CHAPTER V.

SUBTERRANEAN WATERS AND ARTESIAN WELLS.

Subterranean Distribution of the Waters-Admirable Provisions of NatureHydrostatic Laws regulating the Flow of Springs-Thermal Springs-Intermittent Springs-The Geysir-Bunsen's Theory-Artesian Wells-Le Puits de Grenelle-Deep Borings-Various Uses of Artesian Wells-Artesian Wells in Venice and in the Desert of Sahara.

IN

N every zone the evaporating power of the sun raises from the surface of the ocean vapours, which hover in the air until, condensed by cold, they descend in rain upon the earth. Here part of them are soon restored to the sea by the swollen rivers; another part is once more volatilised; but by far the larger quantity finds its slow way into the bowels of the earth, where it serves for the perennial supply of wells and springs.

The distribution of these subterranean waters, and the simple laws which regulate their circulation, afford us one of the most interesting glimpses into the physical economy of our globe. We know that the greater part of the earth's surface is composed of stratified rocks, or alternate beds of impermeable clay and porous limestone or sand, which were originally deposited in horizontal layers, but have since been more or less displaced and set on edge by upheaving forces. Wherever permeable beds of limestone or sand crop out on the surface of the land, the residuary portions of rain-water which are not disposed of by floods or by evaporation, must necessarily penetrate into the pores and fissures, and descend lower and lower, until they finally reach an impermeable stratum which forbids their further progress to a greater depth.

The granite, gneiss, porphyry, lava, and other unstratified and crystalline rocks of igneous origin, which cover about a

third part of the habitable globe, are likewise intersected by innumerable fissures and interstices, which, in a similar manner, collect and transmit rain-water.

Thus the plutonic or volcanic forces which have gradually moulded the dry land into its present form have also provided it with the necessary filters, drains, reservoirs, and conduits, for the constant replenishment of springs, brooks, and rivers. As every porous layer is more or less saturated with moisture, the stratified rocks are frequently traversed at various depths by distinct sheets of water, or rather, in most cases, by permanently drenched or waterlogged sheets of chalk or sand. Thus, in a boring undertaken in search of coal at St. Nicolas d'Aliermont, near Dieppe, no less than seven very abundant aquiferous layers or beds of stone were met with from about 75 to 1,000 feet below the surface. In an Artesian boring at Paris, five distinct sheets of water, each of them capable of ascension, were ascertained; and similar perforations executed in the United States, and other countries, have in the same manner traversed successive stages of aqueous deposits.

Thus there can be no doubt that vast quantities of water are everywhere accumulated in the porous strata of which a great part of the superficial earth-rind is composed, the rapidity with which they circulate varying of course with the amount of hydrostatic pressure to which they are subjected, and the more or less porous and permeable nature of the beds through which they percolate. Were the ground we stand on composed of transparent crystal, and the subterranean water-courses tinged with some vivid colour, we should then see the upper earth-crust traversed in every direction by aqueous veins, and frequently as saturated with water as the internal parts of our body are with blood. But Nature not only perennially feeds our springs and brooks from the inexhaustible fountains of the deep; it is also one of her infinitely wise provisions that the same water which, if placed in casks or open tanks, becomes putrid, continues fresh so long as it remains in the cavities and interstices of the terrestrial strata. While filtering through the earth, it is generally cleansed of all the organic substances whose decay would inevitably taint its purity, and comes forth

salubrious and refreshing, a source of health and enjoyment to the whole animal creation.

The extreme limits to which the waters descend into the earth of course escape our direct observation, as the lowest point to which the subterranean regions have been probed is less than 2,000 or 2,500 feet below the level of the sea; but as we know from the formation of many basins that the strata of which they are composed attain in many cases a thickness of from 20,000 to 30,000 feet, there can hardly be a doubt that they are permeated by water to an equal depth. As steam plays so great a part in volcanic phenomena, the seat or effective cause of which must needs be sought for at an immense distance below the surface of the earth, we have another proof of the vast depth to which the subterranean migrations of water are able to attain.

After this brief glimpse into the reservoirs of the deep, we have to ascertain the power which raises their liquid contents and forces them to reappear upon the surface of the earth. If we pour water into a tube, bent in the form of the letter U, it will rise to an equal level in both branches. We will now suppose that the left branch of the tube opens at the top into a vast reservoir, which is able to keep it constantly filled, and that the right branch is cut off near the bottom, so that only a small vertical piece remains. The pressure of the water column in the left branch will in this case force the liquid to gush out of the orifice of the shortened right branch to the level which it occupied while the branch was still entire.

