_Crushers and Breakers._--Crushers usually consist of grooved iron rollers revolving on horizontal axes. One of the rollers is fixed, the other being adjustable by screws, in order that lumps of different sizes may be treated in one and the same machine, which may be employed either to turn out a roughly crushed product, or to reduce it to a certain degree of fineness. If several pairs of crushing rollers be mounted in series, and each set a little closer than its predecessor, the material can be reduced progressively from large lumps to a fairly fine powder. Each pair of rollers is geared together by pinions, and is turned in such a way as to draw the material in between. If the gear pinions have the same number of teeth, the two rollers will revolve at the same speed and will then merely crush the material into lumps of a size depending on the distance at which the rollers are set apart. Nevertheless, by simply altering the gear ratio of the pinions, the crushing action of the rollers can be supplemented by a grinding action, a much finer powder being then obtainable than otherwise, the one roller running at a higher speed than the other. These crushers differ in strength of construction, very strongly built machines being required for dealing with large lumps of hard material, whereas substances of low crushing strength, such as clay or other earthy materials, can be treated in much lighter machines. In any case, however, it is advisable to have the machine stronger than is absolutely necessary for the work in view; for, although the prime cost is thus increased, the outlay on repairs will be reduced, and the machines can, if necessary, be used on harder material as well. The framework supporting the rollers should always consist of a strong iron casting; and the machine should be set up as close as possible to the engine or motor, to minimise the loss of power in transmission through long shafting, etc. [Illustration: FIG. 1.] Fig. 1 represents a breaker (made by the Badische Maschinenfabrik, Durlach), suitable for the rough crushing of clayey materials supplied in large lumps. It can, however, also crush shale, lime, chalk, as well as hard, sticky masses which would clog up a stone-breaker. The material fed into this breaker is gripped at once by the powerful projecting teeth, which are connected together by sharp-edged ridges, and is crushed in such a way that it can be easily reduced still further by a succeeding pair of smooth rollers. [Illustration: FIG. 2.] The granulator (Fig. 2), made by the same firm, is an example of a machine for crushing harder materials. It is similar in construction to a stone-breaker, but differs in the movement of the jaws, and combines the properties of breaker and grinder, inasmuch as it tears the material as well as crushes it. The figure shows the machine adapted for direct electric drive. If necessary, these granulators can be fitted with classifying jig screens.