mathematical scrutiny. Strangely enough, the physical objections to the
theory of fluidity appear to have been entirely overlooked. Before we
could accept such a theory, we must admit the existence of elements
differing entirely from those with which we are familiar. No fluid known
to us could retain the form of the rings of Saturn under the conditions
to which they are exposed. But the mathematical examination of the
subject disposed so thoroughly of the theory that the rings can consist
of continuous fluid masses, that we need not now discuss the physical
objections to the theory.
There remains only the theory that the Saturnian ring-system consists of
discrete masses analogous to the streams of meteors known to exist in
great numbers within the solar system. The masses may be solid or fluid,
may be strewn in relatively vacant space, or may be surrounded by
vaporous envelopes; but that they are discrete, each free to travel on
its own course, seemed as completely demonstrated by Pierce's
calculations as anything not actually admitting of direct observation
could possibly be. The matter was placed beyond dispute by the
independent analysis to which Clerk Maxwell subjected the mathematical
problem. It had been selected in 1855 as the subject for the Adams Prize
Essay at Cambridge, and Clerk Maxwell's essay, which obtained the prize,
showed conclusively that only a system of many small bodies, each free
to travel upon its course under the varying attractions to which it was
subjected by Saturn itself, and by the Saturnian satellites, could
possibly continue to girdle a planet as the rings of Saturn girdle him.
It is clear that all the peculiarities hitherto observed in the
Saturnian ring-system are explicable so soon as we regard that system as
made up of multitudes of small bodies. Varieties of brightness simply
indicate various degrees of condensation of these small satellites. Thus
the outer ring had long been observed to be less bright than the inner.
Of course it did not seem impossible that the outer ring might be made
of different materials; yet there was something bizarre in the
supposition that two rings forming the same system were thus different
in substance. It would not have been at all noteworthy if different
parts of the same ring differed in luminosity--in fact, it was much more
remarkable that each zone of the system seemed uniformly bright all
round. But that one zone should be of one tint, another of an entirely
different tint, was a strange circumstance so long as the only available
interpretation seemed to be that one zone was made (throughout) of one
substance, the other of another. If this was strange when the difference
between the inner and outer bright rings was alone considered, how much
stranger did it seem when the multitudinous divisions in the rings were
taken into account! Why should the ring-system, 30,000 miles in width,
be thus divided into zones of different material? An arrangement so
artificial is quite unlike all that is elsewhere seen among the
subjects of the astronomer's researches. But when the rings are regarded
as made up of multitudes of small bodies, we can quite readily
understand how the nearly circular movements of all of these, at
different rates, should result in the formation of rings of aggregation
and rings of segregation, appearing at the earth's distance as bright
rings and faint rings. The dark ring clearly corresponds in appearance
with a ring of thinly scattered satellites. Indeed, it seems impossible
otherwise to account for the appearance of a dusky belt across the globe
of the planet where the dark ring crosses the disc. If the material of
the dark ring were some partly transparent solid or fluid substance, the
light of the planet received through the dark ring added to the light
reflected by the dark ring itself, would be so nearly equivalent to the
light received from the rest of the planet's disc, that either no dark
belt would be seen, or the darkening would be barely discernible. In
some positions a bright belt would be seen, not a dark one. But a ring
of scattered satellites would cast as its shadow a multitude of black
spots, which would give to the belt in shadow a dark grey aspect. A
considerable proportion of these spots would be hidden by the satellites
forming the dark ring, and in every case where a spot was wholly or
partially hidden by a satellite, the effect (at our distant station
where the separate satellites of the dark ring are not discernible)
would simply be to reduce _pro tanto_ the darkness of the grey belt of
shadow. But certainly more than half the shadows of the satellites would
remain in sight; for the darkness of the ring at the time of its
discovery showed that the satellites were very sparsely strewn. And
these shadows would be sufficient to give to the belt a dusky hue, such
as it presented when first discovered.[37]
The observations which have recently been made by Mr. Trouvelot
indicate changes in the ring-system, and especially in the dark ring,
which place every other theory save that to which we have thus been led
entirely out of the question. It should be noted that Mr. Trouvelot has
employed telescopes of unquestionable excellence and varying in aperture
from six inches to twenty-six inches, the latter aperture being that of
the great telescope of the Washington Observatory (the largest refractor
in the world).
He has noted in the first place that the interior edge of the outer
bright ring, which marks the outer limit of the great division, is
irregular, but whether the irregularity is permanent or not he does not
know. The great division itself is found not to be actually black, but,
as was long since noted by Captain Jacob, of the Madras Observatory, a
very dark brown, as though a few scattered satellites travelled along
this relatively vacant zone of the system. Mr. Trouvelot has further
noticed that the shadow of the planet upon the rings, and especially
upon the outer ring, changes continually in shape, a circumstance which
he attributes to irregularities in the surface of the rings. For my own
part, I should be disposed to attribute these changes in the shape of
the planet's shadow (noted by other observers also) to rapid changes in
the deep cloud-laden atmosphere of the planet. Passing on, however, to
less doubtful observations, we find that the whole system of rings has
presented a clouded and spotted aspect during the last four years. Mr.
