_Strainers._—The strainers are for the purpose of removing from the pulp all lumps formed by the intertwining of the fibres, and all pieces of unboiled fibre, which, if allowed to pass on, would show in the paper as inequalities in the surface, or as dark specks. The slits are made narrow at the top, and gradually increasing in width, so as to prevent them from getting choked up. These slits allow only the individual fibres to pass through, and their width varies according to the quality of the paper. They are from 2 to 3 in. long, and they vary in width from ·007 to ·05 in. They are put at distances of about 1⁠/⁠4 in. apart. Several plates, each containing about 500 slits, are bolted together, and form a strainer. The whole strainer receives a violent shaking motion, to assist the passage of the fibres through the slits. In the machine represented, two of these strainers are shown at C. The shaking motion is produced by the ratchet-wheel or cams _a_ acting on the hammer _b_. An improved form, called the “revolving strainer,” has of late years been introduced. The pulp generally passes first through one of these, and then through the ordinary or “flat” strainers, as they are called. A revolving strainer is shown at D. It consists of a rectangular box, the sides of which are formed of plates perforated with slits. Inside this box, a slight vacuum is formed by means of an indiarubber bellows worked by a crank on the shaft _d_. The vacuum is intended to serve the purpose of the shake in the ordinary form. The box revolves slowly inside a vat containing the pulp, and the strained pulp flows into the box D^1, and thence on to the flat strainers. [Illustration: FIG. 45.] Various patents have been taken out from time to time for {148} flat strainers worked by means of a vacuum underneath the plates caused by the motion of discs of indiarubber or thin metal. Fig. 45 shows in plan a set of strainers, as manufactured by Messrs. G. & W. Bertram, similar to those in Plate II., but illustrated somewhat more in detail. The pulp first passes through the flat strainer B, and from thence to the two revolving strainers A. From these it flows along the shoots placed at the side on to the paper-machine at E. [Illustration: FIG. 46.]