_RECENT COMETS_ (_continued_)
The mystery of comets' tails had been to some extent penetrated; so far,
at least, that, by making certain assumptions strongly recommended by
the facts of the case, their forms can be, with very approximate
precision, calculated beforehand. We have, then, the assurance that
these extraordinary appendages are composed of no ethereal or
supersensual stuff, but of matter such as we know it, and subject to the
ordinary laws of motion, though in a state of extreme tenuity.
Olbers, as already stated, originated in 1812 the view that the tails of
comets are made up of particles subject to a force of electrical
repulsion proceeding from the sun. It was developed and enforced by
Bessel's discussion of the appearances presented by Halley's comet in
1835. He, moreover, provided a formula for computing the movement of a
particle under the influence of a repulsive force of any given
intensity, and thus laid firmly the foundation of a mathematical theory
of cometary emanations. Professor W. A. Norton, of Yale College,
considerably improved this by inquiries begun in 1844, and resumed on
the apparition of Donati's comet; and Dr. C. F. Pape at Altona[1267]
gave numerical values for the impulses outward from the sun, which must
have actuated the materials respectively of the curved and straight
tails adorning the same beautiful and surprising object.
The _physical_ theory of repulsion, however, was, it might be said,
still in the air. Nor did it even begin to assume consistency until
Zöllner took it in hand in 1871.[1268] It is perfectly well ascertained
that the energy of the push or pull produced by electricity depends
(other things being the same) upon the _surface_ of the body acted on;
that of gravity upon its _mass_. The efficacy of solar electrical
repulsion relatively to solar gravitational attraction grows,
consequently, as the size of the particle diminishes. Make this small
enough, and it will virtually cease to gravitate, and will
unconditionally obey the impulse to recession.
This principle Zöllner was the first to realise in its application to
comets. It gives the key to their constitution. Admitting that the sun
and they are similarly electrified, their more substantially aggregated
parts will still follow the solicitations of his gravity, while the
finely divided particles escaping from them will, simply by reason of
their minuteness, fall under the sway of his repellent electric power.
They will, in other words, form "tails." Nor is any extravagant
assumption called for as to the intensity of the electrical charge
concerned in producing these effects. Zöllner, in fact, showed[1269]
that it need not be higher than that attributed by the best authorities
to the terrestrial surface.
Forty years have elapsed since M. Brédikhine, director successively of
the Moscow and of the Pulkowa Observatories, turned his attention to
these curious phenomena. His persistent inquiries on the subject,
however, date from the appearance of Coggia's comet in 1874. On
computing the value of the repulsive force exerted in the formation of
its tail, and comparing it with values of the same force arrived at by
him in 1862 for some other conspicuous comets, it struck him that the
numbers representing them fell into three well-defined classes. "I
suspect," he wrote in 1877, "that comets are divisible into groups, for
each of which the repulsive force is perhaps the same."[1270] This idea
was confirmed on fuller investigation. In 1882 the appendages of
thirty-six well-observed comets had been reconstructed theoretically,
without a single exception being met with to the rule of the three
types. A further study of forty comets led, in 1885, only to a
modification of the numerical results previously arrived at.
In the first of these, the repellent energy of the sun is fourteen times
stronger than his attractive energy;[1271] the particles forming the
enormously long straight rays projected outward from this kind of comet
leave the nucleus with a mean velocity of just seven kilometres per
second, which, becoming constantly accelerated, carries them in a few
days to the limit of visibility. The great comets of 1811, 1843, and
1861, that of 1744 (so far as its principal tail was concerned), and
Halley's comet at its various apparitions, belonged to this class. Less
narrow limits were assigned to the values of the repulsive force
employed to produce the second type. For the axis of the tail, it
exceeds by one-tenth (= 1·1) the power of solar gravity; for the
anterior edge, it is more than twice (2·2), for the posterior only half
as strong. The corresponding initial velocity (for the axis) is 1,500
metres a second, and the resulting appendage a scimitar-like or plumy
tail, such as Donati's and Coggia's comets furnished splendid examples
of. Tails of the third type are constructed with forces of repulsion
from the sun ranging from one-tenth to three-tenths that of his gravity,
producing an accelerated movement of attenuated matter from the nucleus,
beginning at the leisurely rate of 300 to 600 metres a second. They are
short, strongly bent, brush-like emanations, and in bright comets seem
to be only found in combination with tails of the higher classes.
Multiple tails, indeed--that is, tails of different types emitted
simultaneously by one comet--are perceived, as experience advances and
observation becomes closer, to be rather the rule than the
exception.[1272]
Now what is the meaning of these three types? Is any translation of them
into physical fact possible? To this question Brédikhine supplied, in
1879, a plausible answer.[1273] It was already a current surmise that
multiple tails are composed of different kinds of matter, differently
acted on by the sun. Both Olbers and Bessel had suggested this
explanation of the straight and curved emanations from the comet of
1807; Norton had applied it to the faint light tracks proceeding from
that of Donati;[1274] Winnecke to the varying deviations of its more
brilliant plumage. Brédikhine defined and ratified the conjecture. He
undertook to determine (provisionally as yet) the several kinds of
matter appropriated severally to the three classes of tails. These he
found to be hydrogen for the first, hydro-carbons for the second, and
iron for the third. The ground of this apportionment is that the atomic
weights of these substances bear to each other the same inverse
proportion as the repulsive forces employed in producing the appendages
they are supposed to form; and Zöllner had pointed out in 1875 that the
"heliofugal" power by which comets' tails are developed would, in fact,
be effective just in that ratio.[1275] Hydrogen, as the lightest known
element--that is, the least under the influence of gravity--was
naturally selected as that which yielded most readily to the
counter-persuasions of electricity. Hydro-carbons had been shown by the
spectroscope to be present in comets, and were fitted by their specific
weight, as compared with that of hydrogen, to form tails of the second
type; while the atoms of iron were just heavy enough to compose those of
the third, and, from the plentifulness of their presence in meteorites,
might be presumed to enter, in no inconsiderable proportion, into the
mass of comets. These three substances, however, were by no means
supposed to be the sole constituents of the appendages in question. On
the contrary, the great breadth of what, for the present, were taken to
be characteristically "iron" tails was attributed to the presence of
many kinds of matter of high and slightly different specific
weights;[1276] while the expanded plume of Donati was shown to be, in
reality, a whole system of tails, made up of many substances, each
spreading into a separate hollow cone, more or less deviating from, and
partially superposed upon the others.
Yet these felicities of explanation must not make us forget that the
chemical composition attributed to the first type of cometary trains
has, so far, received no countenance from the spectroscope. The emission
lines of free, incandescent hydrogen have never been derived from any
part of these bodies. Dissentient opinions, accordingly, were expressed
as to the cause of their structural peculiarities. Ranyard,[1277]
Zenker, and others advocated the agency of heat repulsion in producing
them; Kiaer somewhat obscurely explains them through the evolution of
gases by colliding particles;[1278] Herz of Vienna concludes tails to be
mere illusory appendages produced by electrical discharges through the
rare medium assumed to fill space.[1279] But Hirn[1280] conclusively
showed that no such medium could possibly exist without promptly
bringing ruin upon our "dædal earth" and its revolving companions.
On the whole, modern researches tend to render superfluous the chemical
diversities postulated by Brédikhine. Electricity alone seems competent
to produce the varieties of cometary emanation they were designed to
account for. The distinction of types rests on a solid basis of fact,
but probably depends upon differences rather in the mode of action than
in the kind of substance acted upon. Suggestive sketches of electrical
and "light-pressure" theories of comets have been published respectively
by Mr. Fessenden of Alleghany,[1281] and by M. Arrhenius at
Stockholm.[1282] Although evidently of a tentative character, they
possess great interest.