Trouvelot specially describes this appearance as observed on the parts
of the ring outside the disc, called by astronomers the _ansae_ (because
of their resemblance to handles), and it would seem, therefore, that the
spotted and cloudy portions are seen only where the background on which
the rings are projected is black. This circumstance clearly suggests
that the darkness of these parts is due to the background, or, in other
words, that the sky is in reality seen through those parts of the
ring-system, just as the darkness of the slate-coloured interior ring is
attributed, on the satellite theory, to the background of sky visible
through the scattered flight of satellites forming the dark ring. The
matter composing the dark ring has been observed by Mr. Trouvelot to be
gathered in places into compact masses, which prevent the light of the
planet from being seen through those portions of the dark ring where the
matter is thus massed together. It is clear that such peculiarities
could not possibly present themselves in the case of a continuous solid
or fluid ring-system, whereas they would naturally occur in a ring
formed of multitudes of minute bodies travelling freely around the
planet.
The point next to be mentioned is still more decisive. When the dark
ring was carefully examined with powerful telescopes during the ten
years following its discovery by Bond, at which time it was most
favourably placed for observation, it was observed that the outline of
the planet could be seen across the entire breadth of the dark ring. All
the observations agreed in this respect. It was, indeed, noticed by
Dawes that outside the planet's disc the dark ring showed varieties of
tint, its inner half being darker than its outer portion. Lassell,
observing the planet under most favourable conditions with his two-feet
mirror at Malta, could not perceive these varieties of tint, which
therefore we may judge to have been either not permanent or very
slightly marked. But, as I have said, all observers agreed that the
outline of the planet could be seen athwart the entire width of the
dark ring. Mr. Trouvelot, however, has found that during the last four
years the planet has not been visible through the whole width of the
dark ring, but only through the inner half of the ring's breadth. It
appears, then, that either the inner portion is getting continually
thinner and thinner--that is, the satellites composing it are becoming
continually more sparsely strewn--or that the outer portion is becoming
more compact, doubtless by receiving stray satellites from the interior
of the inner bright ring.
It is clear that in Saturn's ring-system, if not in the planet itself,
mighty changes are still taking place. It may be that the rings are
being so fashioned under the forces to which they are subjected as to be
on their way to becoming changed into separate satellites, inner members
of that system which at present consists of eight secondary planets.
But, whatever may be the end towards which these changes are tending, we
see processes of evolution taking place which may be regarded as
typifying the more extensive and probably more energetic processes
whereby the solar system itself reached its present condition. I
ventured more than ten years ago, in the preface to my treatise upon the
planet Saturn, to suggest the possibility 'that in the variations
perceptibly proceeding in the Saturnian ring-system a key may one day be
found to the law of development under which the solar system has reached
its present condition.' This suggestion seems to me strikingly confirmed
by the recent discoveries. The planet Saturn and its appendages, always
interesting to astronomers, are found more than ever worthy of close
investigation and scrutiny. We may here, as it were, seize nature in the
act, and trace out the actual progress of developments which at present
are matters rather of theory than of observation.
VIII.
_COMETS AS PORTENTS_
The blazing star,
Threat'ning the world with famine, plague, and war;
To princes death; to kingdoms many curses;
To all estates inevitable losses;
To herdsmen rot; to ploughmen hapless seasons;
To sailors storms; to cities civil treasons.
Although comets are no longer regarded with superstitious awe as in old
times, mystery still clings to them. Astronomers can tell what path a
comet is travelling upon, and say whence it has come and whither it will
go, can even in many cases predict the periodic returns of a comet, can
analyse the substance of these strange wanderers, and have recently
discovered a singular bond of relationship between comets and those
other strange visitants from the celestial depths, the shooting stars.
But astronomy has hitherto proved unable to determine the origin of
comets, the part they perform in the economy of the universe, their real
structure, the causes of the marvellous changes of shape which they
undergo as they approach the sun, rush round him, and then retreat. As
Sir John Herschel has remarked: 'No one, hitherto, has been able to
assign any single point in which we should be a bit better or worse off,
materially speaking, if there were no such thing as a comet. Persons,
even thinking persons, have busied themselves with conjectures; such as
that they may serve for fuel for the sun (into which, however, they
never fall), or that they may cause warm summers, which is a mere fancy,
or that they may give rise to epidemics, or potato-blights, and so
forth.' And though, as he justly says, 'this is all wild talking,' yet
it will probably continue until astronomers have been able to master the
problems respecting comets which hitherto have foiled their best
efforts. The unexplained has ever been and will ever be marvellous to
the general mind. Just as unexplored regions of the earth have been
tenanted in imagination by
anthropophagi and men whose heads
Do grow beneath their shoulders,
so do wondrous possibilities exist in the unknown and the ill-understood
phenomena of nature.
