_RECENT COMETS_
On the 2nd of June, 1858, Giambattista Donati discovered at Florence a
feeble round nebulosity in the constellation Leo, about one-tenth the
diameter of the full moon. It proved to be a comet approaching the sun.
But it changed little in apparent place or brightness for some weeks.
The gradual development of a central condensation of light was the first
symptom of coming splendour. At Harvard, in the middle of July, a strong
stellar nucleus was seen; on August 14 a tail began to be thrown out. As
the comet wanted still over six weeks of the time of its
perihelion-passage, it was obvious that great things might be expected
of it. They did not fail of realisation.
Not before the early days of September was it generally recognised with
the naked eye, though it had been detected without a glass at Pulkowa,
August 19. But its growth was thenceforward surprisingly rapid, as it
swept with accelerated motion under the hindmost foot of the Great Bear,
and past the starry locks of Berenice. A sudden leap upward in lustre
was noticed on September 12, when the nucleus shone with about the
brightness of the pole-star, and the tail, notwithstanding large
foreshortening, could be traced with the lowest telescopic power over
six degrees of the sphere. The appendage, however, attained its full
development only after perihelion, September 30, by which time, too, it
lay nearly square to the line of sight from the earth. On October 10 it
stretched in a magnificent scimitar-like curve over a third and upwards
of the visible hemisphere, representing a real extension in space of
fifty-four million miles. But the most striking view was presented on
October 5, when the brilliant star Arcturus became involved in the
brightest part of the tail, and during many hours contributed, its
lustre undiminished by the interposed nebulous screen, to heighten the
grandeur of the most majestic celestial object of which living memories
retain the impress. Donati's comet was, according to Admiral Smyth's
testimony,[1188] outdone "as a mere _sight_-object" by the great comet
of 1811; but what it lacked in splendour, it surely made up in grace,
and variety of what we may call "scenic" effects.
Some of these were no less interesting to the student than impressive to
the spectator. At Pulkowa, on the 16th September, Winnecke,[1189] the
first director of the Strasburg Observatory, observed a faint outer
envelope resembling a veil of almost evanescent texture flung somewhat
widely over the head. Next evening, the first of the "secondary" tails
appeared, possibly as part of the same phenomenon. This was a narrow
straight ray, forming a tangent to the strong curve of the primary tail,
and reaching to a still greater distance from the nucleus. It continued
faintly visible for about three weeks, during part of which time it was
seen in duplicate. For from the chief train itself, at a point where its
curvature abruptly changed, issued, as if through the rejection of some
of its materials, a second beam nearly parallel to the first, the rigid
line of which contrasted singularly with the softly diffused and waving
aspect of the plume of light from which it sprang. Olbers's theory of
unequal repulsive forces was never more beautifully illustrated. The
triple tail seemed a visible solar analysis of cometary matter.
The processes of luminous emanation going on in this body forcibly
recalled the observations made on the comets of 1744 and 1835. From the
middle of September, the nucleus, estimated by Bond to be under five
hundred miles in diameter, was the centre of action of the most
energetic kind. Seven distinct "envelopes" were detached in succession
from the nebulosity surrounding the head, and after rising towards the
sun during periods of from four to seven days, finally cast their
material backward to form the right and left branches of the great
train. The separation of these by an obscure axis--apparently as black,
quite close up to the nucleus, as the sky--indicated for the tail a
hollow, cone-like structure;[1190] while the repetition of certain spots
and rays in the same corresponding situation on one envelope after
another served to show that the nucleus--to some local peculiarity of
which they were doubtless due--had no proper rotation, but merely
shifted sufficiently on an axis to preserve the same aspect towards the
sun as it moved round it.[1191] This observation of Bond's was strongly
confirmatory of Bessel's hypothesis of opposite polarities in such
bodies' opposite sides.
The protrusion towards the sun, on September 25, of a brilliant luminous
fan-shaped sector completed the resemblance to Halley's comet. The
appearance of the head was now somewhat that of a "bat's-wing" gaslight.
There were, however, no oscillations to and fro, such as Bessel had seen
and speculated upon in 1835. As the size of the nucleus contracted with
approach to perihelion, its intensity augmented. On October 2, it
outshone Arcturus, and for a week or ten days was a conspicuous object
half an hour after sunset. Its lustre--setting aside the light derived
from the tail--was, at that date, 6,300 times what it had been on June
15, though _theoretically_--taking into account, that is, only the
differences of distance from sun and earth--it should have been only
1/33 of that amount. Here, it might be thought, was convincing evidence
of the comet itself becoming ignited under the growing intensity of the
solar radiations. Yet experiments with the polariscope were interpreted
in an adverse sense, and Bond's conclusion that the comet sent us
virtually unmixed reflected sunshine was generally acquiesced in. It
was, nevertheless, negatived by the first application of the
spectroscope to these bodies.
Very few comets have been so well or so long observed as Donati's. It
was visible to the naked eye during 112 days; it was telescopically
discernible for 275, the last observation having been made by Mr.
William Mann at the Cape of Good Hope, March 4, 1859. Its course through
the heavens combined singularly with the orbital place of the earth to
favour curious inspection. The tail, when near its greatest development,
lost next to nothing by the effects of perspective, and at the same time
lay in a plane sufficiently inclined to the line of sight to enable it
to display its exquisite curves to the greatest advantage. Even the
weather was, on both sides of the Atlantic, propitious during the period
of greatest interest, and the moon as little troublesome as possible.
The volume compiled by the younger Bond is a monument to the care and
skill with which these advantages were turned to account. Yet this
stately apparition marked no turning-point in the history of cometary
science. By its study knowledge was indeed materially advanced, but
along the old lines. No quick and vivid illumination broke upon its
path. Quite insignificant objects--as we have already partly seen--have
often proved more vitally instructive.
Donati's comet has been identified with no other. Its path is an
immensely elongated ellipse, lying in a plane far apart from that of the
planetary movements, carrying it at perihelion considerably within the
orbit of Venus, and at aphelion out into space to 5-1/2 times the
distance from the sun of Neptune. The entire circuit occupies over 2,000
years, and is performed in a retrograde direction, or against the order
of the Signs. Before its next return, about the year 4000 A.D., the
enigma of its presence and its purpose may have been to some
extent--though we may be sure not completely--penetrated.
