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 are devoted to
terrestrial phenomena, the physical condition of the sun determines
the course of all the more important events which take place on the
surface of the earth. It is therefore fit that in this preliminary
study of the celestial bodies, which is especially designed to make
the earth more interpretable to us, we should give a somewhat special