Electricity is, of course, the great revolutionary factor in modern mining. There is scarcely a department of mining in which electric power has not wrought revolutionary changes in recent years; and the subject has become so important and so thoroughly specialized as to "create a literature and a technology of its own." From the electric drill, working hundreds of feet below the surface of the earth, to the delicate testing-instruments in the laboratory of the assaying offices, the effect of this electrical revolution is being felt progressively more and more every year. Moreover, electricity, on account of its transmutability, has made accessible many important mining sites hitherto unworkable. Rich mines are now in operation on an economical basis which, thirty years ago, were worthless on account of their isolation. When such mines were situated in mountainous regions where there was no coal supply at hand for creating steam power, and where the only available water power was perhaps several miles away, operations on a paying basis were out of the question before the era of electric power. At present, however, the question of distance of the seat of power has been practically eliminated by the possibilities of electric conduction. A stream, situated miles away, when harnessed to a turbine and electric motors may afford a source of power more economical than could be furnished a few years ago by a power plant supplied with fuel at the very door of the mine. We need not enter into the details of this transmission of power, however, since the subject has been discussed in a general way in another place. Our subject here is rather to deal with the application of electricity to certain mining implements of special importance. One of the most useful acquisitions to the equipment of the modern miner is a portable mechanical drill, which makes it possible for him to dispense with the time-honored pick, hammer, and hand-drill. But it is only recently that inventors have been able to produce this implement. The great difficulty has lain in the fact that a reciprocating motion, which is essential for certain kinds of drilling, is not readily secured with electric power. The use of steam or compressed air for operating such reciprocating drills presents no mechanical difficulties, and the fact that power of this kind can be transmitted long distances by the use of flexible tubes made such drills popular for several years. But the cost of operating such drills is so much greater than that of the new electric drills that they are rapidly being replaced in mining work.