On June 30, 1861, the earth passed, for the second time in the century,
through the tail of a great comet. Some of our readers may remember the
unexpected disclosure, on the withdrawal of the sun below the horizon on
that evening, of an object so remarkable as to challenge universal
attention. A golden-yellow planetary disc, wrapt in dense nebulosity,
shone out while the June twilight of these latitudes was still in its
first strength. The number and complexity of the envelopes surrounding
the head produced, according to the late Mr. Webb,[1192] a magnificent
effect. Portions of six distinct emanations were traceable. "It was as
though a number of light, hazy clouds were floating round a miniature
full moon." As the sky darkened the tail emerged to view.[1193] Although
in brightness and sharpness of definition it could not compete with the
display of 1858, its dimensions proved to be extraordinary. It reached
upwards beyond the zenith when the head had already set. By some
authorities its extreme length was stated at 118°, and it showed no
trace of curvature. Most remarkable, however, was the appearance of two
widely divergent rays, each pointing towards the head, though cut off
from it by sky-illumination, of which one was seen by Mr. Webb, and both
by Mr. Williams at Liverpool, a quarter of an hour before midnight.
There seems no doubt that Webb's interpretation was the true one, and
that these beams were, in fact, "the perspective representation of a
conical or cylindrical tail, hanging closely above our heads, and
probably just being lifted up out of our atmosphere."[1194] The cometary
train was then rapidly receding from the earth, so that the sides of the
"outspread fan" of light shown by it when we were right in the line of
its axis must have appeared (as they did) to close up in departure. The
swiftness with which the visually opened fan shut proved its vicinity;
and, indeed, Mr. Hind's calculations showed that we were not so much
near as actually within its folds at that very time.
Already M. Liais, from his observations at Rio de Janeiro, June 11 to
14, and Mr. Tebbutt, by whom the comet was discovered in New South Wales
on May 13, had anticipated such an encounter, while the former
subsequently proved that it must have occurred in such a way as to cause
an immersion of the earth in cometary matter to a depth of 300,000
miles.[1195] The comet then lay between the earth and the sun at a
distance of about fourteen million miles from the former; its tail
stretched outward just along the line of intersection of its own with
the terrestrial orbit to an extent of fifteen million miles; so that our
globe, happening to pass at the time, found itself during some hours
involved in the flimsy appendage.
No perceptible effects were produced by the meeting; it was known to
have occurred by theory alone. A peculiar glare in the sky, thought by
some to have distinguished the evening of June 30, was, at best,
inconspicuous. Nor were there any symptoms of unusual electric
excitement. The Greenwich instruments were, indeed, disturbed on the
following night, but it would be rash to infer that the comet had art or
part in their agitation.
The perihelion-passage of this body occurred June 11, 1861; and its
orbit has been shown by M. Kreutz of Bonn, from a very complete
investigation founded on observations extending over nearly a year, to
be an ellipse traversed in a period of 409 years.[1196]
Towards the end of August, 1862, a comet became visible to the naked eye
high up in the northern hemisphere, with a nucleus equalling in
brightness the lesser stars of the Plough and a feeble tail 20° in
length. It thus occupied quite a secondary position among the members of
its class. It was, nevertheless, a splendid object in comparison with a
telescopic nebulosity discovered by Tempel at Marseilles, December 19,
1865. This, the sole comet of 1866, slipped past perihelion, January 11,
without pomp of train or other appendages, and might have seemed hardly
worth the trouble of pursuing. Fortunately, this was not the view
entertained by observers and computers; since upon the knowledge
acquired of the movements of these two bodies has been founded one of
the most significant discoveries of modern times. The first of them is
now styled the comet (1862 iii.) of the August meteors, the second (1866
i.) that of the November meteors. The steps by which this curious
connection came to be ascertained were many, and were taken in
succession by a number of individuals. But the final result was reached
by Schiaparelli of Milan, and remains deservedly associated with his
name.
The idea prevalent in the eighteenth century as to the nature of
shooting stars was that they were mere aerial _ignes fatui_--inflammable
vapours accidentally kindled in our atmosphere. But Halley had already
entertained the opinion of their cosmical origin; and Chladni in 1794
formally broached the theory that space is filled with minute
circulating atoms, which, drawn by the earth's attraction, and ignited
by friction in its gaseous envelope, produce the luminous effects so
frequently witnessed.[1197] Acting on his suggestion, Brandes and
Benzenberg, two students at the University of Göttingen, began in 1798
to determine the heights of falling stars by simultaneous observations
at a distance. They soon found that they move with planetary velocities
in the most elevated regions of our atmosphere, and by the ascertainment
of this fact laid a foundation of distinct knowledge regarding them.
Some of the data collected, however, served only to perplex opinion, and
even caused Chladni temporarily to renounce his. Many high authorities,
headed by Laplace in 1802, declared for the lunar-volcanic origin of
meteorites; but thought on the subject was turbid, and inquiry seemed
only to stir up the mud of ignorance. It needed one of those amazing
spectacles, at which man assists, no longer in abject terror for his own
frail fortunes, but with keen curiosity and the vivid expectation of new
knowledge, to bring about a clarification.
On the night of November 12-13, 1833, a tempest of falling stars broke
over the earth. North America bore the brunt of its pelting. From the
Gulf of Mexico to Halifax, until daylight with some difficulty put an
end to the display, the sky was scored in every direction with shining
tracks and illuminated with majestic fireballs. At Boston the frequency
of meteors was estimated to be about half that of flakes of snow in an
average snowstorm. Their numbers, while the first fury of their coming
lasted, were quite beyond counting; but as it waned, a reckoning was
attempted, from which it was computed, on the basis of that much
diminished rate, that 240,000 must have been visible during the nine
hours they continued to fall.[1198]
Now there was one very remarkable feature common to the innumerable
small bodies which traversed, or were consumed in our atmosphere that
night. _They all seemed to come from the same part of the sky._ Traced
backward, their paths were invariably found to converge to a point in
the constellation Leo. Moreover, that point travelled with the stars in
their nightly round. In other words, it was entirely independent of the
earth and its rotation. It was a point in inter-planetary space.
The _effective_ perception of this fact[1199] amounted to a discovery,
as Olmsted and Twining, who had "simultaneous ideas" on the subject,
were the first to realize. Denison Olmsted was then Professor of
Mathematics in Yale College. He showed early in 1834[1200] that the
emanation of the showering meteors from a fixed "radiant" proved their
approach to the earth along nearly parallel lines, appearing to diverge
by an effect of perspective; and that those parallel lines must be
sections of orbits described by them round the sun and intersecting that
of the earth. For the November phenomenon was now seen to be a
periodical one. On the same night of the year 1832, although with less
dazzling and universal splendour than in America in 1833, it had been
witnessed over great part of Europe and in Arabia. Olmsted accordingly
assigned to the cloud of cosmical particles (or "comet," as he chose to
call it), by terrestrial encounters with which he supposed the
appearances in question to be produced, a period of about 182 days; its
path a narrow ellipse, meeting, near its farthest end from the sun, the
place occupied by the earth on November 12.
