Engineering Wonders of the World
Volume III

THE DRAINAGE -SYSTEM OF LONDON. 219 not expect to find on a vessel designed for such a purpose. A vessel is able to discharge its 1,ßOO-ton burden in a minimum time of six minutes. In practice the operation takes about an hour, the boat steaming along at Dumping the normaj speed meanwhile to distribute the sludge over a large area. The sea-water has been, analyzed after deposit of the sludge without revealing any traces of the impurity, nor has a par- ticle of sludge been discovered on the shore of the Maplin Sands. This proves conclusively enough that all organic matter must be well assimilated by the German Ocean, though the sludge carried out to sea annually would suffice to cover Hyde Park to a depth of nearly five feet. Each of the sludge ships re- ports to the Mouse Lightship every time it passes outward or inward bound, by flags in the daytime, by flashed Morse signals at night. The time of passing the Mouse is noted for comparison with reports sent by the London County Council to the Thames Conservancy (now the Port Authority), which body is also informed of the number of times each, vessel is loaded at the outfall works. The effluent from the sewage is, when it passes into the Thames, remarkably clear and transparent. In fact, it has been said that it is the clearest water that enters the Thames below Richmond. Fish, which previously to the establishment of the precipitation works did not come farther up the river than Graves- end, now pass up to London Bridge—a strik- ing testimony to the improvement effected by the new system of sewage disposal. The outfall works at Crossness are in prin- ciple identical with, those at Barking as regards both their arrangement and the treatment of sewage, but more compact. It should be noted, however, that whereas on the north side of the river all the sewage flows by gravi- tation from Abbey Mills to the precipitation channels, at Crossness it has to be pumped. Recently two new sewers, each 9 feet by 9 feet, have been added to the northern outfall sewer between Old Ford and Barking, and are now in use. These, like ,, . _ New Sewers, the old culverts, are carried in embankment some 20 feet above the sur- rounding district, and cross over numerous roads, railways, and water-ways by means of iron tubes carried on girders and supported by abutments. A new middle-level sewer, which, will discharge by gravity, is being made from Willesden to Old Ford, a distance of nine miles ; and a new low-level sewer, 12| miles long, is under construction between Hammersmith and Bow. On the south side of the river a new outfall, 11| feet in diam- eter, now runs from Deptford to Crossness, and a new high-level sewer from Catford to Crossness. Plans have been drawn up for an additional low-level sewer between Battersea and Deptford. The map on page 210 shows the positions of all the intercepting and out- fall sewers quite clearly. The older intercepting outfall sewers were made of brickwork, either in cut-and-cover tunnel or in embankments, according to the level of the ground surface. For the new northern low- $ewer C°n = struction. level intercepting sewer the tunnelling shield and a cast-iron lining, con- creted on the inside, have been used. In fact, its construction differs little from that of the tunnels of a tube railway except that the spaces between the flanges of the cast- iron lining are filled in and rendered to a smooth surface. The cross-section of the intercepting sewers increases gradually east- wards. Thus, the old middle-level begins with a 4| by 3 feet section, at the western end. North of Kensington Gardens the figures increase to 6 feet by 4 feet. Abreast of London Bridge there is a 9-foot barrel; and by the time the junction with the high-level sewer at Old Ford is reached the dimensions have risen to 9| feet by 12 feet.