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[From the Companion to the British Almanac for 1833.]


THE year which has just passed away has been distinguished by the predicted appearance* of two comets, the most remarkable which have yet fallen under the notice of astronomers. These are what are commonly called the comets of Encket and Biela ‡. The latter has been an object of fear to many on account of the nearness with which it has approached, not the earth, but a point of the earth's path. As public attention has thus been turned to this subject in an unusual degree, we seize this opportunity of laying before our readers a slight account of the present state of cometary astronomy, distinguishing that which we really know of these bodies from the many surmises to which they have given rise.

The signification of the word comet has varied, as new bodies have appeared which analogy has led astronomers to include under that name. It was first given, as the word denotes, to bodies which appeared in the heavens with a train of light, or tail, and thus included some of the meteors which belong to our own atmosphere. We now apply the word to those heavenly bodies, without the limits of our own atmosphere, which are nebulous in their appearance, and with or without a tail. We may divide all which have been observed into three classes: 1. Those whose returns have been predicted, and the prediction verified by the fact. These are three in number, viz. the celebrated comet

* Mr. Henderson has observed Encke's comet at the Cape of Good Hope, and Sir John Herschel that of Biela. We mention these facts here, as neither body is visible to the naked eye, and many of our readers may not be aware of their having been seen by any one.

↑ First discovered by M. Pons, November 26, 1818, but justly named by astronomers after Professor Encke, from his success in detecting its orbit, motion, and perturbations, ↑ First discovered by M. Biela, an Austrian officer, February 28th, 1826.

of Halley, observed by him in 1682, which returned, according to his conjecture (for it could then hardly be called more) in 1759, and will appear again in 1835; its time of revolution is about seventy-six years. The other two are those above-mentioned, of Encke and Biela, which perform their revolutions respectively in about three years fifteen weeks, and six years thirty-eight weeks. 2. Comets whose return has been predicted unsuccessfully. Of these there is only one of any note, viz. that which appeared in 1770. This, it was found, should have returned in five years and a half, if the observations made of it were correct; however, it never could be found again. This phenomenon threw doubt upon the return of comets, until the success of Laplace in devising methods for the calculation of the effects arising from the mutual attractions of our system, recalled the attention of astronomers to this almost forgotten failure. It had been found that the comet of 1770, in its approach to the Sun, had passed so near to Jupiter, that, on the theory of gravitation, the attraction of the latter was 200 times as great as that of the former. On applying the methods of Laplace to this case, it was found that, in 1767, while the comet was describing an orbit of more than 50 years, its motion was changed by the action of Jupiter so that it described the orbit observed in 1770; and that in 1779, it came again so near Jupiter that the preceding effect was reversed, and the orbit was again changed into one of long duration. 3. Comets which have been observed, the predicted return of which is yet to be expected. The most remarkable of these is the one observed by Olbers in 1815, which we may now safely say will return in 1887. 4. Comets which were observed at a time when neither theory nor observation was in a state sufficiently perfect to enable the observers to say whether they would return or not; and others, the orbits of which are uncertain, owing to the weather or other accidents not permitting them to be sufficiently well observed. Of these there are a great many, some of which may yet be recovered. For, long before the time of revolution of a comet could be found, astronomers knew how to determine, 1. The magnitude and position of its least distance from the sun. 2. Where its orbit cut the ecliptic. 3. The inclination of its orbit to the ecliptic. If a future comet should strongly resemble any one already observed in these particulars, and if its time of revolution as hereafter determined, should permit of its having been seen about the time of the former comet, we shall have sufficient reason to conclude that the two are one and the same. We must not, however, expect that the accounts of ancient writers on this subject with regard to the form of comets will ever be verified; such, for example, as those which describe comets in the shape of a sword, or surrounded with a shaggy mane. To say nothing of our never having observed such appearances in the course of the last century and a half, we must recollect the well-known fact, that

comets were formerly considered as warnings of impending evils, or, at least, of remarkable changes. Thus Bodin, who died in 1596, gives it as his opinion that they are the souls of illustrious men, who having remained many ages upon the earth in the capacity of guardian angels, (for so the context must be interpreted,) are called to heaven in the shape of flaming stars. He attributes the plagues, famines, &c. which were supposed to follow, to the want of the prayers of these superannuated intercessors. Pope Calixtus the Third, in the fifteenth century, directed the thunders of the church, not only against the Turks, who had gained some successes, but also against a comet, which was supposed to have had some hand in, or at least to have foretold, them. When such impressions prevailed, it was natural that the appearance of the warning body should be somewhat exaggerated.

If from all that has been said upon comets, we take that which we certainly know, we shall have left a mass of conjectures of every grade of probability, from the one which may be considered as nearly proved, to those which, in point of evidence, might be placed side by side with the opinions of Bodin or Calixtus. We shall try to give some notion of the manner in which we come to know that which we do know, and some reasons for the most probable among the cenjectures. Those who would read more of the history of surmises on this point, are recommended to consult the Annuaire of the French Board of Longitude for 1832, in which will be found a most amusing, as well as instructive, article on this subject, by M. Arago; an English version of which appeared in the Times newspaper some months ago.


