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only seen at Paramatta, by Rumker. The approach to the sun was in some degree confirmed by these observations, but it was impossible to reconcile all the observations with the hypothesis of a medium of given density. The return in 1825 was not favourable for deciding the question, which had now become one of the deepest interest.

Its re-appearance in 1828-9 was awaited with great anxiety by the friends and opponents of the new theory. The comet came round, and passed its perihelion approximately in accordance with the predictions, but the discrepancies from 1819 up to 1829, with any theory, were so great, as to give much perplexity to those engaged in the computations. After long and patient examination, the cause of this difficulty was finally detected. The plane of the comet's orbit makes but a small angle with the orbit of Jupiter, and when the comet is in aphelion, or farthest from the sun, it always approaches very near to the path described by the planet.

A time may then come when Jupiter shall be in the act of passing that part of its orbit very near to the aphelion point of the cometary eclipse, while the comet occupies its aphelion, bringing these bodies into close proximity, and producing excessive perturbations in the movements of this almost spiritual mass. Such, indeed, was the configuration between the returns of 1819 and 1829, on which occasion the comet was delayed in its return to its aphelion by nearly nine days, by the powerful attraction of Jupiter. Under these circumstances, any error in the assumed mass of the planet would exhibit itself in an exaggerated form in the perturbations of the comet. But it was believed in the outset of this investigation, that the mass of Jupiter, employed by Laplace in his theory of the planets, and computed by Bouvard, could be relied on as accurate. Indeed, Laplace had applied the calculus of probabilities, and had found that there was but one chance out of eleven millions that the mass he had adopted could be in error by the one hundredth part of its value.

Suspicion, however, having been aroused with reference to the mass of Jupiter, efforts were at once commenced to sift thoroughly the matter, and three different computers of high reputation undertook the determination of Jupiter's mass by different processes. Encke obtained a mass from the perturbations of the small planet Vesta, Nicolai from the perturbations of Juno, and

Airy re-examined the original measures of the elongations of Jupiter's satellite, made new measures, and thus obtained new data for the resolution of the problem of Jupiter's mass. The results obtained by the three astronomers agreed in a most remarkable manner, and proved incontestably that Laplace's value of the mass of this planet was in error more than four times the hundredth part of its value, and that, instead of requiring 1,070 globes of the magnitude of Jupiter to balance the sun, only 1,049 were necessary.

With the new mass of Jupiter it seemed possible, by admitting a resisting medium, to account for all the perturbations of Encke's comet, and for a time this theory seemed to receive greater consideration from distinguished men. The appearance of Halley's comet in 1835 again threw great doubt over the subject, for it was found impossible to reconcile the movements of the two comets with any assumed density of a resisting medium. Some have been disposed to adopt the idea that the revolution of the planets for ages in the same direction, in this supposed ethereal fluid, has impressed upon it a certain amount of motion in the same direction, and that those comets which chance to revolve with the current will be found to be operated upon differently from those which may happen to come into our system in a direction opposed to the current.

I confess, frankly, that my own mind has always revolted against the doctrine of a resisting fluid. There are so many ways in which the single phenomenon of the gradual approach of Encke's comet to the sun may be accounted for, without resorting to an hypothesis which involves the entire destruction of the planetary system, whose perpetuity has been so effectually provided for by the great Architect of the universe, that it would require the most unequivocal testimony to secure the full consent of my own mind to the adoption of this remarkable theory. It is proper, however, to say, that it has long been received with favour by men to whose judgment I am generally disposed to yield with implicit confidence.

Leaving the further consideration of this subject for the present, we proceed to the examination of another comet of short period, which has excited great attention, especially in its recent return. As early as 1805, Professor Gauss, in computing the elements of the orbits of the comets of that year, found one which seemed to complete its revolution in about six years.

This comet, however, was lost sight of, and it was not until 1826 that M. Biela discovered the same comet on its return to its perihelion. This discovery appears to have been the result of computation, but how far the investigation was carried I have never been able to learn.

The same object was also discovered by M. Gambart about the same time, who, on fixing its elements, found that it performed its revolution about the sun in an ellipse, with a period of six and three-quarter years. This comet, like Encke's, is only to be seen with the telescope. It presents no solid, or even well defined nucleus, and appears to be a mere vapoury mass, of exceeding tenuity. Taking into account the disturbing influence of Jupiter, the returns of Biela's comet, as predicted, agreed well with observation, and gave confidence in the theory on which the predictions were founded.

