Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/470

Rh 452 MINING [EXPLOITATION. by a windlass. One great disadvantage of this method is that the ore and water have to be drawn up some distance by hand labour ; much timber is required &quot; for the stulls if there is a large quantity of worthless stuff in the vein, or if the sides are weak. The advantages are that ore can be worked away as soon as the level is driven, that the men are always bor ing downwards, and, lastly, that the ore can Fig. 60. be carefully picked after it is broken, without fear of any valuable particles being lost. A more economical method of working by underhand stopes, and one largely employed in Cornwall at the present day, consists in reserving any attack upon .. ////////////// the ore-ground until a lower / / // ////////////// level has been driven. An &quot; intermediate shaft (winze) between the two levels is then made, either by sinking from the upper level or rising from the lower one. The work of stoping is com menced at the two upper ends of the winze, and the lode is removed in a sue- // / //// Fig. 61. cession of steps, the workings assuming the appearance exhibited in fig. 61. The steps are generally made steep, so that the ore may readily roll into the winze, and so that the bore-holes may do better execution; but these steep stopes are dangerous if a man happens to slip and fall. The huge open chasms left by the removal of a large lode in this way are also a source of danger ; for there is always a risk of falls of rock, and from places which cannot easily be examined. Figs. 62 and 63, kindly supplied by Captain Josiah Thomas, 1 explain the general arrangement of the workings of the largest tin mine in Cornwall. The lode after producing copper ores to a considerable depth changed its character and became rich in tin. The workings for tin ore are confined almost entirely, to the granita. The section fig. 62 shows that the main shaft of the mim is at first vertical and then carried down on the dip of the lode. SOUTH Fathoms 100 60 100 Fathoms. FIG. 62. Transverse Section, Dolcoath Mine, Cornwall. The process of overhand stoping is precisely the reverse of that --. w .-ir-....-:-V-| -4hiv&quot; Over hand stoping TIN Fathoms 100 60 100 ~ JOO 300 Fid. 63. Longitudinal Section, Main Lode, Dolcoath Mine, Cornwall, which has been just described ; the work is commenced from a rise 100 Fathoms. 1 See Report of Miners Association of Devon and Cornwall for 1882, and R. J. Frecheville, Trans. Roy. Geol. Soc. Cornwall, roL x. part v. (fig. 64 A), or better from the two bottom ends of a winze (fig. 64 B). As soon as the men have excavated a sufficient height of the roof of the level, they put in strong pieces of timber from wall to wall, and