Renzoval of the tools have to be lifted out, the cable is disengaged from the clamps on the temper-screw, and is wound upon the bull-wheel shaft, which for the purpose is thrown into gear with the steam engine, the pitman being at the same time removed from the crank-pin, so that the walking-beam is at rest. For catching and withdrawing broken bits or articles accidentally dropped into the drill-hole, for enlarging or strengthening the bore, etc., ingenions special tools are provided.
It may sometimes be found advantageous not to commence the boring directly at the surface, but, instead, an open pit or shaft may be dug of any convenient depth and with sloping sides (widest at the top), and from the bottom of this the boring may be started. By this plan tube-sections of considerable length may be used without the aid of a very high derrick.
The Diamond Drill is employed to some extent for artesian-well boring. With this ingenious apparatus the hardest rock may easily be penetrated. The drilling-tool consists of a metallic stock or holder in the form of a cylindrical tube, either open at the cutting-end, with angular fraguients of black diamonds embedded in the metal at intervals around the inner and outer edges of the ring in such a manner that cutting-faces of the stone shall project (pi. 53, fig. tz), or so closed at its extremity as to present a convex surface studded around with the fragments of the cutting sub stance. The tubular form .of the drilling-tool affords the special advan tag,e of preserving a perfect record of the various strata through which the drill has passed, since the "core " (fig. II) which it cuts out of the rocks may be broken off and lifted with special tools from time to time, and its parts, being properly placed together, will form a continuous solid cylinder clearly exhibiting the character and successive order of the strata passed through.
Lona'on Artesian obtain the pure water-supplies stored in the underlying chalk-formation at a depth of from 6o to 9oo feet, the strata of the London basin have been penetrated by great numbers of artesian wells. From these sources the Bank of England, the fountains of Trafalgar Square, a number of public institutions, and many of the great breweries of London obtain their supplies. So numerous are the wells in this cir cumscribed area, and so great is their drain upon the underground reser voirs, that it is not surprising that the general level to which the water for merly rose in the London district has within recent years so very sensibly lowered that from many wells whence it formerly flowed the water now requires to be raised by pumping.
French Arles ian "ells. —In France there are many artesian wells, some of them of notable size and productiveness. That at Grenelle, near Paris, commenced in 1834 and finished in 1841, is carried to the depth of 1798 feet and flows at the rate of six hundred gallons per minute, furnishing a val uable source of water-supply to this suburb. The water of this well is horizontally conveyed to the centre of the Place de Bretenil, Paris, where the open-work tower of bronzed cast iron, of elegant design Go. 6, fig. 3), encloses the ascension and distributing pipes. (See p. 63.) The artesian well at Passy (also near Paris), commenced in 1855, was carried through the chalk into the lower greensand to the entire depth of 1923 feet, the bore finishing with the enormous diameter of 2 feet. This is probably the largest artesian well in the world. It required nearly seven years to complete the boring-. ..Nt one time the well yielded 5,582,000 gallons of water every twenty-four hours, but this fell off to 3,795,000 gallons, at which rate it has since continued to flow, and feeds the lakes of the Bois de Boulogne. The well at St. °nen delivers water from two levels, a smaller pipe being- passed inside the first and carried some distance farther down. Both of these streams supply the canal-basin at St. Ouen, which lies above the level of the Seine.
German 21rIesian Germany the artesian salt-spring at Kis singen, in Bavaria, (1832-185o), is one of the most noteworthy. It reaches a depth of about z000 feet, and ejects a stream of brine, at the rate of six hundred and fifty gallons per minute, to a height of 58 feet. The piping in this well is concentric, the water being forced to pass up between the outer and the middle tube, then down between the middle and the inner one to the rock-salt stratum, where it saturates itself with salt, and finally np through the inner tube, from which it is delivered. The pressure in this well is believed to be cattsed by carbonic-acid gas. The boring at Speren berg, near Berlin, was undertaken to obtain a supply of rock-salt. The well-bore was carried down with a diameter of •16 feet to the depth of 2So feet, at which depth the salt-bed was reached. It was continued with the same diameter through the salt 6So feet farther, then reduced to 13 inches and driven to the unparalleled depth of 4194 feet without having perforated the salt-bed.