Dredging, the process of excavating the sediment that collects in harbors and channels; the term is also applied to the scooping up of oysters, or anything else, from the bottom. The drainage waters and even the ebb tide have sometimes been held back by floodgates, and the waters at last let out have rushed with great violence through the channels, sweeping forward the materials that obstructed them. This is the principle of flashing or flushing applied to sewers, and is the most efficient mode of dredging. In some of the harbors in England scouring basins have been constructed especially for this purpose, as at Ramsgate and Dover. To loosen the sediment, the Dutch long since contrived a floating frame to which bars were attached, that went down to the bottom and stirred up the mud, as the machine moved along with the current. These are perhaps the oldest dredging machines. The dredging machine now generally used for deepening channels and harbors is an endless chain with scoop buckets, placed in a frame which may be raised or lowered through a well in the middle of the scow upon which the apparatus and the machinery for moving it are placed. The diagram, fig. 1, will afford an idea of the general principles of its construction and use.
The mud may be received in a shoot, and conveyed to an adjoining scow, or the frame may be made long enough to reach beyond the side of the vessel, and discharge itself without the intervention of the shoot. Another kind of dredging machine, which is used in soft bottoms, especially where old piles remain that would interfere with the working of the endless chain and scoops, is shown in fig. 2. It is a very efficient form, sinking into the mud and filling itself readily, and. is strong enough to draw old piles from their beds. The same form of dredge on a smaller scale, usually worked by hand by means of a windlass, although sometimes by steam power, is used for dredging for oysters in their natural beds. A smaller kind, usually called tongs, is used to gather oysters from artificial beds. It is constructed by placing two rakes, having iron heads and long wooden handles, with their teeth turned toward each other, and uniting them by a hinge like that in a blacksmith's tongs or a pair of shears.