decide between the merits of a discovery made by direct observation and
one effected by means of abstract reasoning. It was not until Saturn had
been examined with much higher telescopic power than Galileo could
employ, that the appendage which had so perplexed the Florentine
astronomer was seen to be a thin flat ring, nowhere touching the planet,
and considerably inclined to the plane in which Saturn travels. We
cannot wonder that the discovery was regarded as a most interesting one.
Astronomers had heretofore had to deal with solid masses, either known
to be spheroidal, like the earth, the sun, the moon, Jupiter, and Venus,
or presumed to be so, like the stars. The comets might be judged to be
vaporous masses of various forms; but even these were supposed to
surround or to attend upon globe-shaped nuclear masses. Here, however,
in the case of Saturn's ring, was a quoit-shaped body travelling around
the sun in continual attendance upon Saturn, whose motions, no matter
how they varied in velocity or direction, were so closely followed by
this strange attendant that the planet remained always centrally poised
within the span of its ring-girdle. To appreciate the interest with
which this strange phenomenon was regarded, we must remember that as yet
the law of gravity had not been recognised. Huyghens discovered the
ring (or rather perceived its nature) in 1659, but it was not till 1666
that Newton first entertained the idea that the moon is retained in its
orbit about the earth by the attractive energy which causes unsupported
bodies to fall earthwards; and he was unable to demonstrate the law of
gravity before 1684. Now, in a general sense, we can readily understand
in these days how a ring around a planet continues to travel along with
the planet despite all changes of velocity or direction of motion. For
the law of gravity teaches that the same causes which tend to change the
direction and velocity of the planet's motion tend in precisely the same
degree to change the direction and velocity of the ring's motion. But
when Huyghens made his discovery it must have appeared a most mysterious
circumstance that a ring and planet should be thus constantly
associated--that during thousands of years no collision should have
occurred whereby the relatively delicate structure of the ring had been
destroyed.
Only six years later a discovery was made by two English observers,
William and Thomas Ball, which enhanced the mystery. Observing the
northern face of the ring, which was at that time turned earthwards,
they perceived a black stripe of considerable breadth dividing the ring
into two concentric portions. The discovery did not attract so much
attention as it deserved, insomuch that when Cassini, ten years later,
announced the discovery of a corresponding dark division on the southern
surface, none recalled the observation made by the brothers Ball.
Cassini expressed the opinion that the ring is really divided into two,
not merely marked by a dark stripe on its southern face. This conclusion
would, of course, have been an assured one, had the previous observation
of a dark division on the northern face been remembered. With the
knowledge which we now possess, indeed, the darkness of the seeming
stripe would be sufficient evidence that there must be a real division
there between the rings; for we know that no mere darkness of the ring's
substance could account for the apparent darkness of the stripe. It has
been well remarked by Professor Tyndall, that if the moon's whole
surface could be covered with black velvet, she would yet appear white
when seen on the dark background of the sky. And it may be doubted
whether a circular strip of black velvet 2000 miles wide, placed where
we see the dark division between the rings, would appear nearly as dark
as that division. Since we could only admit the possibility of some
substance resembling our darker rocks occupying this position (for we
know of nothing to justify the supposition that a substance as dark as
lampblack or black velvet could be there), we are manifestly precluded
from supposing that the dark space is other than a division between two
distinct rings.
Yet Sir W. Herschel, in examining the rings of Saturn with his powerful
telescopes, for a long time favoured the theory that there is no real
division. He called it the 'broad black mark,' and argued that it can
neither indicate the existence of a zone of hills upon the ring, nor of
a vast cavernous groove, for in either case it would present changes of
appearance (according to the ring's changes of position) such as he was
unable to detect. It was not until the year 1790, eleven years after his
observations had commenced, that, perceiving a corresponding broad black
mark upon the ring's southern face, Herschel expressed a 'suspicion'
that the ring is divided into two concentric portions by a circular gap
nearly 2000 miles in width. He expressed at the same time, very
strongly, his belief that this division was the only one in Saturn's
ring-system.