In old times, when the appearance and movements of comets were supposed
to be altogether uncontrolled by physical laws, it was natural that
comets should be regarded as signs from heaven, tokens of Divine wrath
towards some, and of the interposition of Divine providence in favour of
others. As Seneca well remarked: 'There is no man so dull, so obtuse, so
turned to earthly things, who does not direct all the powers of his mind
towards things Divine when some novel phenomenon appears in the heavens.
While all follows its usual course up yonder, familiarity robs the
spectacle of its grandeur. For so is man made. However wonderful may be
what he sees day after day, he looks on it with indifference; while
matters of very little importance attract and interest him if they
depart from the accustomed order. The host of heavenly constellations
beneath the vault of heaven, whose beauty they adorn, attract no
attention; but if any unusual appearance be noticed among them, at once
all eyes are turned heavenwards. The sun is only looked on with
interest when he is undergoing eclipse. Men observe the moon only under
like conditions.... So thoroughly is it a part of our nature to admire
the new rather than the great. The same is true of comets. When one of
these fiery bodies of unusual form appears, every one is eager to know
what it means; men forget other objects to inquire about the new
arrival; they know not whether to wonder or to tremble; for many spread
fear on all sides, drawing from the phenomenon most grave prognostics.'
There is no direct reference to comets in the Bible, either in the Old
Testament or the New. It is possible that some of the signs from heaven
recorded in the Bible pages were either comets or meteors, and that even
where in some places an angel or messenger from God is said to have
appeared and delivered a message, what really happened was that some
remarkable phenomenon in the heavens was interpreted in a particular
manner by the priests, and the interpretation afterwards described as
the message of an angel. The image of the 'flaming sword which turned
every way' may have been derived from a comet; but we can form no safe
conclusion about this, any more than we can upon the question whether
the 'horror of great darkness' which fell upon Abraham (Genesis xv. 12)
when the sun was going down, was caused by an eclipse;[38] or whether
the going back of the shadow upon the dial of Ahaz was caused by a mock
sun. The star seen by the wise men from the east may have been a comet,
since the word translated 'star' signifies any bright object seen in the
heavens, and is in fact the same word which Homer, in a passage
frequently referred to, uses to signify either a comet or a meteor. The
way in which it appeared to go before them, when (directed by Herod, be
it noticed) they went to Bethlehem, almost due south of Jerusalem, would
correspond to a meridian culmination low down--for the star had
manifestly not been visible in the earlier evening, since we are told
that they rejoiced when they saw the star again. It was probably a comet
travelling southwards; and, as the wise men had travelled from the east,
it had very likely been first seen in the west as an evening star,
wherefore its course was retrograde--that is, supposing it _was_ a
comet.[39] It may possibly have been an apparition of Halley's comet,
following a course somewhat similar to that which it followed in the
year 1835, when the perihelion passage was made on November 15, and the
comet running southwards disappeared from northern astronomers, though
in January it was '_received_' by Sir J. Herschel, to use his own
expression, 'in the southern hemisphere.' There was an apparition of
Halley's comet in the year 66, or seventy years after the Nativity; and
the period of the comet varies, according to the perturbing influences
affecting the comet's motion, from sixty-nine to eighty years.
Homer does not, to the best of my recollection, refer anywhere directly
to comets. Pope, indeed, who made very free with Homer's references to
the heavenly bodies,[40] introduces a comet--and a red one, too!--into
the simile of the heavenly portent in Book IV.:--
As the red comet from Saturnius sent
To fright the nations with a dire portent
(A fatal sign to armies in the plain,
Or trembling sailors on the wintry main),
With sweeping glories glides along in air,
And shakes the sparkles from its blazing hair:
Between two armies thus, in open sight,
Shot the bright goddess in a trail of light.
But Homer says nothing of this comet. If Homer had introduced a comet,
we may be sure it would not have shaken sparkles from its blazing tail.
