this, the Latin name is coma, and in consequence, these bodies are called comets, to distinguish them from the other luminaries, which, whether near or remote, apparently fixed or movable, have not this train-like accompaniment. Comets are one of the three classes into which astronomers divide those celestial bodies that adorn the sky during the night. The stars, which retain their relative positions with regard to each other, and are at so great distances from the earth, that no means or instruments hitherto invented can measure them, are one class, and a class not apparently connected with our sun, or deriving light or heat from that luminary. The planets, which change their relative positions among the stars, and of which our earth is one, form the second class. They are solid bodies, and not luminous in themselves, but shine merely by reflecting the light of the sun. The masses of the planets, their magnitudes, and their motions, have been all determined with the greatest accuracy; and the place that any one of them will occupy at any proposed point of time, can be calculated with the greatest ease, by any one acquainted with practical astronomy. The planets are, in their motions, governed by one uniform law. In the early ages, the planets were held to have certain influences upon individuals and nations. The comets, which are more singular in their form, and more varied in the times of their appearance, were still better adapted for superstitious purposes; and, accordingly, we find that their visits have been attempted to be connected with the great, more especially the calamitous, events of nations. The appearance of a comet is, however, no more a prodigy, and has no more influence upon the fate of men or of nations, than the appearance of the moon, or of a deciduous leaf upon a tree in spring. They are so distant, and either their motions are so rapid, or their substance is so rare, that none of them have been found to have any material action upon such of the planets as they have come near, although the planets have had a considerable influence upon them. What the comets are, or what purposes they serve in the economy of creation, we do not know. As far as observation has gone, they are subject to the same laws as the planets, revolving about the sun in orbits or paths, with this difference, that their orbits are much more eccentric, or differ much more from circles, than the orbits of the planets; and thus, while they approach much nearer to the

sun at one time of their revolutions, they recede correspondingly farther from it at another. The time since men had rational opinions on the subject has, however, been too short for verifying, by observation, the theory as applicable to the whole, or even the greater number of these bodies that have, from time to time, made their appearance. Tycho Brahe was the first who expressed a decidedly rational opinion on the subject of comets. Finding, by careful observation, that the comet of 1577 had no diurnal parallax, which he could detect,—that is, that its place, when viewed from the surface of the earth, was not different from what it would have been if viewed from the centre,-he properly concluded that its distance from the earth must be greater than that of the moon, in which this parallax was apparent to him. This was one step; and it was an important one: it removed comets to such a distance from the earth, that their use could not well be supposed to be for it, or their influence upon it very great. The general law of the motion of bodies in free space, as well as his own particular observations on the comet of 1680, led Newton to conclude that the orbits of the comets must, like those of the planets, be ellipses, having the sun in one focus, but far more eccentric, and having their aphelions, or greatest distances from the sun, far remote in the regions of space. The idea thus thrown out by Newton was taken up by Dr. Halley, who collated the observations which had been made of all the twenty-four comets, of which notice had been taken previous to 1680. The results were abundantly curious; with but few exceptions, they had passed within less than the earth's shortest distance from the sun; some of them within less than one third of it; and the average about one half. Out of the number, too, nearly two thirds had had their motions retrograde, or moved in the opposite way to the planets. While Halley was engaged on these comparisons and deductions, the comet of 1682 made its appearance, and he set about observing it with great care, in order to determine the elements of its orbit. Having done so, he found that there was a wonderful resemblance between it and three other comets that he found recorded-the comets of 1456, of 1531, and of 1607. The times of the appearance of these comets had been at very nearly regular intervals,—at least, the differences had been only fractional parts of a year, the average period being between 75 and 76 years. Their

