As the Ourcq Canal had no regular period of closing, it was necessary to construct the bridge without hinderance to navigation. The idea of altering the canal's course could not be thought of, for the proximity of the fortifications and of the bridge over the military road was opposed to it. Moreover, the canal administration insisted upon a free width of 26 feet, which is that of the sluices of the St. Denis Canal, and which would have led to the projection of a revolving bridge of 28 feet actual opening in order to permit of building foundations with caissons in such a way as to leave a passageway of 26 feet during operations. For these reasons it was decided to construct a metallic bridge that should be lifted by means of counterpoises and balanced after the manner of gasometers. The free width secured to navigation is 28 feet. The bridge is usually kept raised to a height of 16 feet above the level of the water in order to allow boats to pass (Fig. 2). In this position it is balanced by four counterpoises suspended from the extremities of chains that pass over pulleys. These counterpoises are of cast iron, and weigh, altogether, 44,000 pounds--the weight of the bridge to be balanced, say 11,000 pounds per counterpoise. Moreover, each of the four chains is prolonged beneath the corresponding counterpoise by a chain of the same weight, called a compensating chain. The pulleys, B and C, that support the suspension chains have projections in their channels which engage with the links and thus prevent the chains from slipping. They are mounted at the extremity of four latticed girders that likewise carry girder pulleys, D. The pulleys that are situated at the side of the bridge are provided laterally with a conical toothing which gears with a pinion connected with the maneuvering apparatus. The two pinions of the same side of the bridge are keyed to a longitudinal shaft which is set in motion at one point of its length by a system of gearings. The winch upon which is exerted the stress that is to effect the lifting or the descent of the bridge is fixed upon the shaft of the pinion of the said gearing, which is also provided with a flywheel, c. The longitudinal shafts are connected by a transverse one. e, which renders the two motions interdependent. This transverse shaft is provided with collars, against which bear stiff rods that give it the aspect of an elongated spindle, and that permit it to resist twisting stresses. The windlasses that lift the bridge are actuated by manual power. Two men (or even one) suffice to do the maneuvering.