Engineering Wonders of the World
Volume III

204 ENGINEERING WONDERS OF THE WORLD. PLACING THE STEEL RODS FOR A REINFORCED CONCRETE BRIDGE OVER THE RIVER LEE DIVERSION. Mechanical Sand Washer. The Washing is done either by subjecting the sand to high- pressure water jets, or by passing it through a mechani- cal washer of the type shown in one of our illustrations. runs on rails round the edge of the central well. It con- sists of a large horizontal tube about 15 inches in. diameter, inside which is an Archimedean screw driven through gearing by a high - pressure three- on the edge of the first of the four groups of sand is lifted cylinder hydraulic engine at- tached to the carriage. The from the bed of the filter filter beds. Three of the groups contain six Filter Beds. beds, arranged round a great circular covered well like the petals of an irregularly-shaped flower. Strained water is admitted into these through culverts from an open aqueduct fed by the great Wal- thamstow reservoirs, 1| miles away to the north.. Were our vision able to penetrate opaque ob- jects, we should see the concrete floor of the filter, on that a 9-inch layer of large gravel, above that again 9 inches of small gravel, and top of all a couple of feet of sea sand. Every twenty-four hours about 1,000,000 gallons of water percolate through every acre of filter to the concrete bottom, along which it flows to a culvert communicating with the central well. From the well it passes to the sumps of the several pumps. Once a month during the summer, and once in six weeks in the winter,* a bed is drained and a top layer, half an inch or so thick, of sand is scraped off, together with the super- incumbent mud and other impurities—such as weeds—and washed for further use. * The period may be much longer or much shorter (in ex- treme cases, three weeks or several months), according to the weather prevailing. by means of a hydraulic ejector and de- posited in a bin at one end, where it is caught by the screw and moved slowly along the tube, encountering in its passage the en- gine’s exhaust water travelling in the opposite direction. The water picks up all the dirt and carries it away to a shoot emptying into a concrete conduit running parallel to the rails. The cleansed sand falls into a bin, from which it is scooped by an endless' chain of buckets —also driven by the engine—and deposited at the edge of the filter bed, or in some other convenient place. One of these washers will deal with 50 cubic yards in a day. The filter beds of the station have a combined area of 24 acres. If all were in use simultaneously— an infrequent occurrence—they could deal with about one-tenth of the total London water supply. In other engine-houses on the Essex side are a pair of compound ver- tical engines ; a pair of hori- zontal Worthington engines ; a single horizontal tandem ; a The other Pumping1 Engines. triple expansion engine of the marine type, known as the “ Prince Consort,” operating three pumps ; and three vertical triple ex-