6 Artesian It'eas are perpendicular perforations or borings through a suc cession of strata deep enough to reach subterranean bodies of water whose sources are higher than the places where the perforations are made, the water being forced to the surface, and often to a considerable height above it by hydrostatic pressure. The name " artesian " is derived from Artois (the ancient Artesitnn), in France, where the art of sinking these wells is believed first to have been practised in Europe. But traces of deep borings discovered in Lombardy, Asia Minor, Egypt, and the Desert of Sahara tes tify that the art was known in more ancient times. In China it has been practised from an unknown period. In the province of On-Tong-Kiao it is reported that there are artesian wells from 15oo to 'Soo feet deep.
Phenomenon ana' Principle.—The phenomenon of artesian wells can be exhibited only when the underground configuration of the strata is favor able, as where they form a basin. The principle of their action is about as follows: A succession of strata, some permeable and others imperme able to water, incline downward so as to form a species of trough or basin (fig. 13). The rain-water, falling on the exposed edges of these strata i.vliere they outcrop in the more elevated land that forms the rim of the basin, permeates the pervious layers of sand, gravel, chalk, etc., until it is arrested in its downward course by an impervious stratum of rock or clay. The water thus imprisoned accumulates in the pervious stratum as in a reservoir; and if a well be bored within the boundaries of the basin, so as to tap this stratum, the water, by the hydrostatic pressure of the semi liquid column acting upon it, will be forced up to, and sometimes even con siderably above, the surface of the g-round, the height depending upon the elevation of the rim of the basin above the location of the well. London and Paris are both situated upon natural basins of this description, and artesian wells in great numbers have been bored in these cities and the contiguous country.
Double Artesian term " artesian well," as is apparent from what has preceded, should be applied only to bored wells in which. the water rises to the surface or to some distance above the surface. It frequently happens, as in the London basin, where numerous wells have been bored in the same neighborhood, that those toward the rim of the basin (j5i. 53,fig. i3)—that is, in its hig,her portion—cease to flow natural ly. Furthermore, it often happens that a number of water-bearing strata will be penetrated before one is reached that will deliver the water from the surface; or it may be that the boring will require to be continued throug-li a number of productive strata because the water yielded by them is insufficient in quantity or objectionable in quality. Occasionally the
supply from two levels is utilized through the same bore-hole by carrying a smaller tube down inside a larger one and continuing it to a lower strat um. A double artesian well of this kind is shown in Figure 8.
Boring: inethod of boring artesian wells has been greatly improved in recent years, more especially in the United States, where vast numbers have been bored in search of petroleum. The modern practice of artesian-well boring is given in the following- description, which is abridged from Trautwine: Deep vertical perforations in earth and rock from 6 to 8 inches in diameter, such as are required for artesian wells for water and oil, and for mining explorations, are drilled by repeatedly lifting and dropping in the same vertical line a heavy iron bit with a steel cutting edge (fig. 5). To insure roundness of the bore, the bit is partly revolved horizontally after each blow. The length of the cutting-edge of the bit is a little greater than the diameter of the bit itself, and consequently the well-bore is made sufficiently large to prevent the binding of the bit. The bit is the lowest member of a series of iron and steel bars screwed together at the ends and called a string- of tools (figs. 4-7).
Tbe String of Tools varies in length from 25 to 6o feet, according to the size and depth of the well-bore and the hardness of the rock; and its diameter throughout (above the cutting-edge) is an inch or two less than that of the bore. The weig-ht of the string- of tools is from 800 to 4000 pounds. The uppermost member is always a " rope-socket " without a swivel (fig. 6), to which the lower end of the supporting rope-cable is attached. This cable passes up and out to a horizontal lever, which by means of a horse-power or steam engine is kept constantly moving up and down with a see-saw motion. The string of tools, with the cutting edge of the bit at its lower end, is thus alternately lifted from 2 to 4 feet and lct fall from thirty to fifty times per minute, and so drills its way into the rock or earth. A depth of from 4 to 10 feet of water is required in the well to facilitate the drilling and the removal of the debris. After water is reached the drilling may be continued even if the bore-hole is full of water; but a great depth of water diminishes the force of the blows.