grade movement, and thus the comets were seen to oscillate about each other, according to some mysterious law which has never been revealed. Such is a brief sketch of the phenomena presented by Biela's comet in its late return. Its next appearance will be looked for with deep interest, to confirm or destroy certain theories which have been propounded to explain its duplex character.

While the periods of the comets which we have thus far considered are comparatively short, those of others which have visited our system have been ascertained to extend to many thousands of years. The great comet of 1811, one of the most brilliant of modern times, in consequence of its remaining visible for nearly ten months, gave ample opportunity for the investigation of the elements of its orbit. After a careful investigation, M. Argelander fixes its period of revolution at 2,888 years. Bessel had examined the same subject previously, and probably with less attention, but obtained a period even greater than Argelander's, amounting to 3,383 years.

The comet of 1807 also occupied the attention of Bessel. A long series of observations furnished the data for computing its elements. The periodic time was fixed at 1,543 years. These computations are necessarily only approximate. The difficulty of ob taining accurately the periodic time increases with the length of the period, and all that can be done is to fix a limit below which it cannot fall. These vast periods give to us the means of learning somewhat of the great distance to which these objects penetrate into space. The comet of 1811, having a period probably three thousand times greater than that of

our earth, must revolve at a mean distance from the sun of more than 80,000,000,000 of miles, and in consequence of its very near approach to the sun at its perihelion, its greatest distance cannot fall below 160,000,000,000 of miles!

Great as this distance is, it is perfectly certain that there are many comets which revolve in orbits far more extensive than the one described by the comet of 1811. Indeed, there seems to be no limit to the distance to which these bodies may sweep outward from the sun; and their return depends simply on the fact whether they recede so far as to fall within the attractive influence of some other sun, towards which they begin to urge their flight, and through whose system of planets they carry the same apprehensions of danger which have been caused in our own.

In reflecting on these singular objects, we are led to inquire what they are, whence their origin, and by what laws are the vast trains of light which occasionally distinguish them developed? Arago divides comets into three classes, with reference to their physical constitution. He thinks they occasionally appear round, and with well defined planetary discs, showing them to be soild opaque bodies, in all respects resembling planets, and only differing from these in the great eccentricity of their orbits. In confirmation of this opinion, he asserts that comets have been seen to transit the sun, and when passing between this luminary and the eye of the spectator, they appear round and black, like the planets Mercury and Venus, when seen under the same circumstances. An example of this kind occurred on the 18th of November, 1826, when the transit of a comet across the sun was wit

nessed by two persons, widely separated from each other.

A second class of comets comprehends those in which there is a nucleus, but devoid of opacity, permitting the light to penetrate through even that portion which may possibly be solid. The third class, and that by far the most numerous, comprehends those comets destitute, entirely, of any solid nucleus, consisting of matter so attenuated as to compare fairly with nothing of which we have any knowledge on the earth's surface. The comets named for Encke and Biela appear to belong to this class; and even Halley's comet, according to the opinion of Sir John Herschel, seems, at its last return, to have been entirely turned into vapor in its perihelion passage.

No theory, with the exception of Laplace's nebular hypothesis, has ever been framed to explain the origin of these wandering bodies. This is not the place to enter into a full development of this subject.—A few hints only can be given. Laplace, following up the speculation of Sir William Herschel, applied the theory of that astronomer to the formation of the solar system, comprehending the comets, as well as the planets and their satellites. This theory supposes that the original chaotic condition of the matter of all suns and worlds was nebulous, like the matter composing the tails of comets. Under the laws of gravitation, this nebulous fluid, scattered throughout all space, commences, to condense towards certain centres. The particles moving towards these central points, not meeting with equal velocities, and in oprosite directions, a motion of rotation is generated in

the entire fluid mass, which, in figure, approximates the spherical form.

The spherical figure once formed, and rotation commenced, it is not difficult to conceive how a system of planets might be produced from this rotating mass, corresponding, in nearly all respects, to the characteristics which distinguish the planets belonging to our own system. If, by radiation of heat, this nebulous mass should gradually contract in size, then a well known law of rotating bodies would insure an increased velocity of rotation. This might continue until the centrifugal force, which increases rapidly with the velocity of the revolving body, would finally come to be superior to the force of gravity at the equator, and from this region a belt of nebulous fluid would thus be detached, in the form of a ring, which would be left in space by the shrinking away of the central globe. The ring thus left would generally coalesce into a globular form, and thus would present a planet with an orbit nearly, if not quite circular, lying in a plane nearly coincident with the plane of the equator of the central body, and revolving in its orbit in the same direction in which the central globe rotates on its axis.

As the globe gradually contracts, its velocity of rotation continually increasing, another ring of matter may be thrown off, and another planet formed, and so on, until the cohesion of the particles of the central mass may finally be able to resist any further change, and the process ceases.

The planetary masses, while in the act of cooling and condensing, may produce satellites in the same manner, and by the operation of the same laws by

which they were themselves formed. Strange and fanciful as this speculation may appear, there are many facts which tend strongly to give it more than probability. It accounts for all the great features of the solar system, which, in its organization, presents the most indubitable evidence that it has resulted from the operation of some great law.

The sun rotates on an axis in the same direction in which the planets revolve in their orbits; the planets all rotate on their axes in the same direction; they all circulate around the sun in orbits nearly circular, in the same direction, and in planes nearly coinci dent with the plane of the sun's equator. The satel iites of all the planets, with one single exception, revolve in orbits nearly circular, but little inclined to the equators of their primaries, and in the same direction as the planets. So far as their rotation on axes has been ascertained, they follow the general law.— In one instance alone we find the rings of matter have solidified in cooling, without breaking up or becoming globular bodies. This is found in the rings of Saturn, which present the very characteristics which would flow from their formation according to the preceding theory. They are flat and thin, and re volve on an axis nearly, if not exactly, coincident with the axis of their planet. Their stability, as we have seen, is guaranteed by conditions of wonderful complexity and delicacy, and the adjustment of the rings to the planet, (humanly speaking), would seem to be impossible after the formation of the planet.— At least it is beyond our power to conceive how this could be accomplished by any law of which we have