Brédikhine's hypothesis was promptly and profusely illustrated. Within
three years of its promulgation, five bright comets made their
appearance, each presenting some distinctive peculiarity by which
knowledge of these curious objects was materially helped forward. The
first of these is remembered as the "Great Southern Comet." It was never
visible in these latitudes, but made a short though stately progress
through southern skies. Its earliest detection was at Cordoba on the
last evening of January, 1880; and it was seen on February 1, as a
luminous streak, extending just after sunset from the south-west horizon
towards the pole, in New South Wales, at Monte Video, and the Cape of
Good Hope. The head was lost in the solar rays until February 4, when
Dr. Gould, then director of the National Observatory of the Argentine
Republic at Cordoba, caught a glimpse of it very low in the west; and on
the following evening, Mr. Eddie, at Graham's Town, discovered a faint
nucleus, of a straw-coloured tinge, about the size of the annular nebula
in Lyra. Its condensation, however, was very imperfect, and the whole
apparition showed an exceedingly filmy texture. The tail was enormously
long. On February 5 it extended--large perspective retrenchment
notwithstanding--over an arc of 50°; but its brightness nowhere exceeded
that of the Milky Way in Taurus. There was little curvature perceptible;
the edges of the appendage ran parallel, forming a nebulous causeway
from star to star; and the comparison to an auroral beam was
appropriately used. The aspect of the famous comet of 1843 was forcibly
recalled to the memory of Mr. Janisch, Governor of St. Helena; and the
resemblance proved not merely superficial. But the comet of 1880 was
less brilliant, and even more evanescent. After only eight days of
visibility, it had faded so much as no longer to strike, though still
discoverable by the unaided eye; and on February 20 it was invisible
with the great Cordoba equatoreal pointed to its known place.
But the most astonishing circumstance connected with this body is the
identity of its path with that of its predecessor in 1843. This is
undeniable. Dr. Gould,[1283] Mr. Hind, and Dr. Copeland,[1284] each
computed a separate set of elements from the first rough observations,
and each was struck with an agreement between the two orbits so close as
to render them virtually indistinguishable. "Can it be possible," Mr.
Hind wrote to Sir George Airy, "that there is such a comet in the
system, almost grazing the sun's surface in perihelion, and revolving in
less than thirty-seven years. I confess I feel a difficulty in admitting
it, notwithstanding the above extraordinary resemblance of
orbits."[1285]
Mr. Hind's difficulty was shared by other astronomers. It would, indeed,
be a violation of common-sense to suppose that a celestial visitant so
striking in appearance had been for centuries back an unnoticed
frequenter of our skies. Various expedients, accordingly, were resorted
to for getting rid of the anomaly. The most promising at first sight was
that of the resisting medium. It was hard to believe that a body,
largely vaporous, shooting past the sun at a distance of less than a
hundred thousand miles from his surface, should have escaped powerful
retardation. It must have passed through the very midst of the corona.
It might easily have had an actual encounter with a prominence. Escape
from such proximity might, indeed, very well have been judged beforehand
to be impossible. Even admitting no other kind of opposition than that
dubiously supposed to have affected Encke's comet, the result in
shortening the period ought to be of the most marked kind. It was proved
by Oppolzer[1286] that if the comet of 1843 had entered our system from
stellar space with parabolic velocity it would, by the action of a
medium such as Encke postulated (varying in density inversely as the
square of the distance from the sun), have been brought down, by its
first perihelion passage, to elliptic movement in a period of
twenty-four years, with such rapid diminution that its next return would
be in about ten. But such restricted observations as were available on
either occasion of its visibility gave no sign of such a rapid progress
towards engulfment.
Another form of the theory was advocated by Klinkerfues.[1287] He
supposed that four returns of the same body had been witnessed within
historical memory--the first in 371 b.c., the next in 1668, besides
those of 1843 and 1880; an original period of 2,039 years being
successively reduced by the withdrawal at each perihelion passage of
1/1320 of the velocity acquired by falling from the far extremity of its
orbit towards the sun, to 175 and 37 years. A continuance of the process
would bring the comet of 1880 back in 1897.
Unfortunately, the earliest of these apparitions cannot be identified
with the recent ones unless by doing violence to the plain meaning of
Aristotle's words in describing it. He states that the comet was first
seen "during the frosts and in the clear skies of winter," setting due
west nearly at the same time as the sun.[1288] This implies some
considerable north latitude. But the objects lately observed had
practically _no_ north latitude. They accomplished their entire course
_above_ the ecliptic in two hours and a quarter, during which space they
were barely separated a hand's-breadth (one might say) from the sun's
surface. For the purposes of the desired assimilation, Aristotle's comet
should have appeared in March. It is not credible, however, that even a
native of Thrace should have termed March "winter."
With the comet of 1668 the case seemed more dubious. The circumstances
of its appearance are barely reconcilable with the identity attributed
to it, although too vaguely known to render certainty one way or the
other attainable. It might however, be expected that recent observations
would at least decide the questions whether the comet of 1843 could have
returned in less than thirty-seven, and whether the comet of 1880 was to
be looked for at the end of 17-1/2 years. But the truth is that both
these objects were observed over so small an arc--8° and 3°
respectively--that their periods remained virtually undetermined. For
while the shape and position of their 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. Brédikhine 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 astronomers by the
Great Southern Comet of 1880 was unexpectedly renewed in the following
year.
On the 22nd of May, 1881, Mr. John Tebbutt of Windsor, New South Wales,
scanning the western sky, discerned a hazy-looking object which he felt
sure was a strange one. A marine telescope at once resolved it into two
small stars and a comet, the latter of which quickly attracted the keen
attention of astronomers; for Dr. Gould, computing its orbit from his
first observations at Cordoba, found it to agree so closely with that
arrived at by Bessel for the comet of 1807 that he telegraphed to
Europe, June 1, announcing the unexpected return of that body. So
unexpected that theoretically it was not possible before the year 3346;
and Bessel's investigation was one which inspired and eminently deserved
confidence. Here, then, once more the perplexing choice had to be made
between a premature and unaccountable reappearance and the admission of
a plurality of comets moving nearly in the same path. But in this case
facts proved decisive.
Tebbutt's comet passed the sun, June 16, at a distance of sixty-eight
millions of miles, and became visible in Europe six days later. It was,
in the opinion of some, the finest object of the kind since 1861. In
traversing the constellation Auriga on its _début_ in these latitudes,
it outshone Capella. On June 24 and some subsequent nights, it was
unmatched in brilliancy by any star in the heavens. In the telescope,
the "two interlacing arcs of light" which had adorned the head of
Coggia's comet were reproduced; while a curious _dorsal spine_ of strong
illumination formed the axis of the tail, which extended in clear skies
over an arc of 20°. It belonged to the same "type" as Donati's great
plume; the particles composing it being driven _from_ the sun by a force
twice as powerful as that urging them _towards_ it.[1291] But the
appendage was, for a few nights, and by two observers perceived to be
double. Tempel, on June 27, and Lewis Boss, at Albany (N.Y.), June 26
and 28, saw a long straight ray corresponding to a far higher rate of
emission than the curved train, and shown by Brédikhine to be a member
of the (so-called) hydrogen class. It had vanished by July 1, but made a
temporary reappearance July 22.[1292]
The appendages of this comet were of remarkable transparency.
Small stars shone wholly undimmed across the tail, and a very nearly
central transit of the head over one of the seventh magnitude on the
night of June 29, produced--if any change--an increase of brilliancy in
the object of this spontaneous experiment.[1293] Dr. Meyer, indeed, at
the Geneva Observatory, detected apparent signs of refractive action
upon rays thus transmitted;[1294] but his observations remain isolated,
and were presumably illusory.
The track pursued by this comet gave peculiar advantages for its
observation. Ascending from Auriga through Camelopardus, it stood, July
19, on a line between the Pointers and the Pole, within 8° of the
latter, and thus remained for a lengthened period constantly above the
horizon of northern observers. Its brightness, too, was no transient
blaze, but had a lasting quality which enabled it to be kept steadily in
view during nearly nine months. Visible to the naked eye until the end
of August, the last telescopic observation of it was made February 14,
1882, when its distance from the earth considerably exceeded 300 million
miles. Under these circumstances, the knowledge acquired of its orbit
was of more than usual accuracy, and showed conclusively that the comet
was not a simple return of Bessel's; for this would involve a period of
seventy-four years, whereas Tebbutt's comet cannot revisit the sun until
after the lapse of two and a half millenniums.[1295] Nevertheless, the
twin bodies move so nearly in the same path that an original connection
of some kind is obvious; and the recent example of Biela readily
suggested a conjecture as to what the nature of that connection might
have been. The comets of 1807 and 1881 are, then, regarded with much
probability as fragments of a primitive disrupted body, one following in
the wake of the other at an interval of seventy-four years.
