discord. We have shown that the rain is an immediate cause of wind;
but how is the rain itself produced? For so marked an effect we
naturally seek a special cause; but no adequate single cause has
ever been discovered. The combination of many conditions, probably,
is necessary, such as a peculiar distribution of heat and moisture
and atmospheric movements; though the immediate cause of the fall of
rain is doubtless the rising, and consequent expansion and cooling,
of the saturated air.
The winds that blow hither and thither, vainly striving to restore
equilibrium to the atmosphere, burden themselves with the moisture
they absorb from the seas; and this moisture absorbs their heat,
retards their motion, and slowly modifies the forces which impel them.
Now when the saturated air, extending far above the surface of the
earth, and carried in its movements still higher, is relieved of an
incumbent weight of air, it becomes rarefied, and its temperature
and capacity for moisture are simultaneously diminished; its moisture,
suddenly precipitated, appears as a cloud, the particles of which
collect into rain-drops and fall to the earth.
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