there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years later, the case
was different. Edmund Halley, who afterwards succeeded Flamsteed as
Astronomer Royal, calculated the elements of its orbit on Newton's
principles, and found them to resemble so closely those similarly
arrived at for comets observed by Peter Apian in 1531, and by Kepler in
1607, as almost to compel the inference that all three were apparitions
of a single body. This implied its revolution in a period of about
seventy-six years, and Halley accordingly fixed its return for 1758-9.
So fully alive was he to the importance of the announcement that he
appealed to a "candid posterity," in the event of its verification, to
acknowledge that the discovery was due to an Englishman. The prediction
was one of the test-questions put by Science to Nature, on the replies
to which largely depend both the development of knowledge and the
conviction of its reality. In the present instance, the answer afforded
may be said to have laid the foundation of this branch of astronomy.
Halley's comet punctually reappeared on Christmas Day, 1758, and
effected its perihelion passage on the 12th of March following, thus
proving beyond dispute that some at least of these erratic bodies are
domesticated within our system, and strictly conform, if not to its
unwritten customs (so to speak), at any rate to its fundamental laws.
Their movements, in short, were demonstrated by the most unanswerable of
all arguments--that of verified calculation--to be _calculable_, and
their investigation was erected into a legitimate department of
astronomical science.

This notable advance was the chief _result_ obtained in the field of
inquiry just now under consideration during the eighteenth century. But
before it closed, its cultivation had received a powerful stimulus
through the invention of an improved _method_. The name of Olbers has
already been brought prominently before our readers in connection with
asteroidal discoveries; these, however, were but chance excursions from
the path of cometary research which he steadily pursued through life. An
early predilection for the heavens was fixed in this particular
direction by one of the happy inspirations of genius. As he was
watching, one night in the year 1779, by the sick-bed of a
fellow-student in medicine at Gottingen, an important simplification in
the mode of computing the paths of comet