Imperfect photographs were taken of Donati's comet both in England and
America;[1296] but Tebbutt's comet was the first to which the process
was satisfactorily applied. The difficulties to be overcome were very
great. The chemical intensity of cometary light is, to begin with,
extraordinarily small. Janssen estimated it at 1/300000 of
moonlight.[1297] Hence, if the ordinary process by which lunar
photographs are taken had been applied to the comet of 1881, an exposure
of at least _three days_ would have been required in order to get an
impression of the head with about a tenth part of the tail. But by that
time a new method of vastly increased sensitiveness had been rendered
available, by which dry gelatine-plates were substituted for the wet
collodion-plates hitherto in use; and this improvement alone reduced the
necessary time of exposure to two hours. It was brought down to half an
hour by Janssen's employment of a reflector specially adapted to give an
image illuminated eight or ten times as strongly as that produced in the
focus of an ordinary telescope.[1298]
The photographic feebleness of cometary rays was not the only obstacle
in the way of success. The proper motion of these bodies is so rapid as
to render the usual devices for keeping a heavenly body steadily in view
quite inapplicable. The machinery by which the diurnal movement of the
sphere is followed, must be especially modified to suit each eccentric
career. This, too, was done, and on June 30, 1881, Janssen secured a
perfect photograph of the brilliant object then visible, showing the
structure of the tail with beautiful distinctness to a distance of
2-1/2° from the head. An impression to nearly 10° was obtained about the
same time by Dr. Henry Draper at New York, with an exposure of 162
minutes.[1299]
Tebbutt's (or comet 1881 iii.) was also the first comet of which the
spectrum was so much as attempted to be chemically recorded. Both
Huggins and Draper were successful in this respect, but Huggins was more
completely so.[1300] The importance of the feat consisted in its
throwing open to investigation a part of the spectrum invisible to the
eye, and so affording an additional test of cometary constitution. The
result was fully to confirm the origin from carbon-compounds assigned to
the visible rays, by disclosing additional bands belonging to the same
series in the ultra-violet; as well as to establish unmistakably the
presence of a not inconsiderable proportion of reflected solar light by
the clear impression of some of the principal Fraunhofer lines. Thus the
polariscope was found to have told the truth, though not the whole
truth.
The photograph so satisfactorily communicative was taken by Sir William
Huggins on the night of June 24; and on the 29th, at Greenwich, the
tell-tale Fraunhofer lines were perceived to interrupt the visible range
of the spectrum. This was at first so vividly continuous, that the
characteristic cometary bands could scarcely be detached from their
bright background. But as the nucleus faded towards the end of June,
they came out strongly, and were more and more clearly seen, both at
Greenwich and at Princeton, to agree, not with the spectrum of
hydro-carbons glowing in a vacuum tube, but with that of the same
substances burning in a Bunsen flame.[1301] It need not, however, be
inferred that cometary materials are really in a state of combustion.
This, from all that we know, may be called an impossibility. The
additional clue furnished was rather to the manner of their electrical
illumination.[1302]
The spectrum of the tail was, in this comet, found to be not essentially
different from that of the head. Professor Wright of Yale College
ascertained a large percentage of its light to be polarized in a plane
passing through the sun, and hence to be reflected sunlight.[1303] A
faint continuous spectrum corresponded to this portion of its radiance;
but gaseous emissions were also present. At Potsdam, on June 30, the
hydro-carbon bands were indeed traced by Vogel to the very end of the
tail;[1304] and they were kept in sight by Young at a greater distance
from the nucleus than the more equably dispersed light. There seems
little doubt that, as in the solar corona, the relative strength of the
two orders of spectra is subject to fluctuations.
The comet of 1881 iii. was thus of signal service to science. It
afforded, when compared with the comet of 1807, the first undeniable
example of two such bodies travelling so nearly in the same orbit as to
leave absolutely no doubt of the existence of a genetic tie between
them. Cometary photography came to its earliest fruition with it; and
cometary spectroscopy made a notable advance by means of it. Before it
was yet out of sight, it was provided with a successor.
At Ann Arbor Observatory, Michigan, on July 14, a comet was discovered
by Dr. Schaeberle, which, as his claim to priority is undisputed, is
often allowed to bear his name, although designated, in strict
scientific parlance, comet 1881 iv. It was observed in Europe after
three days, became just discernible by the naked eye at the end of July,
and brightened consistently up to its perihelion passage, August 22,
when it was still about fifty million miles from the sun. During many
days of that month, the uncommon spectacle was presented of two bright
comets circling together, though at widely different distances, round
the North pole of the heavens. The newcomer, however, never approached
the pristine lustre of its predecessor. Its nucleus, when brightest, was
comparable to the star Cor Caroli, a narrow, perfectly straight ray
proceeding from it to a distance of 10°. This was easily shown by
Brédikhine to belong to the hydrogen type of tails;[1305] while a
"strange, faint second tail, or bifurcation of the first one," observed
by Captain Noble, August 24,[1306] fell into the hydro-carbon class of
emanations. It was seen, August 22 and 24, by Dr. F. Terby of
Louvain,[1307] as a short nebulous brush, like the abortive beginning of
a congeries of curving trains; but appeared no more. Its well-attested
presence was significant of the complex constitution of such bodies, and
the manifold kinds of action progressing in them.
The only peculiarity in the spectrum of Schaeberle's comet consisted in
the almost total absence of continuous light. The carbon-bands were
nearly isolated and very bright. Barely from the nucleus proceeded a
rainbow-tinted streak, indicative of solid or liquid matter, which, in
this comet, must have been of very scanty amount. Its visit to the sun
in 1881 was, so far as is known, the first. The elements of its orbit
showed no resemblance to those of any previous comet, nor any marked
signs of periodicity. So that, although it may be considered probable,
we do not _know_ that it is moving in a closed curve, or will ever again
penetrate the precincts of the solar system. It was last seen from the
southern hemisphere, October 19, 1881.
The third of a quartette of lucid comets visible within sixteen months,
was discovered by Mr. C. S. Wells at the Dudley Observatory, Albany,
March 17, 1882. Two days later it was described by Mr. Lewis Boss as "a
great comet in miniature," so well defined and regularly developed were
its various parts and appendages. Discernible with optical aid early in
May, it was on June 5 observed on the meridian at Albany just before
noon--an astronomical event of extreme rarity. Comet Wells, however,
never became an object so conspicuous as to attract general attention,
owing to its immersion in the evening twilight of our northern June.
But the study of its spectrum revealed new facts of the utmost interest.
All the comets till then examined had been found (with the two
transiently observed exceptions already mentioned) to conform to one
invariable type of luminous emission. Individual distinctions there had
been, but no specific differences. Now all these bodies had kept at a
respectful distance from the sun; for of the great comet of 1880 no
spectroscopic inquiries had been made. Comet Wells, on the other hand,
approached its surface within little more than five million miles on
June 10, 1882; and the vicinity had the effect of developing a novel
feature in its incandescence.
During the first half of April its spectrum was of the normal type,
though the carbon bands were unusually weak; but with approach to the
sun they died out, and the entire light seemed to become concentrated
into a narrow, unbroken, brilliant streak, hardly to be distinguished
from the spectrum of a star. This unusual behaviour excited attention,
and a strict watch was kept. It was rewarded at the Dunecht Observatory,
May 27, by the discernment of what had never before been seen in a
comet--the yellow ray of sodium.[1308] By June 1, this had kindled into
a blaze overpowering all other emissions. The light of the comet was
practically monochromatic; and the image of the entire head, with the
root of the tail, could be observed, like a solar prominence, depicted,
in its new saffron vesture of vivid illumination, within the jaws of an
open slit.
At Potsdam, the bright yellow line was perceived with astonishment by
Vogel on May 31, and was next evening identified with Fraunhofer's "D."
