in the
distant future the prospect of a glorious career as the residence of
organic life. The time will assuredly come when the internal heat must
decline, when the clouds will gradually condense into oceans. On the
surface dry land may then appear, and Jupiter be rendered habitable.

From this sketch of the planet itself we now turn to the interesting and
beautiful system of five satellites by which Jupiter is attended. We
have, indeed, already found it necessary to allude more than once to
these little bodies, but not to such an extent as to interfere with the
more formal treatment which they are now to receive.

The discovery of the four chief satellites may be regarded as an
important epoch in the history of astronomy. They are objects situated
in a remarkable manner on the border-line which divides the objects
visible to the unaided eye from those which require telescopic aid. It
has been frequently asserted that these objects have been seen with the
unaided eye; but without entering into any controversy on the matter, it
is sufficient to recite the well-known fact that, although Jupiter had
been a familiar object for countless centuries, yet the sharpest eyes
under the clearest skies never discovered the satellites until Galileo
turned the newly invented telescope upon them. This tube was no doubt a
very feeble instrument, but very little power suffices to show objects
so dose to the limit of visibility.

[Illustration: Fig. 61.--Jupiter and his Four Satellites as seen in a
Telescope of Low Power.]

The view of the planet and its elaborate system of satellites as shown
in a telescope of moderate power, is represented in Fig. 61. We here see
the great globe, and nearly in a line parsing through its centre lie
four small objects, three on one side and one on the other. These little
bodies resemble stars, but they can be distinguished therefrom by their
ceaseless movements around the planet, which they never fail to
accompany during his entire circuit of the heavens. There is no more
pleasing spectacle for the student than to follow with his telescope the
movements of this beautiful system.

[Illustration: Fig. 62.--Disappearances of Jupiter's Satellites.]

In Fig. 62 we have represented some of the various phenomena which the
satellites present. The long black shadow is that produced by the
interposition of Jupiter in the path of the sun's rays. In consequence
of the great distance of the sun this shadow will