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be inferred from the tendency of the gaseous matter to concentrate round one part, and so form the head, or at all events to indicate the referring point for observation of the place and orbit of the comet.

Both the nucleus and the gaseous semi-transparent body, shine by reflected light, as proved by Arago's polarizing experiment, and cannot therefore be supposed to have any notable temperature of their own; certainly not any heat of incandescence. The small weight that the nucleus can possibly be of, is indicated sufficiently in mere observation by its excessively small dimensions; and the little weight of all the body is shown in its transparency; for it can be seen through in every direction, and so easily, that a thickness of sixty thousand miles of it (according to Sir John Herschel in the case of Biela's comet) does not sensibly affect the brightness of the smallest stars. Hence there was probably not so much substance and matter in all that depth and thickness of comet as there would be in a depth of a few feet, or even inches, in an ordinary cloud.

Comets were anciently divided into twelve classes, in this fashion-bearded, torch-like, sword-like, tun-like, javelin-like, horn-like, &c. &c.—and even still a distinction is kept up between the large comets and the small ones, or those with tails and those with none: but this would seem an unnecessary division, for the one sort merge so imperceptibly into the other, that the features of each may be represented merely by different degrees of the same form or quality.

Comets of long period, as that of 1811, with a period of two thousand five hundred years, are generally large; the nucleus of this was distinct, the tail long, broad, and bright; and, being circumpolar, remained constantly visible for upwards of ten months. And comets of very excentric orbits, as in the instance of 1843, which had hardly any perihelion distance, have very excentric bodies, or long narrow tails, this appendage being at one period 70° long by 1° broad.

Comets of short period and small excentricity, as Encke's, with a period only one thousandth of that of 1811, have small bodies, and these not very

excentrically dispersed about the nucleus. Thus Encke's, as exhibited in the engraving, presents only a small oval mass of vapour, without any visible nucleus, but with a sensible concentration towards the place of it. A small star is shown, as it was seen, clearly and brightly, through the very thick of the comet, and must not be confounded with the nucleus. The rest of the stars are inserted rather to show the telescopic characters of them, and their distinction from the comet; they so sharp, precise, and pungent,—and it so weak, diffuse, and ill defined. While planetary bodies may be fixed in space by means of their boundaries, may, in fact, be defined in the strict geometrical sense of a definition, the comet admits of no such fixation, but recourse must be had to the new natural-history method of the type; which, instead of drawing the circumference of a circle, and stating all that it excludes, rather fixes the place of the centre of it, and gives all that it eminently includes : so the place of the comet cannot be defined by its borders, but by the position of its centre, or we may say the excentric focus, carrying the nucleus and the more condensed matter immediately around it.

Although I have referred the reader to the account which I have given in my Cycle, of Encke's comet, still the interest attached to its discovery, or rather to the determination of the nature of its orbit, and consequent rate of its motion, warrants a Parthian glance. As Encke's investigations produced the surprisingly short period of one thousand two hundred and eight days, a retrospective view was obtained, and by trying back it was found to have been seen in 1786 by Messier-Le Furet des Comètes, by Miss Herschel in 1795, and by two or three observers in 1805: but it was so small and so difficult to be seen even with telescopic aid, and the duration of its appearance was so brief, that the only wonder is, that it did not remain unnoticed in the heavens.

The whole astronomical world hailed with the greatest delight the discovery of a comet of short period, one so very short as to keep the body constantly within, and far within, our planetary region, thus bringing it frequently to our view, instead of driving it to such distant regions beyond, as to lead some men to think that the other focus of the orbit must be formed by some star,

or remote sun. But, though it might be interesting, we can neither lug in the argument, nor the question as to the rare elastic ethereal medium diffused through universal space (which its frequently observed returns somewhat countenance, to the great joy of the undulatory theorists), on the shoulders merely of a drawing of the physical appearance of the now well-known vagrant. The same delicacy, however, and wish to confine ourselves to our own subject, will not prevent an allusion to the contraction observed in the size of the body of the comet on approaching the sun; for with this is intimately bound up all and every change and characteristic in the wanderers.