Once for all, then, as the result of the star-fall of 1833, the study of
luminous meteors became an integral part of astronomy. Their membership
of the solar system was no longer a theory or a conjecture--it was an
established fact. The discovery might be compared to, if it did not
transcend in importance, that of the asteroidal group. "C'est un nouveau
monde planétaire," Arago wrote,[1201] "qui commence à se révéler à
nous."
Evidences of periodicity continued to accumulate. It was remembered that
Humboldt and Bonpland had been the spectators at Cumana, after midnight
on November 12, 1799, of a fiery shower little inferior to that of 1833,
and reported to have been visible from the equator to Greenland.
Moreover, in 1834 and some subsequent years, there were waning
repetitions of the display, as if through the gradual thinning-out of
the meteoric supply. The extreme irregularity of its distribution was
noted by Olbers in 1837, who conjectured that we might have to wait
until 1867 to see the phenomenon renewed on its former scale of
magnificence.[1202] This was the first hint of a thirty-three or
thirty-four year period.
The falling stars of November did not alone attract the attention of the
learned. Similar appearances were traditionally associated with August
10 by the popular phrase in which they figured as "the tears of St.
Lawrence." But the association could not be taken on trust from mediæval
authority. It had to be proved scientifically, and this Quetelet of
Brussels succeeded in doing in December, 1836.[1203]
A second meteoric revolving system was thus shown to exist. But its
establishment was at once perceived to be fatal to the "cosmical cloud"
hypothesis of Olmsted. For if it be a violation of probability to
attribute to one such agglomeration a period of an exact year, or
sub-multiple of a year, it would be plainly absurd to suppose the
movements of _two_ or more regulated by such highly artificial
conditions. An alternative was proposed by Adolf Erman of Berlin in
1839.[1204] No longer in _clouds_, but in closed _rings_, he supposed
meteoric matter to revolve round the sun. Thus the mere circumstance of
intersection by a meteoric of the terrestrial orbit, without any
coincidence of period, would account for the earth meeting some members
of the system at each annual passage through the "node" or point of
intersection. This was an important step in advance, yet it decided
nothing as to the forms of the orbits of such annular assemblages; nor
was it followed up in any direction for a quarter of a century.
Hubert A. Newton took up, in 1864,[1205] the dropped thread of inquiry.
The son of a mathematical mother, he attained, at the age of
twenty-five, to the dignity of Professor of Mathematics in Yale
University, and occupied the post until his death in 1896. The diversion
of his powers, however, from purely abstract studies stimulated their
effective exercise, and constituted him one of the founders of meteoric
astronomy.
A search through old records carried the November phenomenon back to the
year 902 A.D., long distinguished as "the year of the stars." For in the
same night in which Taormina was captured by the Saracens, and the cruel
Aghlabite tyrant Ibrahim ibn Ahmed died "by the judgment of God" before
Cosenza, stars fell from heaven in such abundance as to amaze and
terrify beholders far and near. This was on October 13, and recurrences
were traced down through the subsequent centuries, always with a day's
delay in about seventy years. It was easy, too, to derive from the dates
a cycle of 33-1/4 years, so that Professor Newton did not hesitate to
predict the exhibition of an unusually striking meteoric spectacle on
November 13-14, 1866.[1206]
For the astronomical explanation of the phenomena, recourse was had to a
method introduced by Erman of computing meteoric orbits. It was found,
however, that conspicuous recurrences every thirty-three or thirty-four
years could be explained on the supposition of five widely different
periods, combined with varying degrees of extension in the revolving
group. Professor Newton himself gave the preference to the shortest--of
354-1/2 days, but indicated the means of deciding with certainty upon
the true one. It was furnished by the advancing motion of the node, or
that day's delay of the November shower every seventy years, which the
old chronicles had supplied data for detecting. For this is a strictly
measurable effect of gravitational disturbance by the various planets,
the amount of which naturally depends upon the course pursued by the
disturbed bodies. Here the great mathematical resources of Professor
Adams were brought to bear. By laborious processes of calculation, he
ascertained that four out of Newton's five possible periods were
entirely incompatible with the observed nodal displacement, while for
the fifth--that of 33-1/4 years--a perfectly harmonious result was
obtained.[1207] This was the last link in the chain of evidence proving
that the November meteors--or "Leonids," as they had by that time come
to be called--revolve round the sun in a period of 33·27 years, in an
ellipse spanning the vast gulf between the orbits of the earth and
Uranus, the group being so extended as to occupy nearly three years in
defiling past the scene of terrestrial encounters. But before it was
completed in March, 1867, the subject had assumed a new aspect and
importance.
Professor Newton's prediction of a remarkable star-shower in November,
1866, was punctually fulfilled. This time, Europe served as the main
target of the celestial projectiles, and observers were numerous and
forewarned. The display, although, according to Mr. Baxendell's
memory,[1208] inferior to that of 1833, was of extraordinary
impressiveness. Dense crowds of meteors, equal in lustre to the
brightest stars, and some rivalling Venus at her best,[1209] darted from
east to west across the sky with enormous apparent velocities, and with
a certain determinateness of aim, as if let fly with a purpose, and at
some definite object.[1210] Nearly all left behind them trains of
emerald green or clear blue light, which occasionally lasted many
minutes, before they shrivelled and curled up out of sight. The maximum
rush occurred a little after one o'clock on the morning of November 14,
when attempts to count were overpowered by frequency. But during a
previous interval of seven minutes five seconds, four observers at Mr.
Bishop's observatory at Twickenham reckoned 514, and during an hour
1,120.[1211] Before daylight the earth had fairly cut her way through
the star-bearing stratum; the "ethereal rockets" had ceased to fly.
This event brought the subject of shooting stars once more vividly to
the notice of astronomers. Schiaparelli had, indeed, been already
attracted by it. The results of his studies were made known in four
remarkable letters, addressed, before the close of the year 1866, to
Father Secchi, and published in the _Bulletino_ of the Roman
Observatory.[1212] Their upshot was to show, in the first place, that
meteors possess a real velocity considerably greater than that of the
earth, and travel, accordingly, to enormously greater distances from the
sun along tracks resembling those of comets in being very eccentric, in
lying at all levels indifferently, and in being pursued in either
direction. It was next inferred that comets and meteors equally have an
origin foreign to the solar system, but are drawn into it temporarily by
the sun's attraction, and occasionally fixed in it by the backward pull
of some planet. But the crowning fact was reserved for the last. It was
the astonishing one that the August meteors move in the same orbit with
the bright comet of 1862--that the comet, in fact, is but a larger
member of the family named "Perseids" because their radiant point is
situated in the constellation Perseus.
