depend on the depth removed compared with that of the rock face, on the method adopted in removing the soil, and on the distance to which it has to be conveyed for disposal. 185. Examples of Quarrying taken from Actual Practice.-In the following example taken from actual practice the quantity of material required was 1500 tons, the depth of rock face being 20 feet; it was desired to determine what width would be required when the length of face is known. In fig. 66 the positions of the bore-holes are shown, Nos. 1 to 9 inclusive being vertical, and 10 to 14 horizontal or bottom holes, the length of face being 40 feet. These bore-holes were all drilled to a depth of 14 feet, and placed at an average distance apart of 11 feet, the measurement from the face of the rock to the first line of bore-holes being the same. The bore-holes in the second line were placed 9 feet further back and parallel with those in front. The distance from the face for stripping purposes is determined in the following manner: the total quantity of rock required is divided by the length of face multiplied by the depth of the rock, which in this case is about 20 feet. An allowance, however, must be made for the line of rupture extending beyond the position of the bore-holes, as the necessity of clearing the soil well back from this point to avoid its falling on to the quarried material is obvious; it is therefore advisable to make the width greater than the calculated figure by 6 or 8 feet. Fig. 67 is a cross-section of the quarry face, showing the position of the vertical and horizontal bore-holes; the actual line of rupture, or crater form, of the mass dislodged is shown by the line a, b, and c. Fig. 68 is another example representing road-stone quarry No. 1, where the depth of the rock varied from 27 to 40 feet. In this case it was imperative to blast the total quantity required (4000 tons) at one operation, so that a system of bottom and breast bore-holes in conjunction with the vertical ones became necessary, whereas under ordinary circumstances it would have been worked in benches from the top of the rock surface downwards. It is usual to mark off roughly the area of the rock surface which must be operated upon to produce the quantity of rock required for a given length and depth of face. It will be observed that many of the breast and bottom holes are shown at a considerable angle to the general line of the quarry face; this was necessary, owing to the position and varying direction of the veins and fissures running through the rock mass. The bottom holes are indicated by the numbers 1 to 13 inclusive; the breastholes, at an average of from 7 to 10 feet high above the floor level of the quarry, are numbered from 14 to 22 inclusive, and the top or vertical holes, 23 to 42. Fig. 69 represents a cross-section at A A on plan, and shows the method adopted in arranging the position of the bore-holes. They were all put in 14 feet deep. and charged with dynamite; the total quantity of material actually quarried and passed over the steelyard amounted to 4158 tons. There is no universal rule for placing bore-holes. The best plan is to study the conditions already mentioned, carefully note the position, depth, and direction of the bore-holes relatively to each other, and the results obtained from previous blasting operations. The number of boreholes required will depend on the tenacity of the rock, on the development of the joint planes, and, in some measure, on the lines of fracture started by the preceding blast. With electrical firing, bore-holes can be placed further apart, consequently fewer are required to effect similar results. FIG. 69.-Cross-section of roadstone quarry No. 1 at A A. 186. In quarrying for 'dimension stones,' such as those used for kerbs, channels, and setts, the system of drilling few holes is usually adopted, but these are made deep and of large diameter, the object being to apply the blasting agent so that it will simply push the mass of rock outwards and yield the largest blocks possible. In obtaining materials for macadamizing purposes, however, it is of primary importance to reduce the mass of rock to a suitable size for handling; and therefore a large number of boreholes are drilled, while the explosive used should be specially adapted for the end in view, which will be noticed under blasting. 187. Boring the Shot-holes.-The position and direction in which a bore-hole is intended to be drilled having been determined, the tripod is so placed that when the drill proper is attached and fixed to the saddle the chuck end of the piston carrying the cutting tool will coincide exactly with the spot selected for the bore-hole. In drilling vertical, and even at times bottom bore-holes, it is a common occurrence to encounter loose rock, which should be cleared away previous to fixing the position of the machine. The setting of the tripod for vertical holes is commenced by placing it in an approximate position with the saddle at a fixed height or distance from the rock. The irregular or rough nature of the surface may necessitate placing the legs of the tripod at different angles and of varying lengths. This is readily accomplished by means of the saddle bolt, front leg bolt, and the telescopic pointers fixed by bands and screws. A small hole is made in the rock at three points with a chisel or quarry pick for the pointers of the tripod to sit in. The jaw of the saddle at the apex of the tripod is loosened, and the conical spigot attached to the cradle of the drill is then placed upon it, and properly secured. The position and direction of the bore-hole are then definitely fixed, all the bolts of the tripod being tightened, and the weights placed on the legs by means of the hangers or clips, which keep the drill perfectly rigid when working. Fig. 70 shows the machine drilling a vertical hole. In drilling bottom or breast-holes the tripod legs are placed so that they are in a flat position, the under part of saddle resting on a block of wood. The pointers of the two front legs are placed against the rock face, while the other, or back leg, is placed directly behind the line of the bore-hole to be drilled, and is prevented from receding, by reason of the action of the drill, by wedges or by a heavy block of stone. The weights are fixed on all the legs in a manner similar to that adopted in drilling a vertical bore-hole. Fig. 71 shows the machine drilling a horizontal or breast-hole. The first length of steel is then placed in the recess or chuck end of the piston, and properly tightened by means of the U-bolt and nuts. This operation has to be performed as each succeeding length of steel is required. The connection between the pipes conveying the motive power, whether steam or compressed air, is made by a suitable length of heavy hose marline wound for the former, and wire armoured when compressed air is used. Before attaching the hose to the drill it is advisable to turn on the steam or air temporarily, and clear out any foreign matter which may have by accident gained access into the feed pipes or connections, thereby minimizing any chance of injuring the working of the valve. In commencing the drilling of a bore-hole, whether the rock is at right angles or in a slanting direction to the axis of the drill-bit, it is advisable to go slow at first; this is accomplished by feeding the machine with the cutting tool up to within |