In March of the year 1843, a remarkable beam of light shot suddenly
out from the evening twilight, trailing itself along the surface of
the heavens, beneath the belt stars of Orion. That glimmering beam was
the tail of a comet just whisked into our northern skies, as the rapid
wanderer skirted their precincts in its journey towards the sun. To
the watchful eyes of our latitudes, the unexpected visitant presented
an aspect that was coy and modest in the extreme; its head, indeed,
was scarcely ever satisfactorily in sight. But it dealt far otherwise
with the more favoured climes of the south. At the Cape of Good Hope,
it was seen distinctly in full daylight, and almost touching the solar
disk; and at night appeared with the brilliancy of a first-class star,
with a luminous band flowing out from it to a distance some hundred
times longer than the moon's face is wide. Few persons who caught a
glimpse of that shining tail, either as it fitfully revealed itself in
our heavens, or as it steadily blazed upon the opposite hemisphere of
the earth, were led to form adequate notions of the magnificence of
the object they were contemplating. No one, unaided by the teaching of
science, could have conceived that the streak of light, so readily
compressed within the narrow limits of an eye-glance, stretched out
170 millions of miles in length.
The comet comes from regions of unknown remoteness, and rushes, with
continually increasing speed, towards our own source of warmth and
light--the genial sun. When it has reached within a certain distance
of this object, it appears, however, to overshoot the mark of its
desire, as if too ardent in the chase, and then sways round with
fearful impetus, beginning reluctantly to settle out into space again,
and moving with less and less velocity as it goes, until its misty
form is once more withdrawn by distance from human sight. When the
comet of 1813 swept round the sun in this way, it was so near to the
shining surface of the solar orb, that it must have been rushing for
the time through a temperature forty-seven thousand times higher than
any which the torrid region of the earth ever feels. Such heat would
have been twenty-four times more than enough to melt rock-crystal. The
overburdened sense experiences a feeling of relief in the mere
knowledge, that the comet passed this fiery ordeal as the lightning's
flash might have done. In two short hours, it had shifted its place
from one side to the other of the solar sphere. In sixty little
minutes, it had moved from a region in which the heat was forty
thousand times greater than the fiercest burning of the earth's torrid
zone, into another, in which the temperature was four times less. The
comet might well have a glowing tail as it came from such a realm of
fire. Flames that were colder by many hundred times, would make the
dull black iron shine with incandescent brightness.
As, however, it is the comet's nature to guard its ornamental
appendages with jealous care, it may be conceived that this tail of
170 million miles might prove a somewhat troublesome travelling
companion in so rapid a journey. Comets always turn their tails
prudentially out of harm's way as they whisk through the neighbourhood
of the solar blaze. In whatever direction these bodies may be moving,
they are always seen to project their caudal beams directly _from_ the
sun. Imagine the case of a rigid straight stick, held by one end in
the hand, and brandished round through a half-circle. The outer end of
the stick would move through a considerable sweep. 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 considerable interest in a scientific point of
view, as well as of considerable importance in a dynamic one, for it
is a principle evidently antagonistic to the great prevailing
attribute of gravitation, so universally present in matter. The
comet's tail is the only substance known that is repelled instead of
being attracted by the sun.
The repulsive power to which the development of the comet's tail is
due, is one of extraordinary energy. The comet of 1680 shot out its
tail through something like 100 million miles in a couple of days.
Most probably, much of the matter that is thus thrown off from the
cometic nucleus is never collected again, but is dissipated into
space, and lost for ever to the comet. The tail of the comet of 1680
was seen in its greatest brilliancy soon before the solar approach;
this was, however, an exception to the general rule. Comets nearly
always have the finest tails, and present altogether the most
beautiful appearance, immediately _after_ they have been in the
closest proximity to the sun.
The comet's tail seems, in reality, to be a thin oblong case of
vapour, formed out of the cometic substance by the increasing
intensity of the sunshine, and enclosing the denser portion of that
substance at one end. The diverging streams which it displays upon the
sky are merely the retiring edges of the rounded case, where the
greatest depth of luminous matter comes into sight. As the comet nears
the sun, much of its substance is vaporised for the construction of
this envelope; but as it goes off again into remoteness, the vaporous
envelope is once more condensed. The tail may then be seen to flow
back towards the head, out of which it was originally derived.
