take, which is called the water-shed. Thus, in the Pyrenees, the direction of the streams flowing either north o south, marks the limits of the two kingdoms of France and Spain.

The majestic rivers of Asia, the Ganges, the Indus or Scind, the Irawaddy, and the Brahmapootra, have their sources in the Himalayan range. The rivers of China, also, take their rise in the continuation of the same vast line of mountains. The northern rivers of Asia, the Irtish, the Obi, and the Yenesei, all originate in the Altai mountains.

In Africa, the Nile has one of its sources among the lofty mountains of Abyssinia, and the other in the more distant central range. Respecting the sources of the other principal rivers of Africa, there is still much obscurity.

In North America, the Arkansas, the Red River, and the Missouri, all tributaries of the Mississippi, take their rise in the Rocky Mountains. In the same great range, we also meet in close connexion, the sources of the river Colombia, and of the Saskatchewan; the former of which, taking a westerly course, falls into the Pacific; whilst the latter, flowing in an opposite or easterly direction, enters Lake Winnepeg, and passing through the vast chain of lakes, of which the St. Lawrence forms the outlet, finally falls into the Atlantic Ocean.

In South America, the majestic Marañon, or river of the Amazons, the Oronooko, the Magdalena, and other vast rivers, spring from the mighty range of the Andes. The Parana and the Paraguay take their rise in a range of less elevation, which stretches across the continent to the coast of Brazil.

When the sources of rivers lie among mountains, they are subject to sudden descents, which give rise to rapids, cascades, and cataracts. Rapids are caused by the greater slope or inclination of the bed of a river, which causes the water to rush down in that particular part, sometimes with fearful velocity. Cascades and cataracts are formed by water precipitating itself from a ledge or mass of rock, with a perpendicular, or nearly perpendicular, descent. When in

their most impetuous character, they are denominated cataracts; when more gentle, they are termed cascades.

Although England may not boast of any waterfalls that can vie with those of countries of more mountainous character, it nevertheless possesses some cascades of considerable beauty. Cumberland, Westmoreland, Durham, Yorkshire, and Devonshire, present many instances of picturesque waterfalls. One of the most beautiful in this country is the fall of the Tees, in Durham.

The river, obstructed and divided by a mass of rock, descends in a double cataract from the top; but reuniting its waters before they reach the bottom, the whole dashes into the basin with a force and grandeur scarcely inferior to the cataracts of Switzerland, or even America.

The falls of the Cayne and the Mawddach, in Merionethshire, called Pistil-y-Cayne and Pistil-y-Mawddach, are highly picturesque. The former is described as particu

larly magnificent, Scotland presents us with some very grand waterfalls. The most stupendous is that of Glomach, in the county of Ross. In Ireland we meet with the Fall of Powerscourt, in the county of Wicklow, which is remarkable for the beauty of the surrounding scenery.

Among the most celebrated waterfalls of Europe are the cataracts of the Dahl, in Sweden; of the Rhine, near Schaffhausen; of the Staubbach, in Switzerland; of Tivoli, and Terni, in Italy; and of Ceresoli, in Savoy. The latter is said to have a fall, not unbroken, however, of 2,400 feet.

The most remarkable known waterfalls of Asia are situated among the Himalayan mountains. A cataract of the Shirawati, in the Indian province of Canara, is described as being of surpassing beauty and sublimity, and as having a fall of 1,150 feet.

We have remarked that North America has been called the country of lakes; it may certainly with equal propriety be termed the region of waterfalls. Besides a very large number of rapids and minor falls, it contains an unusual number of principal cataracts. This is in great measure owing to the geological character of the country, to which we shall, in a future page, direct our attention. These falls are, however, generally more remarkable for the vast body of water they pour down, than for the depth of the falls. The stupendous Fall of Niagara is formed by a large and rapid river, 1650 feet in width, which precipitates itself by two channels in one leap, 160 feet perpendicular.

Smooth to the shelving brink, a copious flood
Rolls fair and placid; where collected all

In an impetuous torrent, down the steep

It thundering shoots, and shakes the country round;

At first an azure sheet, it rushes broad,

Then whitening by degrees, as prone it falls,
And from the long-resounding rocks below
Dashed in a cloud of foam, it sends aloft

A hoary mist, and forms a ceaseless shower.

South America contains several waterfalls; the most celebrated of these is the cataract of Tequendama, near

Bogota, situated amongst some of the most picturesque scenery of the Andes, and which has a fall of 540 feet. Rivers are fed either by springs or by the melting of the ows. They do not, therefore, receive their largest supplies from the actual summits of mountains, for large springs are not of ordinary occurrence near the summits of mountains; nor are the vast accumulations of ice and snow, called glaciers, met with on the highest points, or peaks of mountains, but usually on the declivities, or slopes of the upper mountain valleys.

Springs derive their supplies from water, raised into the atmosphere by evaporation, and again deposited on the earth in the form of showers, or mist. This moisture, entering the ground by means of fissures in rocks, or through porous beds, continues to sink, until arrested in its progress by rocks impermeable to water, when it gushes forth as a spring, larger or smaller, according to the supplies it has received. Mountains and hills arrest and condense clouds, and consequently a greater deposition of moisture takes place in mountainous districts; but it will be evident that a peak, or pointed summit, on account of the small surface it presents, for collecting the waters from above, will not be favourable to the formation of copious springs.

The largest spring in Great Britain is that of St. Winifred's Well, at Holywell, which is said to throw up about twenty-one tuns of water per minute, or 30,240 tuns daily. All springs contain some solid matter, either saline or mineral; when this is in excess, they are termed mineral springs. Such are the waters of Tunbridge, Cheltenham, and Harrowgate.

Thermal, or hot springs, occur in almost every region of the globe. Some are of very high temperature; the hottest in this country are those of Bath, which have a temperature of 116°. The Carlsbad springs, in Bohemia, have a temperature of 167°; and those of Coquinas, in Sardinia, a temperature of 198°. The boiling springs of Iceland are well known; and the magnificent jets or fountains of boiling

D

water periodically thrown up by the Geysers, entitle them to be ranked among some of the most extraordinary phenomena of the natural world. The Great Geyser shoots up vast columns of boiling water to the height of ninety or one hundred feet, which succeed each other with great rapidity, after which a pause ensues. There are generally four great series of such eruptions in the course of twenty-four hours, but the intermissions of this extraordinary fountain are not absolutely regular.

CHAPTER V.

THE ATMOSPHERE; ITS CONSTITUTION, AND WEIGHT OR DEN

SITY.

WINDS.

EVAPORATION.-DEW.-MISTS.-CLOUDS.-RAIN,

God made

The firmament, expanse of liquid, pure,
Transparent, elemental air, diffused

In circuit to the uttermost extent
Of this great round.

MILTON.

THE earth is surrounded by an invisible, and highly elastic fluid, termed its atmosphere, which may be considered as a body, accompanying and revolving with the earth. The extent of the atmosphere is supposed to be about forty or fifty miles.

Atmospheric air was long considered to be an "elemental" or simple body, but it is now well known to consist of a mixture of oxygen and nitrogen gases, in regular proportions, usually estimated at twenty-one parts of oxygen to seventy-nine of nitrogen. The atmosphere also always contains a small proportion of carbonic acid, the quantity of which is subject to slight variations. It likewise holds water suspended, in the state of vapour, the proportions of which are still more fluctuating.

Air is ponderable, or has weight; its weight, however, differs according to its temperature. At the temperature of