'il fallait bien que cette apparition eût en effet quelque chose de miraculeux puisqu'elle balança la terreur, que les rapides succèsde Mahomet II., qui venait de s'emparer de Constantinople, répandaient à cette époque dans toute la Chrétienté, et que le Pape Calixte II., ordonna des prières publiques et formula une bulle où il exorcisait à la fois la comète et les Turcs: ce qui n'empêcha pas toutefois, il faut bien l'avouer, la comète de poursuivre paisiblement son cours, et Mahomet de faire de Sainte Sophie sa principale mosquée.'

At its subsequent returns much more of its splendour seems to have vanished, for in 1706 it had hardly any tail, and in 1682 it scarcely attracted the attention of astronomers, so much so, that Lalande apprehended it would be so diminished at its next return, as to be invisible;-happily for science, however, this was not the case.

Since the year 1682 the comet has come within the attraction of Jupiter, Saturn, Uranus, and the Earth. Of these Jupiter, from being of greater magnitude, has had by much the greatest effect in disturbing its motions. The tendency of his attraction has been to increase the length of the comet's path, consequently to augment the time of its revolution, whereas the attraction of Saturn, Uranus, and the Earth, acting in a contrary direction, has tended to shorten the comet's path, and to diminish the time of its revolution. The real change produced is, therefore, the difference of the two, so that the disturbing action of these four planets has enlarged the comet's orbit, and augmented its periodic time.

Fifteen years have now elapsed since Halley's comet began again to engage the attention of astronomers, and all who have computed the time of its arrival at its perihelion from ancient observations, had fixed on the beginning of November of 1835 for the accomplishment of that event. The inaccuracy of the data has, no doubt, led to some discrepancy in the path assigned to the comet by different astronomers, yet their computations, which can only be regarded as approximations, from our ignorance of the motions of the comet during its former revolutions, may be considered as the most splendid triumphs of science, since, notwithstanding all the known and unknown causes of derangement, the comet appeared at the very time, and in the exact spot that was predicted, and its actual passage at its perihelion only differed from its computed passage by a very few days, and that in a period of no less than seventy-five years.

The orbit, determined by Lieutenant Stratford, of the navy, from recent observations, represents with great accuracy the comet's true path in the heavens, and that calculated by Professor Rosenberger very nearly coincides with Mr. Stratford's; according to both of these astronomers, the comet came to its perihelion about noon on the 16th of November. From

From the numerous observations that have been made throughout the world, all the circumstances of this comet's motion will now be perfectly known, and the time of its return to its perihelion seventy-five years hence will be accurately ascertained. There may, indeed, be bodies of a planetary or cometary nature in the far distant regions of the heavens where the comet wanders, which may disturb its motions and change the period of its revolution, and it is even not impossible that another planet may be revolving about the sun beyond the orbit of Uranus, which may at different times alter the comet's orbit; but so accurately can the disturbing action of all the known bodies of our system be calculated, that the existence of these unknown sources of perturbation would be made evident to future astronomers, should any difference be then found between the computed and actual orbit of the comet.

The influence of the ethereal medium on the motions of Halley's comet cannot be known till after it has accomplished another revolution. Professor Rosenberger has, indeed, computed its effect on the duration of its last period, but that is a point which, in the present state of our knowledge, can only be ascertained by experience; it must depend upon the magnitude and density of the comet, and upon the law by which the density of the fluid varies with the distance from the sun, points of which we are ignorant. Moreover, the opinion is generally received, that the ethereal fluid has a rotatory motion about the sun from west to east, communicated to it by the planets revolving in that direction since their creation, or that this motion is coeval with creation itself. Should such rotation exist, it must have a widely different effect upon Halley's comet, which moves from east to west, from that which it produces upon Encke's and Biela's comets revolving in a contrary. direction; therefore, although the influence of the fluid upon Encke's comet, computed on hypothesis, accords with observation, we are still too little acquainted with the nature and motions of Halley's comet to decide what part of the variations in the period of its revolution are due to that cause.

Some idea may be formed of the vast size of the comet's orbit by comparing it with our distance from the sun. The mean distance of the sun from the earth is about ninety-five millions of miles : now the length of the comet's orbit is thirty-six times as much, and its breadth is a fourth of its length. The comet moves in this vast orbit from east to west with a speed subject to great variations. In approaching the sun, its velocity continually increases, till it darts round him with astonishing rapidity, coming as near as within forty-seven millions of miles of his centre. Its velocity gradually diminishes after leaving the sun, till it reaches the most remote point of its path, where its distance from him is about three thou

sand,

sand, three hundred and seventy-two millions of miles, but even there the sun's attraction is sufficient to recall the comet towards him, its velocity being so much reduced that his influence prevails. The sun viewed from the comet at this enormous distance cannot

appear larger than a star, but at the nearest point of the orbit his apparent magnitude will be four times greater than at the earth.

