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in its consequences. Since stars appear to be suns, and suns, according to the common opinion, are bodies that serve to enlighten, warm, and sustain a system of planets, we may have an idea of numberless globes that serve for the habitation of living creatures. But if these suns themselves are primary planets, we may see thousands at different periods with the naked eye, and millions with the help of telescopes; and at the same time, the analogical reasoning still remains in full force with regard to the planets which these suns may support. See Transactions of the Royal Society for the years 1794 and 6. But to return to the Solar System.

The planets perform their revolutions about the sun, in elliptical curves, differing but little from circles, and of which the centre of the sun, or rather the common centre of gravity of the whole system, occupies one of the foci. Each of the planetary orbits is in a plane, which passes through the centre of the sun. The plane of the earth's orbit, that is, the path along which it travels, is denominated the plane of the ecliptic; it is that circle, which, to an inhabitant of the earth, the sun appears to describe in a year. By considering this plane indefinitely extended on all sides, the planes of the orbits of the other planets may be supposed to be cut by it; and hence the positions of these, that is, the angles of their obliquity, are all referred to the ecliptic, or plane of the earth's orbit, as a scale by which they are measured. Thus the inclination of the orbit of Mercury to that of the earth is 7°, that is, one half of the orbit of Mercury rises above the orbit of the earth, and the other half falls below it. The following easy illustration of this principle has been given in some of our elementary treatises. Suppose a bowl, or other concave vessel nearly filled with water, the surface of which, when at rest, will trace a circular line round the inner surface of the bowl, which may represent the elliptic, while the surface of the water is the plane of the ecliptic, and the bowl one half of the concave sky. If now a circular ring, hoop, &c. be immersed in the vessel of water, so that one half of it rises

above the surface, at an angle of 7°, the other half will be as much below, and the ring may represent the orbit of Mercury. Another ring rising above the surface of the water at an angle of about 31° will represent the orbit of Venus; and so of the rest.

All the planets move in their orbits from west to east. The velocities with which they move, are not invariable, but the areas described by their radii vectores, that is, by lines supposed to be drawn from them to the centre of the Sun, are always equal in equal times, or proportional to the times of description. Therefore the motions of the planets are likewise so much the more rapid, as they are less remote from the sun; so that the magnitude of the orbit, and the slowness of the motion, concur in augmenting the durations of their sidereal revolutions.

Mercury and Venus are nearer the sun than the earth, therefore by us they are called inferior planets; those, which move in orbits beyond the orbit of the earth, are called superior planets. To the inhabitants of other planets, the terms inferior and superior, will be differently applied: to the earth there are, independently of the four small planetary bodies lately discovered, two inferior and four superior planets; but to an inhabitant of Jupiter, there are four inferior planets, and two superior.

Mercury is about 3,224 miles in diameter, and revolves round the sun in 87 days, at the distance of $7 millions of miles from that body. He is not often seen by the inhabitants of the earth, on account of his nearness to the sun, and to whom he always appears on the same side of the heavens with the sun; of course he can be seen in the east, only in the morning a little before sun-rise; and in the west, in the evening a little after sun-set. When Mercury is viewed with a telescope of high magnifying powers, he exhibits nearly the same phases as the moon, being sometimes horned, and sometimes full, &c. It has never been completely ascertained whether he turns on his axis, though Schroeter suspected that he had discovered

his diurnal period to be equal to more than 24 of our days. The characteristics of this planet are thus described by the poet Mallet:

-Mercury, the first,

Near bordering on the day, with speedy wheel

Flies swiftest on, inflaming where he comes
With seven-fold splendour.

