Another method somewhat different from over hand sloping is sometimes adopted that is called underhand stoping. Suppose levels one and two had advanced some distance from the shaft, a small connecting shaft is then eut between them called a winze. It may be °pencil by sinking from the upper level or by an upraise from the lower. After it is eut. a party may begin on the upper level. and drilling in its floor may blast away the vein into the winze and allow it to fall to the level below to be removed. They may take off a vertical slice of the vein in this way. and gradually work each way from the winze. The upper level must then be kept passable with a floor of timber.
As these inclined shafts deepen and the vein is found to be rich and permanent. it is often advantageous no longer to use the inclined shaft, but rather to go out from the vein into the hang ing wall on the surface. and sink a vertical shaft that will intersect the vein at some desirable depth. Above this point connections are made with the levels by cross-cuts through the hanging and below it by cross-ems through the foot wall. Vertical shafts are always to be preferred, on account of the greater case and speed of hoist ing, but in a new enterprise the safer rule is to follow the ore until its quantity is proved. Va riations oil the above simple methods arc intro duced by the character of the wall-rock and the size of the ore body. If the Wall-rock is bad, and tends to scale off and impede the workings. it must be propped up with heavy timbering. If the vein is thick, the timbers are built up either rough or synired, and so mortised at the ends that they lit together like the edges of a cube, six feet on the side. Milers fit in with them, each stick entering into the four adjacent cubes, and in the end a framework of timber of great strength is built up. As soon as possible this is Mkt in with waste rock, which filially settles down and is practically as solid as the original vein. Unless precautions are observed in con nection with keeping the walls firm and im movable. they may settle and do great damage both to surface buildings and underground work ings.
In the Lake Superior iron mines produeing soft ore, that lie: under a too heavy burden of gravel to warrant stripping, a system has been adopted called the 'caving system.' The ores of this character on Lake Superior lie in great troughs or elongated basins. A shaft is sunk in the rock beyond the limits of the ore and drifts at various levels are run out into it. From the uppermost level upraises are made to the top of the ore and minor drifts extended to its outer limits. Light timbering and lagging protect the miner, who then at these outer limits begins to mine out the ore on each side of the end of his drift, letting the burden gradually cave in to the place whence the ore is taken. By multiplying these drifts in every direction all the ore is re moved. and the burden, closing in all the time.
keeps the mine shut and the miners protected from the weather. In the end a great pit results, sunk in the natural surface.
In small mines no particular system of timber ing, or taking out the ore is necessary. especially if the wall-rock is firm. Beyond the general plan of shafts and levels the workings follow the ore. and, without much systematic exploration, blast it and remove it to the surface. The objection to this method is that when the known rich spots are exhausted, further operations until more ore is located are all dead work, yielding no return and often causing the enterprise to shut down. In large mines where the wall-rock is firm. great excavations may be made with no timbering whatever.
If the vein or series of veins outcrop on a hill side. either parallel with its surface or (Tossing the neighboring valley. the ore may be won by adits or tunnels run in on a slight up grade. Such a tunnel will automatically drain all the portions of the vein above it and will make it an easy matter to take out the ore, which is merely loosened and sent down to the tunnel in winzes and shoots. But the portions below the tunnel will of necessity be reached by shafts from it and will require pumping. For this reason, unless the advantages of a tunnel are very great. most engineers prefer a vertical shaft at as early a stage in the mine as possible, because it is so easy and convenient to handle ore quickly and cheaply by vertical hoisting. Nevertheless some long and fatuous tunnels have been excavated in former years to drain important veins.
In the handling and transportation of ore underground. important problems are met in large works. It is accomplished in the levels by small ears, usually built of boiler plate to with stand the pounding that they receive, and these are pushed along by men on light tracks to the shaft. The operation is called tramming. If the shaft is vertical the ears are run directly on the cage, and hoisted to the surface, where they arc dumped and returned. Large mines may have cages with two or even three decks, bringing thus two or three 1rani-ears at a trip. If the shaft is imbued. the tram-cars are dumped at landing of the level into a ear in the shaft that is open at the end instead of at the top. This is called a skip and its track is the skipway. The skip dumps automatically at the top of the Alinft. In small mines an iron bucket is used instead of a skip or cage, but as soon as the output becomes at all large, buckets have to be abandoned. The transportation of the miners up and down deep shafts is also an important matter. They may, and as a rule do, ride on the skips, cages, or buckets used for the ore, special trips being made for them. Ladders, except for shallow depths. are no longer used in good practice unless in emergeneies, as the climbing is too slow and haust ing.