motions, certain imaginary lines are drawn round it, which are called great circles when they divide it into two equal hemispheres, and small circles when they divide it unequally.

The Great Circles are the Equator, Horizon, Meridians, Ecliptic, and the two Colures.

The Equator, a large circle, equidistant from the poles, divides the earth into a Northern and a Southern Hemisphere. The latitude of places is measured from it, north and south; and their longitude, on it, east and west.

The Horizon separates the visible half of the celestial concave from the half that is invisible, and is either rational or sensible. The rational, or true horizon, by which the rising and setting of all the heavenly bodies are determined, is an imaginary plane passing through the centre of the earth, and prolonged in imagination till it attains the region of the stars. Parallel to it, and coextensive with it, is the sensible horizon, whose plane is a tangent to the surface at the point on which the spectator is placed. These two planes, although separated throughout their whole extent by a semi-diameter of the earth, will yet, on account of the vast distance at which that interval is seen, be confounded together, and appear as one line in the heavens.

As applied to the earth, however, the sensible or apparent horizon is the small circle which terminates our view of the surface, where earth and sky appear to meet. It enlarges or contracts, according as the spectator's eye is elevated or depressed: thus, if the eye be elevated 6 feet above the sea, the circular expanse of water visible to it will be 3 miles in diameter. The Cardinal Points of the horizon are north, south, east, and west; the Zenith is the upper pole of our horizon, and the Nadir the lower pole.

The Meridians, or lines of longitude, are great circles passing through the poles, and cutting the equator at right angles. Each of them divides the earth into two hemispheres which, in respect to each other, may be termed east and west. There are 12 meridians commonly drawn on globes, each 15° apart, equal to a difference in time of one hour; and 18 meridians, on maps of the world, each 10° apart, corresponding to a difference in time of 40 minutes. But every place is supposed to have a meridian passing through it; and when the sun comes to that meridian, it is noon or mid-day at that place. The longitude of a place is its distance east or west from the first meridian, or that one from which we agree to count. This has varied with different nations, for nature supplies no particular spot from which longitude should be counted in preference to all others. Thus the French reckon from the meridian of Paris; the Spaniards, from that of Cadiz; and the English, from the Royal Observatory at Greenwich. When the latitude and longitude of a place are known, its exact position on the globe may at once be pointed out. The value of a degree of longitude varies according to the latitude, and is nowhere equal to a degree of latitude, except on the equator. At 60° lat. a degree of longitude is equal to 30 geographical miles,

or just the half of its length on the equator; while at the poles it vanishes to nothing.

TABLE SHOWING THE LENGTH OF A DEGREE OF LONGITUDE FOR EVERY 5 DEGREES OF LATITUDE IN GEOGRAPHICAL AND ENGLISH MILES.

The Ecliptic is a great circle, which represents the sun's apparent annual track among the fixed stars. It derives its name from being the circle on or near which the moon must be in the case of an eclipse. Its plane makes an angle of 23° with the plane of the equator. The sun is in the north, or highest point of the Ecliptic, on 21st June; and he is then vertical at the tropic of Cancer; he is in the south, or lowest point, on 21st December, and is then vertical at the tropic of Capricorn. The Ecliptic is divided into twelve equal parts, called signs, of 30° each, named from the constellations or groups of stars through which the sun appears successively to pass. These, with the days on which the sun enters them, are as follows:

The Colures are two meridians which divide the Ecliptic into four equal parts, marking the four Seasons of the year. One of them intersects the equinoctial points, Aries and Libra, and is thence called the Equinoctial Colure; the other intersects the solstitial points, Cancer and Capricorn, and is called the Solstitial Colure.

The Small Circles are the Tropics, the Parallels of Latitude, and the Polar Circles.

The Tropics are two small circles parallel to the equator, and at the distance of 234°, north and south. They are so named because the sun, arrived at them in his apparent annual course, seems to turn away, either northward or southward, as the case may be. The northern is called the Tropic of Cancer, and the southern the Tropic of Capricorn, because they coincide with the ecliptic in the beginning of those signs.

The Parallels of Latitude are small circles parallel to the equator, the object of which is to indicate the latitude of places, and to con nect together all places on the globe having the same latitude. Though on globes and maps of the world they are usually drawn at intervals of 10°, every place is supposed to have a parallel of latitude passing through it.

The Polar Circles are two small circles, drawn around the North and South Pole respectively the former being called the Arctic, and the latter the Antarctic Circle. Their distance from the Poles is 234, that being the angle formed by the earth's axis and a line drawn perpendicular to the earth's orbit. When the sun is vertical to places situated on the Tropic of Cancer, his rays extend beyond the Pole to the Arctic Circle, and all countries within the Antarctic Circle are then in darkness.

Zones.-The Tropics and Polar Circles divide the surface of the earth into five great Climatal Zones or Belts-viz.

1. One Torrid Zone, 47° in breadth, or 23° on either side of the equator, and bounded by the Tropics of Cancer and Capricorn. Every place in this wide region has the sun vertical to it twice a-year; and as the sun's rays never fall very obliquely on any part of it, the temperature at the surface of the earth is here always very high.

