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WELL, the name given to an artificial boring in the earth through which water can be obtained. Two classes may be distinguished: shallow or ordinary wells, sunk through a permeable stratum until an impermeable stratum is reached; and deep and Artesian wells (q.v.), the latter named from Artois in France, which are sunk through an impermeable stratum down into a water-bearing stratum which overlies an impermeable stratum. Obviously ordinary wells can supply water very cheaply, but, since impurities readily reach them, there is great risk of contamination. The same does not apply to deep wells, such water being usually free from organic impurities. In ordinary wells, and in deep wells, the water requires pumping to the surface; in artesian wells, on the other hand, the water usually spouts up to a greater or less height above it.

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 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. 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 this pressure 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.

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 also be continuity of the permeable bed for the uninterrupted passage of the water, and no breach in either of the confining layers by which the water might escape. 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. Faults also have an effect on the supply, which in many cases has been found to increase by cutting headings or adits. The most suitable strata in England are the Chalk, Oolite, New Red Sandstone and Lower Greensand; London is in part supplied by the Chalk, whilst Liverpool utilizes the New Red. The theoretical determination of the existence of artesian conditions can be arrived at only by a thorough acquaintance with the geology of the district. Although water from deep wells is free from organic matter, it usually contains salts such as calcium bicarbonate, &c., which make the water unsuitable for washing and certain manufacturing purposes although it is fit for drinking.

The mechanical appliances employed in boring for water are practically the same as in boring for petroleum (q.v.). The upper part of a deep well may be of brick, the continuation being lined with steel pipes, or, better, it may be lined with metal for its entire length.

One of the most remarkable artesian wells is at Grenelle, near Paris. The operation of boring extended from 1834 to 1841; after a depth of 1254 ft. had been reached (May 1837), a length of 270 ft. of the boring rods broke 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 ft. had been reached without any satisfactory result; but Arago prevailed on them to prosecute the work, and an additional depth of about 300 ft. proved the correctness of Arago's theory. On the 26th February 1841, at a depth of 1798 ft., the boring rods suddenly sank a few yards, 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 F. Prior to this no artesian boring had reached even 1000 ft.; and that of Grenelle was 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 ft. from a depth of 1878½ ft. 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 ft. 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 water-bearing stratum as that of Grenelle, was bored by Kind in a very short time, having been begun on 1 5th September 1855, and carried to a depth of 1732 ft. by March 1857. Its total depth is now about 1923 ft. with the diameter of 2 ft. 4 in. at the bottom; and it throws up a continuous stream of water at the rate of five and a half million gallons per day to a height of 54 ft. 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 ft. 7 in. in diameter, was begun in January 1866 at La Chapelle, and by November 1869 had reached a depth of 1811 ft., the intention being to extend it to a depth of 2950 ft. A bore of 19 in. diameter was carried down to a depth of 1570 ft. in about two and a half years (1864-1867), for the purpose of obtaining a water-supply for the sugar refinery of Say in Paris; and the same engineer who executed this work (Dru) began in 1866 an artesian boring of the huge diameter of 6½ ft. at the part of Paris named Butte aux Cailles, to be carried down to a depth of 2600 to 2900 ft. In the Paris basin there are a great many other wells, varying from 300 to 400 ft. in depth, and from 2 to 8 in. 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. Many of the large London factories, railways, institutions are supplied by artesian wells over 300 ft. deep. At Merton in 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. 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 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. In Chicago there are several wells more than 2000 ft. deep; and at West Chicago in Dupage county, Illinois, there is one 3081 ft. deep. In the city of St Louis, Missouri, there is an artesian well 3843½ ft. deep, yielding a few gallons of salty water (temperature, 105° F.) a minute; boring was stopped in September 1868. Among the deepest borings in the world are: a well in Putnam Heights, Windham county, Connecticut, 6004 ft. deep and 6 in. in diameter, yielding 2 gallons per minute with water rising to 4 ft. from the surface; one at Schladenbach (5735 ft.), near Leipzig; one 12 m. south-east of Pittsburg, which is 5575 ft. deep and 6¼ in. in diameter; one in Lawrence county, Alabama, 5120 ft. deep and 6 in. in diameter, yielding gas, oil and salt water; and one (about 4200 ft.) at Sperenberg 20 m. from Berlin, sunk for the purpose of obtaining a supply of rock salt, — the salt deposit here is 3907 ft. thick.

The following are some of the other most important artesian sinkings that have been made. At Louisville, Kentucky, a bore of 3 in. was carried to a depth of 2086 ft. between April 1857 and the summer of 1858; it yields 264 gallons a minute and its fountain rises 170 ft. high. At Charleston, South Carolina, there are: one well 2050 ft. deep and 4 in. in diameter, yielding 450 gallons a minute; another 1945 ft. deep and 5 in. in diameter, yielding 695 gallons a minute; and three more each exceeding 1900 ft. in depth. In 1858 a well at Neusalwerk, near Minden, had reached the depth of 2288 ft. At Bourne, Lincolnshire, there is a well 95 ft. 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 in Philadelphia (Mount Vernon and 13th sts.), 3031 ft. deep and 8 in. in diameter, yielding 2600 gallons a minute. There are several deep wells in South Dakota: in Aberdeen City there are two 1300 ft. deep with flows of 1350 and 1000 gallons, respectively, per minute. 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 ft. 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 ft., 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 has already made itself apparent. The importance of deep wells in such cases cannot be over-estimated.

Artesian wells have been made to supply warm water, for keeping hospitals, &c., at a constant temperature. Invariably the temperature of water from great depths is higher than that at the surface. The temperature of the water in the well at Grenelle is 82° F., and that of Passy the same, showing that they have a common source. Kissingen well has a temperature, of 66° F., that of St Louis

one of 73°.4 F. that of Louisville 76½° F., and that of Charleston 87° F. The average rate of increase of temperature is 1° for a descent of from 40 to 55 ft. In Württemberg 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 temperature in fish-ponds.