55, see
Humboldt, 'Examen Critique de l'Hist. de Geographie', t. iv., p. 253.
The difference of temperature between the
p 315
equatorial and polar regions engenders two opposite currents in the upper
strata of the atmosphere and on the Earth's surface. Owing to the
difference between the rotatory velocity at the poles and at the equator,
the polar current is deflected eastward, and the equatorial current
westward. The great phenomena of atmospheric pressure, the warming and
cooling of the strata of air, the aqueous deposits, and even, as Dove has
correctly represented, the formation and appearance of clouds, alike depend
on the opposition of these two currents, on the place where the upper one
descends, and on the displacement of the one by the other. Thus the figures
of the clouds, which form an animated part of the charms of a landscape,
announce the processes at work in the upper regions of the atmosphere, and,
when the air is calm, the clouds will often present, on a bright summer sky,
the "projected image" of the radiating soil below.
Where this influence of radiation is modified by the relative position of
large continental and oceanic surfaces, as between the eastern shore of
Africa and the western part of the Indian peninsula, its effects are
manifested in the Indian monsoons, which change with the periodic variations
in the sun's declination,* and which were known to the Greek navigators
under the name of 'Hippalos'.
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