The irregular distribution of land and water, and the peculiar
action of each in imparting the heat of the sun to the incumbent air,--
the irregular distribution of plains and mountains, and their various
effects in different positions and at different altitudes,--the
distribution of heat effected by ocean currents,--all these tend to
produce permanent derangements of climate and great irregularities
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.
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