4. Those which receive streams, but have no visible outlet, as the Caspian Sea and Lake Aral.

Lakes are also sometimes divided into fresh-water lakes and salt-water lakes.

7. The term chain is applied to lakes, when they are disposed in a line, communicating with one another; and cluster, when several small ones are grouped together; and lake region, to the country in which they are numerous and of considerable extent. North America contains the largest lakes on the globe, particularly the chain of Superior, Huron, Michigan, Erie, and Ontario.

8. The lakes of flat countries, such as the lowlands of Northern Germany, are very shallow, but those of mountainous regions occupy deep clefts or basins in the rocks on which these countries are based. Some of the lakes of the Alps, Norway, Sweden, Scotland, and Finland, are of great depth.

Some lakes are considerably depressed below the level of the sea, as the Caspian, which is from 70 to 90 feet lower than the Mediterranean; and the Dead Sea in Palestine, which is 1,312 feet below the same level. Others again are greatly above the sea-level, as the Swiss lakes, and Lake Titicaca in Peru, which is at an elevation of 12,847 feet.

9. Oceanic Movements.-The extent and distribution of the oceanic waters having been fully described in a former chapter, we shall here consider some of the movements to which they are subject. These are of three kinds, namely, waves, tides, and currents.

Waves are produced by the unequal pressure of the air upon contiguous portions of the surface of the ocean. The parts most pressed upon sink beneath the natural level, while those least pressed upon rise above it; when the former portions have sunk to a certain depth, and the latter risen to a certain height, their motions are reversed, and thus a reciprocating motion is produced, and a series of ridges and hollows formed, which are called The waves of the ocean are generally raised by the action of the wind upon its surface, and the height of such waves depends in a great measure upon the depth of the water in which they are produced.

waves.

10. Tides are the regular alternate risings and fallings of the waters of the ocean and of bays, rivers, &c., which communicate freely with it. These elevations and depressions, which generally occur twice in the course of a lunar day, or 24h. 50m. 28s. mean solar time, are produced by the attraction of the moon, combined with that of the sun, but they are chiefly regulated by the former body. The moon by her attraction raises the waters of the ocean under her, and at the same time causes an elevation of the waters on the opposite side of the earth. The sun by his attraction raises similar waves, but in consequence of his great distance they are of less extent. When the sun and moon are in conjunction and opposition, that is, at new or full moon, their combined actions produce spring tides, and when the moon is in quadrature, neap tides are produced.

The time of high water, like the moon's rising, is about 50 minutes later every day. The interval between the times of the transit of the moon and the times of high water on full and change days is called "Establishment of the Port," and when this is known at any seaport the time of high water on any other day can, in most cases, be ascertained. If lines be drawn upon a map connecting those points upon the earth's surface at which the "Establishment," or time of high water, is the same, a series of lines will then be traced called co-tidal lines.

11. The attraction of the sun and moon is felt most at the equator, and the height of the tides diminishes in receding from that circle, until near the poles they are scarcely perceptible. In open situations, as the islands of the Pacific Ocean, the tides rise at regular periods to the height of one or two feet; but when the tide-wave enters a bay, or rushes through a narrow channel, the time of high water is much varied, and the height greatly increased. In the British Channel the tide sometimes rises to 40 or 50 feet, and at the entrance of the Channel to 30 feet. In the Bay of Fundy it rises to 60 feet, and often so rapidly that cattle feeding on the shore have been drowned before they could escape. In inland seas

and lakes, as the Black Sea, the Baltic, and the Caspian, there is no perceptible tide, and in the Mediterranean it is inconsiderable. In the open ocean, near the equator, the tides follow the apparent motion of the moon from east to west, but in the north temperate zone their direction and velocity are greatly varied and modified by the outlines of the shores.

12. Currents.-The waters of the ocean are perpetually moving in currents which appear to complete the circuit of the globe. Some are permanent, others are variable, and change with the seasons, winds, or tides.