distances from the sun, when in perihelion, or nearest to that luminary, had been also nearly the same, being nearly six tenths of that of the earth, and not varying more than one sixtieth from each other. The inclination of their orbits to that of the earth had also been nearly the same, between 17° and 18°; and their motions had all been retrograde. Putting them together, Dr. Halley concluded, that the comets of 1456, 1531, 1607, and 1682, were re-appearances of one and the same comet, which revolved in an elliptic orbit round the sun, performing its circuit in a period varying from a little more than 76 years to a little less than 75; or having, as far as the observation had been carried, a variation of about 15 months in the absolute duration of its year, measured according to that of the earth. For this variation in the time of its revolution, Dr. Halley accounted upon the supposition that the form of its orbit had been altered by the attraction of the remote planets, Jupiter and Saturn, as it passed near to them; and thence he concluded, that the period of its next appearance would be lengthened, but that it would certainly re-appear in 1757 or 1758. Its doing so was, of course, the fact that was to be decisive of the orbits of comets, and that they were regular and permanent bodies, obeying the general laws of matter. Halley did not live to see the verification of his prediction; he died in the year 1742, at the advanced age of 84. Soon after his death, Clairault, D'Alembert and Euler, three of the most eminent mathematicians of Europe, set about the solution of what is

called "the problem of the three bodies;" that is, to determine the paths described by three bodies, projected from three given points, in given directions, and with given velocities, their gravitating forces being directly as their quantities of matter, and inversely as the squares of their distances. The object of this problem is to find the disturbing effects that the bodies composing the solar system have upon each other; and it applies to comets, when within the limits of planetary action, as well as to the planets themselves. After some errors, into which all the three had been led, and which gave a result that seemed to overturn the whole doctrine of gravitation, Clairault succeeded in obtaining an approximate solution, which agreed with and confirmed that theory. Having done so, he applied it to the calculation of the disturbing influence of Jupiter and Saturn, which Halley had predicted would retard the comet of 1682, in its re-appear

ance about 1758. The results of Clairault's calculations were, that the comet would be retarded 100 days by the attraction of Saturn, and 518 days by that of Jupiter, so that it would not come to the perihelion, or point of its orbit nearest the sun, till the 13th of April, 1759. Clairault, however, fixed certain limits, within which his calculations might probably be erroneous. It was eventually found that the difference between calculation and observation was less than that which he assigned. Clairault read his investigations to the academy of sciences in November, 1758; and, in little more than a month afterwards, the comet made its appearance; and it reached its perihelion on the 13th of March, in the following year, being 30 days earlier than he had calculated. Subsequent calculations enabled him to reduce the error to 19 days; and, though the calculations of the disturbing forces were only approximations, enough had been done to prove the return, and determine the orbit of one comet, and give every reason for concluding that all comets, being bodies of the same class, are subject to the same general laws as the planets, and only vary from each other in the proportion and magnitude of their orbits. There was one further confirmation. Clairault had calculated that the node of the comet's orbit, or the point in which it cut the plane of the orbit of the earth, would advance 2° 33′ in absolute space, or 1° 29′ more than the equinoctial points, the precession of which, in the time of the comet's revolution, was 1° 4' ; and observation gave exactly the same result; so that the only difficulty that remained in the doctrine of comets was in the estimation of the disturbances to which they are exposed from the other bodies of the system, more especially in the parts of their orbits most remote from the sun, where their motions are comparatively slow. Along with the period of this comet, and its perihelion distance, the magnitude and form of its path were known. Estimating the mean distance of the earth from the sun at 95,000,000 miles, the mean distance of the comet is 1,705,250,000 miles; its greatest_distance from the sun, 3,355,400,000; its least distance, 55,100,000; and the transverse, or largest diameter of its orbit, 3,410,500,000. Therefore, though its aphelion distance be great, its mean distance is less than that of Herschel; and, great as is the aphelion distance, it is but a very trifling fraction less than one five thousandth part of that distance from the sun, nearer than which