Its character led him to infer a very considerable density in the
glowing vapour emitting it.[1309] Hasselberg founded an additional
argument in favour of the electrical origin of cometary light on the
changes in the spectrum of comet Wells.[1310] For they were closely
paralleled by some earlier experiments of Wiedemann, in which the
gaseous spectra of vacuum tubes were at once effaced on the introduction
of metallic vapours. It seemed as if the metal had no sooner been
rendered volatile by heat, than it usurped the entire office of carrying
the discharge, the resulting light being thus exclusively of its
production. Had simple incandescence by heat been in question, the
effect would have been different; the two spectra would have been
superposed without prejudice to either. Similarly, the replacement of
the hydro-carbon bands in the spectrum of the comet by the sodium line
proved electricity to be the exciting agent. For the increasing thermal
power of the sun might, indeed, have ignited the sodium, but it could
not have extinguished the hydro-carbons.
Sir William Huggins succeeded in photographing the spectrum of comet
Wells by an exposure of one hour and a quarter.[1311] The result was to
confirm the novelty of its character. None of the ultra-violet carbon
groups were apparent; but certain bright rays, as yet unidentified, had
imprinted themselves. Otherwise the spectrum was strongly continuous,
uninterrupted even by the Fraunhofer lines detected in the spectrum of
Tebbutt's comet. Hence it was concluded that a smaller proportion of
reflected light was mingled with the native emissions of the later
arrival.
All that is certainly known about the _extent_ of the orbit traversed by
the first comet of 1882 is that it came from, and is now retreating
towards, vastly remote depths of space. An American computer[1312] found
a period indicated for it of no less than 400,000 years; A. Thraen of
Dingelstädt arrived at one of 3617.[1313] Both are perhaps equally
insecure.
We have now to give some brief account of one of the most remarkable
cometary apparitions on record, and--with the single exception of that
identified with the name of Halley--the most instructive to astronomers.
The lessons learned from it were as varied and significant as its aspect
was splendid; although from the circumstance of its being visible in
general only before sunrise, the spectators of its splendour were
comparatively few.
The discovery of a great comet at Rio Janeiro, September 11, 1882,
became known in Europe through a telegram from M. Cruls, director of the
observatory at that place. It had, however (as appeared subsequently),
been already seen on the 8th by Mr. Finlay of the Cape Observatory, and
at Auckland as early as September 3. A later, but very singularly
conditioned detection, quite unconnected with any of the preceding, was
effected by Dr. Common at Ealing. Since the eclipse of May 17, when a
comet--named "Tewfik" in honour of the Khedive of Egypt--was caught on
Dr. Schuster's photographs, entangled, one might almost say, in the
outer rays of the corona, he had scrutinized the neighbourhood of the
sun on the infinitesimal chance of intercepting another such body on its
rapid journey thence or thither. We record with wonder that, after an
interval of exactly four months, that infinitesimal chance turned up in
his favour.
On the forenoon of Sunday, September 17, he saw a great comet close to,
and rapidly approaching the sun. It was, in fact, then within a few
hours of perihelion. Some measures of position were promptly taken; but
a cloud-veil covered the interesting spectacle before mid-day was long
past. Mr. Finlay at the Cape was more completely fortunate. Divided from
his fellow-observer by half the world, he unconsciously finished, under
a clearer sky, his interrupted observation. The comet, of which the
silvery radiance contrasted strikingly with the reddish-yellow glare of
the sun's margin it drew near to, was followed "continuously right into
the boiling of the limb"--a circumstance without precedent in cometary
history.[1314] Dr. Elkin, who watched the progress of the event with
another instrument, thought the intrinsic brilliancy of the nucleus
scarcely surpassed by that of the sun's surface. Nevertheless it had no
sooner touched it than it vanished as if annihilated. So sudden was the
disappearance (at 4h. 50m. 58s., Cape mean time), that the comet was at
first believed to have passed _behind_ the sun. But this proved not to
have been the case. The observers at the Cape had witnessed a genuine
transit. Nor could non-visibility be explained by equality of
lustre. For the gradations of light on the sun's disc are amply
sufficient to bring out against the dusky background of the limb any
object matching the brilliancy of the centre; while an object just
equally luminous with the limb must inevitably show dark at the centre.
The only admissible view, then, is that the bulk of the comet was of too
filmy a texture, and its presumably solid nucleus too small, to
intercept any noticeable part of the solar rays--a piece of information
worth remembering.
PLATE III.
[Illustration: The Great Comet of September, 1882.
Photographed at the Royal Observatory, Cape of Good Hope]
On the following morning, the object of this unique observation showed
(in Sir David Gill's words) "an astonishing brilliancy as it rose behind
the mountains on the east of Table Bay, and seemed in no way diminished
in brightness when the sun rose a few minutes afterward. It was only
necessary to shade the eye from direct sunlight with the hand at arm's
length, to see the comet, with its brilliant white nucleus and dense
white, sharply bordered tail of quite half a degree in length."[1315]
All over the world, wherever the sky was clear during that day,
September 18, it was obvious to ordinary vision. Since 1843 nothing had
been seen like it. From Spain, Italy, Algeria, Southern France,
despatches came in announcing the extraordinary appearance. At Cordoba,
in South America, the "blazing star near the sun" was the one topic of
discourse.[1316] Moreover--and this is altogether extraordinary--the
records of its daylight visibility to the naked eye extend over three
days. At Reus, near Tarragona, it showed bright enough to be seen
through a passing cloud when only three of the sun's diameters from his
limb, just before its final rush past perihelion on September 17; while
at Carthagena in Spain, on September 19, it was kept in view during two
hours before and two hours after noon, and was similarly visible in
Algeria on the same day.[1317]
But still more surprising than the appearance of the body itself were
the nature and relations of the path it moved in. The first rough
elements computed for it by Mr. Tebbutt, Dr. Chandler, and Mr. White,
assistant at the Melbourne Observatory, showed at once a striking
resemblance to those of the twin comets of 1843 and 1880. This
suggestive fact became known in this country, September 27, through the
medium of a Dunecht circular. It was fully confirmed by subsequent
inquiries, for which ample opportunities were luckily provided. The
likeness was not, indeed, so absolutely perfect as in the previous case;
it included some slight, though real differences; but it bore a strong
and unmistakable stamp, broadly challenging explanation.
Two hypotheses only were really available. Either the comet of 1882 was
an accelerated return of those of 1843 and 1880, or it was a fragment of
an original mass to which they also had belonged. For the purposes of
the first view the "resisting medium" was brought into full play; the
opinion of its efficacy was for some time both prevalent and popular,
and formed the basis, moreover, of something of a sensational panic. For
a comet which, at a single passage through the sun's atmosphere,
encountered sufficient resistance to shorten its period from
thirty-seven to two years and eight months, must, in the immediate
future, be brought to rest on his surface; and the solar conflagration
thence ensuing was represented in some quarters, with more licence of
imagination than countenance from science, as likely to be of
catastrophic import to the inhabitants of our little planet.
But there was a test available in 1882 which it had not been possible to
apply either in 1843 or in 1880. The two bodies visible in those years
had been observed only after they had already passed perihelion;[1318]
the third member of the group, on the other hand, was accurately
followed for a week before that event, as well as during many months
after it. Finlay's and Elkin's observation of its disappearance at the
sun's edge formed, besides, a peculiarly delicate test of its motion.
The opportunity was thus afforded, by directly comparing the comet's
velocity before and after its critical plunge through the solar
surroundings, of ascertaining with approximate certainty whether any
considerable retardation had been experienced in the course of that
plunge. The answer distinctly given was that there had not. The computed
and observed places on both sides of the sun fitted harmoniously
together. The effect, if any were produced, was too small to be
perceptible.
