The distance thus traversed by first-class
comets, in rambling about from sun to sun,
is expressed by a numeration-sum of figures
so long that the mind fails to appreciate
their value. The tails of these great comets
are transparent and composed of very
rarified nebulous matter, through which the
smallest stars can shine without losing their
brightness; but is the head or nucleus of
these astral vagabonds equally nebulous and
equally transparent? That is one of the
points in question: a question not easy to
settle, so rare is it to observe the head of
a comet passing over a fixed star. If we
trust to observations more or less worthy
of confidence which are recorded in
astronomical works, small stars as far as
the sixth or seventh magnitude have been
seen to shine through the central bodies
of certain comets, whilst the bodies of
other comets have completely eclipsed
the stars before which they passed. There
are, therefore, contradictory observations,
which prevent us from laying down any
absolute principle. Consequently, we are
obliged to admit, provisionally, that there
exist comets without a nucleus and comets
with a nucleus.

There can be no doubt about the physical
constitution of the bodiless comets; Sir John
Herschel compares them to the tail-part of
the great comets; they are immense heaps
of cloudy matter excessively rarified, very
variable in their form and in the intensity
of their light, and illumined by the sun in
the interplanetary spaces. Sir John declared
that the most enormous of these tails could
only be trifling in its mass, weighing two or
three pounds altogether; perhaps less.
Consequently, the shock of such a comet upon a
planet, supposing them to meet, would be
feebler than that of a swallow dashing against
a railway train at full speed. Our illustrious
countryman founded his theory on an
experiment related by Newton in the third book
of his Principia; namely, that a globe of air
of the ordinary density and of the diameter
of a small orange, if rarified to the degree
that would be produced by its elevation to a
height equal to the earth's radius, would
occupy a sphere whose radius would be longer
than that of Saturn's orbit. A philosopher
of the present day, Monsieur Babinet, makes
great use of this deduction which Herschel
has drawn from the principle laid down by
Newton. He endeavours to put a stop to
those epidemic terrors which always occur
whenever a comet appears; he attempts to
prove not only that the small periodical
comets and the tails of the great comets are
light collections of vapour; visible nothings,
incapable of causing the slightest injury to
our globe: also that the case is exactly
the same with respect to the nucleus of the
great comets. To some the doctrine appears
adventurous, and that, by trying to prove too
much, it proves nothing. It may be accepted
safely as to the little comets and the tails of
the great ones; but a certain amount of
reserve is prudent, as far as the bodies of the
great comets are concerned.

During the greater part of the month of
September everybody's eyes have been
directed towards the heavens to admire the
brilliant stranger who shines in the
neighbourhood of the Great Bear; but, what
everybody does not know is, that besides this
comet visible to the naked eye, two others
might be seen by the aid of the telescope.
Neither of these three celestial wanderers is
the famous comet of Charles the Fifth, so
much talked of last year. The first comet,
that visible to the unassisted eye, Donati's
comet, is the fifth whose appearance has been
recorded during the present year.

It is useless to describe the aspect of a
heavenly body which has been so recently
admired by all beholders. While this is written,
it is still visible to the left of the Great Bear,
and below it, till between eight and nine in the
evening, re-appearing in the north-east about
four in the morning. Its motion has been a
simultaneous approach both towards the
earth and towards the sun. On the fifteenth
of September it was distant from the earth a
hundred and fifteen millions of miles. At
that date the comet, as seen through one of
Monsieur Foucault's cheap new-invented
telescopes, presented a brilliant spherical
nucleus whose diameter may be estimated at
about two thousand five hundred miles.
This body of the comet, if such it may be
called, was surrounded by an extremely
transparent nebulosity or cloudiness which
stretched itself out in the form of a tail
towards the side opposed to the sun. The
length of the tail might be roughly reckoned
at between thirty or forty millions of miles,
it being difficult to come within the limits of
an odd million or so with material so fugitive
and attenuated. The comet has passed in
front of numerous fixed stars, which are seen
shining through its tail, even quite close to
the body, without suffering any but a very
slightly appreciable diminution of the intensity
of their light, so transparent is the
gauzy or gaseous substance of this long-
extended tail; compared to which the dust
which follows a stage-coach on the road is
solid and substantial. On the thirtieth, the
comet had advanced to within seventy
millions of miles. At the latter date, its
perihelion, or nearest approach to the sun, is
measured by fifty-five millions of miles.

The two other comets, not being visible to
the naked eye, are less interesting to common
observers. Encke's comet, famous for the
theories deduced from the gradual shortening
of its period,* was first perceived in Europe
at the beginning of September. Towards the
end of September, it was situated near the
middle of a straight line joining the stars
alpha and beta of the Lion. It approached

* See "The Ether," Household Words, volume xvii.,
page 558.