The periodical revolution of Vesta is 3 years, 2 months, and 5 days, and its distance from the sun 206,596,000 miles. These late discoveries indicate that there are probably other planets belonging to the Solar System, which have yet escaped the most discerning eyes of Astronomers.

COMETS.

Beside the planets, there are other bodies belonging to the solar system, which have more irregular motions. These are the Comets, which, descending from the far distant parts of the system, with inconceivable velocity, appear to us sometimes bright and round, like the planet Jupiter; sometimes with a transparent, fiery tail, projecting from the part opposite to the sun; and sometimes emitting beams on all sides like hair. They approach much nearer to the sun than any of the planets; and after a short stay, fly off again with equal rapidity, and disappear. Their orbits are more elliptical than those of the planets, and their bodies of much greater density than the earth; for some of them are heated to such a degree as would vitrify or dissipate any substance known to us. Sir Isaac Newton computed the heat of the comet, which appeared in 1680, when nearest to the sun, to be 2000 times greater than that of red hot iron.

The number, magnitudes, and motions of the comets, belonging to our system, are very imperfectly known. Conjecture has limited them to 450. The elements of 97 have been imperfectly determined. The periods of only three have been ascertained. They are found to return at intervals of 75, 129, and 575 years.

THE FIXED STARS

Are those heavenly bodies, which, at all times, preserve the same situation with regard to each other. Though they form no part of the solar system, yet they must be considered here, as they are of great use in the practice of geography. To facilitate their computations, astrono

mers consider these stars as all equally distant from our sun, and forming the concave surface of a sphere inclosing our system. They are distinguished from the planets by their twinkling.

A number of stars, which appear to lie in the neighbor. hood of each other, are called a Constellation. These are 90 in number, and are called by the name of some animal, or other object, whose figure the outline of the constellation is supposed so represent. Some stars are not included in any of the constellations, and are denominated unformed

stars.

According to their different apparent magnitudes, they are called stars of the first, second, third, fourth, fifth, or sixth magnitudes.

The GALAXY, or MILKY WAY, is a broad circle, encompassing the heavens, which appears to consist of innumerable stars, forming one continued dense cluster, and by their blended rays, diffusing a whiteness over that part of the firmament.

The number of stars is unknown; La Land estimates them at 75,000,000; but this is supposed to fall far short of their real number; and we have reason to suppose, that those which are visible bear no comparison in number to those that are invisible. They increase in our view, as the telescope approximates to perfection.

With respect to the distances of the fixed stars, they are so extremely remote, that nothing in the planetary system can compare with them. Though the diameter of the earth's orbit be not less than 190,000,000 of miles; yet this immense space makes not the smallest difference in their appearance; for they seem to be as large when viewed from the farthest, as from the nearest point of the earth's orbit. Their distance is too great for human beings to conceive, and gives rise to the pleasing and sublime conjecture, that each of the fixed stars is a sun, illuminating a complete system of planets, which revolve about it as their

centre.

ECLIPSES.

An Eclipse is a total or partial privation of the light of the sun or moon.

An eclipse of the moon is caused by its entering into the earth's shadow; and consequently, it must happen at the full moon, or when it is opposite to the sun.

An eclipse of the sun is caused by the interposition of the moon between the earth and sun, and, of course, must happen when the moon is in conjunction with the sun, or at the new moon.

The diameters of the sun and moon are supposed to be divided into 12 equal parts, called digits; and an eclipse is said to be so many digits, according to the number of those parts which are involved in the greatest darkness.

DOCTRINE OF THE SPHERE.

PRINCIPLES.

The fundamental principles of Geography are, the spherical figure of the earth; its rotation on its axis; the posi tion of its axis, with regard to the celestial luminaries; and its revolution round the sun.

1.

That the figure of the earth is spherical, is evident. Because such a figure is best adapted to motion. 2. From analogy; all the other planets are spherical. 3. From the appearance of its shadow, in eclipses of the moon, which is always circular 4. From its having been several times circumnavigated. It is not, however, a perfect: sphere, but an oblate spheroid, depressed or flattened at the poles, and elevated at the equator. Its diameter from east to west, is reckoned to be about 34 miles longer than that from north to south.

DEFINITIONS.

A Sphere literally signifies a ball or globe. The appar ent concave orb, which invests the earth, and in which all the heavenly bodies appear to be situated, at equal distances from the eye, is called the Celestial Sphere.

In Geography, the circles which the sun appears to describe on the concave surface of the celestial sphere, are

supposed to be extended to the earth, and marked on its surface. Each circle is divided into 360 equal parts, called degrees; each degree is divided into 60 minutes, and each minute into 60 seconds.

The circles, supposed by Geographers to be described on the sphere, are denominated great and small circles. Great circles divide either the celestial or terrestrial sphere, into two equal parts. Small circles divide the sphere into two unequal parts.

An imaginary line passing through the centre of the earth, from north to south, and on which its diurnal rotation is performed, is called the Axis of the earth. The extreme points of the axis are called the Poles.

A line or circle, passing round the middle of the earth, from west to east, is called the Equator, or Equinoxial. The equator divides the earth into Northern and Southern Hemispheres.

The Meridian is a circle, cutting the equator at right angles and passing through the poles of the earth. The me ridian divides the earth into Eastern and Western Hemispheres.

The Ecliptic is a great circle, in the plane of which the earth performs her annual revolution round the sun; or in which the sun appears to move round the earth, once in a year.

The Ecliptic is divided into 12 equal parts, of 30 degrees each, called Signs. The names of these signs, the characters which distinguish them, and the months in which the sun. enters into them, are as follows, viz.

The ecliptic makes an angle of 23° 28' with the equator, and intersects it in the two points and called the Equinoxial points, because, when the sun is in either of these points he shines equally to both poles, and the day is then equal to the night in all parts of the world. The times when the sun passes these points, are the 20th of March and 20th of September; the former is called the Vernal and the latter the Autumnal equinox.

The points and, where the ecliptic is at the greatest distance from the equator, are called the Solstitial points.

The sun passes the first of these, called the Summer Solstice, on the 21st of June, which is the longest day; the other, called the Winter Solstice, on the 21st of December, which is the shortest day.

The Horizon is a circle which separates the visible from the invisible part of the heavens. The circle, which limits our prospect, where the sky and land, or water, seem to meet, is called the Sensible Horizon. The Rational or Real Horizon is a circle parallel thereto, whose plane passes through the centre of the earth, and divides it into Upper and Lower Hemispheres.

The Horizon is divided into 32 equal parts of 11° 15′ each, called the Points of the Compass.

The four quartering points, East, West, North, and South, are called the Cardinal Points.

The point of the heavens directly over our head, is called the Zenith; and the opposite point, or that directly under our feet is called the Nadir.

The two meridians, that pass through the four cardinal points and cut each other at right angles, in the poles, are called Colures. That which passes through the first degrees of and is called the Equinoxial Colure; and that, which passes through the first degrees of and , is called the Solstitial Colure.

Circles drawn parallel to the equator, at the distance of 8° on each side of it, include the space called the Zodiac, which comprehends within it the orbits of all the planets. The Tropics are two circles drawn parallel to the equator, at the distance of 23° 28′ on each side of it. That on the north side touches the ecliptic in the sign and is called the Tropic of Cancer; that on the south side touches the ecliptic in the sign and is called the Tropic of Capricorn.