These two hydrostatic laws, or rather these two modifications of the same law, have been frequently put to practical uses, as, for instance, in the communicating tubes which distribute the waters of an elevated source or reservoir to the various districts of a town, or in the subterranean conduits which serve to create fountains, such as those of Versailles or the Crystal Palace.

When the Romans intended to lead water from one hill to another, they constructed, at a vast expense, magnificent aqueducts across the intermediate valley; but the Turks, whom we look upon as ignorant barbarians, obtain the same result in a much more economical manner, and in this respect far

surpass the ancients, who, had they been better acquainted with the first principles of hydrostatics, would indeed have left us fewer specimens of their architectural skill, but would at the same time have saved themselves a great deal of unnecessary expense.

Down the slope of the hill from which the water is to be conducted, the Turks lay a tube of brick or metal, which, crossing the valley, moulds itself to its different inflections, and ultimately ascends the declivity of the hill on the opposite side, where, in virtue of the law above cited, the water rises as high as on that from which it descended. If we suppose the descending branch of the tube to be prolonged only as far as the level of the valley, with a superficial orifice, then the liquid will of course gush forth in a vertical column, and form a jet d'eau, or fountain, its height being determined by the elevation of the sheet of water by which it is fed, and the consequent degree of pressure which acts upon it. This is the principle on which all artificial fountains are constructed. The conduit, for instance, which feeds the grand fountain of the Tuileries receives its water from a reservoir situated on the heights of Chaillot.

Whatever the form of the tube may be in which the liquid is contained, the simple hydrostatic law which regulates its level remains unmodified. Let the tube be circular, elliptic, or square, with a single orifice or with many-let it be open or choked with pebbles or permeable sand-in every case the water will invariably rise to the same height, provided the tube be perfectly water-tight; or else gush forth wherever it finds an opening below the highest level.

This hydrostatic principle so perfectly illustrates the origin of springs, that it is almost superfluous to enter into any further details on the subject.

When we consider that porous or absorbent strata, alternating with impermeable strata, frequently crop out on the back or on the slope of hills or mountains, and then, having reached their base, extend horizontally beneath the plain, there can be no doubt that they are placed in the same hydrostatic conditions as ordinary water-conducting tubes, and that wherever any fissure or opening occurs in the superincumbent impervious strata at any point below

HOT AND COLD SPRINGS.

43

the highest level of the water, springs must necessarily be

formed.

As the same strata often extend over many hundreds of miles, we cannot wonder that sources frequently issue from the centre of immense plains, for the hydrostatic pressure which causes them to gush forth may have its seat at a very considerable distance.

As the waters by which the springs are fed have often vast subterranean journeys to perform, their temperature is naturally independent of that of the seasons or of the changes of the atmosphere. Thus, cold springs occur in a tropical climate, when their subterranean channels descend from high mountains, and boiling sources gush forth in the Arctic regions when forced upwards from a considerable depth.

While the waters filter through the earth, they also naturally dissolve a variety of substances, and hence all springs are more or less impregnated with extraneous particles. But many of them, particularly such as are of a higher temperature, contain either a larger quantity or so peculiar a combination of mineral substances as to acquire medicinal virtues of the highest order.

The geological phenomena which favour the production of thermal springs are extremely interesting, and point to a deep-seated origin. By far the greater number of these fountains arise near the scene of some great subterranean disturbance, either connected with volcanic action, or with the elevation of a chain of mountains, or lastly by cliffs and fissures caused by disruption. Thus the thermal springs of Matlock and Bath accompany great natural fissures in the mountain limestone, and the hot springs of Wiesbaden and Ems, of Carlsbad and Toeplitz, all lie contiguous to remarkable dislocations, or to great lines of elevation, or to the neighbourhood of a volcanic focus.

One of the most remarkable phenomena of thermal springs is the constant invariableness of their temperature and their mineral impregnation. During the last fifty or sixty years, ever since accurate thermometrical observations and chemical analyses have been made, the most celebrated mineral sources of Germany have been found to contain the same proportion of mineral substances. This is truly astonishing