448 MINING [FORMATION OP EXCAVATIONS. passing through a nut attached to the cylinder; r is the cradle carrying the feed-screw and supporting the cylinder. It is centred on the clamp s. As this clamp can be fixed in any position on the bar, and as the cradle can be turned on the clamp, it is evident that holes can be bored in any direction. In driving a level with the Darlington drill it is usual to fix the stretcher bar horizontally across the level so as to command the upper part of the face ; holes can then be bored with the cradle above the bar or below it. The bar is then shifted low enough to bore the bottom holes. It is found that all the necessary holes can be bored from two positions of the bar. The bar therefore has simply to be fixed twice ; the alterations in position for boring holes in various directions are managed by shifting the clamp on the bar and turning the cradle on the clamp. Fig. 31 shows the stretcher bar fixed in a vertical position, which is sometimes convenient. In order to clear out the sludge from holes that are "looking downwards," a jet of water, supplied from a hose attached to a half- inch gas-pipe leading from a cistern at a higher level, is made to play into the holes during the process of boring. For sinking shafts Mr Darlington has the drill fixed in a cylin drical case with a large external thread which works in a nut on the clamp. The drill is fed forwards by turning a hand-wheel attached to the case. Rotating Eotating machine drills are also used in mines as well drills, as those with percussive action. Stapff pointed out some years ago that, if a rock may be chipped off by power com municated by a blow, it may also be chipped off by a similar amount of power communicated by pressure. Brandt s rotatory boring-machine consists of a hollow borer which has a steel crown with cutting edges screwed on. The tool is kept tight against the rock by the pressure of a column of water, and is at the same time made to rotate by two little water-pressure engines, whilst a stream of water passing down through the borer washes away the debris and keeps the cutting edges cool. In principle, therefore, this drill resembles the original diamond boring machine of De la Roche-Tolay and Ferret, save that the crown is made of steel and not of diamonds. During the last few years it has been tried with success in railway tunnels and in mines. Jarolimek s drill l acts also by rotation, but the borer is fed forwards and pressed against the rock by a differential screw arrangement. The machine can be worked by hand, or by a little water-pressure or compressed-air engine or an electro-motor. In working certain minerals occurring in seams the undercutting may be performed by machines similar to those used in coal mines (see vol. vi. p. 68). We now come to the application of the tools and machine drills to the purpose of breaking ground for driving levels and sinking shafts. A level or drift is a more or less horizontal passage or tunnel, whilst a shaft is a pit either vertical or inclined. In driving a level by hand labour in hard ground, the first thing the miner has to do is to take out a cut, i.e., blast out a preliminary opening in the "end" or " forebreast." The position of this cut is determined by the joints, which the miner studies carefully so as to obtain the greatest advantage from these natural planes of division. Thus fig. 32 shows a case in which, owing to joints, it was Driving levels, Fig. 32. Fig. 33. advisable to begin with a hole No. 1, and then bore and blast 2, 3, and 4 one after the other. The miner as a rule does not plan the position of any hole until the previous Ocsterreichische Zeitschrift fur Berg- und Huttenwesen, 1881. one has done its work; in fact he regulates the position and depth of each hole by the particular circumstances of the case. Though a vein and its walls may be hard, there is occasionally a soft layer of clay (DD, DD, fig. 33) along one wall (dig, Cornwall ; gouge, United States). The miner then works this away with the pick, and, having excavated a groove as deep as possible, he can now blast down the lode by side holes and so push the level forward. In sinking a shaft a similar method of proceeding is Sinking observed. A little pit (sink) is blasted out in the most snafts - convenient part, and the excavation is widened to the full size by a succession of blasts, each hole being planned according to circumstances. This series of operations is repeated, and the shaft is thus gradually deepened. Where boring machinery is employed, less attention, and sometimes no attention, is paid to natural joints, because when once the drill is in its place it is very little trouble to bore a few more holes, and the work can then be carried on according to a system which is certain of effecting the desired result. A common method of procedure for hard ground is Driving shown in figs. 34 and 35. Four centre holes are bored le ls about a foot apart at first, but con verging till at a depth of 3 feet they are within 6 inches, or less, of each other. Other holes are then bored around them until the end is pierced by twenty or thirty holes in all. The four centre holes are then charged and fired simultaneously, either by electricity or by Bickford s instantaneous fuse, and the result is the removal of a large core of rock. The holes round the opening are then charged and fired, generally in volleys of several holes at a time, and the level is thus carried forward for a distance of 3 feet. If the ground is more favourable fewer holes are required, and they may be bored deeper, in fact as much as 6 feet in some instances. Occasionally the four centre holes are directed so that they meet at the apex of an acute pyramid, and then, after all have been charged with blasting gelatin, only one of them receives a primer and cap ; the shock of the explosion of one charge fires the other three adjacent charges simultaneously. The preliminary opening is not necessarily made in the centre of a level, and sometimes it is blasted out in the bottom or one side. In sinking shafts by boring machinery operations are conducted Sinking much in the same way as in levels, save of course that the holes shafts are directed downwards. Figs. 36 and 37 are a section and plan of with shaft which is ^^ v//, machine drill?. a shatt wnicii is now being sunk at the Foxdale mines in the Isle of Man. %%. About forty-five ^| holes are bored in the bottom of the shaft before the drills are removed ; two of the holes A, B, and occasion ally four, are bored only 4 feet deep, and are blasted with ordinary fuse. up and weaken the core ; then the six holes nearest the centre, which are 8 feet deep, are blasted all together with Bickford s instantaneous fuse, and the result is the removal of a large core leaving a deep sink. The remaining holes are fired in volleys of four at a time in the ordinary way. In this manner the shaft, which is in hard granite, is being deepened at the rate of 3 or 4 fathoms a month. Tonite is the explosive used. Sundry machines have been invented and used for driving levels without blasting. Some cut up the face into small chips which can 3t>. ym F ; g. 37.
They serve simply to smash