Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/481

461 water or with a very slight current the dredge of course anchors the boat, and oars or sails are necessary ; but if the boat be moving at all it is all that is required. It is perhaps most pleasant to dredge with a close-reefed sail before a light wind, with weights, against a very slight tide or current ; but these are conditions which cannot be com manded. The dredge may remain down from a quarter of an hour to twenty minutes, by which time, if things go well, it ought to be fairly filled. In dredging from a small boat the simplest plan is for two or three men to haul in, hand over hand, and coil in the bottom of the boat. For a large yaul or yacht, and for depths over 50 fathoms, a winch is a great assistance. The rope takes g. couple of turns round the winch, which is worked by two men, while a third hand takes it from the winch and coils it down. The dredge comes up variously freighted according to the locality, and the next step is to examine its contents and to store the objects of search for future use. In a regularly organized dredging expedition a frame or platform is often erected with a ledge round it to receive the con tents of the dredge, but it does well enough to capsize it on an old piece of tarpauliug. There are two ways of emptying the dredge ; we may either turn it up and pour out its contents by the mouth, or we may have a con trivance by which the bottom of the bag is made to unlace. The first plan is the simpler and the one more usually adopted ; the second has the advantage of letting the mass slide out more smoothly and easily, but the lacing intro duces rather a damaging complication, as it is apt to loosen or give way. Any objects visible on the surface of the heap are now carefully removed, and placed for identification in jars or tubs of sea-water, of which there should be a number secured in some form of bottle basket, standing ready. The heap should not be much disturbed, for the delicate objects contained in it have already been unavoid ably subjected to a good deal of rough usage, and the less friction among the stones the better. Close to the place where the dredge is emptied there ought to be a tub about 2 feet in diameter and 20 inches deep, provided with a set of sieves so arranged that the lowest sieve fits freely within the bottom of the tub, and the three remaining sieves fit freely within one another. (fig. 2.) Each sieve has a pair of iron handles through which the hand can pass easily, and the handles of the largest sieve are made long, so that the whole nest can be lifted without stooping or putting the arms into the water. The upper smallest sieve is usually deeper than the others; it is made of a strong open net of brass wire, the meshes half an inch to a side. The second sieve is finer, the meshes a quarter of an inch to a side ; the third is finer still ; and the fourth is so close as only to allow the passage of mud or fine sand. The sieves are put into the tub, and the tub filled up to the middle of the top sieve with sea- water. The top sieve is then filled with the contents cf the dredge, and the set of sieves are gently moved up and down by the handles of the bottom sieve in the water. It is of great importance not to give a rotatory motion to the sieves in this part of the process, for this is very ruinous to fragile organisms ; the sieves should be gently churned up and down, whether singly or together. The result is faat the rougher stones and gravel and the larger organisms ere washed and retained in the upper sieve ; the fine mud or sand passes through the whole of the sieves and subsides to the bottom of the tub ; while the three lower sieves contain, in graduated series, the objects of intermediate FIG. 2. Set of Sieves. size. The sieves are examined carefully in succession, and the organisms which they contain are gently removed with a pair of brass or bone forceps into the jars of sea-water, where their movement and their natural colours may ba observed, or placed at once in bottles of strong or weak spirit of wine or dried, according to the object for which they have been collected. The scientific value of a dredging depends mainly upon two things, the care with which the objects procured are preserved and labelled for future identification and reference, and the accuracy with which all the circum stances of the dredging the position, the depth, the nature of the ground, the date, the bottom-temperature, &c. are recorded. Every specimen, whether dry or in spirit, should be labelled at once with the number under which this particular dredging is entered in the dredger s note-book. Up to the middle of last century the little that was known of the inhabitants of the sea beyond low-water mark seems to have been gathered almost entirely from the objects found thrown on the beach after storms, and from chance captures on lead-lines, or by fishermen on their long lines, and in trawls and oyster and clam dredges. The naturalist s dredge does not appear to have been used for investigating systematically the fauna of the bottom of the sea until it was employed by Otho Frederick Muller in the researches which afforded material for the publication, in 1779, of his admirable Descriptions and History of the rarer and less hiou n Animals of Denmark and Nonvdy. In the preface to the first volume Muller gives a quairjt description and figure of a dredge (fig. 3) not very unlike that used by Hall and Forbes, only the mouth of the dredge was square, a form which, unless used with great caution, gives fatal facilities for " wash ing out " in the process of hauling in. At the Birmingham meeting of the British Association in 1839 an im portant committee was appointed " for researches with the dredge with a view to the investigation of the marine zoology of Great Britain, the illustration of the geographical distribution of marine animals, and the more accurate determination of the fossils of the FIG. 3. Otho Freue- Pliocene period." Of this committee rick_ Muller s Dredge Edward Forbes was the ruling spirit, " ) and under the genial influence of his contagious enthu siasm great progress was made during the next decade in the knowledge of the fauna of the British seas, and many wonderfully pleasant days were spent by the original committee and by many others w r ho from year to year were "added to their number." Every annual report of the British Association contains communications from the English, the Scottish, or the Irish branches of the committee; and in 1850 Edward Forbes submitted its first general report on British marine zoology. This report, as might have been anticipated from the eminent qualifica tions of the reporter, was of the highest value ; and, taken along with his remarkable memoirs previously published, " On the Distribution of the Mollusca and Radiata of ^Egean Sea," and " On the Zoological Relations of the existing Fauna and Flora of the British Isles," may be said to mark an era in the progress of human thought. The dredging operations of the British Association committee were carried on generally under the idea that at the 100-fathom line, by which amateur work in small boats was practically limited, the zero of animal life was approached a notion which was destined to be gradually undermined, and finally overthrown. From time to time,

however, there were not wanting men of great skill and ex-