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 snows. 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

60° Fahrenheit, 100 cubic inches of air weigh 30 grains troy; and a perpendicular column of the whole atmosphere weighs about 14 pounds. This is just balanced by a mercurial column of about 30 inches in height; and it is on this principle that the mercurial barometer is constructed.

The weight or density of atmospheric air is not uniform throughout its whole extent; a gradual diminution taking place as we ascend above the level of the sea. The rate of

decreasing density is such, that at the height of seven miles, the atmosphere would have a density equal only to onefourth of that at the surface of the sea: and at the height of fourteen miles, it would have only one-sixteenth, and so on. These heights have never been attained by man, but the ratio is the same at smaller elevations, and the consequence is, that on ascending elevated ground, the atmosphere exerts less pressure on any substance on which it rests.

The average pressure of the atmosphere (though subject to temporary alterations from meteorological causes), is found to be the same, or very nearly so, at any one place from year to year; and over the whole globe its lines of equal density, or pressure, are parallel to the sea level. If, therefore, we know the mean density at any one station, for instance, 2,000 feet above the sea, we may conclude that any other place, having the same density, has a similar elevation above the sea. This decreasing density is indicated by the barometer, the mercury in the tube falling at greater elevations. Thus, at the level of the sea, near the foot of Chimborazo, M. de Humboldt found that the barometer stood at exactly thirty inches; whilst at the elevation of 19,332 feet, to which he ascended on that mountain, it was no higher than fourteen inches and eight lines. And in December, 1831, when M. Boussingault, accompanied by Colonel Hall, ascended the same mountain, to the elevation of 19,699 feet (the greatest terrestrial height yet accomplished), the barometer fell to thirteen inches, eight lines. At all other places having the same elevation above the sea (allowing for some variation of temperature), the indication

of the barometer would be similar; and thus, by the use of that instrument, we are enabled to obtain a very near approximation to the heights of all places which are accessible to the foot of man.

The mean height of the barometer in London is twentynine inches and nine lines, nearly. The height of the mercurial column varies in this climate, from about twentyeight to thirty-one inches.

Another mode of measuring heights, dependent also on the density or pressure of the atmosphere, is by the boiling point of water. In London, and at all other places having a similar elevation, water boils at 212° Fahrenheit; and we are so accustomed to associate the notion of its ebullition or boiling up, with that particular temperature, that perhaps at first we may find some difficulty in reconciling the idea of "boiling water" with any other degree of heat. This ebullition, or boiling up of water is, however, dependent on the pressure of the air above it; and the boiling point of water decreases in a ratio nearly equivalent to the decrease of atmospheric density. And accordingly, at considerable elevations above the sea level, water, when boiled, does not acquire so high a temperature as 212°, and, under ordinary circumstances, cannot be raised to that temperature. At great elevations it is, therefore, not so available for some culinary purposes; and we much doubt (allowing all other conditions to be similar) whether so good a cup of tea could be made at the Hospice de St. Bernard, which is 8,600 feet above the level of the sea, as in the metropolis of England, at the former place the temperature of boiling water being only 203°: and still less could this be accomplished at the Cerro de Pasco, in Bolivia (celebrated for its silver mines), the height of which is 13,673 feet above the sea, and where the boiling point does not exceed 189°. The boiling point of water forms, however, a highly useful standard for measuring the altitude of any accessible locality.

Although a certain proportion of water or aqueous vapour is always present in the atmosphere, the quantity is liable