Page:The American Cyclopædia (1879) Volume III.djvu/563

 CAISSON 557 and so carried down. In 1867 Gen. Sooy Smith planned a caisson of an annular elliptical form, with two air locks by which the founda- tions of the lighthouse at Waugoshance were sunk. The cylinders of the Omaha bridge were sunk in 1868-'9 by Mr. Sickles on Gen. Smith's plan. In 1869-'70 Oapt. Eads sunk the foundations of the St. Louis bridge, using very large caissons and going to the great depth of 110 ft. below the surface of the water with one of them. The caisson of the east pier served a twofold purpose, a coffer dam being erected on the top of the inverted lower portion, in which the masonry was built. This caisson was made of iron, of a hexagonal form, with the air cham- ber under its whole area. The air locks were placed partly within the air chamber, to which access was had both by stairs and an eleva- tor running down the air shafts. The excava- tion was made by a water siphon designed by Oapt. Eads, by which the sand was carried out by the force of water, which passed down one pipe and returned through another, bringing the sand with it. This is probably the most effective method of removing sand or soft ma- terial under such circumstances. Capt. Eads also introduced glass globes in which lights were burned under the normal air pressure, and the smoke conveyed out of the caisson. He practically demonstrated the possibility of carrying down a larger mass of masonry to a greater depth than had ever before been ac- complished; and when it is understood that the consequent maximum air pressure was 54 Ibs. per square inch within the air chamber, the great hazard of the undertaking may be ima- gined ; but it was entirely successful. Among the many other caissons worthy of note may be mentioned those at Leavenworth by Gen. Smith and Mr. Sickles, at St. Joseph by Col. E. D. Mason, at St. Charles by C. Shaler Smith, all on the Missouri river ; while on the Fio. 2. Caisson of East Elver Bridge. Danube alone may be added those at Steyeregg, Mannshausen, and Nussdorf. The largest cais- son that has ever been sunk was for the New York tower of the East river bridge, by Col. W. A. Roebling, in 1872 ; and as it embraces a variety of features, a view of the longitudinal section is presented in fig. 2. Its base is rec- tangular, being 172 ft. long and 102 ft. wide, with an air chamber 9J ft. high, the roof 22 ft. thick, and the sides carried up to a height of 82 ft. from the extreme lower edge. It was used in a double capacity, having a coffer dam above as well as an air chamber below. It was built of timber and lined with thin boiler iron, the whole held together by angle irons and bolts. It contained 4,200,000 ft. board measure of timber, 235 tons of iron, exclusive of 385 tons of bolts, and weighed when completed 13,271 tons, in which was already laid 30,000 tons of masonry. It had two double air locks extending into the air chamber, similar to those in the Saint Louis caisson, in which coils of steam pipe were introduced for keeping an equable temperature. Two air shafts extended up through well holes in the masonry, with an elevator in one and a double circular staircase in the other. Two water shafts, each 7 ft. 9 in. in diameter, extended below the level of the edge of the caisson, in which powerful dredges grappled the stones and coarser mate- rials that were deposited beneath them, and raised them to cars above, which conveyed them away ; while the sand was blown out by the air pressure, on Gen. Smith's plan, through pipes, of which there were more than 40 in various parts of the caisson. Gas was employ- ed to illuminate the interior, which was forced down into tanks and from thence distributed by pipes below. Communication was con- stantly kept up with the interior by means of