The writer is indebted to the Proceedings of the Institution of Civil Engineers, and especially to the paper of Mr J. J. Webster (Proc. Inst. C.E. vol. 89), for much valuable information upon the subject treated. He is also indebted to many manufacturers who have furnished him with particulars and photographs of dredging plant.
(W. H.*)
Plate I.
| Fig. 3.—Barge-loading dredger, “St Austell,” constructed for the British Government by Wm. Simons & Co. | Fig. 4.—Stern-well hopper-dredger “La Puissante,” by Wm. Simons & Co. Length 275 ft., breadth 47 ft., depth 19 ft. |
| Fig. 5.—Dredger constructed for the Lake Copais Co. by Hunter & English. | Fig. 6.—Light-draught dredger, with delivery apparatus working round an arc of 210°, by Hunter & English. |
| Fig. 7.—Twin-screw sand-pump dredger, “Kate,” built for the East London Harbour Board by Wm. Simons & Co. | Fig. 8.—Twin-screw hopper-dredger, “Percy Sanderson,” built for the European Danube Commission by Wm. Simons & Co. |
| Fig. 9.—Twin-screw grab-dredger, “Miles K. Burton,” built for the Mersey Docks and Harbour Board by Wm. Simons & Co. | Fig. 10.—Hopper-dredger, “David Dale,” with buckets of 54 cub. ft. capacity (see fig. 11) built for the North Eastern Railway Company by Lobnitz & Co. |
Plate II.
| Fig. 11.—BUCKETS OF 5 AND 54 CUBIC FEET CAPACITY COMPARED. The latter, the largest ever made, were for the hopper-dredger “David Dale” (Plate I. fig. 10), built by Lobnitz & Co. |
| Fig. 12.—MODEL OF ROCK-CUTTING DREDGER, “DEROCHEUSE.” Built for special work on the Suez Canal by Lobnitz & Co. Length 180 ft., breadth 40 ft., depth 12 ft. |
2. Marine Biology
The naturalist’s dredge is an instrument consisting essentially of a net or bag attached to a framework of iron which forms the mouth of the net. When in use as the apparatus is drawn over the sea-bottom mouth forwards, some part of the framework passes beneath objects which it meets and so causes them to enter the net. It is intended for the collection of animals and plants living on or near the sea-bottom, or sometimes of specimens of the sea-bottom itself, for scientific purposes.
Until the middle of the 18th century, naturalists who studied the marine fauna and flora relied for their materials on shore collection and the examination of the catches of fishing boats. Their knowledge of creatures living below the level of low spring tides was thus gained only from specimens cast up in storms, or caught by fishing gear designed for the capture of certain edible species only. The first effort made to free marine biology from these limitations was the use of the dredge, which was built much on the plan of the oyster dredge.
| Fig. 13.—Otho Frederick Müller’s Dredge (1770). |