But we have yet to say a few words respecting Comets. These bodies were so termed from the Greek noun (kome) the hair, doubtless in order to designate the appearance of the nebulous or dusky light with which they are attended, or of which some of them appear wholly to consist. Comets do not, like the planets, all move in the same directions. Some move from west to east, some from east to west; while others dash down from the upper regions of space, and passing round the Sun mount up to their former elevation; and the paths they describe, instead of approaching the circle, considerably resemble what is termed by the geometrician, the parabolic curve; which, in familiar language may be described as a long narrow oval. Some of the comets approach the Sun, when in the narrowest part of the oval, much nearer than any of the planets ;-but when in the opposite extremity of the oval, they are prodigiously more distant from the Sun than any of the planets. Nearly 500 comets are recorded to have been seen, and the paths of about 100 have been determined. Two or three have been recog

nized on their return.

In this short yet comprehensive view of the Solar System, there is indeed enough to astonish by its magnificence, and to affright by its apparent uncertainty. In regard to the planets, all is order and harmony; but as the movements of comets are less understood, is it not a reflection sufficient to cause a certain shrinking at the heart, that about 500 of them are perpetually dashing around the Sun in every direction; lest two enormous bodies, proceeding with inconceivable rapidity, should meet together and by their concussion destroy the equal balance of the whole system? But we may confidently rely

on this-that the wisdom and power which spake them into existence, spake them also into order, though their precise order be not yet wholly understood. We have already learned enough to convince us that this wisdom and this power are unlimited; and as we proceed, we shall be further assured that, from their exertion, have resulted a series of causes and effects most propitious to our welfare.

The direction in which a planet, or a comet moves round the sun has been denominated its orbit, from the Latin, orbis, a circle, or hollow sphere; and in most books treating on Astronomy, the orbits of the planets are drawn as though they were regular circles, which is not a just representation. It has been said that not one of the planets moves in a circle, but that each describes an ellipsis or oval. Several of the orbits, however, approach very nearly to the circle; as is the case with those of Venus, the Earth, Mars, Jupiter, Saturn, and the Georgium Sidus ;-but the lesser planets, Mercury, Juno, Vesta, Ceres, and Pallas, describe ovals that differ greatly from the circle. It is a fact, meriting our recollection, as arising from the long and often repeated observations of astronomers, that each of the planets moves, and has continued for ages to move, in the same track (if it may be so said) round the Sun; in other words, each of the planets continues to move (with slight occasional variations from causes hereafter to be noticed) over or between the same stars in its progress round the Sun. The size, and even the form of the oval described by any one of them differs from all the rest; and as the orbit of each planet is so invariably the same in respect of size and figure, we might conclude that the Sun is in the centre of each orbit. In forming this conclusion we

should err, for though in one particular part of each of these ovals, there is a spot that may be termed the station of the Sun, which never varies, yet he is not stationed in the middle of any one of them. This is a point essential to be fully comprehended, and retained in the recollection. If we place two pins at some distance from each other upright, in a sheet of pasteboard, and put round them a thread tied at each end; then if a pencil be carried round within the thread, its point will describe an oval, or ellipsis on the pasteboard. The places occupied by the pins, therefore, form two important points in the oval, and are termed its foci, or focuses: precisely in one of these foci is the station of the Sun-and his station is always called the lower focus of the orbit. In the middle between these two foci, is what may be termed the centre of the oval or orbit; and the distance from the centre to either of the foci, is termed the eccentricity of the planet. The more nearly the orbit approaches the circle, the less will be the eccentricity. That of the Earth's orbit is small, but that of Venus is still less, because it more nearly approaches the circle; while the eccentricity of Mercury, Juno, and Pallas is great, because the two foci of their orbits divide them nearly into four equal parts.

The first of the two preceding figures then may serve to represent the orbit of the Earth or of Venus; and the

latter, in which the eccentricity is greater, that of Mercury, Juno, or Pallas.*

The eccentricity of Mercury ....is 7,565,940 miles

In familiar language, the orbit of a planet might be termed its path, if that term did not rather tend to give the notion that the orbit is solid, which is not the fact.

If then we imagine a spectator to be placed far above the Sun, he would see the primary planets revolving in orbits more or less approaching to the circle; and the comets in long narrow ovals, one of the narrow ends of which would pass near the Sun, while the opposite end would be at an immense distance from him. But let us suppose the situation of the spectator to be

* A line drawn across the larger diameter of the orbit of a planet, and connecting the two opposite and most distant parts, is called the transverse axis; the extremities of the line are termed the apsides, and are the places of perihelion and aphelion—of the planets nearest approach to, and greatest distance from the Sun. Another line drawn across the lesser diameter of the orbit, and therefore connecting its two nearest places, is termed the conjugate axis. When a planet is in either of the two places at which the extremity of the conjugate axis touches the orbit, it is then at its mean distance from the Sun.

changed; let us suppose him to be placed not above or below, but on one side of the Sun, so as to observe the nearly horizontal motions of the planets, and he would see that their orbits are not on the same plane. We may familiarly explain this, by saying, that if a number of wires, bent into the form of ovals resembling the orbits of the planets, were placed within one another upon a table, we should have a representation of them, all on the same plane,-the table being the plane; they would all be in a perfectly horizontal position. But it has been ascertained that the orbits of the planets are not so. Let us however suppose one of them to be horizontal, in order to shew how much the others differ from it. Let the horizontal one be the orbit of the earth; and as the rest all incline more or less from the horizontal direction, the difference is termed the inclination of the orbit.

But as

It has been said that Mercury is nearer the Sun than the Earth is. Let the outer of the two ovals in the first of the two preceding figures, represent the orbit of the Earth, and the inner one that of Mercury. In this figure the two orbits are on the same plane. astronomers for the sake of convenience, always suppose the orbit of the Earth to be in a horizontal position, let the outer oval shew it in that position in the second of the above figures, and the inner oval, which represents the orbit of Mercury, is in that figure inclined to the orbit of the Earth;-it shews the inclination of the orbit of Mercury.