Engineering Wonders of the World
Volume I

304 ENGINEERING WONDERS OF THE WORLD. sists generally of a wrought-iron or steel boiler about 5 feet 6 inches in diameter and 13 feet long, built through a brick or concrete dia- phragm wall closing the tunnel at a con- venient point before water is encountered. This air-lock has two doors, an outer and an air is furnished by a compressor or com- pressors at the surface, through a pipe about eight inches in diameter carried through the diaphragm wall to one side of the lock. It has a hanging valve placed at its extreme end, which projects some distance into the tunnel. PALRYMPLE HAY’S HOODED SHIELD, AS USED ON THE BAKER STREET AND WATERLOO RAILWAY UNDER THE RIVER THAMES, WITH PORTION OF SAFETY DIAPHRAGM REMOVED. (Photo, Bolas and Company, 5 and 7 Old Queen Street, S. PF.) inner door, both opening in the direction in which the tunnel is intended to be driven, and of which one at least is always closed. When compressed air is Principle applied to the workings, the .. , , inner door — that is, that Air lock. nearest to the working face— is first shut tightly against an indiarubber joint, which air cannot pass. The compressed The face of the workings being impervious, the air in the tunnel between the air-lock and the working face gradually becomes compressed as the engine forces air down the shaft and through the air-delivery pipe into the tunnel. The compression is indicated in pounds per square inch by a gauge fixed on the outside of the diaphragm wall. The air pressure needed to exclude water from a heading is