As the orbit of this planet includes that of the Earth, it seems to move from west to east round the Earth. Its apparent motion is, however, very unequal; when it begins to be visible in the morning, a little after the conjunction, its motion is direct and most rapid; it becomes gradually slower, and the planet when it arrives at about 136° 48' from the Sun, is stationary; the motion then becomes retrograde, increasing in velocity till Mars is 180° distant from the Sun, or in opposition, so as to be on the meridian at midnight. This velocity then becomes a maximum, diminishes, and again becomes nothing, when Mars, approaching the Sun, is distant from it 1360 48. Its motion then becomes again direct, after having been retrograde during 73 days, and in this interval the planet describes an arc of retrogradation of about 16° 12', continuing to approach the Sun, it finishes by immerging in the evening into the Sun's rays. These singular phenomena are renewed at every opposition of Mars, but with a considerable difference as to the extent and duration of his retrogradations. See La Place's System of the World. The period in which all those changes take pla place, or the interval between two successive conjunctions, or oppositions, is about 780 days, which is the length of the synodical revolution of this planet.

The irregularities of Mars in its orbit, being the most considerable of all the primary planets, Kepler fixed upon it as the first object of his investigations respecting the nature of the planetary orbits; and after extraordinary labour, he at last discovered that the orbit of this planet was elliptical; that the Sun is placed in one of the foci; and that there is no point round which the angular motion is uniform. In the pursuit of this inquiry he found the same thing of the Earth's orbit; hence, by analogy, it was reasonable to think that all the planetary orbits are elliptical, having the Sun in one of the foci.

2. Continued and accurate observations show that the figure of Mars is not an exact sphere, but an oblate spheroid, whose axis or polar diameter is to its equatorial one as 1272 to 1355, or as 15 16 nearly. The mean diameter of Mars is about 4200 miles, and this planet revolves on its axis, from west to east in 24 hours, 39 minutes, and 21 seconds, which is the length of its day.

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Mars is of a red fiery colour, and gives a much duller light than Venus, though when he passes the meridian about midnight, Mars equals Venus in size.

The magnitude or size of Mars, according to Francœur, is .1886, its mass.1294, and its density .715076; the size, mass, and density of the Earth being respectively considered as unity, or 1. And a body weighing one pound on the surface of the Earth, will weigh nearly a quarter of a pound on the surface of Mars.

Mars is the smallest and most eccentric of all the ancient planets, except Mercury, and he is usually represented by the character, which is said to be rudely formed from a man holding a spear protruded, representing the god of war, which is the title of Mars in the heathen mythology. The red colour of this planet is ascribed to the density of its atmosphere; for the atmosphere which surrounds Mars is not only of great density, but of great height: that is, extends a great way from his surface, as appears from the occultations to which the fixed stars are subject on approaching his disc. Cassini observed a small star in the constellation Aquarius, at the distance of 6' from the disc of Mars, that became so faint before its occultation, that it could not be seen by the naked eye, nor by a three feet telescope.

3. The inclination of this planet's orbit to the plane of the ecliptic is 1° 51' 7"; and the inclination of its axis is 59° 41′ 50′′. Its mean distance from the Sun is nearly 15, that of the Earth being considered as 10; and the eccentricity of its orbit is about one-eleventh of its mean distance from the Sun.

From a series of observations, Dr. Herschel found that the poles of Mars were distinguished by very remarkable luminous spots. These he employed to determine the situation of the axis of the planet, and its inclination to the ecliptic, &c. Their magnitude and splendour were sometimes very considerable, but subject to very great variations. He supposes that they are produced by the reflection of the Sun's light from the snow near the poles; and that the variations in their size and brightness is owing to the melting of the polar ice.

The quantity of light and heat which Mars receives from the Sun, is only about half what the Earth receives from

that luminary; and the sun appears only half as large to Mars as to the Earth. The light or heat upon the surface of Mars depends, however, on the density and height of its atmosphere. To Mars, the Earth and Moon appear like two moons, changing places with each other, and appearing sometimes horned, sometimes half and three quarters enlightened, but never full; and never above a quarter of a degree from one another, although they are, at their mean distance 237,519 miles asunder.

If any satellite revolve round Mars, it must be very small, aş it has not yet been discovered, notwithstanding the great number of observations which have been made on this planet with the most powerful telescopes.

4. The variations in the apparent diameter of Mars is very great; its diameter when in opposition is 29", and its mean diameter about 9". Асcording to Dr. Brinkley, the apparent diameter, when Mars is in opposition, is 26", and in conjunction, 5".

These changes in the apparent diameter of Mars, prove that his distance from the Earth is continually changing. When Mars is in conjunction, he is never seen to transit or pass over the Sun's disc. He is not subject to the same limitations in his angular distance from the Sun as Mercury and Venus; but recedes from the Sun to all possible angular distances. He is sometimes in opposition; then the apparent diameter of Mars is nearly five times larger than when in conjunction; and, therefore, he is.five times nearer the Earth in the former position than in the latter. When Mars is viewed in a telescope, his disc changes its form and becomes sensibly oval, according to his relative position with respect to the Sun; sometimes appearing round, at other times gibbous, but never horned. These phenomena show, that Mars revolves in an orbit which includes that of the Earth; and that he receives his light from the Sun.

5. The telescopic views of this planet present a more diversified appearance than any of the other planets; the spots on its surface are at once numerous and extensive. Cassini observed several spots in both hemispheres, by which he determined