cause an upheaval or disturbance of the valley, and there can be little doubt that many earthquakes, that arc mani
festly 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 foun tains, 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 fre quently 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, &c.), and referred by him to the circulation of the subterranean waters in irregular trenches between imper meable 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 dis trict. Still more difficult, as can readily be imagined, is tho 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 tiie 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 ara not essentially different from those used in the sinking of mining shafts ; and it may be remarked that the ex pense and tediousncss 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, f i >rmed*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 with the gimlet-head. Most of the accidents, which formerly rendered well-sinking a hazardous t.usk, 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 w r orked 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 5rV 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 -^th 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 opera tions are through clay or sand, the pipes must follow the progress of the chisel as closely as possible.
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; aftera 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 representa tions 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 1000 feet; and that of Crenelle 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 feet from a depth of 1878^ feet. The most remarkable feature of this spring is that the projecting force is due, not to hydro static pressure, but to that of carbonic acid gas generated at the j unction 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 water bearing 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 em
ployed are detailed in pp. 401 and 413 of vol. viii. (1869)
