MECHANICS AND USEFUL AKTS. 5 



Q 



proposals to construct the caisson were invited from all the shii> 

 builders in this vicinity, and their bid proved to be the lowest. 

 A more than ordinary depth of water in front of the yard was 

 required, — not less than 23 feet, as, when launched, the caisson 

 will draw fully 17 feet of water. It is to be 170 feet long, 102 

 feet wide, — as already stated, — and 15 feet deep, with a top 5 

 feet thick, and sides of a thickness tapering from 9 feet at the top 

 to a foot below. The time required to build it will be about 4 

 months. As soon as it has been set alloat it will sink to within 18 

 inches of the surface of the water; and when the proper time ar- 

 rives it will be towed down to the ferry and placed in position 

 ready for being submerged. This is to be accomplished by l>uild- 

 ing on the top of the caisson successive laj'ers of timber and con- 

 crete to a height of 20 feet. The weightof the caisson, with this 

 20 feet of timber and cement above the ' air-chamber,' will be 

 11,000 tons. 



•'The material excavated is hoisted from the * air-chamber' 

 through two water-shafts by means of dredges, and as it is raised 

 the caisson sinks, being uniform-undermined round the 4 edijes 

 and throughout its whole extent. As the caisson thus gradually 

 sinks, the mason-work, enclosed in a coffer-dam, is in progress on 

 the top of the timber, thus adding the necessary weight. Access 

 is had to the ' air-chamber' by means of two air-shafts 3 feet in 

 diameter. The depth to which it will be probably necessary to go 

 into the bed of the river will ?je about 55 feet below high-water 

 mark, so that all the timber of the foundation will be enclosed in 

 the sand and other material through which an excavation has been 

 made." — Journal of Gas LigJitiny. 



Beginning with a span of 822 in 1854, one of 1,057 in 1867, 

 the bridge proposed for the East river by Mr. Roebling, a steel- 

 wire suspension bridge, is to have a span of 1,000 feet between 

 the towers, 135 feet above the water; it is calculated to bear six 

 times the strain which can, under any circumstances, be brought 

 to bear on it. 



Bridfje over the Schuylkill. — The plans of Mr. Kneats have been 

 adopted for an iron bridge over the Schuylkill, at South Street, 

 Philadelphia. The centre and river piles will be of iron sunk by 

 the pneumatic process ; the length of the bridge will be 2,488 feet, 

 and the clear height 32 feet above hio^h water. 



The Kansas Ctty Bridge was formally opened July 3d, with ap- 

 propriate ceremonies ; the municipal authorities of Chicago and 

 St. Louis and an immense concourse of i>eople participating. 



The bridge consists of comljination wood and iron trusses for 

 spans 130 feet, 177 feet, 200 feet, and 250 feet, an iron span 70 

 feet, and a Linville & Piper patent wrought-iron pivot span 360 

 feet in length. The superstructure was erected by the Keystone 

 Bridge Company, of Pittsljurgh. 



Missouri Iron Bridge. — The draw spans ^3 feet long, and 

 weighs 360 tons. The spans are respectively 200, 250, 200, and- 

 117 feet in length. — Engineering and Mining Journal, July 20. 



Bridge at Louisville. — The largest span of any truss bridge in 

 the United States is that of the great bridge across the Ohio 



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