« SebelumnyaLanjutkan »
Thomas Brisbane, who was then governor of New South Wales, and had fitted up an observatory there, and provided himself with the able assistance of Mr. Rumker. The latter gentleman appears to have discovered the comet on June 2, 1822; and his accurate observations afforded Encke the means of reconsidering the true elements of its orbit, and with additional confidence to compute its return for 1825. This occurred as was expected; the fresh data afforded by that return were carefully collated by the professor, to enable him still more satisfactorily to define the orbit, and with increased confidence to predict its return this year. It was observed by Mr. South first 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 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 considering 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, February 27, 1826, and which was afterwards seen by Gambart and others. It seems to possess similar claims to the attention of astronomers as that of Encke, it being conceived to revolve about the sun in about six years 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, on 10th January, 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 7o 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 the hundred, or thereabouts, that have appeared, 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 acording to any law that has been discov. ered. From 1299 to 1532, all that are mentioned were retrograde; and five that were observed 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 almost 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 prop 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 ?° or 3°. The length of the tail varies, however, not only with the time at which it is observed, but with the place of ob. servation ;-a difference probably depending on the difference of clearness and purity in the air. The tail of the comet of 1759, was 25° long, as measured at Montpelier 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 more 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 half of miles in a day; and as Clairault, with all his care, did not come nearer the truth than nineteen 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 establishec, 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.
MISCELLANEOUS DIRECTIONS, HINTS, AND RE
XIV. WASHINGTON'S AGRICULTURAL NOTES.
NOTWITHSTANDING the numerous public avocations and duties, in which Washington was engaged for a large portion of his life, it is known, that to no one object did he give so much of his time and attention as to Agriculture. The frequency and minuteness of his directions to his managers on this head, and the unceasing correspondence, which he kept up during his absence from Mount Vernon, are truly astonishing, when it is considered in what important and absorbing interests his mind was perpetually occupied. We have selected a few particulars from his manuscript papers, which, at the same time they illustrate his agricultural habits, may in part serve as practical hints, or salutary maxims, to farmers in general.
1. DIRECTIONS TO THE MANAGER OF HIS FARMS.
A system closely pursued, although it may not in all its parts be the best that could be devised, is attended with innumerable advantages. The conductor of the business, in this case, can never be under any dilemma in his proceedings. The overseers, and even the laborers, know what is to be done, and what they are capable of doing, in ordinary seasons. The force to be employed may be in due proportion to the work which is to be performed, and a reasonable and tolerably accurate estimate may be made of the product. But when no plan is fixed, when directions flow from day to day, the business becomes a mere chaos, frequently shifting, and sometimes at a stand, for want of knowing what to do, or the manner of doing it. Thus is occasioned a waste of time, which is of more importance, than is generally imagined.