reover, since a body thus affected should necessarily return at
each revolution to the scene of encounter, the same process of
retardation may, in some cases, have been repeated many times, until the
more restricted cometary orbits were reduced to their present
dimensions. The prevalence, too, among periodical comets, of direct
motion, is shown to be inevitable by M. Callandreau's demonstration that
those travelling in a retrograde direction would, by planetary action,
be thrown outside the probable range of terrestrial observation. The
scarcity of hyperbolic comets can be similarly explained. They would be
created whenever the attractive influence of the disturbing planet was
exerted in a forward or accelerative sense, but could come only by a
rare exception to our notice. The inner planets, including the earth,
have also unquestionably played their parts in modifying cometary
orbits; and Mr. Plummer suggests, with some show of reason, that the
capture of Encke's comet may be a feat due to Mercury.[268]

No _great_ comet appeared between the "star" which presided at the birth
of Napoleon and the "vintage" comet of 1811. The latter was first
described by Flaugergues at Viviers, March 26, 1811; Wisniewski, at
Neu-Tscherkask in Southern Russia, caught a final glimpse of it, August
17, 1812. Two disappearances in the solar rays as the earth moved round
in its orbit, and two reappearances after conjunction, were included in
this unprecedentedly long period of visibility of 510 days. This
relative permanence (so far as the inhabitants of Europe were concerned)
was due to the high northern latitude attained near perihelion, combined
with a certain leisureliness of movement along a path everywhere
external to that of the earth. The magnificent luminous train of this
body, on October 15, the day of its nearest terrestrial approach,
covered an arc of the heavens 23-1/2 degrees in length, corresponding to
a real extension of one hundred millions of miles. Its form was
described by Sir William Herschel as that of "an inverted hollow cone,"
and its colour as yellowish, strongly contrasted with the bluish-green
tint of the "head," round which it was flung like a transparent veil.
The planetary disc of the head, 127,000 miles across, appeared to be
composed of strongly-condensed nebulous matter; but somewhat
eccentrically situated within it was a star-like nucleus of a reddish
tinge, which Herschel presumed to be solid, and ascertaine