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 caps 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 pre sented 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

Dous matter at this distance from the centre, we find that it could not be less than 248 thousand miles; whereas in the place where the atmosphere was darkest, its thickness would be only about 50 thousand; so that a superior intensity of light in the ratio of about 5 to 1, could not fail to produce the remarkable appearance of a bright semi-circle, enveloping the head of the comet at the distance at which it was observed *..

I have entered so fully into the formation of the envelope, as the argument, by which its construction has been analysed, will completely explain the appearance of the streams of light inclosing the tail of the comet, and indeed its whole construction.

The luminous matter as it arises from the envelope, of which it is a continuation, is thrown a little outwards, and assumes the appearance of two diverging bright streams or branches; but if the source from which they rise be the circular rim of an hemispherical hollow shell, the luminous matter in its diverging progress upwards can only form a hollow cone; and the appearance of the two bright streams inclosing the tail, after what has been said of the envelope, will want no farther explanation,

Add to this that, having actually seen these brilliant, streams remain at the borders of the tail in the same diverging situation during a motion of the comet through more than 130 degrees, the hollow conical form of the comet's tail is in fact established by observation.

!,

The feebler light of the tail between its branches is sufficiently accounted for by the thinness of the luminous matter of the hollow cone through which we look towards the middle of the tail: com pared with its great depth about the sides; and indeed the comparative darkness of the inside of the cone and transparency of the atmosphere seen through the envelope, bear witness to their hollow

* From the measures of the envelope, whose diameter the 6th of October was 613032 miles, we have the radius a b, 321516. Then if ed be 25000, we find the angle bac, of, which a c is cosine 22° 44' 37"; and the sine bc, which is the depth, will be to the versed sine cd, which is the thickness, as 4,972 to 1. And if ad is 9′30′′, the greatest brightness which is at c will give the distance ac equal to 8' 45",7. This calculation being made for that part which is convex towards us, the addition of the concave opposite side will double the dimensions of the depth and thickness.

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, conse. quently, 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 1912. 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 an 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.

.1

er

« Such fluctuations 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 Schrotter, 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 se parated 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 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 coudensation of the inatter surrounding

it. The successive observation of some comets, in which it may be 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. ]

CHAP. XII.

OBSERVATIONS ON THE

PRECEDING PHENOMENON

MADE AT THE OBSERVATORY AT GLASGOW.

SIR,

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 approxima tion. 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 Lours may have been obtained at his second calculation. The inaċcuracy 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,