angles to the ecliptic, they must in many instances, if not actually intersect, at least pass very near to the orbits of some of the planets. We have already seen, for instance, that the orbit of Biela's comet so nearly intersects that of the earth, that an actual collision is not impossible, and indeed (supposing neither orbit variable) must in all likelihood happen in the lapse of some millions of years. Neither are instances wanting of comets having actually approached the earth within comparatively short distances, as that of 1770, which on the 1st of July of that year was witin little more than seven times the moon's distance. The same comet in 1767 passed Jupiter at a distance only one 58th of the radius of that planet's orbit, and it has been rendered extremely probable that it is to the disturbance its former orbit underwent during that appulse that we owe its appearance within our own range of vision. This exceedingly remarkable comet was found by Lexell to describe an elliptic orbit with an excentricity of 0.7858, with a periodic time of about five years and a half, and in a plane only 1° 34′ inclined to the ecliptic, having passed its perihelion on the 13th of August 1770. Its return of course was eagerly expected, but in vain, for the comet has never been seen since. Its observation on its first return in 1776 was rendered impossible by the relative situations of the perihelion and of the earth at the time, and before another revolution could be accomplished (as has since been ascertained,) viz: about the 23d of August 1779, by a singular coincidence it again approached Jupiter within one 491st part of its distance from the sun, being nearer to that planet by one-fifth than its fourth satellite. No wonder, therefore, that the planet's attraction (which at that distance. would exceed that of the sun in the proportion of at least 200 to 1) should completely alter the orbit and deflect it into a curve, not one of whose elements would have the least resemblance to those of the ellipse of Lexell. It is worthy of notice that by this rencontre with the system of Jupiter's satellites, none of their motions suffered any perceptible derangement,—a sufficient proof of the smallness of its mass. Jupiter indeed, seems, by some strange fatality, to be constantly in the way of comets, and to serve as a perpetual stumbling-block to them.

(586.) On the 22nd of August, 1844, Signor De Vico, director of the observatory of the Collegio Romano, discovered a comet, the motions of which, a very few observations sufficed to show, deviated remarkably from a parabolic orbit. It passed its perihelion on the 2nd of September, and continued to be observed until the 7th of December. Elliptic elements of this comet, agreeing remarkably well with each other, were accordingly calculated by several astronomers; from which it appears that the period of revolution is about 1990 days, or 5 (5·4357) years, which (supposing

its orbit undisturbed in the interim) would bring it back to the perihelion on or about the 13th of January, 1850. As the assemblage and comparison of these elements thus computed independently, will serve better, perhaps, than any other example, to afford the student an idea of the degree of arithmetical certainty capable of being attained in this branch. of astronomy, difficult and complex as the calculations themselves are, and liable to error as individual observations of a body so ill-defined as the smaller comets are for the most part; we shall present them in a tabular form, as on the next page: the elements being as usual; the time of perihelion passage, longitude of the perihelion, that of the ascending node, the inclination to the ecliptic, semiaxis and excentricity of the orbit, and the periodic time.

This comet, when brightest, was visible to the naked eye, and had a small tail. It is especially interesting to astronomers from the circumstance of its having been rendered exceedingly probable by the researches of M. Leverrier, that it is identical with one which appeared in 1678 with some of its elements considerably changed by perturbation. This comet is further remarkable, from having been concluded by Messrs. Laugier and Mauvais, to be identical with the comet of 1585 observed by Tycho Brahe, and possibly also with those of 1743, 1766, and 1819.

(587.) Elliptic elements have in like manner been assigned to the comet discovered by M. Brorsen, on the 26th of February, 1846, which, like that last mentioned, speedily after its discovery began to show evident symptoms of deviation from a parabola. These elements, with the names of their respective calculators, are as follow. The dates are for February 1846, Greenwich time.

This comet is faint, and presents nothing remarkable in its appearance. Its chief interest arises from the great similarity of its parabolic elements to those of the comet of 1532, the place of the perihelion and node, and the inclination of the orbit, being almost identical.

(588.) Elliptic elements have also been calculated by M. D'Arrest, for a comet discovered by M. Peters, on the 26th of June, 1846, which go to assign it a place among the comets of short period, viz. 58044-3, or very nearly 16 years. The excentricity of the orbit is 0.75672, its semiaxis 6-32066, and the inclination of its plane to that of the ecliptic 31° 2' 14". This comet passed its perihelion on the 1st of June, 1846.

