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manner by perennial springs. Hence the numerous alternations of porous beds with beds impenetrable to water, that occur throughout the entire series of stratified rocks, produce effects of the highest consequence in the hydraulic condition of the Earth, and maintain a universal system of natural Reservoirs, from which water overflows incessantly in the form of Springs, that carry with them fertility into the adjacent valleys. (See PI. 67, fig. 1, S.)
The discharges of water from these reservoirs are much facilitated, and increased in number, by the occurrence of Faults or Fractures that intersect the strata.*
There are two systems of Springs which have their origin in Faults, the one supplied by water descending from the higher regions of strata adjacent to a fault, by which it is simply intercepted in its descent, and diverted to the surface in the form of perennial springs; (see PI. 67, fig. 1, H.) the other maintained by water ascending from below by Hydrostatic pressure, (as in Artesian Wells,) and derived from strata, which at their contact with the fault, are often at a great depth; the water is conducted to this depth either by percolation through pores and crevices, or by small subterraneous channels in these strata, from more elevated distant regions whence it descends, until its progress is arrested by the Fault. (See PI. 67. Fig. 2, d, and PI. 69. Fig. 2. H. L.)
* Mr. Townsend, in his Chapter on Springs, states, that there are six distinct systems of springs in the neighbourhood of Bath, which issue from as many regular strata of subterraneous water, formed by filtration through either sand or porous rocks, and placed each upon its subjacent bed of clay. From these, one system of springs is produced by overflowing in the direction towards which the strata are inclined, or have their dip; whilst another system results from the dislocation of the strata, and breaks out laterally through the fractures by which they are intersected.
It is stated by Mr. Hopkins, (Phil. Mag. Aug. 1834, p. 131.) that all the great springs in the Limestone District of Derbyshire are found in conjunction with great Faults, "I do not recollect (says he) a single exception to this rule, for I believe in every instance where I observed a powerful spring, I had independent evidence of the existence of a great fault,"
Besides the advantages that arise to the whole of the Animal Creation, from these dispositions in the structure of the Earth, whereby natural supplies of water are multiplied almost to infinity over its surface, a farther result, of vast and peculiar importance to Man, consists in the facilities which are afforded him of procuring artificial wells, throughout those parts of the world which are best adapted for human habitation.
The Causes of the rise of water in ordinary artificial wells, are the same that regulate its discharge from the natural apertures which give origin to springs; and as both these effects will be most intelligibly exemplified, by a consideration of the causes of the remarkable ascent of water to the surface, and often above the surface, in those peculiar perforations which are called Artesian Wells, our attention may here be profitably directed to their history.
The name of Artesian Wells is applied to perpetually flowing artificial fountains, obtained by boring a small hole, through strata that are destitute of water, into lower strata loaded with subterraneous sheets of this important fluid, which ascends by hydrostatic pressure, through pipes let down to conduct it to the surface. The name is derived from Artois (the ancient Artesium,) where the practice of making such wells has for a long time extensively prevailed.*
* The manner of action of an Artesian Well is explained by the Section PI. 69. Fig, 3, copied from Mr. Hericart de Thury's representation of a double Fountain at St. Ouen, which brings up water, from two waterbearing strata at different levels below the surface. In this double fountain, the ascending forces of the water in the two strata A and B are different: the water from the lowest stratum B rising to the highest level l>"; that from the upper stratum A rising only to a'. The water from both strata is thus brought to tho surface by one Bore Hole of sufficient size to contain a double pipe, viz. a smaller pipe included within a larger one, with an interval between them for the passage of water; thus, the smaller pipe b brings up the water of the lower stratum B, to the highest level of the fountain b", whilst the larger pipe o brings up the water from stratum A to the lower level a': both these streams are employed to supply the Canal-basin at St. Ouen, above the level of the Seine. Should the lower stratum B contain pure water, and that in the upper stratum A be tainted, the pure water might by this apparatus be brought to the surface through the impure, without contact or contamination.
Artesian Wells are most available, and of the greatest use, in low and level districts where water cannot be obtained from superficial springs, or by ordinary wells of moderate depth. Fountains of this kind are known by the name of Blow wells, on the Eastern coast of Lincolnshire, in the low district covered by clay between the Wolds of Chalk near Louth, and the Sea-shore. These districts were without any springs, until it was discovered that by boring through this clay to the subjacent Chalk, a fountain might be obtained, which would flow incessantly to the height of several feet above the surface.
In the King's well at Sheerness sunk in 1781 through the London clay, into sandy strata of the Plastic clay formation, to the depth of 330 feet, the water rushed up violently from the bottom, and rose within eight feet of the surface. (See Phil. Trans. 1784.) In the years 1828 and 1829 two more perfect Artesian wells were sunk nearly to the same depth in the Dock yards at Portsmouth and Gosport.
