confidence in science, and Halley's prediction, after seeing and
studying the motion of the comet which appeared in 1682, that it would
prove to be a regular member of the sun's family and would be seen
returning after a period of about seventy-six years, together with the
fulfillment of that prediction, produced a revulsion from the
superstitious notions which had so long prevailed.

Then the facts were made plain that comets are subject to the law of
gravitation equally with the planets; that there are many which
regularly return to the neighborhood of the sun (perihelion); and that
these travel in orbits differing from those of the planets only in
their greater eccentricity, although they have the peculiarity that
they do not, like the planets, all go round the sun in the same
direction, and do not keep within the general plane of the planetary
system, but traverse it sometimes from above and sometimes from below.
Other comets, including most of the ``great'' ones, appear to travel
in parabolic or, in a few cases, hyperbolic orbits, which, not being
closed curves, never bring them back again. But it is not certain that
these orbits may not be extremely eccentric ellipses, and that after
the lapse of hundreds, or thousands, of years the comets that follow
them may not reappear. The question is an interesting one, because if
all orbits are really ellipses, then all comets must be permanent
members of the solar system, while in the contrary case many of them
are simply visitors, seen once and never to be seen again. The
hypothesis that comets are originally interlopers might seem to derive
some support from the fact that the certainly periodic ones are
associated, in groups, with the great outer planets, whose attraction
appears to have served as a trap for them by turning them into
elliptical orbits and thus making them prisoners in the solar system.
Jupiter, owing to his great mass and his commanding situation in the