A special interest attached at that time to the question whether the
ring is divided or not, for Laplace had then recently published the
results of his mathematical inquiry into the movements of such a ring as
Saturn's, and, having _proved_ that a single solid ring of such enormous
width could not continue to move around the planet, had expressed the
_opinion_ that Saturn's ring consists in reality of many concentric
rings, each turning, with its own proper rotation rate, around the
central planet. It is singular that Herschel, who, though not versed in
the methods of the higher mathematics, had considerable native power as
a mathematician, was unable to perceive the force of Laplace's
reasoning. Indeed, this is one of those cases where clearness of
perception rather than profundity of mathematical insight was required.
Laplace's equations of motion did not express all the relations
involved, nor was it possible to judge, from the results he deduced, how
far the stability of the Saturnian rings depended on the real structure
of these appendages. One who was well acquainted with mechanical
matters, and sufficiently versed in mathematics to understand how to
estimate generally the forces acting upon the ring-system, could have
perceived as readily the general conditions of the problem as the most
profound mathematician. One may compare the case to the problem of
determining whether the action of the moon in causing the tidal wave
modifies in any manner the earth's motion of rotation. We know that as a
mathematical question this is a very difficult one. The Astronomer
Royal, for example, not long ago dealt with it analytically, and deduced
the conclusion that there is no effect on the earth's rotation,
presently however, discovering by a lucky chance a term in the result
which indicates an effect of that kind. But if we look at the matter in
its mechanical aspect, we perceive at once, without any profound
mathematical research, that the retardation so hard to detect
mathematically must necessarily take place. As Sir E. Beckett says in
his masterly work, _Astronomy without Mathematics_, 'the conclusion is
as evident without mathematics as with them, when once it has been
suggested.' So when we consider the case of a wide flat ring surrounding
a mighty planet like Saturn, we perceive that nothing could possibly
save such a ring from destruction if it were really one solid structure.
To recognise this the more clearly, let us first notice the dimensions
of the planet and rings.
We have in Saturn a globe about 70,000 miles in mean diameter, an
equatorial diameter being about 73,000 miles, the polar diameter 66,000
miles. The attractive force of this mighty mass upon bodies placed on
its surface is equal to about one-fifth more than terrestrial gravity if
the body is near the pole of Saturn, and is almost exactly the same as
terrestrial gravity if the body is at the planet's equator. Its action
on the matter of the ring is, of course, very much less, because of the
increased distance, but still a force is exerted on every part of the
ring which is comparable with the familiar force of terrestrial gravity.
The outer edge of the outer ring lies about 83,500 miles from the
planet's centre, the inner edge of the inner ring (I speak throughout of
the ring-system as known to Sir W. Herschel and Laplace) about 54,500
miles from the centre, the breadth of the system of bright rings being
about 29,000 miles. Between the planet's equator and the inner edge of
the innermost bright ring there intervenes a space of about 20,000
miles. Roughly speaking, it may be said that the attraction of the
planet on the substance of the ring's inner edge is less than gravity at
Saturn's equator (or, which is almost exactly the same thing, is less
than terrestrial gravity) in about the proportion of 9 to 20; or, still
more roughly, the inner edge of Saturn's inner bright ring is drawn
inwards by about half the force of gravity at the earth's surface. The
outer edge is drawn towards Saturn by a force less than terrestrial
gravity in the proportion of about 3 to 16--say roughly that the force
thus exerted by Saturn on the matter of the outer edge of the
ring-system is equivalent to about one-fifth of the force of gravity at
the earth's surface.