13. Currents of the Pacific Ocean.-The Antartic drift current, which is caused by the melting of ice and the prevailing south-west winds, flows from the south pole. It first sets from south to north, but is gradually deflected eastward towards the coast of Chili. About the meridian of 80° W. it divides, one part flowing south to Cape Horn, the other northward along the western coast of South America, where it receives the name of the Peruvian Current, a subdivision of which is called Mentor's Counter-current. The Peruvian Current favours the voyage along the coast to such an extent that vessels sail from Valparaiso to Callao (the port of Lima), a distance of more than 1,600 miles, in eight or nine days, while several weeks, and in some cases even months, are spent in the return voyage. Between Lima and Cape Blanco the current suddenly leaves the coast, and mixes with the general rotation or Equatorial Current. Near the Galapagos Islands the cool current coming from the southward meets a warmer stream from the neighbourhood of the Bay of Panama. Of these currents Captain Fitzroy observes that, " in some places they run three, four, or even five miles an hour, generally but not always, to the north-west. On one side of an island the temperature of the sea is sometimes found to be near 80°, while on the other side the water is at less than 60°."

14. The Equatorial Current of the Pacific is the movement of the waters of the Pacific from east to west. The stream begins to be felt at a distance from the con

tinent of about 10° W. long.; thence it runs westward, and following the impulse of the trade winds, flows, according to Humboldt, at the rate of from 7 to 8 miles in 24 hours. The southern limit of this mighty seastream coincides with the parallel of 26° S., while its northern border may be reckoned at 24° N.; so that the oceanic waters move in a westerly direction over a space of 500, or nearly one-third part of the distance from pole to pole. Between latitudes 5° and 100° N. there is a current flowing in the opposite direction, called the North Equatorial Counter-current.

15. The Cape Horn Current flows round the Cape towards the Falkland Islands, and is gradually lost in the Atlantic Ocean.

The Mexican Coast Current flows in the direction of south-east in winter and north-west in summer; this change of direction seems to be dependent upon the monsoons.

The Behring's Strait Current flows from the Pacific to the Arctic Ocean through the narrow channel which separates Asia from America, and then runs along the northern shores of the latter continent.

The Japanese Current.-This is a branch of the Equatorial Current which flows from the Island of Formosa northward along the islands of Japan, and is finally lost in the ocean.

The Carolinian Monsoon Current is the central part of the Equatorial current near the Caroline Islands. It is influenced by the monsoons running east in winter and west in summer.

16. The New South Wales alternating Current.— From the parallel of 28° S. to the south of Van Diemen's Land, this current flows during the summer months (from August to May) as follows:-From the land, to a distance of seven leagues, it sets to the south-west one and one-fourth miles per hour in the space of 27 leagues distance from the land, it sets towards the northeast at a rate of three-fourths of a mile per hour, and in the winter in an opposite direction. These alternating currents are supposed to depend on the monsoon winds.

17. Currents of the Indian Ocean. Numerous and variable currents are produced by the Equatorial

Current as it rushes through the islands of Asia to the coast of Africa and the Cape of Good Hope. The principal of these are as follow :

The Bengal Current, which flows along the Coromandel coast, is altogether dependent for its force and direction upon the monsoons. In summer it flows to the north-east, and in winter to the south-west.

The Malabar Current is also dependent upon the monsoons, running southward in summer and northward in winter along the Malabar coast, between Bombay and Cape Comorin.

The United Bengal and Malabar Current.-This sea-stream is formed by the united waters of the abovenamed currents: from Cape Comorin it proceeds westerly towards the coast of Africa.

The Passage Drift Current.-This is perceptible off the south-west of Australia, from which it flows northerly to about the Tropic of Capricorn. It then runs in a westerly direction towards the coast of Africa. As it approaches the Island of Madagascar it separates into two branches, one of which flows round the north of that island, the second and smaller branch advances towards the Cape of Good Hope.

The Mosambique Current, which is a continuation of the Passage Drift, runs with great velocity between the Island of Madagascar and the mainland of Africa, and thence southward to the southern extremity of the continent. This current is most rapid at Cape Corientes, as its name implies. The mean velocity varies from 18 to 20 miles per day, but this rate is greatly exceeded under particular circumstances. Horsburgh states that the ship "Northampton" was carried 139 miles in 24 hours by this current.

The L'Agullas, or Cape Current, also forms a strong stream, and runs with great velocity between the Cape and the L'Agullas sand-bank.

The Cape Counter-Current.—This is formed by a large portion of the waters of the preceding current, which being resisted by the L'Agullas bank, is returned to the Indian Ocean by a counter-current. It has been observed as far eastward as 65° 15′ E. long.