The secondary and tertiary geological formations, such as those underneath London and Paris, often present the appearance of immense basins, the boundary or rim of the basin having been formed by an upheaval of the subjacent strata. In these formations it often happens that a porous stratum, consisting of sand, sandstone, chalk, or other calcareous matter, is included between two impermeable layers of clay, so as to form a flat porous U tube, continuous from side to side of the valley, the outcrop on the surrounding hills forming the mouth of the tube. The rain filtering down through the porous layer to the bottom of the basin forms there a subterranean pool, which with the liquid or semi-liquid column pressing upon it constitutes a sort of huge natural hydrostatic bellows. Sometimes the pressure on the superincumbent crust is so great as to cause an upheaval or disturbance of the valley, and there can be little doubt that many earthquakes, that are manifestly not of volcanic origin, are due to this simple cause.

It is obvious, then, that when a hole is bored down through the upper impermeable layer to the surface of the lake, the water will be forced up by the natural law of water seeking its level, to a height above the surface of the valley greater or less according to the elevation of the level in the feeding column, thus forming a natural fountain on precisely the same principle as that of most artificial fountains, where the water supply comes from a considerable height above the jet.

In the Tertiary formations, the porous layers are not so thick as in the Secondary, and consequently the occurrence of underground lakes is not on so grand a scale; but there being a more frequent alternation of these sandy beds, we find a greater number of them, and often a series of natural fountains may be obtained in the same valley, proceeding from water-bearing strata at different depths, and rising to different heights.

It does not follow that all the essentials for an Artesian well are present, though two impermeable strata with a porous one between may crop out round a basin. There must be, in the first place, continuity of the permeable bed for the uninterrupted passage of the water; and there must be, on the other hand, no flaw or breach in either of the confining layers by which the water might escape. To one or other of these causes is due the failure of many attempts to find Artesian wells where from appearances they might be expected. It has occasionally happened that on deepening the bore, with the hope of increasing the flow of water, it has ceased altogether, doubtless from the lower confining layer being pierced, and the water allowed to escape by another outlet. The subterranean pool is frequently of small extent, and of the nature of a channel rather than of a broad sheet of water; and the existence of one spring is no guarantee that another will be found by merely boring to the same depth in its neighbourhood. Several such failures are recorded by Arago in his notices on Artesian wells in the Annuaire du Bur. des Long. (1835, <fcc.), and referred by him to the circulation of the subterranean waters in irregular trenches between impermeable masses. The preliminary theoretical determination of the existence of these Artesian conditions is in itself a difficult matter, and can be arrived at only by a thorough acquaintance with the geological disposition of the district. Still more difficult, as can readily be imagined, is the practical execution of the boring of a hole of a few inches diameter to the depth of hundreds of feet. The keeping of the bore quite vertical, the ready elevation of the loosened rubbish, the prevention of the breakage of the boring rods at great depths, are some of the difficulties that beset the operation, and have tried the ingenuity of engineers.

The mechanical appliances employed in boring deep wells are not essentially different from those used in the sinking of mining shafts; and it may be remarked that the expense and tediousness of Artesian boring at the beginning of this century are to be ascribed not so much to the want of steam engines, as to the awkward mode of using the boring tool. In the old method a continuous boring bar was made use of, the steel boring tool or chisel being fixed to iron rods, which were screwed together in lengths of 10 to 15 feet. This, with a strong cross handle at the top, formed*a sort of large gimlet, and was turned by two men, the tool being raised and suddenly dropped, as required, by a third man at the end of a lever, which was connected by a rope or chain w ith the gimlet-head. Most of the accidents, w hich formerly rendered well-sinking a hazardous task, were due to breakages of the borer or boring bar by the sudden fall which it was necessary to give the too in pecking through a hard stratum. The modern mode of boring is but an adaptation of steam power to a simple method practised for ages by the Chinese. They bore their deep wells by a steel head worked up and down by means of a cord suspended from a lever—a wooden pipe being used to guide the borer and keep the hole perfectly vertical. Free falling tools, worked by steam power, are now employed when bore-holes of large diameter have to be executed, the weight of the tool giving sufficient percussion to pierce the hardest rock. In the enormous Artesian boring, of 5⅜ feet diameter, begun, in 1866, at La Chapelle, in Paris, a boring tool of about 4 tons weight was employed. It was raised by steam power, and allowed to fall freely from twenty to twenty-five times a minute, a twist of - 1 ⅛th of a turn being given after each stroke. (See Engineering, vol. viii. pp. 401, 413.) The borer is withdrawn at intervals, and the rubbish removed by means of a cylindrical augur with a valve at the bottom. Permanent pipes of cast or wrought iron must be fixed in the bore-hole to keep it open; and if the operations are through clay or sand, the pipes must follow the progress of the chisel as closely as possible.

