A most valuable source of water supply are the artesian wells, which 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 success 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 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 existence, 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 be obtained from them nor to pass through them. The most famous artesian well, perhaps, is that of Grenelle, near Paris, which was bored in 1833-41, and whose water is brought from the gault at a depth of 1,798 feet. It yields 5161/2 gallons of water in a minute, propelled thirty-two feet above the surface; temperature, 81°'7 F. An artesian well bored at Pesth in 1868-79 yields, at a depth of 3,182 feet, water of a temperature of 165° F. In the United States numerous artesian wells have been sunk, some of great depth, among which are two in St. Louis, Missouri, 2,197 and 3,8431/2 feet deep respectively; several in Chicago of from 700 to 1,200 feet in depth; one in Louisville, Kentucky, 2,086 feet deep; one in Columbus, Ohio, 2,7751/3 feet in depth, with many others from 500 to 2,000 feet deep.