Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/483

Rh it a block through which the dredge-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 consisted of a pair of direct-acting, high- pressure, 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 centre of the blocks, and the movable block, obtaining a backward and forward motion from the piston-rod, acted 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 feet 6 inches above it. At each end of this shaft a large and a small drum were fixed, the larger having three sheaves cast upon it of different sizes, the lesser being a common barrel only. To these diums the line was led, two or three turns being taken round the drum selected. In hauling in, the dredge-rope was taken to a gin-block secured to a spar on the forecastle, then aft to the drum of the donkey-engines 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 handling of the dredge had certain advantages. Instead of attaching the weights directly to the dredge-rope, and sending them down with the dredge, a &quot; toggle,&quot; a small spindle-shaped piece of hard wood, was attached transversely to the rope at the required distance, 200 to 300 fathoms in advance of the dredge. A &quot; messenger,&quot; consisting of a figure of eight of rope, with two large thimbles in the loops, had one of the 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-K&amp;gt; deep- sea leads in three canvas covers, were attached to the other thimble of the traveller, 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 some what 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 misadventure 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. The great risk in dredging in very deep water is that of the dredge running down nearly vertically and sinking at once into the soft mud, and remaining imbedded until hauling in commences. Duringthe earlier part of the voyage of the &quot; Challenger &quot; this accident seemed too often to defeat, at all events partially, the object of the operation and, after various suggestions for modifying the dredge, it was proposed to try some form of the trawl in order to insure, so far as possible, the capture of any of the larger marine animals which might be present, and thus to gain a better general idea of the nature of the fauna. A 15-feet beam-trawl was sent down off Cape St Vincent to a depth of 600 fathoms ; the experiment looked hazardous, but the trawl came up in due time all right, and contained, along with many of the larger Invertebrata, several fishes, The trawl seemed to answer so well that it was tried again a little farther south in 1090 fathoms, and again it was per fectly successful, and during the remainder of the voyage it was employed almost as frequently, and in nearly as deep water, as Ball s dredge. The deepest successful haul of the trawl was in the Pacific in 3125 fathoms, and the deepest haul of Ball s dredge was in the Atlantic at 3150 fathoms. During the voyage of the &quot; Challenger &quot; a course of about 70,000 nautical miles was traversed in three years and a half, and 362 observing stations were established at intervals as nearly uniform as circumstances would permit ; and at the greater number of these dredging or some modifi cation of the process was successfully performed 52 times at depths greater than 2000 fathoms, and thrice at depths beyond 3000 fathoms. So fully convinced were the &quot; Challenger &quot; officers that they could dredge at any depths, that it was only want of time and daylight which pre vented their doing so at their deepest sounding, 4575 fathoms. The Atlantic was crossed five times, and an erratic route through the Pacific gave a good idea of the condi tions of the abysses of that ocean, while in the South Indian Ocean dredging and trawling were carried down close to the Antarctic ice-barrier. The expedition was successful, and the results were of the most interesting nature. Animal life was found to exist at all depths, although probably in diminishing abundance as the depth becomes extreme ; and in all parts of the world at depths beyond 400 or 500 fathoms the fauna had much the same general character. The species usually differed in widely separated areas, but the great majority of forms, if not identical, were so nearly allied that they might be regarded as representative and genetically related. Although all marina invertebrate classes were represented, echinoderms in their different orders, sponges, and Crustacea preponderated, while corals and Mollusca were comparatively scarce. In the first two groups named many forms occurred allied to families which had been pre viously regarded as extinct or nearly so : thus among the echinodenns, stalked crinoids were by no means rare, and many species of regular Echinidea related to the Chalk genus Echinothuria, and many irregular species allied to Ananchytes and Dysaster occurred. The sponges were mainly represented by the Hexactinellid, the beautiful order to which the glass-rope sponge of Japan and the marvellous &quot; Venus s Flower Basket&quot; of the Philippines belong, the order to which the Ventricidites of the Chalk must also be referred. Dredging at these great depths is a difficult and critical operation, and, although by its means some idea of the nature and distribution of the abyssal fauna of the oce^n has already been attained, it will be long before the blanks are filled up; for of the area of 140,000,000 square miles forming the &quot; abyssal province &quot; the actual amount hitherto traversed by the naturalist s dredge may still be readily reckoned by the square yard.  DREDGING. 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. It is a process which has been useful from very early times in works of marine and hydraulic engineering, and it has of late years, by improved appliances, been brought to high perfection.

Bag and Spoon Dredge.—The first employment of machinery to effect this object is, like the discovery of the canal lock, claimed alike for Holland and Italy, in both of which countries dredging is believed to have been practised before it was introduced into Britain. The Dutch at a very early period used what is termed the &quot; bag and spoon &quot; dredge for cleaning their canals. It was 