the last two thousand years or thereabouts, only a few instances of the kind, certainly not so many as twenty, have been recorded, while there is reason to believe that some of these relate to the same star which has blazed out more than once, we may fairly consider the chance exceedingly small that during the next two thousand, or even the next twenty thousand years, our sun will be exposed to a catastrophe of the kind.

We might arrive at this conclusion independently of any considerations tending to show that our sun belongs to a safe class of system-rulers, and that all, or nearly all, the great solar catastrophes have occurred among suns of a particular class. There are, however, several considerations of the kind which are worth noting.

In the first place, we may dismiss as altogether unlikely the visit of a comet from the star-depths to our sun, on a course carrying the comet directly upon the sun's surface. But if, among the comets travelling in regular attendance upon the sun, there be one whose orbit intersects the sun's globe, then that comet must several times ere this have struck the sun, raising him temporarily to a destructive degree of heat. Now, such a comet must have a period of enormous length, for the races of animals now existing upon the earth must all have been formed since that comet's last visit on the assumption, be it remembered, that the fall of a large comet upon the sun, or rather the direct passage of the sun through the meteoric nucleus of a large comet, would excite the sun to destructive heat. If all living creatures on the earth are to be destroyed when some comet belonging to the solar system makes its next return to the sun, that same comet at its last visit must have raised the sun to an equal, or even greater intensity of heat, so that either no such races as at present exist had then come into being, or, if any such existed, they must at that

time have been utterly destroyed. We may fairly believe that all comets of the destructive sort have been eliminated. Judging from the evidence we have on the subject, the process of the formation of the solar system was one which involved the utilisation of cometic and meteoric matter; and it fortunately so chanced that the comets likely otherwise to have been most mischievous—those, namely, which crossed the track of planets, and still more those whose paths intersected the globe of the sun-were precisely those which would be earliest and most thoroughly used up in

this way.

Secondly, it is noteworthy that all the stars which have blazed out suddenly, except one, have appeared in a particular region of the heavens-the zone of the Milky Way (all, too, on one half of that zone). The single exception is the star in the Northern Crown, and that star appeared in a region which I have found to be connected with the Milky Way by a well-marked stream of stars, not a stream of a few stars scattered here and there, but a stream where thousands of stars are closely aggregated together, though not quite so closely as to form a visible extension of the Milky Way. In my map of 324,000 stars this stream can be quite clearly recognised; but, indeed, the brighter stars scattered along it form a stream recognisable with the naked eye, and have long since been regarded by astronomers as such, forming the stars of the Serpent and the Crown, or a serpentine streak followed by a loop of stars shaped like a coronet. Now the Milky Way, and the outlying streams of stars connected with it, seem to form a region of the stellar universe where fashioning processes are still at work. As Sir W. Herschel long since pointed out, we can recognise in various parts of the heavens various stages of development, and chief among the regions where as yet Nature's work seems incomplete, is the Galactic zone

especially that half of it where the Milky Way consists of irregular streams and clouds of stellar light. As there is no reason for believing that our sun belongs to this part of the galaxy, but on the contrary good ground for considering that he belongs to the class of insulated stars, few of which have shown signs of irregular variation, while none have ever blazed suddenly out with many hundred times their former lustre, we may fairly infer a very high degree of probability in favour of the belief that, for many ages still to come, the sun will continue steadily to discharge his duties as fire, light, and life of the solar system.

VII.

THE RINGS OF SATURN.

THE rings of Saturn, always among the most interesting objects of astronomical research, have recently been subjected to close scrutiny under high telescopic powers by Mr. Trouvelot, of the Harvard Observatory, Cambridge, U.S. The results which he has obtained afford very significant evidence respecting these strange appendages, and even throw some degree of light on the subject of cosmical evolution. The present time, when Saturn is the ruling planet of the night, seems favourable for giving a brief account of recent speculations respecting the Saturnian ring-system, especially as the observations of Mr. Trouvelot appear to remove all doubt as to the true nature of the rings, if indeed any doubt could reasonably be entertained after the investigations made by European and American astronomers when the dark inner ring had but recently been recognised.

It may be well to give a brief account of the progress of observation from the time when the rings were first discovered.

In passing, I may remark that the failure of Galileo to ascertain the real shape of these appendages has always seemed to me to afford striking evidence of the importance of careful reasoning upon all observations whose actual significance is not at once apparent. If Galileo had been

thus careful to analyse his observations of Saturn, he could not have failed to ascertain their real meaning. He had seen the planet apparently attended by two large satellites, one on either side, 'as though supporting the aged Saturn upon his slow course around the sun.' Night after night he had seen these attendants, always similarly placed, one on either side of the planet, and at equal distances from it. Then in 1612 he had again examined the planet, and lo, the attendants had vanished, 'as though Saturn had been at his old tricks, and had devoured his children.' But after a while the attendant orbs had reappeared in their former positions, had seemed slowly to grow larger, until at length they had presented the appearance of two pairs of mighty arms encompassing the planet. If Galileo had reasoned upon these changes of appearance, he could not have failed, as it seems to me, to interpret their true meaning. The three forms under which the rings had been seen by him sufficed to indicate the true shape of the appendage. Because Saturn was seen with two attendants of apparently equal size and always equi-distant from him, it was certain that there must be some appendage surrounding him, and extending to that distance from his globe. Because this appendage disappeared, it was certain that it must be thin and flat. Because it appeared at another time with a dark space between the arms and the planet, it was certain that the appendage is separated by a wide gap from the body of the planet. So that Galileo might have concluded—not doubtfully, but with assured confidence—that the appendage is a thin flat ring nowhere attached to the planet, or, as Huyghens said some forty years later, Saturn 'annulo cingitur tenui, plano, nusquam cohærente.' Whether such reasoning would have been accepted by the contemporaries of Galileo may be doubtful. The generality of men are not content with reasoning which is logically sound, but require