The longest swing bridges ever constructed are the Interstate Bridge at Omaha, Neb., which has a total length of 520 feet; the Thames River Drawbridge at New London, Conn., which has a length of 503 feet; and the Arthur Kill Drawbridge between Staten Island, N. Y., and New Jersey, which has a swing span of 496 feet.
The most notable of the hinged or pivot bascule bridges in Europe is the Tower Bridge across the Thames, near the Tower of London, England, which consists of a suspension and drawbridge combined with a central bascule span of 200 feet, formed by two leaves or trusses, hinged at the opposite towers. The United States has many examples of the bas cule type of bridge, one of the principal ones being a great single-leaf bridge at Chicago, carrying three tracks of the Chicago and North western Railway across the Chicago River. This bridge is 185 feet long, covering with one leaf nearly as great a span as the London Bridge does with two leaves. The largest bas cule bridge in the world is that carrying the Canadian Pacific Railway from Sault Sainte Marie, Ontario, to Sault Sainte Marie, Mich., over the United States Ship Canal. This bridge is of the heel trunnion type, 330 feet long, in two leaves. When closed it forms virtually a simple fixed span, with a compres sion joint at the top of the truss and a tension joint at the bottom. The counterweights are of concrete, each of 550 cubic yards and weigh ing 1,000 tons. One of the largest bascule bridges is that at Salmon Bay, Seattle, carrying the Great Northern Railway. It is of the Strauss low trunnion type and is a single span of 200 feet. Its total movable weight is 2,350 tons, which is counterweighted by a mass of concrete measuring 21 feet by 34 feet by 28 feet —735 cubic yards in all. The wind pres sure of this bridge when open is calculated at 6,955,500 foot-pounds.
The modern pontoon bridges are a develop ment of the ancient bridge-of-boats principle. A notable example of the type is the pontoon bridge built in 1873 across the Hooghly River, at Calcutta, India. It is 1,530 feet in length between the abutments on each bank of the river and consists of a superstructure carried on 14 pairs of rectangular iron pontoons with rounded bilges and wedge-shaped ends. These pontoons are 10 feet wide, 8 to 11 feet deep and 160 feet long and are built in 11 water tight compartments. They are held in position across the river by means of 134-inch chain cables laid across and anchors laid up and down the stream. The superstructure consists of
trestle-work, which carries a plank roadway and footpath platform having a total width of 62 feet at a height of 27 feet above the surface of the river. This height is sufficient to allow ordinary boat navigation, but the passage of large vessels is provided for by arrangements which permit the opening of a span 200 feet wide, by the temporary removal of four of the pontoons and the superstructure carried on them. A pontoon bridge over the Golden Horn at Constantinople is 1,531 feet long and 82 feet in width. An opening span gives 205 feet clear way: it is pivoted at one side and swings through an arc of 180 degrees. The longest pontoon bridge in existence is the railroad bridge over the Mississippi River at Prairie du Chien, Wis. It is 7,000 feet long and crosses an island. There are two pontoon sections, one 2,000 feet long to the east of the island and another 1,500 feet long west of it. This bridge was rebuilt in 1898. The pontoon bridges of the United States army are of two kinds: one built of wood throughout and the other a wood frame with canvas covering. The pontoons are 31 feet long, 5 feet 8 inches wide and 3 feet deep. The wooden type weigh 1,600 pounds apiece, the canvas ones 510 pounds.
Military bridges are temporary structures erected to facilitate the movements of troops, their supplies and their armament, during the course of extensive field operations. The proper equipment of a modern army includes the material required for the construction of pontoon bridges of a limited length. This ma terial consists of the necessary cables and pon toons of canvas, the metal or wooden frames of which are capable of being down' and packed for transportation. Spar bridges are usually made with round timbers cut near the location of the bridge. The most efficient and useful are those built in the form of tres tles consisting of timber frames on which the stringers carrying the roadway are placed. The most notable examples of military bridge con struction in the United States are the pontoon bridge built across the Potomac at Harper's Ferry, under the direction of General Banks, in February 1862, which was composed of 60 boats, and the trestle 80 feet high and 400 feet long, built at another period of the Civil War across the Potomac Creek, Virginia, for rail road purposes.