us to interpret the nature of
this neighbour, and thereby to extend the conception of planetary
histories which we derive from our own earth.

[Illustration: Fig. 5.--Comparative Sizes of the Planets (Chambers).]

Beyond Mars we find one of the most singular features of our solar
system in a group of small planetary bodies, the number of which now
known amounts to some two hundred, and the total may be far greater.
These bodies are evidently all small; it is doubtful if the largest is
three hundred and the smaller more than twenty miles in diameter. So
far as it has been determined by the effect of their aggregate mass in
attracting the other spheres, they would, if put together, make a
sphere far less in diameter than our earth, perhaps not more than five
hundred miles through. The forms of these asteroids is as yet unknown;
we therefore can not determine whether their shapes are spheroidal, as
are those of the other planets, or whether they are angular bits like
the meteorites. We are thus not in a position to conjecture whether
their independence began when the nebulous matter of the ring to which
they belonged was in process of consolidation, or whether, after the
aggregation of the sphere was accomplished, and the matter solidified,
the mass was broken into bits in some way which we can not yet
conceive. It has been conjectured that such a solid sphere might have
been driven asunder by a collision with some wandering celestial body;
but all we can conceive of such actions leads us to suppose that a
blow of this nature would tend to melt or convert materials subjected
to it into the state of vapour, rather than to drive them asunder in
the manner of an explosion.

The four planets which lie beyond the asteroids give us relatively
little information concerning their physical condition, though they
afford a wide field for the philosophic imagination. From this point
of view the reader is advised to consult the writings of the late R.A.
Proctor, who has brought to the task of interpreting the planetary
conditions the skill of a well-trained astronomer and a remarkable
constructive imagination.

The planet Jupiter, by far the largest of the children of the sun,
appears to be still in a state where its internal heat has not so far
escaped that the surface has cooled down in the manner of our earth.
What appear to be good observations show that the equatorial part of
its area, at least, still glows from its own heat.