Homer said simply that 'Pallas rushed from the peaks of heaven, like the
bright star sent by the son of crafty-counselled Kronus (as a sign
either to sailors, or the broad array of the nations), from which many
sparks proceed.' Strangely enough, Pingre and Lalande, the former noted
for his researches into ancient comets, the latter a skilful astronomer,
agree in considering that Homer really referred to a comet, and they
even regard this comet as an apparition of the comet of 1680. They cite
in support of this opinion the portent which followed the prayer of
Anchises, 'AEneid,' Book II. 692, etc.: 'Scarce had the old man ceased
from praying, when a peal of thunder was heard on the left, and a star,
gliding from the heavens amid the darkness, rushed through space
followed by a long train of light; we saw the star,' says AEneas,
'suspended for a moment above the roof, brighten our home with its
fires, then, tracing out a brilliant course, disappear in the forests of
Ida; then a long train of flame illuminated us, and the place around
reeked with the smell of sulphur. Overcome by these startling portents,
my father arose, invoked the gods, and worshipped the holy star.' It is
impossible to recognise here the description of a comet. The noise, the
trail of light, the visible motion, the smell of sulphur, all correspond
with the fall of a meteorite close by; and doubtless Virgil simply
introduced into the narrative the circumstances of some such phenomenon
which had been witnessed in his own time. To base on such a point the
theory that the comet of 1680 was visible at the time of the fall of
Troy, the date of which is unknown, is venturesome in the extreme. True,
the period calculated for the comet of 1680, when Pingre and Lalande
agreed in this unhappy guess, was 575 years; and if we multiply this
period by five we obtain 2875 years, taking 1680 from which leaves 1195
years B.C., near enough to the supposed date of the capture of Troy.
Unfortunately, Encke (the eminent astronomer to whom we owe that
determination of the sun's distance which for nearly half a century held
its place in our books, but has within the last twenty years been
replaced by a distance three millions of miles less) went over afresh
the calculations of the motions of that famous comet, and found that,
instead of 575 years, the most probable period is about 8814 years. The
difference amounts only to 8239 years; but even this small difference
rather impairs the theory of Lalande and Pingre.[41]
Three hundred and seventy-one years before the Christian era, a comet
appeared which Aristotle (who was a boy at the time) has described.
Diodorus Siculus writes thus respecting it: 'In the first year of the
102d Olympiad, Alcisthenes being Archon of Athens, several prodigies
announced the approaching humiliation of the Lacedaemonians; a blazing
torch of extraordinary size, which was compared to a flaming beam, was
seen during several nights.' Guillemin, from whose interesting work on
Comets I have translated the above passage, remarks that this same comet
was regarded by the ancients as having not merely presaged but produced
the earthquakes which caused the towns of Helice and Bura to be
submerged. This was clearly in the thoughts of Seneca when he said of
this comet that as soon as it appeared it brought about the submergence
of Bura and Helice.
In those times, however, comets were not regarded solely as signs of
disaster. As the misfortunes of one nation were commonly held to be of
advantage to other nations, so the same comet might be regarded very
differently by different nations or different rulers. Thus the comet of
the year 344 B.C. was regarded by Timoleon of Corinth as presaging the
success of his expedition against Corinth. 'The gods announced,' said
Diodorus Siculus, 'by a remarkable portent, his success and future
greatness; a blazing torch appeared in the heavens at night, and went
before the fleet of Timoleon until he arrived in Sicily.' The comets of
the years 134 B.C. and 118 B.C. were not regarded as portents of death,
but as signalising, the former the birth, the latter the accession, of
Mithridates. The comet of 43 B.C. was held by some to be the soul of
Julius Caesar on its way to the abode of the gods. Bodin, a French lawyer
of the sixteenth century, regarded this as the usual significance of
comets. He was, indeed, sufficiently modest to attribute the opinion to
Democritus, but the whole credit of the discovery belonged to himself.
He maintained that comets only indicate approaching misfortunes because
they are the spirits or souls of illustrious men, who for many years
have acted the part of guardian angels, and, being at last ready to die,
celebrate their last triumph by voyaging to the firmament as flaming
stars. 'Naturally,' he says, 'the appearance of a comet is followed by
plague, pestilence, and civil war; for the nations are deprived of the
guidance of their worthy rulers, who, while they were alive, gave all
their efforts to prevent intestine disorders.' Pingre comments justly on
this, saying that 'it must be classed among base and shameful
flatteries, not among philosophic opinions.'
Usually, however, it must be admitted that the ancients, like the men of
the Middle Ages, regarded comets as harbingers of evil. 'A fearful star
is the comet,' says Pliny, 'and not easily appeased, as appeared in the
late civil troubles when Octavius was consul; a second time by the
intestine war of Pompey and Caesar; and, in our own time, when, Claudius
Caesar having been poisoned, the empire was left to Domitian, in whose
reign there appeared a blazing comet.' Lucan tells us of the second
event here referred to, that during the war 'the darkest nights were lit
up by unknown stars' (a rather singular way of saying that there were no
dark nights); 'the heavens appeared on fire, flaming torches traversed
in all directions the depths of space; a comet, that fearful star which
overthrows the powers of the earth, showed its horrid hair.' Seneca also
expressed the opinion that some comets portend mischief: 'Some comets,'
he said, 'are very cruel and portend the worst misfortunes; they bring
with them and leave behind them the seeds of blood and slaughter.'