It being impracticable to carry steam or the steam engine to the bottom of the mine for work there, compressed air is there employed, which is compressed by a steam engine up at the mouth. By this compressed air operated in a cylinder to drive a piston, and a connecting rod and a pick, a massive steel pick attached to the rod may be driven in any direction against the wall of coal at the rate of from ninety to one hundred and twenty blows per minute; and at the same time the discharged compressed, cold, pure, fresh air flows into and through the mine, affording ventilation when and where most needed. In addition to these great drills, more recent inventors have brought out small machines for single operators, worked by the electric motor. After the coal is lifted out, broken and assorted, it needs to be washed free of the adhering dust and dirt; and for this purpose machines are provided, as well as for screening, loading and weighing. The operations of breaking, assorting and washing are often combined in one machine, while an intermediate hand process for separating the pieces of slate from the coal may be employed; but additional automatic means for separating the coal and slate are provided, consisting in forcing with great power water through the coal as it falls into a chamber, which carries the lighter slate to the top of the chamber, where it is at once drawn off. The chief of machines with _ores_ is the _ore mill_, which not only breaks up the ore but grinds or pulverises it. Some chemical and other processes for reducing ores have been referred to in the Chapter on Metallurgy. Other mechanical processes consist of _separators_ of various descriptions--a prominent one of which acts on the principal of centrifugal force. The crushed material from a spout being led to the centre of a rapidly rotating disk is thrown off by centrifugal force; and as the lighter portions are thrown farther from the disk, and the heavier portions nearer to the same, the material is automatically assorted as to size and weight. As the disk revolves these assorted portions fall through properly graded apertures into separate channels of a circular trough, from whence they are swept out by brushes secured to a support revolving with the disk. Many forms of ore washing machines have been invented to treat the ore after it has been reduced to powder. These are known by various names, as jiggers, rifflers, concentrators, washing frames, etc. A stream of water is directed on, into, and through the mass of pulverised ore and dirt, the dirt and kindred materials, lighter than the ore, are raised and floated towards the top of the receptacle and carried away, while the ore settles.