Dredging
2 of the 7 encyclopedias on this shelf carry an entry for Dredging. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the operation of removing mud, silt and other deposits from the bottom of harbors, canals, rivers, docks, etc., by mechanical means. The steam-dredging machine now in common use is said to have been first applied by Boulton 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. The excavating buckets are sometimes placed on the perimeter of a large wheel instead of on an endless chain. The so-called clamshell bucket consists of two parts hinged together which descend through the water open and return closed with its clutch of material amounting sometimes to five cubic yards. In order to work in clay or hard sand it is provided with teeth. 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. Great improvements aprovements have been made in hydraulic 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 ing the dragging 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 suspending 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 somewhat similar to the oyster dredge. LADDER BUCKET BUCKET AND SUCTION DREDGE
Aiton's Encyclopedia (1910)
the deepening of channels and harbors by the removal of loose earth. There are two typical kinds of dredging machines. The first consists essentially of a series of buckets, running on an endless chain or belt, such as may be seen in a grain elevator, only on a larger scale. As the buckets pass around the lower pulley, they scoop up earth; as they run over an elevated pulley at the other end of the circuit, they discharge the earth into a scow provided for the purpose of receiving it. Huge dredges of this sort are in use in digging the Panama Canal . A second sort of dredge consists of a steam shovel, such as is used for loading earth on flat cars. The shovel, which is really a huge, metal scoop, has the size of a barrel. It is swung on a derrick and is worked by steam. It drops to the bottom and scrapes up a load of earth. It is then elevated by means of a chain and ratchet work, and swung around by means of the derrick until in position to discharge its contents through a hinged bottom into a scow as before. Both kinds of dredge are mounted on a flat boat constructed for the purpose. It carries an engine and a supply of fuel. It advances as the work progresses and may be taken from place to place as its services are needed. Still another kind of dredge may be mentioned. A colossal contrivance, used in deepening the channel of the Willamette and Columbia rivers, churns up the soil to be removed and pumps it up like a mosquito, through a forty-inch tube. Mammoth dredges are kept at work constantly in the Suez and other canals.