The drum is provided with two strong lateral shields or cheeks (Fig. 10), one of which carries the interchangeable cross-arm and the charging hopper. Both cheeks are lined with detachable chill-cast plates, to take up the wear. The bed is formed of heavy steel bars (which can be turned round), between which are arranged adjustable slits for the discharge of the reduced material. Guard sieves are mounted all round, and close to, the bed, and interchangeable fine screens surround these in turn. The mesh of the fine screens determines the fineness of the product, and the residue falls down on to a plate which returns it to the interior of the drum. The reduction of the charge is effected by a number of very hard, forged steel balls of various sizes. The mill must be run in the direction marked by the arrow on the outer shell, so that the residue on the screens can be returned to the drum by the plate provided for that purpose; and the prescribed working speed must be maintained. The mill must not be overloaded. The impact of the balls should be mild, but distinctly audible. Overloading reduces the output. Idle running causes the most wear, since the balls then roll directly on the bed, which, of course, should be prevented as far as possible. The feed is continuous; and, of course, only dry material should be introduced. When the balls have lost size and weight through wear, they must be replaced by a fresh set. _Pulverisers._--Pulverisers are the best form of crusher for tough and not over-hard materials. They are simple and strong in construction, of high capacity with comparatively small consumption of power, and furnish a good, uniform product, the size of which ranges from fine powder to coarse granules, according to the screens used and the class of material treated. [Illustration: FIG. 11.] The crushing is effected by a cross-arm beater, composed of four to six radial steel arms on a divided, cast-steel hub, keyed on to the horizontal shaft. The arms are hardened, and are adjustably and detachably mounted on the hub. The beating action of the arms, which run at high speed, forces the material against the studded surface of the hardened cheeks of the machine and also against the hardened square steel bars forming the periphery, the repeated impact of the material on itself, as well as against the arms and bars, progressively reducing it until small enough to fall through the screen on the under half of the casing, into a closed receptacle below. The screen mesh varies according to the degree of fineness required. The peripheral bars are mounted in a very simple manner, and in such a way that when one edge of the bars is worn, a quarter turn brings a fresh, sharp edge into operation, so that all four edges of each bar can be utilised.