return. It revolves in an ellipse of great excentricity (though not comparable to that of Halley's), the plane of which is inclined at an angle of about 13° 22' to the plane of the ecliptic, and in the short period of 1211 days, or about 3 years. This remarkable discovery was made on the occasion of its fourth recorded appearance, in 1819. From the ellipse then calculated by Encke, its return in 1822 was predicted by him, and observed at Paramatta, in New South Wales, by M. Rümker, being invisible in Europe: since which it has been re-predicted and re-observed in all the principal observatories, both in the northern and southern hemispheres, as a phenomenon of regular occurrence.

(577.) On comparing the intervals between the successive perihelion passages of this comet, after allowing in the most careful and exact manner for all the disturbances due to the actions of the planets, a very singular fact has come to light, viz. that the periods are continually diminishing, or, in other words, the mean distance from the sun, or the major axis of the ellipse, dwindling by slow and regular degrees at the rate of about 0.11 per revolution. This is evidently the effect which would be produced by a resistance experienced by the comet from a very rare ethereal medium pervading the regions in which it moves; for such resistance, by diminishing its actual velocity, would diminish also its contrifugal force, and thus give the sun more power over it to draw it nearer. Accordingly this is the solution proposed by Encke, and at present generally received. It will, therefore, probably fall ultimately into the sun, should it not first be dissipated altogether, a thing no way improbable, when the lightness of its materials is considered.

(578.) By measuring the apparent magnitude of this comet at different distances from the sun, and thence, from a knowledge of its actual distance from the earth at the time, concluding its real volume, it has been ascertained to contract. in bulk as it approaches to, and to expand as it recedes from, that luminary. M. Valz, who was the first to notice this fact, accounts for it by supposing it to undergo a real compression or condensation of volume arising from the

pressure of an æthereal medium which he conceives to grow more dense in the sun's neighbourhood. But such an hypothesis is evidently inadmissible, since it would require us to assume the exterior of the comet to be in the nature of a skin or bag impervious to the compressing medium. The phenomenon is analogous to the increase of dimension above described as observed in the comet of Halley when in the act of receding from the sun, and is doubtless referable to a similar cause, viz. the alternate conversion of evaporable matter into the states of visible cloud and invisible gas by the alternating action of cold and heat. This comet has no tail, but offers to the view only a small ill-defined nucleus, excentrically situated within a more or less elongated oval mass of vapours, being nearest to that vertex which is towards the sun.

(579.) Another comet of short period is that of Biela, 80 called from M. Biela, of Josephstadt, who first arrived at this interesting conclusion on the occasion of its appearance in 1826. It is considered to be identical with comets which appeared in 1772, 1805, &c., and describes its very excentric ellipse about the sun in 2410 days or about 6 years; and in a plane inclined 12° 34′ to the ecliptic. It appeared again according to the prediction in 1832, and in 1846. Its orbit, by a remarkable coincidence, very nearly intersects that of the earth; and had the latter, at the time of its passage in 1832, been a month in advance of its actual place, it would have passed through the comet, a singular rencontre, perhaps not unattended with danger.

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Should calculation establish the fact of a resistance experienced also by this comet, the subject of periodical comets will assume an extraordinary degree of interest. It cannot be doubted that many more will be discovered, and by their resistance questions will come to be decided, such as the following:- What is the law of density of the resisting medium which surrounds the sun? Is it at rest or in motion? If the latter, in what direction does it move? Circularly round the sun, or traversing space? If circularly, in what plane? It is obvious that a circular or vorticose motion of the ether would accelerate some comets and retard others, according as their revolution was, relative to such motion, direct or retrograde. Supposing the neighbourhood of the sun to be filled with a material fluid, it is not conceivable that the circulation of the planets in it for ages should not have impressed upon it some degree of rotation in their own direction. And this may preserve them from the extreme effects of accumulated resistance. Author.

