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—are mere masses of vapour, totally divested of solidity, and so small and faint, that they are not at all discoverable by the massisted sight, and frequently cannot be detected without considerable difficulty even with telescopic aid. In 1832, Sir John Herschel, with the aid of a reflecting telescope of twenty feet in length, and possessing an enormous illuminating power, could barely see Biela's comet; and he asserts, that if he had not discovered its position by such means, he would have found it quite impossible to have detected it with a refracting telescope, although he did see it afterwards with an equatorial instrument of that kind.
The extreme lightness of comets compared with the smallest masses of the solar system,-as, for example, the new planets and the satellites of the old ones,-is fully established by the observed fact, that on their nearest approach to these bodies they never by their attraction cause them to deviate in the slightest perceptible degree from their usual course; yet this cannot be accounted for by denying to the comets the quality of gravitation, since the attraction of the planets upon them is very considerable. In the year 1767 a comet, previously unknown, entered our system by a course so near the planet Jupiter, that the attraction of that body threw the comet completely into a new orbit; which was found by calculation, made by Lexell on the observations of Messier, to be an oval or ellipse, in which, had the comet continued to move, its period would have been about five years and a half. While passing round the sun the comet followed this orbit, but on receding from the sun it passed, in 1779, among the satellites of Jupiter, and was again thrown into another orbit by the attraction which it suffered, and was never afterwards seen. This circumstance, which was not understood at the time, occasioned considerable difficulty to astronomers; but the problem has since been solved by the methods given by Laplace; and it has been ascertained that previous to 1767 the comet moved in an orbit in which its period must have been at least fifty years, and at its nearest approach to the sun its distance would have been about six times the earth's distance. In such an orbit it is impossible the comet would ever have been visible. The next disturbance of Jupiter, in 1779, threw it into a new orbit, in which its period would have been twenty years, and its least distance from the sun four times the earth's distance. Consequently, in such an orbit it never could be visible from the earth. In this case not the slightest effect was produced upon the motion of Jupiter's satellites by the attraction of the comet; from whence we must infer that the mass of the comet must have had an infinitely small proportion to the mass of the smallest of the satellites.
It is an interesting and well-ascertained fact, so far as any evidence can be collected from the periodic comets, that these bodies are undergoing a gradual decrease of magnitude. This has been particularly observable in the successive returns of Halley's comet; in which, from its very long period, such an effect might be expected to be conspicuous. But in the comets of Biela and Encké, of shorter periods, a like effect has been observed. The inference which must be drawn from this, is, that the constituent parts of comets are gradually scattered through space: possibly the formation of their tails, by the operation of the sun, may expel matter from their masses, which the gravitation of the mass does not possess sufficient coercion to recall. Unless, however, we admit that a period will come when comets will altogether vanish from our system, we can scarcely attribute this declension of magnitude and splendour to comets universally; if they have a decay, they must have a growth; if there be a decrease, there must be an increase, and a maximum; otherwise, on tracing back such effects, we must, by assuming a sufficient duration of time, find a set of bodies of infinite magnitude and infinite splendour.
May it not happen, that in their excursions through the abyss of space they may be fed with cometic matter, so that the waste of individual comets may be repaired? Under certain circumstances, comets, whose courses may intersect, may coalesce ;—a larger may attract and carry with it a smaller. However this be, we are not warranted in hastily generalizing the fact of the decay of magnitude observed in the cases just mentioned. It is true that in the five last appearances of Halley's comet, its magnitude and splendour appear to be on the decline. But if we apply the same reasoning to appearances antecedent to 1456, how, it may be asked, did its return so little attract the notice of historians in 1380? Also, between the year 1305 and 399, although some returns are mentioned which correspond in time with the period of Halley's comet, yet we have no accounts of the same terrific object. The spirit of the times was nevertheless such, that had it so appeared, it could scarcely have passed without exciting the usual superstitious terrors. Must we not then admit the possibility of growth or increase as well as decline and diminution?
It is a curious, and not uninteresting circumstance, that the periodical path of Biela's comet passes very close to that of the earth; so close, that at the moment the centre of the comet is at the point nearest to the earth's path, the matter of the comet extends beyond that path, and includes a portion within it. Thus, if the earth were at that point of its orbit which is nearest to the path of the comet, at the same moment that the comet should be at that point of its orbit which is nearest to the path of the earth,
the earth would be enveloped in the nebulous atmosphere of the comet. As this comet has no nucleus nor solidity, a collision in such a case would, of course, be out of the question. The effect produced would be merely an intermixture of the cometic atmosphere with that of the earth. The extremely light mass of the comet would, notwithstanding its proximity, render it impossible that it could produce any sensible effect, either on the annual or diurnal motion of the earth; so that our years, seasons, and days would remain unchanged. With respect to the effect which might be produced upon our atmosphere by such a circumstance, it is impossible to offer any thing but the most vague conjecture. We have already shown that the nebulous matter of this comet must be infinitely more attenuated than our atmosphere; so that the two fluids, when mixed, would be combined in a proportion in which our atmosphere would prevail to the extent perhaps of millions to one. For a single particle, therefore, of the cometary matter which we should inhale, we should inspire millions of particles of atmospheric air. Under such circumstances, it is scarcely probable that we should be conscious of the presence of the cometic matter at all. But even against the occurrence of such a circumstance as this, there are many thousand chances. It is certain that every year the earth must pass through the point in question; but the comet can only pass through the corresponding point of its path once in seven years. The earth moves in its orbit at the rate of about two millions of miles per day; it consequently could remain within the limits of danger for a very brief period; but unless that brief period precisely coincided with the moment in its seven years' circuit, at which the comet should pass through the corresponding point, the effect which we have now alluded to could not take place."
