But with certain of the piers for the Forth Bridge the water was too deep for timber cofferdams, and the usual diving-bell was not sufficiently large. The piers were to be of immense size, no less than 55 feet in diameter, and the diving-bell of ordinary size would not cover that great width. Huge caissons were therefore made, 70 feet wide, constructed of iron plates and rising in height, according to the depth of water, up to 150 feet. The lower part of the immense caisson or tank was fitted as a water-tight division and filled with compressed air, the object being to resist the pressure of the water. Two shafts communicated with this air-tight division or mining chamber, one for the removal of the earth excavated, and the other for the men to pass up and down. The escape of the air through the shafts was prevented by the use of an air-lock, working on the same principle as a water-lock on rivers or canals. There were two doors in the lock, one communicating with the shaft and the other with the outside air. When the latter was closed and the lock filled with compressed air by opening a valve or tap, the door of the shaft could be opened and the man could descend to his work below. That work consisted chiefly of excavation in the bed of the river. Drills, hydraulic cutters, and dynamite blasting were all utilised until huge holes, many feet below the river bed, were hollowed out. As the caisson was filled with concrete above the air-tight chamber where the men worked it was exceedingly heavy, and sank by its own weight into the space prepared. The mining chamber was lit by electricity, and was about seven feet high. The mud of the river bed was mixed with water and blown away by the compressed air which seems to have been about 33 lbs. to the square inch. The caissons were sunk down to rock or boulder clay, and when they had reached the required distance the mining chamber was filled with concrete, and the same material used to the level of the water; the piers were then built up with huge stones placed in cement, the whole forming a magnificent mass of concrete and masonry, carried down in some cases to about 40 feet below the bed of the river. [Illustration: THE FORTH BRIDGE.] The three chief piers consist of groups of four columns of masonry, each gradually tapering from 55 feet in diameter to 49 feet at the top, and about 36 feet high. From these rise the huge cantilevers connected together by girders 350 feet in length.