All persons who have paid the slight est attention to geology are aware that in stratified countries (and it is in such only that Artesian wells exist) different beds of rocks are superposed on one another, and ranged in a certain constant order. The strata sometimes follow a horizontal direction for a considerable extent of country ; at other places they are inclined, and even placed perpen to the horizon, having the ap pearance of having been bent and burst through by the action of a powerful force from beneath. In those cases the edges of the strata are often exposed, especially on the summits and flanks of hills, to the action of the atmosphere.
The following diagram illustrates this : it represents a basin composed of perme able strata (a a a) separated by imper meable layers (bbb). The water which falls on the edges of a on the side x will sink down and the beds until the water rises on the other side, and has a tendency to run out at A. If tubes be sunk in the middle, as at c d e f, the water will flow up these tubes until it attains the level of the beds of clay at b; and as the ground at the place of sinking is on a lower level, the water will rise in the tube considerably above the surface, and will be thrown up with great force.
Between the strata are frequently found beds of permeable sand, through which water, coming in contact with them, must necessarily pass, first, through the in clined part by virtue of its specific gra vity, and then in the horizontal brunches, by virtue of the pressure of the water remaining in the elevated portions of the strata. In this manner the water insinu ates itself between the different strata ; and hence we may expect that in locali ties where the tertiary stratification pre vails, as many distinct sources of subter raneous water will be met with in pene trating perpendicularly through the sur face, as there are distinct layers of a sandy or gravelly nature reposing on im permeable strata. This consequence of the theory is perfectly confirmed by ex perience. M. Arago mentions, that in digging for coal near St. Nicholas d' Alier moat, a short distance from Dieppe, seven distinct and copious sources of water were found, the respective depths of which were : 1st, between 80 and 100 feet ; 2d, 328 feet ; 8d, from 570 to 590 feet ; 4th, from 690 to 710 feet ; 5th, 820 feet ; 6th, 940 feet ; 7th, 1090 feet ; and the occasional force of each source wee very great. Similar occurrences are fre quent in the neighborhood of London, and are familiar to all miners. But it is
not enough that the structure of the country is snob that water can percolate between different strata ; the phenomena of Artesian fountains could not be ex plained supposing it to be col lected in large quantities, and forming subterranean reservoirs of immense ex tent. That such reservoirs exist, no doubt can be entertained. The cele brated fountain of Vauclase sends forth at all times a stream of water sufficient to form a considerable river. Even in the driest seasons, when the water is least plentiful, it produces 5780 cubic feet per minute. After great rains, its pro duct is thrice as great. The mean quan tity emitted is 9360 cubic feet per minute, or about 5032 millions of cubic feet in a year. Many other examples of the same kind might be cited; showing that water must not only be collected in subter raneous cavities in immense quantities, but that it also passes freely from one place to another. In fact, the disposition of the rocks in strata permits the water to be collected under the surface, and to be conveyed without waste, as if in close pipes.
According to the view which has now been taken of the manner in which sub terraneous water is collected, its eleva tion to the surface through a natural fis sure or artificial perforation is a simple result of hydrostatic pressure. Generally speaking, it is only on the acclivities of hills, or in elevated places, that the edges of the strata are exposed, and where, consequently, the rain water can be re ceived under beds of impermeable ma terials. Conceive two strata of clay, or rocks, as a and b, having a bed of sand or other matter permeable to water inter posed, and suppose that d is the place where the edges of the strata drop out, or where a fissure allows a free entrance of the water to the permeable stratum. The water at first descends through the effect of gravity ; it then passes along to wards b in consequence of the pressure exercised by the superior part of the column near d. Now suppose a perfora tion to be made at e, and continued till it reaches through the stratum a, the water will naturally continue to rise till it gains the same altitude as d, or at least till it reaches the surface, if below that alti tude. The water in fact between the two impermeable strata is in the same circumstances as in an artificial pipe ; and if the surface of the ground at e is con siderably lower than d, the ascensional force may be sufficient to cause a con siderable jet.