One of the most remarkable examples of the patience and skill required to surmount the difficulties of deep boring is the Artesian well at Grenelle, in the vicinity of Paris. The operation of boring extended from 1834 to 1841; after a depth of 1254 feet had been reached (May 1837), a length of 270 feet of the boring rods suddenly broke off and fell to the bottom of the hole, and nearly fifteen months’ constant labour was required to pick it up again. Discouraged by the delay, the French Government was to have abandoned the project after a depth of 1500 feet had been reached without any satisfactory result; but the urgent representations of Arago prevailed on them to prosecute the work. And it was fortunate, for an additional depth of about 300 feet proved the correctness of Arago’s theory. On the 26th February 1841, at a depth of 1798 feet, the boring rods suddenly sank a few yards. The subterranean water-bearing stratum had been reached, and within a few hours a vast column of water spouted up at the rate of 600 gallons per minute, and at a temperature of nearly 82° Fahr., furnishing a valuable source of supply for a suburb of Paris where it was greatly wanted. Prior to this no Artesian boring had reached even 1C00 feet; and that of Grenelle waβ the deepest executed till the completion (12th August 1850) of the salt-spring at Kissingen, in Bavaria, which throws up a column of water to the height of 58 feet from a depth of 1878⅜ feet. The most remarkable feature of this spring is that the projecting force is due, not to hydrostatic pressure, but to that of carbonic acid gas generated at the junction of the gypsum with the magnesian limestone, about 1680 feet down. Modern mechanical improvements have enabled engineers to exceed these Artesian dimensions considerably, and at a greatly diminished cost. The well at Passy, near Paris, which is supplied from the same waterbearing stratum as that of Grenelle, was bored by the Saxon engineer Kind in a very short time, having been begun on 15th September 1855, and carried to a depth of over 1700 feet by March 1857. Its total depth is now about 1923 feet, with the enormous diameter of 2 feet 4 inches at the bottom; and it throws up a continuous stream of water at the rate of 5,582,000 gallons per day to a height of 54 feet above the ground. Among other deep wells sunk in the Paris basin subsequently to those of Grenelle and Passy, the following may be mentioned. A gigantic bore, 5 feet 7 inches in diameter, was begun in January 1866 at La Chapelle, and by November 1869 had reached a depth of 1811 feet, the intention of the engineers being to extend it to a depth of 2950 feet. The mechanical processes employed are detailed in pp. 401 and 413 of vol. viii. (1869) of Engineering. A bore of 19 inches diameter was carried down to a depth of 1570 feet in about two and a half years (1864-7), for the purpose of obtaining a water supply for the sugar refinery of Μ. Say in Paris; and the same engineer who executed this work (Μ. Dru), began in 1866 an Artesian boring of the huge diameter of 6∣ feet at the part of Paris named Butte aux Cailles, to be carried down to a depth of 2600 to 2900 feet. (See Engineering, vol. iii. p. 605.) Besides these monster wells, there are in the Paris basin a great many others, varying from 300 to 400 feet in depth, and from 2 to 8 inches in the diameter of the bore-hole.

The Tertiary chalk strata over which London stands have been riddled with Artesian borings for the sake of pure water supply. The source of the New River (which was opened in 1613) is one of the chalk springs, at Chadwell, near Ware; this alone yields London a twenty-fourth part of its water supply, some 4 J million gallons a day. Many of the large London breweries obtain the water employed in the manufacture of their beer from Artesian wells over 300 feet deep. The Kentish Town water-works have a well 900 feet deep, which penetrates the chalk bed some 580 feet. Artesian wells also supply water to the Bank of England, the fountains at Trafalgar Square, the Mint, Pentonville Prison, Colney Hatch Asylum, Shoreditch Workhouse, the North Western Railway Station, Holloway Prison, &c. On the duke of Northumberland’s property at Sion House there is a chalk well which descends 620 feet, and spouts the water up about 4 feet above the surface. At Merton ⅛ Surrey, at Brighton, at Southampton, all along the east coast of Lincolnshire, and in the low district between the chalk wolds near Louth and the Wash, Artesian borings have long been known, and go by the name of blow-wells among the people of the district. It is worthy of notice that the general level to which the water rises in the London district has been very sensibly lowered by the immense number of perforations that have been made; and in several wells where the water formerly rose to the surface, it now stands considerably below it, and requires to be pumped up.

