Page:The American Cyclopædia (1879) Volume VI.djvu/265

 DREDGING (DEEP-SEA) 257 3 or 4 in. wide, sharp at the forward edge, and pierced with holes at their hinder and thicker edge. To these holes the net, made of strong twine, and a protecting cover of canvas open at the bottom, are fastened with copper wire. The iron rings passing through the holes in the frame, and the iron rod running through the rings as used in recent English expeditions, ap- pear to be unnecessary complications. The net ought to have small meshes, about half an inch to the side, and may be lined near the bottom with some open canvas to retain the smaller specimens. In the United States coast survey dredgings it was found convenient to have the net open at the bottom and to tie it up with a rope yarn ; this gives greater facil- ity for emptying it, and the numerous folds formed in the bottom of the bag have a great retentive power for small objects. A stout wire bent in a bow inside the net, and hooked by its ends to the frame, is useful in preventing the net from being turned inside out in going down. The bag has also been made occasion- ally of strong canvas, with windows of copper or brass wire gauze on each side ; but this construction seems to afford too much facility for washing out, unless the bag is of consider- able length. Dredges have also been made of two sides of hide, laced all round with a small line ; this works well on coral ground, but not elsewhere. Some of the Scandinavian and German naturalists use triangular dredges, having three scrapers and three arms. The two arms of an ordinary dredge are made each of a double or forked rod of iron, hinged on the small ends of the frame, and forming a ring at the outer end. The dredge rope passes through the ring of one of the arms only, the other arm being made fast or stopped to the first by a few turns of spun yarn. By this arrangement, in case the dredge is caught, the stop, being the weakest part, breaks first, and the rope pulling on one arm only, the dredge disengages itself and comes up endwise. For use from a steamer the dimensions of the dredge may be larger. Those used in the Por- cupine expedition were 4 ft. 6 in. long by 6 in. wide. Capt. Calver of the Porcupine, having observed that frequently objects came up adher- ing to the outside of the bag and even to the line, attached to the dredges some of the swabs used for washing decks. The frayed hemp swept up every rough or spiny object with which it came in contact ; and among the dwellers of the deep, Crustacea, bryozoa, echinoderms, polyps, and sponges nearly all partake of those qualities. Smooth shells, and those which bur- row in the sand, alone escape the " hempen tangles." In one case Prof. "W. Thomson es- timates that not fewer than 20,000 specimens of echinus Norvegicus came up in the tangles at one haul. These are best attached to an iron bar fixed transversely to the bottom of the bag, or in very rocky bottom they can be used with- out the dredge. In the expedition of the United States steamer Hassler they were successfully used attached to a long bundle of rattans, which yields when in contact with rocks, and adapts itself to the inequalities of the bottom ; it is of course loaded sufficiently with lead. The salabre of the coral fishers of the Mediter- ranean is constructed on the same principle. It is a heavy wooden cross, carrying at the end of the arms large bunches of old tattered nets and swabs. It is kept horizontal, and lifted and dropped alternately a number of times, thus breaking off the branches of coral which are entangled in the nets. In the re- cent expedition of the British ship Challenger it was found practicable to use in great depths the trawl net, with which European fishermen pro- cure fishes living close to the bottom. It con- sists of a wooden beam 12 to 15 ft. long, pro- vided at the ends with iron supports like the runners of a sled, which keep it one or two feet from the ground. A large bag net is laced to the beam by one edge of its mouth, while the other, kept down by leaden sinkers, drags on the ground. Besides fishes, many crustace- ans can be procured with it, which either swim close to the bottom or rise from the sand at the approach of the beam. A miniature trawl net is used by some naturalists in small depths for the lesser Crustacea or radiata, the bag being made of bunting and the frame of hoop iron. The rope or line used in dredging must be made of the best materials, Russian or Italian hemp, and of a size to resist at times a very heavy strain. The deeper the dredge is to be used, the stronger the line must be. In the Porcupine, lines from 2 to 2 in. in cir- cumference were used, the latter having a breaking strain of 2-J- tons. In the United States coast survey, lines from 1 to 2 in., of Italian hemp, were mostly used. In dredging from a boat or a yacht depths from 100 to 200 fathoms can be reached. If the boat drifts considerably by the effect of the wind or cur- rent, it is necessary, unless the dredge is very heavy, to attach a lead to the line a few fath- oms above the dredge ; otherwise it is apt to be lifted clear of the bottom. The length of the line paid out is usually about twice the depth. Deep-sea dredging, that is, dredging in more than 200 fathoms, necessitates the use of a steamer and of a donkey engine to bring up the dredge. The line passes over a large block suspended from a derrick or boom. The rope to which this block is attached passes through another block or a bull's-eye, and thence through an accumulator to the deck, where it is made fast. This accumulator is a com- bination of 30 or 40 strong India-rubber springs held apart by two wooden disks, and is very useful in indicating the strain on the line, which when too great can be relieved by a few turns of the engine. The line (which is marked for every hundred fathoms in the manner of a sounding line) is con- veniently coiled in succession on a row of large iron pins projecting inward from the top of the bulwark. On board the Hassler stout