DREDGING

the excavation of materjal under water by mechanical devices. The operation dislodges and removes mud, silt, rock and other deposits from the beds of harbors, canals, rivers and docks. Out at sea the same proce is utilized to scour the bottom and bring up oysters, shells, plants and other marine growths for scientific investigation. Modern engineering has developed very effective methods of dredging, notably in the sea-going dredge. There are a number of appliances used in dredging, from the single bucket or scoop of the dipper or grapple dredges, which alternately dig and lift the load, to the more elaborate types (ladder, hydraulic or suction and scowing dredges) which dig continually, raising the material at the same time. The first named group are among the simpler forms of dredges. They include the ‘bag-and-spoon’ dredge, which has a leather bag attached to a metal hoop, both being fastened to a'long pole, which guides their scraping of the bottom. Another device is the clam-shell dredge, so named because it has hinged iron jaws or scoops resembling a clam shell. This is the grapple type. The scoops spread open by means of chains as they descend, dig into the material at the bottom by the force of their own weight, and are then lifted up by a steam engine, and the contents deposited into a scow or barge alongside. The ladder bucket dredge is formed of a moving iron ladder attached at one end to a high superstructure raised on the deck of the dredging ve el, while the other end is lowered through a well in the ve el’s center by means of a steam windla and chains. Metal buckets are attached to the ladder, operating as an endle chain. As the ladder digs into the bottom the chain of buckets, operating at high speed, scoops up the dislodged material, and on returning loaded they discharge their contents into shoots, which empty it into scows. The hydraulic or suction dredge has a powerful pump operating on a flexible pipe, which is sent to the bottom and through it the material is pumped up and deposited into hopper barges moored alongside. This type of dredge removes much debris that is semi-liquid; hence the receiving hoppers have overflow pipes through which the water pumped up with the material is drawn off. It is serviceable in dredging sand, gravel, alluvial deposits and earth free from large stones, stumps, piling and similar obstructions. The dipper or bucket ledge is an application of the land steam shovel to under water excavation. It is extensively used in the’ United States and is the most familiar of all dredging devices. The bucket is suspended from a long support or arm and lowered to the bottom, where its teeth cuts into the material and, on being raised, deposits its load into a hopper or both the hydraulic principle of suction pump and pipe and the endle chain of buckets used in the ladder type.