Observed at Highbury, by T. W. Burr, Esq.

The comet was first seen on the 5th Sept. without a telescope. On the 8th the tail was quite half a degree long, of a brushlike form, and the nucleus bright and stellar in appearance. On the 12th the comet had wonderfully increased in brilliancy. The nucleus was more planetary and discoid in character, of a golden tint, and having the coma, or nebulous haze, extending round it on one side, and stretching away to a tail of about 3° of length in the opposite direction. There was a well-marked, comparatively dark, separation between the nucleus and the exterior outline of the head; and the sides of the tail were more brilliant than the central portions, conveying the impression that the bright nucleus was near the extremity of a nebulous tubular envelope with slightly divergent sides, and therefore approximating to a funnel shape. The growth of the tail now became exceedingly rapid; and the brilliancy of the head increased likewise, it being but little inferior to that of the stars composing the tail of Ursa Major. By the 21st September the light of the head exceeded that of any star in Ursa Major, and closely approached that of Procyon; while the tail was 8° long, and curved like a scimetar, a characteristic which it preserved during the remainder of its appear

ance.

On October 1st the nucleus had become brighter, but smaller, with projecting wings or jets of light sweeping backwards: while on October 2d a fine dark division in the tail was apparent, reaching from the head for about a degree in length, which subsequently became wider and larger. On October 1st, the tail was 21° in length; on October 5th, 28°; and by the end of the week, on October 9th, fully 36° or 38°, as measured by the declination-circle to the point where the eye estimated its appearance to cease.

The following micrometrical measures of the position of the comet with respect to Arcturus were made on the evening of the 5th of October :

The observations were made with a telescope of 3 inches

aperture, the powers ranging from 20 to 173.

Observed at York, by W. Gray, Esq.

The accompanying sketches of the comet show what particularly impressed me; and though I am unable to draw, they yet may be of use in confirming the observations of better artists. Sept. 30. The second stratum has its crescent prolonged on the following side.

Oct. 3. The first stratum is a sector of 240°, a dark welldefined space between the branches of the tail, a black spot distinct on the preceding side of the glowing nucleus, and on the second area I was under the 'impression that a similar spot, very faint, existed. (I did not see the latter spot on subsequent evenings, but the spot on the first area was always visible.)

Observed at Spalding, Lincolnshire, by F. Selby, Esq.

The observations were made with a six-and-a-half-foot refracting equatoreal, of five inches aperture, by Slater, of London. Owing to the obstruction offered by high buildings, the view of the comet was limited to an interval of time, varying from an hour and a half to half-an-hour each evening.

Sept. 13. A sudden and momentary emanation from the nucleus was remarked. At first it was supposed to be due to atmospheric causes; but from its recurring in precisely the same form, the author felt convinced that it was really attributable to a change in the nucleus.

Appearances of a similar nature continued to be observed during the visibility of the comet. The author arrives at the conclusion that the nucleus is a dense body, emitting igneous or gaseous emanations.

The Great Comets of 1811 and 1858.

The following extract of a letter from Admiral Smyth to the Editor, in reference to the great comets of 1811 and 1858 will be read with interest. It may be mentioned that the comet of 1811 was observed for several months by the gallant admiral while employed on active service in the Mediterranean:

"In re the magnificent comet, I have been closely attending to its fine figure; and am asked on various sides, as I had the advantage of having closely watched both, which I thought the most splendid in appearance, this, or that of 1811? Now, to my memory, which is very distinct, the palm must be given to the latter. As a mere sight-object, the branched tail was of greater interest, the nucleus with its head-veil' was more distinct, and its circumpolarity was a fortunate incident for gazers.

But recollect that in these remarks, I mean nothing disrespectful to the Donati. On the contrary, with those exceptions, it is one of the most beautiful objects I have ever seen in the heavens. The head is certainly not so fully pro

nounced as in that of 1811; but greatly its physical interest is increased by segments of light and a dark hollow, giving the aspect a resemblance to the gaslight called a bat's-wing.

"This dark line, or space down the centre of the brilliant phenomenon, not only had the direct tendency to strengthen the luminosity of the jets of light, in the manner observable in the burning of a wax taper, but also, on a fuller scrutiny of this singular characteristic, to recall its striking resemblance to the similar feature seen in water-spouts, and in the pillars raised in sand-storms which I have witnessed in North Africa."

There has been forwarded to the Apartments of the Society a drawing by Mr. De La Rue, on a 20-inch celestial globe, of the appearance presented by the comet at the Cranford Observatory on the evening of the 5th of October. The time to which the drawing corresponds is 8h P.M., when the tail of the comet was passing over Arcturus. The drawing gives a good idea of the apparent dimensions and curvature of the tail. It is seen passing over g, 5, 7, λ, X,, and Bootis, extending a little beyond the three last-mentioned stars. At its greatest breadth it extends from a Bootis to a small star, h 2, in the same constellation. The tail, roughly speaking, extends from 20° to 56° of north declination, measuring about 36° on the hour-circle passing through Arcturus.

This drawing was executed by Mr. De La Rue before proceeding to Russia, whence he intends shortly forwarding the results of his observations.

