Page:Popular Science Monthly Volume 23.djvu/399

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THE New York Bridge Company, having for its object the building of the suspension-bridge between New York and Brooklyn across the East River, was chartered by the State Legislature April 16, 1867. The work was begun by this company, and continued until its control was transferred to the two cities, June 5, 1874, from which time until the completion of the structure it was carried on by a board of trustees. John A. Roebling was at first chosen chief-engineer, but, dying two years later, he was succeeded by his son and associate, Colonel Washington A. Roebling.

The engineer's plans and estimates were submitted in September, 1867, and were finally approved in the spring of 1869. The mechanical work was begun on the site of the Brooklyn tower January 3, 1870. The finished tower rises 278 feet above high-water mark, and measures from top to foundation 316 feet. It is faced above water with granite, but is built partly of blue limestone. At the water-level the tower is 140 feet wide and 59 feet thick; the roadway passes through it at a height of 119 feet 3 inches by means of two archways each 117 feet high, and 33 feet 9 inches wide at the base. Where this tower stands, the river-bed is a compact conglomerate of clay, sand, and bowlders, in which its foundation rests at a depth of 4412 feet below high-water mark. The lowest course of masonry rests on a layer of pine-beams 15 feet thick, i. e., the roof of the caisson used to carry down the foundation. Under the roof of the caisson are built 72 brick pillars, 912 feet high, and the rest of the space is filled in with a solid concrete. The Brooklyn tower was finished in May, 1875. The New York tower differs from this in being three feet wider, and in extending down for 7812 feet below high-water mark, where it reaches some spurs of the bed-rock, making the total height of the tower 350 feet. The roof of the caisson was made 22 feet thick, so as to support the greater weight of masonry to be built upon it during its descent. This tower was finished in July, 1876. Neither has as yet settled two inches.

The four cables are each 1534 inches in diameter, over two thirds of a mile long (3,57812 feet), and each consists of 5,282 galvanized steel wires, not twisted as in a small wire rope, but lying parallel from end to end. No. 7 wire was used, which is a little over one eighth of an inch thick, and each cable was made in nineteen strands. The coils of wire for one strand were spliced together, so that each strand consists of a continuous wire running back and forth across the river, and at each end passing around a grooved piece of iron called a shoe. The