the very nearest of the fixed stars cannot be situated; and, as the determination of their distance is negative and not positive, -a distance within which they cannot be, and not one at which they actually are,the nearest of them may be at twice or ten times that distance. The comet of 1759 is, therefore, a body belonging to the solar system, and quite without the attraction of any body which does not belong to that system; and, as this is determined of one comet, analogy points it out as being the case with them all.-Besides the comet of 1759, of which there have been four authenticated returns, and which may be expected again about 1833, there are two others, of which something like a return has been traced at long intervals. One of these passed its perihelion at about 8 o'clock on the morning of the 6th of July, 1264, reckoning mean time at Greenwich; and again, at a little past 8 o'clock, on the evening of the 21st of April, 1556. Thus its period is about 292 years, and it may be expected in 1848. The perihelion distance, however, of this comet, which was more than half that of the earth, in 1264, had diminished an eighth part by 1556; and, as this must have caused a great elongation of its orbit, and as, from the length of its period, it must go far into the regions of space, there is no knowing how both the time of its revolution, and the form and position of its orbit, may have been altered.-The other comet, in the elements of whose orbit there is a similarity, from which its identity might be with probability inferred, appeared in 1532, and again in 1661, having thus a period of about 129 years. The return of that comet should, therefore, have been about 1790. In that year, three comets made their appearance; but neither of them resembled the one of 1661. Two of them moved in the opposite direction; and the remaining one was more than twice the distance from the sun in its perihelion, and its orbit at nearly double the angle with that of the earth.-The comet denominated Encke's comet, which has engrossed the public mind generally, and the scientific world in particular, has justly claimed and received the careful attention of astronomers, since its appearance in 1818 engaged professor Encke to consider the elements of its orbit. He was enabled to identify it with a comet described by Messrs. Mechain and Messier in 1786, in the constellation Aquarius; also with a comet discovered in 1795, by Miss Herschel, in the constellation Cycnus; and with the comet in 1805. The

investigation of the diligent professor enabled him to foretell its re-appearance in 1822, and to state the probability of its not being observable in our climate. This anticipation was realized by its discovery in New South Wales, in the observatory of the governor, sir Thomas Brisbane, June 2, 1822; and the accurate observations of Mr. Rumker, who discovered it, afforded Encke the means of reconsidering the true elements of its orbit, and with additional confidence computing its return for 1825. This occurred as was expected. The fresh data afforded by that return were carefully collated by the professor. It was observed again on October 30, 1828. This comet affords particular interest to the mind of the astronomer, though it does not offer a splendid object to his eye. Its orbit is an ellipse of comparatively small dimensions, wholly within the orbit of Jupiter: its period is about three years and three tenths-a much shorter period than has hitherto appeared to comprise the revolution of any other comet, with the exception of one seen in 1770, which did not satisfy, as far as observation has been able to show, the prediction of the period of five years and a half, which was attributed to it. In the opinion of Encke and other astronomers, this comet may afford an opportunity of proving that the heavens oppose a resisting medium to the motion of bodies. The subject has been discussed in the Transactions of the astronomical society of London, by the able mathematician Massotti; and that gentleman offers reasons for con sidering comets capable of affording a demonstration of a resisting medium in the heavens, though planets may give no indication of it.—Another comet, which encourages the anticipation of much astronomical gratification, is one which Biela discovered, Feb. 27, 1826, and which was afterwards seen by Gambart and others. It seems to possess claims to the attention of astronomers similar to that of Encke, it being conceived to revolve about the sun in about six years and seven tenths, and to be the same as the comet which appeared in 1772, and that which appeared in 1806. Encke's comet was in its perihelion, by computation, Jan. 10, 1829.— The comet of 1770, to which allusion has been made, would lead us to conclude that we are still ignorant of many of the causes by which the form of the orbits of comets, and the times of their revolution and return, may be disturbed. That comet moved almost in the plane of the earth's orbit, having an inclination of only about