But here, again, a difficulty presents itself. The comet's tail is
believed by most of the illustrious astronomers of the day, to be the
body converted into vapour by solar influence. If it be so, the
vaporising process must be a much more subtile one than any that could
be performed in our alembics, for the comet's substance is already all
vapour before the distillation commences. The faintest stars have been
seen shining through the densest parts of comets without the slightest
loss of light, although they would have been effectually concealed by
a trifling mist extending a few feet from the earth's surface. Most
comets appear to have bright centres--nuclei, as they are called; but
these nuclei are not solid bodies, for as soon as they are viewed by
powerful telescopes, they become as diffused and transparent as the
fainter cometic substance. Comets are properly atmospheres without
contained spheres; enormous clouds rushing along in space, and bathed
with its sunshine, for they have no light excepting sunlight. They
become brighter and brighter as they get deeper within the solar
glare, and dimmer and paler as they float outwards from the same. The
light of the comet only differs from the light of a cloud that is
drifted across the cerulean sky of noon, in the fact, that it is
reflected from the inside as well as the outside of the vaporous
substance. The material illuminated reflects light, and is permeated
by light, at once. In this respect it resembles air as much as
cloud--the blueness of the sky is the sunlit air seen through the
lower and inner strata of itself. In the same way, the whiteness of
the comet is sunlit vapour seen through portions of itself. The
sunbeams pass as readily through the entire thickness of the cometic
substance as they do through our own highly permeable atmosphere.
The belief in the comet's surpassing thinness and lightness is not a
mere speculative opinion. It rests upon incontrovertible proof. In
1770, Lexell's Comet passed within six times the moon's distance of
the earth, and was considerably retarded in its motion by the
terrestrial attraction. If its mass had been of equal amount with the
earth's mass, its attraction would have influenced the earth's
movement in a like degree in return, and the earth would have been so
held back in its orbitual progress in consequence, that the year would
have been lengthened to the extent of three hours. The year was not,
however, lengthened on that occasion by so much as the least
perceptible fraction of a second; hence it can be shewn, that the
comet must have been composed of some substance many thousand times
lighter than the terrestrial substance. Newton was of opinion, that a
few ounces of matter would be sufficient for the construction of the
largest comet's tail.
Light as the comet's substance is, it is not, however, light enough to
escape the grasp of the sun's gravitating attraction. When the mass of
thin vapour is rushing through the obscurity of starlit space, so far
from the sun that the solar sphere looks but the brightest of the
stellar host, it feels the influence of the solar mass, remote as it
is, and is constrained to bend its course towards it. Onwards the thin
vapour goes, the sun waxing bigger and bigger with each stage of
approach, until at last the little star has become a fiery globe,
filling up half the heavens with its vast proportions, and stretching
from the horizon to the zenith of the visible concave. The great comet
of 1680 came in this way from a region of space where the sun looked
but half as wide as the planet Mars in the sky, and where the solar
heat was imperceptible, the surrounding temperature being 612 degrees
colder than freezing water, into another in which the sun filled up
140 times greater width of the sky than it does with us, and where the
heat was some hundred times higher than the temperature of boiling
water. It was then only 880,000 miles away from the solar surface, and
would have fallen to it in three minutes, in obedience to its
attraction, if the impetus of its motion in a different direction had
been on the instant destroyed or arrested. But this impetus proved too
great for the attraction, light as the material of the moving body
was. When the comet has approached comparatively near to the grand
source of attraction, the speed of its accelerating motion has become
so excessive, that it is able to withstand the augmented solicitation
it is subjected to, and move outwards in a more direct course. It
goes, however, slower and slower, and curving its journey less and
less, until at last its motion in remote obscurity is again so
sluggish, that the sun's attraction is once more predominant, and able
to recall the truant towards its realms of light. Such is the history
of the comet's course.
Thin comet vapours drift through space, sustained by exactly the same
influences that uphold dense planetary spheres. They are supported in
the void by the combined effects of motion and attraction. Their own
impetus strives to carry them one way, while the sun's attraction
draws them another, and they are thus constrained to move along paths
that are intermediate to the lines of the two impulses. Now, when
bodies are driven in this way by two differently acting powers, they
must travel along curved lines, if both the driving forces are in
continued operation, for a new direction of motion is then impressed
on them at each succeeding instant. There are three kinds of curved
lines along which bodies thus doubly driven may move: the _circular_
curve, which goes round a central point at an unvarying equal
distance, and returns into itself; the _elliptical_ curve, which
returns into itself by a route that is drawn out considerably in one
direction; and the _hyperbolic_ curve, that never returns into itself
at all, but has, on the other hand, a course which sets outwards each
way for ever. The _parabolic_ curve, as it is called, is a line
partaking of the closeness of the ellipse on the one hand, and the
openness of the hyperbola on the other. A parabola is an ellipse
passing into a hyperbola; or, in other words, it is a part of an
ellipse whose length, compared with its breadth, is too great to be
estimated, and is consequently deemed to be endless for all practical
purposes.
