construction; for, were these parts solid, both the cone and the hemispherical termination of it must have been much brighter in the middle than towards the circumference, which is contrary to observation. CHAP. XI. CONSTRUCTION OF THE COMET OF 1811, AS DETERMINED BY PROFESSOR BURCKHARDT, OF GOTTINGEN, MEMBER OF THE NATIONAL INSTITUTE. THE following observations on the comet were made at Gottingen, and published there on the 20th of September, 1811: "The comet which is now visible on the horizon in the northern part of the heavens, is one of the most remarkable which has ever been observed. None has ever been so long visible, and, cousequently, none has ever afforded such certain means of information with respect to its orbit. Accordingly, since the end of March last, when it was first perceived by M. Flauguergues in the south of France, its course has been regularly traced; nor shall we lose sight of it till the month of January 1812. Its train, which occupies a space of 12 degrees, exhibits several curious phænomena. It is not immediately counected with the comet, as if it were au emanation from it, but forms, at a distance from the nucleus, a wide belt, the lower part of which girds without coming in contact with it, much in the same manner as the ring of Saturn; and this belt extends itself in two long luminous fasces, one of which is usually rectilinear, while the other, at about the third of its length, shoots forth its rays with a slight curve like the branch of a palm-tree; nevertheless this configuration is subject to change. It has, been observed that the space between the body of the comet and its train is occasionally filled, and of the two fasces, that which is generally rectilinear sometimes arches its rays, while those of the other assume the form of right lines. Finally, rays, or, as it were, plumes of ignited matter, have been seen to issue from the lower extremities of the fasces or flakes, and again unite. "Such Aluctuations and accidents in that sort of luminous atmosphere which must occupy in the regions of space a scope of about eight millions of leagues, are immense, and may well impress the imagination with astonishment. The celebrated astronomer of Lilienthal, Mr. De Schrætter, remarked variations of the same kind in the tail of the last comet of 1807, and inserted, in the work he published with respect to it, plates of the successive configurations. "Professor Harding has also observed and delineated with care the present comet under its various aspects, and his design will appear in one of the succeeding numbers of the "Geographical and Astronomical Correspondence, edited at the Observatory of Gotha by Mr. De Lindenau. " They will show that when the comet first appeared, and was yet at a distance from the sun, the two flakes of its train were separated so as to form a right angle; but as that distance decreased, they approached each other till they became parallel. This phenomenon, however, may be nothing more than an optical illusion. "As to the nucleus, or the comet itself, it has been found im possible, as yet, even with the aid of the best telescopes, to make observations on its disk, as on that of a solid body and of determined circumference. We can only discern a vague circular mass, more luminous than the train, particularly towards the centre; but the verge of which is doubtful, furnishing to the eye no fixed line of demarcation. "This mass is without doubt composed of a very subtile substance, as is probably that of all comets. This hypothesis receives much support from the fact, that one of these stars, of very considerable magnitude, (the first comet in 1770,) passed and re-passed through the very middle of the satellites of Jupiter without occasioning amongst them the slightest disorder. There is every reason to believe, that the nucleus of the present comet is nothing more than a conglomeration of vapours of very little density, so little perhaps as to be transparent. Whether this be the case or not, might be easily ascertained, if those who are in the habit of observing it would watch the moment of its transit athwart the disk of some star, the rays of which would have sufficient power to perforate it, if transparent. Such a body might very possibly be an incipient world, just past its gaseous state, and which was to derive solidity from the precipitation and condensation of the matter surrounding it. The successive observation of some comets, in which it may be possible to distinguish the different stages of claos and progressive formation, can alone furnish any knowledge with respect to this point." [Moniteur, Oct. 4. Tilloch's Phil. Mag. ] Glasgow Observatory, Oct. 7. I HOPE the following facts relative to the comet will not be unacceptable to your readers: Since my communication to you of the 4th, relative to the comet, announcing the determination of the elements of its orbit made at this establishment, I am happy to perceive in the London papers which arrived to-day, the result of Burckhardt's second approximation. The talents of this gentleman as a computor are well known, and highly appreciated by the learned world, Between his time of the perihelion passage and ours there is a difference of no more than three days, and the whole period of the comet's revolution, I am satisfied, exceeds considerably 100 years. It is to be remarked too, that Burckhardt never