Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/480

460 460 DREDGE animals living on the bottom of the sea at greater or less depths, for the purpose of determining their structure and zoological relations, and ascertaining their geographical dis tribution. The instrument usually employed in this and other northern countries for dredging oysters and clams is a light frame of iron about 5 feet long by a foot or so in width, with a scraper like a narrow hoe on one side, and a suspending apparatus of thin iron bars which meet in an iron ring for the attachment of the dredge-rope on the other. From the frame is suspended a bag about 2 feet in depth of iron chain netting, or of wide-meshed hempen-cord netting, or of a mixture of both. Natu ralist dredgers at first used the oyster dredge, but it is scarcely suitable for scientific purposes. Having a scraper on one side only, it is liable in a current, in deep water, or in unskilled hands, to fall on its back and conse quently to come up empty, the scraper not having come into play. Oyster dredgers are not allowed to take oysters below a certain size, and the commercial dredge is so contrived as to allow all small bodies to fall through, and, as many of these are of the highest interest to the naturalist, his object is thus in a great measure defeated. The remedy for these defects is to have a scraper on each side, with the arms attached in such a way that one or other of the scrapers must reach the ground in whatever position the dredge may fall ; and to have the dredge-bag deeper in proportion to the size of the frame, and of a material which is only sufficiently open to allow the water to pass freely through, with the openings so distributed as to leavo a part of the bag close enough to bring up the finest rnud. The late Dr Robert Ball of Dublin devised the modifica tion which has since been used almost universally by naturalists in this country and abroad under the name of &quot; Ball s Tredge &quot; (fig. 1). The dredges on this pat tern, used in Britain for ten years after their first introduction, about the year 1838, were usually small and rather heavy not more than 12 to 15 inches in length, by 4 or 4-J inches in width at the mouth. Two scrapers, the length of the dredge-frame, and Jj 1|- to 2 inches wide, were set at an |z angle of about 110 to the plane of the dredge s mouth, so that when the dredge was gently hauled along it took hold of the ground and secured anything loose on its surface, Latterly Ball s dredges of con siderably larger size have been used. Perhaps the most convenient form for dredging from a row boat or FIG. I. ^Ball s Natural- a yawl is that represented in the ists&amp;gt; Dred s e - figure. The frame is 18 inches long, and its width is 5 inches. The scrapers are 3 inches wide, and these are so set that the distance across between their scraping edges is 7- inches. The ends of the frame connecting the scrapers are round bars of iron five-eighths of an inch in diameter, and from, these bars two curved arms of round iron of the same thickness, dividing beneath into two branches, which are attached to the ends of the cross-bars by eyes allowing the arms to fold down over the dredge-mouth, meet in two heavy eyes at a point 18 inches above the centre of the frame. The total weight of the dredge frame and arms is 20 ft) ; it ought to be of the best Lowmoor or Swedish wrought iron. The thick inner edges of the scrapers are perforated by round holes at distances of about an inch, and through these strong iron rings about an iuch in diameter are passed, and two or three similar rings run on the short rods which form the ends of the dredge-frame. A light iron rod, bent to the form of the dredge opening, usually runs through these rings, and to this rod arid to the rings the mouth of the dredge-bag is securely attached by stout cord or strong copper wire. The dredge-bag for a dredge of this size should be about 2 feet deep ; and probably the most suitable material is hand-made netting of very strong twine, the meshes half an inch to the side, the inter-spaces contracting to a third of an inch across when the twine is thoroughly soaked. So open a network would let many of the smaller things through, and to avoid this, and at the same time to give free egress to the water, the bottom of the bag, to the height of about G inches, is lined with &quot; bread-bag,&quot; a light open kind of canvas. It may be said that in such a dredge many valuable small objects may be washed through the meshes of the upper part of the dredge along with the mud and thus lost; but, on the other hand, if the bag be very close it is apt to get filled up with rnuJ. at once, and to collect nothing more. For work round the coasts of Europe, at depths attain able from a row-boat or yawl, probably the best kind of line is bolt-rope of the best Russian hemp, not less than l- inches in circumference, containing eighteen to twenty yarns in three strands. Each yarn should bear nearly a hundred weight, so that the breaking strain of such a rope ought to be about a ton. Of course it is never voluntarily exposed to such a strain, but in shallow water the dredge is often caught among rocks or coral, and the rope should be strong enough in such a case to bring up the boat, even if there were some little way on. It is always well, when dredging, to ascertain the approximate depth with the lead before casting the dredge ; and the lead ought always to be accompanied by a registering thermometer, for the sub sequent haul of the dredge will gain greatly in value as an observation in geographical distribution, if it be accompanied by an accurate note of the bottom-temperature. For depths under 100 fathoms the amount of rope paid out should be at least double the depth ; under 30 fathoms, where one usually works more rapidly, it should be more nearly three times ; this gives a good deal of slack before the dredge if the boat be moving very slowly, and keeps the lip of the dredge well down. When there is anything of a current, from whatever cause, it is usually convenient to attach a weight, varying from 14 Ib to half a hundred weight, to the rope 3 or 4 fathoms in front of the dredge. This prevents in some degree the lifting of the mouth of the dredge ; if the weight be attached nearer the dredge it is apt to injure delicate objects passing in. In dredging in sand or mud, the dredge-rope may simply be passed through the double eye formed by the ends of the two arms of the dredge-frame ; but in rocky or unknown ground it is better to fasten the rope to the eye of one of the arms only, and to tie the two eyes together with three or four turns of rope-yarn. This stop breaks much more readily than the dredge-rope, so that if the dredge get caught it is the first thing to give way under the strain, and in doing so it often alters the position of the dredge so as to allow of its extrication. The dredge is slipped gently over the side, either from the bow or from the stern in a small boat more usually the latter while there is a little way on, and the direction which the rope takes indicates roughly whether the dredge is going down properly. When it reaches the ground and begins to scrape, an experienced hand upon the rope can usually detect at once a tremor given to the dredge by the scraper passing over the irregularities of the bottom. ^ The due amount of rope is then paid out, and the rope hitched to a bench or rollock-pin. The boat should move very slowly, probably not faster than a mile an hour. In still