IN In the ages of ignorance and superstition, very wild and extraordinary opinions have been entertained regarding the nature of comets, and the purposes for which they appear in the heavens. The ancient Chaldeans, and Pythagoras, with some of his followers, believed them to be of the nature of the planets. Aristotle supposed them to be meteors in the upper heavens, generated when they appeared, and vanishing out of sight by being destroyed. In later times, Kepler, noted for the wildness of his imagination, as well as for the strength of his genius, fancied comets to be monstrous and uncommon animals generated in the celestial spaces. Bodin, a French writer of the sixteenth century, maintained that comets "are spirits, or geni different from men, yet mortal, which, having lived on the earth innumerable ages, and being at length arrived on the confines of death, celebrate their last triumph, or are recalled to the firmament, like shining stars." An opinion something like this, extravagant though it be, prevailed in remote ages; for we find that the people generally believed that a great comet which appeared immediately after the assassination of Julius Cæsar, was the soul of that celebrated man. The conjecture for all was yet only conjecture of Bernoulli, an Italian philosopher, was nearer the truth, and yet far from it: He thought that comets were the satellites of some far-off planet, invisible on account of its distance, which wander betimes within the sphere of our vision.

Sir Isaac Newton believed that the orbit of the comet of 1680 would become gradually less, and that at last the comet would fall into the sun; thus, in his opinion, fulfilling one of the uses of comets, namely, to furnish fuel for the sun, which was supposed to be necessary, to prevent the sun from being wasted away by giving out so much light. Mr. Brydone seemed to hold the same opinion with regard to almost all the comets without tails, which he alleges are seen approaching the sun, but of whose return from the sun no evidence satisfactory to him was ever given. Such were the opinions of philosophers regarding comets, before the accurate observations, superior instruments, and profound calculation of modern science made us better acquainted with them.

Whilst the learned formed conjectures regarding comets that were wide of the truth, ignorance and superstition invested them with a mysterious and awful significance. They were regarded as omens or forerunners of war, or pestilence, or famine; of the birth of mortals who were to be great for good or evil among their fellows; or of those calamities which too often followed the death of the great conquerors of antiquity. Princes, popes, peoples, were perplexed and alarmed by the appearance of these strange wanderers in the heavens, as they glared down in their fiery splendor, or gave forth their pale, livid, watery light, the very emblem, as men thought, of plague and famine; or as their

Speaking of Mithridates the Great, king of Pontus, the historian Justin says, "The celestial signs had foretold the future greatness of this man. For, in that year in which he was born, and in that in which he first began to reign, the star of a comet, through each time, so shone for seventy days, that all the heaven seemed to be in a blaze. For it had taken up the fourth part of the heaven by the greatness of it, and had overcome the brightness of the sun by the splendor of it; and it occupied the space of four hours in rising and setting." A comet which appear ed in 837 so alarmed the reigning king of France, Louis I., that he ordered a number of churches and monasteries to be built throughout his dominions, in the hope that thereby he would appease the wrath of Heaven, of which he supposed the comet to be an indication. And in 1446, the pope of that day, Calixtus III., hard pressed by the successes of the Turks under Mahomet II., was greatly alarmed by the appearance of a remarkable comet, and he appointed a form of prayer to be used against its supposed baleful influence; and, for the same end, he ordered the bells in all the churches to be rung every day at noon. But, "Thus saith the Lord, be not dismayed at the signs of heaven, for the heathen are dismayed at them."

Though much has yet to be learned regarding comets, the observations and labors of modern science have led to a good deal of trustworthy knowledge of them, and have shown them to be very harmless, but very interesting celestial phenomena. In this paper we shall attempt to give a rapid sketch of what is known regarding comets, and especially of the results of recent investigations, drawing our materials chiefly from the work of Mr. Hind.

Some comets are distinctly, and even conspicuously visible to the naked eye; others (and these by far the greater number) are only to be seen by the aid of the telescope. In almost every instance in which they are conspicuously visible to the naked eye, comets consist of a head composed of a nucleus of light surrounded by a mass of nebulous matter, and of a tail or train of nebulous light stretching out from the head or nucleus. When we direct the eye or the instrument to the head of a comet, we see sometimes a star-like body in the centre, sometimes a disc-like appearance, and sometimes evident ly only the nebulous matter, in a state of

VOL. XXXIV.—NO. III.

French philosopher Arago was of opinion that some comets are solid bodies like the planets; and he rested that opinion very much on an alleged fact, that a comet has been observed as a round black spot on the body of the sun, in its transit across the disc of that luminary, like the planets Mercury and Venus in similar circumstances. There is the strongest reason to believe, however, that what have sometimes been taken for opaque or planetary nuclei, were nothing but the nebulous or gaseous matter of which comets are composed, in a very high state of condensation. It having been computed that the comet of 1827 would cross the sun's disc, the occurrence was looked forward to with much interest, as fitted to furnish the best information regarding the nuclei of comets. Unfortunately, the sun at the computed time was hidden by clouds in this country ; he was seen, however, by the French observers at Verviers and Marseilles; but no spot or cloud could be discovered on the sun's disc; and hence it was concluded that the comet had no solid or opaque part whatever. Besides, when comets have presented to the observer what seemed to be a solid planetary disc, it has been found that the appearance of that disc underwent changes of shape and character altogether inconsistent with the fact of it being a solid substance. And it is held to be doubtful whether a single instance can be produced of a comet with a planetary disc presenting the same appearance throughout the whole period that it was observed. Indeed, the changes exhibited by the central portion of the heads of cometsin other words, the different appearances presented by the disc, in those of them which are so furnished-a -are among the most puzzling of the phenomena connected with these bodies.

The nuclei of different comets present very different appearances, and even the nucleus of the same comet evidently undergoes, as we have just stated, great and surprising changes. A remarkable comet appeared A. D. 389, whose head seemed composed of several small stars. The nucleus of the comet of 1835-36, usually called Halley's comet, presented at one time the appearance of a fanshaped flame, proceeding from a bright point: at another time, it was like a red-hot coal of an oblong form; at another time, it was seen as a well-defined disc, with an apparent diameter of not less than 97,000 miles; and at another time, as a brilliant kernel of light, with a diameter varying from 250 to 1,000

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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 ap peared 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 ob; served 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. 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.

In some

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. The 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.

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 heavens. 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 toward the sun, a portion of the cometic

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 heavens-about the poles, as well as in