ARTESIAN WELLS are perpendicular borings into the ground, through which water rises, from various depths, according to circumstances, above the surface of the soil. The possibility of obtaining water in this way in a particular district depends on its geological structure. All rocks contain more or less water. Arenaceous rocks receive water mechanically, and according to their compactness and purity, part with a larger or smaller proportion of it. A cubic yard of pure sea-sand can contain, in addition to the quantity of dry sand which occupies that space, about one third of its bulk of water. It would part with nearly the whole of this into a well sunk in it,- and regularly pumped from. Chalk and other rocks, composed of fine particles, closely compacted together, contain as large a proportion of water; but from the power of capillary attraction, little or none of this water would be drained into a well sunk in such rock. From the exist ence, however, of numerous crevices in chalk through which the water freely flows, and from the general presence of a larger quantity of water than the porous rock is able to retain, wells sunk in chalk often yield water. There is yet a third class of rocks, which are perfectly impervious to water: such are clays, which are absolutely retentive, neither allowing water to he obtained from them nor to pass through them. When such rocks occur in basins (q.v.) in alternating layers, and in such order that pervious beds are in serted between impervious ones, it is evident, that if a perforation is made through the retentive barrier-bed in the lower portiOn of the basin, the water contained in the water logged strata will rise through the bore to a height depending upon the pressure of water which has accumulated in the confined sloping space between the two impervious beds. There is a number of porous beds composing the cretaceous measures, resting on the impervious gault, and these, again, are covered by the equally impervious series of the London clay, which form the strata on the surface, and extend to a considerable depth. The edges of the (balk-beds are largely exposed in the higher grounds around London; the water falling on the whole area of these exposed edges, sinks into the more or less porous cretaceous beds, and would, in course of time, by continued accessions, fill up the basin, were it not prevented by the clay above. By driving a bore through this superior bed, the inferior water-logged strata are reached; and the subterranean water rises to the surface, and flows continuously, by means of hydrostatic pressure.
Many such wells exist in London and its vicinity; those which form the ornamental fountains in Trafalgar square descend into the upper chalk to a depth of 393 ft. The most famous artesian well, prhaps, is that of•Grenelle, in the :outskirts of Paris, where the water is brought from the gault at a depth of 1798 ft. It yields 5161 gallons of water in a minute, propelled 32 ft. above the surface; temperature, 81°.7 F. An artesian well in course of construction at Pesth, yielded, at a depth of 3100 ft., 175,000 gallons
of water per day, of a temperature of 161° F., projected 35 ft. above the surface. It is to be sunk till the water reach 178' F.
It is believed that the Chinese have been long acquainted with A. W. They have been in use for centuries in Austria, especially in the neighborhood of Vienna, where they are very abundant. No knowledge existed as to their source, and consequently the boring for them was engaged in and conducted in a rude and empirical manner. .An excavation was made till a bed of clay was reached; on this a perforated mill-stone was laid, and through the hole the clay was bored until water rose. As soon as geology took the position of a science, and the theory of A. W. was propounded, the engineer was able, after the geological survey of a district, to discover whether a supply of water could there be obtained in this way. Already, districts formerly dry and arid have received a plentiful supply of water by means of such wells, and many more applications have yet to be made: it seems likely that erelong Africa's deserts may thus be converted into fertile plains. In an official report of the Algerian government for 1856-57, it is stated that artesian borings had been executed in the Sahara of the province of Constan tine with remarkable success. The first attempt, after a few weeks' labor, produced a constant stream, forming a perfect river, and yielding 4010 quarts of water per minute, at. a temperature of F. There are now upwards of 75 such borings in the Sahara, yielding an aggregate of 600,000 gallons per hour. The result is proving beneficial not only to the country materially, but also to the character and habits of its nomadic Arab inhabitants. Several tribes have already settled down around these wells, and forming thus the centers of settlements, have constructed villages, planted date-palms, and entirely renounced their previous wandering existence.
A. W. have supplied a portion of the data upon which the internal temperature of the earth has been calculated. They have their origin below that zone which is affected by the changing superficial temperature of the seasons, and conseqmmtly the water is of a constant temperature. Thus the .Grenelle artesian well has a temperature of F., while the mean temperature of the air in the cellar of the Paris observatory is only 53°. MM. Arago and IValferdin observed the temperature as the work proceeded, and found that there was a gradual and regular increase downwards. The latter gentleman made a series of very accurate and careful observations on the temperature of two borings at Creuzot, within a mile of each other, commencing at a height of 1030 ft. above the sea, and going down to a depth, the one of 2678 ft., the other about 1900 ft. The results, after every possible caution bad been taken to insure correctness, gave a rise of 1° F. for every 55 ft. down to a depth of 1800 ft., beyond which the rise was more rapid, being 1° for every 44 ft. of descent. There are several very deep borings in the United States.