The relation of the percentage of gelatine to the period of beating thus becomes a matter of interest, and well worth investigation. The figures are suggestive of further experimental research along definite lines. [Illustration: FIG. 48.--Plan and Sectional Elevation of a "Hollander."] _Developments in Beating Engines._--Since the introduction of the Hollander beating engine, about A.D. 1670, other types of beater almost too numerous to mention have been devised to supersede it, but the fact remains that the principle of the original Hollander and its general design are still adhered to in the engines used by paper-makers for high-class work. The alterations and improvements which have taken place during the last fifty years relate chiefly to the modifications naturally arising from the introduction of fibres not requiring such drastic treatment as rags. The machines now in use for reducing half-stuff to beaten pulp ready for the paper machine may be classified as follows:-- (1) Beaters of the Hollander type, in which the circulation of the pulp in the engine and the actual beating process are both effected by the beater roll. (2) Beaters of the circulator type, in which the movement of the pulp is maintained by a special contrivance, and the beater roll used only for beating. (3) Beaters of the stone roll type in which the roll and bedplate are either or both composed of stone, granite, or similar non-metallic substance. (4) Refiners, containing conical shaped beater rolls working in a conical shell fitted with stationary knives. [Illustration: FIG. 49.--Beating Engine with Four Beater Rolls.] _The Hollander._--This beating engine in its simplest form consists of an oval shaped trough, divided into two channels by a "midfeather," which does not, however, reach completely from one end to the other. In one of the channels the bed of the trough slopes up slightly to the place where the "bedplate" is fixed. The bedplate consists of a number of stout metal bars or knives firmly fastened into an iron frame, which lies across this channel. The beater roll, a heavy cast-iron roll provided with projecting knives or blades arranged in clumps of three around the circumference, and supported on bearings at each side of the engine, revolves above the bedplate with the knives adjusted to any required distance from it, the raising or lowering of the beater roll for this purpose being effected by the use of adjustable bearings. The bed of the trough behind the beater roll rises sharply up from the bedplate and then falls away suddenly, as shown in the diagram, forming the "backfall." When the engine is in operation the mixture of water and pulp is drawn between the knives and circulated round the trough. The material is disintegrated into fibres of the required condition, discharged over the backfall, and kept in a state of continual circulation, and the beating maintained until the stuff has been sufficiently treated.