The question of the near approach of comets to the earth, and of the effects of such an occurrence, has been very fully and satisfactorily investigated by Du Sejour.† He shows that of all the comets whose paths had been then ascertained, none could pass nearer to the earth than about twice the moon's dis
In the year 1832, Biela's comet arrived at the point of its orbit nearest the earth on the 30th of October, and enveloped within its limits a part of the earth's path; but the earth did not arrive at the corresponding point of its orbit until the 30th of November; and since the earth moves at the rate of two millions of miles per day, its distance from the comet on the 30th of October must have been sixty millions of miles. + Traité analytique des mouvemens apparens des corps célestes. Paris, 1786-1789.
tance; and that none ever did pass nearer to the earth than nine times the moon's distance. This occurred with the comet of 1770, already mentioned as having been changed in its course twice by the action of Jupiter. The least unreasonable ground of apprehension from the proximity of a comet would be the possible production of a tide in the ocean, which would so disturb its equilibrium as to submerge considerable tracts of land. But to accomplish this, or indeed to raise a tide at all, it is necessary (even admitting that the disturbing body can exert sufficient attraction) that the angular motion of the attracting body, with respect to the earth, should not exceed a certain rate. The moon only produces the tides because its angular velocity is considerably under this limit. Du Sejour has proved that a comet could not, by possibility, remain more than two hours and a half so near the earth as a fourth part of the moon's distance. And it could not remain even so long unless it passed the earth under a very peculiar and improbable combination of circumstances. For example, if its orbit were nearly perpendicular to that of the earth, it could not remain more than half an hour in such a position. Under such circumstances, the production of a tide would be impossible. He shows that eleven hours at least would be necessary to enable a comet to produce an effect on the waters of the earth, from which the injurious consequences so much dreaded could follow. The conclusion to which he arrives is, therefore, that although in strict geometrical rigour, it is not physically impossible that a comet should encounter the earth, yet the moral possibility of such an event is absolutely nothing.' The determination of the number of comets connected with our system is a question, which, although not admitting of a demonstrative solution, may be solved upon grounds of a high degree of probability; and it is one of so much interest, that we are induced here to extend the limits we had intended for this article, in order to lay before our readers the views of M. Arago and others on this point.
The total number of distinct comets, whose paths during the visible part of their course, had been ascertained up to the year 1832, was one hundred and thirty-seven. In order to discover whether bodies of this nature prevail more in any particular regions of space than in others,-whether, like the planets, they crowd into a particular plane, or are distributed through the universe without preference of any one region to any other,it was necessary to examine and compare the paths of these hundred and thirty-seven bodies. After a close examination of the planes of their orbits with respect to that of the earth, it appears, that the numbers inclined at various angles, from 0 to
90°, is pretty nearly the same. Thus, at angles between 80° and 90° there are fifteen comets; while at angles between 10° and 20° there are thirteen; and between 30° and 40° there are seventeen. Again, the points, where they pass through the plane of the earth's orbit, are found to be uniformly distributed in every direction round the sun. The points where they pass nearest to the sun are likewise distributed uniformly round that body. Their least distances from the sun also vary in such a manner as leads to the supposition of their uniform distribution through space. Thus, if we suppose a globe of which the sun is the centre to pass through the orbit of Mercury, so as to enclose the space round the sun, extending to a distance on every side equal to the distance of Mercury, thirty of the ascer tained comets, when at their least distance from the sun, pass within that globe. Between that globe and a similar one through the orbit of Venus, forty-four comets pass under like circumstances. Between the latter globe and a like one through the orbit of the earth, thirty-four pass: between the globe through the orbit of the earth and one through the orbit of Mars, twentythree pass; and between the latter and a globe through the orbit of Jupiter, six pass. No comet has ever been visible beyond the orbit of Jupiter. It must be here observed, that beyond the orbit of Mars it is extremely difficult to discern comets; and this may account for the comparatively small number of ascertained comets which do not come nearer to the sun than that limit. A comparison of the above numbers with the spaces included between these successive imaginary globes, and with the relative facility or difficulty of discerning comets in the different situations thus assigned, leads to a demonstration, that, so far as these hundred and thirty-seven observed comets can be considered as an indication of the general distribution of comets through space, that distribution ought to be regarded as uniform; that is, an equal number of comets have their least distances included in equal portions of space.
Adopting then this conclusion, M. Arago reasons in the following manner: The number of ascertained comets, which at their least distances pass within the orbit of Mercury, is thirty. Now, our most remote planet, Herschel, is forty-nine times more distant from the sun than Mercury; consequently a globe, of which the sun is the centre, and whose surface would pass through the orbit of Herschel, would include a space greater than a similar globe through the orbit of Mercury in the proportion of the cube of forty-nine to one, or of a hundred and seventeen thousand six hundred and forty-nine to one. Assuming the uniform distribution of comets, it will follow, that for every comet inclu