_Edge-runners._--This type of crusher is highly suitable for reducing earth colours in large works. The special feature of the type is that both stones are mounted vertically and turn on a common shaft in the same way that a cart wheel does on its axle. These runners are particularly useful for reducing clay, chalk and other earth colours, which have to be dealt with in large quantities. They will also crush fairly large lumps, and they can therefore be used for the further reduction of materials roughly crushed in a breaker, etc. The material may be treated in either the wet or dry state, only slight alteration being needed to change from one method to the other. There are numerous different patterns of edge-runner, but all of them can be divided into two groups, viz.: mills with stationary troughs, whilst the shaft carrying the runners rotates; and those in which the trough revolves, and the stones merely turn on the stationary horizontal shaft. [Illustration: FIG. 4.] Comparison of the efficiency of the two types has shown that the revolving-trough type is the better, giving a larger output per unit time with a reduced consumption of power. Figs. 4 and 5 show a vertical section and plan respectively of this type of edge-runner. The trough _G_ is turned by means of a toothed crown gearing with the bevel pinion _O_ mounted on an overhead shaft _C_ driven by a belt pulley _N_. [Illustration: FIG. 5.] The bearings of the vertical shaft _J_ of the trough are situated at _L_ and _M_. The runners _H_ are loosely mounted on the fixed horizontal shaft _E_ and revolve in consequence of the friction between them and the material in the trough. As the latter revolves, the material is continuously pushed aside by the runners, and is again brought under them by the action of scrapers. The great advantages afforded by edge-runners, in consequence of their simplicity, easy management and low wear in comparison with other grinding appliances, have led to their reintroduction on a large scale. It should, however, be borne in mind that the edge-runner mill must be of a pattern suitable to the materials it will have to treat. The method of drive usually depends on local conditions. The revolving-trough type is chiefly useful for mixing, on account of the ease with which the materials can be charged. The capacity of edge-runner mills depends on the nature of the material, the diameter and weight of the runners, the speed at which they are run, and also on the rate at which the reduced material is discharged in order to give place to fresh portions of the charge. This is effected by means of two sets of scrapers, the individual members of which can be adjusted in any direction. Their ploughing action also greatly assists the mixing effect. [Illustration: FIG. 6.]