The grinding-plates (pl. 8, figs. 8--it) are of peculiar construction or dress, which makes them particularly valuable in respect to the quality of the work they will do. The principle is that of gradual reduction. On the running plate and close around the shaft are large ribs, which lack less than an inch of coining in contact with the corresponding ribs of the stationary plate; then conic numerous small A-like projections (A; fig. 8), and finally, on the outer edge (B), a ring of fine reversed inclines. The ribbed plates engage the grain, small pieces of cob, etc., which have been broken by the double breaker and further reduced by the crusher, and reduce them to small uniform pieces. The material then passes to the plates at A (fig. 9), whose sharp-edged projections cut or shear the small pieces of grain, etc. as they pass from one to the other; and finally the material, now reduced to small gritty particles, is taken by the cor rugated edges of the plates and is rubbed to the desired quality of feed. These plates are made of very hard cast iron; and when worn, the direc tion of running them may be reversed, which brings into service their opposite edges. This mill, therefore, possesses in one form the self-sharp ening principle.
of may observe at this point that, as every cutting-tool implies an adjoining void, the cutting principle in millstones is due to the hard edges of porous substances. Thus, French burr-stone and the various forms of lava and sandstone of which the ancient and modern millstones are made have these characteristics, and those kinds which are the hardest are the most highly prized. Even fine-grained por celain, which has been used in the rolls of roller-mills, has been found, on trial, to possess a cutting-surface too sharp for the bran-particles, which are too much reduced for easy separation from the flour, and these rolls have for this reason, in certain milling-plants, given way to chilled-iron rolls, which have smoother surfaces, and which ent and wear less. (See p. 47.)
The Selfsharfiening Principle, in so far as it depends upon hard and soft materials alternately arranged, is illustrated in the conformation of the crowns of horses' teeth and of those of other grain-eating animals, and in the teeth of beavers, which are admirably adapted for cutting wood. The convolutions of the projecting harder portions of the crowns, from which the softer intervening matter has been worn away while masticat ing grain, are shown in Figure 13 (y51. 8), and in Figure 14 is exhibited a cross-section of a tooth which has been surfaced, and from which the dentine has been partly removed, thus showing the ramifications of the enamel.
Natural Crushers. —In the solution of numerous mechanical problems Nature furnishes many suggestive examples. We may refer, by way of illus tration, to a natural crusher—namely, a lobster's claw (pt. 8, fig. 15). This consists of two hinged parts so formed as to possess great power and wonder ful adaptation to the purposes intended. The general appearance suggests the parabola, which is the favorite and usual shape adopted by the scientific mechanic and proved by mathematicians to be a form possessing the great est strength and resistance with the least amount of material. Further examination in detail will reward the student with natural examples of the best disposition of material, of adaptation of means to an end, of pro tection to delicate parts, of marvellous joints and means of operating them, the whole being managed at arm's length, delicately hinged and folding, prehensile, and of unusual interest to the constructing mechanic.