This result is, in itself, a memorable one. It seems to give the _coup
de grâce_ to Encke's theory--discredited, in addition, by Backlund's
investigation--of a resisting medium growing rapidly denser inwards. For
the perihelion distance of the comet of 1882, though somewhat greater
than that of its predecessors, was nevertheless extremely small. It
passed at less than 300,000 miles of the sun's surface. But the ethereal
substance long supposed to obstruct the movement of Encke's comet would
there be nearly 2,000 times denser than at the perihelion of the smaller
body, and must have exerted a conspicuous retarding influence. That none
such could be detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the question of
identity. The "Great September Comet" of 1882 was in no hurry to
withdraw itself from curious terrestrial scrutiny. It was discerned with
the naked eye at Cordoba as late as March 7, 1883, and still showed in
the field of the great equatoreal on June 1 as an "excessively faint
whiteness."[1319] It was then about 480 millions of miles from the
earth--a distance to which no other comet--not even excepting the
peculiar one of 1729--had been pursued.[1320] Moreover, an arc of 340
out of the entire 360 degrees of its circuit had been described under
the eyes of astronomers; so that its course came to be very well known.
That its movement is in a very eccentric ellipse, traversed in several
hundred years, was ascertained.[1321] The later inquiries of Dr.
Kreutz,[1322] completed in a volume published in 1901,[1323]
demonstrated the period to be of about 800 years, while that of its
predecessor in 1843 might possibly agree with it, but is much more
probably estimated at 512 years. The hypothesis that they, or any of the
comets associated with them, were returns of an individual body is
peremptorily excluded. They may all, however, have been separated from
one original mass by the divellent action of the sun at close quarters.
Each has doubtless its own period, since each has most likely suffered
retardations or accelerations special to itself, which, though trifling
in amount, would avail materially to alter the length of the major axis,
while leaving the remaining elements of the common orbit virtually
unchanged.[1324]
A fifth member was added to the family in 1887. On the 18th of January
in that year, M. Thome discovered at Cordoba a comet reproducing with
curious fidelity the lineaments of that observed in the same latitudes
seven years previously. The narrow ribbon of light, contracting towards
the sun, and running outward from it to a distance of thirty-five
degrees; the unsubstantial head--a veiled nothingness, as it appeared,
since no distinct nucleus could be made out; the quick fading into
invisibility, were all accordant peculiarities, and they were confirmed
by some rough calculations of its orbit, showing geometrical affinity to
be no less unmistakable than physical likeness. The observations secured
were indeed, from the nature of the apparition, neither numerous nor
over-reliable; and the earliest of them dated from a week after
perihelion, passed, almost by a touch-and-go escape, January 11. On
January 27, this mysterious object could barely be discerned
telescopically at Cordoba.[1325] That it belonged to the series of
"southern comets" can scarcely be doubted; but the inference that it was
an actual return of the comet of 1880, improbable in itself, was
negatived by its non-appearance in 1894. Meyer's incorporation with this
extraordinary group of the "eclipse-comet" of 1882[1326] has been
approved by Kreutz, after searching examination.
The idea of cometary systems was first suggested by Thomas Clausen in
1831.[1327] It was developed by the late M. Hoek, director of the
Utrecht Observatory, in 1865 and some following years.[1328] He found
that in quite a considerable number of cases, the paths of two or three
comets had a common point of intersection far out in space, indicating
with much likelihood a community of origin. This consisted, according to
his surmise, in the disruption of a parent mass during its sweep round
the star latest visited. Be this as it may, the fact is undoubted that
numerous comets fall into groups, in which similar conditions of motion
betray a pre-existent physical connection. Never before, however, had
geometrical relationship been so notorious as between the comets now
under consideration; and never before, in a comet still, it might be
said, in the prime of life, had physical peculiarities tending to
account for that affinity been so obvious as in the chief member of the
group.
Observation of a granular structure in cometary nuclei dates far back
into the seventeenth century, when Cysatus and Hevelius described the
central parts of the comets of 1618 and 1652 respectively as made up of
a congeries of minute stars. Analogous symptoms of a loose state of
aggregation have of late been not unfrequently detected in telescopic
comets, besides the instances of actual division offered by those
connected with the names of Biela and Liais. The forces concerned in
producing these effects seem to have been peculiarly energetic in the
great comet of 1882.
The segmentation of the nucleus was first noticed in the United States
and at the Cape of Good Hope, September 30. It proceeded rapidly. At
Kiel, on October 5 and 7, Professor Krüger perceived two centres of
condensation. A definite and progressive separation into _three_ masses
was observed by Professor Holden, October 13 and 17.[1329] A few days
later, M. Tempel found the head to consist of _four_ lucid aggregations,
ranged nearly along the prolongation of the caudal axis;[1330] and Dr.
Common, January 27, 1883, saw _five_ nuclei in a line "like pearls on a
string."[1331] This remarkable character was preserved to the last
moment of the comet's distinct visibility. It was a consequence,
according to Dr. Kreutz, of violent interior action in the comet itself
While close to the sun.
There were, however, other curious proofs of a disaggregative tendency
in this body. On October 9, Schmidt discovered at Athens a nebulous
object 4° south-west of the great comet, and travelling in the same
direction. It remained visible for a few days, and, from Oppenheim's and
Hind's calculations, there can be little doubt that it was really the
offspring by fission of the body it accompanied.[1332] This is rendered
more probable by the unexampled spectacle offered, October 14, to
Professor Barnard, then of Nashville, Tennessee, of _six or eight_
distinct cometary masses within 6° south by west of the comet's head,
none of which reappeared on the next opportunity for a search.[1333] A
week later, however, one similar object was discerned by Mr. W. R.
Brooks, in the opposite direction from the comet. Thus space appeared to
be strewn with the filmy débris of this beautiful but fragile structure
all along the track of its retreat from the sun.
Its tail was only equalled (if it were equalled) in length by that of
the comet of 1843. It extended in space to the vast distance of 200
millions of miles from the head; but, so imperfectly were its
proportions displayed to terrestrial observers, that it at no time
covered an arc of the sky of more than 30°. This apparent extent was
attained, during a few days previous to September 25, by a faint, thin,
rigid streak, noticed only by a few observers--by Elkin at the Cape
Observatory, Eddie at Grahamstown, and Cruls at Rio Janeiro. It diverged
at a low angle from the denser curved train, and was produced, according
to Brédikhine,[1334] by the action of a repulsive force twelve times as
strong as the counter-pull of gravity. It belonged, that is, to type 1;
while the great bifurcate appendage, obvious to all eyes, corresponded
to the lower rate of emission characteristic of type 2. This was
remarkable for the perfect definiteness of its termination, for its
strongly-forked shape, and for its unusual permanence. Down to the end
of January, 1883, its length, according to Schmidt's observations, was
still 93 million miles; and a week later it remained visible to the
naked eye, without notable abridgment.
Most singular of all was an anomalous extension of the appendage
_towards_ the sun. During the greater part of October and November, a
luminous "tube" or "sheath," of prodigious dimensions, seemed to
surround the head, and project in a direction nearly opposite to that of
the usual outpourings of attentuated matter. (See Plate III.) Its
diameter was computed by Schmidt to be, October 15, no less than four
million miles, and it was described by Cruls as a "truncated cone of
nebulosity," stretching 3° or 4° sunwards.[1335] This, and the entire
anterior part of the comet, were again surrounded by a thin, but
enormously voluminous paraboloidal envelope, observed by Schiaparelli
for a full month from October 19.[1336] There can be little doubt that
these abnormal effluxes were a consequence of the tremendous physical
disturbance suffered at perihelion; and it is worth remembering that
something analogous was observed in the comet of 1680 (Newton's), also
noted for its excessively close approach to the sun, and possibly moving
in a related orbit. The only plausible hypothesis as to the mode of
their production is that of an opposite state of electrification in the
particles composing the ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a repetition of
that of its immediate predecessor, thus confirming the inference that
the previously unexampled sodium-blaze was in both a direct result of
the intense solar action to which they were exposed. But the D line was,
this time, not seen alone. At Dunecht, on the morning of September 18,
Drs. Copeland and J. G. Lohse succeeded in identifying six brilliant
rays in the green and yellow with as many prominent iron-lines;[1337] a
very significant addition to our knowledge of cometary constitution, and
one which lent countenance to Brédikhine's assumption of various kinds
of matter issuing from the nucleus with velocities inversely as their
atomic weights. All the lines equally showed a slight displacement,
indicating a recession from the earth of the radiating body at the rate
of 37 to 46 miles a second. A similar observation, made by M. Thollon at
Nice on the same day, gave emphatic sanction to the spectroscopic method
of estimating movement in the line of sight. Before anything was as yet
known of the comet's path or velocity, he announced, from the position
of the double sodium-line alone, that at 3 p.m. on September 18 it was
increasing its distance from our planet by from 61 to 76 kilometres per
second.[1338] M. Bigourdan's subsequent calculations showed that its
actual swiftness of recession was at that moment 73 kilometres.
