ARTESIAN (ăr-te'zhan) WELLS, bor ings of considerable depth which tap a subter ranean stream or sheet of water. The name is derived from Artois (Latin artesium), a prov ince in France where the first deep borings in Europe were made. Strictly speaking the term artesian is applicable only to such wells as dis charge water at the surface under natural con ditions (that is, self-flowing wells), but in America the term is commonly applied to any wells of more than ordinary depth. The United States Geological Survey restricts the term to a well in which the water rises under its own pressure above the point at which it was first encountered, even though it does not flow out at the surface, and this is the sense in which the term is here used. The conditions which determine the presence of artesian water in a region relate to the geological structure of the underlying strata. It is essential in the first place that a pervious stratum be overlaid by an impervious layer. The pervious bed, usually sandstone or sand, serves as a reservoir for the accumulation of water, while the impervious bed prevents this water from escaping. The second requisite is that the strata have a gentle pitch toward the site of the well and that they outcrop at some place above the well. The dis tance of the outcropping edges, which receive the water supply from rains, is sometimes very great. Owing to friction the water column of the well never reaches the level of the outcrop ping source. The conditions for artesian water are particularly favorablt when the strata are arranged in the form of a geological basin dip ping in all directions toward the well, as there then is no opportunity for the water to escape at a lower level. From these considerations it is evident that the discharge from an artesian well depends upon the rainfall of the region and upon the area of the exposed porous stratum. At first the discharge is usually very abundant owing to the long accumulation, and unless this drain is constantly supplied the flow will gradually decrease until a balance is estab lished. When several wells are bored in the same vicinity, the flow from each may be di minished, but the total discharge will increase until the limit of supply is reached. This is
well illustrated in the wells bored in the Lon don basin which in 1838 gave a total daily sup ply of 6,000,000 gallons; in 1851 with a larger number of borings the supply was about doubled, while the force had diminished very markedly; also in Denver, Colo., where some years ago there were many flowing wells which yielded water in large volume and with suf ficient head to rise to the upper floors of the buildings. As wells multiplied the head and volume decreased so that in 1916 all wells in the centre of the city had to be pumped and artesian water was available only in the lower parts of some of the surrounding country. The large area in which flows were obtained in southern California has also been greatly de creased by the heavy draft of water to irrigate orange orchards. Artesian water is valuable not only for domestic use, for which it is usually adapted by its purity, but it is also ex tensively employed in the irrigation of arid regions. Some parts of the Sahara Desert have been reclaimed by making use of the subter ranean stores of water, and recent investiga tions have shown that there are many areas which may yet be brought under cultivation. It is, however, in the United States that irriga tion by artesian waters has reached its greatest development. Special surveys of the Great Plains region have been undertaken by the United States Geological Survey for the pur pose of defining the areas where successful bor ings may be made, and artesian wells are now largely employed for irrigation in South Dakota, New Mexico, Texas, California and Montana. The supply is drawn mostly from the Cretaceous sandstone, which is reached at a depth varying from less than 100 to more than 1,500 feet. When the flow of water is sufficiently strong it may be utilized for power purposes as is done in some parts of Europe and at several points in the Dakotas. In Wiir temberg a supply of warm water is applied to the heating of buildings.