in the weather. To these we must add what the astronomer calls
disturbing actions of the second order,--effects of the disturbances
themselves upon the action of the disturbing agencies,--effects of
the irregular winds upon the distribution of heat and rain, and upon
the action of lands and seas, mountains and plains. Though such
disturbances are comparatively insignificant in the motions of the
planets, yet in the weather they are often more important than the
primary causes.

The aggregate and permanent effect of all these disturbing causes,
primary and secondary, is seen in that irregular distribution of
climates, which the tortuous isothermal lines and the mottled
raincharts illustrate. The isothermal lines may be regarded as the
topographical delineations of that bed of temperatures down which
the upper atmosphere flows from the equator toward the poles, till
its downward tendency is balanced by the centrifugal force of its
eastward motion. 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.