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because the line is able to reach to but a comparatively small depth. If we were to found our opinion upon analogy, we might conclude that the greatest depth of the ocean is, at least, equal to the height of the loftiest mountains, that is, between 20,000 and 30,000 feet. Along the coast, its depth has always been found proportioned to the height of the shore. When the coast is high and mountainous, the sea that washes it is deep; but when the coast is low, the water is shallow. If we reckon its average depth at two miles, the ocean will contain 296 millions of cubical miles of water. We shall have a more specific idea of this enormous mass of water, if we consider that it is sufficient to cover the whole globe to the height of more than 8000 feet; and if this water were reduced to one spherical mass, it would form a globe of more than 800 miles in diameter.

The general colour of the sea is a deep bluish green, which becomes clearer towards the coasts. This colour is thought to arise entirely from the same cause as the azure of the sky; the rays of blue light, being the most refrangible, pass in the greatest quantity through the water, which, on account of its density and depth, makes them undergo a strong refraction. The other colours exhibited in parts of the sea, depend on causes which are local, and sometimes deceptive. The Mediterranean, in its upper part, is said to have at times a purple tint. In the gulf of Guinea, the sea is white; around the Maldive islands it is black; and in some places it has been observed to be red. These appearances are probably occasioned by vast numbers of minute marine insects, by the nature of the soil, or by the infusion of certain earthy substances in the water. The green and yellow shades of the sea proceed frequently from the existence of marine vegetables at or near the surface.

The water of the sea contains several extraneous substances, in proportions varying in different places. The component parts, in addition to pure water, are

commonly muriatic acid, sulphuric acid (vitriol), fixed mineral alkali, magnesia, and sulphated lime. By boiling or evaporation in the air, common salt (muriate of soda) is obtained, which, for salting meat, is preferred to the salt of springs. The saltness of the sea appears, with some local exceptions, to be less towards the poles than near the tropics; and, in particular places, it varies from temporary causes. The violent tropical rains have an effect in diminishing it, especially near coasts, where an increased volume of fresh water is brought down by the rivers. The Baltic is at all times less salt than the ocean, and when a strong east wind keeps out the North Sea, its waters are said to become almost fit for domestic uses. The most curious phenomenon of all, is that of springs of fresh water rising up in the midst of the sea. In the bay of Xagua, on the southern coast of Cuba, springs of this kind gush up with great force, at the distance of two or three miles from the land; and others occur near Goa, on the western coast of Hindostan, and in the Mediterranean Sea, not far from Marseilles. Various theories have been advanced to account for the saltness of the ocean. Some assert the existence of vast beds of salt at its bottom. Others have supposed that the sea may have originally received all its saline particles from those existing on the surface of the earth, which were dissolved and carried down to the ocean by the action of the rivers. The most probable solution of the matter is, that it is an essential and absolute quality impressed upon it from the creation of the world, by the Great Author of nature. Its presence, united to the action of the tides and waves, preserves the vast mass of waters from corruption, and at the same time gives it a specific gravity sufficient to float the large bodies which move in it, or upon its surface. The bitterness which exists in sea water, but apparently not beyond a certain depth, is, with much probability, considered to be owing to the vegetable and animal matter held there, in a state of decomposition. From the same cause some account

for the luminous appearance which the sea often presents at night, particularly in summer and autumn, while others ascribe it to electricity, or to innumerable minute animals moving rapidly through the water in all directions. Water being a bad conductor of heat, the temperature of the sea changes much less suddenly than that of the atmosphere, and is by no means subject to such extremes as the latter. It is also modified by currents, which mingle together the waters of different depths and regions, and by the neighbourhood of shallows and banks. Thus bays, inland seas, and the spaces among clusters of islands, where the action of the waves is more confined, and the water usually of les3 depth than at a distance from land, are the most favourable places for the production and accumulation of marine ice. It is on this account that the navigation of the Baltic is annually stopped by the ice, in a latitude not more northerly than that of tracts which, in the main ocean, are always open to the passage of ships. In like manner, ice extends from five to eight degrees farther from the south than from the north pole, owing, it is probable, to the almost entire absence of land near the Antarctic Circle; while the north pole is so nearly surrounded by land, that the ice of the Arctic Ocean is shut up, and cannot be carried forward to such a distance by the current, which sets towards the equator.

The ocean has three kinds of motion. The first is that undulation which is produced by the wind, and which is entirely confined to its surface. The second motion is that continual tendency which the whole water in the sea has towards the west, which is greater near the equator than towards the poles. It begins on the west side of America, when it is moderate ; but as the waters advance westward their motion is accelerated; and, after having traversed the globe, they strike with great violence on the eastern shore of America. Being stopped by that continent, they rush, in the form of an impetuous current called by navigators the Gulf-stream, into the Gulf of Mexico, and thence proceed along the coast of North America, till they come to the south side of the great bank of Newfoundland, when they turn suddenly off and run down through the Azores, or Western Isles. This motion is most probably owing to the diurnal revolution of the earth on its axis, which is in a direction contrary to the current of the sea. The third motion is the tide, which is a regular swell of the ocean every 121 hours. This motion is now ascertained to be owing to the attractive influence of the moon, and partly to that of the sun. There is always a flux and reflux at the same time, in two parts of the globe, and these are opposite to each other; so that when our antipodes have high water, we have the same. When the attractive powers of the sun and moon act in the same direction, which happens at the time of new and full moon, we have the highest or spring tides; but when their attraction is opposed to each other, which happens at the quarters, we have the lowest or neap tides.

LESSON XII.

SPECIFIC GRAVITY. The specific gravity of a body means simply its weight compared with that of another body of the same size. When we say that substances, such as lead and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of the substances in nature. Mahogany is a heavy body when compared to most other kinds of wood, but light when compared to stone. Chalk is a heavy body compared to coal, but light if compared to metal. Thus our notions of light and heavy are vague and undefined, and some standard of comparison is required, to which the weight of all other bodies may be referred. The body which has been adopted as a standard of reference, is distilled water. When the specific gravity of bodies is to be estimated, it is necessary simply to weigh the body under trial in water. If a piece of gold be weighed in a glass of water, the gold will displace just as much water as is equal to its own bulk : a cubic inch of water must make way for a cubic inch of gold. The bulk alone is to be considered, the weight having nothing to do with the quantity of water displaced : for a cubic inch of gold does not occupy more space, and therefore will not displace more water than a cubic inch of ivory, or any other substance that will sink in water.

The gold will weigh less in water than it did out of it, on account of the upward pressure of the particles of water, which in some measure supports the gold, and, by so doing, diminishes its weight. If the body under trial be of the same weight as the water in which it is immersed, it will be wholly supported by it; if it be heavier, the water will offer some resistance to its descent; and this resistance will in all cases be the same to bodies of equal bulk, whatever be their weight. All bodies of the same size, therefore, lose the same quantity of their weight when completely immersed in water. A body weighed in water loses as much of its weight as is equal to that the water displaces; so that were this water put into the scale to which the body is suspended, it would restore the balance.

When a body is weighed in water, in order to ascertain its specific gravity, it may either be suspended to a hook at the bottom of the basin of the balance, or, taking off the basin, suspended

to the arm of the balance. Now, supposing that a cubic inch of gold weighed

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