miles. The comet of 1807 was seen by Sir W. Herschel to have a well-defined planetary disc of a circular form. Sir William also saw a similar planetary disc in the head of the great comet of 1811; but, when that comet was examined by glasses of high power, the appearance of the stellar nucleus vanished. In 1819 a comet was observed, which exhibited phases similar to those of a crescent moon, during a part of the time that it was visible. The nucleus of a comet which appeared in 1825, as seen by Professor Santini, appeared to be composed of three bright spots. The great comet of 1843 had at one time a nucleus, small, but extremely bright; at another time it exhibited a well-defined planetary disc. The different appearances thus presented by the nuclei of different comets, and still more the differences observed in the nucleus of the same comet at different times, seem plainly to indicate that the nuclei of comets are not solid bodies, and that they consist of nebulous or gaseous matter considerably or highly condensed. Some comets are exceedingly brilliant; so much so, that they have been distinctly visible during the day, and while the sun was shining, and have also cast a shadow at night. This was the case with the comet mentioned by Diodorus Siculus, and also with the one which appeared at the birth of Mithridates, B.C. 134, to which we have already referred. One of the comets of 1618 was distinctly seen in full day light; and an instance occurred, in July, 418, of a comet that had not been observed before, being detected as a bright body in the neighborhood of the sun during an eclipse. The comet of 1744 was observed with the telescope at noon day, and many persons could distinctly see it with the naked eye for a considerable period after sunrise. Two comets of extraordinary brightness appeared in 1402; they were seen day and night, and caused great alarm. ་ ་ Usually the head in comets exhibits a pale, livid, white light; sometimes it is fiery red; sometimes a dull red, inclining to yellow; and occasionally it presents a greenish, and sometimes a bluish tinge. The comet of 1811 had a disc of a pale ruddy color; the surrounding nebulosity was greenish, or bluish green. The nucleus of the comet of 1843 was of a golden hue, of the color of Venus, or reddish, according to different observers. The comet of 1652 was of a pale livid color. The Chinese describe the comet of 1577 as of a bluish color, with a white vapor. But it has been chiefly the tails of comets that have attracted the attention of mankind, and filled them with astonishment, and often, in the days of ignorance and superstition, with terror. The generality of comets visible to the naked eye are tailed comets. The tail is usually developed as the comet 'approaches the sun; and just after it has passed that body, it for the most part appears to attain its greatest dimensions. In some comets, the tail is an elongated train of light, becoming fainter towards the extremity; in others, it is rounded off, bushy, or fan-shaped. Sometimes the elongated tail seems to be split at the end, so as to present the appearance of two or more tails. The tail generally projects from the head of the comet in a direction away from the sun, so as to be a continuation of the line drawn from the sun to the head of the comet. This, however, is not always the case: for in the comets of 1577 and 1680 the tail deviated from the line joining the sun and the comet--that of the former 21°, and that of the latter 5°. Sometimes, when comets have two tails, the one is in the usual direction, and the other is towards the sun. comets of 1824 and 1845 were of this kind; that of 1824 had two tails, making with each other an angle which varied from 138° to 170°; and the two tails of that of 1845 made with one another an angle of about 140°. The tail of the same comet has been observed to vary greatly in appearance, and that sometimes in the course of a single night. The Occasionally a vibratory or coruscating motion, something resembling certain of the motions of the aurora borealis, has been noticed in the tails of some comets. The Chinese, to whom we are indebted for a large number of careful observations of early comets, were the first to notice these vibrations in the tail of a comet which appeared A. D. 615, and they have often been observed since. Longomontanus described the tail of the comet of 1618 as having "an enormous vibration;" another observer says that it appeared as if agitated by the wind. In the great comet of 1843, the pulsations of the tail were distinctly visible. Some comets have tails not straight, but curved, presenting to the eye of wondering and alarmed nations the appearance of an immense flaming sword hung out in the heav ens. Such a phenomenon was held to portend bloody wars. The apparent and real length of the tail are generally different. If the eye of the observer be in a line drawn through the tail lengthwise the tail will seem to make a part of the head, or, in other words, no tail will be distinguishable; if the eye be a little out of the line, the tail will appear short; and it is only when the eye is in a line at right angles, or