affords one of the many proofs of Design, which arise out of the investigation of its actual condition, and of its relations to the organized beings which are placed upon it.

Nearly three-fourths of the Earth being covered with Sea, whilst the remaining dry land is in need of continual supplies of water, for the sustenance of the animal and vegetable kingdoms, the processes by which these supplies are rendered available for such important purposes, form no inconsiderable part of the beautiful and connected mechanisms of the terraqueous Globe.

The great Instrument of communication between the surface of the Sea, and that of the Land, is the Atmosphere, by means of which a perpetual supply of fresh-water is derived from an Ocean of salt water, through the simple process of evaporation.

By this process, water is incessantly ascending in the state of Vapour, and again descending in the form of Dew and and Rain.

Of the water thus supplied to the surface of the land, a small portion only returns to the Sea directly in seasons of flood through the channels of Rivers;*

A second portion is re-absorbed into the Atmosphere by Evaporation;

A third portion enters into the composition of Animal and Vegetable bodies;

A fourth portion descends into the strata, and is accumulated in their interstices into subterraneous sheets and reservoirs of water, from which it is discharged gradually at the surface in the form of perennial Springs, that form the ordinary supply of Rivers.

As soon as Springs issue from the Earth, their waters commence their return towards the Sea; rills unite into stream

* It is stated by M. Arago, that one-third only of the water which falls in rain, within the basin of the Seine, flows by that river into the sea: the remaining two-thirds either return into the atmosphere by evaporation, or go to the support of vegetable and animal life, or find their way into the sea hy subterraneous passages. Annuaire, pour l'An 1835,

ALTERNATIONS OF CLAY WITH POROUS STRATA.

417

lets, which, by farther accumulation form rivulets and rivers, and at length terminate in estuaries, where they mix again with their parent ocean. Here they remain, bearing part in all its various functions, until they are again evaporated into the Atmosphere, to pass and repass through the same Cycles of perpetual circulation.

The adaptations of the Atmosphere to this important service in the economy of the Globe belong not to the province of the geologist. Our task is limited to the consideration of the mechanical arrangements in the solid materials of the Earth, by means of which they co-operate with the Atmosphere, in administering to the circulation of the most important of all fluids.

There are two circumstances in the condition of the strata, which exert a material influence in collecting subterraneous stores of water, from which constant supplies are regularly giving forth in the form of springs; the first consists in the Alternation of porous beds of sand and stone, with strata of clay that are impermeable by water;* the second circumstance is the Dislocation of these strata, resulting from Fractures and Faults.

The simplest condition under which water is collected within the Earth, is in superficial beds of Gravel which rest on a sub-stratum of any kind of Clay. The Rain that falls upon a bed of gravel sinks down through the interstices of the gravel, and charges its lowest region with a subterraneous sheet of water, which is easily penetrated by wells, that seldom fail except in seasons of extreme drought. The accumulations of this water are relieved by Springs, overflowing from the lower margin of each bed of gravel.

A similar result takes place in almost all kinds of permeable strata, which have beneath them a bed of clay, or of any other impermeable material. The Rain water descends and accumulates in the lower region of each porous stratum next above the clay, and overflows in the same

* See p. 62.

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 Pl. 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 Pl. 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

• 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."

by the Fault. (See Pl. 67. Fig. 2, d, and Pl. 69. Fig. 2. H. L.)

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.

Artesian Wells.

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 Pl. 69. Fig. 3, copied from Mr. Héricart 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 ''; that from the upper stratum A rising only to a'. The water from

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

both strata is thus brought to the 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 a 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.

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.