CHAP. XV.

GENERAL REMARKS ON THE PRECEDING CHAPTERS RELATIVE TO COMETS.

Ir is manifest from Chapter X, that Dr. Herschel supposes the comet of 1811 to have been produced from nebulous light, consistently with his theory noticed in chapters VII and VIII; that such light became progressively consolidated, and that a part of its luminous matter had been exhaled in its approach to some other stars before its perihelion path in our own system. Yet from the length of its tail, and the luminous envelope that surrounded its outer line, compared with those of the comet of 1807, he thinks it must have been of much later date.

The comet of 1807, in its approach to the sun advanced within sixty-one millions of miles of it, and its tail, when longest, covered an extent of nine millions of miles. The late comet, in its perihelion did not pass 'so near the sun by about thirty-six millions of miles, being about two thirds only of the closest approximation of the preceding, and nevertheless acquired a tail of upwards of a hundred millions of miles. May we not then conclude, as he suggests, that the consolidation of the comet of 1807, when it reached its perihelion, had already been carried to a much greater degree of density than that of the last comet, by some former approach to our sun, or to some similarly constructed celestial bodies, such as we have reason to believe the fixed stars to be? And that comets may pass round other suns than ours, is rendered probable from our not knowing, with certainty as yet, the return of more than one comet among the great number that have been observed?

He calculates the bright point, or what we may admit to be the solid or planetary body of the comet, at about 428 miles in diameter: and that its distance from the earth, at the time of making his observations, was 114 millions of miles. It existed in a stelliform nucleus which he calls the head of the comet, the diameter of which he calculates at 127 thousand miles. It was surrounded by

a circular darkish space which he supposes to be an atmosphere, and computes at 507 thousand miles in diameter. The tail varied in length and breadth from September 2, when he could perceive none, to September 9, when it extended 9 or 10 degrees; September 18, when it reached 11 or 12 degrees; October 6, at which it possessed 25 degrees, which was its greatest length. On October 12, it was 17 degrees, and on October 15, 23 degrees, or somewhat more than a hundred million of miles. The breadth of the tail varied as well as its length; on October 12, it was 63 degrees in its broadest part, or nearly 15 millions of miles.

The general shape of the comet he calculates niust have been that of an inverted hollow cone, terminating its vertex in an equally hollow cap of nearly an hemispherical construction; the cap and sides of this hollow cone being of inconsiderable thickness. The tail shortened rapidly in November. On the 5th it was 12 degrees; on the 16th 71, December 10th 5 degrees, and of feeble light; on the 14th nearly the same, but the light very considerably feebler.

To these remarks we may farther add, that the tail of the comet of 1680, was nearly equal to that of 1811, having been calculated at a hundred millions of miles.

Deguignes enumerates two or three hundred comets mentioned by Chinese writers. Doubts, however, have since been thrown upon the authorities referred to.

•Extensive as are orbits of comets, from their eccentricity they have sometimes approached much nearer to the sun than any of the planets; for the comet of 1680, when at its perihelion, was at the distance of only one-sixth of the sun's diameter from its surface. Yet from the very inconsiderable density of their enormous tails, and even of the greater part of the nucleus itself, should it ever happen to a planet, of which there is but very little probability, to fall exactly in the way of a comet, it is supposed that the inconvenience suffered by the inhabitants of the planet might be merely temporary and local: the chances are, however, much greater, that a comet might interfere in such a manner with a planet, as to de flect it a little from its course, and retire again without coming actually into contact with it.

Nearly 500 comets are recorded to have been seen at different times, and the orbits of about a hundred have been correctly as

certained: but we have no opportunity of observing a sufficient portion of the orbit of any comet, to determine with accuracy the whole of its form as an ellipsis, since the part which is within the limits of our observation does not sensibly differ from the parabola, which would be the result of an ellipsis prolonged without end.

Two comets at least, or perhaps three, have been recognized in their return. A comet appeared in 1770, which Prosperin suspected to move in an orbit materially different from a parabola; Mr. Lexell determined its period to be 5 years and 7 months, and its extreme distances to be between the orbits of Jupiter and of Mercury; but it does not appear that any subsequent observations have confirmed his theory. It has, however, been calculated, that supposing the theory correct, it must afterwards have approached so near to Jupiter as to have the form of its orbit entirely changed.

