p. If the stick were
170 million miles long, the extent of the sweep would be not less than
500 million miles! Through such a stupendous curve did the comet of
1843 whirl its tail in two little hours as it rounded the solar orb.
It is hardly possible to believe, that one and the same material
substance could have been subjected to the force of such motion
without being shattered into a myriad fragments. Sir John Herschel
very beautifully suggests, that the comet's tail, during this
wonderful perihelion passage, resembled a negative shadow cast beyond
the comet, rather than a substantial body; a momentary impression made
upon the luminiferous ether where the solar influence was in temporary
obscuration. But this suggestion can only be received as an ingenious
and expressive hint; it cannot be taken as an explanation. There is as
much difficulty, as will be presently seen, in the way of admitting
that comets have shadows of any kind, as there would be in compassing
the idea that bodies of enormous length can be whirled round through
millions of miles in the minute. The truth is, the comet's tail is yet
an unguessed puzzle, and vexes even the wits of the wise. It keeps
grave men seated on the horns of a dilemma, so long as their attention
is fixed upon its capricious charms.

The comet's tail is always thrown out away from the sun, just as the
shadow of an opaque body in the same position would be. But this is
not all that can be said of it. It is not only cast away from the sun:
it is really cast _by the sun_--shadow-like, although not of the
nature of shadow. It only appears when the comet gets near to the
sun's effulgence, and is lost altogether when that body gets far from
the great source of mundane light and heat. It is raised from the
comet's body, by the power of sunshine, as mist is from damp ground.
When Halley's Comet of 1682 approached the fierce ordeal of its
perihelion position, the exhalation of its tail was distinctly
perceived. First, little jets of light streamed out towards the sun,
as if bursting forth elastically under the influence of the scorching
blaze; very soon these streams were stopped, and turned backwards by
the impulse of some new force, and as they flowed in this fresh
direction, became the diverging streaks of the tail. Not only a
vapour-forming power, but also a vapour-drifting power, is brought
into play in the process of tail formation; and this latter must be
some occult agent of consi