ovian literature, so thoroughly and so frequently
have they been discussed.

And, having noticed these recurring features, the observer will begin to
note their relations to one another, and will thus be led to observe
that some of them gradually drift apart, while others drift nearer; and
after a time, without any aid from books or hints from observatories, he
will discover for himself that there is a law governing the movements on
Jupiter's disk. Upon the whole he will find that the swiftest motions
are near the equator, and the slowest near the poles, although, if he is
persistent and has a good eye and a good instrument, he will note
exceptions to this rule, probably arising, as Professor Hough suggests,
from differences of altitude in Jupiter's atmosphere. Finally, he will
conclude that the colossal globe before him is, exteriorly at least, a
vast ball of clouds and vapors, subject to tremendous vicissitudes,
possibly intensely heated, and altogether different in its physical
constitution, although made up of similar elements, from the earth.
Then, if he chooses, he can sail off into the delightful cloud-land of
astronomical speculation, and make of the striped and spotted sphere of
Jove just such a world as may please his fancy--for a world of some
kind it certainly is.

For many observers the satellites of Jupiter possess even greater
attractions than the gigantic ball itself. As I have already remarked,
their movements are very noticeable and lend a wonderful animation to
the scene. Although they bear classical names, they are almost
universally referred to by their Roman numbers, beginning with the
innermost, whose symbol is I, and running outward in regular order II,
III, and IV.[5] The minute satellite much nearer to the planet than any
of the others, which Mr. Barnard discovered with the Lick telescope in
1892, is called the fifth, although in the order of distance it would be
the first. In size and importance, however, it can not rank with its
comparatively gigantic brothers. Of course, no amateur's telescope can
afford the faintest glimpse of it.

[5] Their names, in the same order as their numbers, are Io, Europa,
Ganymede, and Callisto.

Satellite I, situated at a mean distance of 261,000 miles from Jupiter's
center--about 22,000 miles farther than the moon is from the earth--is
urged by its master's overpowering attraction to a speed of 320 miles
per minute, so that it performs a complete revolution