Pontoon Drazebridge.—What is said to be the largest structure of this class in the world is the pontoon drawbridge over the Missouri River at Nebraska City, of which a view is shown in Figure 5 05/. 47). Its length across the navigable channel is 1074 feet, while the back channel is trav ersed by a causeway 1050 feet long supported on cribs. The draw is V-shaped, with the apex down stream. It is operated by the current and controlled by one man. The clear span of the draw is 528 feet. It was completed and opened for traffic in August, ISM It will be removed during the ice season.
Flying Bridges (ferries) are employed in situations where the traffic is not important enoug-h to warrant the erection of a bridge. They consist of a boat or pontoon (or of two coupled boats) and the cable to which it is attached and by which it is guided from side to side of the stream. A cable is sometimes stretched across the stream from bank to bank, and the towing rope for the boat is attached to this by means of a travelling- roller (pi. 39, fig. IS), or, more commonly, the cable is anchored in mid-stream, so that it is free to swing around its fastening when the boat, which is secured to its other end, crosses over (fig. 19). To support the cable, so that it shall not drag along- the bottom of the stream, it is attached along its course to several floats or trailing pontoons. The boat is propelled across stream by the force of the current against its side, which is kept diagonally to the course of the stream, while the head of the boat is kept in the right direction either by the rudder or by a chain secured to the towing cable.
Rolling Bridge at St. Ofalo.—A novel system of communication, that is sufficiently interesting to warrant notice, is the rolling- bridge between the two cities of St. Maio and St. Servan, situated on the northern coast of France, on the English Channel, and separated from each other by a nar row arm of the Channel, which is dry at low tide, but quite deep at high tide. To establish direct intercourse between the two citics the architect Leroyer of St. Maio constructed a light iron framework which carried foot passeng-ers from one jetty to another. This iron structure rests upon a car riage mounted on wheels adapted to run on a railway-track. The carriage is drawn in one direction or another by attached chains, which wind over a dnun actuated by a steam-engine. At the upper part of the framework there is a platform for carriages, wagons, and merchandise, and for the convenience of foot-passengers there is provided a species of cabin, to serve as a protection from the weather. The bridge operates at all states of the
tide. At high tide the appearance of the structure moving across the waterway with nothing visible above the water but the freighted platform and a few slender iron rods of the framework is quite curious. At low tide the passage-way is dry, and the appearance of the bridge making its trip between the jetties is shown in Figure 3 (fi/. 49). The heig-,lit of the plat form floor above the rails is 34,, feet, and at high tide the bridge is sub merged to the depth of 33 feet. The distance to be traversed from side to side of the pass is 295 feet, and the trip is made in two and a half minutes. This rolling bridge has been in operation since 1871 without interrup tion. The system is an exceedingly simple one, and, as there would appear to be no difficulty in extending it considerably in length, it might be found a serviceable expedient for other localities where the conditions are similar.
Railzcay Ferries serve for the transportation of railway trains (exclud ing the locomotive) across rivers, lakes, etc., whereby the unloading and reloading of freight or the transfer of passengers is avoided. They are boats of various dimensions, with a deck provided with several railway tracks, upon which sections of a train are run from the wharf. This spe cies of railway ferriage is used in situations where the building of a bridge is inexpedient for economic or strategic reasons, or to provide facilities for transportation pending the construction of large bridges. These railway ferries may be divided into two classes: (1) Those in which the ferry-boat is steered by its rudder, and is therefore free to follow any course; and (2) Those in which the course of the ferry-boat is directed to the desired point by means of a chain or wire rope. In those of the first class either the rails are placed upon the steam ferry-boat itself or the cars are run upon tracks on special flat-boats, which the ferry-boat conveys to their destination. In those of the second class, where the course of the boat is fixed, the boat traverses the stream with the aid of two chains laid on the bed of the stream and anchored at the ends. Chain-wheels on the boat driven by a steam engine pick lip the chains, and in this manner the boat propels itself to its destination. The chains are paid out as the boat progresses, and sink again to the bed of the stream.