The sphere is
cloud-wrapped, but it is doubtful whether the envelope be of watery
vapour; it is, indeed, quite possible that besides such vapour it may
contain some part of the many substances which occupy the atmosphere
of the sun. If the Jovian sphere were no larger than the earth, it
would, on account of its greater age, long ago have parted with its
heat; but on account of its great size it has been able,
notwithstanding its antiquity, to retain a measure of temperature
which has long since passed away from our earth.

In the case of Saturn, the cloud bands are somewhat less visible than
on Jupiter, but there is reason to suppose in this, as in the
last-named planet, that we do not behold the more solid surface of the
sphere, but see only a cloud wrap, which is probably due rather to the
heat of the sphere itself than to that which comes to it from the sun.
At the distance of Saturn from the centre of the solar system a given
area of surface receives less than one ninetieth of the sun's heat as
compared with the earth; therefore we can not conceive that any
density of the atmosphere whatever would suffice to hold in enough
temperature to produce ordinary clouds. Moreover, from time to time
bright spots appear on the surface of the planet, which must be due to
some form of eruptions from its interior.

Beyond Saturn the two planets Uranus and Neptune, which occupy the
outer part of the solar system, are so remote that even our best
telescopes discern little more than their presence, and the fact that
they have attendant moons.

From the point of view of astronomical science, the outermost planet
Neptune, of peculiar interest for the reason that it was, as we may
say, discovered by computation. Astronomers had for many years
remarked the fact that the next inner planetary sphere exhibited
peculiarities in its orbit which could only be accounted for on the
supposition that it was subjected to the attraction of another
wandering body which had escaped observation. By skilful computation
the place in the heavens in which this disturbing element lay was so
accurately determined that when the telescope was turned to the given
field a brief study revealed the planet. Nothing else in the history
of the science of astronomy, unless it be the computation of eclipses,
so clearly and popularly shows the accuracy of the methods by which
the work of that science may be done.

As we shall see hereafter, in the chapters which