s, to seek fresh universes....

The original parabola is converted into an ellipse, if the imprudent
adventurer in returning to the Sun passes near some great planet, such
as Jupiter, Saturn, Uranus, or Neptune, and suffers its attraction. It
is then imprisoned by our system, and can no longer escape from it.
After reenforcement at the solar focus, it must return to the identical
point at which it felt the first pangs of a new destiny. Henceforward,
it belongs to our celestial family, and circles in a closed curve.
Otherwise, it is free to continue its rapid course toward other suns and
other systems.

* * * * *

As a rule, the telescope shows three distinct parts in a comet. There is
first the more brilliant central point, or _nucleus_, surrounded by a
nebulosity called the _hair_, or _brush_, and prolonged in a luminous
appendix stretching out into the _tail_. The _head_ of the comet is the
brush and the nucleus combined.

[Illustration: FIG. 53.--The tails of Comets are opposed to the Sun.]

It is usually supposed that the tail of a comet follows it throughout
the course of its peregrinations. Nothing of the kind. The appendix may
even precede the nucleus; it is always opposite the Sun,--that is to
say, it is situated on the prolongation of a straight line, starting
from the Sun, and passing through the nucleus (Fig. 53). The tail does
not exist, so long as the comet is at a distance from the orb of day;
but in approaching the Sun, the nebulosity is heated and dilates, giving
birth to those mysterious tails and fantastic streamers whose
dimensions vary considerably for each comet. The dilations and
transformations undergone by the tail suggest that they may be due to a
repulsive force emanating from the Sun, an electric charge transmitted
doubtless through the ether. It is as though Phoebus blew upon them
with unprecedented force.

Telescopic comets are usually devoid of tail, even when they reach the
vicinity of the Sun. They appear as pale nebulosities, rounded or oval,
more condensed toward the center, without, however, showing any distinct
nucleus. These stars are only visible for a minute fraction of their
course, when they reach a point not far from the Sun and the terrestrial
orbit.

The finest comets of the last century were those of 1811, 1843, 1858,
1861, 1874, 1880, 1881, and 1882. The Great Comet of 1811, after
spreading terror over certain peoples, notably in Russia