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the average time of the revolution of comets generally is about a thousand years; which proves that the sun's gravitating force extends very far. La Place estimates that the solar attraction is felt throughout a sphere whose radius is a hundred millions of times greater than the distance of the earth from the sun." "The orbit of Halley's comet is four times longer than it is broad; its length is about 3420 millions of miles, about thirty-six times the mean distance of the earth from the sun. At its perihelion it comes within fiftyseven millions of miles of the sun, and at its aphelion it is sixty times more distant. On account of this extensive range, it must experience 3600 times more light when nearest to the sun than in the most remote point of its orbit. In the one position the sun will seem to be four times larger than he appears to us, and at the other he will not be apparently larger than a star."*

The appearance of this comet, so near the time predicted by astronomers, and in positions so nearly agreeing with those which were previously calculated, is a clear proof of the astonishing accuracy which has been introduced into astronomical calculations, and of the soundness of those principles on which the astronomy of comets is founded. It likewise shows that comets, in general, are permanent bodies connected with the solar system, and that no very considerable change in their constitution takes place while traversing the distant parts of their orbits.t

From the preceding historical sketches and descriptions, the reader will learn something of the general phenomena of com

* Mrs. Somerville's "Connexion of the Physical Sciences;" a work which, though written in a popular style, would do honour to the first philosophers of Europe. Of this lady's profound mathematical work on the "Mechanism of the Heavens," the Edinburgh Reviewers remark, "It is unquestionably one of the most remarkable works that female intellect ever produced in any age or country; and, with respect to the present day, we hazard little in saying, that Mrs. Somerville is the only individual of her sex in the world who could have written it."

The most particular observations on Halley's comet, during its appearance in 1835, which I have seen, are those which were made by the Rev. T. W. Webb, of Tretire, near Ross, an account of which, with deductions and remarks, was read to the Worcestershire Natural History Society. The observations were made with an excellent achromatic telescope by Tulley, of five feet six inches focal length, and 3 and 7-10ths inches aperture. Through the kindness of this gentleman I was favoured with a manuscript copy of these observations, and would have availed myself of many of his judicious remarks had my limits permitted.

ets; and I shall now briefly inquire into the opinions which have been formed respecting the

PHYSICAL CONSTITUTION OF COMETS.

On this subject our knowledge is very imperfect; in fact, we may be said to know little or nothing of the physical construction of those mysterious bodies, or of the nature of the substances of which they are composed. In regard to the nebulosity of comets, where there appears no nucleus, it has been conjectured to be composed of something analogous to globular masses of vapour, slightly condensed towards the centre, and shining either by inherent light or by the reflected rays of the sun. When there is a nucleus in the centre of a comet, it seldom happens that the nebulosity extends to it with a gradually increasing intensity. On the contrary, the parts of the nebulosity near the nucleus are but slightly luminous, and seem to be extremely rarefied and transparent. At some distance from their centre, their shining quality is suddenly increased, so that it looks like a ring of invariable size resting in equilibrium around the centre. Sometimes two, and even three of these concentric rings have been perceived separated by intervals; but what appears to be a ring must in reality be a spherical covering, an idea of which may be formed by imagining, in our atmosphere, at three different heights, three continued layers of clouds entirely covering the globe. The matter of the nebulosity is so rare and transparent that the smallest stars may frequently be seen through it.

As to the nucleus, it is generally considered as the solid or densest part of the comet. The nuclei of comets are sometimes very similar to the disks of planets, both in form and brightness. They are generally small compared with the whole size of the comet, but in some cases they are of considerable magnitude, as we have already stated in respect to the comets of 1807 and 1811. Some suppose that the nuclei of comets are transparent, as well as their nebulosities, and allege as a proof that stars have been seen through a nucleus. Thus, Montaigne is said to have seen a star of the sixth magnitude through the nucleus of a small comet, and Olbers saw a star of the seventh magnitude, although it was covered by a comet, and without its light being rendered less powerful; but the accuracy of such observations has been called in question. On the other hand, it has been concluded that the

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nucleus of a comet has on several occasions eclipsed a star which was in the same line of vision. Messier, when observing the small comet of 1774, perceived a star which was eclipsed by the opaque body of a comet, or, at least, all the circumstances attending it led to that conclusion. On the 28th Nov., 1828, at 10h 30′ P.M., M. Wartmann, at Geneva, perceived a star of the eighth magnitude completely eclipsed by Encke's comet. Comets have likewise been observed to transit the disk of the sun like dark spots. M. Gambart, of Marseilles, calculated that a comet which he had observed would pass across the sun on the morning of the 18th November, 1826, and both he and M. Flaucerques were successful in obtaining a sight of it during its transit. Mr. Capel Llofft, on the 6th June, 1818, at 11 A.M., saw a body passing over the sun's disk, which appears to have been a comet. was likewise seen on the same day by Mr. Acton, at 2h 30', considerably advanced beyond the point in which it was seen at 11 A.M., and its progress over the disk seems to have exceeded that of Venus in transit. These observations seem evidently to indicate that some comets at least have nuclei composed of solid and opaque materials. From all the observations in relation to this point, collected by M. Arago, he deduces the following conclusions: 1. That there exist some comets destitute of the nucleus. 2. That there are other comets, the nuclei of which are transparent. 3. That there are also comets which are more brilliant than the planets, the nuclei of which are probably solid and opaque.

