Lift′ing-bridge.
The lifting-bridge is the simplest of all drawbridges, being merely a platform hinged to the bank, curb, or scarp at one end, the other being lifted by chains or cords worked by windlass or weights. It is used in crossing the canals in Holland, where the roadway is nearly on a level with the water. It is also used in fortifications. The drawbridges (a) of Brussels were balanced by weights attached to chains passing over standards that stood immediately over the walls of the canal, and were braced and stayed by timbers and iron ties. The weight of the draws was borne by struts beneath, which were footed in set-offs in the faces of the masonry, and rested at their upper ends against jogs beneath the sill-timbers of the bridge. Perronet, the great French engineer, 1708 – 94, was the chief engineer of the Ponts et Chaussecs from 1747, and did more for the science of bridge-building than any of his contemporaries. He contrived a drawbridge (b) which had a short middle draw to allow the masts of vessels to pass when the headway beneath the bridge was sufficient for the hulls of the vessels. This middle section is hinged to one portion of the bridge and lifted by a chain and winch. |
| Lifting-bridges. |
