bolae, and any body whose orbit is of this
character can only be seen at a single apparition. The theory of
gravitation, though it admits the parabola as a possible orbit for a
comet, does not assert that the path must necessarily be of this type.
We have pointed out that this curve is only a very extreme type of
ellipse, and it would still be in perfect accordance with the law of
gravitation for a comet to pursue a path of any elliptical form,
provided that the sun was placed at the focus, and that the comet obeyed
the rule of describing equal areas in equal times. If a body move in an
elliptic path, then it will return to the sun again, and consequently we
shall have periodical visits from the same object.

An interesting field of enquiry was here presented to the astronomer.
Nor was it long before the discovery of a periodic comet was made which
illustrated, in a striking manner, the soundness of the anticipation
just expressed. The name of the celebrated astronomer Halley is,
perhaps, best known from its association with the great comet whose
periodicity was discovered by his calculations. When Halley learned from
the Newtonian theory the possibility that a comet might move in an
elliptic orbit, he undertook a most laborious investigation; he
collected from various records of observed comets all the reliable
particulars that could be obtained, and thus he was enabled to
ascertain, with tolerable accuracy, the nature of the paths pursued by
about twenty-four large comets. One of these was the great body of 1682,
which Halley himself observed, and whose path he computed in accordance
with the principles of Newton. Halley then proceeded to investigate
whether this comet of 1682 could have visited our system at any previous
epoch. To answer this question he turned to the list of recorded comets
which he had so carefully compiled, and he found that his comet very
closely resembled, both in appearance and in orbit, a comet observed in
1607, and also another observed in 1531. Could these three bodies be
identical? It was only necessary to suppose that a comet, instead of
revolving in a parabolic orbit, really revolved in an extremely
elongated ellipse, and that it completed each revolution in a period of
about seventy-five or seventy-six years. He submitted this hypothesis to
every test that he could devise; he found that the orbits, determined on
each of the three occasions, were so nearly identical that it would be
contr