It is clear, first, that if the ring-system did not rotate, the forces
thus acting on the material of the rings would immediately break them
into fragments, and, dragging these down to the planet's equator, would
leave them scattered in heaps upon that portion of Saturn's surface. The
ring would in fact be in that case like a mighty arch, each portion of
which would be drawn towards Saturn's centre by its own weight. This
weight would be enormous if Bessel's estimate of the mass of the
ring-system is correct. He made the mass of the ring rather greater than
the mass of the earth--an estimate which I believe to be greatly in
excess of the truth. Probably the rings do not amount in mass to more
than a fourth part of the earth's mass. But even that is enormous, and
subjected as is the material of the rings to forces varying from
one-half to a fifth of terrestrial gravity, the strains and pressures
upon the various parts of the system would exceed thousands of times
those which even the strongest material built up into their shape could
resist. The system would no more be able to resist such strains and
pressures than an arch of iron spanning the Atlantic would be able to
sustain its own weight against the earth's attraction.
It would be necessary then that the ring-system should rotate around the
planet. But it is clear that the proper rate of rotation for the outer
portion would be very different from the rate suited for the inner
portion. In order that the inner portion should travel around Saturn
entirely relieved of its weight, it should complete a revolution in
about seven hours twenty-three minutes. The outer portion, however,
should revolve in about thirteen hours fifty-eight minutes, or nearly
fourteen hours. Thus the inner part should rotate in little more than
half the time required by the outer part. The result would necessarily
be that the ring-system would be affected by tremendous strains, which
it would be quite unable to resist. The existence of the great division
would manifestly go far to diminish the strains. It is easily shown that
the rate of turning where the division is, would be once in about eleven
hours and twenty-five minutes, not differing greatly from the mean
between the rotation-periods for the outside and for the inside edges of
the system. Even then, however, the strains would be hundreds of times
greater than the material of the ring could resist. A mass comparable in
weight to our earth, compelled to rotate in (say) nine hours when it
ought to rotate in eleven or in seven, would be subjected to strains
exceeding many times the resistances which the cohesive power of its
substance could afford. That would be the condition of the inner ring.
And in like manner the outer ring, if it rotated in about twelve hours
and three-quarters, would have its outer portions rotating too fast and
its inner portions too slowly, because their proper periods would be
fourteen hours and eleven hours and a half respectively. Nothing but the
division of the ring into a number of narrow hoops could possibly save
it from destruction through the internal strains and pressures to which
its material would be subjected.
Even this complicated arrangement, however, would not save the
ring-system. If we suppose a fine hoop to turn around a central
attracting body as the rings of Saturn rotate around the planet, it may
be shown that unless the hoop is so weighted that its centre of gravity
is far from the planet, there will be no stability in the resulting
motions; the hoop will before long be made to rotate eccentrically, and
eventually be brought into destructive collision with the central
planet.
It was here that Laplace left the problem. Nothing could have been more
unsatisfactory than his result, though it was accepted for nearly half a
century unquestioned. He had shown that a weighted fine hoop may
possibly turn around a central attracting mass without destructive
changes of position, but he had not proved more than the bare
possibility of this, while nothing in the appearance of Saturn's rings
suggests that any such arrangement exists. Again, manifestly a multitude
of narrow hoops, so combined as to form a broad flat system of rings,
would be constantly in collision _inter se_. Besides, each one of them
would be subjected to destructive strains. For though a fine uniform
hoop set rotating at a proper rate around an attracting mass at its
centre would be freed from all strains, the case is very different with
a hoop so weighted as to have its centre of gravity greatly displaced.
Laplace had saved the theoretical stability of the motions of a fine
ring at the expense of the ring's power of resisting the strains to
which it would be exposed. It seems incredible that such a result
(expressed, too, very doubtingly by the distinguished mathematician who
had obtained it) should have been accepted so long almost without
question. There is nothing in nature in the remotest degree resembling
the arrangement imagined by Laplace, which indeed appears on _a priori_
grounds impossible. It was not claimed for it that it removed the
original difficulties of the problem; and it introduced others fully as
serious. So strong, however, is authority in the scientific world that
none ventured to express any doubts except Sir W. Herschel, who simply
denied that the two rings were divided into many, as Laplace's theory
required. As time went on and the signs of many divisions were at times
recognised, it was supposed that Laplace's reasoning had been justified;
and despite the utter impossibility of the arrangement he had suggested,
that arrangement was ordinarily described as probably existing.
