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derful, because more uncommon, phenomena of the natural world, but leading its student to observe and to reflect, to derive subjects for thought and admiration from all that surrounds him; thus enhancing his pleasure, both in the perusal of every description of the earth's surface, and in the contemplation of all the works of creation. Evident traces of beneficent design present themselves on every side,—the student himself may perceive them; the more he knows the more does he wish to know,-his spirit becomes athirst for knowledge; and it is well that it should be so,-for

The desire which tends to know

The works of God, thereby to glorify

The great Work-master, leads to no excess
That reaches blame, but rather merits praise
The more it seems excess.

He has entered on a vast and inexhaustible field, full of
treasures, which those who seek shall find, hidden treasures
of wisdom, power, and goodness: and if these are pursued
with a proper spirit, he will learn that "the mind employed
on that which is great, becomes itself greater."

CHAPTER II.

THE EARTH; ITS POSITION IN THE UNIVERSE, FIGURE, MAG-
NITUDE, AND DENSITY; GENERAL STRUCTURE OF ITS SUR-
FACE. PHENOMENA OF EARTHQUAKES AND VOLCANOS.

THE earth belongs to a system of planetary bodies, of which the sun forms the centre, and which is hence termed the solar system. We are at present acquainted with eleven

principal or primary planets, which revolve round the sun, and eighteen secondary planets, called moons, or satellites, revolving round the planets of which they are the attendants. The orbit of the earth, or path in which it moves round the sun, is elliptical, though it differs very little from a circle; the mean, or average distance of the earth from the sun, being ninety-five millions of miles.

The earth is of a globular form; its figure, however, is not that of a perfect sphere, but somewhat elliptical, or flattened at the poles, though the flattening, or difference of diameter is so trifling, that "if a model of similar proportions were turned in wood," says Sir John Herschel, "and placed before us on a table, the most correct eye or hand would not detect the deviation from the spherical form."

The globular form of the earth is proved by the circumstance that ships steering in a general direction, either eastward or westward, arrive again at the point of departure. Few individuals, it is true, have it in their power to verify this in their own experience, but there is another mode of observation, by which many may have the opportunity of actually perceiving the shape of the earth; this cannot, however, be accomplished with certainty on land,—the hills, trees, and other objects, which diversify its surface, and break and vary the line of the horizon, although bearing a most minute proportion to the whole earth, are, nevertheless, too considerable with respect to ourselves, to admit of our forming any judgment of the whole. But with the surface of the sea, the case is otherwise; if we stand upon the sea-shore, we observe the surface of the ocean, not losing itself in misty distance, but terminated by a sharp, clear, well-defined line, or offing, as it is called, which constitutes the visible horizon. If we sail out of sight of land, this offing extends in a circle round us, of which our station in the vessel forms the centre. The appearance of this visible horizon, or sea-offing, is a consequence of the curvature of the surface.

That such is the case, is in fact proved by the pheno

1

mena exhibited by vessels receding from the shore, and sailing beyond our visible horizon.

The vessel, as it recedes, diminishes in size, but the whole is visible until it reaches this water-line. Beyond this, though the masts and sails still remain in view, the hull of the ship is below the horizon; the portion of the ocean on which it is floating, being concealed from our view by the spherical surface of the sea, which protrudes between the hull and ourselves. If the vessel proceed a little further, the lower sails disappear, and only the upper are descried; and at length the whole is lost to our view. If, however, we immediately ascend some eminence, (the mast of a ship, if at sea-a tower, or the upper rooms of a house, if on land,) the vessel will again be visible, the elevation we have reached enabling us to see to a greater distance; and this will be in exact proportion to the height above the water, both of the object and of the observer. It is on this principle that, when a vessel is outward-bound, and

The sailor sighs as sinks his native shore,

And all its lessening turrets bluely fade:

He climbs the mast to feast his eyes once more.

When the eye is elevated six feet above the water, an object may be seen on its surface at the distance of three miles. Two points, each ten feet above the level of the water, cease to be visible to each other at a distance of eight miles. If we ascend any great elevation, such as the Peak of Teneriffe, Mowna Roa, (in the island of Hawaii, one of the Sandwich group,) or any other mountain com

manding a vast extent of ocean, the distance at which any object is visible is proportionably increased, and the observer commands a much larger portion of the earth's surface. Thus, from the Peak of Teneriffe, or from Mowna Roa, a four-thousandth part of the whole surface of the earth is visible. The greatest extent of the earth's surface ever seen at once by man, was that displayed to the view of MM. Biot and Gay Lussac, in their celebrated aëronautic expedition, to the height of 22,900 feet above the sea; the visible area, in this case, having been a sixteen-hundredth of the earth's surface. On ascending to great elevations, while the absolute visible range, or quantity of the surface seen at once, has increased, the apparent size of the earth will have sensibly diminished. The visible horizon invariably extends in a circle round the observer; the same appearances being observed universally, in every part of the earth's surface, that have been visited by man. Now, the figure of a body, which, however seen, appears always circular, can be no other than a sphere, or globe.

The earth, as already stated, is not a perfect sphere, but a spheroid; its ellipticity, or the deviation in its figure from that of a perfect sphere, being equal to about one threehundredth part; the compression at the poles being estimated at twenty-six miles in the diameter of the whole earth. Its greater, or equatorial diameter is, therefore, 7,926 miles, nearly, and the polar diameter 7,899 miles. The equatorial circumference of the earth is a little less than 25,000 miles.

The revolution of the earth on its axis is performed in a natural day, or, more correctly speaking, once in 23h 56′ 4′′; any point on its equator having a rotatory motion of more than 1000 miles per hour. This velocity gradually diminishes as we approach the poles, where it altogether ceases. In the parallel of London, the rate of this motion is 648 miles in the hour. Whilst

The planet earth, so steadfast though she seem,

is thus revolving on her axis, she has also a progressive mo

tion in her orbit, and is therefore at the same time passing through space with almost inconceivable velocity: for, the length of the earth's orbit being estimated at 600,000,000 miles, her motion must exceed 68,400 miles in the hour.

The subject of the density of the earth has long occupied the attention of the learned. It is supposed to increase towards the centre, the average density of the rocks on the surface scarcely exceeding two and a half times that of water, whilst the mean density of the whole earth is equal to about five times that of water.

The rocks on the earth's surface are, as we have seen in our review of geology*, divided into two great classes, the aqueous and the igneous. Aqueous rocks are such as appear to have been formed by gradual deposits in water, and to have become more or less consolidated, or hardened into solid rock; such are sandstones, clays, limestones, &c. Igneous rocks are such as appear to have been formed through the agency of heat, by the melting down or fusion of the materials of which they are composed. The principal rocks of this class are granite, basalt, lava, &c.

Granite is a very universally-diffused rock; not that it appears at the surface in every locality, being in many places concealed from our view by beds of aqueous rocks of enormous thickness; but it is supposed to constitute the base on which all other rocks have been deposited. In many places, however, it towers pre-eminently above all other rocks; thus forming the summits of some of the loftiest mountains on the face of the globe.

The aqueous rocks, which are also called stratified, are arranged in three divisions, primary, secondary, and tertiary. The primary are very widely spread over the earth's surface, almost all lofty mountain ranges being in part formed of rocks of this description. They are the principal, though by no means the sole depositories of metallic ores, but are devoid of fossils. The secondary, and tertiary rocks-which again have their appropriate sub-divisions, each characterize

*Recreations in Geology, pp. xxix., xxxi.