Observed at Carbost, Isle of Skye (Lat. 57° 16' 32"; Long. 6° 22′ 40′′ W.), by Lieut. Chinmo, R.N.

This paper contains a delineation of the position of the comet, and of the length and curvature of the tail relatively to the principal stars of Ursa Major. The time of observation is, September 30, 9h 10m P.M. The place of the comet was laid down by means of sextant observations of its distance from a and ʼn Ursa Majoris. The scale employed is two-tenths of an inch to a degree. The tail of the comet is seen extending to Ursa Majoris. The curvature is very well shown. The author has also given a delineation of the position of the comet relatively to the constellation of Ursa Major for the evening of October 5 (6h 39m P.M.), when it approached very near to Arcturus. The stars to which the comet was referred were a and Ursa Majoris.

Note on Comet V. 1858. By W. R. Grove, Esq. F.R.S.

The following extract from a letter addressed by Mr. Grove to Mr. De La Rue refers to the phenomena observed during the transit of the tail of the comet over Arcturus on the

evening of the 5th of October. The telescope employed by Mr. Grove was a small instrument of only two inches aperture, and his remarks are offered under an impression that the weather was not generally favourable for observing the comet on the interesting occasion to which they refer :

"When the comet had entered well within the margin of the tail a dark notch was formed cutting out a portion of the tail round the star; and as the star got further in, this became a dark areola surrounding the star, and in diameter equal to about one-tenth of the line of transit. This continued until the star reached the middle; at this part there is a broad dark line which extends from the nucleus to a distance considerably above the point where the star crossed. When Arcturus arrived here, this dark space was perfect up to the star, but on the other side the white light of the tail appeared to come quite up to the star; in short, as the bright part of the tail had been darkened in the vicinity of the star, the dark part was brightened, at least so much of it as was on the side furthest from the nucleus.

"I saw the notch again on the opposite side previous to emersion, and then lost it by clouds.

"The effects I have described are, doubtless, optical, and the notch and areola evidently due to the bright light of this star: the effect on the dark central part is not so easy to explain."

Admiral Smyth has forwarded a series of interesting drawings of the comet by Baron De Forester, representing its appearance as seen to the naked eye at Oporto on various evenings included between September 26 and October 15. The drawings are carefully executed upon a dark ground; and as the stars in the vicinity of the comet are laid down, they serve to give a good idea of the varying apparent dimensions, and curvature of the tail.

Thoughts on the Formation of the Tail of a Comet.
By J. J. Waterston, Esq.

Any attempt to account for the formation of the tail of a comet upon exact physical principles would be assisted, if, in the drawings of the telescopic appearances of the nucleus and adjacent coma— -which are sometimes provided when circumstances are favourable-we were presented with the projection on the plane of vision of the radius vector, and of the tangent to the orbit. In some cases it might be possible to take such observations as would fix the actual curve of the exterior line of the tail, referred to the radius vector, passing through the

centre of the nucleus; i.e. supposing the central axis of the tail to be in the plane of the orbit. This line is very marked in the front side of Donati's comet, and might probably be easily fixed by the equatoreal.

If we view the tail as composed of molecules as free from the force of cohesion as the molecules of an uncondensable gas, and raised from the nucleus by the heat of the sun; and that that heat as it strikes upon each molecule is converted into a force centrifugal, that not only effectually counteracts the force centripetal of the sun's gravity, but that greatly exceeds it; such molecules will be quickly removed from the feeble attraction of the nucleus, and assume the motion of bodies entirely free from its influence. This hypothesis is suggested by the mechanical theory as a possibility.

If we inquire as to the centrifugal power of the sun's rays, we may, with the data afforded by modern research, easily compute the accelerative force it is capable of engendering on a single chemical molecule, if their whole heating power were converted into such a force. This is surpassingly great-no less than 800 miles per second! The data are, value of sun's radiation in a solar day equal to 1.8 ft. thickness of ice melted; the mechanical equivalent of which is about 13 lb. raised 1 foot high per second by the heat impinging on a square foot. If this heat impinges on a superficial foot of gold-leaf, one molecule thick (about one 200-millionth of an inch, as deduced from the relation of capillarity to latent heat, both being the measure of liquid cohesion, the first that of the superficial stratum of molecules, the second that of a cubic mass of molecules), and is converted into an impulsive force in one direction, it would in one second communicate a velocity of 800 miles a second; and in about four minutes a velocity equal to light itself. It is, of course, an extreme case to suppose the whole heat absorbed and converted; but it seems right to have in view the quantitative elements of the question. While the earth's atmosphere absorbs heat from the sun's rays, we have reason to suppose that heatvibrations are converted into rectilineal velocity of gaseous molecules; but there is no reason to suppose that the impulse is in the specific direction from the sun, but equally from and to, as in vibratory impulses generally. It may be shown that the mechanical equivalent of the solar heat that impinges on the earth amounts to one-hundredth of its gravity towards the sun; so that, if any part of it acted centrifugally, the orbit would be disturbed. The same remark applies to a comet; but it cannot be denied that the phenomena of the tail, more especially as it turns sharp round in the perihelion passage, are such as require for their explanation a very active exhibition of such a centrifugal force as might be engendered by the conversion of heat acting upon single and free molecules.

Oct. 7, 1858.