In most instances, comets move in space, about the sun, in ellipses so
very lengthened, that their paths seem to be parabolas as long as the
cloudy bodies are visible in the sky. Two of them, Ollier's Comet and
Halley's, are known to return into sight after intervals of
seventy-four and seventy-six years, during which they have visited
portions of space a few hundred millions of miles further than the
orbit of Neptune. Six comets travel in elliptical orbits that are
never so far from the sun as the planet Neptune, and return into
visibility in short periods that never exceed seven or eight years.
These interior comets of short period seem to be regular members of
our world-system in the strictest sense. Their paths, although more
eccentric, are all contained in planes that nearly correspond with the
planes of the planetary orbits, and they travel in these paths in the
same general direction with their planetary brethren in every case.
The planetoid comets of short period are--Encke's, De Vico's,
Brorsen's, D'Arrest's, Biela's, and Fage's. The comet of 1843 is half
suspected to belong to the group, and to be also a periodic body,
revisiting our regions punctually at intervals of twenty-one years.
The comet's motions strikingly illustrate the almost absolute voidness
of space. If the thin vapour experienced any resistance while moving,
its free passage would be checked, although that resistance was many
thousand times less than the one the hand feels when waved in the air.
It is found, however, that Encke's Comet does indicate the presence of
some such resistance. It goes slower and slower with each return, and
contracts the dimensions of its elliptical journey progressively. But
it must be remembered, that this is one of the close comets that never
gets well out of the solar domain in which our neighbouring planets
float. The resisting medium which opposes its journey may be merely an
ethereal solar atmosphere surrounding the sun, as our air surrounds
the earth, but spreading to distances of millions instead of tens of
miles. On the other hand, it must be remembered also that starlight
passes through universal space, and is everywhere spread out therein,
and that it is hardly possible to think of starlight as an existence
without some sort of material reality. Some physicists believe that
Encke's Comet, with its retarded motions, will some day fall into the
sun; while others fancy that such a consummation can never take place,
because successive portions of its substance will be thrown off by the
tail-forming process with each perihelion return; so that long before
the cometic mass could reach the sun, it will have been altogether
dissipated into space, and nothing will be left to accomplish the
final state of the fall.
The great peculiarity of cometic paths, as compared with the planetary
ones, is, that they consist of ellipses of very much more eccentric
proportions; and that, therefore, the bodies moving in them, go
alternately to much greater and less distances from the sun than the
planets do. It must not be imagined, however, that all comets revolve
about the sun even in the most lengthened ellipses. Three at
least--the comets of 1723, 1771, and 1818--are known to have moved
along hyperbolic paths instead of parabolic or elliptical ones. These
comets, therefore, can make but one appearance in our skies. Having
once shewn themselves there, and vanished, they are lost to us for
ever. They are but stray and chance visitors to the domains of our
sun, and refuse to submit themselves, with the more regular members of
their fraternity, to the regulation-arrangements of our system, or to
appear punctually at the systematic roll-call therein instituted. They
are the true free-wanderers of the Infinite, passing from shore to
shore of immensity, and presenting themselves, for short and uncertain
intervals, to star after star. When they flit through our skies, they
shew themselves in all possible positions, and move along all possible
directions. They sometimes, however, yield too much to temptation, and
have to suffer the penalty of a short imprisonment in consequence.
Lexell's Comet, for instance, rushed in its hyperbolic path too near
to Jupiter, and was caught in the attraction of its mass, and made to
dance attendance on the sun through two successive elliptical
revolutions. At the end of the second, the influence that had
impounded the comet came, however, into play oppositely, and restored
it again to its wandering life and hyperbolic courses. Its cloudy form
has not presented itself amongst our stars since 1770, when its visit
was thus strangely received by Jupiter.
Twenty-three comets were seen by the naked eye during the sixteenth
century, 12 were seen in the seventeenth, 8 in the eighteenth, and 9
in the first half of the nineteenth. This does not, however, give
anything like an adequate idea of the number of comets really in
existence. When Kepler was asked how many comets he thought there
were, he answered: 'As many as there are fishes in the sea.' And
modern science seems determined, that the sagacious German shall not
be at fault even in this predication. Two or three fresh telescopic
comets are now usually found out every year. In 1847, 178 comets were
known to be moving in parabolic orbits, and therefore to be in some
way permanent connections of our world-system. Lalande has enumerated
700 comets, but Arago believes that not less than 7,000,000 exist,
which fall at some time or other within the reach of our sun's
influence.