surface through some natural channels, it forms a spring. Springs are of two kinds-cold, and thermal or hot, springs. Cold springs, again, may be either surface or deep-seated. When the bed of rock-using the term in its geological sense-is perfectly open and pervious 2 to water, like sand or gravel, and rests upon some impervious rock, such as a bed of clay, the rain-water soaks through the sand or gravel, and collects in pools on the impervious stratum. If a hole be dug then through the upper strata, water will rise to a certain height, much in the same way as sea-water filtrates into any hollow which we dig in the sands at low tide. Such surface-springs are very common on the sides of hills or mountains, especially where the upper strata consists of chalk, sand, or other permeable substances. The supply of such springs depends almost directly upon the rainfall of the immediate district, and therefore they often entirely cease during a long drought. Deep-seated springs are such as depend for their supply upon water, which has sunk to a great depth, and has then again risen to the surface. Their nature is illustrated by the formation of an Artesian Well. When a pervious stratum lies between two impervious strata, the water absorbed along the line of outcross collects in pools. If now a boring is made through the upper impervious stratum, the water will rush up to find its level. Such wells are called Artesian, from the town of Artois, where such borings attracted attention during the Middle Ages. There are many such wells in the neighbourhood of London, some of them of considerable depth. A celebrated experiment of this kind was tried at Grenelle, in the suburbs of Paris, in 1834, when a depth of 1600 feet was reached without finding water; when the boringrod had descended to 1800 feet, water, with a temperature

of 82°, rushed up in great quantities. Now if, instead of this artificial channel, the water had found an outlet through some fissure in the upper impervious bed, it would have formed a deep-seated spring. At Tours, in 1830, a well was sunk, and when the water rushed up, it brought a great quantity of fine sand, shells, seeds, and other vegetable matter, and it was thought from the nature of the shells and vegetable remains, that the water had come from the valleys of Auvergne 150 miles distant. In some cases, small fish, with perfect eyes, unlike those found in underground channels, have been brought up alive. These facts seem to suggest the idea that leaky beds of rivers are often the feeders of springs; and it is very probable that the ocean itself supplies large quantities of water to the underground circulation.

3

Intermittent springs are common, and occur under various circumstances. On the banks of the Thames, between Richmond and London, the wells regularly ebb and flow with the tide, owing to the porous nature of the river's banks, which are alternately saturated and drained by the tide. In other cases, the intermission may arise from the fact that the spring issues from an opening in the side of a reservoir fed from above. If the supply be not equal to the waste, the water will sink below the opening, and the spring will stop until the reservoir be replenished. Or, if the opening happen to form a natural syphon, the discharge will continue until the cavity is empty, and then cease until the water once more reaches the highest point of the curved channel through which it issues.

Thermal springs, in many instances, derive their temperature from the depth to which their water has descended; this was the case with the water which

issued from the Artesian Well at Grenelle; and that experiment seemed to fix the increase of temperature at the rate of 1° Fahrenheit for every 60 feet of descent. In many instances, however, hot springs seem to be connected with centres of volcanic disturbances. During the earthquake of Lisbon the temperature of a spring called La Source de la Reine (the Queen's spring), at Bagnères de Luchon, in the Pyrenees, was suddenly raised as much as 70°, or changed from a cold spring to one of 122° Fahrenheit, a heat which it has since retained. Indeed, it may be accepted as a general truth, that in regions where volcanic eruptions still occur, hot springs are abundant, and occasionally attain a boiling temperature while in proportion as we recede from such centres of igneous 5 activity, the thermal waters decrease in frequency and average heat.

Geysers. The most striking phenomena connected with boiling springs may be seen in the geysers of Iceland. They occur in the south-western district of the island, where nearly 100 of them are said to break out in a circuit of two miles. The great geyser rises out of a spacious basin at the top of a circular mound, composed of siliceous incrustations deposited from the spray of the water. The basin is about fifty feet in diameter, and four feet deep, terminating downward in a smooth pipe eight or ten feet in diameter. The water in the pipe appears to be in a constant state of ebullition. rises and gradually fills both the pipe and the basin. Subterranean noises are then heard, the ground is shaken slightly, and a column of water is thrown up with loud explosions, to the height of 100 or 200 feet. After playing for a while like a fountain, and giving off great clouds of vapour, the pipe is emptied, and a column of steam rushing up with amazing force finishes the eruption. Few of

It

the geysers play longer than five or six minutes at a time, though sometimes half an hour.

"Physical Geography, by W. LAWSON.”

1. HYDRAULIC, relating to water in motion. (Gr. hydōr, water.)

2. PERVIOUS, that which can be penetrated; lit., affording a way through. (Lat. per, through, and via, a way.)

3. INTERMITTENT, ceasing at intervals; lit., going between. (Lat. mitto, to cause to go.)

4. SYPHON, a bent tube for drawing off liquids from one vessel into another.

5. IGNEOUS, pertaining to, consisting of, or like fire. (Lat. ignis, fire.)

6. EBULLITION, the act of boiling. (Lat. bullio, to boil.)

WINDS.

VAPOUR is continually rising from all parts of the earth's surface, and mingles in an invisible form with the atmosphere. Now as dry air is 40 per cent. heavier than the same volume of vapour at the same temperature, it follows that the varying amount of this vapour in the air must have a marked effect upon the rise and fall of the column of mercury in the barometer. The amount of aqueous vapour in the atmosphere attains its maximum1 at or near the hottest part of the day, and its minimum2 at or near the coldest. On the other hand, as heat expands the air and cold condenses it, the density of the atmosphere will be greatest about midnight, and least about noon. The result of these causes, when combined, is that there are two maxima and two minima heights of the barometer every twenty-four hours. Within the tropics where these variations are very regular, the mercury attains its greatest height at

nine or half-past nine in the morning: it then sinks till four in the afternoon, after which it again rises and attains a second maximum at half-past ten or eleven in the evening. It then begins to fall, and reaches its second minimum about four in the morning. These variations are very slight, scarcely exceeding the one-tenth of an inch in the whole day, but the change is so regular in the tropics that, according to Humboldt, the time of day may be inferred from the height of the barometer with considerable approach to accuracy.

We have stated that heat expands the air and makes it lighter, while cold condenses it and makes it heavier. Now, it is the property of all fluids to preserve a state of equilibrium, and one of the chief conditions of the equilibrium of the atmosphere is, that every stratum should be of the same density. If therefore, the air in any particular locality becomes rarefied3, the denser air in the vicinity will flow in to restore equilibrium, and a wind is produced. This may be illustrated by the phenomena of the land and sea breezes which are experienced on the sea-coast in every part of the world. Land is heated more readily than water, but cools more rapidly. During the day, therefore, the land is warmer than the sea, and the atmosphere which is above the land partakes of its higher temperature. The colder air from off the sea therefore rushes in to restore equilibrium, and a sea breeze is felt. After sunset, the land cools more rapidly than the water, the atmosphere over the land is chilled, and becomes heavier than that of the sea, and a land breeze begins to blow, and continues till both atmospheres become of the same density.

The trade-winds, which prevail within the tropies, are produced in a similar manner. The equatorial parts of the globe being the hottest, the atmosphere there is more