bridge are so adjusted that they are at rest when opened at an inclination of about 40°, instead of in the horizontal position which they occupy when closed. Thus, as soon as the locks are withdrawn, the leaves will, without the application of any power whatever, roll back and upward and open a channel of sufficient width for the passage of vessels. An interesting record is that of the Rush Street Bridge, at Chicago, said to be the most active movable bridge in the world. During an average season of lake navigation, comprising a little over eight months, this bridge is opened between 10,000 and 11,000 times, or fully 40 times every 24 hours. Yet the power expense for the operation of this bridge by electricity does not exceed 67 cents a day.

Another form of movable bridge is that known as a swing bridge. It consists of a bridge balanced on a pivot or on a circle of rollers situated on a pier in the centre of the river. When open the bridge is swung around until it lies within the central pier and, with it, points up and down instead of athwart stream. This gives a passage on each side of the pier. The swing bridge over the Raritan River, New Jersey, gives two passages, each 216 feet wide. A similar bridge in Kansas City crosses two passages each 160 feet wide. The total moving weight is 303 tons. The bridge is opened by steam power in about one and a half minutes, or by manual power in two minutes. From two thirds to three fourths of the moving weight rests on the central pivot. Another form of swing bridge is made in two leaves which swing in to either bank and meet in the middle.

Traversing or telescope bridges are occasionally employed. They are so constructed as to be capable of being rolled horizontally backward or in an oblique direction. The bridge across the Arun, near Arundel, England, on the South Coast Railway, is 144 feet long. It is traversed on wheels, and acts as a sliding cantilever, the overhanging portion resting on the opposite abutment when in place.

Bascule or draw bridges are raised on horizontal hinges, and are made in one leaf, or in two leaves which meet in the middle. The most ancient form of the bascule was that of a flap of framed timber used to cross the moat of a castle, and capable of being drawn up by means of chains from the inside.

Lift bridges are contrived so as to rise perpendicularly without changing their horizontal position. This motion may be imparted by hydraulic power, by counterbalancing weights, or by means of a winch operated by hand, steam, or electrical power. Bridges of this form are used largely on the Erie Canal. One in Chicago has a clear lift of 155 feet above the water. 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 an angle of about 55° with the current.

Pontoon Bridges.-These are a development from the ancient bridge of boats, which consisted of boats moored so as to form a continuous line, across which a roadway of planks was laid. The longest floating bridge in the world, probably, is the pontoon bridge across the Hooghly, at Calcutta, designed and constructed by Sir Bradford Leslie. The bridge is 1,530 feet long between the abutments, and is carried on 14 pairs of pontoons, which are held in position

by means of chain cables one and three quarters inches thick, and anchors weighing three tons each, laid on the up stream and down stream sides, 900 feet asunder. By their great length the cables afford the necessary spring to allow for the ordinary rise and fall of the river, the stress on each cable varying from 5 to 25 tons, according to the state of the weather and the stage of the tide, the maximum velocity of which is six miles an hour. The pontoons are rectangular iron boxes having rounded bilges and wedge-. shaped ends. They are each 160 feet long, made of such considerable length in order to obviate pitching motion in rough weather, with a beam of 10 feet, and depth of from 8 to 11 feet, presenting a side of from 32 to 4 feet above the water, according to the state of the traffic. For perfect safety each pontoon is divided by bulkheads into II compartments. They are made of iron plates one fourth of an inch and five sixteenths of an inch in thickness, riveted together. The platform of the bridge is supported by trestle-work on the pontoons at a clear height of 27 feet above the water, a convenient height for boat navigation. The roadway platform is of 3-inch teak planks, and is 48 feet wide, with a footpath at each side 7 feet wide. An opening 200 feet wide, for the passage of ships, is made by removing, when occasion requires, four of the pontoons with their superstructure, and sheering them clear of the opening. The portion so removed is in two divisions, which are separately secured, right and left, and, when in place, are connected by drawbridges with the fixed portions of the bridge. Before launching, the pontoons were ballasted sufficiently to make them float upright; and were afterward coupled in pairs by the sills of the main trusses, when the ballast was removed. The floating bridge is connected with the shore at each end by adjusting ways hinged to the shore. The ordinary time taken to open the bridge is 15 minutes, and to close it, 20 minutes. It is opened only twice a week. An excellent instance of pontoon structure, though not a bridge, is the Great Landing Stage at Liverpool, England.

Military Bridges are temporary constructions 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, cotton-bales, 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. Enough material for 100 yards of length accompanies each army corps. All military bridges have their roadway formed in the following manner: Five to nine roadbearers of stout timber support chesses or flat planks 10 feet long, held in position, so as to form a level surface, by two ribands placed above them and over the outer road-bearers, to which they are fastened by rack lashings. The road-bearers are supported by the pontoons, casks, boats, trestles, or piles, which form the piers, usually 10 to 15 feet apart, or by transoms on the ropes in the case of suspension bridges. To prevent injury to the boats, balks of timber are built up along the keel of each for the road-bearers to rest upon. A saddle on pontoons and gunnels on casks answer the same pur