two streams of nearly equal size, that is considered the main stream whose direction is unchanged, and that the tributary whose course is altered. If you follow the river as far as it goes, you will come to where it enters the sea; that is its mouth. A steep descent, or precipice, in the bed of a river causes a Waterfall. A great waterfall, like Niagara, is called a cataract. Smaller ones are called cascades.

Turn now and go up the stream, in order to find whence it has come. You pass where one tributary after another joins it. Each tributary swells it. Above each confluence the stream is smaller. At last you come to mere threads or rills of water trickling from a mountain-side. You have reached

the source of the stream.

All streams cannot be traced up to such small rills. Some come in greater body from springs; others issue from the glacier-ice of great mountains; others from broad marshes in a flat country. But all these sources derive their supplies of water in the first instance from the moisture in the atmosphere. All streams begin in some kind of rain.

5. RIVER-BASINS AND WATER-SHEDS.—A great river and all its tributaries form a River-system. Take a map; mark all the sources of a river and its tributaries, and join the points: the enclosed space is the Riverbasin.* The high land between two adjoining river-basins, which, like the

* Scotland may be taken as an example of a small country with well-defined riverbasins. The chief are those of the Solway, the Clyde, the Tweed, the Forth, the Tay and Esk, the Dee and Don, and the Moray Firth.

The Solway Basin includes the counties of Wigton, Kirkcudbright, and Dumfries. It is bounded by the western part of the Cheviot Hills, the Lowther Hills, the Lead Hills, and the line of high ground extending thence along the extreme south of Ayrshire.

The Clyde Basin includes the counties of Argyle, Bute, Dumbarton, Lanark, Renfrew, and Ayr. It also is bounded by the line of high ground just named, by the Lead and Lowther Hills, by the high ground connecting the latter with the Pentland Hills and extending along the north-east border of Lanarkshire, by the Campsie Fells and the heights east of Loch Lomond.

The Tweed Basin includes the counties of Berwick, Roxburgh, Selkirk, and Peebles. It is bounded by the Cheviot and Lowther Hills, by the Pentland Hills and the high ground connecting these with the Lowther Hills, by the Moorfoot Hills, and by the Lammern uir Hills.

The Forth Basin includes the counties of Haddington, Mid-Lothian, Linlithgow, Stirling, Clackmannan, Kinross, and Fife. It is bounded by the Lammermuir, Moorfoot, and Pentland Hills; by the high ground on the north-east border of Lanarkshire, the Campsie Fells, and the hills on the east side of Loch Lomond; by the Ben More and Ben Voirlich groups of the Grampians; and by the Ochil Hills.

The Tay and Esk Basin includes Perthshire and Forfarshire, and part of Kincardine. It is bounded by the Ochil Hills on the south and the Grampians on the north.

The Dee and Don Basin, which includes the north of Kincardine, and Aberdeen, is bounded by the two branches of the Grampians that separate Aberdeenshire from Forfar and Perth on the one side, and from Banff on the other.

The Moray Firth Basin includes the counties of Banff, Elgin, Nairn, Inverness, Ross and Cromarty, Sutherland and Caithness. It is bounded on the east and south by the Grampians, on the west by the mountains of Ross-shire. It includes many smaller basins, the chief of which are those of the Spey, the Findhorn, the Ness, and the Dornoch Firth.

See Nelsons' Wall Map of Scotland, Coloured to show the River Basins.

ridge on the roof of a house, divides the water flowing from it, and sends the rivers in opposite directions, is called the Water-shed, or water-parting of the country. When the water-shed lies east and west, as in Europe and Asia, the general direction of the rivers is north and south. When the water-shed lies north and south, as in South America, the general direction of the rivers is east and west. The more distant the mountain system of a country is from the sea, the gentler and more varied is its slope, and the greater are its rivers. The most remarkable illustration of this is afforded by the Andes in South America. These mountains lie very near the west coast, and thousands of miles from the east coast. Consequently, there are on the west of South America only a few mountain torrents; on the east there are three of the greatest rivers in the world—the Orinoco, the Amazon, and the La Plata.

In Asia the greatest rivers are in the north, because the mountain system is nearer the south; but there, as in Europe, the high land occupies a central position, from which considerable rivers flow in nearly all directions.

