metal, and movable gratings which are constructed of bars. The form manufactured by John Smith & Co. of Carshalton, and shown in figs. 22 and 23, has been adopted in various English towns, and, so far as I could learn, has everywhere given satisfaction. On account of its simplicity, the apparatus impressed me favourably. A sieve of woven wire (a) is placed over two movable cylinders (6), which are generally operated by means of d FIG. 22. Revolving Screen. John Smith & Co., Carshalton. an under-driven water-wheel (d) placed in the current of the sewage. The solids are swept from the sieve into a trough by a rotating brush (c). A similar form of apparatus was constructed by Herzberg at Göttingen, and put into use in 1903. The wire sieve is not simply woven, but constructed more like a spring mattress, and with a -inch mesh. Pieces of brass are attached to the sieve, which is of copper, at distances about FIG. 23.-Revolving Screen. John Smith & Co., Carshalton. a yard apart, in order to prevent the solids from sliding down. The sieve is driven by steam power at a velocity of about 2 yards per minute. As in Smith's apparatus, the solids are swept from the sieve by a brush which rotates in the opposite direction to the screen. The action of the brush is aided by little jets of water, supplied from underneath by a perforated tube. The solids fall into waggons from which the excess water drains back into the sewer. The drained solids are then mixed with peat dust and street sweepings, and made into a compost for agricultural purposes. Besides wire, other woven materials have been recommended for the removal of the solid matters from sewage. For over ten years a rotating screen has been in use at Glasgow. It is Screen Frame. நிடியர்கள் constructed of flattened iron bars, placed about three-quarters of an inch apart, and provided with projecting angle irons to prevent the solids from sliding down. At Hamburg a screen of similar construction was introduced a few years ago. In appearance it resembles the rotating sieve manufactured by Smith & Co. It is not a sieve, however, but a movable grating constructed of bars (fig. 25). The separate bars are about 15 inches long, and they are placed about 3-inch apart, so as to form sections of the screen 3 yards wide. The whole screen consists of forty-six such sections. The small bars Comb. Rubber. FIG. 25.-Bars of Hamburg Screen. were first constructed of caoutchouc, but this was later rejected in favour of a soft alloy. The screen is so constructed that the teeth of the rubber comb which removes the solids can pass between the separate bars. From the rubber comb the solids are scraped on to a travelling band by a strip of rubber. The velocity of the screen is from 1 inch to 13 inches per second--a velocity which allows the apertures to be partially clogged and thus to remove finer suspended matter. The whole apparatus is built in a grit chamber, 18 yards long, 10 yards wide, and 2 yards deep, from which the sediment is dredged out and discharged on to a travelling band. The dredger is mounted on rails, which permit of its action at any point of the detritus chamber. The apparatus shown in figs. 26 and 27 was constructed some years ago by Latham for the town of Croydon. It consists of a woven wire sieve, stretched in the form of a disc, which is placed across the sewer and rotated by means of water power. The solids are removed from the disc by means of a worm conveyor. At first two such discs were provided at Croydon, one with a coarse and the other with a fine mesh, but the coarser one was unnecessary and therefore was dispensed with. Latham's apparatus has also been adopted at Rhyl. Friedrich's apparatus (figs. 28 and 29) also consists of a sieve in the form of a disc; it is not, however, placed vertically in the sewer, like Latham's apparatus, but horizontally. A coarse sieve and a fine sieve are provided, and these are shaken by means of springs. This shaking is intended to loosen the solids and to carry them to a discharging channel. Riensch has lately abandoned the lines which he pursued for over ten years, and has adopted forms of apparatus similar to the above. He places a disc sieve nearly horizontally in the sewage, but sufficiently inclined to permit of the retained solids being brought out of the sewage by the rotation of the sieve (fig. 30). The solids are then removed from the sieve by means of rotating brushes (fig. 31). The advantage of this form of construction over his previous forms is ascribed by Riensch to the fact that the solids are not so liable to be crushed by the apparatus and so to escape through the sieves. The apparatus depicted in figs. 32, 33, and 34 has been in use for several years in an experimental installation at Dresden. At this plant the sewage first passes through a detritus tank 6 yards long (described on p. 46), and then reaches the disc sieve, which is 5 yards in diameter and The Stendal sugar factory, at which Riensch made his first experiments, has adopted a form of apparatus constructed by Joerning and Sauter (fig. 35), which there gives every satisfaction. It consists of a drum sieve having openings -inch wide. The solids retained on the surface of the drum form a filtering medium, which retains particles much less than -inch in size. The drum is cleansed by means of a steam blower. Simplicity and durability are among the features of this apparatus. Finally, it should be mentioned that centrifugal force has been recommended for the removal of suspended matters from sewage. C. Grease Extraction. Grease in Sewage.-Although it is the custom in sewered towns to insist upon the provision of grease traps at all places where grease is likely |