In some Continental mines the experiment has been tried of constantly consuming the fire damp, before it has had time to accumulate in explosive quantities, by means of numerous open lights kept constantly burning. This method is effective, but since the numerous lights consume the precious oxygen of the air as well as the damp, the method has never become popular. Obviously, then, the question of mine ventilation is closely associated with that of lighting. Probably the simplest method of properly ventilating a mine is that of having two openings at the surface, one on a much higher level than the other if the mine is on a hillside, the lower one corresponding to the lowest portion of the mine where possible. By such an arrangement natural currents will be established, and may be controlled and distributed through the mine by doors or permanent partitions, or aided by fans. But of course only a comparatively small number of mines are so situated that this system can be used. It is possible, of course, to ventilate a mine from a single shaft or opening by use of double sets of pipes, one for admitting air and the other for expelling it; but this system is obviously not an ideal one, and is prohibited by law in most mining districts. Such laws usually stipulate that there must be at least two openings situated at some distance from each other. The older method of creating air currents was by means of furnaces, but this method, while very effective, is expensive and dangerous. In using this system a furnace is built near the outlet of the air shaft, the combustion of the fuel creating the necessary draught. But in the nature of things this furnace is a constant menace to the mine, besides being an extremely wasteful expenditure of energy. The modern method of ventilating is by means of rotary fans, the electric fan having practically solved the problem. The air currents established by such fans are controlled either by the doors in the passages, or by means of auxiliary fans. In addition, jets of compressed air are sometimes used, and have become very popular. Another important problem that constantly confronts the mining engineer is that of drainage. Mines are, of course, great reservoirs for the accumulation of water, which must be drained or pumped out continually; and as the shafts are sunk deeper and deeper it becomes increasingly difficult to raise the water to the surface. Special means and machinery are employed for this purpose which will be considered more in detail in a moment. ELECTRIC MACHINERY IN MINING