lengths of the principal mountain chains. The Andes, the Cordilleras, the Rocky Mountains, upwards of 10,000 miles. The Altai Mountains 5000. The Mountains of the Moon probably 2000. The Oural, Atlas, and Himaleh ranges, about 1500. The Dorfrine Mountains about 1000. The Caucasian range, the Alps, and Appenines, from 700 to 600. The Carpathians about 500. The Pyrenees about 250.

38. Every principal chain has one side very steep, and on the other a very gradual slope. The Alps, for instance, are much more rapid in their descent on the Italian side than on that of Switzerland. The Scandinavian masses are steepest towards the west and north-west; the Pyrenees towards the south. Mount Atlas and Mount Libanus present their bold and craggy declivities towards the Mediterranean. The two mountain chains that border the northern and the southern coasts of Asia Minor present very abrupt faces towards the Black Sea and the Mediterranean, while their inland sides merge more gently into high table-lands. In like manner,

the Western Ghauts, on which the table-land of the Deccan rests, have precipitous declivities directly towards the west, and long and gentle slopes towards the east.

39. The height of mountains is an important point in physical geography; for elevation determines climate, and a single mountain side will exhibit great varieties of vegetables and animals at different heights. The mean height of the Himalayan chain is 15,600 feet; the highest mountain of the chain being Dhawalagiri, which attains 28,077 feet. Of the Andes, 11,800 feet; Nevada di Sorata, in the Bolivian Andes,

being 25,250. Of the Alps, 7700 feet; Mount Blanc, in the Pennine Alps, being 15,646 feet above the level of the sea.

40. The Ocean.-The depth of the ocean is unknown. But it has been calculated to be about 120 feet around the coasts of England, 360 around those of Scotland, 2000 on the western coast of Ireland, and 50,000 in the main Atlantic. The greatest probable depth hardly exceeds ten miles; while from four to five miles is the estimated mean depth.

41. The waters of the ocean are kept in perpetual motion by winds, currents, and tides. The effect of wind is chiefly confined to the surface: the wildest tempest does not agitate it at the depth of an hundred feet. But the appearance presented by the long ridgy waves, rising with an uniform and rapid swell, and curling over in foaming surges, which tumble headlong with prodigious force, is at once grand and terrific; and if waves meet with resistance, as when they dash against some lofty cliff, their actual height becomes much greater than in the deep sea.

42. The most general currents are produced by the movement of the waters perpetually setting from the polar regions towards the equator, and by the progression of the tropical seas towards the west. In the Atlantic, the Equatorial Current produces that remarkable movement of the waters called the gulph stream. This current, perceptible at the Canaries and travelling westward, passes between the West Indian islands, and sweeps along the northern coasts of South America to the Isthmus of Darien; thence it stretches towards Yucatan, and, after wheeling round the Mexican Gulph, rushes through the channel between Cuba and Florida, passing between the Baha

mas and the United States towards Newfoundland, where it encounters the great current from the arctic regions, and is arrested. From Newfoundland the retrograde current travels eastwards, deflecting towards the south, sends a branch through the Straits of Gibraltar, and re-enters itself on the coast of Africa; having performed, in the course of thirty-five months, a magnificent circuit of more than 10,000 miles. There is also a general western current in the Great Pacific. This is greatly broken by the vast chain of islands and shoals which stretch from China to New Zealand. The huge mass of Australia produces separate and dangerous currents around its shores. The general current, passing through Bass's Straits and from among the Australasian islands, flows with weakened force through the Indian Ocean towards the Bay of Bengal. Deflected thence, it forms the current in the Mozambique Channel, and sweeps round the Cape of Good Hope in a powerful stream, not less than forty-three leagues in breadth. These great oceanic currents are chiefly caused by the centrifugal force of the earth's diurnal rotation.

43. It belongs to astronomy to teach the theory of tides; to explain the two floods and two ebbs daily; how the united influence of the sun and moon causes the two spring and two neap tides during each lunation; and why the highest tides take place about the equinoxes. The phenomenon of the tides has been observed in every part of the earth which is washed by the sea. For about six hours the sea gradually swells, so that it enters the mouths of harbours and rivers, advancing and rising upon the coasts: this is called flood tide. For about twelve

neap.

minutes it is stationary; and is said to be at high water. It then begins to ebb, or flow back again, for about six hours; again becomes stationary for about twelve minutes, during which it is low water; and then begins to flow again as before. The highest tides in the month are called spring tides; the lowest These phenomena are much affected by local causes. The tides of the German Ocean take twelve hours to reach London Bridge. In the Baltic and the Mediterranean scarcely any tides are perceptible. But in bays, harbours, and seas open to the tidal currents, as Baffin's Bay, Hudson's Bay, and the Red Sea, there are regular and often very high tides. In the British Channel, the tide, which in the open sea does not exceed one or two feet, sometimes rises 40 or 50 feet. The tides often rise with dangerous rapidity; as in the Bay of Fundy, and at the mouths of the Indus and the Ganges.

44. The sea, when viewed near land, is generally of a greenish colour, as in the German Ocean; but when viewed out of soundings, is of a beautiful deep blue. The occasional red colour of the sea, as of the Red Sea, is said to be produced by myriads of minute animalculæ of a peculiar species. In some parts of the Mediterranean the sea is occasionally purple; in the Gulph of Guinea white; amid the Maldive islands black.

45. The phosphorescence of the sea is a magnificent spectacle. Sometimes a vessel cleaving the dark waters appears to trace a long line of fire, and every stroke of the oar produces flashes of lambent flame. In other cases, thousands of stars seem to float and gambol on the watery surface; at one instant blend

ing into a single vast sheet of light, at another breaking into luminous waves and dissolving in a brilliant foam. This splendid phenomenon is chiefly owing to innumerable microscopic animals, to electricity, and to animal and vegetable decomposition.

46. The saltness of the ocean chiefly arises from the abundant presence of common salt, and seems to be greatest in the Southern Ocean and the Mediter

ranean.

47. Rivers.-The size and the course of rivers are chiefly determined by the height and direction of the mountain-chains in which they originate, and the extent and nature of the country drained by them and their tributaries. Thus the Rhine, the Danube, and the Rhone, the largest rivers in Europe, rise in the Swiss Alps; the great rivers of Asia-the Ob, the Yang-tse Kiang, the Irawaddy, the Brahmaputra, and the Ganges-originate in some of its lofty chains; in Africa, the Nile has one of its sources in the mountains of Abyssinia; while, in America, the Amazon, the Missisippi, and La Plata rise in the highest mountain ranges in the world.

48. The velocity of rivers is, generally speaking, proportionate to the declivity of the ground over which they flow. When river-courses lie among mountains they are subject to sudden breaks, which form rapids, or, in the case of precipitous descents, cataracts. The velocity of rivers is also accelerated by the volume of water which they convey.

49. The channel of rivers is greatly determined by the action of their waters; even rocky beds being eroded in the lapse of years. Thus the Falls of Niagara have receded about eighteen feet in thirty years.