Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/482

462 462 D K E D G E perience to maintain, with Sir James Clark Ross, that &quot; from however great a depth we may be enabled to bring up mud and stones of the bed of the ocean we shall find them teeming with animal life.&quot; Samples of the sea- bottom, procured with great difficulty and in small quantity from the first deep soundings in the Atlantic, chiefly by the use of Brooke s sounding machine, an instru ment which by a neat contrivance disengaged its weights when it reached the bottom, and thus allowed a tube, so arranged as to get filled with a sample of the bottom, to be recovered by the sounding line, were eagerly examined by miscroscopists ; and the singular fact was established that these samples consisted over a large part of the bed of the Atlantic of the entire or broken shells of certain Foraminifera. Dr Wallich, the naturalist to the &quot; Bull dog &quot; sounding expedition under Sir Leopold M Clintock, reported that star-fishes, with their stomachs full of the deep-sea Foraminifera, had come up from a depth of 1200 fathoms on a sounding line ; and doubts began to be enter tained whether the bottom of the sea was in truth a desert, or whether it might not present a new zoological region open to investigation and discovery, and peopled by a peculiar fauna suited to its special conditions. In the year 1868, while the question was still undecided, two testing investigations were undertaken independently. In America Count L. F. Pourtales, one of the officers employed in the United States Coast Survey under Pro fessor Pierce, commenced a series of deep dredgings across the Gulf Stream off the coast of Florida, which were con tinued in the following year, and were productive of most valuable results ; and in Great Britain the Admiralty, on the representation of the Eoyal Society, placed the &quot;Lightning,&quot; a small gun vessel, at the disposal of a small committee to sound and dredge in the North Atlantic between Shetland and the Faroe Islands. In the &quot; Lightning,&quot; with the help of a &quot; donkey- engine &quot; for winding in, dredging was carried on with comparative ease at a depth of 600 fathoms, and at that depth animal life was found to be still abundant. The results of the &quot; Lightning s &quot; dredgings were regarded of so great importance to science that the Eoyal Society pressed upon, the Admiralty the advantage of continuing the researches, and accordingly, during the years 1869 and 1870, the gun-boat &quot; Porcupine&quot; was put under the orders of a committee consisting of Dr Carpenter, F.B.S., Dr Gwyn Jeffreys, F.li.S., and Professor Wyville Thomson, F.E.S., one or other of whom superintended the scientific work of a series of dredging trips in the North Atlantic to the north and west of the British Islands, which occupied two summers. In the &quot; Porcupine,&quot; in the summer of 1869, dredging was carried down successfully to a depth of 2435 fathoms, upwards of two miles and a half, in the Bay of Biscay, and the dredge brought up well-developed representatives of all the classes of marine invertebrates. During the cruises of the &quot; Porcupine &quot; the fauna of the deep water off the western coasts of Great Britain and of Spain and Portugal was tolerably well ascertained, and it was found to differ greatly from the fauna of shallow water in the same region, to possess very special characters, and to show a very marked relation to the faunce of the earlier Tertiary and the later Cretaceous periods. In the winter of 1872, as a sequel to the preliminary cruises of the &quot; Lightning &quot; and &quot; Porcupine,&quot; by far the most considerable expedition in which systematic dredging had ever been made a special object left Great Britain. H.M.S. &quot;Challenger,&quot; a corvette of 2306 tons, with auxiliary steam working to 1234 horse-power, was despatched to in vestigate the physical and biological conditions of the great ocean basins. The &quot; Challenger &quot; was provided with a most complete and liberal organization for the purpose ; she had powerful deck engines for hauling in the dredge, workrooms, laboratories, and libraries for investigating the results on the spot, and a staff of competent naturalists to undertake such investigations and to superintend the packing and preservation of the specimens reserved for future study. In these deep-sea dredgings it was frequently found that, while few objects of interest were brought up within the dredge, many echinoderms, corals, and sponges came to the surface sticking to the outside of the dredge, and even to the first few fathoms of the dredge-line. This suggested many expedients, and finally a long transverse iron bar was attached to the bottom of the dredge-bag, and large bunches of teazed-out hemp were fastened to the free ends of the bar (fig. 4). The &quot;hempen-tangles&quot; are now regarded as an essential part of the dredge, nearly as important as the dredge- bag, and often much more con spicuous in its results. This addi tion to Ball s dredge is not, how ever, generally available in dredg ing from a boat or in shallow water; the tangles are apt to catch on rocks or coral, and a turn of the drum of the donkey-engine is re quired to free them. Ball s dredge was still employed, with some slight modifications, the result of further experience. Fig. 4 represents the form of dredge which was found most suit able for great depths. The dredge- frame of hammered iron is 4 feet 6 inches long and 1 foot 3 inches broad ; the scrapers are 3 inches wide, and are connected at the ends by bars of 1 inches round iron. The arms are of inch round iron, and slightly curved ; they are bolted together to a stout iron bar which ends above in a swivel and ring. Two bars of square iron of some strength are attached lay eyes to the round cross-bars at the ends of the dredge -frame, and have the other ends lashed to the iron bar which bears the tangles. These rods keep the dredge-bag at its full length, and prevent it or the tangles from folding over the mouth of the dredge. The dredge-bag is 4 feet 6 inches in length ; the lower half is of twine netting so close as to retain everything except the finest mud, which indeed only partially washes through, and the upper half is of twine netting with the meshes an inch to the side. The bag is guarded by three loops of bolt-rope attached to the frame of the dredge, to the bottom of the bag, and finally to the tangle-bar. The canvas pads represented in the figure on the dredge-frame are only to protect the seizings of the loops. The dredge is suspended by an iron chain, which forms the first few fathoms of the dredge-line. The chain is not, however, directly fastened to the ring at the end of the arms, but is made fast to one of the end bars of the dredge-frame, arid it is stopped to the ring by a single strand of bolt-rope. If the dredge get caught the stop carries away, the direction of the strain on the dredge is altered^ and it probably relieves itself and comes up end upwards. In deep water a 28 Ib deep-sea lead is usually hung from the centre of the tangle-bar with four tangles on each side. Dredging was carried on in the &quot; Challenger &quot; 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 a patent accumulators was hung to the pendant, and beneath 4. Deep-sea Dredge, with Tangle-bar.