This discovery was quickly capped by others of the same kind. Leverrier
published, January 21, 1867,[1213] elements for the November swarm,
founded on the most recent and authentic observations; at once
identified by Dr. C. F. W. Peters of Altona with Oppolzer's elements for
Tempel's comet of 1866.[1214] A few days later, Schiaparelli, having
recalculated the orbit of the meteors from improved data, arrived at the
same conclusion; while Professor Weiss of Vienna pointed to the
agreement between the orbits of a comet which had appeared in 1861 and
of a star-shower found to recur on April 20 (Lyraïds), as well as
between those of Biela's comet and certain conspicuous meteors of
November 28.[1215]
These instances do not seem to be exceptional. The number of known or
suspected accordances of cometary tracks with meteor streams contained
in a list drawn up in 1878[1216] by Professor Alexander S. Herschel (who
has made the subject peculiarly his own) amounts to seventy-six;
although the four first detected still remain the most conspicuous, and
perhaps the only absolutely sure examples of a relation as significant
as it was, to most astronomers, unexpected.
There had, indeed, been anticipatory ideas. Not that Kepler's comparison
of shooting stars to "minute comets," or Maskelyne's "forse risulterà
che essi sono comete," in a letter to the Abate Cesaris, December 12,
1774,[1217] need count for much. But Chladni, in 1819,[1218] considered
both to be fragments or particles of the same primitive matter,
irregularly scattered through space as nebulæ; and Morstadt of Prague
suggested about 1837[1219] that the meteors of November might be
dispersed atoms from the tail of Biela's comet, the path of which is cut
across by the earth near that epoch. Professor Kirkwood, however, by a
luminous intuition, penetrated the whole secret, so far as it has yet
been made known. In an article published, or rather buried, in the
_Danville Quarterly Review_ for December, 1861, he argued, from the
observed division of Biela, and other less noted instances of the same
kind, that the sun exercises a "divellent influence" on the nuclei of
comets, which may be presumed to continue its action until their
corporate existence (so to speak) ends in complete pulverisation. "May
not," he continued, "our periodic meteors be the débris of ancient but
now disintegrated comets, whose matter has become distributed round
their orbits?"[1220]
The gist of Schiaparelli's discovery could not be more clearly conveyed.
For it must be borne in mind that with the ultimate destiny of comets'
tails this had nothing to do. The tenuous matter composing them is, no
doubt, permanently lost to the body from which it emanated; but science
does not pretend to track its further wanderings through space. It can,
however, state categorically that these will no longer be conducted
along the paths forsaken under solar compulsion. From the central, and
probably solid parts of comets, on the other hand, are derived the
granules by the swift passage of which our skies are seamed with
periodic fires. It is certain that a loosely agglomerated mass (such as
cometary nuclei most likely are) must gradually separate through the
unequal action of gravity on its various parts--through, in short, solar
tidal influence. Thenceforward its fragments will revolve independently
in parallel orbits, at first as a swarm, finally--when time has been
given for the full effects of the lagging of the slower moving particles
to develop--as a closed ring. The first condition is still, more or
less, that of the November meteors; those of August have already arrived
at the second. For this reason, Leverrier pronounced, in 1867, the
Perseid to be of older formation than the Leonid system. He even
assigned a date at which the introduction of the last-named bodies into
their present orbit was probably effected through the influence of
Uranus. In 126 A.D. a close approach must have taken place between the
planet and the parent comet of the November stars, after which its
regular returns to perihelion, and the consequent process of its
disintegration, set in. Though not complete, it is already far advanced.
The view that meteorites are the dust of decaying comets was now to be
put to a definite test of prediction. Biela's comet had not been seen
since its duplicate return in 1852. Yet it had been carefully watched
for with the best telescopes; its path was accurately known; every
perturbation it could suffer was scrupulously taken into account. Under
these circumstances, its repeated failure to come up to time might
fairly be thought to imply a cessation from visible existence. Might it
not, however, be possible that it would appear under another form--that
a star-shower might have sprung from and would commemorate its
dissolution?
An unusually large number of falling stars were seen by Brandes,
December 6, 1798. Similar displays were noticed in the years 1830, 1838,
and 1847, and the point from which they emanated was shown by Heis at
Aix-la-Chapelle to be situated near the bright star Gamma
Andromedæ.[1221] Now this is precisely the direction in which the orbit
of Biela's comet would seem to lie, as it runs down to cut the
terrestrial track very near the place of the earth at the above dates.
The inference was, then, an easy one, that the meteors were pursuing the
same path with the comet; and it was separately arrived at, early in
1867, by Weiss, D'Arrest, and Galle.[1222] But Biela travels in the
opposite direction to Tempel's comet and its attendant "Leonids"; its
motion is direct, or from west to east, while theirs is retrograde.
Consequently, the motion of its node is in the opposite direction too.
In other words, the meeting-place of its orbit with that of the earth
retreats (and very rapidly) along the ecliptic instead of advancing. So
that if the "Andromedes" stood in the supposed intimate relation to
Biela's comet, they might be expected to anticipate the times of their
recurrence by as much as a week in half a century. All doubt as to the
fact may be said to have been removed by Signor Zezioli's observation of
the annual shower in more than usual abundance at Bergamo, November 30,
1867.
The missing comet was next due at perihelion in the year 1872, and the
probability was contemplated by both Weiss and Galle of its being
replaced by a copious discharge of falling stars. The precise date of
the occurrence was not easily determinable, but Galle thought the
chances in favour of November 28. The event anticipated the prediction
by twenty-four hours. Scarcely had the sun set in Western Europe on
November 27, when it became evident that Biela's comet was shedding over
us the pulverised products of its disintegration. The meteors came in
volleys from the foot of the Chained Lady, their numbers at times
baffling the attempt to keep a reckoning. At Moncalieri, about 8 p.m.,
they constituted (as Father Denza said[1223]) a "real rain of fire."
Four observers counted, on an average, four hundred each minute and a
half; and not a few fireballs, equalling the moon in diameter, traversed
the sky. On the whole, however, the stars of 1872, though about equally
numerous, were less brilliant than those of 1866; the phosphorescent
tracks marking their passage were comparatively evanescent and their
movements sluggish. This is easily understood when we remember that the
Andromedes _overtake_ the earth, while the Leonids rush to meet it; the
velocity of encounter for the first class of bodies being under twelve,
for the second above forty-four miles a second. The spectacle was,
nevertheless, magnificent. It presented itself successively to various
parts of the earth, from Bombay and the Mauritius to New Brunswick and
Venezuela, and was most diligently and extensively observed. Here it had
well-nigh terminated by midnight.[1224]
It was attended by a slight aurora, and although Tacchini had
telegraphed that the state of the sun rendered some show of polar lights
probable, it has too often figured as an accompaniment of star-showers
to permit the coincidence to rank as fortuitous. Admiral Wrangel was
accustomed to describe how, during the prevalence of an aurora on the
Siberian coast, the passage of a meteor never failed to extend the
luminosity to parts of the sky previously dark;[1225] and an enhancement
of electrical disturbance may well be associated with the flittings of
such cosmical atoms.
