282 



ENGINEERING. 



fft-tlier The four cables enter the anchor- 

 walls situated 1,337 feet from the pier on the 

 New York side, and 837 from the pier on the 

 Brooklyn side, at the height of nearly 80 feet 

 above lii-h water. Passing into the anchorages 

 to the distance of 20 feet, they connect with the 

 anchor-chains, which are composed of ten links, 

 each 12 or more feet in length, 130 feet in to- 

 tal leiiirth, which describe a quarter of the arc 

 of a circle, thus converting a part of the ten- 

 Hon into vertical pressure. The strain of the 

 cables on the anchorages is about 5,600 tons, 

 while the structures are calculated for ten times 

 that tension. The cables only sustain a part 

 of the load of the bridge. They are not placed 

 parallel, but, in order to hold the bridge stiff 

 against side pressure from the wind, the out- 

 side cables are carried over the towers at points 

 wider apart than the width of the flooring, and 

 converge as they approach the middle of the 

 span ; while the inside cables pass over the 

 piers near together, and diverge till they meet 

 the center. The steel wire composing the 

 cables weighs one pound per 11 feet. Each 

 strand of the 19 composing the cable is made up 

 of 261 wires. The cables are about 3,500 feet 

 in length each. The ropes are compressed into 

 perfect cylindrical form, the strands losing 

 their roundness by the compression^ and form- 

 ing segments of the cylinder in two layers 

 around the central strand, the effect being to 

 bring all the wires into one compact cylindri- 

 cal mass. 



The towers, as stated, are 268 feet in height 

 above high-water mark. That on the Brooklyn 

 side is situated immediately north of Fulton 

 Ferry-house ; the New York tower is at Pier 

 29, near the foot of Roosevelt Street. Each of 

 the piers rests upon a caisson sunk down to 

 the solid rock, which is 82 to 92 feet below the 

 surface of the water on the New York side, 

 and 45 feet on the opposite side of the river. 

 The towers are each 134 feet in length by 56 

 in width at the water-line. These dimensions 

 are made gradually smaller up to the upper 

 cornice by sloped offsets ; at the cornice they 

 are 120 feet by 40 feet. The roadway rests on 

 the piers at the height of 130 feet above the 

 water-line, and their height above the floor is 

 130 feet, not counting the ornamental capping 

 and balustrades. The roadway passes under two 

 archways in the piers, each 32 feet wide and 

 120 feet high. The piers are built entirely of 

 granite, and have hollow chambers; each con- 

 tains about 900,000 cubic feet of stone, weigh- 

 ing over 70,000 tons ; the foundations under the 

 water are extended to about 17,000 square 

 feet, making the load a little more than four 

 tons per square foot. The dead weight of the 

 ridge is to be about 3,600 tons, and the esti- 

 ited moving load is 1,400 tons. The founda- 

 lons for the piers were excavated by means of 

 JWHons, to aid in lowering which an air-cham- 

 was constructed at the bottom of each, 

 and dredging operations were conducted below 

 t.ie sinking mass by men within the chambers. 



Each of the anchorages contains about 35,000 

 cubic yards of masonry. 



The caisson sunk for the foundation of the 

 New York pier in 1872 was the largest ever 

 made. It had a rectangular base, 172 feet long 

 and 102 in width. At the bottom was an air- 

 chamber 9 feet high. The roof was 22 feet- 

 thick, and the sides were carried up to a height 

 of 82 feet from the bottom. The upper part 

 served as a coffer-dam. The caisson was made 

 of timber and lined with boiler-iron. The 

 weight of the timber and iron part was 13,271 

 tons, in which had been laid 30,000 tons of 

 masonry. There were two double air-locks 

 running into the air-chamber and two air-shafts 

 extending through well-holes in the masonry, 

 and in these an elevator and staircase. The 

 temperature was kept even by steam-piping. 

 Below the bottom edge of the cnisson extend- 

 ed two water-shafts, in which worked power- 

 ful dredges to grapple the large stones and 

 coarser materials under the caisson and convey 

 them into cars. The earth and sand were blown 

 out by compressed air on the plan of General 

 William Sooy Smith. There were over 40 

 pipes for this purpose. This caisson was sunk 

 to a depth of 78 feet below mean high tide. 

 The pressure of 34 pounds above the normal 

 pressure of the atmosphere was kept up by aid 

 of thirteen large steam compressors. The cais- 

 son on the Brooklyn side, sunk the previous 

 year, was 168 feet long by 102 wide. 



The new bridge which is being built over 

 the Missouri River at Glasgow for the Chicago 

 and Alton Railroad, according to the designs 

 of General Sooy Smith, is made of steel, being 

 the first example of a truss bridge composed 

 solely of that metal in America. It is a single- 

 track bridge of five spans of 315 feet each, 

 formed by Pratt trusses with pin connections. 

 In the sinking of the timber caissons for the 

 foundations of the piers the pneumatic process 

 was employed. The excavated sand and dirt 

 were discharged by steam ejectors. 



The longest span crossed by a swing-bridge 

 is that over the channel which connects the 

 graving docks with the harbor at Marseilles, 

 a model of which was exhibited at the world's 

 fair in Paris. The bridge is formed by three 

 main girders, which support a platform 203 

 feet in length and 41 feet in breadth, on which 

 are a railway, a carriage-way, and a footpath, 

 the last being carried outside one of the outer 

 girders on cantilevers. Over the pivot is a 

 cross-girder, under which is an hydraulic press 

 which lifts the bridge off its bearings and sup- 

 ports it while it swings. At the counterbal- 

 ance end of the bridge each girder is furnished 

 with a wheel, which moves on an iron rail. 

 The total weight of the bridge is 760 tons. 

 The bridge can be turned by one man in three 

 minutes. The hydraulic "machinery which 

 actuates it consists of the central press, which 

 raises the bridge and forms the pivot of ro- 

 tation, of cylinders for . working supporting 

 wedges, and of two machines which turn the 



