Page:EB1911 - Volume 08.djvu/596

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571
DREDGE AND DREDGING

up from a depth of 1200 fathoms on a sounding line; and doubts began to be entertained 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 1867, while the question was still undecided, two testing investigations were undertaken independently. In America Count L. F. de Pourtales (1824-1880), an officer employed in the United States Coast Survey under Benjamin Peirce, commenced a series of deep dredgings across the Gulf Stream off the coast of Florida, which were continued in the following year, and were productive of most valuable results; and in Great Britain the Admiralty, on the representation of the Royal Society, placed the “Lightning,” a small gun-vessel, at the disposal of a small committee to sound and dredge in the North Atlantic between Shetland and the Faröe Islands.

In the “Lightning,” with the help of a donkey-engine 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 “Lightning’s” dredgings were regarded of so great importance to science that the Royal Society pressed upon the Admiralty the advantage of continuing the researches, and accordingly, during the years 1869 and 1870, the gun-boat “Porcupine” was put under the orders of a committee consisting of Dr W. B. Carpenter, Dr Gwyn Jeffreys, and Professor (afterwards Sir Charles) Wyville Thomson, 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 “Porcupine,” 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 “Porcupine” 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 faunae of the earlier Tertiary and the later Cretaceous periods.

In the winter of 1872, as a sequel to the preliminary cruises of the “Lightning” and “Porcupine,” by far the most considerable expedition in which systematic dredging had ever been made a special object left Great Britain. H.M.S. “Challenger,” a corvette of 2306 tons, with auxiliary steam working to 1234 h.p., was despatched to investigate the physical and biological conditions of the great ocean basins.

The “Challenger” 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. Since the “Challenger” expedition the use of wire rope has enabled far smaller vessels to undertake deep sea work. The “Challenger,” however, may be said to have established the practicability of dredging at any known depth.

Operating Dredges and Trawls in deep Seas.—Dredging operations from large vessels in deep seas present numerous difficulties. The great weight of the ship makes her motion, whether of progress or rolling, irresistible to the dredge. The latter tends to jump, therefore, which both lowers its efficiency and causes it to exert a sudden strain on the dredge rope.

The efficiency or evenness of dredging was secured, therefore, by the special device of fastening a heavy weight some 200 or 300 fathoms from the dredge end of the dredge rope. This was either lowered with the dredge or sent down after by means of a “messenger,” a ring of rope fixed round, but running freely on, the dredge rope. The latter plan was used on the “Challenger”; the weights were six 28 ℔ leads in canvas covers: their descent was arrested by a toggle or wooden cross-bar previously attached to the rope at the desired point. When, however, the rope used is of wire this front weight is unnecessary.

The possibility of sudden strain necessitates a constant watching of the dredge rope, as the ship’s engines may at any moment be needed to ease the tension by stopping the vessel’s way, and the hauling engines by paying out more rope. The use of accumulators both renders the strain more gradual and gives warning of an increase or decrease; indeed they can be calibrated and used as dynamometers to measure the strain. One of the best forms of accumulator consists of a pile of perforated rubber disks, which receive the strain and become compressed in doing so. The arrangement is in essence as follows. The disks form a column resting on a cross-bar or base, from which two rods pass up one on each side of the column. Another cross-bar rests on the top disk, and from it a rod passes freely down the centre perforation of disks and base. Eyes are attached to the lower end of this rod and to a yoke connecting the side rods at the top: a pull exerted on these eyes is thus modified by the elasticity of the dredge. In the “Porcupine” and other early expeditions the accumulator was hung from the main yard arm, and the block through which the dredge rope ran suspended from it. In more recent ships a special derrick boom is rigged for this block, and a second accumulator is sometimes inserted between the topping lift by which this is raised and the end of the boom.

The margin of safety of steel wire rope is much larger than is that of hempen rope, a fact of importance both in towing in a rough sea and in hauling. Galvanized steel wire with a hempen core was first used by Agassiz on the “Blake.” He states that his wire weighed one pound per fathom, against two pounds per fathom of hempen rope, and had a breaking strain nearly twice that of hempen rope, which bore two tons. Thus in hauling the wire rope has both greater capability and less actual strain. It has also the advantages of occupying a mere fraction (19) of the storage space needed for rope, of lasting much longer, and its vibrations transmit much more rapid and minute indications of the conduct of the dredge.

Wire rope is kept wound on reels supplied with efficient brakes to check or stop its progress, and an engine is often fitted for winding it in and veering it out. From the reel it passes to the drum of the hauling engine, round which it takes some few turns; care is taken by watching or by the use of an automatic regulator (Tanner) that it is taken at a rate equal to that at which it is moving over the side. From the hauling engine it passes over leading wheels (one of which should preferably be a registering wheel and indicate the amount of rope which has passed it), and so it reaches the end of the derrick boom.

The dredge is lowered from the derrick boom, which has been previously trained over to windward so that its end is well clear of the ship, while the ship is slowly moving forward. The rope is checked until the net is seen to be towing clear, and then lowered rapidly. Where a weight is used in front of the trawl Captain Calver successfully adopted the plan of backing after sufficient line had been paid out: the part of the rope from weight to surface thus became more vertical, while the shorter remainder, previously in line with it, sank to the bottom without change of relative position of weight and dredge. The ship was then ready for towing. When no front weight is used the manœuvre is unnecessary.

There should be a relation maintained between speed of vessel onward and of rope downward, or a foul haul may result owing to the gear capsizing (in the case of a trawl), or getting the net over the mouth (in a dredge). The most satisfactory method of ensuring this relation seems to be so to manage the two speeds that the angle made by the dredge rope is fairly constant. This angle can be observed with a simple clinometer. The following table abridged from Tanner most usefully brings together the requisite angles with other useful quantities.

Depth of water. Speed of ship
while shooting
dredge or trawl.
Length of
rope
required.
Angle of dredge
rope while
lowering trawl.
Angle of dredge
rope while
dragging trawl.
Fathoms. Knots. Fathoms.    
 100 3   200 60 55
 200 3   400 60 55
 400 3   700 60 52
 600 1000 55 50
 800 1200 50 44
1000 1500 50 40
1500 2166 50 40
2000 2  2670 45 35
3000 2  4000 40 35