None of the Artesian borings in England approach the depths frequent on the Continent and in America. The average depth of the water-bearing stratum around Paris is six times that of the London chalk beds; and in some parts of Germany and of America, wells have been sunk to even double the depth of the Parisian wells of Grenelle and Passy. At Chicago there are two wells, one 700 feet and the other 1000 feet deep, with a diameter of 5 inches, which supply the city with 800,000 gallons of water daily. At the town of St Louis, in the State of Missouri, an Artesian boring was carried down for 3147 feet, but as only brine was obtained, it was ordered by Government to be stopped in September 1868, when the depth was approaching three-quarters of a mile. The deepest boring in the world is at Sperenberg, twenty miles from Berlin, having been sunk for the purpose of obtaining a supply of rock salt. A bore-hole of 16 feet diameter was carried down to the depth of 280 feet, where the salt bed began; after a further descent of 680 feet was attained, the bore was reduced to 13 inches diameter, and then continued till the extraordinary depth of 4194 feet was reached, without having pierced through the salt deposit, which has thus the enormous thickness of at least 3907 feet.

The following are some of the other most important Artesian sinkings that have been made. At Louisville, in Kentucky, a bore of 3 inches was carried to a depth of 2086 feet between April 1857 and the summer of 1858, when a supply of water almost as plentiful as that of Grenelle was obtained. At Charleston, South Carolina, there is a well 1250 feet deep, which was begun in 1848, and which discharges water some 10 feet above the surface at the rate of 1200 gallons an hour. In 1858 a well at Neusalwerk, near Minden, had reached the depth of 2288 feet. At Bourne, Lincolnshire, there is a well 95 feet deep, which yields over half a million gallons of water per day, the pressure being sufficient to supply the town and force the water to the tops of the highest houses. There is one, on the property of the Continental Hotel, in Philadelphia, over 200 feet deep, and 8 inches in diameter, which yields 50,000 gallons a day. Two Artesian wells at Croydon supply a million gallons of water per day; and Brighton draws over a million gallons from Artesian sinkings. There is a well at Bages, near Perpignan, which gives 330 gallons per minute; and one at Tours, which jets about 6 feet above ground, and gives 237 gallons per minute.

The boring of wells in the great-desert of Sahara is a very ancient industry; and some oases are supplied with water wholly from Artesian wells. The average depth of these is from 160 to 200 feet, and the upper strata have only to be pierced to give a constant stream. With their primitive methods of boring, the Arabs often labour for years before they reach the wished-for pool; and with only palm wood as a casing, they have great difficulty in keeping the bore-hole from closing up by the drifting of the sand, and they require to scour them out periodically. Since 1858 an immense number of perforations have been made by French engineers, and the fertilising effect upon the sandy desert plains is already making itself apparent. Villages are built and palm trees are growing where formerly there was but waste and solitude. (See Geological Magazine, vol. i. p. 31.) The importance of deep wells in such cases cannot be over-estimated.

Artesian wells have been made to supply warm water, for keeping hospitals, <fcc., at a constant temperature. Invariably the temperature of water from great depths is higher than that at the surface, this being due to some unknown source of heat in the interior of the globe. The temperature of the water in the well at Grenelle is 82° Fahr., and that of Passy the same, showing that they have a common source. Kissingen well has a temperature of 66° Fahr., that of St Louis one of 73 0, 4 Fahr., that of Louisville 76⅛ o Fahr., and that of Charleston 87° Fahr. The average rate of increase of temperature is 1° for a descent of from 40 to 55 feet. In Würtemberg the water of Artesian wells is employed to maintain in large manufactories a constant temperature of 47° when it is freezing outside. Artesian waters have also been employed to reduce the extreme variations of cold in fish-ponds, which are so fatal to their finny inhabitants.

See Héricart de Thury, Considérations sur la cause du Jaillissement des Eaux des Puits-forés, Paris, 1829; Bruckmann, Ueler Artesische Brunnen, Heilb., 1833; Arago, “Notices Scientifiques,” Annuaire du Bureau des Long., 1834; Engineering, vols. ii. iii. and viii.; Transactions of the Academy of St Louis, vol. i. No. 1; American Jour, of Science and Art, 1859; Beardmore’s Manual of Hydrology; Notice by General Desvaux in the Annales de Mines, 1858; Burnell’s Hydraτdic Engineering (Weale’s Series); Les Pτιits artésiens des oasis méridionales de l’Algérie, 2d edition, 12mo, 1861, by Berbrugger.