(589.) By far the most remarkable comet, however, which has been seen during the present century, is that which appeared in the spring of 1843, and whose tail became visible in the twilight of the 17th of March in England as a great beam of nebulous light, extending from a point above the western horizon, through the stars of Eridanus and Lepus, under the belt of Orion. This situation was low and unfavourable; and it was not till the 19th that the head was seen, and then only as a faint and ill-defined nebula, very rapidly fading on subsequent nights. In more southern latitudes, however, not only the tail was seen, as a magnificent train of light extending 50° or 60° in length; but the head and nucleus appeared with extraordinary splendour, exciting in every country where it was seen the greatest astonishment and admiration. Indeed, all descriptions agree in representing it as a stupendous spectacle, such as in superstitious ages would not fail to have carried terror into every bosom. In tropical latitudes in the northern hemisphere, the tail appeared on the 3d of March, and in Van Diemen's Land, so early as the 1st, the comet having passed its perihelion on the 27th of February. Already on the 3d the head was so far disengaged from the immediate vicinity of the sun, as to appear for a short time above the horizon after sunset. On this day when viewed through a 46-inch achromatic telescope it presented a planetary disc, from which rays emerged in the direction of the tail. The tail was double, consisting of two principal lateral streamers, making a very small angle with each other, and divided by a comparatively dark line, of the estimated length of 25°, prolonged, however, on the north side by a divergent streamer, making an angle of 5° or 6° with the general direction of the axis, and traceable as far as 65° from the head. A similar though fainter lateral prolongation appeared on the south side. A fine drawing of it of this date by C. P. Smyth, Esq., of the Royal Observatory, C. G. H., represents it as highly symmetrical, and gives the idea of a vivid cone of light, with a dark axis, and nearly rectilinear sides, inclosed in a fainter cone, the sides of which curve slightly outwards. The light

of the nucleus at this period is compared to that of a star of the first or second magnitude; and on the 11th, of the third; from which time it degraded in light so rapidly, that on the 19th it was invisible to the naked eye, the tail all the while continuing brilliantly visible, though much more so at a distance from the nucleus, with which, indeed, its connexion was not then obvious to the unassisted sight—a singular feature in the history of this body. The tail, subsequent to the 3d, was, generally speaking, a single straight or slightly curved broad band of light, but on the 11th it is recorded by Mr. Clerihew, who observed it at Calcutta, to have shot forth a lateral tail nearly twice as long as the regular one, but fainter, and making an angle of about 18° with its direction on the southern side. The projection of this ray (which was not seen either before or after the day in question) to so enormous a length, (nearly 100°) in a single day conveys an impression of the intensity of the forces acting to produce such. a velocity of material transfer through space, such as no other natural phænomenon is capable of exciting. It is clear that if we have to deal here with matter, such as we conceive it, viz. possessing inertia-at all, it must be under the dominion of forces incomparably more energetic than gravitation.

(590.) There is abundant evidence of the comet in question having been seen in full daylight, and in the sun's immediate vicinity. It was so seen on the 28th of February, the day after its perihelion passage, by every person on board the H. E. I. C. S. Owen Glendower, then off the Cape, as a short, dagger-like object, close to the sun, a little before sunset. On the same day, at 3h 6m P. M., and consequently in full sunshine, the distance of the nucleus from the sun was actually measured with a sextant by Mr. Clarke, of Portland, United States, the distance, centre from centre, being then only 3° 50′ 43′′. He describes it in the following terms: "The nucleus, and also every part of the tail, were as well defined as the moon on a clear day. The nucleus and tail bore the same appear ance, and resembled a perfectly pure white cloud, without any variation, except a slight change near the head, just sufficient to distinguish the nucleus from the tail at that point." The denseness of the nucleus was so considerable, that Mr. Clarke had no doubt it might have been visible upon the sun's disc, had it passed between that and the observer. The length of the visible tail resulting from these measures was 59′, or not far from double the apparent diameter of the sun; and as we shall presently see that on the day in question the distance from the earth of the sun and comet must have been very nearly equal, this gives us about 1700000 miles for the linear dimensions of this, the densest portion of that ap