A singular incident connected with the meteors of 1872 has now to be
recounted. The late Professor Klinkerfues, who had observed them very
completely at Göttingen, was led to believe that not merely the débris
strewn along its path, but the comet itself must have been in immediate
proximity to the earth during their appearance.[1226] If so, it might be
possible, he thought, to descry it as it retreated in the diametrically
opposite direction from that in which it had approached. On November 30,
accordingly, he telegraphed to Mr. Pogson, the Madras astronomer, "Biela
touched earth November 27; search near Theta Centauri"--the
"anti-radiant," as it is called, being situated close to that star. Bad
weather prohibited observation during thirty-six hours, but when the
rain clouds broke on the morning of December 2, there a comet was, just
in the indicated position. In appearance it might have passed well
enough for one of the Biela twins. It had no tail, but a decided
nucleus, and was about 45 seconds across, being thus altogether below
the range of naked-eye discernment. It was again observed December 3,
when a short tail was perceptible; but overcast skies supervened, and it
has never since been seen. Its identity accordingly remains in doubt. It
seems tolerably certain, however, that it was _not_ the lost comet,
which ought to have passed that spot twelve weeks earlier, and was
subject to no conceivable disturbance capable of delaying to that extent
its revolution. On the other hand, there is the strongest likelihood
that it belonged to the same system[1227]--that it was a third fragment,
torn from the parent-body of the Andromedes at a period anterior to our
first observations of it.
In thirteen years Biela's comet (or its relics) travels nearly twice
round its orbit, so that a renewal of the meteoric shower of 1872 was
looked for on the same day of the year 1885, the probability being
emphasised by an admonitory circular from Dunecht. Astronomers were
accordingly on the alert, and were not disappointed. In England,
observation was partially impeded by clouds; but at Malta, Palermo,
Beyrout, and other southern stations, the scene was most striking. The
meteors were both larger and more numerous than in 1872. Their numbers
in the densest part of the drift were estimated by Professor Newton at
75,000 per hour, visible from one spot to so large a group of spectators
that practically none could be missed. Yet each of these multitudinous
little bodies was found by him to travel in a clear cubical space of
which the edge measured twenty miles![1228] Thus the dazzling effect of
a luminous throng was produced without jostling or overcrowding, by
particles, it might almost be said, isolated in the void.
Their aspect was strongly characteristic of the Andromede family of
meteors. "They invariably," Mr. Denning wrote,[1229] "traversed short
paths with very slow motions, and became extinct in evolved streams of
yellowish sparks." The conclusion seemed obvious "that these meteors are
formed of very soft materials, which expand while incalescent, and are
immediately crumbled and dissipated into exiguous dust."
The Biela meteors of 1885 did not merely gratify astronomers with a
fulfilled prediction, but were the means of communicating to them some
valuable information. Although their main body was cut through by the
moving earth in six hours, and was not more than 100,000 miles across,
skirmishers were thrown out to nearly a million miles on either side of
the compact central battalions. Members of the system were, on the 26th
of November, recorded by Mr. Denning at the hourly rate of about 130;
and they did not wholly cease to be visible until December 1. They
afforded besides a particularly well-marked example of that diffuseness
of radiation previously observed in some less conspicuous displays.
Their paths seemed to diverge from an area rather than from a point in
the sky. They came so ill to focus that divergences of several degrees
were found between the most authentically determined radiants. These
incongruities are attributed by Professor Newton to the irregular shape
of the meteoroids producing unsymmetrical resistance from the air, and
hence causing them to glance from their original direction on entering
it. Thus, their luminous tracks did not always represent (even apart
from the effects of the earth's attraction) the true prolongation of
their course through space.
The Andromedes of 1872 were laggards behind the comet from which they
sprang; those of 1885 were its avant-couriers. That wasted and disrupted
body was not due at the node until January 26, 1886, sixty days, that
is, after the earth's encounter with its meteoric fragments. These are
now probably scattered over more than five hundred million miles of its
orbits;[1230] yet Professor Newton considers that all must have formed
one compact group with Biela at the time of its close approach to
Jupiter about the middle of 1841. For otherwise both comet and
meteorites could not have experienced, as they seem to have done, the
same kind and amount of disturbance. The rapidity of cometary
disintegration is thus curiously illustrated.
A short-lived persuasion that the missing heavenly body itself had been
recovered, was created by Mr. Edwin Holms's discovery, at London,
November 6, 1892, of a tolerably bright, tailless comet, just in a spot
which Biela's comet must have traversed in approaching the intersection
of its orbit with that of the earth. A hasty calculation by Berberich
assigned elements to the newcomer seeming not only to ratify the
identity, but to promise a quasi-encounter with the earth on November
21. The only effect of the prediction, however, was to raise a panic
among the negroes of the Southern States of America. The comet quietly
ignored it, and moved away from instead of towards the appointed
meeting-place. Its projection, then, on the night of its discovery, upon
a point of the Biela-orbit was by a mere caprice of chance. North
America, nevertheless, was visited on November 23 by a genuine Andromede
shower. Although the meteors were less numerous than in 1885, Professor
Young estimated that 30,000, at the least, of their orange fire-streaks
came, during five hours, within the range of view at Princeton.[1231]
Brédikhine estimated the width of the space containing them at about
2,700,000 miles.[1232] The anticipation of their due time by four days
implied--if they were a prolongation of the main Biela group, the
nucleus of which passed the spot of encounter five months previously--a
recession of the node since 1885 by no less than three degrees. Unless,
indeed, Mr. Denning were right in supposing the display to have
proceeded from "an associated branch of the main swarm through which we
passed in 1872 and 1885."[1233] The existence of separated detachments
of Biela meteors, due to disturbing planetary action, was contemplated
as highly probable by Schiaparelli.[1234] Such may have been the belated
flights met with in 1830, 1838, 1841, and 1847, and such the advance
flight plunged through in 1892. A shower looked for November 23, 1899,
did not fall, and no further display from this quarter is probable until
November 17, 1905, although one is possible a year earlier.[1235]
The Leonids, through the adverse influence of Jupiter and Saturn,
inflicted upon multitudes of eager watchers a still more poignant
disappointment. A dense part of the swarm, having nearly completed a
revolution since 1866, should, travelling normally, have met the earth
November 15, 1899; in point of fact, it swerved sunward, and the
millions of meteorites which would otherwise have been sacrificed for
the illumination of our skies escaped a fiery doom. The contingency had
been forecast in the able calculations of Dr. Johnstone Stoney and Dr.
