Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/469

Rh EXPLOITATION.] MINING 451 of timbering. Fig. 53 is a plan of the shaft: &quot; S, S are the longi tudinal or sill-timbers, T, T the transverse end-timbers, P partition- timbers, r guide-rods between which the cage moves, g gains cut in the sill-timbers, to receive the ends of the posts. The sheathing or lagging is seen enclosing the whole frame.&quot; Fig. 54 is a trans verse section through the partition P of iig. 53, between the [lumping compartment and the adjoining hoisting compartment, looking towards the latter. In this figure, G, G are the posts, S the sill-timbers, P the partition-timbers, the ends of which are framed with short tenons that are received in gains cut in the sill- timbers and the ends of the posts, r guide-rod, I lagging or sheathing.&quot; Fig. 55 is an end view of the frame shown in fig. 53. &quot;The single piece T forms the end, while the double pieces P forming the partitions are seen beyond.&quot; &quot;The outer timbers of each set, that is, the two sides and ends of the main frame, are 14 inches square; the posts, ten in number, four at the corners and two at each end of the three partitions, are of the same size. The dividing timbers, forming the partitions, are 12 inches square.&quot; When ground is loose or running, recourse must be had to a spilling process like that described for levels. Strong balks of timber are fixed at the surface or in solid ground, and then the first frame is hung from these bearers, and each successive frame from the one above it. Iron bars with cotters may be used for suspend ing the sets ; but on the Comstock lode each bolt is made in two parts with a tightening screw in the middle, and the sets can thus be kept very firmly together. The laths are driven in advance, in the manner explained in the case of levels, and a new frame is put in as soon as the excavation has been sufficiently deepened within the protecting sheath of boards. In very unstable ground it may be necessary to put in the frames touching each other, so that the shaft becomes encased in a solid box of timber, occasionally 14 inches thick. Masonry Like levels, shafts may be lined with masonry or brickwork, and for these have the advantage of being far more permanent than timber, shafts. an( l f requiring fewer repairs. This kind of shaft-lining is especially desirable in the loose ground near the surface; because, if the working is discontinued temporarily, the shaft still remains secure and available for use at any future time, whereas if timber is put in it often decays, the top of the shaft collapses, and much ex pense is incurred in the process of reopening it. The section of the shafts that are walled is generally circular as affording the best resistance to pressure; but elliptical walling is also met with. Another shape is like a rectangle, save that the sides, instead of being straight, form curves of large radius. The walling may be dry or with mortar, according to circumstances. The masonry is put in either in one length or in successive por tions in descending order, and this is the usual plan. The shaft is sunk a certain depth, with temporary timbering if necessary, and when firm ground has been reached a bed is cut out round the shaft, and on this is placed a crib or curb AB (fig. 56) 1 con sisting of segments of timber which form a ring. This serves for a foundation for the brickwork, which is built up to the surface ; the temporary timbering is re moved, and the space filled up with earth or concrete. Sinking is then resumed below the curb, and for a certain distance of a smaller diameter, so as to leave a bracket, or ledge, to support the first curb. On arriving, after a certain depth of sink ing, at another firm bed, a second curb CD is put in and a portion of brickwork built up. When the ledge of rock is reached, it is carefully removed in small sections and the brickwork brought up to the first curb. This process is repeated till the shaft is completed, or reaches rock in which no masonry is requisite. If, owing to the nature of the ground, it is impossible at first to find a firm seat for the curbs, it becomes necessary to han; them by iron bolts from a strong bearin Fig. 56. _ frame at the surface. When shafts pass through very watery strata, it is most desirable to stop all influx into the mine for the purpose of saving the heavy expense of pumping. The manner in which this is effected by a watertight lining, known as tubbing, is described in the article COAL, vol. vi. p. 62, where will also be found an account of Triger s plan of sinking shafts with compressed air, and the very successful method of boring shafts through water-bearing ground invented by Messrs Kind & Chaudron. xploi- 7. Exploitation, or Working Aivay of Veins, Beds, and tion. Masses. We have described how shafts are sunk and levels driven, and we now come to the processes employed in removing the mineral. 1 J. Gallon, Lectures on Mining, vol. i., Atlas, plate xxviii. The deposit must first of all be reached by a shaft, or, where the contour of the country permits it, by a level. In the case of a vein an exploratory shaft is often sunk on the course of the lode for 20 or 30 fathoms, and, if the indications found in a level driven out from this shaft warrant further prosecution of the mine, a first working shaft is sunk to intersect the lode at a depth of 100 fathoms or more from SOUTH. the surface. Crosscuts are then driven out at intervals of 10, 15, or 20 fathoms to reach the lode, as shown in fig. 57, which represents a section at right angles to the line of strike. Sometimes the main shafts are carried down all the way along the dip of the deposit, though perpendicular shafts have the advan tages of quicker and cheaper winding and cheaper pumping, to say Fig. 57. nothing of the possibility of utilizing the cages for the rapid descent and ascent of the miners. If an inclined shaft appears to be advisable, great care should be taken to sink it in a straight line. In either case levels are driven out along the strike of the lode as shown in the longitudinal section fig. 58, in the hopes of meeting with valuable ore- Fig. 58. bodies such as are represented by the stippled portions of the figure. For the purpose of affording ventilation, and still further exploring the ground and working it, inter mediate shafts, called winzes (Cornwall) or sumps (North Wales), are sunk in the lode. The actual mode of removing the lode itself depends a good deal upon circumstances, viz., its width, the nature of its contents, and the walls that enclose it ; but the methods of working may generally be brought under one of two heads, viz., underhand stoping or overhand stoping. The word sfopc is equivalent to step, and the term stoping means working away any deposit in a series of steps. Underhand or bottom stapes are workings arranged like the steps of a staircase seen from above, whilst orcrhand or back slopes are like similar steps seen from underneath. Both methods have their advantages and //////////////////////////////// disadvantages, and both are largely //////////////////////////////// used. We will first take underhand stop- 7r/////&amp;gt; 5 2 , / 2 ing, as this is the older method. /. Wffk Mr-i In the old days the miner began in // the floor of the level (fig. 59), and sank down a few feet, removing the part 1 ; he followed with 2, 3, 4, &c., until the excavation finally presented the appearance shown in fif 60 Any valueless rock or mineral was deposited upon plat forms of timber (stiills), and the ore was drawn up into the level Under hand stoping.