it would alter the _size_ of our
globe to a very considerable extent. If these oceans were transformed
into vapour, then the atmosphere, charged with mighty clouds, would have
a bulk some hundreds of times greater than that which it has at present.
Viewed from a distant planet, the cloud-laden atmosphere would indicate
the visible size of our globe, and its average density would accordingly
appear to be very much less than it is at present.

From these considerations it will be manifest that the discrepancy
between the size and the weight of Jupiter, as contrasted with our
earth, would be completely removed if we supposed that Jupiter was at
the present day a highly heated body in the condition of our earth
countless ages ago. Every circumstance of the case tends to justify this
argument. We have assigned the smallness of the moon as a reason why the
moon has cooled sufficiently to make its volcanoes silent and still. In
the same way the smallness of the earth, as compared with Jupiter,
accounts for the fact that Jupiter still retains a large part of its
original heat, while the smaller earth has dissipated most of its store.
This argument is illustrated and strengthened when we introduce other
planets into the comparison. As a general rule we find that the smaller
bodies, like the earth and Mars, have a high density, indicative of a
low temperature, while the giant planets, like Jupiter and Saturn, have
a low density, suggesting that they still retain a large part of their
original heat. We say "original heat" for the want, perhaps, of a more
correct expression; it will, however, indicate that we do not in the
least refer to the solar heat, of which, indeed, the great outer planets
receive much less than those nearer the sun. Where the original heat may
have come from is a matter still confined to the province of
speculation.

A complete justification of these views with regard to Jupiter is to be
found when we make a minute telescopic scrutiny of its surface; and it
fortunately happens that the size of the planet is so great that, even
at a distance of more millions of miles than there are days in the year,
we can still trace on its surface some significant features.

Plate XI. gives a series of four different views of Jupiter. They have
been taken from a series of admirable drawings of the great planet made
by Mr. Griffiths in 1897. The first picture shows the appearance of the
globe at 10h. 20m. Greenwich time on