A. M. W. Downing,[1236] superintendent of the Nautical Almanac Office;
but the verification scarcely compensated the failure. Nor was the
situation retrieved in the following years. Only ragged fringes of the
great tempest-cloud here and there touched our globe. As the same
investigators warned us to expect, the course of the meteorites had been
not only rendered sinuous by perturbation, but also broken and
irregular. We can no longer count upon the Leonids. Their glory, for
scenic purposes, is departed. The comet associated with them also evaded
observation. Although it doubtless kept its tryst with the sun in the
spring of 1899, the attendant circumstances were too unfavourable to
allow it to be seen from the earth.[1237] By an almost fantastic
coincidence, nevertheless, a faint comet was photographed, November 14,
1898,[1238] by Dr. Chase, of the Yale College Observatory, close to the
Leonid radiant, whither a "meteorograph" was directed with a view to
recording trails left by precursors of the main Leonid body. A promising
start, too, was made on the same occasion with meteoric researches from
sensitive plates.[1239] Indeed, Schaeberle and Colton[1240] had already,
in 1896, determined the height of a Leonid by means of photographs taken
at stations on different ridges of Mount Hamilton; and Professor
Pickering has prosecuted similar work at Harvard, with encouraging
results. Everything in this branch of science depends upon how far they
can be carried. Without the meteorograph, rigid accuracy in the
observation of shooting stars is unattainable, and rigid accuracy is the
_sine quâ non_ for obtaining exact knowledge.
Biela does not offer the only example of cometary disruption. Setting
aside the unauthentic reports of early chroniclers, we meet the "double
comet" discovered by Liais at Olinda (Brazil), February 27, 1860, of
which the division appeared recent, and about to be carried
farther.[1241] But a division once established, separation must
continually progress. The periodic times of the fragments will never be
identical; one must drop a little behind the other at each revolution,
until at length they come to travel in remote parts of nearly the same
orbit. Thus the comet predicted by Klinkerfues and discovered by Pogson
had already lagged to the extent of twelve weeks, and we shall meet
instances farther on where the retardation is counted, not by weeks, but
by years. Here original identity emerges only from calculation and
comparison of orbits.
Comets, then, die, as Kepler wrote long ago, _sicut bombyces filo
fundendo_. This certainty, anticipated by Kirkwood in 1861, we have at
least acquired from the discovery of their generative connection with
meteors. Nay, their actual materials become, in smaller or larger
proportions, incorporated with our globe. It is not, indeed, universally
admitted that the ponderous masses of which, according to Daubrée's
estimate,[1242] at least 600 fall annually from space upon the earth,
ever formed part of the bodies known to us as comets. Some follow
Tschermak in attributing to aerolites a totally different origin from
that of periodical shooting-stars. That no clear line of demarcation can
be drawn is no valid reason for asserting that no real distinction
exists; and it is certainly remarkable that a meteoric fusillade may be
kept up for hours without a single solid projectile reaching its
destination. It would seem as if the celestial army had been supplied
with blank cartridges. Yet, since a few detonating meteors have been
found to proceed from ascertained radiants of shooting-stars, it is
difficult to suppose that any generic difference separates them.
Their assimilation is further urged--though not with any demonstrative
force--by two instances, the only two on record, of the tangible descent
of an aerolite during the progress of a star-shower. On April 4, 1095,
the Saxon Chronicle informs us that stars fell "so thickly that no man
could count them," and adds that one of them having struck the ground in
France, a bystander "cast water upon it, which was raised in steam with
a great noise of boiling."[1243] And again, on November 27, 1885, while
the skirts of the Andromede-tempest were trailing over Mexico, "a ball
of fire" was precipitated from the sky at Mazapil, within view of a
ranchman.[1244] Scientific examination proved it to be a "siderite," or
mass of "nickel-iron"; its weight exceeded eight pounds, and it
contained many nodules of graphite. We are not, however, authorised by
the circumstances of its arrival to regard the Mazapil fragment of
cosmic metal as a specimen torn from Biela's comet. In this, as in the
preceding case, the coincidence of the fall with the shower may have
been purely casual, since no hint is given of any sort of agreement
between the tracks followed by the sample provided for curious study,
and the swarming meteors consumed in the upper air.
Professor Newton's inquiries into the tracks pursued by meteorites
previous to their collisions with the earth tend to distinguish them, at
least specifically, from shooting-stars. He found that nearly all had
been travelling with a direct movement in orbits the perihelia of which
lay in the outer half of the space separating the earth from the
sun.[1245] Shooting-stars, on the contrary, are entirely exempt from
such limitations. The Yale Professor concluded "that the larger
meteorites moving in our solar system are allied much more closely with
the group of comets of short period than with the comets whose orbits
are nearly parabolic." They would thus seem to be more at home than
might have been expected amid the planetary family. Father Carbonelle
has, moreover, shown[1246] that meteorites, if explosion-products of the
earth or moon, should, with rare exceptions, follow just the kind of
paths assigned to them, from data of observation, by Professor Newton.
Yet it is altogether improbable that projectiles from terrestrial
volcanoes should, at any geological epoch, have received impulses
powerful enough to enable them, not only to surmount the earth's
gravity, but to penetrate its atmosphere.
A striking--indeed, an almost startling--peculiarity, on the other
hand, divides from their congeners a class of meteors identified by
Mr. Denning during ten years' patient watching of such phenomena at
Bristol.[1247] These are described as "meteors with stationary
radiants," since for months together they seem to come from the same
fixed points in the sky. Now this implies quite a portentous velocity.
The direction of meteor-radiants is affected by a kind of
_aberration_, analogous to the aberration of light. It results from a
composition of terrestrial with meteoric motion. Hence, unless that of
the earth in its orbit be by comparison insignificant, the visual line
of encounter must shift, if not perceptibly from day to day, at any
rate conspicuously from month to month. The fixity, then, of many
systems observed by Mr. Denning seems to demand the admission that
their members travel so fast as to throw the earth's movement
completely out of the account. The required velocity would be, by Mr.
Ranyard's calculation, at least 880 miles a second.[1248] But the
aspect of the meteors justifies no such extravagant assumption. Their
seeming swiftness is very various, and--what is highly significant--it
is notably less when they pursue than when they meet the earth. Yet
the "incredible and unaccountable"[1249] fact of the existence of
these "long radiants," although doubted by Tisserand[1250] because of
its theoretical refractoriness, must apparently be admitted. The first
plausible explanation of them was offered by Professor Turner in
1899.[1251] They represent, in his view, the cumulative effects of the
earth's attraction. The validity of his reasoning is, however, denied
by M. Brédikhine,[1252] who prefers to regard them as a congeries of
separate streams. The enigma they present has evidently not yet
received its definitive solution.
The Perseids afford, on the contrary, a remarkable instance of a
"shifting radiant." Mr. Denning's observations of these yellowish,
leisurely meteors extend over nearly six weeks, from July 8 to August
16; the point of radiation meantime progressing no less than 57° in
right ascension. Doubts as to their common origin were hence freely
expressed, especially by Mr. Monck of Dublin.[1253] But the late Dr.
