These mills, Fig. 68, consist of a “bell” or inner cone, covered with blades or teeth arranged at a slight angle to the vertical section of the cone, and which are made finer and increased in number towards its lower and wider part. This cone rotates within an outer hollow cone or casing, also provided with blades or teeth which are sloped slightly in the opposite direction to those of the inner cone, so as to meet them at an angle, like the cutting-blades of a pair of scissors, and the angles of the cone are so chosen that the blades approach each other more closely towards their base. The outer cone is fixed, and is provided with a hopper like a coffee-mill, while the inner cone is so rotated on its axis that bark placed in the hopper is screwed down between the two, and cut finer and finer till it reaches the lower edge, when it drops out. The blades or teeth are usually cast in one piece with the metal cones, and sharpened when required by chipping with cold chisels. This operation should not be conducted in the mill-house, or small chippings of iron may get mixed with the bark, and cause stains on the leather. This form of mill, which is run in England at about 30 revolutions per minute, and at nearly three times this speed in America, works very well with dry material, but clogs badly if it be appreciably damp. On this account it is always well to run the mill with a fairly slack belt which will slip before exerting sufficient pressure to break the machine, as in such operations as grinding, safety clutches are of but little use. A type of mill varying somewhat from the above, consists of a pair of discs or very obtuse cones, the inner one of which runs on a horizontal axis. The teeth are generally arranged in concentric rings and interlock with each other. The material to be ground is fed at or near the centre of the fixed disc, and escapes at the edges. The construction of this class of mill will be easily understood from Fig. 69. Very small pieces of iron or steel which get caught between the teeth will often result in the breaking of the latter, and the formation of iron dust, which is a serious objection to the employment of this type of mill (to which the Schmeija “Excelsior,” the Glaeser “Favorita,” and the “Devil Disintegrator” of the Hardy Patent Pick Co. belong) for grinding barks. [Illustration: FIG. 69.--“Excelsior” Mill.]