At length, however, a discovery was made which caused the whole question
to be reopened.
On November 10, 1850, W. Bond, observing the planet with the telescope
of the Harvard Observatory, perceived within the inner bright ring a
feeble illumination which he was at a loss to understand. On the next
night the faint light was better seen. On the 15th, Tuttle, who was
observing with Bond, suggested the idea that the light within the inner
bright ring was due to a dusky ring inside the system of bright rings.
On November 25, Mr. Dawes in England perceived this dusky ring, and
announced the discovery before the news had reached England that Bond
had already seen the dark ring. The credit of the discovery is usually
shared between Bond and Dawes, though the usual rule in such matters
would assign the discovery to Bond alone. It was found that the dark
ring had already been seen at Rome so far back as 1828, and again by
Galle at Berlin in May 1838. The Roman observations were not
satisfactory. Those by Galle, however, were sufficient to have
established the fact of the ring's existence; indeed, in 1839 Galle
measured the dark ring. But very little attention was attracted to this
interesting discovery, insomuch that when Bond and Dawes announced their
observation of the dark ring in 1850, the news was received by
astronomers with all the interest attaching to the detection of before
unnoted phenomena.
It may be well to notice under what conditions the dark ring was
detected in 1850. In September 1848 the ring had been turned edgewise
towards the sun, and as rather more than seven years are occupied in
the apparent gradual opening out of the ring from that edge view to its
most open appearance (when the outline of the ring-system is an eclipse
whose lesser axis is nearly equal to half the greater), it will be seen
that in November 1850 the rings were but slightly opened. Thus the
recognition of the dark ring within the bright system was made under
unfavourable conditions. For four preceding years--that is, from the
year 1846--the rings had been as little or less opened; and again for
several years preceding 1846, though the rings had been more open, the
planet had been unfavourably placed for observation in northern
latitudes, crossing the meridian at low altitudes. Still, in 1838 and
1839, when the rings were most open, although the planet was never seen
under favourable conditions, the opening of the rings, then nearly at
its greatest, made the recognition of the dark ring possible; and we
have seen that Galle then made the discovery. When Bond rediscovered the
dark ring, everything promised that before long the appendage would be
visible with telescopes far inferior in power to the great Harvard
refractor. Year after year the planet was becoming more favourably
placed for observation, while all the time the rings were opening out.
Accordingly it need not surprise us to learn that in 1853 the dark ring
was seen with a telescope less than three inches and a half in aperture.
Even so early as 1851, Mr. Hartnup, observing the planet with a
telescope eight inches and a half in aperture, found that 'the dark ring
could not be overlooked for an instant.'
But while this increase in the distinctness of the dark ring was to be
expected, from the mere fact that the ring was discovered under
relatively unfavourable conditions, yet the fact that Saturn was thus
found to have an appendage of a remarkable character, perfectly obvious
even with moderate telescopic power, was manifestly most surprising.
The planet had been studied for nearly two centuries with telescopes
exceeding in power those with which the dark ring was now perceived.
