that its orbit was elliptical, and its period of revolu tion so short as to fall fairly within the limits of per. petual examination.

In 1818, a comet was discovered by Pons, not at all remarkable for its magnitude, for it was even invisible to the naked eye, but when the attempt was made to represent its places by a parabolic orbit, which had thus far been invariably applied to the comets, it was found impossible to assign any elongated orbit which would embrace the observed positions of the comet. After a very elaborate investigation, Professor Encke at length reached the conclusion that the orbit was not a parabola, but an ellipse of comparatively small dimensions, and that this comet was actually revolving around the sun in a period of about three years. This discovery excited a great deal of interest, for it was the first in which a short period had been detected, and efforts were at once made to identify the new member of the solar system in its preceding revolutions. Olbers determined its identity with a comet which appeared in 1795, and subsequently ascertained that another, which had been observed but twice in 1786, and from which observations no elements could be computed, could be no other than the new comet of that period. In this way, observations on this interesting object were obtained, stretching through some thirty-three years, or about ten of its revolutions. This extended series of observations furnished the data for a critical examination of the elements of the comet's orbit, and Prof. Encke, having discussed them with elaborate care, reached the astonishing conclusion that the magnitude of the orbit was gradually diminishing, the periodic time growing shorter

from revolution to revolution, and that the comet was certainly falling nearer and nearer to the sun!

To account for this extraordinary phenomenon, the learned astronomer, having exhausted all causes known to exist in the solar system, finally, with much hesitation, announced the theory of the comet's motion in a resisting medium. The existence of such a me dium was in direct opposition to all the received doctrines of astronomy, and the absolute necessity for its use in this instance was looked upon by astronomers with feelings of strong distrust. But Encke argued that such a medium might exist, of such exceeding tenuity as not sensibly to affect the movements of the ponderous planets, while a filmy mass of vapor, such as this comet undoubtedly was, might be very sensibly retarded in its original velocity, which would diminish continually the centrifugal force, and give to the central attraction of the sun a constantly increasing power, which would produce precisely the phenomena exhibited by the comet.

With these views, Encke predicted the reäppear ance of the comet in 1822. In consequence of its great southern declination at that period, it escaped all the European observers, and was 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 favorable for deciding the question, which had now become one of the deepest interest.

Its reäppearance in 1828-9 was awaited with great anxiety by the friends and opponents of the new

theory. The comet came round, and passed its per helion approximately in accordance with the predic tions, 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 ellipse, 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 ref.

orence 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 reëxamined 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 di rection 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 favor 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, Prof. 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