2. Two Temperate Zones, one northern and the other southern, each 43° in breadth, lying between the Tropics and the Polar Circles. Never having the sun vertical, they are characterised by a lower temperature than tropical regions; the fruits of the earth are less luxuriant and spontaneous; and man, compelled to exercise his corporeal and thinking powers, attains to a higher degree of intelligence and civilisation than in those regions where his wants are supplied without any exertion on his part.

3. The Two Frigid Zones, each 234 in radius, are included within the Polar Circles. They are deprived of the influence of the sun for long intervals in winter, and have a correspondingly greater length of day in summer, when his rays fall very obliquely on the surface. These conditions, coupled with the extreme cold of the long winters, are so unfavourable to human culture and human happiness, that the tribes who inhabit the frigid zone have not been able to attain to any considerable degree of civilisation.

PART II.

PHYSICAL GEOGRAPHY.

1.-MATERIALS, DENSITY, AND ATTRACTIVE POWER OF THE EARTH.-Of the interior of the planet which we inhabit we know almost nothing, our observation being confined to a portion of its external crust, or rind, rarely exceeding 10 miles in depth, or of the distance from the surface to the centre. Small as this portion is, when compared with the immense, volume enclosed by it, it presents to our view a vast variety of substances, each of which has a character peculiar to itself. On examination, they are nearly all found to be compound bodies, which, on being analysed, are reducible to sixty-two constituent elements.

These the chemist divides into two groups, -the Metallic, and the Non-metallic. The metals are fifty in number, the best known of them being gold, silver, copper, iron, lead, tin, zinc, and mercury; while the non-metallic class consists of only twelve, the principal of which are oxygen, hydrogen, nitrogen, carbon, sulphur, and phosphorus. Each of these elementary substances has properties peculiar to itself; and, what is more remarkable, on each of them the Creator has stamped, in deep and indelible characters, a particular number, which forms, as it were, the law of its being, and determines in what proportions it shall combine with other substances. This law of definite proportions serves in the mineral kingdom the same end as the laws which regulate the propagation of species do in the vegetable and animal kingdoms; the identity of species is rigidly preserved, and, notwithstanding the prodigious number of combinations, all confusion is avoided. The fundamental laws which regulate all chemical combinations are four in number: 1. All chemical compounds are definite in their nature, the ratio of the elements being constant. 2. When any body is capable of uniting with a second in several proportions, these proportions bear a simple relation to each other. 3. If a body, A, unite with other bodies, B, C, D, the quantities of B, C, D, which unite with A, represent the relations in which they unite among themselves, in the event of union taking place. 4. The combining quantity of a compound is the sum of the combining quantities of its components.

Each of the sixty-two elementary substances has a density or specific gravity peculiar to itself, ranging from hydrogen, which is

the lightest, to platinum, which is the heaviest; but the resulting mean density of the Earth is 5.67, or about five and two-thirds the weight of its own bulk of distilled water at the temperature of 60°. Thus, while the specific gravity of Mercury is nearly thrice as great, that of the Sun, Jupiter, Uranus, and Neptune, is four times less. As the specific gravity of the substances forming the crust of the Earth rarely exceeds 3.0, the obvious inference seems to be, that the interior of the planet cannot be hollow, but, on the contrary, must consist of materials in a highly condensed state.

103.

1

The Attractive Energy which the Earth exercises on all material substances near its surface is such that, when freely suspended, they are drawn towards it with a velocity of 16 feet in the first second of time; three times 16 feet the next second; five times 16 feet the third second; and so on, following the order of the odd numbers of the scale. Comparing the Earth, in this particular, with the other planets, we find that bodies falling towards the surface of Mars descend with only the half of this velocity; while in Jupiter the velocity is nearly three times greater. At great elevations above the surface the intensity of the force of gravitation decreases in the inverse ratio of the square of the distance. Thus, a body which (in a spring balance) weighs 16 ounces at the surface, will weigh only 4 ounces at the distance of two semi-diameters from the centre, or one semi-diameter above the surface; while at the distance of four semi-diameters it will weigh only 1 ounce. Under the surface the law of decrease is very different, it being there directly as the distance from the centre. Thus, at one thousand miles below the surface the body will weigh 12 ounces; half-way towards the centre, 8 ounces; at the distance of a thousand miles from the centre, 4 ounces; while at the centre the pressure on the balance will be nothing. The accompanying Diagram will render these observations more intelligible to the pupil.

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2. CONFIGURATION OF THE SURFACE.

Division into Land and Water. The surface of the earth, which, as we have already seen, comprises an area of 197,000,000 square miles, is very unequally divided into land and water. The total area of the land is estimated at 51,500,000 sq. m., or a little more than of the entire surface; while the waters cover 145,500,000 sq. m., or nearly of the whole. The land is, moreover, very unequally distributed over the surface: thus, the northern hemisphere contains three times as much land as the southern; the eastern hemisphere, or Old World, contains twice as much as the western; and if a great circle be drawn round the globe, having London as its centre, it will