6. USES AND EFFECTS OF RIVERS.-Rivers are of the greatest importance to a country, not only as a natural drainage for its soil, but also as the natural means of internal communication; though in this respect they are now superseded to a great extent by railways. The Rhine, the Danube, and the Ganges, are still great highways of traffic. The Mississippi and its tributaries are navigable to a distance of 3000 miles from its mouth; while the inland navigation of the Amazon system extends to no less than 50,000 miles. The greatest sea-ports in the world are at or near the mouths of rivers. Some of the most striking physical changes on the Earth's surface are due to the action of rivers. A river is constantly gnawing away and undermining its banks, large portions of which are often swept away by floods. The soil thus set free is carried down by the river in its course. When its basin broadens, and its speed slackens, much of this is deposited on the bottom of its channel, or on the flats which it has overflowed. Thus what it has borrowed from the solid earth at one part of its course, it repays at another. Sometimes this deposit of rubbish takes place at the mouth of a river, and is so regular and constant as to raise up a barrier in the river's way, and cause its stream to divide. The river thus empties itself into the sea by two or more mouths, instead of one. These mouths or arms enclose the triangular bed of deposited debris; which, from its resemblance to the Greek letter D (A), is called a Delta. The most remarkable deltas are those of the Nile and the Mississippi.

7. GLACIERS AND ICEBERGS.-This, however, is not the only effect of water-force in the past history of the globe. Men of science are now generally of opinion that mountains have been formed by the hollowing out of the valleys, and that this hollowing out has been the work of rain and streams and ice, which wear channels for themselves with resistless power, even in solid rock.

The form in which ice does this work is that of a Glacier, or ice-river, which, very gradually, but very surely, rolls down the valleys from the regions of perpetual snow. A glacier as truly drains a region over-burdened

with snow, as a river drains a region over-saturated with water. While the debris that falls into a river sinks for the most part to the bottom-only the light sand and soil being carried down-the debris that falls on the surface of the glacier, or bars its course, is carried forward bodily and with the utmost certainty. Huge boulders, which no human engineering could move, have thus been carried in the mass over miles, and then deposited on the margin of fertile valleys. For when the glacier reaches a warmer region, it gradually melts, and a muddy stream issues from it. At the same time the debris, which forms a long line of rubbish on its surface, called a moraine, is deposited in the valley.

In the region of glaciers, as elsewhere, fragments of rock are detached in various ways by the action of the atmosphere. The carbonic acid contained in rain causes some rocks, especially limestone, to crumble into sand. The oxygen in the air, which makes iron rust, also forms a crust on the surface of many rocks. This falls off, and a new crust is formed; that too falls off, and thus coat after coat is peeled away. Lastly, when water gets into crevices and clefts of rocks, and freezes, it expands, and breaks off fragments, which fall into the valleys.

When a glacier meets a lake, as happens in some parts of Switzerland, large masses of it break off, and are hurled into it, and float upon its surface till they gradually melt away. These floating masses are called Icebergs, or icemountains. The icebergs of the ocean have the same origin. They are

detached masses of Arctic glaciers which glide down to the sea. It requires a very long time to melt an iceberg. It is often floated two thousand miles from the place where it was launched before it disappears. It carries with it over all that distance whatever rocks and rubbish have been frozen into its substance. As it gradually melts, its debris is dropped into the sea. Sometimes an iceberg is stranded in comparatively deep water; for it is found that there are two feet under the water for every one foot that appears above it. An iceberg so stranded melts away by degrees, and deposits where it melts the moraines and boulders which it may have borne with it from its

native mountains. This is but another example of how water-force operates in changing the face of nature.

The bottom of every glacier is covered with loose stones and sand, which cause it to leave peculiar scratchings on the rocks over which it has passed. These scratchings, as well as moraines and boulders, are found in many countries where glaciers no longer exist. The inference is, that glaciers must once have existed there,-for example, in Scotland, in Cumberland and Wales, and in Ireland. This has led to the further conclusion that many lakes owe their origin to ice-force. For a mountain-lake is simply a valley which has been dammed up at its lower end, so that the river flowing into it could not proceed further until the valley was filled with water. The damming up has in many cases been the work of ice.

A few lakes, like Titicaca in South America, have no rivers running into them, and no outlet to the sea. Such lakes are supposed to be the craters of extinct volcanoes. Some are supplied by rivers, but send none out. They are usually salt lakes, such as the Caspian Sea, the Sea of Aral, and the Dead Sea in Asia; and the Great Salt Lake in North America. They are supposed to get rid of their superfluous water by evaporation. The largest freshwater lake in the world is Lake Superior in North America. It covers 43,000 square miles, and is thus one-third larger than Ireland.

