DREDGE AND DREDGING. The word "dredge" is used in two senses. (1) From Mid. Eng. _dragie_, through Fr. _dragée_, from Gr. [Greek: tragêmata], sweetmeats, it means a confection of sugar formed with seeds, bits of spice or medicinal agents. The word in this sense is obsolete, but survives in "dredger," a box with a perforated top used for sprinkling such a sugar-mixture, flour or other powdered substance. "Dredge" is also a local term for a mixed crop of oats and barley sown together ("maslin" or "meslin," cf. Fr. _dragée_), and in mining is applied to ore of a mixed value. (2) Connected with "drag," or at least derived from the same root, dredge or dredger is a mechanical appliance for collecting together and drawing to the surface ("dredging") objects and material from the beds of rivers or the bottom of the sea. In the following account the operations of dredging in this sense are discussed (1) as involved in hydraulic engineering, (2) in connexion with the work of the naturalist in marine biology. 1. HYDRAULIC ENGINEERING Dredging is the name given by engineers to the process of excavating materials under water, raising them to the surface and depositing them in barges, or delivering them through a shoot, a longitudinal conveyor, or pipes, to the place where it is desired to deposit them. It has long been useful in works of marine and hydraulic engineering, and has been brought in modern times to a state of high perfection. The employment of dredging plant and the selection of special appliances to be used in different localities and in varying circumstances require the exercise of sound judgment on the part of the engineer. In rivers and estuaries where the bottom is composed of light soils, and where the scour of the tide can be governed by training walls and other works constructed at reasonable expense, so as to keep the channel clear without dredging, it is manifest that dredging machinery with its large cost for working expenses and for annual upkeep should be as far as possible avoided. On the other hand, where the bottom consists of clay, rock or other hard substances, dredging must, in the first instance at any rate, be employed to deepen and widen the channel which it is sought to improve. In some instances, such as the river Mississippi, a deep channel has for many years been maintained by jetties, with occasional resort to dredging to preserve the required channel section and to hasten its enlargement. The bar of the river Mersey is 11 m. from land, and the cost of training works would be so great as to forbid their construction; but, by a capital expenditure of £120,000 and an annual expense of £20,000 for three years, the depth of water over the bar at low tide has been increased by dredging from 11 ft. to 27 ft., the channel being 1500 ft. wide.