MECHANICS AND USEFUL ARTS. 49 



is separated from it by a revolving drawbridge, so that the 

 lino can be rendered impassable at an}^ moment. On the right 

 bank a fort is being built, which will command the bridge. 



Bridge at Omaha, U. S. — One of the most important works on 

 the Union Pacilic Railroad — the construction of a bridge across 

 the Missouri River, at Omaha, 400 miles west of Chicago — is 

 about to be commenced by General G. M. Dodge, engineer of 

 the Union Pacific Railway. The bridge is about 2,800 feet long, 

 and is divided into 11 spans of 250 feet each, the piers being cyl- 

 inders of cast iron, 8 feet 6 inches in diameter, and filled with 

 concrete. The t^eacherous bottom of the Missouri River presents 

 more than ordinary dilficulties in obtaining a reliable foundation, 

 from the great depth of the shifting sand, which is constantly fill- 

 ing up old channels, and opening fresh ones, so that the section 

 of the bed is ever varying. Where it is possible, the cylinders will 

 be lowered on to the rock, and elsewhere, to a depth of 70 feet below 

 low water, in the sand, the bases being enlarged from 8 feet 6 inches 

 to 12 feet in diameter, to spread the bearing surface, which will 

 also be increased by iiat bai-s projecting from the foot of the cyl- 

 inder into the surrounding sand. Foundations of this class have 

 been successfully employed by the Hon. W. J. McAlpine, in 

 various bridges he has constructed. The length of the cylinders, 

 from low water to the underside of the girders, will be G9 feet, 

 making a total height of the main colimms of 139 feet. The 10 

 piers, each withi two cylinders, will be braced transversely, and 

 protected up stream with ice-breakers attached to columns 5 feet 

 diameter, and j)laced 20 feet in advance of the piers. The faces 

 will be of cast-iron plates, meeting at an angle of 45 degrees, in 

 front of the columns to which they are braced with oak timber, 

 the intermediate sj^aces being filled with rubble and concrete. 

 From below low water to the highest flood levels, the cylinders 

 will be cased by plates, and the enclosed space will be enclosed 

 with concrete, to prevent any accumulation of ice, or other ob- 

 structions, Avhich may be carried down the stream, from getting 

 betVv'een the cylinders, and straining them on the intermediate 

 bracing. The girders of the superstructure will be trusses made 

 of wrought iron, with the exception of a cast upper chord. The 

 approaches to the bridge on both shores will be on a gradient of 

 one in oO, made in embankment on the eastern side to a height of 

 40 feet above the ground, the remainder being a viaduct of trestle- 

 work. The total length of the whole, including the river cross- 

 ing, will be about 3f^ miles. — Journal of the Franklin Institute,. 

 March, 18G9. • 



Concrete Bridge. — The tests applied to the experimental bridge 

 of concrete, set in cement, erected over that branch of the Metro- 

 politan District Railway which forms one of the junctions between 

 the circular line and the West London Extension, prove conclu- 

 sively the reliable character of concrete exposed to compressive 

 strains. The structure experimented upon spans the open cutting 

 between Gloucester-Road Station andEarle's Court Road. Itisaflat 

 arch of 75 feet span, and 7 feet 6 inches rise in the centre, where the 

 concrete is 3 feet 6 inches in thickness, increasing towards the 



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