eau has shown
experimentally that by rotating a globe of oil in a mixture of water and
spirit having the same density this process may be actually repeated in
miniature.

This brilliant, and yet simple, hypothesis is consistent with, and
explains many other circumstances connected with the position,
magnitude, and movements of the Planets and their satellites.

The Planets, for instance, lie more or less in the same plane, they
revolve round the Sun and rotate on their own axis in the same
direction--a series of coincidences which cannot be accidental, and for
which the theory would account. Again the rate of cooling would of
course follow the size; a small body cools more rapidly than a large
one. The Moon is cold and rigid; the Earth is solid at the surface, but
intensely hot within; Jupiter and Saturn, which are immensely larger,
still retain much of their original heat, and have a much lower density
than the Earth; and astronomers tell us on other grounds that the Sun
itself is still contracting, and that to this the maintenance of its
temperature is due.

Although, therefore, the Nebular Theory cannot be said to have been
absolutely proved, it has certainly been brought to a high state of
probability, and is, in its main features, generally accepted by
astronomers.

The question has often been asked whether any of the heavenly bodies are
inhabited, and as yet it is impossible to give any certain answer. It
seems _a priori_ probable that the millions of suns which we see as
stars must have satellites, and that some at least of them may be
inhabited. So far as our own system is concerned the Sun is of course
too hot to serve as a dwelling-place for any beings with bodies such as
ours. The same may be said of Mercury, which is at times probably ten
times as hot as our tropics. The outer planets appear to be still in a
state of vapour. The Moon has no air or water.

Mars is in a condition which most nearly resembles ours. All, however,
that can be said is that, so far as we can see, the existence of living
beings on Mars is not impossible.

COMETS

The Sun, Moon, and Stars, glorious and wonderful as they are, though
regarded with great interest, and in some cases worshipped as deities,
excited the imagination of our ancestors less than might have been
expected, and even now attract comparatively little attention, from the
fact that they are always with us. Comets, on the other hand, both as
rare and occasio