Ross found a depth of more than 27,000 feet near St. Helena. Still greater depths are reported to have been discovered, but are not to be relied on.

Near to the land the sea generally is less deep than in midocean. The Straits of Dover are scarcely 200 feet deep in the deepest part. The North Sea attains to a depth of 400 feet. The greatest depths in the Mediterranean are about 6,000 feet. The Black Sea soundings give about 3,000 feet for the deepest part.

The pressure of the water at the bottom of the ocean is immense. It may be calculated by taking the weight of a cubic foot of water at sixty-two pounds. Living creatures at the bottom of the sea could not sustain this pressure were it not that (by a law of hydrostatics) it acts in every direction, so that the force of a liquid on one side of a body is balanced by an equal one on the other side.

Various marine animals, shells, sand, clay, chalk, and other substances have been brought up from the bed of the ocean. Vegetable life exists only near the surface. Beyond a depth of 1,200 feet the rays of the sun do not penetrate. It is probable therefore that perpetual darkness reigns in the lowest waters of the sea, except where phosphorescent animals shed an uncertain light.

WAVES. The sea is in perpetual motion. This continual movement, together with its saltness, keeps it pure. There are three kinds of movement visible in the waters of the ocean; namely, (1) waves, (2) currents, (3) tides.

Waves are temporary movements of the surface of the water when agitated by the wind or any similar disturbing influence. Waves are generally produced by the wind, but they are sometimes occasioned by other agencies acting on a smaller scale, as the passage of a vessel through the water, the falling of an iceberg or other body into the sea, etc. Sometimes the wind

only causes a gentle ripple on the surface; at other times, when a tempest is raging, the waves rise to such an extent that huge ships are wrecked by the violence of the motion.

Waves do not disturb the waters of the sea to a great depth. Even in a great storm, all is tranquil beyond the depth of about three or four hundred feet.

Waves rise to a greater or less height according to the strength of the disturbing causes. In very violent tempests they have been observed to attain a height of between thirty and forty feet.

Waves dash with great violence against the rocks and the shore. The rugged appearance of a rocky coast is caused by the force of the waves, which have washed away masses of earth and stones. Countless millions of pebbles worn round and smooth by the ceaseless action of the waves, and vast tracts of the finest sand produced by the same restless agency, line the low parts of the coast. Sometimes the sea makes inroads on the land by washing away a portion of the coast. On the east coast of England, for example, villages which were once situated two or three miles inland are now found close to the

sea.

CURRENTS. A current (as applied to the ocean) is a stream flowing in a certain direction, and conveying the water from one part of the ocean to another. It is simply a flow of waters; an oceanic river. The chief causes of currents are :— (1) The influence of temperature.

(2) The force of prevailing winds.

(3) The revolution of the earth on its axis.

The water of the ocean near the equator is expanded and made lighter by the heat. Hence, the equilibrium being destroyed, the cold and consequently heavier water from the northern and southern regions flows beneath the water of the tropics. There is, therefore, a general movement of the surface water towards the poles, and of the deeper water towards the equator.

This general flow of the surface water is modified by the action of the winds, which tend to give their own direction to the water exposed to their influence.

Since the earth turns on its axis from west to east, carrying

the water with it, this rotatory motion becomes gradually less from the equator to the poles, since the circles of rotation gradually diminish. Hence the water from the equator flowing north and south, acted on by the rotatory motion it received at the equator, arriving at regions where the rotatory motion of the earth is less than its own, travels faster to the east than the parts of the earth to which it has flowed. The northerly therefore becomes a north-easterly, and the southerly a southeasterly motion. Similarly the undercurrents towards the equator find themselves left behind when they arrive in the parts of the world which have a greater velocity than their own, and from southerly and northerly they are converted into southwesterly and north-westerly currents.

The direction of currents is also greatly modified by the shape and position of the continents, islands, shoals, sandbanks, and other obstacles which they meet in their course.

