DREDGE (ante). Naturalists use an instrument constructed on the general plan of an oyster-dredge for obtaining specimens of animals living at the bottom of the sea, to determine their structure and geographical distribution. In working, the dredge is slipped gently over the side of the boat, either from the bow or the stern. When it reaches the bottom and begins to scrape, an experienced hand upon the rope can usually detect by the tremor of the line when the dredge is passing over an irregular surface. The boat should move not more than a mile in an hour. The dredge may remain down from 15 to 20 minutes, within which time, in favorable circumstances, it may be fairly filled. It comes up variously freighted, according to the locality, and the contents are examined. The scientific value of dredging depends mainly upon two things : the care with which objects procured are preserved and labeled for future identification, and the accuracy with which all the circumstances of the dredging—the position, the depth, the nature of the ground, the date, temperature, etc.—are recorded.
Until the middle of the 18th c. the little that was known of the inhabitants of the sea beyond low-water mark seems to have been gathered almost entirely from objects thrown on the beach after storms, and from the chance captures of fishermen. The dredge was used to aid natural history, first by Otto Frederick Muller, in the researches which furnished material for his and History of the rarer and less knou.tr, Animals of Denmark and Norway, 1779. Thenceforward much advance was made in knowledge of deep-sea life, mainly by the efforts of the British association; but the first important undertaking was in the winter of 1872. At that time The Chal lenger, a steam-corvette of 2,306 tons, and 1234 horse-power, was sent out to investi gate the physical and biological conditions of the ocean basins. This vessel was thoroughly equipped, and carried a corps of distinguished scientists. Dredging was done from the main yard-arm. A strong pendant was attached by a hook to the cap of the main-mast, and, by a tackle to the yard-arm, a compound arrangement of 55 to 70 of Hodge's patent accumulators was hung to the pendant, and beneath it a block through which the dredging-rope passed. The donkey-engines for hauling in the dredging and sounding gear were placed at the foot of the main-mast on the port side. They con sisted of a pair of direct-acting horizontal engines, in combination of 18 horse power nominal. Instead of a connecting rod to each, a guide was fixed to the end of the piston-rod, with a brass block working up and down the slot of the guide. The
crank axles ran through the center of the blocks, and the movable block, obtaining a backward and forward motion from the piston-rod, acting on the crank as a connecting rod would do. This style of engine is commonly used for pumping, the pump-rods being attached to the guide on the opposite side from the piston-rod_ At one end of the crank a small-toothed wheel was attached, which drove one thrice the multiple on a horizontal shaft extending nearly across the deck, and about 3 ft. and 6 in. above it. At each end of this shaft a large and small drum were fixed, the larger having three sheaves cast upon it of different sizes; the small being a common barrel only. To these drums the line was led, two or three turns being taken round the drum selected. In hauling in, the dredging-rope was taken to a gin-block secured to a spar on the forecastle, then aft to the drum of the donkey-engine on the port side, then to a leading-block on the port side of the quarter-deck, and across the deck to a leading-block on the starboard side corresponding in diameter with the drum used on the port side, and from this it was finally taken by the hands and coiled. The strain is of course greatest at the yard arm and the first leading-block, and by this arrangement it is gradually diminished as the line passes round the series of blocks and sheaves. A change made latterly in the handing of the dredge had certain advantages. Instead of attaching the weights directly to the dredge-rope, and sending them down with the dredge, a "toggle," a small spindle-shaped piece of hard wood, was attached transversely to a rope at the required distance, 200 to 300 fathoms in advance of the dredge. A " messenger," consisting -of a figure of eight of rope, with two large thimbles in the loops, had one of its thimbles slipped over the chain before the dredge was hung, and the other thimble made fast to a lizard. When the dredge was well down and had taken its direction from the drift of the ship, the weights, usually six 28-lb. deep-sea leads in three canvas covers, were attached to the other thimble of the traveler, which was then cut adrift from the lizard and allowed to spin down the line until it was brought up by the toggle. By this plan the dredge took a somewhat longer time to go down; but after it was adopted not a single case occurred of the fouling of the dredge in the dredge-rope, a misadven ture which had occurred more than once before, and which was attributed to the weights getting ahead of the dredge in going down, and pulling it down upon them entangled in the double part of the line.