Kleiber[1254] showed, by strict geometrical reasoning, that the
forty-nine radiants successively determined for the shower were all, in
fact, comprised within one narrowly limited region of space. In other
words, the application of the proper correction for the terrestrial
movement, and the effects of attraction by which each individual
shooting-star is compelled to describe a hyperbola round the earth's
centre, reduces the extended line of radiants to a compact group, with
the cometary radiant for its central point; the cometary radiant being
the spot in the sky met by a tangent to the orbit of the Perseid comet
of 1862 at its intersection with the orbit of the earth. The reality of
the connection between the comet and the meteors could scarcely be more
clearly proved; while the vast dimensions of the stream into which the
latter are found to be diffused cannot but excite astonishment not
unmixed with perplexity.
The first successful application of the spectroscope to comets was by
Donati in 1864.[1255] A comet discovered by Tempel, July 4, brightened
until it appeared like a star somewhat below the second magnitude, with
a feeble tail 30° in length. It was remarkable as having, on August 7,
almost totally eclipsed a small star--a very rare occurrence.[1256] On
August 5 Donati admitted its light through his train of prisms, and
found it, thus analysed, to consist of three bright bands--yellow,
green, and blue--separated by wider dark intervals. This implied a good
deal. Comets had previously been considered, as we have seen, to shine
mainly, if not wholly, by reflected sunlight. They were now perceived to
be self-luminous, and to be formed, to a large extent, of glowing gas.
The next step was to determine what _kind_ of gas it was that was thus
glowing in them; and this was taken by Sir William Huggins in
1868.[1257]
A comet of subordinate brilliancy, known as comet 1868 ii., or sometimes
as Winnecke's, was the subject of his experiment. On comparing its
spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by
electricity, he found the agreement exact. It has since been abundantly
confirmed. All the eighteen comets tested by light analysis, between
1868 and 1880, showed the typical hydro-carbon spectrum[1258] common to
the whole group of those compounds, but probably due immediately to the
presence of acetylene. Some minor deviations from the laboratory
pattern, in the shifting of the maxima of light from the edge towards
the middle of the yellow and blue bands, have been experimentally
reproduced by Vogel and Hasselberg in tubes containing a mixture of
carbonic oxide with olefiant gas.[1259] Their illumination by disruptive
electric discharges was, however, a condition _sine quâ non_ for the
exhibition of the cometary type of spectrum. When a continuous current
was employed, the carbonic oxide bands asserted themselves to the
exclusion of the hydro-carbons. The distinction has great significance
as regards the nature of comets. Of particular interest in this
connection is the circumstance that carbonic oxide is one of the gases
evolved by meteoric stones and irons under stress of heat.[1260] For it
must apparently have formed part of an aeriform mass in which they were
immersed at an earlier stage of their history.
PLATE II.
[Illustration: Great Comet.
Photographed, May 5, 1901, with the thirteen-inch Astrographic Refractor
of the Royal Observatory, Cape of Good Hope.]
In a few exceptional comets the usual carbon-bands have been missed. Two
such were observed by Sir William Huggins in 1866 and 1867
respectively.[1261] In each a green ray, approximating in position to
the fundamental nebular line, crossed an otherwise unbroken spectrum.
And Holmes's comet of 1892 displayed only a faint prismatic band devoid
of any characteristic feature.[1262] Now these three might well be set
down as partially effete bodies; but a brilliant comet, visible in
southern latitudes in April and May, 1901, so far resembled them in the
quality of its light as to give a spectrum mainly, if not purely,
continuous. This, accordingly, is no symptom of decay.
The earliest comet of first-class lustre to present itself for
spectroscopic examination was that discovered by Coggia at Marseilles,
April 17, 1874. Invisible to the naked eye till June, it blazed out in
July a splendid ornament of our northern skies, with a just perceptibly
curved tail, reaching more than half way from the horizon to the zenith,
and a nucleus surpassing in brilliancy the brightest stars in the Swan.
Brédikhine, Vogel, and Huggins[1263] were unanimous in pronouncing its
spectrum to be that of marsh or olefiant gas. Father Secchi, in the
clear sky of Rome, was able to push the identification even closer than
had heretofore been done. The _complete_ hydro-carbon spectrum consists
of five zones of variously coloured light. Three of these only--the
three central ones--had till then been obtained from comets; owing, it
was supposed, to their temperature not being high enough to develop the
others. The light of Coggia's comet, however, was found to contain all
five, traces of the violet band emerging June 4, of the red, July
2.[1264] Presumably, all five would show universally in cometary
spectra, were the dispersed rays strong enough to enable them to be
seen.
The gaseous surroundings of comets are, then, largely made up of a
compound of hydrogen with carbon. Other materials are also present; but
the hydro-carbon element is probably unfailing and predominant. Its
luminosity is, there is little doubt, an effect of electrical
excitement. Zöllner showed in 1872[1265] that, owing to evaporation and
other changes produced by rapid approach to the sun, electrical
processes of considerable intensity must take place in comets; and that
their original light is immediately connected with these, and depends
upon solar radiation, rather through its direct or indirect electrifying
effects, than through its more obvious thermal power, may be considered
a truth permanently acquired to science.[1266] They are not, it thus
seems, bodies incandescent through heat, but glowing by electricity; and
this is compatible, under certain circumstances, with a relatively low
temperature.
The gaseous spectrum of comets is accompanied, in varying degrees, by a
continuous spectrum. This is usually derived most strongly from the
nucleus, but extends, more or less, to the nebulous appendages. In part,
it is certainly due to reflected sunlight; in part, most likely, to the
ignition of minute solid particles.
FOOTNOTES:
[Footnote 1188: _Month. Not._, vol. xix., p. 27.]
[Footnote 1189: _Mém. de l'Ac. Imp._, t. ii., 1859, p. 46.]
[Footnote 1190: _Harvard Annals_, vol. iii., p. 368.]
[Footnote 1191: _Ibid._, p. 371.]
[Footnote 1192: _Month. Not._, vol. xxii., p. 306.]
[Footnote 1193: Stothard in _Ibid._, vol. xxi., p. 243.]
[Footnote 1194: _Intell. Observer_, vol. i., p. 65.]
[Footnote 1195: _Comptes Rendus_, t. lxi., p. 953.]
[Footnote 1196: _Smiths. Report_, 1881 (Holden); _Nature_, vol. xxv., p.
94; _Observatory_, vol. xxi., p. 378 (W. T. Lynn).]
[Footnote 1197: _Ueber den Ursprung der von Pallas gefundenen
Eisenmassen_, p. 24.]
[Footnote 1198: Arago, _Annuaire_, 1836, p. 294.]
