328 B K I D G E S [EXAMPLES. more thorough protection against scour is provided by cover- in"- the centre bed of the river with a concrete or stone FIG. 99. Aqueduct of Guetin over the Allier. floor, as in fig. 99, representing a pier of the aqueduct crossing the Allier at Guetin. Cast-iron guide piles (usually hollow tubes) are used for the same purpose as wooden guide piles. They may be cast with grooves down one edge in which sheet piling is held. Cast-iron and wooden piles are frequently used as part of the open-work metal or wooden framing of piers. In situations where these piles are not liable to injury by ice, iloating timber, or barges, this construction is very econo mical. Fig. 4, Plate XIX. shows the Crumlin viaduct, with piers of this character, the construction of which will be more fully described in paragraph 79 Fig. 100 shows the Portage bridge (234 feet high) span- FIG. 100. Portage Bridge. The ning the Genesce River, in the State of New York, piers are large wooden frames. 70. Screiv piles are cast-iron piles which are screwed into the soil instead of being driven in. At their end is fixed a blade of cast-iron from two to eight times the diameter of the shaft of the pile ; the pitch of the screw varies from one-half to one-fourth of the external diameter of the blade. The pile is turned by levers radiating from its head. In one example of their use (Rankine) the pile was screwed in by four levers, each 40 feet in length, with eight bullocks harnessed to it. The screws were 4 feet G inches in diameter, and the working load borne by them was 100 Ib per square inch. The piles were screwed from 20 to 45 feet deep in earth. Disc 2)iles have been used in sand. These piles had a flat flange at the bottom, and water was pumped in at the top of the pile, which was weighted to prevent it from rising. Sand was thus blown or pumped from below the piles, which were thus easily lowered in ground which had baffled all attempts to drive in piles by blows. In ground which is of the nature of quicksand, piles will often slowly rise to their original position after each blow. 71. Wells. In some soils foundations may be obtained by the device of building a masonry casing like that of a well and excavating the soil inside ; the casing gradually sinks and the masonry is continued at the surface. This method is applicable in running sands. The interior of the well is generally filled up with concrete or brick when the required depth has been reached, but in some cases a inert floor or inverted arch would be preferable. VIII. EXAMPLES. 72. The task of selecting a limited number of bridges v v FIG. 101. Pons SuLlicius. which shall represent the gradual progress in the art of construction and illustrate our present practice is one of much difficulty. Many very admirable and interesting structures must necessarily be passed over in silence, and space will not admit of full details being given even of those bridges which arc not iced. 73. Bridges ~bvi.lt before (he year 1000 A.D. Herodotus mentions a bridge erected by Xitocris over the Euphrates at Babylon. It appears to have consisted of stone piers con nected by planking, which was removed at night. The river was diverted to allow the piers to be built. Diodorus Siculus ascribes the work to Semiramis. The first bridge constructed at Rome was called the Pons Siiblic.ius, or wooden bridge (sul/ica meaning a stake or pile). It is said to have been built by Ancus Martins, and rebuilt by the chief priests, who from this 1 circumstance were called " Pontifices." Fig. 101 shows the design of
this bridge as restored by Colonel Emy (Traite da Part