That a comet is a material body is proved by the same sort of reasoning which is applied to the planets. Firstly, it either reflects the light of the sun, or shines by its own light; which of the two has never been distinctly proved; perhaps both suppositions may be true. Matter is always present where light is either emitted or reflected, at least on our globe. Secondly, comets are found to be acted upon by the laws of gravitation exactly as all other material bodies are, they are attracted by the sun, and move (so far, at least, as we can make out) in ellipses, or other conic sections, and this motion is disturbed, or, technically speaking, perturbed by the attraction of the planets, especially by the larger planets, Jupiter or Saturn. In this manner they have furnished one of the most decisive proofs of the Newtonian theory of gravitation. We

[* A translation of this "article" was also made by Professor Farrar, of the University in Cambridge, and published by Hilliard, Gray, & Co. Boston, 1832; under the following title:

"Tract on Comets; and particularly on the Comet that is to intersect the Earth's Path in October, 1832, by M. Arago, attached to the Royal Observatory at Paris ; — Translated by John Farrar." Am. Ed,]


have already mentioned the comet of 1770; but this, it may be those who cannot examine the calculations for themselves, wa of the astronomers, to account for their own failure. We wi fore cite another instance, in which the effects of planetary pert were very great, were predicted before the event, and verifi The comet of 1682, or of Halley, it is well known, was predicted as likely to appear in 1757. This he concluded from observin comet with a similar orbit had appeared in 1531 and 1607. He remarked, that as the comet would, if his supposition were tr near to Jupiter and Saturn, some alteration might be expected t attractions of these planets. In 1757, while astronomers wer ning to look for the expected body, with no very great hopes of pearance, Lalande proposed to Clairaut to undertake the compu the effect of the planets upon the comet. These names may well known to our readers as to mathematicians and astro we will, therefore, inform them, that Lalande was a practical mer of great eminence, and that Clairaut was a mathematician ural philosopher of even greater celebrity. So little wedded we men to the system of gravitation, that the first discarded, or threw doubt upon, the theory of the return of comets, on of the non-appearance of that of 1770, already mentioned the second, on account of some unexplained phenomena, imagi Newton had mistaken the law according to which the mutua tions of planets depend upon their distance. The two under enormous labor above-mentioned; and the result was, that announced, in the year 1758, that the revolution which was taking place, would be 618 days longer than the preceding one the one which took place between 1607 and 1682. At the sa he observed, that the methods of calculation were yet so inc that the result could not be depended upon within thirty days conclusion had been quite correct, the comet would have co perihelion, or nearest point to the sun, about the middle of Apr and it did arrive there on the 13th of March of the same year the thirty days which had been allowed for errors. We may remark, that the comets of 1832, of which that of Encke has ond appeared, according to prediction, and that of Biela has been observed by Sir J. Herschel, both very near their predicted could not have had their tables constructed without a strict a

On the subject of this comet, we derive our information from a communica by Sir John Herschel to the Royal Astronomical Society, and read at the that body on the 9th of November last. An account of this interesting pap found in the Monthly Notice of the transactions of the Society for November, by Priestley and Weale, Holborn.

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e planetary perturbations. From such facts we are justified in ing that comets are material bodies, subject, like the planets, to traction of the sun and other bodies of our system, and describing iptic orbit round the sun nearly, the difference being attributable action of the planets, or, perhaps, in some degree, to a resisting


e next question is, comets being material, what is their quantity of er: that is, if brought to the earth without alteration of their dimen, would they be light or heavy in proportion to their size. On this we have sufficient evidence, not as to the actual quantity of matany comet, but as to limits below which it must fall, at least in e comets of which the times of revolution are known. It results the theory of gravitation, that of two bodies, the first cannot affect econd, without being itself more or less affected by the second. of two bodies, one of which is very great compared with the other, fect which the smaller produces upon the greater is small comI with that which the greater produces upon the less. This is anals (though the two phenomena must not be confounded) to a fact of day observation, that a light body striking against a heavy one, gh with with great velocity, produces, nevertheless, but a small ge in the velocity of the greater one, and rice versa. For example, e motion of Jupiter and Saturn it is observed, that the average vey of Jupiter is accelerated, while that of Saturn is retarded more twice as much. And it is shown, by a process independent of this rvation, that Jupiter contains more than twice the quantity of matf Saturn. After some ages, the motion of Jupiter will cease to be erated, and that of Saturn to be retarded. After which, that of ter will begin to be retarded, while that of Saturn will begin to be erated. Hence, if a comet so large, or rather so heavy, as to bear ppreciable proportion to the mass of a planet, were to be disturbed e latter in any considerable degree, the comet itself would produce gree of disturbance in the motion of the planet, which would be eptible to our instruments. Thus, if Halley's comet, which was ded between 1682 and 1579, more than 500 days by the action of ter, had been only the twenty-thousandth part of the mass of Jupiits effect upon the latter would have been even then most distinctly eptible by good instruments. The same thing would take place if the mass of that comet were very much less, and yet, in the er case, it would be less than one sixtieth part of the earth. But e are two much more conclusive arguments. Laplace found, that e comet of 1770 had only been the five-thousandth part of the earth, ould have lengthened our year by three seconds. No such alterahas taken place, and the comet must, therefore, have been less than

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