Changes in the inverse order to those seen in the spectrum of comet
Wells soon became apparent. In the earlier body, carbon bands had died
out with _approach_ to perihelion, and had been replaced by sodium
emissions; in its successor, sodium emissions became weakened and
disappeared with _retreat_ from perihelion, and found their substitute
in carbon bands. Professor Riccò was, in fact, able to infer, from the
sequence of prismatic phenomena, that the comet had already passed the
sun; thus establishing a novel criterion for determining the position of
a comet in its orbit by the varying quality of its radiations.
Recapitulating what was learnt from the five conspicuous comets of
1880-82, we find that the leading facts acquired to science were these
three. First, that comets may be met with pursuing each other, after
intervals of many years, in the same, or nearly the same, track; so that
identity of orbit can no longer be regarded as a sure test of individual
identity. Secondly, that at least the outer corona may be traversed by
such bodies with perfect apparent impunity. Finally, that their chemical
constitution is highly complex, and that they possess, in some cases at
least, a metallic core resembling the meteoric masses which occasionally
reach the earth from planetary space.
A group of five comets, including Halley's, own a sort of cliental
dependence upon the planet Neptune. They travel out from the sun just to
about his distance from it, as if to pay homage to a powerful protector,
who gets the credit of their establishment as periodical visitors to the
solar system. The second of these bodies to affect a looked-for return
was a comet--the sixteenth within ten years--discovered by Pons, July
20, 1812, and found by Encke to revolve in an elliptic orbit, with a
period of nearly 71 years. It was not, however, until September 1, 1883,
that Mr. Brooks caught its reappearance; it passed perihelion January
25, and was last seen June 2, 1884. At its brightest, it had the
appearance of a second magnitude star, furnished with a poorly developed
double tail, and was fairly conspicuous to the naked eye in Southern
Europe, from December to March. One exceptional feature distinguished
it. Its fluctuations in form and luminosity were unprecedented in
rapidity and extent. On September 21, Dr. Chandler[1339] observed it at
Harvard as a very faint, diffused nebulosity, with slight central
condensation. On the next night, there was found in its place a bright
star of the eighth magnitude, scarcely marked out, by a bare trace of
environing haze, from the genuine stars it counterfeited. The change was
attended by an eight-fold augmentation of light, and was proved by
Schiaparelli's confirmatory observations[1340] to have been accomplished
within a few hours. The stellar disguise was quickly cast aside. The
comet appeared on September 23 as a wide nebulous disc, and soon after
faded down to its original dimness. Its distance from the sun was then
no less than 200 million miles, and its spectrum showed nothing unusual.
These strange variations recurred slightly on October 15, and with
marked emphasis on January 1, when they were witnessed with amazement,
and photometrically studied by Müller of Potsdam.[1341] The entire cycle
this time was run through in less than four hours--the comet having, in
that brief space, condensed, with a vivid outburst of light, into a
seeming star, and the seeming star having expanded back again into a
comet. Scarcely less transient, though not altogether similar, changes
of aspect were noted by M. Perrotin,[1342] January 13 and 19, 1884. On
the latter date, the continuous spectrum given by a reddish-yellow disc
surrounding the true nucleus seemed intensified by bright knots
corresponding to the rays of sodium.
A comet discovered by Mr. Sawerthal at the Royal Observatory, Cape of
Good Hope, February 19, 1888, distinguished itself by blazing up, on May
19, to four or five times its normal brilliancy, at the same time
throwing out from the head two lustrous lateral branches.[1343] These
had, on June 1, spread backward so as to join the tail, with an effect
like the playing of a fountain; ten or eleven days later, they had
completely disappeared, leaving the comet in its former shape and
insignificance. Its abrupt display of vitality occurred two full months
after perihelion.
On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, New York,
eminent as a successful comet-hunter, secured one of his customary
trophies. The faint object in question was moving through the
constellation Cetus, and turned out to be a member of Jupiter's numerous
family of comets, revolving round the sun in a period of seven years.
Its past history came then, to a certain extent, within the scope of
investigation, and proved to have been singularly eventful; nor had the
body escaped scatheless from the vicissitudes to which it had been
exposed. Observing from Mount Hamilton, August 2 and 5, Professor
Barnard noticed this comet (1889, v.) to be attended in its progress
through space by four _outriders_, "The two brighter companions" (the
fainter pair survived a very short time) "were perfect miniatures,"
Professor Barnard tells us,[1344] "of the larger comet, each having a
small, fairly defined head and nucleus, with a faint, hazy tail, the
more distant one being the larger and less developed. The three comets
were in a straight line, nearly east and west, their tails lying along
this line. There was no connecting nebulosity between these objects, the
tails of the two smaller not reaching each other, or the large comet. To
all appearance they were absolutely independent comets." Nevertheless,
Spitaler, at Vienna, in the early days of August, perceived, as it were,
a thin cocoon of nebulosity woven round the entire trio.[1345] One of
them faded from view September 5; the other actually outshone the
original comet on August 31, but was plainly of inferior vitality. It
was last seen by Barnard on November 25, with the thirty-six inch
refractor, while its primary afforded an observation for position with
the twelve-inch, March 20, 1890.[1346] A cause for the disruption it had
presumably undergone had, before then, been plausibly assigned.
The adventures of Lexell's comet have long served to exemplify the
effects of Jupiter's despotic sway over such bodies. Although bright
enough in 1770 to be seen with the naked eye, and ascertained to be
circulating in five and a half years, it had never previously been seen,
and failed subsequently to present itself. The explanation of this
anomaly, suggested by Lexell, and fully confirmed by the analytical
inquiries both of Laplace and Leverrier,[1347] was that a very close
approach to Jupiter in 1767 had completely changed the character of its
orbit, and brought it within the range of terrestrial observation; while
in 1779, after having only twice traversed its new path (at its second
return it was so circumstanced as to be invisible from the earth), it
was, by a fresh encounter, diverted into one entirely different. Yet the
possibility was not lost sight of that the great planet, by inverting
its mode of action, might undo its own work, and fling the comet once
more into the inner part of the solar system. This possibility seemed to
be realized by Chandler's identification of Brooks's and Lexell's
comet.[1348] An exceedingly close approach to Jupiter in 1886 had, he
found reason to believe, produced such extensive alterations in the
elements of its motion as to bring the errant body back to our
neighbourhood in 1889. But his inference, though ratified by Mr. Charles
Lane Poor's preliminary calculations, proved dubious on closer inquiry,
and was rendered wholly inadmissible by the circumstances attending the
return of Brooks's comet in 1896.[1349] The companion-objects watched by
Barnard in 1889 had by that time, perhaps, become dissipated in space,
for they were not redetected. They represented, in all likelihood,
wreckage from a collision with Jupiter, dating, perhaps, so far back as
1791, when Mr. Lane Poor found that one of the fateful meetings to which
short-period comets are especially subject had taken place.
The Lexell-Brooks case was almost duplicated by the resemblance to De
Vico's lost comet of 1844[1350] of one detected November 20, 1894, by
Edward, son of Lewis Swift. Schulhof[1351] announced the identity, and
Chandler,[1352] under reserve, vouched for it. Had the comet continued
to pursue the track laboriously laid down for it at Boston, and shown
itself at the due epoch in 1900, its individuality might have been
considered assured; but the formidable vicegerent of the sun once more
interposed, and, in 1897, swept it out of the terrestrial range of view.
Hence the recognition remains ambiguous.
On the morning of March 7, 1892, Professor Lewis Swift discovered the
brightest comet that had been seen by northern observers since 1882.
