Next in importance to the matter of ventilation in mines is the matter of drainage. The first difficulty experienced from water is while the shaft or slope is in process of sinking. It is usually necessary to hold the water in one side of the opening while work is going on in the other side. A small pumping engine is generally sufficient to keep the pit clear until the bottom is reached, but occasionally the amount of water is such that a large engine and pumping appliances have to be put in place at once. In Europe much trouble is often experienced from the excessive flow of water while sinking the shaft, and a watertight casing has frequently to follow the shaft downward in order that work may go on at all. Such appliances are not as a rule necessary in this country, though much difficulty has been encountered in sinking shafts through the quicksand deposits of the Susquehanna basin in the Wyoming valley. The general principle of mine drainage has been already explained. It is, in brief, that the floor of the mine shall be so graded that all water will gravitate to a certain point. That point is near the foot of the shaft or slope, and is at the mouth of the drift or tunnel. But from the sump of the shaft or slope the water must be raised by artificial means. A powerful steam pumping engine, located at the surface, is employed to do this work, and one compartment of the shaft or slope, known as the pump-way, is set aside for the accommodation of pipe, pump-rods, and supporting timbers, which extend from the top to the bottom of the shaft. The most powerful of these pumps will throw out a volume of twelve hundred gallons of water per minute. It is seldom that the tonnage of water pumped from a mine falls below the tonnage of coal hoisted, and in some of the wet collieries of the Lehigh district eight or ten tons of water are pumped out for every ton of coal hoisted. In the Wyoming district a thousand tons of water a day is not an unusual amount to be thrown out of a mine by a single pump.