DREDGING, the operation of removing mud, silt and other deposits from the bottom of harbors, canals, rivers, docks, etc., by me chanical means. The most simple dredging apparatus is the spoon apparatus, which consists of a strong iron ring or hoop, properly formed for making an impression upon the soft matter at the bottom, so as to scoop it into a large leather bag attached to the ring and perforated with a number of small holes. The means for working it is a long handle, a sus pending rope, and a crane or sweep-pole planted in a boat. This primitive apparatus was formerly used in the canals and ditches of the Netherlands. Much more effective is the steam dredging-machine now in common use. It is said to have been first applied by Boultoti and Watt for use on the weir at Sunderland, England, in 1796. It has a succession of strong iron buckets on an endless chain running on a frame, the lower end of which is vertically adjustable so as to regulate the depth at which it works. The buckets tear up the matter at the bottom, raise it and discharge it into barges or hoppers stationed close to the dredging vessel. The Suez Canal was excavated by means of a ladder dredge with a long chute and supporting girder. The material excavated was carried in boxes on a sort of tramway and tipped out on the bank.
In excavating at Hell Gate and Flood Rock Reefs, at New York, a barge measuring 120 feet in length, 44 in beam, and 15 in depth, used a grapple which weighed 15 tons and lifted boulders of 70 tons. In some cases the
current of river or tides has been utilized in dredging channels. In bottoms of mud or loose sand the steam pump or hydraulic dredger may be used. The watery material is pumped out and deposited on the shore; the water drains away, leaving the sand or mud. By this means more than 3,000 cubic yards a day can be excavated at an extremely low cost.
Great improvements have been made in hy draulic dredges, and some built in recent years for use in the Mississippi River have a capacity of over 1,000 cubic yards per hour.
Dredging is also the operation of dragging the bottom of the sea in order to bring up oysters, or to procure shells, plants and other objects for scientific observation. The oyster dredge is a light iron frame with a scraper like a narrow hoe on one side, and a suspend ing apparatus on the other. To the frame is attached a bag made of some kind of netting to receive the oysters. The dredges used by naturalists are mostly modifications of or some what similar to the oyster dredge. Scientific dredging has of late assumed a high importance as making us acquainted with the life of deep sea areas. See DEEP-SEA LIFE.