282 



ENGINEERING. 



a movable bulk-head at the end. The shell is 

 of boiler-iron, in sections 15 feet long. One 

 plate of the temporary boiler-plate bulk-head is 

 removed and the earth dug away from behind 

 it. A second plate is in like manner taken out, 

 if no difficulty has been experienced. Then a seg- 

 ment of the sheet-iron shell is bolted on to the 

 completed part. The air-pressure, employed 

 to keep out water, is 26 pounds to the square 

 inch. When there is an escape of air, dry 

 cement is applied to the hole and is carried 

 in by the current until the hole is stopped. 

 In the case of larger leaks much ingenuity 

 is required, as also in getting in the shell- 

 plates. 



The Severn Tunnel, which will carry the 

 Great Western Railway into the heart of the 

 coal and iron district of South Wales, will 

 probably be completed in 1884, 7,000 feet hav- 

 ing been dug and arched at the close of the 

 year. Since the irruption of water from a 

 land spring, which seriously delayed the works, 

 no similar mishap has occurred. The work 

 progresses at the rate of 500 feet a month. 

 With 20 pumps of various capacities, only half 

 of which are operated continuously, nearly 

 12,000,000 gallons of water are raised per 

 diem. The tunnel will be 4| miles in length, 

 2 miles under the Severn and 2 miles under 

 the land. The strata under the river have 

 been found sound and dry latterly. More 

 water enters in the land-sections. The brick- 

 work is made of vitrified brick set in Portland 

 cement. There are 3,000 men employed and 

 14,000 pounds of explosives are used monthly. 

 The tunnel passes through red sandstone under 

 the river and hard pennant under the Glouces- 

 tershire shore. The minimum thickness of the 

 roof, between the tunnel and the river-bed, is 

 40 feet, and the depth of the tunnel below high 

 water 163 feet. 



The shorter tunnel, which is being bored 

 under the Mersey, resembles the other in being 

 excavated in red sandstone. It is three fourths 

 of a mile in length. The floor is 144 feet be- 

 low high water. As in the Severn works, an 

 enormous quantity of water has to be pumped 

 out. The machine devised by Colonel Beau- 

 mont, for excavating the Channel Tunnel, has 

 been adopted. It is expected to advance the 

 driftway 30 yards per week. 



The construction of the Brooklyn Bridge was 

 first begun January 2, 1870. In 1877 the mak- 

 ing of the cables was commenced, and in 1882 

 the approaches were nearly completed, and 

 work begun on the superstructure. The cais- 

 sons for the towers were 172 by 102 feet on 

 the New York, and ]68 by 102 feet on the 

 Brooklyn side. The New York caisson weighed 

 7,000 tons, and the concrete filling 8,000 tons. 

 The tower on the New York side contains 46,- 

 945 cubic yards of masonry, the Brooklyn 

 tower 38,214 yards. The span over the river 

 is 1,595 feet. The land-spans are each 930 

 feet long. The Brooklyn approach is 971, and 

 the New York approach 1,562* feet. The total 



length of the bridge is 5,989 feet. The sus- 

 taining cables, four in number, are 15| feet in 

 diameter, and will each stand a strain of 12,- 

 200 tons. The wire which composes them is 

 12 feet to the pound. There are 5,296 wires 

 in each cable. They 'are of galvanized steel 

 and oil-coated. Each wire is 3,578 feet long. 

 The wires are not twisted, but closely wrapped 

 to form a solid cylinder. The tower on the 

 Brooklyn side is laid on a foundation 45 feet 

 below high water, while the New York tower 

 rests on a foundation sunk 78 feet below the 

 high-tide mark. The towers have each a sec- 

 tion at the high-water line of 140 by 59 feet. 

 At the roof-course they are 136 by 53 feet. 

 The height of the towers above high water is 

 278 feet. The clear height of the bridge above 

 high water in the center of the river is 135 

 feet at 90 Fahr. The height of the floor at 

 the towers is 119 feet at high water. The top 

 of the towers is 159 feet above the roadway. 

 The anchorages are in masses of masonry 129 

 by 119 feet at the base, and 89 feet high at the 

 front, and 85 feet at the rear. At the top 

 they are 117 by 104 feet. The anchor-plates 

 weigh 23 tons. The depots at the ends of the 

 bridge will be of glass and iron, the one on 

 the New York side 260 feet long and 59 feet 

 wide. The cars will be propelled by station- 

 ary engines and wire-traction ropes. 



Besides the great suspension-bridge at New 

 York, a second bridge over the East River has 

 been commenced farther up. It is to cross in 

 two suspended spans from Ravenswood, Long 

 Island, to the upper part of Blackwell's Island, 

 and from BlackwelFs Island to the New York 

 shore. The bridge, with its approaches of iron 

 trestle-work, will be 10,043 feet long. The 

 suspension spans will be respectively 734 and 

 618 feet in length, and will have a clear height 

 above the water at mean tide of 154 feet. 

 There will be three chain-cables strung over 

 iron towers, and holding up the iron trusses 

 which support the floor of the bridge. The 

 roadway will be 76 feet wide, giving room for 

 double railroad-tracks, carriage-roads, and side- 

 walks. The towers will be formed by twelve 

 wrought-iron columns of 2^-inch iron about 

 24 inches in diameter, braced in every direc- 

 tion. Each of the four towers will be 46 feet 

 long at the top, 100 feet long at the base, and 

 260 feet high. The towers rest upon masonry 

 piers 60 feet wide and 120 feet long. The ca- 

 bles, one on each side and one in the middle, 

 will be double, consisting each of two chain- 

 cables, one above the other, crossing each other 

 in the center of the bridge on a pin- joint, and 

 joining each other by symmetrical curves. The 

 two cables are braced together with diagonal 

 braces and divide the load between them. The 

 natural advantages of the location are un- 

 equaled. At each abutment solid rock is found 

 a short distance below the surface. The an- 

 chorage on Long Island and in New York will 

 be in 40 feet of natural rock, and on Black- 

 well's Island it will be in 12 feet of rock re-en- 