nearly so, to the tail, that the whole length of it will be seen. In some comets, the apparent length of the tail has been enormous, stretching over 40°, 50°, 60°, and 100° of an arc of the heavens. A comet, observed in the southern hemisphere in 1689, had a tail that extended over 60°, and was two hours and a-half in rising. And we are told of the great comet which appeared in 1264, that, when its head was just above the eastern horizon, the tail stretched away westward to beyond the midheavens. The nearer that the comet is to the eye of the observer favorably situated, the greater will be the apparent length of the tail. But the real length of the train of light that accompanies these wonderful bodies, is even more confounding than its apparent length. The tail of the celebrated comet of 1680 was considerably more than 100,000,000 of miles long; that of the comet of 1744 was 19,000,000 of miles; that of the comet of 1769 about 40,000,000 of miles. The great comet of 1811 had a tail of upwards of 100,000,000 of miles in length; and the second comet of that year was accompanied by a tail 130,000,000 of miles in length; whilst the comet of March, 1843, had a train of light stretching away from the nucleus, or head, to the astonishing distance of 200,000,000 of miles. There is something in such enormous measures of distance which the mind can hardly grasp, that swells out our conceptions of the wisdom and power of Him who created and rules these glorious wanderers. And in connection with that wisdom and power, what thoughts should we have of the first sleep of the Infant in the manger of the stable at Bethlehem, of the agony in the garden, of the ignominy on the cross, of the humiliation and helplessness in the tomb which a stranger's kindness supplied! In some comets there is a rim or border of light on the head next the sun, and passing round on each side to form the commencement of the tail. That border is called the envelope. Of the manner in which the envelope and tail are formed, astronomers can as yet do little more than conjecture. But it is supposed that the nucleus, or more condensed part of the head, being acted upon by the sun, throws off, or outwards atmosphere, which is straightway driven back by some repelling power, and, passing along the sides of the comet, forms the tail. If this conjecture be correct, then the repelling power, whether coming from the sun, or whencesoever it comes, must be prodigiously great, for the tail generally retains a rectilineal direction away from the sun, whether the comet be sweeping down in space towards that luminary, or be whirling with lightning speed round him in its perihelion, or nearest distance to him, or be receding away into space after it has passed him. There is considerable difficulty in finding the real magnitude of comets, from its being impossible to determine precisely where the nebulous matter of which the head is composed terminates, inasmuch as it appears to thin off from the centre, and gradually to shade away into darkness. Besides, the nebulous matter evidently contracts and expands, so that the same comet has different dimensions at different periods. Still, the measurement of a number of comets ha been accomplished, we believe, with considerable accuracy. But, besides the contractions and expansions which take place in the heads of comets from causes altogether unknown to us, there are other contractions and expansions which some at least of these bodies undergo with considerable regularity, and which apparently depend on their approach to and departure from the sun. In approaching the sun they are observed to contract, in receding from the sun they are observed to expand. This is certainly contrary to what we would expect, and it is one of the many intimations which we have that the physical constitution of comets-the matter of which they are composed-is something of which we on earth have no experience, and can form no conception. All comets, most probably, move in an ellipse, often of very high eccentricity, although it may be that some of them, from the very great velocity with which they pass their perihelion, move in a parabola, or in a hyperbola. Some comets have a direct motion in the order of the signs of the zodiac-i. e., in the same direction as the earth and the planets; others move in a contrary or retrograde direction. Of the 206 comets whose orbits have been calculated down to the end of 1852, 105 have a direct, and 101 have a retrograde motion. The paths of comets are not confined to the ecliptic, as those of the planets are, but comets are seen in all parts of the zodiacal regions. They come darting Of the number of comets we can have no The discovery that comets move in an ellipse, having the sun in one of the foci, and that they will, therefore, periodically visit the neighborhood of the sun, was one of surpassing interest, and the determination of the periods of their return is one of the greatest triumphs of human intellect. Let us briefly advert to that discovery. fer friend Dr. Halley. "A ion," says Mr. Mitchell ley had searched all | records for the purpose torical details touching aspects of comets from down to his own time. of the comet of 1682, tion with great care, pains computed the elen found it moving in a pla to the ecliptic, and in a elongation. In its aph the sun to the enormou 000,000 of miles. He nature of its orbit was su belief that the comet wo intervals of about seve recurring to his historic found it possible to tra tainty several hundred y bility even to the time ridates, one hundred an Christ." From his computation believed to be the histor had of its former appear predicted its return in believed that before th rived he would be in his thirst for fame which is we call it ?