ir orbits could be and were fixed
with a very close approach to accuracy, the length of those orbits might
vary enormously without any very sensible difference being produced in
the small part of the curves traced out near the sun. Dr. Wilhelm Meyer,
however, arrived, by an elaborate discussion, at a period of
thirty-seven years for the comet of 1880,[1289] while the observations
of 1843 were admittedly best fitted by Hubbard's ellipse of 533 years;
but these Dr. Meyer supposed to be affected by some constant source of
error, such as would be produced by a mistaken estimate of the position
of the comet's centre of gravity. He inferred finally that, in spite of
previous non-appearances, the two comets represented a single regular
denizen of our system, returning once in thirty-seven years along an
orbit of such extreme eccentricity that its movement might be described
as one of precipitation towards and rapid escape from the sun, rather
than of sedate circulation round it.

The _geometrical_ test of identity has hitherto been the only one which
it was possible to apply to comets, and in the case before us it may
fairly be said to have broken down. We may, then, tentatively, and with
much hesitation, try a _physical_ test, though scarcely yet, properly
speaking, available. We have seen that the comets of 1843 and 1880 were
strikingly alike in general appearance, though the absence of a formed
nucleus in the latter, and its inferior brilliancy, detracted from the
convincing effect of the resemblance. Nor was it maintained when tried
by exact methods of inquiry. M. Bredikhine found that the gigantic ray
emitted in 1843 belonged to his type No. 1; that of 1880 to type No.
2.[1290] The particles forming the one were actuated by a repulsive
force ten times as powerful as those forming the other. It is true that
a second noticeably curved tail was seen in Chili, March 1, and at
Madras, March 11, 1843; and the conjecture was accordingly hazarded that
the materials composing on that occasion the principal appendage having
become exhausted, those of the secondary one remained predominant, and
reappeared alone in the "hydro-carbon" train of 1880. But the one known
instance in point is against such a supposition. Halley's comet, the
only _great_ comet of which the returns have been securely authenticated
and carefully observed, has preserved its "type" unchanged through many
successive revolutions. The dilemma presented to astronomer