Our
clouds are formed by the action of the sun, producing evaporation of
water; on Jupiter, whose mean distance from the sun is more than five
times as great as ours, the intensity of the solar rays is reduced to
less than one twenty-fifth part of their intensity on the earth, so that
the evaporation can not be equally active there, and the tendency to
form aerial currents and great systems of winds must be proportionally
slight. In brief, the clouds of Jupiter are probably of an entirely
different origin from that of terrestrial clouds, and rather resemble
the chaotic masses of vapor that enveloped the earth when it was still
in a seminebulous condition, and before its crust had formed.
Although the strongest features of the disk of Jupiter are the great
cloud belts, and the white or colored spots in the equatorial zone, yet
the telescope shows many markings north and south of the belts,
including a number of narrower and fainter belts, and small light or
dark spots. None of them is absolutely fixed in position with
reference to others. In other words, all of the spots, belts, and
markings shift their places to a perceptible extent, the changes being
generally very slow and regular, but occasionally quite rapid. The main
belts never entirely disappear, and never depart very far from their
mean positions with respect to the equator, but the smaller belts toward
the north and south are more or less evanescent.
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