The introduction of the beaters with special means of circulating the pulp was found to be of the greatest service in the treatment of stuff like esparto and wood pulp, since these materials did not require the drastic measures necessary with rag pulp. In 1890 several engines of this class were being adopted, amongst which may be mentioned Hemmer's, Reed's and Taylor's. The pulp discharged from the beater roll was drawn through an independent pipe or channel by means of an Archimedean screw, or a centrifugal pump. _Stone Beater Rolls._--The substitution of stone for metal in the roll and bedplate of the engine brings about some remarkable changes in the nature of the beaten stuff. The fibre is submitted to the action of rough surfaces rather than that due to the contact of sharp edges, with the result that the disintegration is much more rapid, and produces a "wet" working pulp suitable for imitation parchments and similar papers. The latest materials used for this purpose are basalt lava stone in Germany, and carborundum in America. [Illustration: FIG. 52.--Nugent's Beating Engine with Paddles for Circulating the Pulp.] Care is necessary in the manipulation of these beaters to prevent fracture of the stone parts. In the Wagg Jordan engine this danger is materially reduced by the construction of the working parts. _Refiners._--In these engines the beater roll is a conical shaped drum carrying the knives, which revolve inside a conical shell completely lined with fixed knives. The fibres are thus cut up to the desired length, but before discharge from the engine they pass between two circular discs, one stationary and the other revolving in a vertical position. The effect of the discs is to tear or bruise the fibres rather than to cut them. The refiner is best employed to clear or brush out the mass of pulp after a certain amount of preliminary treatment in the beater, as the refiner cannot produce the effects obtained by actual beating as in the Hollander. [Illustration: FIG. 53.--A "Tower" Beating Engine with Centrifugal Pump for Circulating Pulp.] _Power Consumption._--The long treatment required to thoroughly pulp a strong material demands a great amount of power. Engines differ considerably in their power consumption, and comparisons are frequently made in terms of the power required to beat a given weight of pulp. But this is not always a true criterion of efficient work. Some types of beater are suitable for producing certain results, and the mere substitution of a beater consuming less power is worse than useless unless it can be shown that the same effects are being obtained. The efficiency of the Hollander for the beating of rag pulp, in spite of the high power consumption, is a case in point. [Illustration: FIG. 54.--Working Parts of a Modern Refining Engine.]