ELEMENTS OF GEOGRAPHY.

GEOGRAPHY is a description of the earth.

Mathematical geography treats of the earth as a part of the solar system; illustrates, from astronomical principles, its figure, magnitude, and motion; explains the construction of globes, with their application to the solution of problems; and describes the methods of projecting maps

and charts.

Physical geography embraces a descriptive view of the general features of the earth, or its natural history.

Civil or political geography delineates the empires, kingdoms, and states, which occupy the surface of the earth, and exhibits various monuments of human industry and skill.

MATHEMATICAL GEOGRAPHY.

THE SOLAR SYSTEM, The solar system is composed of the sun, the primary planets, the satellites, and the comets. The Sun is the source of light and heat to all bodies in the solar system. It is situated in the centre of the planets, which revolve around the sun in elliptic orbits, from

west to east.

There are 11 primary planets, namely, Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel or Uranus.

Ceres, Pallas, Juno, and Vesta are small primary planets. They have all been discovered since 1801, and their magnitudes and motions have not been perfectly ascertained.

Mercury and Venus are called inferior planets, because their orbits are included within that of the earth; the others superior, because their orbits are without that of the earth.

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Saturn is remarkably distinguished from the other planets, by being encompassed by a double ring.

There are 18 satellites or secondary planets. The Earth has 1, Jupiter 4/Saturn 7, and Herschel 6. These revolve round their primaries, and accompany them in their annual revolutions round the sun.

Comets are bodies revolving round the sun in very eccentric ellipses. The number of comets belonging to the solar system is unknown. In 1811, the number of those of which the elements had been calculated, was 103.

Table of the sizes, distances, rotations, and periods of the sun and planets.

Earth. The figure of the earth is spherical. This is evident, 1st, from the appearance of a ship wnen approach. ing the shore, the mast and topsails always being seen before the hull; 2dly, from the circular form of the earth's shadow, as observed in eclipses of the moon; and 3dly, from the actual circumnavigation of the globe.

The earth is not a perfect sphere, but is flattened at the poles. The equatorial diameter exceeds the polar diameter by upwards of 26 miles.

The annual revolution of the earth round the sun, in

connexion with the obliquity of the ecliptic, occasions the succession of the four seasons; the diurnal rotation on ts own axis every 24 hours, is the cause of day and night. This motion, being from west to east, causes the sun and stars to appear to rise in the east and set in the west, once in 24 hours.

Eclipses. An eclipse of the sun is caused by the moon's passing between the sun and the earth; and can never happen except at the time of new moon.

An eclipse of the moon is caused by the earth's passing between the sun and the moon; and can never take place except at the time of full moon.

Fixed Stars. Those stars, which always retain nearly the same situation with respect to each other, are called fixed stars. About 1,000 are seen by the naked eye in the visible horizon in each hemisphere; but by the use of a telescope, the number is immensely increased, and it increases in proportion to the magnifying power of the telescope.

GLOBES.

A sphere or globe is a round body, every point of whose surface is equally distant from a point within the body, called the centre. A hemisphere is a half globe.

There are two kinds of artificial globes, the terrestrial and the celestial.

The terrestrial globe is a representation of the earth, having the seas and different countries delineated on it, as they are on the surface of the earth.

The celestial globe is an artificial representation of the heavens, having the fixed stars drawn upon it in their natural situation.

The axis of a globe is an imaginary line passing through its centre from north to south, around which it revolves. The poles are the extremities of the axis terminated by the surface of the globe. One is the north, and the other the south pole; and are each 90° from the equator.

Great circles are such as divide the globe into two equal parts. These are the equator or equinoctial, the horizon, the meridians, the ecliptic, and the two colures,

Less circles are those that divide the globe into two un

equal parts. These are four, the two tropics, and the two polar circles.

The circumference of every circle is supposed to be divided into 360 equal parts, called degrees; each degree into 60 equal parts, called minutes; and each minute into 60 equal parts, called seconds. They are marked thus, 23° 28′ 16′′; i. c. 23 degrees, 28 minutes, and 16 seconds.

The Equator is an imaginary great circle of the earth, equi-distant from the poles, dividing the earth into the northern and southern hemispheres.

The Equator is divided by the principal meridian into two equal parts of 180° each.

Meridans are imaginary great circles passing through the poles, and perpendicular to the equator.

The meridian of the place, or the 12 o'clock hour circle, is the meridian that cuts the horizon in the north and south points. The first meridian of any country is the meridian passing through the capital of that country.

The Horizon is either sensible or real. The sensible horizon is a small circle which limits our prospect, where the sky and land or water appear to meet. The real horizon is a great circle which divides the earth into the upper and lower hemispheres.

The horizon is divided into four equal parts of 90° each, by the four cardinal points, East, West, North, and South. The wooden horizon of the artificial globe is divided into three parts; the innermost is marked with all the points on the mariner's compass; the next has the names, characters, and figures of the twelve signs; and the third is a calendar of months and days. By the two last is instantly seen the sign and degree the sun is in during every day in the year.

The Ecliptic is an imaginary great circle in the heavens, in the plane of which the earth performs her annual revolution round the sun.

The ecliptic is drawn on the artificial globe obliquely to the equator, making with it an angle of 23° 28'.

The twelve signs are the twelve equal parts, into which the ecliptic is divided, each consisting of 30°.