BASINS.-Very few rivers run their course without receiving others, called tributaries, some of which may be as important as the main stream. A great river, with all its tributaries, is described as a river system, and the whole extent of country drained by it is called its basin. Several river basins may descend towards some inland sea, therefore we read of the basin of the Baltic or the Mediterranean. Then, as the ocean basins are the general receptacles (receptus, taken or received) of rivers, all those areas that slope towards any ocean form the river system of that

ocean.

WATER-SHEDS.-The line or ridge that separates one basin or system from another is termed the watershed, because the streams are shed off as if from the ridge of a house. Generally these boundaries are hills and mountains, but in some places there is very little rising ground, and a few feet of fall determines the direction of the rivers.

LENGTH OF RIVERS.-In estimating the length of a river, the shortest distance from its source to its mouth is called its direct course; the length of the principal stream, including its windings but not its tributaries, is termed the total length of its course.

There are a few rivers, as the Amazon, the Nile, the Mississippi, and the La Plata, which are several thousand miles long, and drain above a million square miles. Those of the second class are above one thousand miles long, and drain half a million. There are nearly a hundred that drain about 20,000 square miles.

Rivers may be grouped, according to the ocean which receives their waters, into the Arctic, the Atlantic, the Indian, and the Pacific.

The Arctic system includes all those rivers of Europe, Asia, and America that obey the general slope of the land towards the north. They flow mostly through frozen ground, and there is great sameness in their condition. The large rivers of Northern Asia become very sluggish and monotonous in the lower part of their courses, sometimes flowing between high banks of frozen mud and sand, in which are embedded the remains of the mammoth and other huge animals that seem once to have inhabited the northern plains of Asia. For many months of the year these rivers are themselves frozen and unfit for navigation, and in summer the lower courses thawing later than the higher, from being further north, afford no outlet for the melted snow and ice, which therefore overspreads the country in swamps. The peculiarity of the American section of the Arctic system is, that they find their way through a labyrinth of depressions forming permanent lakes.

The Atlantic system embraces all those rivers of Europe, Africa, and America that find their way,

directly or indirectly, to the Atlantic ocean. The chief of those in Europe that enter directly are the Rhine, Elbe, Weser, in Germany; the Seine, Loire, and Garonne, in France; the Douro, Tagus, Guadiana, and Guadalquiver, in the Spanish peninsula. Through the basin of the Baltic the Atlantic receives the Neva, Niemen, Vistula, and Oder. Through the Mediterranean the Atlantic receives from Europe the Ebro, the Rhone, and the Po, besides the Danube, Dnieper, Dniester, and Don, through the Black Sea.

The Danube, which is the largest of European rivers, has a winding course of 1496 miles, and a drainage area of 234,080. Such of these rivers as rise in the Alps, like the Rhone and the Rhine, are very rapid in their upper stages. Those that flow into the Baltic are ice-locked for part of the year, but generally the rivers of Europe are regular and useful, all more or less navigable in their lower courses, and so situated as to equalise the water supply more than those of any other continent.

Down the westerly slopes of Africa flow the Senegal, Gambia, Niger, Congo, Coanza, and Orange, little known in the interior. The Nile flows almost due north from or beyond the lake Nyanza to the Mediterranean, and is remarkable for its regular inundations, depositing a fine black soil, which has been an unfailing source of wealth to Egypt.

The rivers of South America are by far the most gigantic of the Atlantic system, having plentiful supplies and abundant room for development. At the head of them is the Amazon, by far the largest river

in the world, having a course of 3500 miles, and a drainage of about 1,500,000. After this, the Plata, Orinoco, San Francisco, Paranahyba, Tocantins or Para, and Magdalena. Most of these are subject to periodical inundations from the tropical rains, producing a rank vegetation of wood or grass.

In North America a low watershed divides the Arctic basin from the Atlantic, which includes the Mississippi, with its immense feeders, draining more than a million square miles; the Rio del Norte, St Lawrence, Saskatchewan, and Churchill.

The system of the Pacific is not at all in proportion to the lands that surround it. On account of the rapid counter-slope of the Andes there is not a stream of importance from Tierra del Fuego to the Gulf of California; and from that to Behring Strait, the only rivers of note are those formed where the Rocky Mountains bend somewhat inland-the Colorado, Columbia, and Fraser.

On the Asiatic side, however, the Eastern slope affords room for the development of some large streams descending to the Pacific. Such are the Amoor, Hoangho, Yang-tse-Kiang, Tche-Kiang, Menam, and Mekong.

The river system of the Indian Ocean includes streams from Asia, Africa, and Australia. The principal ones from Asia are the Ganges, Brahmapootra, Indus, Martaban, Irawady, Tigris, and Euphrates.

Australia is singularly destitute of rivers. Scarcely any are known in the west; and the Murray, on the eastern slope, is an insignificant stream in the dry

season.

We know little of the African rivers that discharge into the Indian Ocean. The Zambesi, explored by Dr Livingstone, seems to be the most important, draining an immense extent of inland country. The main water-supply, both of Africa and Australia, appears to be from rains; and the surplus, after supplying the wants of vegetable and animal life, seems to be carried off by evaporation rather than by regular river drainage.

SUMMARY.-A great river with its tributaries forms a river system, and the country which it thus drains a river basin. The high land between two such basins is called a watershed. The rivers which flow into the Arctic Ocean are generally long but sluggish, and for many months of the year are frozen. Those in America form in their course a number of vast lakes. Most of the European rivers flow into the Atlantic; those of them which join the Baltic are frozen part of the year. In Africa, the Atlantic receives the Senegal, Niger, and other rivers; and the Nile, the longest stream of this continent, annually overflows its banks with the waters it receives from the hills of Abyssinia. South America is remarkable for the length of its rivers, the Amazon being 3500 miles long, while North America has the Mississippi and the St Lawrence. The chief rivers flowing into the Pacific are the Hoang-ho, Yang-tse-Kiang, and other Chinese rivers, most of which annually overflow. The Indian Ocean receives the Ganges, Indus, and Euphrates from Asia; and also the Zambesi, which drains an immense but unexplored district of Africa.

QUESTIONS.

Explain what is meant by a river basin and a river system. What is meant by the term watershed, and why is it so called? In what way can rivers be grouped? Which is the largest

river in Europe? in South America? What forms the great, watershed in South America; and what effect has its position on the size of the rivers and the lengths of their courses? Which continent has, as far as is known, fewest rivers?