The return in 1832 excited the liveliest interest throughout the civilised world, in consequence of the fact that it was discovered from computation, that on the night of the 29th of October, this comet would pass a little within the earth's orbit, and those unacquainted

with the subject received the impression from this announcement, that the earth and comet would come into collision, producing the most terrific consequences. Such was the consternation excited, throughout the city of Paris especially, that the Academy of Sciences found it necessary to give to the subject their serious attention, and finally gave the matter in charge to M. Arago, who produced an elaborate report on the subject of comets gener

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ally, which served to calm the popular apprehension, and has proved to be a valuable addition to our knowledge on this difficult subject.

In this report, M. Arago showed that the comet would indeed cross the earth's track at the time predicted, but at the moment of crossing, the earth would be some fifty-five millions of miles distant from the point occupied by the comet, and could not experience the slightest possible influence from such a body, at such a distance.

If the comet had been delayed in its approach for thirty days by any disturbing cause, then indeed the earth and comet would have filled at the same time the point where their orbits intersect, and the dreaded collision would have taken place. The consequences of such a shock it is impossible to conjecture; but reasoning from the known physical condition of the comet, none of the terrible disasters so generally anticipated would have occurred. The exceeding rarity of the matter composing this body may be inferred from the statements of Sir John Herschel. "It passed," says he, "over a small cluster of most minute stars of the 16th and 17th magnitude; and when on the cluster presented the appearance of a nebula resolvable, and partly resolved; the stars of the cluster being visible through the comet. A more striking proof could not have been offered of the extreme translucency of the matter of which the comet consists. The most trifling fog would have effaced this group of stars, yet they continued visible through a thickness of cometic matter, which, calculating on its distance and apparent diameter, must have exceeded 50,000 miles, at least toward its central parts. That any star of the cluster was centrally covered, is indeed more than I can assert; but the general bulk of the comet might be said to have passed centrally over the group."

Such is the nature of the body from whose contact the ignorant apprehended the most fearful convulsions. Olbers, who studied the subject with great care, was disposed to think that in case the earth had passed directly through the comet, no inconvenience would have occurred, and no change beyond a slight influence on the climate would have been experienced.

It is useless to speculate with reference to the probable consequences of a collision, which there is scarcely one chance in millions can ever occur. Science has as yet discovered no guarantee for any planet against the possible shock of a comet ; but an examination of the delicate adjustments of our own system, and those of Jupiter and Saturn, would seem to indicate to us that in all past time no derangement has ever occurred from such a cause.

The last return of Biela's comet was marked by a phenomenon unexampled, so far as I know, in the history of these wandering bodies. True to the predictions of Santini, the comet first became visible on the evening of the 26th of November, 1845, and in the precise point which had been assigned by theory. De Vico, the director of the observatory at Rome, was the first to catch a glimpse of the expected comet. Nothing remarkable in its appearance was noticed until about the 29th of December, when Mr. E. C. Herrick, of New Haven, pointed out to several friends what he regarded to be a sort of anomalous tail, but shooting out from the head of the comet in a direction entirely at variance with the usually received theory, that the tail is always opposite to the sun. In this supposed tail a kind of knot was noticed, brighter and more condensed than any other part. Owing to insufficient optical power, the true character of the phenomenon was not fairly detected by Mr. Herrick.

On the night of the 12th of January, 1848, Lieut. Maury, in charge of the observatory at Washington, United States, discovered that what had hitherto appeared as a single body, was actually composed of two distinct and separate comets. This most extraordinary fact was immediately announced, and the double character was observed at all the principal observatories in Europe and the United States. There can be no doubt whatever as to the reality of the appearance. The comet actually became double, and the two parts, bound together by some inscrutable bond, continued their swift journey through space, pursuing almost exactly the route predicted for the single comet.

From measures obtained by Professor Challis, of Cambridge, England, on the 23rd of January, 1846, the two comets were separated from each other by a distance equal to about onethirteenth the apparent diameter of the sun. On the 28th of the same month, Sir John Herschel records the following notices ::-"The comet was evidently double, consisting of two distinct nebulæ, a larger and a smaller one, both round, or nearly so; the one in advance faint and small, and not much brighter in the middle; the one which followed nearly three times as bright, and one and a half times larger in diameter, and a good deal brighter in the middle, with an approach to a stellar point."

On the evening of the 9th of February, having returned to the observatory at Cincinnati, after an absence of more than two

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