When one fancies the world is about to end,--and this has been
believed for more than a thousand years,--no solicitude is felt in the
work of improving this world; and, by the indifference or disdain into
which one falls, periods of famine and general misery are induced
which at certain times have overtaken our community. Why use the
wealth of a world which is going to perish? Why work, be instructed,
or rise in the progress of the sciences or arts? Much better to forget
the world, and absorb one's self in the barren contemplation of an
unknown life. It is thus that ages of ignorance weigh on man, and
thrust him further and further into darkness, while Science makes
known by its influence on the whole community, its great value, and
the magnitude of its aim.

The history of a comet would be an instructive episode of the great
history of the heavens. In it could be brought together the
description of the progressive movement of human thought, as well as
the astronomical theory of these extraordinary bodies. Let us take,
for example, one of the most memorable and best-known comets, and give
an outline of its successive passages near the Earth. Like the
planetary worlds, Comets belong to the solar system, and are subject
to the rule of the Star King. It is the universal law of gravitation
which guides their path; solar attraction governs them, as it governs
the movement of the planets and the small satellites. The chief point
of difference between them and the planets is, that their orbits are
very elongated; and, instead of being nearly circular, they take the
elliptical form. In consequence of the nature of these orbits, the
same comet may approach very near the Sun, and afterwards travel from
it to immense distances. Thus, the period of the Comet of 1680 has
been estimated at three thousand years. It approaches the Sun, so as
to be nearer to it than our Moon is to us, whilst it recedes to a
distance 853 times greater than the distance of the Earth from the
Sun. On the 17th of December, 1680, it was at its perihelion--that is,
at its greatest proximity to the Sun; it is now continuing its path
beyond the Neptunian orbit. Its velocity varies according to its
distance from the solar body. At its perihelion it travels thousands
of leagues per minute; at its aphelion it does not pass over more than
a few yards. Its proximity to the Sun in its passage near that body
caused Newton to think that it received a heat twenty-eight