"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 Lili enthal, 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 impossible, 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 de marcation.

"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 consi derable 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

be

it. The successive observation of some comets, in which it may possible to distinguish the different stages of chaos and progressive formation, can alone furnish any knowledge with respect to this point."

[Moniteur, Oct. 4. Tilloch's Phil. Mag. ]

I

Glasgow Observatory, Oct. 7.

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; instru ments 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, deter mined 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 New-! ton, that it is not till immediately after the perihelion passage, that comets acquire their maximum of lustre and of size. The enlarge ment 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 in strument is lately dispatched from London for us, adə

1

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:

COMET 1661. car ça att ba endil

BURCKHärdt.

Long, of node.... 82 deg. 30 min.... 140 deg. 13 min. Inclination...... 32 deg. 35 min.... 72 deg. 42 min. Place of perihelion, 145 deg. 58 min....74 deg. 12 min. 1917 Perihelion dist...42,600,000 miles. 96,000,000 miles... ...pis 34... I am, Sir, your obedient servant,

Glasgow Observatory, Oct. 16, ; SIR-In the Glasgow Courier of October 5, I had the honour of submitting to the public the results of the joint labours of Mr. Cross and myself, for the preceding month, on the comet, at the Glasgow Observatory. In The Star newspaper of October, 11, appeared for the first time the elements of the orbit, as determined by the celebrated Burckhardt, Member of the National Institute.

It is a duty, which I owe to the skill and the unwearied exertions, of my associate Mr. Cross, to this patriotic establishment, and also: to this country, hitherto considered by the French mathematicians. and astronomers unequal to the primary solution of this difficult, problem, to state the following facts:-On October, 8, at eight: hours fifteen minutes, by observations made here, with every pre-, caution to insure the utmost accuracy, the comet had deviated 42 degrees 18 minutes from the lougitude which Burckhardt's elements, assign for that instant. On October 14, at two o'clock in the morning, the longitude, as deduced from a most satisfactory transit,, was 206 degrees 42 minutes. By the French computation, it ought, to have been 248 degrees 1 minute, differing from nature by 41 de grees 19 minutes. By our elements, which have received a partial correction from my observations since the 5th, the coincidence on the 8th, at the same time, was within 15 minutes, and on the 14th, within 13 minutes. Our computed latitudes on the 13th agree to a minute with observation, while those of Burckhardt differ by 3 des grees or 180 times that quantity. and s

The examination of both has been made by the excellent tables of the parabola, constructed by Delambre, imperial observer at Paris., It is in the longitude of the perihelion that the chief discordance exists between the French elements and ours, and this amounts to about 31 degrees; the former being, in our judgment, too small by this quantity,

The comet has been continually approaching the earth for many weeks. From September 15th till October 14th, its decrease of distance amounted to 25 millions of miles, yet its brilliancy and the magnitude of its tail have gone on diminishing, as Burckhardt properly remarked, Persons ignorant of astronomy would naturally infer from this diminution, the recedure of the comet from us, as, from its increase they conjectured its approach, Astronomers laugh at such idle dreams when applied to a demonstrative science, in

which conjecture has found no place since the days of Newton. Its first principles teach, that these phænomena arise from the comet's varying distance from the sun. At the period of the perihelion passage these bodies are known uniformly to attain their maximum of size and brightness.

[Tilloch's Phil. Mag.]

ANDREW URE.

CHAP. XIII

OF SOLAR AGENCY IN THE PRODUCTION OF COMETIC PHÆNOMENA.

As we are now in a great measure acquainted with the physical construction of the different parts of the present comet, and have seen many successive alterations that have happened in their ar rangement, it may possibly be within our reach to assign the pro bable manner in which the action of such agents as we are acquainted with has produced the phænomena we have observed.

... In its approach to a perihelion, a comet becomess exposed to the action of the solar rays, which, we know are capable of producing light, heat, and chemical effects. That their influence on the pre sent comet has caused an expansion, and decomposition of the cometic matter, we have experienced in the growing condition of the tail and shining quality of its light, which seems to be of a phos phoric nature. The way by which these effects have been produced may be supposed to be as follows.

The matter contained in the head of the comet would be dilated by the action of the sun, but chiefly in that hemisphere of it which is immediately exposed to the solar influence; and being more increased in this direction than on the opposite side, it would become eccentric, when referred to the situation of the body of the comet; but as the head is what draws our greatest attention, on account of