256 DREDGING Drebbel left two treatises, which appeared first in Dutch (Leyden, 1608); afterward in Latin, under the title Tractatus duo : De Na- tura Elementorum ; De Quinta Essentia (Ham- burg, 1621) ; and in French (Paris, 1673). 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, sweep- ing forward the materials that obstructed them. This is the principle of flashing or flushing applied to sewers, and is the most effi- cient mode of dredging. In some of the har- bors in England scouring basins have been constructed especially for this purpose, as at Ramsgate and Dover. To loosen the sedi- ment, the Dutch long since contrived a float- ing 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 dredg- FlG. 1. ing machines. The dredging machine now generally used for deepening channels and har- bors 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 ma- chinery for moving it are placed. The dia- gram, 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.
- 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, al- though sometimes by steam power, is used for DREDGING (DEEP-SEA) dredging for oysters in their natural beds. A smaller kind, usually called tongs, is used to gather oysters from artificial beds. It is con- structed by placing two rakes, having iron FIG. 2. 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. DREDGING, Deep-Sea, an operation much re- sorted- to by modern naturalists to investigate the bottom of the sea and its inhabitants. It has added vastly to our knowledge of the ani- mal kingdom in general, of its distribution in depth, and of the important part it plays in the formation of the superficial layers of the earth. The first naturalist who appears to have made use of the dredge was Otto Frederik Mtiller, a Danish zoologist of the last century, who ob- tained by its means a large portion of the speci- mens described by him and figured so beauti- fully by his brother in the Zoologia Danica. The description of his sacculus reticularis, as he calls it, and of the alternate joys and disap- pointments of the dredge, is very amusing. A small figure of the instrument is given among the allegorical ornaments of the title page, and has been reproduced in Thomson's "Depths of the Sea." That same dredge was afterward purchased by Tilesius, the naturalist who ac- companied Krusenstern in his journey round the world, and after his return it was deposited in the academy of St. Petersburg. It consisted of a square frame, each side being in the shape of a scraper. A bag of netting was attached to it, and four iron arms were hinged at the corners and met in a ring to which the rope was attached. Little was heard of dredging after Mailer's time until about 1838, when Dr. Robert Ball of Dublin introduced the modern dredge, commonly known as Ball's dredge. This is oblong, with only two scrapers and two arms. For dredging from a rowboat 18 in. by 6 is a convenient size. The scrapers may be
