The two last-named comets would likewise seem to have a period of
revolution not exceeding five or six years, and their aphelia are in the
vicinity of Jupiter's orbit. Among the comets that have a period of
revolution of from seventy to
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seventy-six years, the first in point of importance with respect to
theoretical and physical astronomy is Halley's comet, whose last appearance,
in 1835, was much less brilliant than was to be expected from preceding
ones; next we would notice Olbers's comet, discovered on the 6th of March,
1815; and, lastly, the comet discovered by Pons in the year 1812, and whose
elliptic orbit has been determined by Encke. The two latter comets were
invisible to the naked eye. We now know with certainty of nine returns of
Halley's large comet, it having recently been proved by Laugier's
calculations*, that in the Chinese table of comets, first made known to us
by Edward Biot, the comet of 1378 is identical with Halley's; its periods of
revolution have varied in the interval between 1378 and 1835 from 74.91 to
77.58 years, the mean being 76.1.
[footnote] *Laugier, in the 'Comptes Rendus des Seances de l'Academie',
1843, t. xvi., p. 1006.
A host of other comets may be contrasted with the cosmical bodies of which
we have spoken, requiring several thousand years to perform their orbits,
which it is difficult to determine with any degree of certainty.
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