Wells of this kind have now become frequent in the neighbourhood of London, where perpetual Fountains are in some places obtained by deep perforations through the London clay, into porous beds of the Plastic-clay formation, or into the Chalk.*
In common cases of Artesian wells, where a single pipe alone is used, if the Boring penetrates a bed containing impure water; it is continued deeper until it arrives at another stratum containing pure water; the bottom of the pipe being plunged into this pure water, it ascends within it and is conducted to the surface through whatever impurities may exist in the superior strata. The impure water, through which the boring may pass in its descent, being excluded by the pipe from mixing with the pure water ascending from below.
Important treatises upon the subject of Artesian Wells have lately been published by M. Hericart de Thury and M. Arago in France, and by M. Von Bruckmann in Germany.f
* One of the first Artesian wells near London was that of Norland House on the N. W. of Holland House, made in 1794, and described in Phil. Trans. London, 1797. The water of this well was derived from sandy strata of the plastic clay formation, but so much obstruction by sand attends the admis. sion of water to the pipes from this formation, that it is now generally found more convenient to pass lower through these sandy strata, and obtain water from the subjacent chalk. Examples of wells that rise to the surface of the lowest tract of land on the W. of London may be seen in the Artesian fountain in front of the Episcopal palace at Fulham, and in the garden of the Horticultural Society. Many such fountains have been made in the Town of Brentford, from which the water rises to the height of a few feet above the surface.
This height is found to diminish as the number of perpetually flowing fountains increases; and a general application of them would discharge the subjacent water so much more rapidly than it arrives through the interstices of the chalk, that fountains of this kind when numerous would cease to overflow, although the water within them would rise and maintain its level nearly at the surface of the land.
The Section, PI. 68 is intended to explain the cause of the rise of water in Artesian Wells in the Basin of London, from permeable strata in the Plastic-clay formation, and subjacent Chalk. The water in all these strata is derived from the rain, which falls on those portions of their surface that are not covered by the London Clay, and is upheld by clay beds of the Gault, beneath the Chalk and Fire-stone. Thus admitted and sustained, it accumulates in the joints and crevices of these strata to the line A. B. at which it overflows by springs, in valleys, such as that represented in our section under C. Below this line, all the permeable strata must be permanently filled with a subterranean sheet of water, except where faults and other disturbing causes afford local sources of relief. Where these reliefs do not interfere, the horizontal line A, B, represents the level to which water would rise by hydrostatic pressure, in any perforations through the London Clay, either into sandy beds of the Plastic-clay formation, or into the Chalk; such as those represented at D. E. F. G. H. I. If the Perforation be made at G. or H. where the surface of the country is below the line A. B. the water will rise in a perpetually flowing Artesian fountain, as it does in the valley of the Thames between Brentford and London.
t See Hericart de Thury's Considerations sur la cause du Jaillissement des Eaux des puits fores, 1829, VOL. I.—36
It appears that there are extensive districts in various parts of Europe, where, under certain conditions of geological structure, and at certain levels, artificial fountains will rise to the surface of strata which throw out no natural springs,*'
Notices scientifiques par M. Arago. Annuaire, pour l'An. 1835.
Von Bruckmann fiber Artesische Brunnen. Heilbronn am Neckar, 1833.
* The Diagrams in PI. 69. Figs. 1 and 2. are constructed to illustrate the causes of the rise of water in natural, or artificial springs, within basinshaped strata that are intersected by the side of Valleys, or traversed by Faults.
Supposing a Basin (PI. 69 Fig. 1.) composed of Permeable strata, E. F. G. alternating with impermeable strata, H. I. K. L. to have the margin of all these strata continuous in all directions at one uniformly horizontal level, A, B, the water which falls in rain upon the extremities of the strata E, F, G, would accumulate within them, and fill all their interstices with water up to the line A, B; and if a Pipe were passed down through the upper, into either of the lower strata, at any point within the circumference of this basin, the water would rise within it to the horizontal line A, B, which represents the general level of the margin of the Basin. A disposition so regular never exists in nature, the extremities or outcrops of each stratum are usually at different levels, (Fjg. 1, a. c. e. g.) In such cases the line a. b. represents the water level within the stratum G; below this line, water would be permanently present in G; it could never rise above it, being relieved by springs that would overflow at a. The line c. d. represents the level above which the water could never rise in the stratum F; and the line e, f, represents the highest water level within the stratum E. The discharge of all rain-waters that percolated the strata, E, F, G, thus being effected by overflowing at e. c. a.
If common wells were perforated from the surface, i. k. 1. into the strata G. F. E, the water would rise within them only to the horizontal lines a b, c d, e f.
The upper porous stratum C, also, would be permanently loaded with water below the horizontal line, g, h, and permanently dry above it.
The theoretical section, PI. 69. fig. 2. represents a portion of a basin intersected by the fault II, L, filled with matter impermeable to water. Supposing the lower extremities of the inclined and permeable strata N, O, 1% Q, R, to be intersected by the fault or dike H, L, the rain-water which enters the uncovered portion of these strata between the impermeable clay beds, A, B, C, D, E, would accumulate in the permeable strata up to the horizontal lines, AA", BB", CC", DD", EE". If an Artesian weU was perforated into each of these strata to A', B', C, V, E', through the clay beds A, B, C, D, E, the water from these beds would rise within a pipe ascending from the perforations of the levels A", B", C", D", E".