In many geological basins, the deep subterranean waters are frequently inclosed over a surface of many square miles between impermeable beds of clay or hard rock, which nowhere permit them to escape; but if a hole be bored deep enough to reach a permeable bed, it is evident that they will then gush forth more or less violently, according to the degree of hydrostatic pressure which acts upon them. This is the simple theory of the Artesian Wells, so called from the French province of Artois, where, as far back as the beginning of the twelfth century, springs of water were artificially obtained by perforating the soil to a certain depth in places where no indication of springs existed at the surface. The barbarous inhabitants of the Sahara seem, however, to have long preceded the Artesians in the art of sinking deep wells, for Olympiodorus, a writer who flourished at Alexandria about the middle of the sixth century, mentions pits sunk in the oasis to the depth of 200 or 300 yards, and pouring forth streams of water, used for irrigation. By the aid of geological science, and of greater mechanical skill, Artesian borings[6] are at present frequently undertaken in civilised countries, wherever the nature of the ground promises success, and the want of water is sufficiently great to warrant the attempt. Sometimes the water is reached at a moderate distance from the surface, but not seldom it has been found necessary to bore to a depth of 200 or 300 fathoms. Often efforts, even on this large scale, have proved vain, and the work has been abandoned in despair. Footnote 6: See Chapter on Mines in general, for a short account of earth-boring operations. One of the most remarkable instances on record of a successful sinking for water is that of the Artesian well of Grenelle, one of the Parisian suburbs. [Illustration: POROUS STRATA. ARTESIAN WELL SUNK IN THE LONDON BASIN.]