8. WINDS.-The air-ocean, like the water-ocean, is subject to disturbances, -constant, periodic, and variable. A current of air is called Wind. The chief causes of wind are change of temperature and the Earth's rotation. Heated air is lighter than cold air, and rises through it, just as hot water rises through cold water. Cold air rushes towards the hot air, to supply the place which its rising would leave void. But the place of the cold air also needs to be supplied. This is done from the higher regions of the atmosphere, where the heated air becomes cool and descends again to make way for that which has been more recently heated. There is thus a constant circulation of air going on in the atmosphere, and thus, for the most part, are winds produced. But air-currents are also caused by electrical influences, and by the attraction of the Sun and Moon.

The rate of wind varies from 5 miles an hour,—a light breeze,―to 80 or 100 miles an hour,-a hurricane. From 30 to 40 miles an hour, it is reckoned a high wind or a gale; at 50 miles an hour, it is called a storm.

The air is hottest over the Torrid Zone; and there, therefore, it is constantly rising upwards, while colder air is flowing from the Poles to supply its place. If the globe did not rotate, the result would be a north wind in the northern and a south wind in the southern hemisphere, constantly blowing towards the Equator. But the Earth's rotation gives a greater speed to places near the Equator, because they are turning round a larger circle in the same time as other parts of the Earth; and the cold currents lag behind, because, as they approach the Equator, they are passing into regions which have always a higher velocity than the regions from which they have come. Change of temperature alone would produce north and south winds. Rotation alone would produce east winds. The combination of these two forces produces a north-east and a south-east wind on the north and the south of the Equator respectively. These winds, which are nearly constant in the

great oceans, are called Trade winds, because of their great advantage to navigators in sailing from east to west.

The trade winds are most regular in the Atlantic Ocean. In the Pacific, the number of islands and banks interferes with the constancy of the southeast trade wind. In the Indian Ocean, the south-east trade wind alone blows, owing to the mass of land north of the Equator.

The region of the trade winds extends to 28 or 30 degrees on each side of the Equator. But between the northern and the southern trades there is a zone of variable breezes, or equatorial calms, called Doldrums, where rain, accompanied by thunder-storms, is almost constant. Outside of the trade winds there are also narrow belts of calm, called the Calms of Cancer in the north, and the Calms of Capricorn in the south. Beyond these regions of calm, the winds are variable; but in the North Temperate Zone, westerly and south-westerly winds prevail; in the South Temperate Zone, north-westerly. Monsoons' are periodical winds in the Indian Ocean, owing to the abridgment of that ocean by the continent of Asia, which prevents the formation of a north-east trade wind there.

For one half of the year (from April till October) a south-west monsoon carries moisture to India and the east of Asia. Then the Sun is vertical north of the Equator. The temperature of the land in Asia becomes high. The air over it is rarefied and rises; and cool winds rush in from the Indian Ocean to supply its place.*

During the other half of the year (October till April) the north-east monsoon blows towards Africa. Then the Sun is vertical south of the Equator. It is winter in Asia, and its temperature falls below that of South Africa, towards which, therefore, the currents of air are attracted. The north-east monsoon leaves Asia as a dry wind; but it acquires some moisture in crossing the ocean, and bears rain to the south-east of Africa.

The change of the monsoons is accompanied by violent storms, often by hurricanes. The Chinese farmer watches for the coming of the monsoon as eagerly as the Egyptian rice-grower watches for the inundation of the Nile. The monsoons also regulate the navigation of the Clina Sea and

the Indian Ocean.

Partial and irregular monsoons prevail on the west coast of Africa, on the coast of Brazil, and on the west coast of America, from California to Chili. The alternate Land and Sea Breezes † of tropical countries are due to the greater heat of the land during the day, and the greater heat of the sea during the night.

Certain local winds have special names :

The Simoom' is a dry, noxious Desert wind in Arabia.

The Harmat'tan is the name given, on the west of Africa, to the wind originating in the Sahara.

The Siroc'co is the same wind tempered by the Mediterranean, blowing as a hot south-east wind towards Sicily and Italy. It is called the Sola'no in Spain, and the Sa'miel in Turkey.

Typhoons', or whirlwinds, are hurricanes which sweep over the China

* See lesson on The South-west Monsoon in Ceylon, p. 293.

† See lesson on Land and Sea Bree: es, p. 92.