observation, he endeavoured to educe a system of philosophy from fancies, and to found it upon syllogisms. Aristotle, who may be considered the typical philosopher of the Greek school, included comets in the wide range of phenomena which he claimed the privilege of explaining. To him was due the opinion mentioned above-an opinion confidently maintained during the many centuries in which the philosophy of Aristotle held sway over men's minds. To him, also, was due a yet more remarkable opinion, the view, namely, that the Milky Way is a vast comet which continually reproduces itself! Xenophanes and Theon, in the fifth century, adopted a rather singular view of the Aristotelian theory of comets, when they spoke of these objects as 'travelling light-clouds.'

To these fancies the ancients added the idea that the shapes of comets indicated their character as portents. Thus in fig. I five views of comets are shown, as an arrowhead, a sea monster, a sword, a lance, and in flames.

Tycho Brahe was the first to express doubts respecting the views of Aristotle. From a careful series of observations, he demonstrated that the orbits of comets are certainly situated beyond the moon's orbit. He thought the orbits must be circular, for he lived at a time when none but circular orbits were conceded to the celestial bodies. Dörfel, a native of Upper Saxony, proved that the orbits of comets are either very elongated ovals or parabolas, and that the sun occupies a focus of the curve. It happens, singularly enough, that this discovery was effected but a year or two before Newton propounded the theory of gravitation. Newton himself examined the orbit of the great comet of 1680 (known as 'Newton's comet') and others; and he found that they all accord with the law of gravity.

But before long, Newton's friend and pupil, Halley, effected a yet more remarkable discovery. In hopes of confirming Newton's views by results founded on actual observation, he collected all the records of comets which seemed

twenty-four. Among these, three presented a remarkable similarity. One appeared in 1531, and was described by Appian; another appeared in 1607, and was observed by

entitled to confidence, and attempted-as well as his meagre materials would allow him-to calculate the elements of their orbits. In this way he computed the paths of no less than

Kepler; the third was traced by Halley himself in 1682. The equality of the intervals between these epochs led to the suspicion that the same comet had appeared three times And Halley found, on searching historical records, that a comet appeared in 1305, another in 1380, and a third in 1456. Combining these appearances with those mentioned before, he thought he had satisfactory evidence of identity. For he was sufficiently familiar with the results which might be expected to flow from the law of gravity, to be aware that absolute regularity of motion was not to be expected in a body traversing the solar system in an eccentric orbit, and swayed from its proper path by the attraction of such giant planets as Jupiter and Saturn. Indeed, it happens, singularly enough—one out of many remarkable coincidences in the history of comets-that the comet of 1380 was not Halley's comet, which really appeared in 1378, a date bringing in a yet greater discordance in the intervals than Halley had suspected and accounted for. With remarkable acumen— since no means existed in his day for anything like accurate computation-he not only pointed out the possible influence of the great planets in disturbing the comet in past revolutions, but he made a rough approach to an estimate of the effect that they would have on the period of its next visit. 'Instead of appearing in August 1757, as it would if its period remained unaltered, it will not appear,' he said, 'until the end of 1758 or the beginning of 1759, for it will be retarded by the action of Jupiter. Wherefore,' he adds, with a pardonable anxiety to secure the credit of his ingenious investigations, 'if it should return, according to our prediction, impartial posterity will not refuse to acknowledge that this was discovered by an Englishman.'

As the time for the fulfilment of the prediction approached, an intense interest was excited in the minds of astronomers. In 1757, Clairut, Lalande, and Madame Lepaute undertook the computation of the epoch at which the comet might be expected to return. They applied methods of investigation invented by Clairut himself. It

resulted from their laborious computations that April 13, 1759, was fixed on for the epoch at which the comet should attain its closest approach to the sun, or, as it is technically expressed, should pass its perihelion. But Clairut was careful to allow a month either way, on account of unavoidable omissions in the calculation, and for the effects of unknown forces, such as the action of some planet too far off to be seen' (a happy anticipation of modern discoveries).

6

And now the heavens were swept diligently by all the telescopists of Europe, each eager to be the first to announce the discovery of an object whose appearance or non-appearance was to confirm or to disprove the Newtonian theory. It was actually discovered, however, without telescopic aid, by a Saxon farmer, George Palitsch, on Christmas Day, 1758. It reached its perihelion on March 13, 1759, confirming at once the accuracy of Clairut's computations, and the justice of his caution in assigning rather wide limits of

error.

It was now evident that comets travel, like the planets, in determined paths; and also that the investigation of their motions is a subject worthy the study of the ablest mathematicians, and sufficient to tax their highest powers. An account of their labours would be out of place in such a treatise as the present; but we recommend the subject to the notice of the astronomical student, as one of the most interesting chapters in the history of modern science.

One comet, however, discovered not long after astronomy had achieved this triumph, seemed at first to teach a different lesson. In 1770 a comet appeared whose path turned out to be, not a long oval or parabola, as had been the case with all the orbits yet examined, but an ellipse of moderate extent, and not very eccentric. The orbit lay also much closer than usual to that thin slice of space (so to speak) within which the planets are observed to move. Lexell, who computed the path, found that the period of the comet was about five and a half years. Its return was carefully watched for, but no one has ever seen the comet since. The

cause of its disappearance, and also of its sudden appearance -for this was equally remarkable, when we remember that so conspicuous a comet could not have been circulating long in its small orbit without discovery-was carefully inquired into. The result was singular. On tracing back the path of the comet it was found that it must have passed very near to the great planet Jupiter. 'It had intruded,' says Herschel, 'an uninvited guest into his family circle-actually nearer to him than his fourth satellite.' Accordingly, the comet's path, originally a long oval, had been bent into a curve of less extent. Having once entered on this new path, the comet was free to follow it-always returning, be it noticed, to the point at which it had started on it—so long as Jupiter was not interfered with. But it happened, unfortunately for the stability of the comet's motions, that, after going twice round the new path, it again presented itself near Jupiter's track, when the planet (which had meanwhile gone once round his orbit) was not very far from the scene of his former encounter. He accordingly again exerted his influence upon the unfortunate comet, and this time dismissed it on a path which will not admit of its approaching the earth near enough to be seen.1

Let us return, however, to Halley's comet.

It so chances that the comet which was the first to show. full obedience to the law of gravitation was one which exhibited in a very remarkable and significant manner the characteristics which distinguish comets from other heavenly bodies, and make them so mysterious to the student of science. At the return of Halley's comet, in 1836, all that had signalised the return in 1759 was repeated, but the

'It must be noticed, however, that Leverrier, who very carefully reexamined the question, was led to deny the accuracy of the results recorded above. Admitting that Jupiter has twice disturbed the comet, he thinks there is no certainty (for want of sufficiently accurate observations) respecting either the original path of the comet, or that in which it is at present circulating unobserved--if, indeed, it has not been absorbed by Jupiter.