a degree and a half; it had been observed with great care; and the result of the observations was, that it should return about every five years and a half. Instead of going out of the system, as may be presumed to be the case with those comets that have long periods and eccentric orbits, its greatest distance could not be much greater than that of Jupiter, while its mean distance from the sun was not much more than three times the perihelion distance of the earth. No comet, at all answering to that one, has, however, been again discovered; and therefore the conclusion is, that there are, within the system itself, causes which can completely alter the motions of these bodies; but what those causes are, other than the attraction of the planets, has not yet been ascertained. One remarkable difference between the comets and the planets is in the angles which their orbits make with that of the earth. Leaving out the small planets that have recently been discovered, all the others are contained within a zone extending only 7° on each side of the earth's orbit; and, with the exception of Mercury (by far the smallest of the old planets), they are within half that space. But the orbits of the comets are at all possible angles; and the number increases with the angle, so that they approximate to an equal distribution, in all directions, round the sun as a centre. The numbers that have been observed are as follows:-Under 10° of inclination, 8; under 20°, 19; under 30°, 26; under 40°, 37; under 50°, 47; under 60°, 63; under 70°, 79; under 80°, 88; and under 90°, about 100. Thus by far the greater number of the comets have their paths out of the direction of those of the planets; and hence, though they be bodies of such consistency as that their collision with the planets would produce serious consequences, there is but little chance that such collision can take place. The comets that have been observed have made their passages through very different parts of the solar system: 24 have passed within the orbit of Mercury; 47 within that of Venus; 58 within that of the Earth; 73 within that of Mars; and the whole within that of Jupiter. Of a hundred, or thereabouts, mentioned by Lalande, about one half have moved from west to east, in the same direction as the planets, and the other half in the opposite direction. The direct and retrograde ones do not appear to follow each other according to any law that has been discovered. From 1299 to 1532, all that are mentioned were retrograde; and five that were ob

served from 1771 to 1780 were all direct. -Being quite ignorant both of the size of the comets, and their quantities of matter, we can form no conclusion as to their effects, even upon the positions of the planets. Hitherto, their influence, if anything, has been very small; for, within the limits that must be allowed for error, even in the best tables that are calculated upon an approximation, the whole of the irregularities are explainable upon the hypothesis of planetary disturbance alone; and the system appears to have gone on just as if there had been no comets in it. That the comets are formed of matter of some sort or other we know, from the dense and opaque appearance of their nucleus, as well as from the action of the planets upon them; but, as their action upon the planets has not been great, or even perceptible, we are led to the conclusion that they are not bodies of the same density or magnitude as even the smallest and rarest of the planets. When a comet is viewed through a telescope of considerable power, there appears a dense nucleus in the centre of the luminous and apparently vaporous matter, of which the external parts are composed; and the opacity of this nucleus varies in different comets. On its first appearance, and again when it recedes, the luminous part of the comet is faint, and does not extend far from the nucleus; but, as it moves on towards the perihelion, the brightness increases, and the luminous matter lengthens into a train, which, in some cases, has extended across a fourth of the entire circumference of the heavens. But, though the general fact of the increased brightness of comets, and length of their tails, with their approach to the sun, and the consequent inclination of their motion, has been established, the observations have not been uniform or minute enough for proving what proportion the increase of brightness bears to the increase of the velocity, and the diminution of the distance from the sun. No doubt, all the comets of which there are well-authenticated accounts, of great brightness and length of tail, have passed near the sun in their perihelion. Thus the comet of 1769, which was not a fifth of the earth's perihelion distance from the sun, had a tail of 60° in length, as seen at Paris; while that of 1759, which was more than half the earth's perihelion distance distant, had a train of only 2° or 3°. The length of the tail varies, however, not only with the time at which it is observed, but with the place of observation-a difference prob