Some among these telescopes were not only of great power, but employed
by observers of the utmost skill. The elder Herschel had for a quarter
of a century studied Saturn with his great reflectors eighteen inches in
aperture, and had at times turned on the planet his monstrous (though
not mighty) four-feet mirror. Schroeter had examined the dark space
within the inner bright ring for the special purpose of determining
whether the ring-system is really disconnected from the globe. He had
used a mirror nineteen inches in aperture, and he had observed that the
dark space seen on either side of Saturn inside the ring-system not only
appeared dark, but actually darker than the surrounding sky. This was
presumably (though not quite certainly) an effect of contrast only, the
dark space being bounded all round by bright surfaces. If real, the
phenomenon signified that whereas the space outside the ring, where the
satellites of the planet travel, was occupied by some sort of cosmical
dust, the space within the ring-system was, as it were, swept and
garnished, as though all the scattered matter which might otherwise have
occupied that region had been either attracted to the body of the planet
or to the rings.[36] But manifestly the observation was entirely
inconsistent with the supposition that there existed in Schroeter's time
a dark or dusky ring within the bright system. Again, the elder Struve
made the most careful measurement of the whole of the ring-system in
1826, when the system was as well placed for observation as in 1856
(or, in other words, as well placed as it can possibly be); but though
he used a telescope nine inches and a half in aperture, and though his
attention was specially attracted to the inner edge of the inner bright
ring (_which seemed to him indistinct_), he did not detect the dark
ring. Yet we have seen that in 1851, under much less favourable
conditions, a less practised observer, using a telescope of less
aperture, found that the dark ring could not be overlooked for an
instant. It is manifest that all these considerations point to the
conclusion that the dark ring is a new formation, or, at the least, that
it has changed notably in condition during the present century.
I have hitherto only considered the appearance of the dusky ring as seen
on either side of the planet's globe within the bright rings. The most
remarkable feature of the appendage remains still to be mentioned--the
fact, namely, that the bright body of the planet can be seen through
this dusky ring. Where the dark ring crosses the planet, it appears as a
rather dark belt, which might readily be mistaken for a belt upon the
planet's surface; for the outline of the planet can be seen through the
ring as through a film of smoke or a crape veil.
Now it is worthy of notice that whereas the dark ring was not detected
outside the planet's body until 1838, nor generally recognised by
astronomers until 1850, the dark belt across the planet, really caused
by the dusky ring, was observed more than a century earlier. In 1715 the
younger Cassini saw it, and perceived that it was not curved enough for
a belt really belonging to the planet. Hadley again observed that the
belt attended the ring as this opened out and closed, or, in other
words, that the dark belt belonged to the ring, not to the body of the
planet. And in many pictures of Saturn's system a dark band is shown
along the inner edge of the inner bright ring where it crosses the body
of the planet. It seems to me that we have here a most important piece
of evidence respecting the rings. It is clear that the inner part of the
inner bright ring has for more than a century and a half (how much more
we do not know) been partially transparent, and it is probable that
within its inner edge there has been all the time a ring of matter; but
this ring has only within the last half-century gathered consistency
enough to be discernible. It is manifest that the existence of the dark
belt shown in the older pictures would have led directly to the
detection of the dark ring, had not this appendage been exceedingly
faint. Thus, while the observation of the dark belt across the planet's
face proves the dusky ring to have existed in some form long before it
was perceived, the same fact only helps to render us certain that the
dark ring has changed notably in condition during the present century.
The discovery of this singular appendage, an object unique in the solar
system, naturally attracted fresh attention to the question of the
stability of the rings. The idea was thrown out by the elder Bond that
the new ring may be fluid, or even that the whole ring-system may be
fluid, and the dark ring simply thinner than the rest. It was thought
possible that the ring-system is of the nature of a vast ocean, whose
waves are steadily advancing upon the planet's globe. The mathematical
investigation of the subject was also resumed by Professor Benjamin
Pierce, of Harvard, and it was satisfactorily demonstrated that the
stability of a system of actual rings of solid matter required so nice
an adjustment of so many narrow rings as to render the system far more
complex than even Laplace had supposed. 'A stable formation can,' he
said, 'be nothing other than a very great number of separate narrow
rigid rings, each revolving with its proper relative velocity.' As was
well remarked by the late Professor Nichol, 'If this arrangement or
anything like it were real, how many new conditions of instability do we
introduce. Observation tells us that the division between such rings
must be extremely narrow, so that the slightest disturbance by external
or internal causes would cause one ring to impinge upon another; and we
should thus have the seed of perpetual catastrophes.' Nor would such a
constitution protect the system against dissolution. 'There is no escape
from the difficulties, therefore, but through the final rejection of the
idea that Saturn's rings are rigid or in any sense a solid formation.'