The seven-fold splendour refers to the quantity of light and heat which he enjoys, compared with the earth: "for light and heat are always diffused round the central body inversely as the squares of the mean distances of the planet from the sun;" thus Mars is about twice the distance from the suu that the earth is, and will enjoy only one fourth as much light and heat from the sun as we enjoy. So in calculating the quantities of light and heat enjoyed by Mercury, compared with what we enjoy, and knowing his distance from the sun to be 37,000,000, and the distance of the earth to be 95,000,000, we say, by the proposition above, calling the light or heat enjoyed by the earth as unity,

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The diameter of Venus is not quite so large as that of the earth, being about 7,648 miles in length: she performs her revolution about the sun in 224 days, at the distance of 68 millions of miles from the sun; so that she will enjoy twice as much benefit from the sun with regard to light and heat, as we enjoy ;

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for it will be as above 68) With almost any telescope this beautiful planet exhibits all

Venus is an evening

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the same phases as those of the moon. star, when she appears east of the sun; when she is west of him. Twice in the course of 120 years, Venus passes over the disc of the Sun; this phenomenon

is denominated the transit of Venus; and, by means of it, astronomers have been enabled to ascertain, with great accuracy, the distance of the earth from the Sun. Venus is thought to turn on her axis in about 23 hours and 20 miles: she is said to have an atmosphere fifty miles in height. nation of the orbit of Venus to the plane of the in an angle of nearly 4 degrees.

The incliecliptic, is

The earth, like the other planets, is spherical, but not an exact sphere. Its diameter is about 7,950 miles in length, but the diameter at the equator is 37 miles longer than that from pole to pole. It turns on its axis in about 24 hours, which causes the succession of day and night, every part of it being in that time, successively brought into, and continued in the light of the Sun. It has another motion in its orbit round the sun, which is performed in 365 days, 5 hours, 48 minutes and 49 seconds.

The diameter of the earth, in its course round the sun, is not perpendicular to its orbit, but inclined to it in an angle of 234°; it is owing to this that the days and nights are of different lengths, in different parts of the globe. The seasons of spring, summer, autumn, and winter depend, (1) on the length of the days and nights; and (2) upon the position of the earth with respect to the sun, that is, upon the perpendicularity with which the rays of the sun fall upon any particular part of the earth. We have observed that the motions of the planets in their orbits, are more rapid in proportion to their nearness to the sun thus the velocity of Mercury is found to be at the rate of 105,000 miles in an hour; that of the earth, at the rate of 68,000 miles in an hour; and the velocity of Mars, is equal to 53,000 miles in an hour. The velocity of the same planet is different in different parts of its orbit: it moves faster when nearer the sun, and slower when farther from it.

Mars, the first of the superior planets, is distinguished from the rest by the redness of its colour, a circumstance that has been attributed to the density of its atmosphere. His figure, like that of the earth, is an oblate spheroid; and he travels

round the sun in about 687 days, at the mean distance of 144 millions of miles from that body. He has likewise a rotation on his axis in 24 hours, 39 minutes, a fact that has been discovered by means of an immensely large spot seen on his surface, when he is in that part of his orbit which is opposite to the sun and the earth. The orbit of Mars is inclined to the plane of the ecliptic in an angle of nearly two degrees.

The planet Ceres, if this and the three following be planets, when viewed through a good telescope, is of a ruddy colour, and appears to be of the size of a star of the eighth magnitude. It performs its revolution about the sun in 4 years, 7 months, and 10 days, at the mean distance of 260 millions of miles from that body. The magnitudes of this and the other bodies between Mars and Jupiter are not by any means clearly ascertained. Dr. Herschel makes the diameter of Ceres to be only 160 miles, whereas Schroeter makes it more than ten times as large, or 1,624 miles in length. The inclination of its orbit to the plane of the ecliptic is in an angle of more than 10 degrees.

Pallas is nearly of the same magnitude with Ceres, from which, and from all the other planets it is chiefly distinguished by the great inclination of its orbit to the plane of the ecliptic, making an angle of 34° nearly with it. Its mean distance from the sun is 266 millions of miles; the length of its year, which depends on its distance from the sun, is a little greater than that of Ceres.

Juno is of a reddish colour, and is less than either Ceres or Pallas, and is nearer to the sun than either of them: its mean distance from it being but two hundred and fifty-three millions of miles, and its annual revolution is equal to 4 years and 128 days of our time. The inclination of its orbit is 21 degrees. Schroeter has observed a very remarkable variation in the brilliancy of this planet, which he thinks is owing to some changes that are going on in its atmosphere, though it may, he says, arise from a diurnal rotation about its axis in 27 hours. Vesta appears to be as large as a star of the fifth or sixth

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