CAISSON (ante), in engineering, a hollow box of iron or wood, open at the bottom, sunk where, piers are to be placed. The largest caisson yet sunk was for the tower of the Brooklyn bridge on the New York side. At the bottom it was 172 ft. long and 102 ft, wide, with an air-chamber 9i ft. high, the roof 22 ft. thick, and the sides carried up 82 ft. from the lower edge. It had a coffer-dam in the upper part; was built of timber lined with boiler-iron, and bolted together. In its construction there were used of lum ber, board measure, 4,200,000 ft., and of iron, including bolts, 620 tons. When com pleted, it weighed 13,271 tons, and there were 30,000 tons of masonry laid within it. There were two double air-locks extending into the air-chamber, in which were steam pipes to keep an even temperature. Two shafts passed up through well-holes in the masonry, with an elevator in one, and two spiral stairways in the other. Below the lowest edge of the caisson extended two water-shafts, each 71 ft. in diameter, in which
dredges and scoops grappled the stones and soil, raising their loads to cars above, which conveyed the refuse away. At the same time sand and fine dirt were blown out by air pressure through 90 or more pipes in various parts of the structure. The interior was illuminated by gas, and constant communication by telegraph was kept up with the workmen inside. There were four shafts, each 2 ft. in diameter, for the introduction of material for the concrete with which the whole interior was finally tilled. The cais son was sunk 78 ft. below mean tide, a work that required a pressure of 34 lbs. per sq. inch. in addition to the normal pressure of air; and to supply this addition, 13 large f...ompressors were used. The earliest caissons for such purposes were used in England in 1738-40 in laying the foundations of the Westminster bridge over the Thames.