MINING                                                      1234                                                      MINING

saves the expense of hoisting it up through a shaft. The tunnel often also saves the expense of pumping out the water, which often is a troublesome factor in mining. In many cases, however, it is necessary to approach the ore by a shaft. The shaft may be either vertical or inclined, according to local conditions, especially according to the position of the ore body. The inclined shaft is sometimes known as an incline. A shaft is usually rectangular in section. In very small mines it may be no more than four feet square, but in large mines the shafts are much larger, large enough to permit the working of at least two hoists, called cages or skips. There must also be room for pipes, both for carrying down compressed air and for the carrying out of water. Large mines frequently have

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shafts as much as 12x18 feet in diameter.

If the material to be mined lies in a horizontal bed, the shaft is sunk down to the level of the material to be mined or a little below. Tunnels are then made, leading sidewise from the shaft into the substance to be mined. The material to be mined is worked out along these tunnels, carried to the shaft and hoisted to the surface. When the material which is being mined is worked out, it often is necessary to put in timbers to keep the mine open. If, for example, a horizontal bed of coal is being mined, the taking out of the coal leaves the rock above unsupported. Timbers may be put in to hold up the roof, or columns of coal may be left here and there to serve the same purpose. Where practicable, the tunnels driven from a shaft are made to rise slightly from the horizontal, so that the water en-

sump or pool at the bottom of the shaft. From the sump it is pumped out. The slight inclination of the tunnel also facilitates the transfer of the substance mined to the shaft. If the tunnels are parallel with a mineral vein, they are called drifts. When they cut the vein, they are called cross-cuts. Tunnel is a rather indefinite name which may be applied either to drifts or cross-cuts. The process of drifting and cross-cutting is called "driving a level." Tunnels are usually as much as 6x6 feet in section. Tunnels are often run at various levels; for example, tnnnels may be run at the 50-foot level (that is, 50 feet below the surface), at the 100-foot level, at the 150-foot level and so on. When the drifts and cross-cuts have been made, and the different levels put in communication by vertical shafts, the ore between the various openings is said to be "blocked out."

Some ore has been extracted in the process of blocking out the ore. When a body of ore has been blocked out, extraction of ore begins in earnest. The processes employed for loosening the substance to be mined are various. Sometimes it is loosened by the pick or some other sort of hand-tool. Sometimes it already is soft or incoherent. Often it is so hard that it must be blasted. Ore is often worked from below rather than from above. That is, ore between the 50-foot level and the 100-foot level is often worked from the latter and carried along it to the main hoisting shaft. The working out of the ore between levels is "stoping." Another problem which the miners have to encounter is the drainage of the mine. In most deep mines the amount of water seeping in is great, and it must be pumped out about as fast as it enters. For this purpose force-pumps are used. In deep mines several or many force-pumps may be needed. Pumps which are able to elevate the water 300 or 400 feet each are in common use. In mines near deep valleys it sometimes is possible to secure drainage by driving a drainage tunnel or "adit" from the shaft to the valley.

Still another problem which has to be faced in mining is that of ventilation. In coal-mines where noxious gases are sometimes plentiful, in mines where many men are at work, in mines where there is much blasting and in deep mines where the temperature is high at the bottom ventilation is of the utmost importance. Ven

countered in the tunnel may drain into the I tilation may be either natural or artificial