MOVABLE BRIDGES.
by reason of the insufficient height of a bridge structure thrown across a river or other stream, navig-ation is obstructed, means must be employed whereby the bridge may be opened to permit the passage of vessels. This is accomplished through various mechanical arti fices by whose aid either the entire bridge (where its length is small) or a section of it is lifted by chains (lift bridge), or slid back (rolling bridge), or made to revolve upon a vertical axis (pivot bridge), so that the length of the movable section is placed parallel with the channel of the stream.
Lift the lift bridges a section of the bridge floor is raised to a vertical position by the aid of some simple mechanical device, such as a windlass secured to some stationary part of the structure and connected with the movable section by a rope or chain. A counter-weight is fre quently used for this purpose, with suitable provision for counteracting the difference in leverage during operation (pi. 49, fig. 2). This form of mov able bridge is adapted only for small structures, such as those spanning creeks, canals, or ferries.
the rolling bridges the moving section is mounted upon cylindrical rollers, by which the friction of the parts is reduced to a m in' inn.
Pivot the pivot bridges, which are by far the most numer ous of this class, the centre bearing is formed of a series of short conical rollers arranged radially in a circle in such a manner that the vertices of the cones, when extended, will intersect at the turning-point or pivot of the draw-span. The draw is turned usually by means of a toothed wheel which is geared into a rack on the centre ring, and which is operated by a capstan worked either by hand or, in the case of a structure of great size, by a steam-engine. The movement of the draw-span is controlled by some form of locking device which prevents it from passing beyond a pre determined limit.
Arthur Kill recently-constructed drawbridge span ning the Arthur Kill and connecting the States of New York and New Jer sey is worthy of notice, as it is at present the largest drawbridge in exist ence. Figure r (fi/. 49) shows this structure with the draw open. The
total length of the bridge proper, exclusive of approaches, is Soo feet. It comprises two shore spans covered by fixed trusses, and two draw spans closed by the great drawbridge, which has a total length of 5oo feet. On each side of its central pier it affords, when open, a clear waterway of 2oS feet. It requires about two minutes to open or to close it. This structure was completed in August, MS.
Pontoon or pontoon bridges, consisting of a number of floating and anchored vessels (boats or pontoons), designed to serve as the support for a roadway across a stream, are of great antiquity. The army of Darius crossed the Bosphorus (493 B. C.) On a bridge of this kind on its way to invade Greece, and his successor, Xerxes, crossed the Helles pont (48o B. c.) in the same manner. Pontoon bridges were also used by the Romans in their campaigns. In modern times they .have acquired importance in connection with military operations, and the armies of vari ous nations have a trained organization (bridge-equipment corps) to facili tate the passage of rivers. For this purpose the corps is provided with various forms of portable floating pontoons and appurtenances, by whose aid a stream of considerable width may be bridged in a few hours with a temporary structure sufficiently strong to bear any weight which would be likely to be brought upon it in service. A number of bridges of this type are in use at the present time for ordinary road traffic, and recently they have even been made to serve for railway traffic, as in the case of the rail way pontoon bridge over the Rhine at Maxau, built in 1865 (pi. 39, fig. 14), the roadway of which is supported upon thirty-four pontoons. Includ ing approaches, it is 1190.3 feet long-, and is traversed with light locomo tives. To permit of the passage of vessels plying the river, certain sec tions are detachable, so that they may be swung open and closed, as may be required.