Fig. 6 illustrates an edge-runner mill with revolving trough and overhead drive; and Fig. 7 one with stationary trough and bottom drive; both made by the Badische Maschinenfabrik, Durlach. The runners are of grey cast-iron, chill-castings or cast-steel being used for crushing hard materials. The trough in all cases is lined with detachable chill-cast plates. Special attention is bestowed on the lubrication of all the moving parts, and all the lubricators are easily accessible. [Illustration: FIG. 7.] The main shafts of the fixed-trough machines have forged cranks, and the metal crank bearings are provided with dust caps. All the shaft journals run in detachable metal bushes. A special advantage attaching to this type is the automatic screening device and the returning of the screen residue. In some cases, complicated appliances are employed to return the coarse residue from the screen, bucket elevators, worm conveyors, etc., all entailing increased motive power, not inconsiderable wear, and a higher prime cost; but in this instance the object is achieved, without extra power or wear, by very simple means. The dust-proof shell enclosing the runners and screen is provided with large doors and charging hoppers. The motive power required to drive edge-runner mills depends on the dimensions of the mill and on the class of material to be treated; the larger the mill and the coarser the material, the more power needed to drive it. This type is the more suitable for raw materials that are of an earthy character, so that all that is necessary is to destroy the cohesion of the particles, as is the case, for example, with clay and all earthy minerals. [Illustration: FIG. 8.] The wet method of crushing with edge runners is particularly suitable as a preliminary to levigation. A machine arranged for this purpose is shown in Fig. 8. It consists of two sets of edge runners, one with fixed, and the other with revolving trough. The material is introduced by hand, or by suitable charging mechanism, into the upper, fixed-trough machine, where it is continuously sprinkled with water and kneaded by the one runner, and is passed thence to the second roller which forces it through the slotted bed into the bed of the lower set. The slotted beds of the upper and lower set are offset; and the chief function of the lower set, with rotating bed, is to secure intimate admixture of the material which, in most cases, is already sufficiently reduced. [Illustration: FIG. 9.] _Ball Mills._--Ball mills are generally used for crushing dry materials to fine powder. The mill shown in Fig. 9 is a typical form of grinding drum enclosed in a dust-proof casing, the latter being provided, at the top, with an opening connected to the dust exhaust pipe. The discharge outlet at the bottom can be closed by a slide. [Illustration: FIG. 10.]