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 characterized

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

by distinct assemblages of organic remains-are of less universal occurrence, and frequently vary in their composition in different localities. Allowing, however, for these local diversities, a general correspondence is observable in the structure and arrangement of rocks in all parts of the earth, and indications everywhere present themselves, that similar agency has been employed in the formation of the whole.

A layer of alluvial soil, or of soil deposited by water, &c. and of recent formation, usually constitutes the upper coating of the earth's surface, especially in plains and valleys. This deposit is chiefly formed of particles abraded, or worn from older rocks, by the action of water, of the atmosphere, or of electricity; the whole surface of the earth being thus in a greater or less degree, in a state of perpetual change. Within this wide great universe

Nothing doth firme and permanent appeare,

But all things tost and turnéd by transverse.

Water is a very powerful and active agent in altering and variously modifying the surface of the earth. The atmosphere, also, effects great changes on rocks; the differently shaped summits of mountains and hills, owing much of their form to the destroying influence of the weather, which acts with varied effects on differently constituted rocks. Electricity, again, both directly and indirectly, produces an infinity of changes. When we speak of electricity, however, we must not be understood as referring solely to the more striking effects of the electric fluid, but rather to its silent and unobserved operation; which in the economy of nature, is far more important than the former, though vast effects are occasionally produced by lightning. Thus, at Cayamba, in Colombia, about forty years ago, the lightning reft a cliff in its entire length, and precipitated a mighty mass into the river Huallaga, whose course was at the time obstructed; and the river now forces its way, with much violence and uproar, through the massive fragments of the shattered cliff.

But it will be evident that the changes produced by the

above-mentioned agents (at least in their external operation,) are limited to the wearing away, and consequent lowering of the more elevated parts of the earth's surface, and that all appear to have a tendency to reduce the whole to one general level. We must, therefore, look for some other agent, some opponent force, to counteract these effects, and accomplish the more remarkable work of elevating mountain ranges, and of raising land above its former level. Such is subterranean heat, displayed in the phenomena of earthquakes and volcanos: and probably also, although less obviously, in the gradual rise which is taking place in some regions. That a connexion exists between earthquakes and volcanos, has been proved by several well-authenticated facts; and that both owe their origin to subterranean heat, is generally admitted, though the mode of its operation may be regarded as still involved in obscurity. The disturbances it produces on the earth's surface, may, however, be regarded as some of the most important phenomena which it belongs to physical geography to describe*.

Some of our readers may, nevertheless, not be aware of the magnificent scale, on which the energy of subterranean heat is at present developed on the surface of the globe; for these commotions in the earth's crust, though, on account of the dreadful catastrophes to which they occasionally give rise, almost unavoidably universally regarded as appalling phenomena, are perhaps very generally considered as detached events of rare occurrence; remarkable, it is true, from the destruction of property, and awful loss of life, with which they have, in many instances, been accompanied, but of no further account in the natural world. Nay, in this highly favoured island, we are so far removed from the theatre of volcanic action, that the very circumstance of these powerful agents being continually at work

* The consideration of these phenomena is included in the department of physical geography; the application of their effects, as illustrations of changes on the earth's surface, belongs to geology.