THE MINOR PLANETS.

THERE is no subject of inquiry to which the improved powers of the telescope have been directed with greater effect than the investigation of the physical condition of the several planets composing the solar system. We shall on the present occasion take a review of some of these bodies, and shall state the chief circumstances which have been discovered respecting them.

In a general survey of the system, the planets composing it will naturally be classed in three distinct groups, the first of which we shall call the minor planets, the second the new planets, and the third the major planets.

Proceeding from the sun outward in the system, the four planets which are nearest to that luminary are Mercury, Venus, the Earth, and Mars. Between these bodies there prevails a striking analogy. We find that they are not very different in magnitude; that they correspond closely, so far as we can discover, in their geographical character; that they receive in not very different proportions the influence of the sun. The close alliance between them has also occurred to other astronomical writers, inasmuch as they are sometimes called the terrestrial planets, from their analogy to the earth.

OF THE PLANET MERCURY.

The planet Mercury revolves at a distance from the sun of about thirty-six millions of miles, completing his periodical revolution in about eighty-eight days, or something less than three of our months. The diameter of this planet is about three thousand two hundred miles, or four tenths of that of the earth, and consequently its volume or bulk is about a sixteenth of that of our globe. As Mercury revolves round the sun in an orbit enclosed within that of the earth, it follows that his illuminated hemisphere, which is always presented to the sun in the course of each revolution, must assume every possible variety of position in regard to the earth. Thus when Mercury is between the sun and earth as at A, in what is called inferior conjunction, his dark hemisphere is turned tow

the sun is never vertical, on the one hand, nor, on the other hand, is ever absent for twenty-four successive hours. How different must be the circumstances attending the planet Venus, if it be true, as there seems reason to believe, that the axis of that planet, instead of being inclined 2310 from the perpendicular, is inclined 75° from it. The polar circles would include a portion of each hemisphere, the extent of which would be five sixths of its entire breadth. Thus the greater portion of such a globe would be subject to vicissitudes somewhat similar to those which are incidental to our frigid zone, but the changes would be much more complicated. Within a certain space of such a planet, the sun would at one season of the year pass through the zenith, and the circumstances of the day would resemble those between our own tropics; while at another riod of the year, the sun would never rise for twenty-four hours. In fact, the polar circle would overlay the tropics, and the phenomena of each zone would alternately prevail at different seasons.

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The position of the axis of Mercury is not ascertained, but there is reason to believe that, like that of Venus, it is inclined at a very large angle from the perpendicular.

ORBITS AND TRANSITS OF MERCURY AND VENUS.

The motion of the planets Mercury and Venus, like that of the other bodies of the system, is very nearly in the plane of the ecliptic. The orbit of Mercury makes with the plane of the ecliptic an angle of 70, and that of Venus an angle of less than 40; the consequence of which is, that these planets are never seen much above or below the ecliptic. The apparent diameter of the sun is about half a degree; consequently the greatest distance to which Venus can depart from the ecliptic, will be less than eight diameters of the sun; and the greatest distance of the planet Mercury from it will be fourteen diameters of the sun. The points at which these planets are seen upon the ecliptic are called the NODES of the orbits; and if at the time they pass near these nodes they happen to be in inferior conjunction, they may be directly between the eye of the observer on the earth and the sun's disk. In that case, they would be seen as a black spot moving in the sun's disk. In order that this remarkable phenomenon, which is called a transit, should take place, it is obviously necessary that the distance of the disk of the planet from the place of the sun's centre should be less than half the sun's apparent diameter; that is, less than fifteen minutes of a degree. If, then, the distance of either of the inferior planets from the ecliptic at the time they are in inferior conjunction be less than fifteen minutes, there must be a transit; and the less that distance is, the greater the extent of the sun's disk over which the planet will be seen moving. If the planet be exactly in its node at the time of the inferior conjunction, then it will be passing directly across the centre of the sun.

It will be evident that the part of the sun's disk in which the planet is seen projected in a transit, will also depend on the position of the observer upon the earth. It may happen that, from some parts of the earth, the planet would not be projected upon the solar disk at all; and, in short, at different parts of the earth, the line of its projected course will necessarily be different. These effects will depend on the extent of the earth, and its distance from the sun and the planet.

These phenomena have, therefore, supplied a very happy expedient by which the distance of the sun from the earth may be exactly ascertained. The transit of Venus is especially applicable to this investigation, and has been used with signal success. When the transit of the planet occurred in 1769, observers were sent by different European governments to the most favorable