We know that in the depths of the earth
there is still plenty of heat, which every now and then makes its
presence felt by bursting up through the vents we call volcanoes, the
weak spots in the earth's crust; but our surface long ago cooled, for
the outside of any body gets cool before the inside, as you may have
found if ever you were trying to eat hot porridge, and circled round the
edge of the plate with a spoon. A large body cools more slowly than a
small one, and it is possible that Jupiter, being so much larger than we
are, has taken longer to cool. One reason we have for thinking this is
that he is so very light compared with his size--in other words, his
density is so small that it is not possible he could be made of
materials such as the earth is made of.

As I said, when we study him through telescopes we see just the
exterior, the outer envelope of cloud, and as we should expect, this
changes continually, and appears as a series of belts, owing to the
rotation of the planet. Jupiter's rotation is very rapid; though he is
so much greater than the earth, he takes less than half the time the
earth does to turn round--that is to say, only ten hours. His days and
nights of five hours each seem short to us, accustomed to measure
things by our own estimates. But we must remember that everything is
relative; that is to say, there is really no such thing as fast or slow;
it is all by comparison. A spider runs fast compared with a snail, but
either is terribly slow compared with an express train; and the speed of
an express train itself is nothing to the velocity of light.

In the same way there is nothing absolutely great or small; it is all by
comparison. We say how marvellous it is that a little insect has all the
mechanism of life in its body when it is so tiny, but if we imagine that
insect magnified by a powerful microscope until it appears quite large,
the marvel ceases. Again, imagine a man walking on the surface of the
earth as seen from a great distance through a telescope: he would seem
less than an insect, and we might ask how could the mechanism of life be
compressed into anything so small? Thus, when we say enormous or tiny we
must always remember we are only speaking by the measurements of our own
standards.

There is nothing very striking about Jupiter's orbit. He takes between
eleven and twelve of our years to get round the sun, so you see, though
his day is shorter, his year is longer than ours. An