INTRODUCTORY

DEFINITIONS AND EXPLANATIONS.

GEOGRAPHY is a description of the Earth, and consists of four parts: MATHEMATICAL GEOGRAPHY, which treats of the form and magnitude of the earth, and of the position of its parts in relation to its own motions and to the heavenly bodies; PHYSICAL or NATURAL GEOGRAPHY, which treats of the natural features of the earth's surface; GEOLOGY, which describes the structure of the earth's substance, and the changes which take place at the surface and in the interior; and ORDINARY or POLITICAL GEOGRAPHY, which treats of the division of the land into countries, and of their inhabitants, towns, natural products, manufactures, and commerce.

DEFINITIONS, &c. IN MATHEMATICAL GEOGRAPHY.

THE EARTH, or world on which we live, is a round body, like a ball or globe. This is proved by the following circumstances:

1. Men have often sailed round it; that is, setting out from one place, they have returned to it without turning back-simply moving a little to right or left to avoid running upon the land. 2. It bulges out everywhere between us and a distant object, so that in receding from the object, its lower parts go first out of view, and in approaching it, its lower parts come last into view. This is best exemplified by ships sailing on the sea.3. The sun does not give light to the whole world at once, but rises successively later to places farther west; which proves that the world is round from east to west. In Britain the sun rises about one minute later for every ten miles west.-4. As we pro

ceed northwards, the stars round the north pole rise higher in the heavens, and sink as we retire southwards, while some stars disappear, and others come into view; thus showing that the world is round from north to south.-5. In eclipses of the moon, the earth's shadow always exhibits a circular edge, whatever part of the earth be turned towards the moon; and none but a round body can give a circular shadow in whatever position it may be placed. We may easily be satisfied that it is the earth's shadow thrown by the sun upon the moon that causes an eclipse of the latter; for there never is an eclipse of the moon, except when the sun, moon, and earth, are in a straight line, with the latter between the two former.

The HORIZON, or sensible horizon, is that circle all round, where earth and sky appear to meet. We cannot see the earth beyond it, nor the sky below it. It bounds or limits our view, and derives its name from a Greek word having this meaning.

The earth has a continual motion of ROTATION, like the spinning of a top, which is completed once every 23 hours, 56 minutes, 4.09 seconds. This is called the earth's diurnal motion: it takes place from west to east, and causes the whole heavens, including sun, moon, and stars, to appear to revolve round us from east to west daily.

When a body rotates, there is a line in it which simply turns round, but does not move out of its place, while every point in the body describes a circle round some point in that line. This line is called the axis of the rotating body. The earth's axis passes through its centre, and is 7899 miles in length.

The ends of the earth's axis are called the POLES of the earth. That nearest Europe is called the NORTH POLE; the other, the SOUTH POLE. No human being has ever been known to have been at either of the poles, which are inaccessible from the extreme cold, the perpetual snow and ice around them, and the consequent absence of animal or vegetable life. At each pole there is six months of day, and six months of night,—that is, six months when the sun is never above the horizon, and six months when he is never below it. These periods are between March 20 and September 23.

A great circle round the world, equidistant from both poles, is called THE EQUATOR. It is 24,897 miles long, and divides the world into two equal parts, called NORTHERN and SOUTHERN

HEMISPHERES. Any circle dividing a globe or sphere into two equal parts is called a great circle; others are termed small circles. The great circles are equal in length.

The Equator is a remarkable line. The people who live there have equal day and night during all the year, the sun rising and setting at six o'clock; the twilight is very short, so that it is dark almost immediately after sunset; the sun is right overhead there at midday on the 20th March and 23d September; the heat is great, and nearly equal at all seasons, the temperature being about 81° Fahr.; and all the heavens may be seen in one night—one-half immediately after sunset, the other half just before sunrise. It is "the Line" of seamen, who practise various ceremonies when they cross it. The world bulges out like an orange at the equator, which is consequently the largest circle that can be drawn round the earth: its diameter being 7925 miles, or 26 miles longer than the earth's axis. Latitude is reckoned from the equator.

Any great circle round the earth through both poles is called a MERIDIAN CIRCLE. Each meridian circle is 24,856 miles long, and divides the earth into two hemispheres, which may be termed east and west in respect to each other.

A MERIDIAN is that half of a meridian circle which extends from one pole to the opposite; and is said to be the meridian of any place through which it passes. People on the same meridian have midday at the same moment, midnight at the same moment, and their time is the same.

Each meridian circle contains two meridians, which are called opposite or lower in respect to each other; and their time is always twelve hours different.

PARALLELS of LATITUDE, called shortly PARALLELS, are small circles round the earth, each of which is everywhere at the same distance from the equator. The daily circle, which every place describes by the earth's rotation, is its parallel of latitude.

In the following figure, representing one-half of the earth's surface, or one hemisphere; let N be the north pole, S the south pole. Then, the outer line NQSE is a meridian circle; and each of the other lines is meant to represent a half-circle. EQ is half of the equator; Ac, mo, Cr, C'n, A'c', the halves of parallels; and all the lines from N to S are meridians.

In all maps or representations of the earth, the upper part is called the North; the lower, the South; the right side, the East; the left side, the West. Any place is said to be north of one below it, south of one above it, east of a place on its left, west of one on its right. The meridians run due north and south; the equator and the parallels east and west. The direction of north at any place may be found by looking towards the north polar star (see Astronomy); by the mariner's compass, which points nearly north; or by the position of the sun at midday, which is then in the south, or exactly opposite to north.

For the purposes of measuring and expressing distance on a circle, it is divided into 360 equal parts, called DEGREES, and marked thus (°);-40° means 40 degrees. To express still smaller parts, each degree is divided into 60 equal parts, called MINUTES, and marked ('); and each minute is divided into 60 equal parts, called SECONDS, and marked ("). Thus, 40° 27' 59" means 40 degrees, 27 minutes, and 59 seconds. In a semicircle there are 180 degrees; in the quarter of a circle, or quadrant, 90 degrees. All circles are divided in this manner, however much they may