Compressed Air Work And Diving 1909

SALTASH BRIDGE. 53 dorne was surrounded by eleven compartments, or cells, making a wall 4 ft. thick, communicating with each othcr, and with a third cylinder 6 ft. in diameter, at the top of which was fixed an air-lock, placecl inside the 10-ft. shaft but out of centre. The caisson was stink partly by being forcecl through the mud, and finally by excavating, under air pressure, the area between the outer and inner walls of the portion below the dorne. On reaching its full bottom the air space was filled with a ring of granite masonry to a height of about 7 ft., and the caisson itself lewisecl to the rock to clo away with any danger of its overturning. An attempt was then made to pump out the water, and to excavate the remainder of the mud through the 10-ft. shaft in the open. The leakage was, however, too great, and as the 10-ft. shaft was too weak to stand air pressure, it was strengthened by a fourth cylinder 9 ft. in diameter, at the top of which was placecl a lock, and the excavation completed with the help of compressed air. The interiör was then fillecl with masonry, the inner shell and dorne being cut away as the work proceecled. The chief defect in the design of this caisson seems to be the unequal level of the cutting edge, which would allow the water to enter to the level of the highest point if in fairly open ground, and this appears to have occurrecl. The design was, however, somewhat complicated, and has not, as a matter of faet, been since imitated. The same clifficulty of a sloping rock bottom had to be con- tended with in the pitching of the two Inchgarvie south caissons at the Forth Bridge, and was met by levelling up by sand bags and piers of concrete. The cylinders for the Londonderry Bridge, 1859, were sixteen in number, two to a pier, and 11 ft. in diameter, of the orelinary single shell cast-iron type, and