ably depending on the difference of clearness and purity in the air. The tail of the comet of 1759 was 25° long, as measured at Montpellier, in the south of France, and considerably more than that as measured at the Isle of Bourbon, in the Indian ocean. That of 1769 was 60° at Paris, 70° at Boulogne, 90° between Teneriffe and Cadiz, and 97° at Bourbon. Generally speaking, they appear to be brighter and larger when seen at sea than on land, and in the warmer regions than in those nearer the poles. When the superstitious fear of comets, as portending harm to the inhabitants of the earth, had vanished before the light of philosophy, that light was in some danger of giving rise to fear of another sort-fear of physical harm to the earth itself, by the collision of some comet that might cross its path. We have no evidence, however, that such a collision ever did happen, either with the earth or with any other planet; and we have not absolutely correct means of so calculating the place of a comet as to be able to say with certainty that, on a given day, during a given month, or even during a given year, it shall cross the orbit of a planet. The motion of the earth in its orbit is, in round numbers, more than a million and a half of miles in a day; and as Clairault, with all his care, did not come nearer the truth than 19 days, though the collision of a comet and the earth should be calculated from any known data, the earth might, in fact, be, at the time, far enough from the comet. Indeed, though the fact of the return of two comets be established, namely, Halley's and Encke's, and the return of every one, if not affected by physical causes that lie beyond the limits of our present knowledge, has been rendered exceedingly probable, yet we can observe them for so short a portion of their courses, and these seem so very apt to be altered, that we ought not to speak of them with anything like the certainty with which we speak of the planets. As far as we have been able to examine them, they appear to obey the same laws as the other distinct masses that make up the known part of the system of the universe. Beyond this we know nothing of their nature; and as for their effects, moral or physical, we need give ourselves no trouble about them, for there is not a trace of the existence of such effects upon any authentic record. Respecting the hypotheses relating to the structure of comets, and particularly to their tail, professor Fischer, of Berlin, has given valuable information in Bode's As

tronomisches Jahrbuch (Astronomical Yearbook), 1823, p. 90. See, also, the French edition of Schubert's Astronomy (Petersburg, 1822, vol. 2, p. 510). To learn their mathematical relations, see Nouvelles Méthodes pour la Détermination des Orbites des Comètes, by Legendre (Paris, 1806, 4to.); and Olbers' Neue Methode die Bahn eines Kometen aus eigner Beobachtung zu berechnen (Weimar, 1797). La Place's Théorie du Mouvement et de la Figure des Planètes et des Comètes has become rare; but Biot, in the Additions to the third book of his Astronomy, p. 185, extracts the part relating to the theory of comets entirely from it.

COMFORT, POINT. (See Point Comfort.) COMFORTABLE; a very expressive word among the English, and people of English descent. It is also found even in recent French publications, probably carried to Paris by the innumerable English who visit the capital of France. Every nation has not only certain words which cannot be rendered precisely by any terms in other languages, but also certain ideas growing out of its customs, wants, &c., which do not exist with other nations, and which are the real cause of this peculiar significance of particular words. Such a word is comfortable, which signifies more than a mere physical feeling of gratification. In fact, it has something of the same indefinable and untranslatable character with the word home-a word which expresses a vast deal of feeling, of a faithful and tender attachment. A comfortable home is an expression, of which it would be impossible to approach to a translation, in some other languages, for instance, in Italian; as an Italian finds his enjoyment in the open air in his lovely climate, and has little regard for the pleasures of home. Many circumstances may have coöperated to produce, among the English, their love of comfort, and the means for ensuring it which we find in their houses. In fact, the comforts of an English dwelling surpass every thing of the kind among other nations. We would confine our observation to the dwelling, because, as respects the whole manner of living, the degree of enjoyment is certainly much greater in France. It is always highly interesting to study those expressions by which a nation describes its habitual likings or dislikings, because they disclose, at once, the general disposition of the people. Such a one is comfortable. The German, in a pleasant state of mind, says he feels gemüthlich, or, of a person, er ist ein gemüthlicher Mensch. The Ameri