[Footnote 1199: Humboldt had noticed the emanation of the shooting stars
of 1799 from a single point, or "radiant," as Greg long afterwards
termed it; but no reasoning was founded on the observation.]
[Footnote 1200: _Am. Journ. of Sc._, vol. xxvi., p. 132.]
[Footnote 1201: _Annuaire_, 1836, p. 297.]
[Footnote 1202: _Ann. de l'Observ._, Bruxelles, 1839, p. 248.]
[Footnote 1203: _Ibid._, 1837, p. 272.]
[Footnote 1204: _Astr. Nach._, Nos. 385, 390.]
[Footnote 1205: _Am. Jour. of Sc._, vol. xxxviii. (2nd ser.), p. 377.]
[Footnote 1206: _Ibid._, vol. xxxviii., p. 61.]
[Footnote 1207: _Month. Not._, vol. xxvii., p. 247.]
[Footnote 1208: _Am. Jour. of Sc._, vol. xliii. (2nd ser.), p. 87.]
[Footnote 1209: Grant, _Month. Not._, vol. xxvii., p. 29.]
[Footnote 1210: P. Smyth, _Ibid._, p. 256.]
[Footnote 1211: Hind, _Ibid._, p. 49.]
[Footnote 1212: Reproduced in _Les Mondes_, t. xiii.]
[Footnote 1213: _Comptes Rendus_, t. lxiv., p. 96.]
[Footnote 1214: _Astr. Nach._, No. 1,626.]
[Footnote 1215: _Ibid._, No. 1,632.]
[Footnote 1216: _Month. Not._, vol. xxxviii., p. 369.]
[Footnote 1217: Schiaparelli, _Le Stelle Cadenti_, p. 54.]
[Footnote 1218: _Ueber Feuer-Meteore_, p. 406.]
[Footnote 1219: _Astr. Nach._, No. 347 (Mädler); see also Boguslawski,
_Die Kometen_, p. 98. 1857.]
[Footnote 1220: _Nature_, vol. vi., p. 148.]
[Footnote 1221: A. S. Herschel, _Month. Not._, vol. xxxii., p. 355.]
[Footnote 1222: _Astr. Nach._, Nos. 1,632, 1,633, 1,635.]
[Footnote 1223: _Nature_, vol. vii., p. 122.]
[Footnote 1224: A. S. Herschel, _Report Brit. Ass._, 1873, p. 390.]
[Footnote 1225: Humboldt, _Cosmos_, vol. i., p. 114 (Otté's trans.).]
[Footnote 1226: _Month. Not._, vol. xxxiii., p. 128.]
[Footnote 1227: Even this was denied by Bruhns, _Astr. Nach._, No.
2,054.]
[Footnote 1228: _Am. Jour._, vol. xxxi., p. 425.]
[Footnote 1229: _Month. Not._, vol. xlvi., p. 69.]
[Footnote 1230: In Schiaparelli's opinion, centuries must have elapsed
while the observed amount of scattering was being produced. _Le Stelle
Cadenti_, 1886, p. 112.]
[Footnote 1231: _Astr. and Astroph._, vol. xi., p. 943.]
[Footnote 1232: _Bull. de l'Acad. St. Petersbourg_, t. xxxv., p. 598.
1894.]
[Footnote 1233: _Observatory_, vol. xvi., p. 55.]
[Footnote 1234: _Le Stelle Cadenti_, p. 133; _Rendiconti dell' Istituto
Lombardo_, t. iii., ser. ii., p. 23.]
[Footnote 1235: Denning, _Memoirs Roy. Astr. Soc._, vol. liii., p. 214;
Abelmann, _Astr. Nach._, No. 3,516.]
[Footnote 1236: _Proc. Roy. Soc._, March 2, 1899; _Nature_, November 9,
1899.]
[Footnote 1237: Berberich, _Astr. Nach._, No. 3,526.]
[Footnote 1238: Elkin, _Astroph. Jour._, vol. ix., p. 22.]
[Footnote 1239: Elkin, _Astroph. Jour._, vol. x., p. 24.]
[Footnote 1240: _Pop. Astr._, September, 1897, p. 232.]
[Footnote 1241: _Month. Not._, vol. xx., p. 336.]
[Footnote 1242: _Revue des deux Mondes_, December 15, 1885, p. 889.]
[Footnote 1243: Palgrave, _Phil. Trans._, vol. cxxv., p. 175.]
[Footnote 1244: W. E. Hidden, _Century Mag._, vol. xxxiv., p. 534.]
[Footnote 1245: _Amer. Jour. of Science_, vol. xxxvi., p. i., 1888.]
[Footnote 1246: _Revue des Questions Scientifiques_, January, 1899, p.
194; Tisserand, _Bull. Astr._, t. viii., p. 460.]
[Footnote 1247: _Month. Not._, vol. xlv., p. 93.]
[Footnote 1248: _Observatory_, vol. viii., p. 4.]
[Footnote 1249: Denning, _Month. Not._, vol. xxxviii., p. 114.]
[Footnote 1250: _Comptes Rendus_, t. cix., p. 344.]
[Footnote 1251: _Month. Not._, vol. lix., p. 140.]
[Footnote 1252: _Bull. de l'Acad. St. Petersb._, t. xii., p. 95.]
[Footnote 1253: _Publ. Astr. Pac. Soc._, vol. iii., p. 114.]
[Footnote 1254: _Month. Not._, vol. lii., p. 341.]
[Footnote 1255: _Astr. Nach._, No. 1,488.]
[Footnote 1256: _Annuaire_, Paris, 1883, p. 185.]
[Footnote 1257: _Phil. Trans._, vol. clviii., p. 556.]
[Footnote 1258: Hasselberg, _Mém. de l'Ac. Imp. de St. Pétersbourg_, t.
xxviii. (7th ser.), No. 2, p. 66.]
[Footnote 1259: Scheiner, _Die Spectralanalyse der Gestirne_, p. 234.
Kayser (_Astr. and Astroph._, vol. xiii., p. 368) refers the anomalies
of the carbon-spectrum in comets wholly to instrumental sources.]
[Footnote 1260: Dewar, _Proc. Roy. Inst._, vol. xi., p. 541.]
[Footnote 1261: _Proc. R. Soc._, vol. xv., p. 5; _Month. Not._, vol.
xxvii., p. 288.]
[Footnote 1262: Keeler, _Astr. and Astrophysics_, vol. xi., p. 929;
Vogel, _Astr. Nach._, No. 3,142.]
[Footnote 1263: _Proc. Roy. Soc._, vol. xxiii., p. 154.]
[Footnote 1264: Hasselberg, _loc. cit._, p. 58.]
[Footnote 1265: _Ueber die Natur der Cometen_, p. 112.]
[Footnote 1266: Hasselberg, _loc. cit._, p. 38.]