ARTESIAN WELLS

(Fr. Puits Artesicns.) Vertical perforations of the exterior crust of the earth, of small diameter, and frequently of great depth, through which subterraneous water arises to the surface, often forming abundant and elevated jets. The name Artesian is derived from Artois, a province of France, where especial attention has been given to this means of obtaining water; but it appears, from suflScient historical evidence, that wells of this kind were perfectly well known to the ancients. Niebuhr cites a pa age from Olympiad or us.who flourished at Alexandria about the middle of the sixth century, in which it is stated that when wells are dug in the Oasis to the depth of two hundred, three hundred, or sometimes five hundred yards, rivers of water gush out from their orifices of which the agriculturists take advantage to irrigate their fields. The oldest Artesian well known to exist in France is in the ancient convent of the Chartreux, at Lillers in Artois. It is said to have been made in 1 126. Others exist at Stuttgard, of great antiquity, though their dates cannot be fixed with precision. The inhabitants of the great desert of Sahara appear also to have been lolig acquainted with this mode of obtaining water, and the Chinese are said (but the truth of the statement is questionable) to have practised it for you sands of years. Various conjectures have been made as to the source of the water which comes from the Artesian wells. It was long believed that the water of the sea must nece arily penetrate by way of infiltration into the interior of the continents, and at length form large bodies of subterraneous waters, which, excepting for capillary influences, would not rise above the general level of the ocean. Another opinion, maintained by Aristotle, Seneca, Cardan, and even Des cartes, was, that the subterraneous water, from which the sources of rivers and springs are supplied, is the product of the condensation of aqueous vapours ascending from the interior parts of the earth in consequence of the central heat. But these hypotheses are founded on mere conjecture, unsupported by the sliglitest evidence, and consequently merit no attention. The simplest and most natural explanation is, that the water of ordinary wells, of Artesian fountains and rivers, is supplied by the rain which falls on the surface at a higher elevation, and which penetrates through the pores and fi ures of the ground till it meets with some impermeable stratum, or is collected in subterranean reservoirs. It has been objected that springs are sometimes situated on or near the summits of mountains, which could not be supplied in this way; but on an attentive examination of all the circumstances, that is to say, on measuring accurately the extent of surface at a greater elevation than the spring, and comparing it with the quantity of rain that falls annually in the same climate, it has been found, in every instance, that the aqueous deposition from the atmosphere greatly exceeds the supply from the spring. It is computed that not more than a third part of the rain which falls in the valley of the Seine is conveyed to the sea by the river; the remaining two-thirds support vegetation, supply fountains and springs, or are returned to ttie atmosphere by evaporation. The imARTESIAN FOUNTAINS. mense bodies of water which some continental rivers roH towards the ocean are but a small part of the rain which falls in the surrounding countries. A uming, then, tliat the subterraneous water is supplied fi-om atmospherical deposition, it remains to be explained how it arrives at the situation it occupies in the interior 5f the earth, and by what forces it is raised from great depths to the surface. All persons who have paid the slightest 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 perpendicularly to the horizon, having the appearance 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. Between the strata are frequently found beds of permeable sand, through which water, coming in contact with them, must nece arily pa , first, through the inclined Cart by virtue of its specific gravitv, and then in the orizontal branches, by virtue of the pre ure of the water remaining in the elevated portions of the strata. In this manner the water insinuates itself between the different strata; and hence we may expect that in localities where the tertiary stratification prevails, as many distinct sources of subterraneous water will be met with in penetrating perpendicularly through the surfacp, as there are distinct laj^ers of a sanclv' or gravelly nature reposing on impermeable strata. This conseouence of the theory is perfectly confirmed by experience. M. Arago mentions, that in digging for coal near St. Nicholas d'Aliermont, 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; 3d, 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 was very great. Similar occurrences are frequent in the neighbourhood of London, and are familiar to all miners. But it is not enough that the structure of the country is such that water can percolate between the different strata; the phenomena cf Artesian fountains could not be explained without supposing it to be collected in large quantities, and forming subterraneous reservoirs of immense extent. That such reservoirs exist, no doubt can be entertained. The celebrated fountain of Vaucluse 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 4780 cubic feet per minute. After great rains, its product is thrice as great. The mean quantity emitted is 9360 cubic feet per minute, or about 6030 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 subterraneous cavities in immense quantities, but that it also pa es 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 subterraneous water is collected, its elevation to the surface through a natural fi ure or artificial perforation is a simple result of hydrostatic pre ure. 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 received under beds of impermeable materials. Conceive two strata of day or rocks as a and b, having a bed of sand or other matter permeable to water interposed, and suppose that d is the place where the edges of the strata crop out, or where a fi ure allows a free entrance of the water to the permeable stratum. The water at first descends through the effect of gravity; it then pa es along towards b in consequence of the pre ure exercised by the superior part of the column near d. Now suppose a perforation" 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 altitude. The water in fact between the two impermeable strata is in the same circumstances as in an artificial pipe; and if the surface [s. 104]