-of noble m triotic jealousy for the country, he expressed event of the comet's ret At an early period it was thought that comets moved in straight lines. When this was found to be an error, and they were observed to move in a curve, that curve was supposed to be a parabola, or a hyperbola. And indeed the parabola and hyperbola difvery little from the ellipse in the small part of a comet's orbit, in which it can be observed. For it is only for a comparatively short space before a comet reaches its perihelion, and after it passes it, that it is within the reach of the astronomer's eye or instruments. But it was by-and-by found that the observations made on the orbits of differ-membered that its perio ent comets would by no means agree with covered by an Englishm the supposition that they were moving in a parabola, and plainly indicated that their orbit was an ellipse. The orbits of a number of such comets have been calculated, and the result has been the determination of the periodical times of several comets; and these calculated periodic times have been verified by the return of the comets as predicted. The most celebrated are the comets known by the name of Halley's Encke's, and Biela's comets. A brief notice of these may not be devoid of interest, and may assist us in obtaining a more distinct notion of the cometary world. Guided by the light which his great theory of gravitation threw on a variety of astronomical observations, Sir Isaac Newton came to the conclusion that the comets, like the planets, revolve round the sun. If this had before been suspected, it had not in any way been proved. To verify it, if really true, became an object of deep interest to Newton, and his As the period of the cated by Halley approa of the elements of its with great care and la guidance of the results quarters were on the ou ed mighty traveller. A Dresden, of the name of on the 25th of Decem winter and spring of 1 numerous observers, and helion on the 12th of before the calculated tim error in these enormo affecting the passage of an orbit of such prodigio in great part to the wan accurate knowledge of planets Jupiter and Sat ing forces considerably the comet. Halley's comet was ag was led to the astounding and singular conclusion, that the orbit of the comet is gradually diminishing, and that ere long it must fall into the sun. Encke's comet presents the appearance of a round nebulous body, with a bright nuclear condensation, and it is destitute of a tail. It can be seen, when most favorably situated, by the naked eye, as a star of the fifth or sixth magnitude. un, and within the sphere of in 1835. As that time computation of its orbit was care and labor by a number mathematicians, and every that might affect it was t. The result established science, and showed how ssfully the human intellect the great problem of the space. Rosenberger fixed ovember, 1835, as the time and the comet having been e on the 6th of August, advards the sun, and made its on the 15th of November, after the calculated time. I wonderful accuracy, in and computing the period zedes into space the amaz70,300,000 miles, which in passes within the range of g forces, and which travels eed so gradually and so It bears magnificent testi- and grasp of the intellect none be proud because of ceived. The thought that vastly, boundlessly more hould keep the wisest, and 1 most vigorous-minded, the noble achievements of stify to man, that he is g better than grovelling lities of the world; for ven than the study of the he universe, and the laws governed; that he will id ultimate occupation in he Creator and Lawgiver nest and only happiness in at Creator and Lawgiver and redemption-a coveChrist-the Saviour and ortal spirit. e. In 1826, the periodicity of the fine comet called Biela's comet was discovered. This comet presents the appearance of a small round nebulous body, with a feeble condensation towards the centre, and without any tail. Its periodical time has been ascertained to be about 6 years, or more accurately 6.617 years. We find in Mr. Hind's work on Comets an interesting account of a very remarkable change observed to take place in this comet during its visit to the region of our system, in the end of 1848 and beginning of 1849. That change consisted in an actual separation into two distinct nebulosities, which travelled in company for more than three months. The apparent distance between them was, at first, little more than two minutes, but subsequently it increased to about fourteen minutes. We can not even mention the other comets