So that when, last May, Wells's comet suddenly began. to show the well-known lines of sodium, promise was at once, and for the first time, afforded, that the problems of cometic changes, in so far as these depend on motions taking place within the comet itself, may before long be solved. We can have very little doubt, for instance, that if such a comet as Donati's were now to appear, and to be studied under favourable conditions during those parts of its course in which it was subject to the most intense disturbing action, the bright lines which would be seen in the comet's spectrum would either by their displacement tell us that the substance of the comet is driven wildly hither and thither in the head and swept swiftly away to form the tail, as it seems to be, or else, by remaining unchanged in position, would show that there are no such movements of disturbance or repulsion.

Now the comet which has recently been seen near the sun has been observed by this method. On September 18, when it was but three degrees (say half-a-dozen sun-breadths) from the sun on the sky, it was examined in the clear sky of Nice by M. Thollon, a skilful French spectroscopist.

The spectrum, notwithstanding the obviously unfavourable conditions under which the observation was made, showed clearly the line (or rather the double line) of sodium. Here, by the way, was at once evidence such as in former times no astronomer could have of the comet's real position in space. Formerly if a comet was observed anywhere, once only, nothing could be certainly known respecting its position, except that it was somewhere in the line of sight in which it was seen. But if we are right in believing that the sodium in a comet is only vaporised and rendered selfluminous when the comet is near the sun, then the new comet on September 18 was not only shown to lie in a certain direction, but within certain tolerably narrow limits of distance.

But Thollon observed something else, not quite so satisfactorily as to be absolutely certain of it, but still so as to

give a considerable degree of assurance. He says that the line of sodium seemed displaced towards the red. This would indicate recession. Observe here again how the spectroscopic method of determining motions of recession or approach may come in to help the astronomer to determine the position of a comet. Supposing this method should ever be so improved that the exact rate of a comet's motion might be determined by it, then instead of merely ascertaining, in any single observation, the direction in which a comet lies at the moment, the astronomer may learn its direction, something (as we have seen) of its distance, and the rate at which it is moving from or towards the observer. The rate of its thwart motion cannot of course be inferred from the spectroscopic observations directly, yet indirectly it can. For the rate of motion at any given distance from the sun for an orbit of known dimensions is known; now the distance of the comet being partly indicated by the spectroscopic observations, the thwart motion is known within the same degree of error. Hence, combining this with our more precise knowledge of the motion of recession or approach, we make a first rather rough approximation to the real motion, both in direction and in amount—which would determine the orbit absolutely. Observations made a day or two later will show whether the body really is moving in this orbit; and if the later observations include spectroscopic ones we shall obtain means of testing and correcting the first estimate of the orbit, which will practically give us the orbit correctly-much more correctly, at any rate, than it can be deduced by the methods at present in use from observations made on four or five different occasions.1

It may be well, perhaps, in conclusion, to inquire how the comet would actually be absorbed by the sun.

First, then, be it noticed that at first there would be no tendency towards a diminution of the perihelion distance

' Theoretically the orbit of a comet can be deduced from three observations; but practically many observations are required to give anything like accuracy.

S

of the comet, as many seem to imagine. The point of nearest approach must remain nearly at the same distance from the sun at each return of the comet, so long as the orbit remains eccentric. Only when the velocity in perihelion (or at the point of nearest approach) is so reduced that the centrifugal tendency no longer balances the centripetal force, would there be any approach towards the sun. This amounts to saying that until the orbit is transformed into a circle (when there will be no perihelion at all) there will be no approach towards the sun. When that transformation is effected, there will be approach at every part of the circuit-in other words, the course of the comet will become a spiral, the coils of which will draw closer and closer in towards the sun's surface; the sun will be within the coils, but the comet itself will be in the toils, and its end not far off. As throughout this approach the comet's substance would be in the form of vapour, there would probably be a rapidly increasing resistance, and hence a rapidly increasing rate of approach towards the sun. Oddly enough, the comet's rate of travelling would be increased notwithstanding this constant resistance, the sun's indrawing action adding more motion than the frictional resistance subtracts. For several days, probably, the comet in each circuit, when off the solar disc, would be a conspicuous object to spectroscopists, though not perhaps visible through the telescope. The comet would appear outside of the sun's disc, first on one side, then on the other, at intervals of about 13 hours-3 hours being the time of circuit of a body close to the sun's surface. As this surface is carried round once in about twentyfive hours, there would be considerable loss of velocity, and resulting heat, in the substance of each part of the comet as it was absorbed. But I believe the whole heat of the sun would be little increased if the whole of the comet were thus absorbed at once; and very little indeed if, as is certain, the absorption took place piecemeal.

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JUPITER'S SATELLITES.

JUPITER surpasses our earth more than 1,400 times in volume. Saturn alone can be compared with him in this respect, but even Saturn is but half as large as Jupiter. In mass, this superb planet is not merely 'facile princeps,' but exceeds much more than twofold all the other planets taken together. We may view, indeed, in Jupiter and his system, a miniature, but instead of being a miniature of our earth it is as a miniature of the whole solar system that he is to be regarded. The sun himself does not so greatly exceed Jupiter in volume as Jupiter does our earth. And the bodies which circle round Jupiter travel with velocities comparable with those of the swiftest members of the solar system. While Mercury and Venus travel 100,000 and 80,000 miles an hour, and our earth travels 68,000 miles an hour round the sun, Jupiter's inner satellite travels upwards of 40,000 miles an hour around its primary. Mars travels 55,000 miles an hour round the sun; the second satellite travels 32,000 miles an hour round Jupiter. Jupiter himself sweeps less swiftly round the sun than these satellites do around him, so that through a portion of their orbits they are actually retrograding. The third satellite also travels so swiftly round Jupiter as to be reduced very nearly to absolute rest when its velocity acts in a contrary direction to that of Jupiter. The fourth satellite travels less swiftly than the third, but yet as swiftly as the planet Saturn in his orbit around the sun.

Nearly every celestial object has an interest attaching

to it other than that derived from its physical aspect-an interest which may be called historical. In the moon, for instance, we see an object without which (it is not too much to say) astronomy would never have approached its present state of exactness and accuracy. Mars, in like manner, afforded evidence such as no other planet could supply, when Kepler was engaged in the series of researches which rendered his name illustrious, and without which Newton's views might never have been directed to gravitation as a universal principle. Venus is connected with the determination of the fundamental element of all astronomical measures the sun's distance from the earth. Mercury, Saturn, Uranus, and Neptune, the sun, fixed stars, comets, asteroids, and nebulæ, all have their historical interest, derived from the evidence which they have afforded on special questions of inquiry. Jupiter is second to none in this. respect. At a critical period in the history of astronomy, when the world of science was divided on the subject of the Copernican Theory of the Universe, and when all without the world of science were steadfastly opposed to the new views, the discovery that Jupiter was the centre of a miniature system, circling around him as the theory in dispute taught that the planets circled around the sun, came opportunely as an illustration, and to those who could grasp the significance of the phenomenon, as a proof, of the views of the German astronomer. Later came a yet more remarkable and important discovery, through the observation of Jupiter's system-the discovery that light does not travel, as had been supposed, instantaneously, but with a measurable, however inconceivable, velocity. Through this discovery, supplemented by Bradley's discovery of the aberration of the fixed stars, came a proof-which is absolutely beyond cavil or question-of the true theory of the solar system. Supplementary proofs of Newton's views have been derived also, as might be expected, from the influence exerted by a planet whose disturbing agency so largely exceeds that of all the other members of the solar system.