a new class of celestial objects, somewhat loosely
distinguished as "comets of short period." These bodies (of which about
thirty have been found to circulate within the orbit of Saturn) are
remarkable as showing certain planetary affinities in the manners of
their motions not at all perceptible in the wider travelling members of
their order. They revolve, without exception, in the same direction as
the planets--from west to east; they exhibit a marked tendency to
conform to the zodiacal track which limits planetary excursions north
and south; and their paths round the sun, although much more eccentric
than the approximately circular planetary orbits, are far less so than
the extravagantly long ellipses in which comets comparatively untrained
(as it were) in the habits of the solar system ordinarily perform their
revolutions.

No _great_ comet is of the "planetary" kind. These are, indeed, only by
exception visible to the naked eye; they possess extremely feeble
tail-producing powers, and give small signs of central condensation.
Thin wisps of cosmical cloud, they flit across the telescopic field of
view without sensibly obscuring the smallest star. Their appearance, in
short, suggests--what some notable facts in their history will presently
be shown to confirm--that they are bodies already effete, and verging
towards dissolution. If it be asked what possible connection can be
shown to exist between the shortness of period by which they are
essentially characterised, and what we may call their _superannuated_
condition, we are not altogether at a loss for an answer. Kepler's
remark,[243] that comets are consumed by their own emissions, has
undoubtedly a measure of truth in it. The substance ejected into the
tail must, in overwhelmingly large proportion, be for ever lost to the
central mass from which it issues. True, it is of a nature inconceivably
tenuous; but unrepaired waste, however small in amount, cannot be
persisted in with impunity. The incitement to such self-spoliation
proceeds from the sun; it accordingly progresses more rapidly the more
numerous are the returns to the solar vicinity. Comets of short period
may thus reasonably be expected to _wear out_ quickly.

They are, moreover, subject to many adventures and vicissitudes. Their
aphelia--or the farthest points of their orbits from the sun--are
usually, if not invariably, situated so near to the path either of
Jupiter or of Saturn, as to permit these giant