Page:The American Cyclopædia (1879) Volume III.djvu/283

 BRIDGE (MlLITABT) 277 nected with each other by steel thimbles or couplings, accurately fitted and firmly secured over and about the joints. The top and bot- tom chords or tubes are 12 ft. apart, connected with each other by a triangular system of bra- cing so arranged as to constitute, with the tubes, an arched truss of great lightness and strength. The width of this bridge is 54 ft., and its cost, together with the tunnel under the city of St. Louis by which access is gained to its western Fio. 17. St. Louis Bridge. end, will be about $9,000,000. (See fig. IT.) Movable bridges are of several kinds, and receive different names from the manner in which they are constructed and operated. The term is usually applied to a platform properly supported between two points of a fixed bridge, and so constructed as to be readily removed and replaced. Drawbridges are those which are raised or lowered by means of a horizontal axis and counterpoise connected with the plat- form. Turning or swinging bridges are those which turn horizontally about a vertical axis, while rolling bridges are those which rest upon rollers, and can be propelled horizontally on them, so as to close or open the passage. We sometimes meet with a still different class of movable bridges, where the platform is sup- ported by boats, or any other buoyant body, and which can be introduced in the waterway, or withdrawn from it, at convenience. BRIDGE, Military. The art of constructing temporary bridges for the passage of troops across large rivers and narrow arms of the sea, was well known to the ancients. Darius pass- ed the Bosporus and Danube, and Xerxes the Hellespont, by bridges of boats. The army of Xerxes constructed two bridges across the lat- ter strait, the first of 360 vessels, anchored head and stern alongside each other, their keels in the direction of the current, the vessels con- nected with each other by strong cables, over which planks were laid, fastened by a rail on either side, and covered in by a bed of earth. The second bridge had 314 vessels, and was similarly constructed. According to Arrian, Alexander had a regular pontoon train of light boats attached to his army. The Romans had wickerwork vessels, covered with hides, des- tined to support the timber platform of a bridge; these formed a part of the train of their armies until the end of the empire. They, however, also knew how to construct a more solid kind of military bridge, whenever a rapid river had to be crossed ; witness the fa- mous bridge on piles, on which Csar passed the Rhine from Gaul into Germany in 55 B. 0. The bridge was built, according to the best authori- ties, somewhere in the region between Coblentz and Andernach. Its construction occupied the army for ten days. During the middle ages we find no notice of bridge equipages, but during the thirty years' war the various armies engaged carried materials with them to form bridges across the large rivers of Germany. The boats used were very heavy, and generally made of oak. The platform of the bridge was laid on trestles standing in the bottoms of these boats. The Dutch first adopted a smaller kind of vessel, flat-bottomed, with nearly vertical sides, point- ed head and stern, and both ends projecting, in an inclined plane, above the surface of the water. They consisted of a framework of wood, covered with sheets of tin, and were called pontoons. The French, too, according to Fo- lard, claim the invention of pontoons made of copper, and are said to have had about 1672 a complete pontoon train. By the beginning of the 18th century all European armies had pro- vided themselves with this kind of vessels, mostly wooden frames, covered in with tin, copper, leather, or tarred canvas. The latter material was used by the Russians. The boats were small, and had to be placed close together, with not more than 4 or 5 ft. clear space be- tween them, if the bridge was to have any buoyancy ; the current of the water was there- by greatly obstructed, the safety of the bridge endangered, and a chance given to the enemy to destroy it by sending floating bodies against it. The pontoons now employed by the conti- nental armies of Europe are of a larger kind, but similar in principle to those of 100 years ago. The French have used since 1829 a flat- bottomed vessel with nearly vertical sides, di-