in which these objects are discovered. It is by minute and diligent
comparison of the heavens with elaborate star charts that these bodies
are brought to light. Such enquiries would be equally efficacious in
searching for an ultra-Neptunian planet; in fact, we could design no
better method to seek for such a body, if it existed, than that which is
at this moment in constant practice at many observatories. The labours
of those who search for small planets have been abundantly rewarded with
discoveries now counted by hundreds. Yet it is a noteworthy fact that
all these planets are limited to one region of the solar system. It has
sometimes been conjectured that time may disclose perturbations in the
orbit of Neptune, and that these perturbations may lead to the discovery
of a planet still more remote, even though that planet be so distant and
so faint that it eludes all telescopic research. At present, however,
such an enquiry can hardly come within the range of practical astronomy.
Its movements have no doubt been studied minutely, but it must describe
a larger part of its orbit before it would be feasible to conclude, from
the perturbations of its path, the existence of an unknown and still
more remote planet.

We have thus seen that the planetary system is bounded on one side by
Mercury and on the other by Neptune. The discovery of Mercury was an
achievement of prehistoric times. The early astronomer who accomplished
that feat, when devoid of instrumental assistance and unsupported by
accurate theoretical knowledge, merits our hearty admiration for his
untutored acuteness and penetration. On the other hand, the discovery of
the exterior boundary of the planetary system is worthy of special
attention from the fact that it was founded solely on profound
theoretical learning.

Though we here close our account of the planets and their satellites, we
have still two chapters to add before we shall have completed what is to
be said with regard to the solar system. A further and notable class of
bodies, neither planets nor satellites, own allegiance to the sun, and
revolve round him in conformity with the laws of universal gravitation.
These bodies are the comets, and their somewhat more humble associates,
the shooting stars. We find in the study of these objects many matters
of interest, which we shall discuss in the ensuing chapters.

CHAPTER XVI.

COMETS.

Comets contrasted with Planets in Nature as well