162 FOUNDRY WORK central shaft, making about 30 r.p.m. A jarring action is produced by the contact of cams, which assists in breaking up the sand, that is also thrown about by internal stays, Coal is ground to dust in mills provided with heavy rollers, or balls, the first being used within closed cylinders, the second (fig. 65) in open pans. The balls, of cast iron, about 10 in. in diameter, are rotated in an annular path having a concave section of rather larger radius than that of the balls. The same mills may be used for pulverizing sands. In the largest foundries these units are associated in one automatic system Fig. 65.—Coal Mill for continuous treatment. In general, the arrangement is as follows: raw sand is thrown into a hopper at the base of an elevator, which discharges it into a drying oven. Thence it goes into the grinding mill, afterwards into a polygonal sieve, and then to a mixing apparatus, where the coal dust is added in the correct proportion. The old sand is treated in another part of the plant, conveyed for admixture with the new, the product elevated into a disintegrator, mixed, and stored in bins for use. Machines for Moulding.—It is not possible to describe here, even in barest outlines, the leading types of these machines, of which the useful varieties must now be numbered by hundreds. The only way to treat this immense subject is to state with brevity the forms and utilities of the principal elements in their designs, with comparisons of the methods and economies of their operations.