The first operation consists in sinking a rectangular shaft, usually 2 metres square; this work is carried out by hand, the ordinary native ‘fass’ being almost the only implement used. As the shaft is cut out it is timbered with lengths of palm-wood strung one below the other, to prevent the walls from falling in. The excavation is carried as deep as possible, the limit generally depending on the amount of sub-surface water met with. In the oasis of Dakhla, where the superficial strata consist almost entirely of clays, it can usually be continued to a depth of about 30 metres; but in Kharga the depth varies considerably, owing to the more frequent intercalations of water-logged sandstones. The success of the prospective bore depends to a very great extent on the depth to which this preliminary work is carried, as the succeeding strata, lying between the base of the shaft and the true artesian beds, have to be pierced by a hole unprotected by casing, and if of any very considerable thickness, caving beds are likely to greatly interfere with, if not prevent, the progress of the work. On completion of the timbered shaft the wooden casing is placed centrally in position, so as to form a vertical pipe from the base of the shaft to the surface of the ground. The pipe may be either square or round in section, and is usually made of acacia, though hollowed trunks of doum-palm are sometimes used. It consists of a number of different sections, joined together in such a way as to leave no projecting portions either within or without, the whole length of casing thus preserving a constant diameter from top to bottom. As a rule, the joints are so well formed that the pipe is, to all intents and purposes, water-tight after a short soaking. The square variety of casing usually has an inside width of 36 centimetres, the thickness of the wood being 4 or 5 centimetres; the circular, and perhaps more common, variety is made with an inside diameter of 35 centimetres, except when required for insertion within a previously fixed string of casing, when smaller sizes have to be used. The casing, of course, eventually forms the actual channel through which the artesian water flows to the surface. The space intervening between the sides of the timbered shaft and the central pipe is then filled in with a mixture of sand and clay, firmly packed down, so as to hold the pipe securely in position, and prevent the escape of water should any of the joints become leaky.