believe, that it represents Diana actively engaged in the chace, and throwing a javelin. She seems to be in the act of advancing rapidly, against the wind, the effects of which are very visible in the folds of the drapery. In this statue a line or ruck is observable above the breasts, resembling that on the knee of No. 5. Neither the attitude nor proportions of the original are to be admired; but the face is fine. Found in 1772, near La Storta. 15. A bust of Hadrian, larger than life.

COMETS

(To be continued.)

ON THE HISTORY AND THEORY OF COMETS. are stars which from time to time discover themselves in the heavens, remain visible for a greater or less space of time, and then disappear. While visible, they are observed to be either approaching the sun, or receding from it. Comets are also distinguishable by the figure of their bodies, and by their laws of motion.

Comets differ very greatly in their figure, not only from other stars, but from each other; so that the portraiture of almost each individual, at least as seen through the telescope, is peculiar to itself. The figures of some have been observed to be as well defined, and as round, as those of the planets; but in general a thin, luminous matter is seen surrounding and projecting from them, various in its form, but constituting that characteristic of these bodies from which they have derived, from the Greeks, the name of Comets, (Kounts,) and, among ourselves, the corresponding one of Blazing Stars. In some instances, no star has been visible, but the whole body of the comet has appeared as if composed of the luminous matter alone. It is said, that the Greek word xounts is derived from xoμáw; comatus sum, 'I am comated,' or, 'have rays', as if the coma were itself luminous, and did not emit light by reflection. The word coma' signifies, literally, a head of hair, but, figuratively, the branches and leaves of trees and plants, the flakes of fire that fall from torches, the sun-beams, and, in general, all beams and radiations. The figurative mode of expression, however, has been very generally employed in the descriptions given us of comets; thus Cicero calls them stella Cincinnata-stellas quasi nostri Cinciunatus vocant'-stars with heads of curly hair. By Pliny and other Latin writers, they are called stella Crinitæ,' an

Portraits of the comets of 1661, 1688, 1744, 1769, are preserved, and may be seen engraved in many astronomical works. Hevelius gives pictures of comets of various shapes, as they have been described by historians, a sword, a buckler, a tun, &c. These are drawn by fancy only, from the descriptions in words. He gives also, however, pictures of some comets, drawn and engraved by himself, from the views he had of them through a long and excellent telescope.

*14

appellation

By

appellation corresponding with the Kounts of the Greeks.* Óvid, a comet is called stella comans,' which implies, literally, the having much hair, but which is strictly synonymous with the name of Blazing Star.'

The form, real and apparent, assumed by the luminous matter which accompanies a comet is various, not only in different comets, but in the same comet, in different parts of its course. When the comet is at a given distance from the sun, it surrounds, in an unequal manner, the nucleus or body, forming the coma. When it is arrived within a given distance, the light begins to project, not toward the luminary, but toward that point in the heavens which is nearly opposite to it; and in this form is called the tail, train, or trail of the comet; for it is now behind the comet; that is, it projects in the direction opposite to its course. As the comet approaches the sun, the quantity of this light increases, and the length to which it projects extends. When the comet has passed the sun, in other words, when the space of time, during which it has remained in the vicinity of the sun, has been increased, the light is observed to be increased also, both in quantity and length; but, as its direction is still toward that point in the heavens which is opposite the sun, it now precedes the comet in its course, and is no longer called its tail, but its beard. As the comet recedes from the sun, the length of the light progressively contracts, and when it has again reached a given distance from that luminary, the light is again seen to surround, in an equal manner, the nucleus or body. Such are the real variations in the form of the light; but there is also another which is only apparent. When, in the course of the comet's evolution, it arrives at a point, in respect of the earth, diametrically opposite to the sun, the earth being between, the longitudinal view of the light is taken from us; that point in the heavens which is opposite to the sun, and toward which it is directed, being then also opposite to ns. At this time, however, the light is not wholly hidden; for the diameter or breadth of the light, even in its projected state, exceeding the diameter or breadth of the nucleus, that portion of the light which fills the excess of diameter or breadth, forming to the eye a border or coma, as may be exemplified in the case where a piece of coin, of larger diameter, is placed behind one of smaller.

Using the word 'coma' as a collective name for the light, in whatever form, which surrounds or is projected from the nucleus of a comet, and the word' nucleus,' to signify the body or star itself, we may describe a comet as composed of two parts, the nucleus and the coma. What is the nature of the substance of the coma,

Cometas Græci vocant, nostri Crinitas. Plin. Hist. Nat. ii. 22.

and of the nucleus itself, are points upon which nothing has been agreed. A recent philosophical writer, in speaking of the coma, has admitted the expression, even if it consists of a material substance; an expression difficult to understand, but which may mean, that in the writer's apprehension, the coma possibly consists only of reflected or refracted light. According to some, the coma is of the nature of the aurora-borealis, and our own earth, accompanied by this meteor, may appear to the inhabitants of other hea venly bodies as comets do to us. With respect to the nucleus, the writer last alluded to observes, that its density is probably comparatively small, and endeavours, upon this hypothesis, to explain the phenomenon of the projection of the coma. It is probable,' he observes, that the density of the nucleus or body of the comet itself is comparatively small, and its attraction for the tail consequently weak, so that it has little tendency to reduce the tail, even if it consists of a material substance, to a spherical form; for since some comets have no visible nucleus at all, there is no difficulty in supposing the nucleus, when present, to be of very moderate density, and perhaps to consist of the same kind of substance as constitutes the tail or coma, in a state of somewhat greater condensation'.t