Hevelius had first of all noticed that comets (large ones, of course, visible to the naked eye) contracted in size on approach to the sun, and vice versá; but the observation was strangely overlooked, and precisely the contrary opinion gained ground. At length, a few years ago, M. Valz pointed out that Encke's comet regularly contracted in diminishing its perihelion distance; this contraction he attributed to the pressure of the dense ether in the neighbourhood of the sun. This cause Sir John Herschel showed to be insufficient, and he supplied a better, but here the matter stopped; and so far from applying the same theory to large comets, the contrary law was still supposed to obtain with them; they were considered to "throw" or "shoot" out their tails in approaching the sun; and to retract them, if not to lose them altogether, on leaving him; thereby proving them to be an entirely different genus to the small comets.

More exact observation, however, aided by the fortunate appearance of so extreme a comet as that of 1843, and a calmer consideration of the facts of the case, independently of any previous theoretical ideas, has shown that all comets decrease in size as they come to perihelion; that the length of the comet varies in a certain proportion with the radius vector; that, consequently, in the case of a large one like that of 1843, with hardly any perihelion distance, it becomes at that part of its orbit, where it remains but for a moment, so very dense, as to be capable of being seen in the day-time, yet expands so rapidly after that, as to be soon lost sight of again even at night. In the


case, however, of a comet like Encke's, small in itself and with little excentricity, it never experiences much concentration of substance, and therefore never becomes particularly bright.

That comet's tails should have been supposed to be produced at perihelion, seems only to be attributed to their becoming then more visible than at other times; they are then nearer to us, and hence seen under a larger angle; they are closer to the sun, and consequently illuminated with a stronger light; and thirdly, they are then more dense, and therefore capable of reflecting greater light. Partly, perhaps, also owing to the quickness of the comet's motion at perihelion, by which it is transferred in twenty-four hours from the daylight part of the sky to the night; and thus a tail, before existing, but not seen, by reason of the light back-ground, is said, when seen brightly on the night-sky, to have been suddenly shot out there: for, be it remembered, no one ever pretends to have seen it shooting forth.

Again, as to the phenomena of the form of a tail, as far as such an ill-defined shape is concerned, this is pretty clearly proved by the comet of 1843 to be an affair of phase; and there is strong reason to believe that every comet is a complete elliptical figure, with the nucleus in the focus nearest to the sun. The phase becomes strongest, or the tail-appearance most manifest, in large and dense comets; and least in small and diffuse ones; or in those which may be concentrated within a very small space when their perihelion distance is very small. But this is not exactly the place to explain the whole range of phenomena; it must suffice to point to our figure of Encke's comet. Its smallness, together with its diffuseness and little excentricity, causes the illuminating rays of the sun to strike upon it, and be reflected back to us with almost equal intensity from every part (independently of course of any inherent and proper form in each portion to reflect light as from greater or less density): here, therefore, we see the whole body of the comet; namely, an oval vaporous mass, equally diffuse on every side, and most condensed about one focus, namely, that nearest to the sun.

The drawing attempts to give the comet just as seen in the telescope, with

the view of rendering every one who has the drawing in possession, to be as well circumstanced for judging of the phenomena as those who saw the comet on the night in question. But, nevertheless, the representation falls much too short of what an astronomical drawing should be, of which the necessary accompaniments ought to be-statements of the probable error of the magnitude and brightness of each part. This, however, cannot be attempted in the present backward state of this branch of astronomy; but the more backward it is, and the more it is neglected by others, the more room for some one to distinguish himself, and perform real good work, in adding to the general stock of astronomical knowledge. Cometography being necessarily joined with the determination of the magnitudes and colours of the stars, I cannot but again recommend both it and them.


My late esteemed friend, John Frederick Daniell, who expired on duty in the Council-room of the Royal Society, observed that "man may with propriety be said to be a meteorologist by nature;" because watching the atmospheric vicissitudes on which he is so dependent not only for his comfort, but even for his subsistence, is a necessary portion of the labour to which he is born. Investigating the weather is but studying the conditions and variations of the air, as influenced by the elements of rest and motion, wind and calm, heat and cold, moisture and drought, together with other similar particulars: still the study is to the philosopher one of interest and delight—to the observer of nature it affords objects of grandeur and sublimity-to the farmer, the traveller, and the physician, it is in some measure a study of necessity, still more to the seaman it is especially and vitally important. But, though these phenomena have consequently occupied the attention of all classes of the community from the earliest ages, it is in comparatively recent times that the

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