When the comet was first seen in England it appeared to be merely a faint nebulous mass with a concentration of light on one side of its centre; still it was brighter than either Encke's or Biela's comets. By the end of August the concentrated light had attained the size of a star of about the tenth maguitude, which in approaching the earth and sun became so well defined that it might have been mistaken for a solid nucleus; and although the whole of the comet increased in brilliancy, it never reached its ancient splendour. These appearances, observed in England, are confirmed by those made in other countries. We have received from a friend in St. Petersburgh the following very interesting account of Halley's comet, by M. Struvé, at Dorpat, in Livonia, an eminent astronomer, possessing the best instruments: in a letter dated the 7th of September, at Dorpat, he says,

'Halley's comet has been discovered here and at Nicolaef on the 20th of August. Since that day I have been most fortunate in the manner in which the place of the comet has been determined on fifteen nights, with a precision that leaves nothing to be desired; being limited only by the form of the comet which presents no solid nucleus, but only a concentration of light towards the centre, which is, however, a little excentric on the side opposite to the sun.' And in a subsequent letter of the 20th of September, he adds— You congratulate me on being favoured with weather quite astonishing for our climate; I have only succeeded yesterday in finishing the copy of a series of observations presenting the phenomenon of a celestial body moving with the velocity of the moon, which might, without doubt, be employed for the determination of longitudes, if many astronomers were to measure the differences in declination between the comet and a neighbouring star. This rapid motion affords a hope of being able to observe from time to time occultations of fixed stars by the comet, which if they were sufficiently luminous, and to pass through the centre of the comet, might throw important light upon its nature. Unfortunately the comet has hitherto passed through constellations poor in stars; nevertheless I observed on the 29th of September a central occultation of a fixed star of the 9th magnitude, which, according to calculation, passed at 1"-5 of a degree from the centre of the comet; the star remained constantly visible without any considerable diminution of light, and it was the nucleus of the comet that disappeared at the moment of conjunction from the brilliancy of the star.

'The

The reductions of the micrometric measurements made during the whole of the passage of the comet will show whether the star experienced refraction from the comet or not.'

But still more extraordinary information is communicated in Mr. Struvé's letter of the 4th of October :

'The observations of the form of the central part of the comet are very curious. I have seen an elliptical nucleus, the greater diameter of which was from 1"5 to 3" of a degree, and the lesser diameter was about 04. It resembled a burning coal: from thence issued, in a direction nearly opposite to that of the tail, a divergent flame varying in intensity, in form, in direction, sometimes even double-one might fancy that luminous gas was issuing from the nucleus.'

This remarkable phenomenon of light diverging from the nucleus peculiar to Halley's comet, as far as is known, has been noticed by many in this country and in France. M. Arago, and other astronomers at the Royal Observatory in Paris, on the 15th of October, saw a luminous sector or diverging light issuing from the head of the comet, a little to the south of the point immediately opposite the tail: it was much more brilliant than the rest of the nebulosity, and was bounded by two lines of fainter light directed towards the centre of the head; next evening not a trace of this sector existed; but at a point diametrically opposite the axis of the tail a new sector was formed of more than 90° of angular extent-more elongated-much more brilliant than the former, and bounded by two very bright lines tending to the centre of the head. On the 17th this appearance had diminished in splendour; and on the evening of the 18th, though the atmosphere was extremely clear, the whole of the comet, together with its tail, had lost much in brightness. The comet was not seen again till the 21st, and then there were three luminous sectors or brushes of light seen in the nebulosity, the most feeble and dilated of which was exactly in the prolongation of the tail. By the 23d of October these had totally vanished; but the whole aspect of the comet had changed so suddenly, and so completely, that at first M. Arago imagined some moisture must have been deposited on the lenses of his telescope. The nucleus, hitherto so brilliant and well-defined, had become large and diffuse; and although it still occupied the centre of the head, the brilliancy of the nebulosity on the eastern side far surpassed that on the western. Diverging flames, such as already described, have not been seen in any other comet, although they were noticed by Hevelius in Halley's comet, in the year 1682. He says,In ipso capite, beneficio longioris telescopii, non nisi unicum nucleum. figuræ ovalis et gibbosæ constanter notavimus; nisi quod die 8° Septembris,

Septembris, ex dicto nucleo clarissimus simul radius, ex parte etiam incurvatus in caudam exiret: quod ut notari meretur (cum ejus generis faciem in nullo adhuc cometâ, quantum memini, observaverim) sic lubens volui faciem capitis et caudæ delineatam dare, uti ex figura subsequente suo loco videbis.'

A rotatory motion may be inferred from the rapid changes of appearance in these sectors.

According to the observations of M. Valz, of Nismes, the nebulosity of Halley's comet dilates on approaching the sun, but the changes in its physical constitution are quite unprecedented. Its tail, before arriving at its perihelion, extended over about thirtyeight or forty degrees, and was curved at the end; it never was very bright, but it will be seen to much greater advantage in the southern hemisphere after leaving the sun.

Although authentic records of Halley's comet go no farther back than the year 1456, yet, by assuming that it returns at intervals of 75 years, it may be traced to much more remote times, and, with some small degree of probability, even to a period before the Christian era. The evidence rests upon its periodic revolution alone, as no observations on comets were made in those days when they were regarded as prodigies rather than astronomical objects. Since the period of Halley's comet may vary as much as 18 months, from the disturbing action of the planets-its identity with comets of remote ages must be regarded as very doubtful, especially as the accounts given of them by historians are evidently written under the influence of fear, and therefore too much exaggerated to be much depended upon.

The description of the comet, which appeared at the birth of Mithridates, 130 years before the Christian era, is so wonderful, that it corresponds with Halley's comet in no circumstance but that of time. We are told that it surpassed the sun in brightness, and occupied one quarter of the heavens, and was of dimensions so enormous that it took four hours to rise and four to set. If this exaggerated account refers to Halley's comet, it must have accomplished six revolutions before it again attracted the notice of historians, for the next that corresponds in its period came to our system in A.D. 323. Another appeared in A.D. 399, after an interval of 76 years, which is mentioned by all the historians of the Lower Empire to have been of prodigious magnitude, horrible aspect, and with a tail that seemed to reach to the earth. A comet again came to the sun after a period of twice 75 years, that is, in the year 550, remarkable for the sack of Rome by Totila. From that period, there is no other comet on record corresponding with Halley's till the years 930, 1005, and 1305.

The