Astronomers and historians have recorded between five and six hundred instances of the appearance of comets. Of these, the dates of between four and five hundred are subsequent to the christian era, and those of about one hundred antecedent. M. Desguines enumerates two or three hundred comets, mentioned by Chinese authors, of which a part are to be considered as belonging to the five or six hundred observed in Europe, and a part may have been recorded by those authors only. The presence of comets, however, has often escaped, and often continues to escape, the observation of all mankind, and innumerable appearances are unrecorded in history. It is to the defect of records alone that we attribute the paucity of numbers before the christian era. In modern times, the progressive improvement of the telescope has led to an apparently progressive increase of the number of comets; of comets, however, which have never become visible to the naked eye. Within the last twenty years, many such have been discovered by Miss Herschel. Many comets may be supposed to have escaped all observation from having had their path too near the sun, or from having appeared during moonlight nights. Many have passed unseen in Europe, because they were visible only in

Young's Lectures on Natural Philosophy, &c. London. 1807. The very loose expression, material substance,' urgently requires correction.

Young's Lectures.

the

the other hemisphere; many have been too small to be discerned by the telescopes formerly in use, and many are so small as to be invisible even through the improved ones of our own times.

The difference in the sizes of comets is one of the additional circumstances not hitherto mentioned, by which these bodies are distinguishable from each other. While some are so small as to almost escape, even when nearest the earth, the search of the most powerful telescope, others have equalled the apparent magnitude of the fixed stars, that of the planets Venus, Jupiter, and Saturn, and even that of the Moon. Further, the nucleuses of comets differ in form, colour, and comparative brightness.

To the early generations of mankind, few natural appearances can be conceived more pardonable objects of terror than comets. Looking instinctively to the skies, where the sun, moon, and stars, claimed so much of the gratitude of every human creature, what stronger occasion of anxiety can be conceived, than the appearance, in those skies, of a new, unknown, and unexperienced light, differing, in its aspect, from all others? Whence did it come, and for what purpose? What a subject of apprehension while present, what a source of conjecture when gone! The sun and moon are lights of greater magnitude, and, were their appearance equally sudden and temporary, of more terror than comets; but these are constant; these are familiar; men see them from their infancy, and know them to be the givers of life and joy. But of comets nothing is known, and therefore from them every thing is to be feared. It has been suggested with some felicity, that a comet, in which Milton has found an image with which to compare his Satan, were a more fit simile for his Raphael, sociably mild,' and whose approach is thus described by Adam:

Haste hither, Eve! and, worth thy sight, behold,
Eastward among those trees, what glorious shape
Comes this way moving; seems another morn

Ris'n on mid-noon; some great behest from heaven
To us perhaps he brings.

But there is an error in this criticism, agreeable as the error is. Such language, on such an occasion, is well fit for Eden, and for Adam still in Eden; but it is no language, either for this nether world, or for its inhabitants. Fear is the foremost of our passions, because self preservation is the most indispensable of our cares. To man, amid the accidents of nature, the dread of concealed and approaching evils is the reasonable inmate of the mind, and every change, every new object, is a reasonable cause of excitement of that dread. We are not to wonder, then, if the appearances of comets alarmed the earlier generations of mankind. On a first consideration, it may be expected, however, that a short acquaintance with these ap

pearances

+

pearances should have dispelled every sentiment of terror, and that the children of these generations should have speedily learned to reject it. But a little reflection will correct this mistake. That acquaintance with comets, which was acquired by the earlier generations, was far from being so satisfactory as this view supposes. When comets appeared, nations, families, and individuals, were filled with apprehension of some sudden or approaching evil; and what great number was there, either of nations, families, or individuals, upon whom, either during the stay of the comet, or after its departure, no evil came, sudden or remote; no evil of which it might be said, that the comet was the sign and forerunner? Wars, inundations, storms, sickness, famine, and death, are evils constantly impending, and some of them have assuredly accompanied or followed the appearance of every comet whatever. Cicero, in an age very different from those from which we are now tracing the impressions of these fears, pronounces, That comets foreshow calamities- Cometæ prænuncii calamitatum'--and how shall we prove that they do not? Comets come, and calamities come after them; and how shall we demonstrate that they do not foreshow them?

The successive generations of mankind, even to the present, have therefore scarcely enjoyed any advantages in this respect over their precursors. If the latter bent under primæval ignorance, the former have been loaded with accumulated errors, with false information, and with erroneous opinions. If the latter were deceived by their own unfounded fears, the former have been deceived by their fathers' unfounded traditions. The latter yielded to doubts, the former yield to pretended proofs. If the latter trembled, through nature, the former have trembled, and still tremble, through education. It is even to be believed, (a question which will presently be particularly considered), that the opinions and writings of men the most illustrious for their science, within the two last centuries, have added to the vain alarms of the multitude upon this subject of comets. Be this as it may, it is a fact which at least, the historian is called upon to record, that at few periods have these vain alarms been more indulged in than in the present, the commencement of the nineteenth century. While this paper is writing, a comet is visible; and at this time, more than the half of Europe is teaching, or being taught, what are the evils with which it threatens the world! It falls within the knowledge of the writer, that an inhabitant of this metropolis, a person of respectable condition in life, is hourly assuring his friends, with perfect gravity and real earnestness, that his own life, and the lives of his neighbours, are now limited to a fortnight's duration; that fortnight being the extreme period which is to elapse before this comet is to burn up the earth.

As