About the time of perihelion, which occurred on April 6, it was
conspicuous, as it crossed the celestial equator from Aquarius towards
Pegasus, with a nucleus equal to a third magnitude star, and a tail
twenty degrees long. This tail was multiple, and multiple in a most
curiously variable manner. It divided up into many thin nebulous
streaks, the number and relative lustre of which underwent rapid and
marked changes. Their permanent record on Barnard's and W. H.
Pickering's plates marked a noteworthy advance in cometary photography.
Plate IV. reproduces two of the Lick pictures, taken with a six-inch
camera, on April 5 and 7 respectively, with, in each case, an exposure
of about one hour. The tail is in the first composed of three main
branches, the middle one having sprung out since the previous morning,
and the branches are, in their turn, split up into finer rays, to the
number of perhaps a dozen in all. In the second a very different state
of things is exhibited. "The southern component," Professor Barnard
remarked, "which was the brightest on the 5th, had become diffused and
fainter, while the middle tail was very bright and broad. Its southern
side, which was the best defined, was wavy in numerous places, the tail
appearing as if disturbing currents were flowing at right angles to it.
At 42° from the head the tail made an abrupt bend towards the south, as
if its current was deflected by some obstacle. In the densest portion of
the tail, at the point of deflection, are a couple of dark holes,
similar to those seen in some of the nebulæ. The middle portion of the
tail is brighter, and looks like crumpled silk in places."[1353] Next
morning the southern was the prominent branch, and it was loaded, at 1°
42' from the head, with a strange excrescence, suggesting the
budding-out of a fresh comet in that incongruous situation.[1354] Some
of these changes, Professor Barnard thought, might possibly be explained
by a rotation of the tail on an axis passing through the nucleus, and
Pickering, who formed a similar opinion on independent grounds, assigned
about 94 hours as the period of the gyrating movement.[1355] He,
moreover, determined accelerative velocities outward from the sun of
definite condensations in the tail, indicating for its materials, on
Brédikhine's theory, a density less than one half that of
hydrogen.[1356] This conclusion applied also to Rordame's comet, which
exhibited a year later phenomena analogous to those remarked in Swift's.
Their photographic study led Professor Hussey[1357] to significant
inferences as to the structure and rapid changes of cometary appendages.
PLATE IV.
[Illustration:
Photographs of Swift's Comet.
By Professor E. E. Barnard.
No. 1. Taken April 4, 1892; exposure 1 h.
No. 2. Taken April 6, 1892; exposure 1 h. 5 m ]
Seven comets were detected in 1892, and all, strange to say, were
visible together towards the close of the year.[1358] Among them was a
faint object, which unexpectedly left a trail on a plate exposed by
Professor Barnard to the stars in Aquila[1359] on October 12. This was
the first comet actually discovered by photography, the Sohag comet
having been simultaneously seen and pictured. It has a period of about
six years. Holmes's comet is likewise periodical, in rather less than
seven years. Its path, which is wholly comprised between the orbits of
Mars and Jupiter, is less eccentric than that of any other known comet.
Subsequently to its discovery, on November 6, it underwent some curious
vicissitudes. At first bright and condensed, it expanded rapidly with
increasing distance from the sun (to which it had made its nearest
approach on June 13), until, by the middle of December, it was barely
discernible with powerful telescopes as "a feebly luminous mist on the
face of the sky."[1360] But on January 16, 1893, observers in Europe and
America were bewildered to find, as if substituted for it, a yellow star
of the seventh magnitude, enveloped in a thin nebulous husk, which
enclosed a faint miniature tail.[1361] This condensation and recovery of
light lasted in its full intensity only a couple of days. The almost
evanescent faintness of Holmes's comet at its next return accounted for
its invisibility previous to 1892, when it was evidently in a state of
peculiar excitement. Mr. Perrine was barely able, with the Lick 36-inch,
to find the vague nebulous patch which occupied its predicted place on
June 10, 1899.
The origin of comets has been long and eagerly inquired into, not
altogether apart from the cheering guidance of ascertained facts. Sir
William Herschel regarded them as fragments of nebulæ[1362]--scattered
débris of embryo worlds; and Laplace approved of and adopted the
idea.[1363] But there was a difficulty. No comet has yet been observed
to travel in a decided hyperbola. The typical cometary orbit, apart from
disturbance, is parabolic--that is to say, it is indistinguishable from
an enormously long ellipse. But this circumstance could only be
reconciled with the view that the bodies thus moving were casual
visitors from outer space, by making, as Laplace did, the tacit
assumption that the solar system was at rest. His reasoning was, indeed,
thereby completely vitiated, as Gauss pointed out in 1815;[1364] and the
objections then urged were reiterated by Schiaparelli,[1365] who
demonstrated in 1871 that a large preponderance of well-marked
hyperbolic orbits should result if comets were picked up _en route_ by a
swiftly-advancing sun. The fact that their native movement is
practically parabolic shows it to have been wholly imparted from
without. They passively obeyed the pull exerted upon them. In other
words, their condition previous to being attracted by the sun was one
very nearly of relative repose.[1366] They shared, accordingly, the
movement of translation through space of the solar system.
This significant conclusion had been indicated, on other grounds, as the
upshot of researches undertaken independently by Carrington[1367] and
Mohn[1368] in 1860, with a view to ascertaining the anticipated
existence of a relationship between the general _lie_ of the paths of
comets and the direction of the sun's journey. It is tolerably obvious
that if they wander at haphazard through interstellar regions their
apparitions should markedly aggregate towards the vicinity of the
constellation Lyra; that is to say, we should meet considerably more
comets than would overtake us, for the very same reason that falling
stars are more numerous after than before midnight. Moreover, the comets
met by us should be, apparently, swifter-moving objects than those
coming up with us from behind; because, in the one case, our own real
movement would be added to, in the other subtracted from, theirs. But
nothing of all this can be detected. Comets approach the sun
indifferently from all quarters, and with velocities quite independent
of direction.
We conclude, then, that the "cosmical current" which bears the solar
system towards its unknown goal carries also with it nebulous masses of
undefined extent, and at an undefined remoteness, fragments detached
from which, continually entering the sphere of the sun's attraction,
flit across our skies under the form of comets. These are, however,
almost certainly so far strangers to our system that they had no part in
the long processes of development by which its present condition was
attained. They are, perhaps, survivals of an earlier, and by us scarcely
and dimly conceivable state of things, when the swirling chaos from
which sun and planets were, by a supreme edict, to emerge, had not as
yet separately begun to be.
FOOTNOTES:
[Footnote 1267: _Astr. Nach._, Nos. 1,172-4.]
[Footnote 1268: _Berichte Sächs. Ges._, 1871, p. 174.]
[Footnote 1269: _Natur der Cometen_, p. 124; _Astr. Nach._, No. 2,086.]
[Footnote 1270: _Annales de l'Obs. de Moscou_, t. iii., pt. i., p. 37.]
[Footnote 1271: _Bull. Astr._, t. iii., p. 598. The value of the
repellent force for the comet of 1811 (which offered peculiar facilities
for its determination) was found = 17·5.]
[Footnote 1272: Faye, _Comptes Rendus_, t. xciii., p. 13.]
[Footnote 1273: _Annales_, t. v., pt. ii., p. 137.]
[Footnote 1274: _Am. Jour. of Sc._, vol. xxxii. (2nd ser.), p. 57.]
[Footnote 1275: _Astr. Nach._, No. 2,082.]
[Footnote 1276: _Annales de l'Obs. de Moscou_, t. vi., pt. i., p. 60.]
[Footnote 1277: _Astr. Register_, March, 1883.]
[Footnote 1278: _Astr. Nach._, No. 3,018.]
[Footnote 1279: _Ibid._, No. 3,093.]
[Footnote 1280: _Constitution de l'Espace Céleste_, p. 224.]
[Footnote 1281: _Astroph. Jour._, vol. iii., p. 36.]
[Footnote 1282: _Physikalische Zeitschrift_, November 10 and 17, 1900;
_Astroph. Jour._, vol. xiii., p. 344. _Cf._ Schwarzschild, _Sitzungsb._,
München, 1901, Heft iii.; J. Hahn, _Nature_, vols. lxv., p. 415; lxvi.,
p. 55.]
[Footnote 1283: _Astr. Nach._, No. 2,307.]
