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. Round or oblong spots,
as distinguished from belts, are still more variable and transient. The
main belts themselves show great internal commotion, frequently
splitting up, through a considerable part of their length, and sometimes
apparently throwing out projections into the lighter equatorial zone,
which occasionally resemble bridges, diagonally spanning the broad space
between the belts.

[Illustration: JUPITER AS SEEN AT THE LICK OBSERVATORY IN 1889. THE
GREAT RED SPOT IS VISIBLE, TOGETHER WITH THE INDENTATION IN THE SOUTH
BELT.]

Perhaps the most puzzling phenomenon that has ever made its appearance
on Jupiter is the celebrated "great red spot," which was first noticed
in 1878, although it has since been shown to be probably identical with
a similar spot seen in 1869, and possibly with one noticed in 1857.
This spot, soon after its discovery in 1878, became a clearly defined
red oval, lying near the southern edge of the south belt in latitude
about 30 deg. Its length was nearly one third of the diameter of the disk
and its width almost one quarter as great as its length. Translated into
terrestrial measure, it was about 30,000 miles long and 7,000 miles
broad.

In 1879 it seemed to