ventured to give to the public his first trials; and therefore, whatever differences exist between his numbers and ours may have been obtained at his second calculation. The inaccuracy of the first he expressly admits in his letter to the editor of the Moniteur, which begins in the following manner:-" Having been requested to correct my first determination," &c. I wish it to be understood, however, that the appearance of his statement has not shaken, in the least degree, the confidence I humbly conceive due to our own results. The observations from which these are derived were performed with the instruments of Troughton; instruments unquestionably superior to any other in the world. But we have still more direct assurance of the accuracy of our observations, by comparing them with the numbers which have been published from the highest authority (that of the Astronomer Royal) in the Philosophical Magazine. The longitudes of the comet, determined at Greenwich and Glasgow Observatories, coincide to the fraction of a minute. The time of the perihelion passage may be considered as pretty accurately fixed, either for Sept. 12 or 9, or, as is more probable, at some intermediate period. From this we can fully explain some of the phænomena generally remarked. From the 9th, as stated in the Glasgow papers by a correspondent, the comet was observed to increase considerably both in brilliancy and in the apparent magnitude of the coma, but particularly of the tail, in the course of eight days. This verifies very happily the observation of Sir Isaac Newton, that it is not till immediately after the perihelion passage, that comets acquire their maximum of lustre and of size. The enlargement therefore uniformly takes place at that time, whether the comet is coming nearer us or moving in the opposite direction. The quantity of increase due to its approximation alone, in six or eight days, can be calculated, and we know that there is no instrument in Scotland capable of measuring the change of apparent magnitude produced by this cause. Whether the exquisite micrometer of Troughton, applied to our great Herschelian telescope, may show any difference, I shall be able to ascertain in a few days, as that instrument is lately dispatched from London for us. I must acknowledge, however, that I entertain very slender hopes of success in this kind of observation on a minute body surrounded with such a nebulosity, and at a distance from us much greater than that of the sun. It has been said, that this comet was ascertained to be the same with that of 1661. The two are as different as can be imagined in every respect. Hence we may see how much safer in the event, scientific investigation is than vague conjectures. I subjoin the elements of the comet of 1661, and those now given by Burckhardt: Long. of node 82 deg. 30 min.... Place of perihelion, 115 deg. 58 min.... BURCKHARdt. 140 deg. 13 min. Perihelion dist.... 42,600,000 miles..... 96,000,000 miles. I am, Sir, your obedient servant, With regard to its transparent cometic atmosphere, we have not only the constant observations of its roundness, during the abovementioned long period of the comet's motion, to prove it to be spherical; but in addition to this, I have already shown that it is of an elastic nature, for which reason alone, had we no other, its globular figure could not be doubted. A most singular circumstance, which however must certainly be admitted, is, that the constant appearance of the bright envelop, with its two opposite diverging branches, can arise from no other figure than that of an inverted hollow cone, terminating at its vertex in an equally hollow cap, of nearly a hemispherical construction; nor can the sides or eaps of this hollow cone be of any considerable thickness. The proof of this assigned construction is, that the bright envelop has constantly been seen in my observation as being every where nearly equidistant from the transparent atmosphere; now if that part of it which in a semi-circular form surrounds the comet, on the side exposed to the sun, were not hemispherical, but had the shape of a certain portion of a ring, like that which we see about the planet Saturn, it must have been gradually transformed from the appearance of a semi-circle into that of a straight line, during the time that we have seen it in all the various aspects presented to us by a geocentric motion of the comet, amounting to 90 degrees. That this hemispherical cap is comparatively thin, is proved from the darkness and transparency of that part of the atmosphere which it covers; for had the curtain of light, which was drawn over it, been of any great thickness, the scattered rays of its lustre would have taken away the appearance of this darkness; nor would the atmosphere have remained sufficiently transparent for us to see extremely small stars through it. It remains now only to account for the semi-circular appearance of the bright envelop; but this, it will be seen, is the immediate consequence of the great depth of light near the circumference, contrasted with its comparative thinness towards the centre. The 6th of October, for instance, the radius of the envelop was 9' 30" on the outside, and 7′ 30′′ on the inside; and as the greatest brightness was rather nearer to the outside, we may suppose its radius to have been about 8'. Then if we compute the depth of the lumi |