[Footnote 1284: _Ibid._, No. 2,304.]
[Footnote 1285: _Observatory_, vol. iii., p. 390.]
[Footnote 1286: _Astr. Nach._, No. 2,319.]
[Footnote 1287: _Ueber die Kometen von 371 v. Chr._, 1668, 1843, I. und
1880 I. Göttingen, 1880.]
[Footnote 1288: _Meteor._, lib. i., cap. 6.]
[Footnote 1289: _Mém. Soc. Phys. de Genève_, t. xxviii., p. 23.]
[Footnote 1290: _Annales de l'Obs. de Moscou_, t. vii., pt. i., p. 60.]
[Footnote 1291: Brédikhine, _Annales_, t. viii., p. 68.]
[Footnote 1292: _Am. Jour. of Sc._, vol. xxii., p. 305.]
[Footnote 1293: Messrs. Burton and Green observed a dilatation of the
stellar image into a nebulous patch by the transmission of its rays
through a nuclear jet of the comet. _Am. Jour. of Sc._, vol. xxii., p.
163.]
[Footnote 1294: _Archives des Sciences_, t. viii., p. 535. _Cf._
Perrine's negative results for Swift's comet in 1899, _Astr. Nach._, No.
3,602.]
[Footnote 1295: Riem concluded in 1896 for a definitive period of 2,429
years; _Observatory_, vol. xix., p. 282.]
[Footnote 1296: Holden, _Publ. Astr. Pac. Soc._, vol. ix., p. 89.]
[Footnote 1297: _Annuaire_, Paris, 1882, p. 781.]
[Footnote 1298: _Annuaire_, 1882, p. 766.]
[Footnote 1299: _Am. Jour. of Sc._, vol. xxii., p. 134.]
[Footnote 1300: _Report Brit. Assoc._, 1881, p. 520.]
[Footnote 1301: _Month. Not._, vol. xlii., p. 14; _Am. Jour. of Sc._,
vol. xxii., p. 136.]
[Footnote 1302: Piazzi Smyth, _Nature_, vol. xxiv., p. 430.]
[Footnote 1303: _Astr. Nach._, No. 2,395.]
[Footnote 1304: _Ibid._]
[Footnote 1305: _Astr. Nach._, No. 2,411.]
[Footnote 1306: _Month. Not._, vol. xlii., p. 49.]
[Footnote 1307: _Astr. Nach._, No. 2,414.]
[Footnote 1308: _Copernicus_, vol. ii., p. 229.]
[Footnote 1309: _Astr. Nach._, Nos. 2,434, 2,437.]
[Footnote 1310: _Ibid._, No. 2,441.]
[Footnote 1311: _Report Brit. Assoc._, 1882, p. 442.]
[Footnote 1312: J. J. Parsons, _Am. Jour. of Science_, vol. xxvii., p.
34.]
[Footnote 1313: _Astr. Nach._, No. 2,441.]
[Footnote 1314: _Observatory_, vol. v., p. 355. The transit had been
foreseen by Mr. Tebbutt, but it occurred after sunset in New South
Wales.]
[Footnote 1315: _Observatory_, vol. v., p. 354.]
[Footnote 1316: Gould, _Astr. Nach._, No. 2,481.]
[Footnote 1317: Flammarion, _Comptes Rendus_, t. xcv., p. 558.]
[Footnote 1318: Captain Ray's sextant observation of the comet of 1843,
a few hours before perihelion, was too rough to be of use.]
[Footnote 1319: _Astr. Nach._, No. 2,538.]
[Footnote 1320: _Nature_, vol. xxix., p. 135.]
[Footnote 1321: _Astr. Nach._, No. 2,482.]
[Footnote 1322: _Vierteljahrsschrift Astr. Ges._, Jahrg. xxiv., p. 308;
_Bull. Astr._, t. vii., p. 513.]
[Footnote 1323: _Observatory_, vol. xxiv., p. 167.]
[Footnote 1324: The attention of the author was kindly directed to this
point by Professor Young of Princeton (N. J.). _Cf._ Rebeur-Paschwitz,
_Sirius_, Bd. xvi., p. 233.]
[Footnote 1325: Oppenheim, _Astr. Nach._, No. 2,902.]
[Footnote 1326: _Astr. Nach._, No. 2,717.]
[Footnote 1327: Gruithuisen's _Analekten_, Heft 7, p. 48.]
[Footnote 1328: _Month. Not._, vols. xxv., xxvi., xxviii. _Cf._ Plummer,
_Observatory_, vol. xiii., p. 263.]
[Footnote 1329: _Nature_, vol. xxvii., p. 246.]
[Footnote 1330: _Astr. Nach._, No. 2,468.]
[Footnote 1331: _Athenæum_, February 3, 1883.]
[Footnote 1332: _Astr. Nach._, Nos. 2,462, 2,466.]
[Footnote 1333: _Ibid._, No. 2,489.]
[Footnote 1334: _Annales_, Moscow, t. ix., pt. ii., p. 52.]
[Footnote 1335: _Comptes Rendus_, t. xcvii., p. 797.]
[Footnote 1336: _Astr. Nach._, No. 2,966.]
[Footnote 1337: _Copernicus_, vol. ii., p. 235.]
[Footnote 1338: _Comptes Rendus_, t. xcvi., p. 371.]
[Footnote 1339: _Astr. Nach._, No. 2,553.]
[Footnote 1340: _Ibid._]
[Footnote 1341: _Astr. Nach._, No. 2,568.]
[Footnote 1342: _Annales de l'Observatoire de Nice_, t. ii., c. 53.]
[Footnote 1343: Fényi, _Astr. Nach._, No. 2,844; Kammermann, _Ibid._,
No. 2,849.]
[Footnote 1344: _Publ. Astr. Pac. Soc._, vol. i., p. 72.]
[Footnote 1345: _Annuaire_, Paris, 1891, p. 301.]
[Footnote 1346: _Astr. Nach._, No. 2,989.]
[Footnote 1347: _Comptes Rendus_, t. xxv., p. 564.]
[Footnote 1348: _Astr. Journ._, Nos. 205, 231.]
[Footnote 1349: _Ibid._, Nos. 228, 244, 380.]
[Footnote 1350: _Observatory_, vol. xviii., pp. 60, 163 (Denning and
Lynn).]
[Footnote 1351: _Astr. Nach._, No. 3,267; Plummer, _Knowledge_, vol.
xix., p. 156.]
[Footnote 1352: _Astr. Jour._, Nos. 333, 338.]
[Footnote 1353: _Astr. and Astroph._, vol. xi., p. 387.]
[Footnote 1354: _Knowledge_, vol. xv., p. 299.]
[Footnote 1355: _Harvard Annals_, vol. xxxii., pt. ii., p. 272.]
[Footnote 1356: _Ibid._, p. 287.]
[Footnote 1357: _Publ. Astr. Pac. Soc._, vol. vii., p. 161.]
[Footnote 1358: H. C. Wilson, _Astr. and Astroph._, vol. xii., p. 121.]
[Footnote 1359: _Observatory_, vol. xvi., p. 92.]
[Footnote 1360: Barnard, _Astr. and Astroph._, vol. xii., p. 180;
_Astroph. Jour._, vol. iii., p. 41.]
[Footnote 1361: Palisa, _Astr. Nach._, No. 3,147; Denning,
_Observatory_, vol. xvi., p. 142.]
[Footnote 1362: _Phil. Trans._, vol. ci., p. 306.]
[Footnote 1363: _Conn. des Temps_, 1816, p. 213.]
[Footnote 1364: _OEuvres_, t. vi., p. 581.]
[Footnote 1365: _Mem. dell' Istit. Lombardo_, t. xii., p. 164;
_Rendiconti_, t. vii., p. 77, 1874.]
[Footnote 1366: W. Förster, _Pop. Mitth._, 1879, p. 7; Fabry, _Étude sur
la Probabilité des Comètes Hyperboliques_, Marseille, 1893, p. 158.]
[Footnote 1367: _Mem. R. A. Soc._, vol. xxix., p. 335.]
[Footnote 1368: _Month. Not._, vol. xxiii., p. 203.]