SUBSTRUCTURE.] B 11 I D G E S 327 110 feet from the surface of the water. Figs. 97 and 97 A show the pier of the bridge of Argenteuil. Excava tion was carried on in the lower chamber, the roof of which was very strongly built of metal, and served as the foundation for masonry and concrete built up round an inner tube, serving as a pass age for men and materials to the upper chamber and "air lock." This lock is formed of two concentric cylinders of 3 9 feet and 10 8 feet diameter respectively (fig. 97). The annular space was divided by two vertical partitions; the doors of communication (which were air-tight) were 1 S feet wide and 2 14 feet high. A small engine worked the hoist by a stuffing-box passing through the shell. A safety- valve is of course required. The upward pressure of the air requires to be counter balanced by weights. In the Argenteuil bridge this necessary weight was afforded by the masonry built in the tube as the tube sank. This plan seems preferable to the method of loading the shell externally with pig or railway Fra 97 ._ rouilda tions, Bridge of iron. Frequently, owing to Argenteuil. the tenacious nature of the soil, the water cannot be driven out below the tube, and in that case a syphon must be provided passing out at the top. The Argenteuil tube was sunk at a mean rate of about 18 inches per diem. This method is not confined to cylindrical tubes. Fig. 98 shows the method em ployed in building the piers of the bridge at Kehl over the Rhine. In this case four rectangular working chambers were o o sunk side by side and bolted together ; each chamber communicated with the surface by two air-passages, and one central elliptical passage which remained full of water. This central passage served for the exit of the excavated material. A mass of concrete was built resting on the working chambers, and contained by wooden framework. The concrete was added at the top above water as the foundations gradually sank. At Mantes and Chalons wrought iron caissons, shaped like the usual masonry piers, have been sunk by analogous methods. The method of sinking cylinders by compressed air was invented by Mr Triger in 1841, and was first used on a large scale by Mr Hughes at Rochester. The tubes at this bridge were designed to be sunk by having the air exhausted inside the tube, a system invented by Dr Potts. G9. Piles are used either to enclose a space or to bear part of the weight of a structure ; for the former purpose a wooden pile may be a round or square pointed piece of timber, G or 9 inches in diameter and 8 or 12 feet long. Bearing piles may be of any dimensions which can practi cally be procured, and several lengths of timber are often jointed so as to form one long strut. The point is armed with metal, and the head protected by a metal ring, which prevents it from spreading when struck by the rammer which drives the pile. Bearing piles are usually placed at a distance from centre to centre not less than 2 feet 6 Via. FIG. 98. Foiuulutioiis, Bridge of Kehl on tlie Rhine. inches or more than 4 feet ; 3 feet is a common distance. The diameter of bearing piles varies from 9 inches to 20 inches ; a pile may be considered to be driven homo when, with thirty blows of a rain weighing 800 Ib and fall ing 5 feet, it does not move ith of an inch (Rankine). A French rule- gives as a limit inch motion with twenty-five blows from a ram weighing G cwt. and falling 4 feet 3 inches. A pile which does not move more than this will bear from 600 to 1000 Ib per square inch. This would give a load of 50 tons for a 1 3-inch pile ; if, as is more usual, the load be only 8 or 10 tons for a 13-inch pile, the ultimate rate of descent may be three, four, or five times as much as the above. Piles are used as foundations in those grounds which are compressible, or which would be squeezed out from beneath masonry under the weight to be borne. The wooden bearing piles are usually sawn oft" so that all the heads may be level, and a wooden grating or platform rests on the heads, over which the concrete or masonry pier may be built ; in other cases the piles come up for some distance into the concrete. An external row of wooden piles is not unfre- quently employed as a precaution against scour, but these should always be further protected by a stone bank, which
will continue to protect the pier if the piles decay. A