design the Britannia tubular bridge over the Menai Straits upon that ugly though sound principle, though in justice it must be admitted that his original suggestion for a cantilever bridge was not accepted. The tubular bridge was the first attempt to use wroughtiron exclusively for long spans. Stephenson and Harrison had finished in 1849 the high-level bridge at Newcastle-upon-Tyne, with its masonry piers, cast-iron arched ribs, and wroughtiron ties-a bridge that remains as the link between the old cast-iron and the modern wrought-iron types. The smaller wrought-iron tubular bridge over the Conway river, of 400 feet span, was successfully opened for traffic in 1848, and the larger bridge was in course of erection. There are two central spans of 460 feet, and two side spans of 230 feet, at a height of 100 feet above high water. There are two separate wroughtiron tubes side by side, each 1,510 feet long, resting on three towers of masonry and the end abutments, so as to give the required spans. The tubes are rectangular in section; the top and bottom booms are cellular; the whole is built up of wrought-iron plates riveted together, supported solidly on the central tower, and on rollers on the side-towers and abutments. The design was closely copied in the Victoria bridge over the St. Lawrence at Montreal, finished in 1859. The webbing connecting the top and bottom flanges of a girder need not be plain plate-iron, as in the 1850 Torksey bridge in Lincolnshire. It may well be made in lattice form, and, in fact, the open-work type was tried by Sir John MacNeill, as long ago as 1843, over a short 84-feet span near Dublin. The type is now very general, by reason equally of its strength, neatness, and simplicity of construction. A good example is given by the Charing Cross railway bridge in London. The completion of the Forth Bridge in 1889 marked a great advance in the history of girder bridges, as it introduced cantilevers for long spans, and was immediately copied for bridges in various parts of the globe. Imagine two enormous steel brackets back to back, forming together a beam or lattice girder 1,700 feet long, tapering each way from the middle, where the whole is supported on a substantial pier. Such is the double cantilever. Its two parts, built up simultaneously, balance each other. Another similar cantilever is at a distance of 1,700 feet, and the short space between their near ends is bridged by ordinary lattice girders, also built outwards from each end. The Forth Bridge is designed with a taper in plan-that is, it is widest at the piers, narrowest at the centre of each large span. This is mainly to withstand lateral wind pressure safely, a lesson learnt from the sad catastrophe to the original Tay Bridge, overthrown by a storm in December, 1879 (p. 877). The operations of building the piers in deep water were unique at the time, and offer the best example of the direction of development of this part of the subject. Hollow wroughtiron caissons, or water-tight cylinders, 70 feet in diameter, were sunk to the rock-bed 72 feet below the water-level. The rock was cut away to receive each caisson, by operations carried on in a chamber at its base, supplied with compressed air at 33 lbs. per square inch to resist the encroachment of the water. Where the bed was of mud, the weight of the caisson carried through to the stiff boulder-clay below. In each case the caisson and mining-chamber were filled with concrete, so that, when set, each caisson constituted a firm stone pillar 70 feet in diameter. The main improvement in suspension bridges has been in the process of stiffening by some form of lattice-work. The suspension principle may well be combined with others, as with the Tower Bridge, where the side spans are half suspension, and the centre span has two permanent lattice-girders and the lower drawbridge. The Telford bridge over the Menai Straits still remains our finest example of the simple type. It was Canals and Supply. THE VYRNWY DAM. opened in 1825; it has a span of 579 feet, and the roadway is 100 feet above the water-level. The growth of railways has been attended with a decline in canal traffic in this country. Yet it is very possible that this generation will see the advantage of rescuing our extensive waterways from desuetude. The only great canal scheme of this century has been that of the Ship Canal connecting Manchester with the Mersey estuary. It is 35 miles in length, 26 feet deep, and with a bottom width of 120 feet. There are four reaches, separated by sets of three locks. Steamers can effect the whole distance in eight hours, including all delays at the locks. In the matter of watersupply, the growing interest in problems of public health, and the municipalisation of water supply, has caused attention to be paid to the purity and sufficiency of the water for large towns, and important alterations have been proposed for Liverpool, Manchester, Birmingham, and London. The Liverpool supply from the River Vyrnwy, in Montgomeryshire, is now an accomplished fact. Manchester now derives its water from Thirlinere, in the Lake district. Birmingham is to have its water from South Wales, and London still (in 1904) supplements its former supplies by storing Thames water and drawing from wells in Kent and Herts. From the 'thirties the Metropolitan supply was in the hands of eight different companies till they were replaced in 1904 by a Water Board. A scheme has long been advocated for an aqueduct from Mid-Wales. The chief engineering difficulties are associated with the formation of the storage reservoir near the source of supply, though the choice of route and construction of easy gradients for the conduit are matters THE MANCHESTER SHIP CANAL AT EASTHAM. Photo: Priestley & Sons, Egremont. H. C. and Mining, 1846. 1885. of importance. The main interest in the Liverpool supply scheme attaches to the masonry dam built across the Vyrnwy valley, on a bed of solid rock, forming a basin for the upland waters to accumulate. The lake thus formed is five miles long, and averages three-quarters of a mile in width. The dam is 1,200 feet long, rising 100 feet above the ground, with 60 feet of foundation below. Along the top is a carriage-road, supported on 33 arches, which give the structure a fine appearance. The aqueduct to Liverpool is 67 miles long, whereas that leading from Thirlmere to Manchester is 96 miles long. THE British trade in pig iron continuously, sometimes rapidly, Metallurgy increased, and several new districts have assumed a position of great importance. Chief amongst them is that of Cleveland, with which Sir Lowthian Bell has long been identified, and where his important work in connection with the theory of iron smelting has been done. The maximum output was in 1883 (8,490,000 tons), but since then the American demand has been met locally and our own output diminished in consequence. The annual output per furnace had increased from the 1,500 tons in the early part of the century to from 20,000 to 25,000 tons, a result due to the increased size of the furnaces and improved appliances used with them. Competition has become much more severe in later years, and the phenomenal fortunes made in early days have ceased to be easily obtained by new comers into the field. Much of the growth of the pig iron industry has been for the purpose of supplying raw material for the production of finished wrought iron and steel. In 1815, steel for the purpose of making tools and cutlery could be obtained of the highest quality. It was, and for these purposes still is, produced by the process of cementation from wrought iron, and it was costly in the extreme. But of cheap steel, or steel suitable for constructive purposes, there was none. The wrought or malleable iron was produced by Cort's process of puddling (Vol. V., p. 632), with the improvements made by Hall, but the rolling-mill with grooved rolls introduced by Cort had made it an easy matter to supply bars of any desired section. It was thus possible to produce the wrought iron rails necessary for the early railways, a requirement that of itself led to a rapid Iron and |