Dr. Halley foretold the return of a comet about 1758, which had appeared in 1531, in 1607, and in 1682, at intervals of about 75 years; and with Clairaut's further correction for the perturbations of Jupiter and Saturn, the time agreed within about a month. The mean distance of this comet from the sun must be less than that of the Georgian planet; so that by improving our, telescopes still more highly, we may, perhaps, hereafter be able to convert some of the comets into planets, so far as their remaining always visible would entitle them to the appellation. Dr. Halley also supposed the comet of 1680 to have been seen in 1106, in 531, and in the year 44 before Christ, having a period of 575 years; and it has been suspected that the comets of 1556 and 1264 were the same, the interval being 292 years; a conjecture which will either be confirmed or confuted in the year 1848. Some persons have even doubted of the perfect coincidence of the orbits of any comets, seen at different times, with each other, and have been disposed to consider them as messengers forming a communication between the neighbouring systems of the sidereal world, and visiting a variety of stars in suc cession, so as to have their courses altered continually, by the attraction towards many different centres; but considering the coincidence of the calculation of Halley and Clairaut with the subsequent appearance of the comet of 1759, this opinion can scarcely be admitted to be in any degree probable with respect to the comets in general, however possible the supposition may be in some particular cases.

The reader may upon this subject consult alsó Bartholinus De Cometis, 4to.Copenhagen, 1665; Hooke's Lectures and Collections, 4to. 1678; Cometa,-Figures, p. 2,3; Heinsius, Ueber den Cometen, 4to. Petersb. 1743; Dionis du Séjour, Essai sur les Cometes, Paris, 1775; Deguignes, S. E. X. 1785. App. 39; Young's Nat. Phil.Vol. I. p. 513.

CHAP. XVI.

SYNOPSIS OF THE PRINCIPAL ELEMENTS OF ASTRONOMY, DEDUCED FROM M. LA PLACE'S EXPOSITION DU SYSTEME DU MONDE.

THE following article is drawn from the third edition of M.La Place's very excellent Exposition (1808) in which the author has given the elements of the planets in a more correct manner than in either of the preceding editions; and wherein he has revised and amended all his former calculations by more recent and exact_observations.

The arrangement of the present memoir is somewhat new; but many persons have frequently found the want of a manual of this kind, where all the different facts, relative to astronomy, might be brought under their respective heads, without the necessity of turn ing to a variety of works for information. Much time is often lost in a research of that kind, which it is the object of the present abstract to prevent.

In the original work, the author has universally adopted the decimal division of the day, and of the quadrant. This method is here preserved in the Tables of the Elements of the Planets; but, in subsequent parts, the common sexagesimal notation is adopted, as being more easily understood in this country.

Some other facts, not mentioned by M. La Place, are inserted in this tract, in order to enlarge the view of the subject: but these pas sages are always kept separate, by being inclosed within brackets.

The Sun.

The Sun, which is the source of light and heat to our system, is the most considerable of all the heavenly bodies, and governs all the planetary motions.

Its diameter is 111.454 times the mean diameter of the earth; whence its volume is 1384472 times greater than that of the earth; but its mass is only 337086 times greater. Whence we conclude that its density is 126, or about that of our globe.

It is surrounded by an atmosphere; and it is oftentimes covered with spots. Some of these spots have been observed so large as to exceed the earth four or five times in magnitude.

The observation of these spots shows that the Sun moves on its axis, which is nearly perpendicular to the ecliptic; and the duration of an entire sidereal rotation of the sun is about 25 days.

Whence we conclude that the sun is flattened at the poles. - The solar equator is inclined 7° 30' to the plane of the ecliptic.

A body, which weighs one pound at the surface of the earth, would, if removed to the surface of the sun, weigh 27.933 pounds. And bodies would fall there with a velocity of 334.65 feet in the first second of time.

The sun, together with the planets, moves round the common centre of gravity of the system; which centre is nearly in the centre of the sun.

This motion changes into epicycloids the ellipses of the planets and comets, which revolve round the sun.

The sun appears to have a particular motion, which carries our system towards the constellation of Hercules.

The apparent diameter of the sun, as seen from the earth, undergoes a periodical variation. It is greatest when the earth is in its perihelion; at which time it is 82' 35",6: and it is least when the earth is in its aphelion; at which time it is 31′31′′,0. Its mean apparent diameter is therefore 32′ 5′′,3.

His horizontal parallax is 8".

The greatest equation of his centre is 1° 55' 27",7; which dimi nishes at the rate of 16′′,9 in a century.

The diurnal motion of the sun from east to west, and his annual motion in the ecliptic, are optical deceptions; arising from the real motion of the earth on its axis, and in its orbit.