The following are the principal currents in the ocean :—

(1) The Equatorial Current. This vast current takes its rise in the Antarctic Ocean, and flows north-east across the Pacific Ocean till it reaches the western coast of America at about 45° south latitude. It here divides into two branches, of which the smaller follows the coast of America southward, and flows round Cape Horn into the Atlantic Ocean, forming the Cape Horn Current. The main current flows north along the coast of Chili and Peru till it reaches the equator, when it bends westward, and traverses the Pacific Ocean from east to west. On reaching the East India Islands, it throws off a branch called the Japan Current, which flows north-east along the coast of Asia till it joins the current flowing through Behring's Straits into the Arctic Ocean. The other branch is much broken in its passage among the East India Islands till it reaches the Indian Ocean, when it joins the great body of water flowing from east to west till it arrives at the coast of Africa. The main stream flows north of Madagascar, turns south through the Mozambique Channel, doubles the Cape of Good Hope, and crosses the Atlantic Ocean in a north-west direction. On reaching the

coast of Brazil near Cape St. Roque, it divides into two streams, of which the smaller turns southward, and flows along the coast of South America. The larger branch pursues its course along the coast of Brazil, enters the Caribbean Sea, and, following the windings of the coast, flows round the Gulf of Mexico, and issues from thence under the name of the Gulf Stream.

(2) The Gulf Stream. This famous current is, as already observed, connected with the great Equatorial Current, which flows round the globe, and it receives its name from the fact that it issues from the Gulf of Mexico. The water flowing through the Caribbean Sea into the Gulf of Mexico is driven through the Straits of Florida with great force, and with a velocity of about five miles an hour. The Gulf Stream flows to the north along the east coast of Florida till it reaches the latitude of 30° N., when it turns to the north-east at some distance from the coast of Georgia and Carolina. As it proceeds it is met by a cold polar current from the north, which flows beneath and between it and the land. It continues its course, bending further to the east when it arrives at Cape Hatteras, increasing in breadth, but decreasing in temperature and velocity. On arriving at the great Bank of Newfoundland, it bends to the east, south-east, and south, crossing the Atlantic in a broad stream till it arrives at the Azores, when it loses itself in the waters of the Atlantic. Its velocity gradually diminishes till, in the neighbourhood of the Azores, it is only about ten miles a day. The temperature of its water is considerably higher than that of the surrounding ocean. Near Newfoundland, for example, the difference of temperature is about 8°. Its breadth increases as it proceeds on its course to the north-east. Off Charleston Harbour it is about 62 miles in breadth; near Cape Hatteras about 75 miles. Its effects, in causing an increase of temperature, are said to be felt as far as the coasts of France and Spain, and even in the British Islands.

Vessels crossing the Atlantic from Europe endeavour to avoid it by sailing further north. On the return voyage the Gulf Stream assists the navigator.

The general effect of this interchange of water between the polar regions and the tropics is the moderating of the severe cold in the former and of the great heat in the latter. The current from the Antarctic Ocean, for example, tempers the heat of Chili and Peru, while the warm waters of the Gulf Stream mitigate the severity of the northern climates.

Currents have sometimes been divided into classes, as follows:

Constant Currents, or those whose course is regular and permanent. These are caused by the combined action of the winds, differences of temperature, the rotation of the earth, and other influences. The great currents described above are

examples of this class.

Periodical Currents, or those which only flow at particular seasons of the year, or during certain hours in the day. They are caused by the action of the tides and of the monsoons and other periodical winds.

Variable Currents, or those which change the direction of their course from time to time. These are caused by the action of the tides or of variable winds, and by the melting of the ice in the polar regions.

Counter Currents, or those which run in an opposite direction to other currents. The polar current which we have already mentioned in connection with the Gulf Stream, is an example.

Drift Currents, or those which consist of a movement of the surface of the water in obedience to the force of the wind. The direction of the movement of the surface water borne by a drift current is sometimes different from or even contrary to that of the current beneath.

TIDES. Tides are the movements of the waters of the ocean when they rise and fall at nearly regular intervals, advance upon the land and up the mouths of the rivers, and then retire to a lower level. The rising of the water is called the Flow, its retirement the Ebb of the tide. There are two flows and two ebbs of the tide in 24 hours 50 minutes and 28