whose periodicity has been recently detected. In some it ranges from five to eight years; in others, if we may credit the calculations, it ranges from 3,000 to upwards of 100,000 years! into the right tract by nd prediction of Halley, tion of that prediction, ointing out other comets In 1819, Professor etected the periodicity of w goes by his name, and a period of about three Many opportunities, eriod of this comet, have ried and accurate obserlating its elements with ncke himself bestowed . In connection with these ascertained periodic times, and these bewildering periods and distances, the question suggests itself, Do all comets move in an elliptical orbitfrom some far turning-point in space directing their course towards the sun with motion gradually accelerated, till, on their burning path, they rush past and round him, and recede away a journey of many, many years, to the immensely distant point whence they must begin again their journey towards him? --or do these travellers in space pass away into the depths of the universe till they come within the sphere of the attraction of some other sun as glorious and powerful, or more so, than ours, round which they sweep with lightning speed, and fly off to seek yet another, and from that other yet another, of the great central suns of God's stupendous universe, forming thus a connecting link between the systems of which that universe is composed ? Comets may have their motion accelerated or retarded, and even their orbits changed, planets or other bodies which they may approach in space. Halley's comet in 1835 had its perihelion passage accelerated by the Earth 15 days, by Venus about 5 days, and by Mercury and Mars together about one day. The perihelion passage of Biela's comet in 1832 was shortened, or its motion accelerated 10.023 days by the united action of the Earth, Jupiter, and Saturn. And Encke's comet, according to the calculations of that philosopher, underwent a retardation of more than nine days, from the influence of the planet Jupiter. It has been thought, also, that comets undergo retardation by the resistance of the medium through which they move, though by many the doctrine of a resisting medium is strongly doubted or denied. But the most extraordinary change produced by planetary perturbations occurs in regard to the comet of 1770, usually called Lexell's comet. As far as could be judged, that comet had never been observed before. Yet Lexell proved, that in the orbit in which it was then moving, it had a periodic time of about five years and seven months. In his investigations, he found that this comet had approached very near to the planet Jupiter in May, 1767; so near, that the influence of Jupiter on the comet must have drawn it aside from the orbit in which it was moving into an entirely new orbit; and this had brought it so near us as to enable us to see it for the first time. As it had such a short period, its return was watched for in 1776, but it escaped observation, probably owing to its position in regard to the sun. It has never been seen again, but its disappearance has been accounted for. Lexell found that the comet, in its aphelion passage, about August, 1779, was so near Jupiter, that the mass of that planet exercised a power on the comet 225 times greater than that of the sun upon it, and must, therefore, have again drawn it quite out of the orbit in which it was moving in 1770. And subsequent investigations have shown that the action of Jupiter would so affect this comet, that when it escaped from the sphere of the activity of the planet in October, 1779, it was moving in an ellipse, with a revolution of rather more than 16 years, and a perihelion of 3 times the semi-diameter of the earth's orbit; and at such a distance there would be no hope of our ever seeing it again. The perturbing force of the planet Jupiter thus seems to have brought this planet within our view by its influence on it in 1767, and again to have withdrawn it from our view by its. influence on it in in 1779. Of all the comets that ed, that of 1729 had the distance-namely, 4.04; comet of 1843 the least, the great comet of 1680 distance of 0.0062. Sir culated that, from its n the comet of 1680 mus heat 2,000 times greater t iron. The heat communic however, would depend proximity to the sun, but rials of which it was com But we probably conce as much interested in the comets approach our ea ness to which they approa indeed a very slight know tions of the earth and of show, not only that they m near to one another, but e actually come in contact. several comets have crosse ecliptic almost in our ear these, the comets of 1680 There was great alarm in garding the comet of 18 discovered and announced of October that year it w within the earth's orbit; a unacquainted with the su posed that a collision woul M. Arago showed, that would actually cross the earth would be at the time of miles from the point a was crossing. This same only a tenth part of that d 000 of miles from the ea approach made by any co is believed to have been m of 1770, which came withi of us. No sensible effect has any part of the solar syste ous comets that have swep |