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 center from center 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 appearance, 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 appendage, making no allowance for the foreshortening, which at that time was very considerable.

(591.) The elements of this comet are among the most remarkable of any recorded. They have been calculated by several eminent astronomers, among whose results we shall specify only those which agree best; the earlier attempts to compute its path having been rendered uncertain by the difficulty attending exact observations of it in the first part of its visible career. The following are those which seem

(592.) What renders these elements so remarkable is the smallness of the perihelion distance. Of all comets which have been recorded this has made the nearest approach to the sun. The sun's radius being the sine of his apparent semidiameter (16' 1" 5) to a radius equal to the earth's mean distance 1, is represented on that scale by 0.00466, which falls short of 0.00534, the perihelion distance found by taking a mean of all the foregoing results, by only 0-00067, or about one seventh of its whole magnitude. The comet, therefore, approached the luminous surface of the sun within about a seventh part of the sun's radius! It is worth while to consider what is implied in such a fact. In the first place, the intensity both of the light and radiant heat of the sun at different distances from that luminary increase proportionally to the spherical area of the portion of the visible hemisphere covered by the sun's disc. This disc, in the case of the earth, at its mean distance has an angular diameter of 32' 3". At our comet in perihelio the apparent angular diameter of the sun was no less than 121° 32'. The ratio of the spherical surfaces thus occupied (as appears from spherical geometry) is that of the squares of the sines of the fourth parts of these angles to each other, or that of 1: 47042. And in this proportion are to each other the amounts of light and heat thrown by the sun on an equal area of exposed surface on our earth and at the comet in equal instants of time. Let any one imagine the effect of so fierce a glare as that of 47000 suns such as we experience the warmth of, on the materials of which the earth's surface is composed. To form some practical idea of it we may compare it with what is recorded of Parker's great lens, whose diameter was 32 inches and focal length six feet eight inches. The effect of this, supposing all the light and heat transmitted, and the focal concentration perfect, (both conditions very imperfectly satisfied,) would be to enlarge the sun's effective angular diameter to 23° 26', which, compared on the same principle with a sun of 32′ in diameter, would give a multiplier of only 1915 instead of 47000. The heat to which the comet was subjected therefore surpassed that in the focus of the lens in question, on the lowest calculation, in

R B

the proportion of 24 to 1. Yet that lens melted carnelian, agate, and rock crystal!

(593.) To this extremity of heat however the comet was exposed but for a short time. Its actual velocity in perihelio was no less than 366 miles per second, and the whole of that segment of its orbit above (i.e. north of) the plane of the ecliptic, and in which, as will appear from a consideration of the elements, the perihelion was situated, was described in little more than two hours; such being the whole duration of the time from the ascending to the descending node, or in which the comet had north latitude. Arrived at the descending node, its distance from the sun would be already doubled, and the radiation reduced to one fourth of its maximum amount. The comet of 1680, whose perihelion distance was 0.0062, and which therefore approached the sun's surface within one third part of his radius (more than double the distance of the comet now in question) was computed by Newton to have been subjected to an intensity of heat 2000 times that of red-hot iron,-a term of comparison indeed of a very vague description, and which modern thermotics do not recognize as affording a legitimate measure of radiant heat.

(594.) Although some of the observations of this comet were vague and inaccurate, yet there seem good grounds for believing that its whole course cannot be reconciled with a parabolic orbit, and that it really describes an ellipse. Previous to any calculation, it was remarked that in the year 1668 the tail of an immense comet was seen in Lisbon, at Bologna, in Brazil, and elsewhere, occupying nearly the same situation among the stars, and at the same season of the year, viz. on the 5th of March and the following days. Its brightness was such that its reflected trace was easily

A transit of this comet over the sun's disc must probably have taken place shortly after its passage through its descending node. It is greatly to be regretted that so interesting a phænomenon should have passed unobserved. Whether it be possible that some offset of its tail, darted off so late as the 7th of March, when the comet was already far south of the ecliptic, should have crossed that plane and been seen near the Pleiades, may be doubted. Certain it is, that on the evening of that day, a decidedly cometic ray was seen in the immediate neighbourhood of those stars by Mr. Nasmyth. (Ast. Soc. Notices, vol. v. p. 270.)

distinguished on the sea. The head, when it at length came in sight, was comparatively faint and scarce discernible. No precise observations were made of this comet, but the singular coincidence of situation, season of the year, and physical resemblance, excited a strong suspicion of the identity of the two bodies, implying a period of 175 years within a day or two more or less. This suspicion has been converted almost into a certainty by a careful examination of what is recorded of the older comet. Locating on a celestial chart the situation of the head, concluded from the direction and appearance of the tail, when only that was seen, and its visible place, when mentioned, according to the descriptions given, it has been found practicable to derive a rough orbit from the course thus laid down: and this agrees in all its features so well with that of the modern comet as nearly to remove all doubt on the subject. Comets, moreover, are recorded to have been seen in A. D. 268, 442-3, 791, 968, 1143, 1317, 1494, which may have been returns of this, since the period above-mentioned would bring round its appearance to the years 268, 443, 618, 793, 968, 1143, 1318, and 1493, and a certain latitude must always be allowed for unknown perturbations.

(595.) But this is not the only comet on record whose identity with the comet of '43 has been maintained. In 1689 a comet bearing a considerable resemblance to it was observed from the 8th to the 23d of December, and from the few and rudely observed places recorded, its elements had been calculated by Pingré, one of the most diligent enquirers into this part of astronomy." From these it appears that the perihelion distance of that comet was very remarkably small, and a sufficient though indeed rough coincidence in the places of the perihelion and node tended to corroborate the suspicion. But the inclination (69°) assigned to it by Pingré appeared conclusive against it. On recomputing the elements, however, from his data, Professor Pierce has assigned to that comet an inclination widely differing from Pingré's, viz.

Author of the "Cométographie," a work indispensable to all who would study this interesting department of the science.