Nearly all the slate-picking machines are based on the fact that the specific gravity of coal is lighter than that of slate or stone. One method brings the principle of friction into play. A section, a few feet in length, of the floor of the shute down which the coal passes is made of stone. At the end of this stone section is a narrow slot cut in the floor, crosswise of the shute, and beyond the slot the iron bottom is continued as before. Now when the shallow stream of broken coal strikes the stone bottom the friction between that bottom and the pieces of slate and stone is so great that these particles are impeded in their progress, and by the time they reach the slot they have not impetus enough to cross it and must therefore drop into it and be carried away. But the friction between coal and stone is slight in comparison, and the pieces of coal retain enough of their impetus to carry them safely across the slot and on down the shute. This is not a perfect separation, and the coal and slate which it divides has usually to be looked over again, to insure satisfactory results. The best and most practicable invention thus far brought into use is that of Mr. Charles W. Ziegler, picker boss at the Von Storch colliery, Scranton. This machine acts somewhat upon the method last described, though by a system of rollers, levers, and screens in connection with it and attached to it, it is able to make quite perfect separation of the coal and slate. Two or three of these machines placed on a single shute should do the work required of them very thoroughly. The experience of domestic buyers of coal would seem to indicate, either that the picker boys do not do their whole duty or that the picking machines have not yet been made perfect. But it must be remembered that the separation of slate and bony coal from good material is made only in a rough and general way in the mine, and that a very large percentage of the output, as it reaches the breaker, is unfit for use. To clean and separate this material thoroughly, therefore, requires much labor, and extreme care and skill. After these separate streams of coal have passed the scrutiny of the picker boys or the test of the picking machine, the shutes in which they run are narrowed into pockets or bins, closed at the end by a gate. The pocket projects over the car track high enough from it for a railroad coal car to stand beneath, and the coal is then fed from the pocket into the car at will. There is also a loading place for the rock and slate which have been separated from the coal on its way through the breaker; and there are two or three points where the coal dirt is gathered from its pockets to be taken away. All this refuse is run out by separate tracks to a convenient distance from the breaker and there dumped.