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 ansæ (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.