Traversing Bridges.—Movable bridges, sometimes called telescope bridges, capa ble of being rolled horizontally backward, or in an oblique direction, are occasion ally employed. The bridge across the Arun, near Arundel, on the South Coast railway, is 144 feet long. It is traversed on wheels, and acts as a sliding canti lever, the overhanging portion resting on the opposite abutment when in place.
Lift Bridges.—A notable example is the one erected over the Surrey canal, which is lifted by the four corners; another over the Royal canal, Dublin.
Pontoon Bridges.—Bridges of boats are made of boats laid over with planks, fastened across the stream by means of anchors or stakes. The bridge at Rouen is 300 yards long, paved with stone for the passage of carriages and horses. The so-called flying bridge is rather a ferry than a bridge of boats.
The longest floating bridge in the world, probably, is the pontoon bridge across the river Hooghly, at Calcutta, ends. They are each of the great length of 160 feet, made of such considerable length in order to obviate pitching motion in rough weather. The roadway plat form is of 3-inch planks of teakwooel from Burma, forming a roadway 48 feet wide, with a footpath at each side 7 feet designed and constructed by Sir Bradford Leslie. The bridge is 1,530 feet long be tween the abutments, and is carried on 14 pairs of pontoons, which are held in position by means of chain cables, 13/4 inches thick, and anchors weighing three tons each, laid on the up stream and down stream sides, 900 feet asunder. By wide. An opening 200 feet wide, for the passage of ships, is made by removing, when occasion requires, four of the pon toons with their superstructure, and sheering them clear of the opening. The floating bridge is connected with the shore at each end by adjusting ways hinged to the shore.
their great length, the cables afford the necessary spring to allow for the or dinary rise and fall of the river, the stress on each cable varying from 5 tons to 25 tons, according to the stage of the weather and of the tide, the maximum velocity of which is 6 miles an hour. The pontoons are rectangular iron boxes, hav ing rounded bilges and wedge-shaped Military Bridges are temporary con structions to facilitate the passage of rivers by troops, to restore a broken arch, or cross a chasm of no very great width. Those over a river are either floating or fixed. The former are made of pontoons, boats, casks, rafts of timber, or anything that will give sufficient buoyancy; and the latter of piles, trestles, or other timber work. Spars, ropes, and planks are used in a variety of ways for spanning narrow chasms. The pontoon bridge is the only one which is carried with an army. Heavy guns are better warped across on specially constructed rafts. A flying bridge is a boat or raft anchored by a long cable up stream, and carried across by the action of the current acting obliquely against its side, which should be kept at about an angle of 55° with the current.
Of the rock formations called natural bridges, the most remarkable is the countries of imperfect civilization even yet they are few.
Following is a list of the notable bridges of the world: Pons Sublicus, across the Tiber at Rome, defended 507 B. C., by Horatius
Codes.
Cxsar's bridge across the Rhine, a wooden trestle-work, built 55 B. C.
Trajan's magnificent bridge over the Danube, 4,770 feet in length, built A. D. 115.
London bridge. One existed at the end of the 10th century; one built of wood, natural bridge over Cedar Creek, in Vir ginia, 125 miles W. of Richmond. The mass of siliceous limestone through which the little river passes is presumably all that remains of a once extensive stratum. The cavern or arch is 200 feet high and 60 feet wide. The solid rock walls are nearly perpendicular, and the crown of the arch is 40 feet thick.
History of Bridges.—Bridges seem to have existed in China from a period of considerable antiquity. The word bridge does not occur in the Authorized Version of the Bible. Temporary bridges, for military purposes, were con structed before permanent structures for the convenience of the inhabitants were erected. The former were often of boats. Thus, Cyrus constructed such bridges about 536 B. C., Darius Hystaspes about 490, and Xerxes about 480 B. C. Bridges of stone or brick seem to have been first used by the Romans; there were none erected in Greece till after the Roman conquest. The first Roman bridge is said to have been one spanning the Tiber, between the Janiculum and the Aventine mountain, built by or under Ancus Martius. Now they are universal ix properly civilized countries, though in 1014; a stone bridge, by Peter of Cole church, begun 1176, was finished 1209. The new London bridge is constructed of granite, from the designs of L. Rennie; it was commenced in 1824 and completed in about seven years, at a cost of $7,290,000.
The bridge at Burton, over the Trent, was formerly the longest bridge in Eng land, being 1,545 feet. It is now partly removed. Built in the 12th century.
The Bridge of the Holy Trinity, at Florence, Italy, was built in 1569. It is 322 feet long, constructed of white marble, and stands unrivaled as a work of art.
The Rialto, at Venice, is said to have been built from the designs of Michael Angelo. It is a single marble arch, feet long, and was completed in 1591.
The Bridge of Sighs, at Venice, over which condemned prisoners were trans ported from the Hall of Judgment to the place of execution, was built in 1589.
Brooklyn Bridge was commenced, under the direction of John A. Roebling, in 1870, and completed in about 13 years.
Coalbrookdale bridge, England, was the first cast-iron bridge. It was built over the Severn in 1779.
High bridge, New York, by which the Croton aqueduct is carried across Har lem river, is of granite throughout.
Victoria bridge, which spans the St. Lawrence at Montreal, Canada. It is tubular and 9,194 feet, or nearly 2 miles, long. The massive tube through which the railway track is laid is 22 feet high and 16 feet wide. It was formally opened in 1860.
as war memorials. A notable one is the State Soldiers' and Sailors' memorial bridge at Harrisburg, Pa., built at a cost of over $2,000,000, and completed in 1919.
Besides the bridges here enumerated, there are many other notable specimens of bridge architecture in this country.