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In respect to the tail, or luminous train which generally accompanies comets, it is found that it is generally in opposi tion to the sun, or on the prolongation of the line which would join the sun and the nucleus. But this is not always the case. Sometimes the direction of the tail has been found at right angles with this line; and in some extraordinary instances, the tails of comets have been observed to point directly towards the sun. This was the case with a comet that appeared in 1824, which for about eight days exhibited a luminous train in opposition to that which assumed the ordinary direction. This anomalous tail, according to Olbers, was 7° long, while the other was only 340, and it was bright enough to be seen with an opera-glass. In general, however, it is found that the tail inclines constantly towards the region last quitted by the comet, as if, in its progress through an ethereal medi

um, the matter forming it experienced more resistance than that of the nucleus. The tail is generally enlarged in proportion to its distance from the head of the comet, and in certain cases it is divided into several branches, as already noticed of the comet of 1807. Some have supposed that the divided tail is nothing more than a perspective representation of the sides of a great hollow cone; but there are certain observations which seem to prove that, in some cases, they have a separate existence as independent branches. The most remarkable instance of a divided tail was in the comet of 1744. On the 6th and 7th of March there were six branches in the tail, each of them about 4° in breadth, and from 30° to 40° long. Their edges were pretty well defined and tolerably bright; their middle emitted but a feeble light, and the intervening spaces were as dark as the rest of the firmament. The tails of comets, as already noticed, sometimes cover an immense space in the heavens. The comet of 1680 had a tail which extended to 68°, that of 1811 to 230, and that of 1769 to 97° in length; so that some of these tails must have reached from the zenith to the horizon. The length of the tail of the comet of 1680, estimated in miles, was 112,750,000; that of 1769, 44,000,000; and that of 1744, 8,250,000 miles. A body moving at the rate of 20 miles every hour would not pass over the space occupied by the tail of the comet of 1680 in less than 643 years. It has been supposed by some astronomers that certain changes in the appearance of the tails of comets arise from the rotation of the cometary body; as some comets have been supposed to rotate about an axis passing through the centre of the tail, such as that of 1825, which was concluded, from certain appearances, to perform its rotation in 20 hours, 30 minutes.

As to the nature of the immense tails of comets, their origin, or the substances of which they are composed, we are entirely ignorant, and it would be wasting time to enter into any speculation on this subject, as nothing could be presented to the view of the reader but vague conjectures, gratuitous hypotheses, and unfounded theories.

MISCELLANEOUS REMARKS ON COMETS.

1. Whether comets shine with their own native light, or derive their light from the sun?—This is a question about which

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there have been different opinions, and at the present moment it may be considered as still undetermined, though the probability is, that, in general, they derive their light from the same source as the planets. It appears to have been the opinion of both Schroeter and Herschel, that the comet of 1811 shone by inherent light; and the rapid variations which have been observed in the brightness of the nucleus and the coruscations of the tail, are considered by some as inexplicable on any other hypothesis. It is likewise supposed, that certain phenomena which have been observed in the case of faint and rarefied comets tend to corroborate the same position. For example, Sir J. Herschel, on September 23, 1832, saw a small group of stars of the 16th and 17th magnitude through the comet of Biela. Though this group could have been effaced by the most trifling fog, yet they were visible through a thickness of more than 50,000 miles of cometary matter; and therefore it is supposed scarcely credible that so transparent a material, affording a free passage to the light of such minute stars, could be capable of arresting and reflecting to us the solar rays. On the other hand, it has been objected to this opinion, that comets have appeared as dark spots on the disk of the sun; that their light exhibits traces of polarization; and that they have been occasionally observed to exhibit phases. M. Arago remarks, that "on the very day that any comet shall appear with a distinct phase, all doubts on this subject will have ceased." But it is considered doubtful whether any decided phase has yet been perceived, although some observers were led, from certain phenomena, to infer that something like a phase was presented to their view. It is found that all direct light constantly divides itself into two points of the same intensity when it traverses a crystal possessing the power of double refraction; reflected light gives, on the contrary, in certain portions of the crystal through which it is made to pass, two images of unequal intensity, provided the angle of reflection is not 90°; in other words, it is polarized in the act of reflection. On this principle M. Arago pointed out a photometrical method of determining whether comets borrow their light from the sun, or are luminous in themselves. On the 23d of October, 1835, having applied his new apparatus to the observation of Halley's comet, he immediately saw two images presenting the complementary colours, one of them red, the other green. By turning the instrument half round, the red image

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