(580.) This comet is small and hardly visible to the naked eye, even when brightest. Nevertheless, as if to make up for its seeming insignificance by the interest attaching to it in a physical point of view, it exhibited at its last appearance in 1846, a phænomenon which struck every astronomer with amazement, as a thing without previous example in the history of our system. It was actually seen to separate itself into two distinct comets, which, after thus parting company, continued to journey along amicably through an arc of upwards of 70° of their apparent orbit, keeping all the while within the same field of view of the telescope pointed towards them. The first indication of something unusual being about to take place, might be, perhaps, referred to the 19th of December 1845, when the comet appeared pear-shaped, the nebulosity being unduly elongated in the north following direction. But on the 13th of January, at Washington in America, and on the 15th and subsequently in every part of Europe, it was distinctly seen to have become double; a very small and faint cometic body, having a nucleus of its own, being observed appended to it, at a distance of about 2' (in arc) from its center, and in a direction forming an angle of about 328° with the meridian, running northwards from the principal or original comet (see art. 204). From this time the separation of the two comets went on progressively, though slowly. On the 30th of January, the apparent distance of the nucleus had increased to 3', on the 7th of February to 4', and on the 13th to 5', and so on, until on the 5th of March the two comets were separated by an interval of 9′ 19′′, the

Perhaps not quite so. To say nothing of a singular surmise of Kepler, that two great comets seen at once in 1618, might be a single comet separated into two, the following passage of Hevelius cited by M. Littrow (Nachr. 564.) does really seem to refer to some phænomenon bearing at least a certain analogy to it. "In ipso disco," he says (Cometographia, p. 326.) quatuor vel quinque corpuscula quædam sive nucleos reliquo corpore aliquanto densiores ostendebat.

According to Mr. Hind's observation. But there can be little doubt that by a mistake of the most common occurrence, when no measure of the position is taken, north following is an error of entry or printing for north preceding (n f for n p). In fact, an elongation from north following to south preceding would agree with the regular direction of the tail and would occasion no remark.

apparent direction of the line of junction all the while varying but little with respect to the parallel.*

(581.) During this separation, very remarkable changes were observed to be going on both in the original comet and its companion. Both had nuclei, both had short tails, parallel in direction, and nearly perpendicular to the line of junction, but whereas at its first observation on January 13th, the new comet was extremely small and faint in comparison with the old, the difference both in point of light and apparent magnitude diminished. On the 10th of February, they were nearly equal, although the day before the moonlight had effaced the new one, leaving the other bright enough to be well observed. On the 14th and 16th, however, the new comet had gained a decided superiority of light over the old, presenting at the same time a sharp and starlike nucleus compared by Lieut. Maury to a diamond spark. But this state of things was not to continue. Already, on the 18th, the old comet had regained its superiority, being nearly twice as bright as its companion, and offering an unusually bright and starlike nucleus. From this period the new companion began to fade away, but continued visible up to the 15th of March. On the 24th the comet was again single, and on the 22d of April both had disappeared.

(582.) While this singular interchange of light was going forwards, indications of some sort of communication between the comets were exhibited. The new or companion comet, besides its tail, extending in a direction parallel to that of the other, threw out a faint arc of light which extended as a kind of bridge from the one to the other; and after the restoration of the original comet to its former preeminence, it, on its part, threw forth additional rays, so as to present (on the 22d and 23d February) the appearance of a comet with three faint tails forming angles of about 120° with each other, one of which extended towards its companion.†

By far the greater portion of this increase of apparent distance was due to the comet's increased proximity to the earth. The real increase reduced to a distance=1 of the comet was at the rate of about 3" per diem.

These last mentioned particulars, rest on the testimony of Lieutenant Maury of Washington, who had the advantage of using a nine-inch object glass

(583.) Professor Plantamour, director of the observatory of Geneva, having investigated the orbits of both these comets as separate and independent bodies, from the extensive and careful series of observations made upon them, has arrived at the conclusion that the increase of distance between the two nuclei, at least during the interval from February 10th to March 22d, was simply apparent, being due to the variation of distance from the earth, and to the angle under which their line of junction presented itself to the visual ray; the real distance during all that interval (neglecting small fractions) having been on an average about thirty-nine times the semi-diameter of the earth, or less than two-thirds the distance of the moon from its center. From this it would appear, that already, at this distance, the two bodies had ceased to exercise any perceptible amount of perturbative gravitation on each other; as, indeed, from the probable minuteness of cometary masses we might reasonably expect. Calculating upon the elements assigned by him, we find 164.4 for the interval of their next perihelion passages. And it will be, therefore, necessary at their next reappearance, to look out for each comet as a separate and independent body, computing its place from these elements as if the other had no existence. Nevertheless, as it is still perfectly possible that some link of connection may subsist between them, (if indeed, by some unknown process the companion has not been actually reabsorbed,) it will not be advisable to rely on this calculation to the neglect of a most vigilant search throughout the whole neighbourhood of the more conspicuous

of Munich manufacture. It does not appear that any large telescope was turned upon it in Europe on the dates in question.