Page:The American Cyclopædia (1879) Volume XI.djvu/591

 MINE 573 coal mining, where the seams are not too thick, steep, or variable, may be employed either with or without gobbing up.) The word stope is probably a corruption of step, and refers to the stair-like appearance presented by the face of the excavation. Overhand stoping is con- ducted as follows: From the level below the ground to be exploited, a " raise " or upward shaft is driven up into the ground, and from this the different " breasts " are driven horizontally on the vein, in one or in both directions. The extraction begins at the bottom, by the exca- vation of a block having the width of the vein, a height of 4 to 9 ft., and a length of not less than 7 nor more than 30 ft. In this work two sides of the rock are always free : the up- right face, toward the central shaft, and the lower horizontal side, over the level.- When the breast has been driven far enough, a new workman or set of workmen may begin with a second breast, while the former still contin- ues to advance. Fig. 1, representing the pro- file of a double stope, shows the order in which the work proceeds. The space behind and be- low the workmen is filled up with the waste rock, broken from the vein in order to get at the ore, or with rock brought from else- where for this special purpose. Openings or " chutes " are left in this, through which the a FIG. 1. Double Overhand Stope. ore can be allowed to fall to the level below, where it is received in cars. This level is usually protected by a roof of stulls and lag- ging, on which the waste rock is piled, as is shown in fig. 2 ; or a portion of the vein is left standing over the level, as a protection. The workmen stand on the waste rock, and stoping goes on in the manner indicated, until the whole of the valuable mineral between the bottom level and the one next above (say 60 to 100 ft., measured on the dip of the vein) has been extracted. Of course, by starting stopes at different points on the lower level, within the limits of the mining claim, or the body of valuable ore, more men can be set at work. But the regular productiveness of a mine is not susceptible of indefinite increase in this way. The maximum rate of exploita- tion which can be maintained until the mine is entirely exhausted, depends upon the rate at which the shaft or shafts can be sunk and new levels opened at greater depths. The too rapid exhaustion of one level would necessitate a suspension of active extraction while the next level below was in course of preparation; and in this work of sinking shafts and runnin"- drifts (sometimes called the " dead work " of the mine) it is not possible to multiply the i FIG 2. Single Overhand Stope, packed with waste rock. number of men, so as to secure more rapid progress. Only so many men can be accom- modated at the bottom of a shaft or the end of a drift; and when their effectiveness has been raised to the highest point by selecting good workmen, dividing them into three "shifts" or gangs, working eight hours each in turn, employing the most suitable tools and explosives, and, if circumstances are favor- able, drills operated by steam or compressed air, the limit of practicable progress has been reached ; and this determines the normal pro- ductiveness of the mine. Driving the stopes faster than the dead work is "robbing" the mine. Underhand stoping is the reverse of 3. Underhand Stope. the method just described. Here the stopes begin from the level above, and may be com- menced (if the presence of water is not too troublesome) before any lower level has been