Part 29

Until the middle of the 18th century, naturalists who studied the marine fauna and flora relied for their materials on shore collection and the examination of the catches of fishing boats. Their knowledge of creatures living below the level of low spring tides was thus gained only from specimens cast up in storms, or caught by fishing gear designed for the capture of certain edible species only. The first effort made to free marine biology from these limitations was the use of the dredge, which was built much on the plan of the oyster dredge.

[Illustration: FIG. 13.--Otho Frederick Müller's Dredge (1770).]

_The Oyster Dredge._--At first naturalists made use of the ordinary oyster dredge, which is constructed as follows. The frame is an iron triangle, the sides being the round iron "arms" of the dredge, the base a flat bar called the shere or lip, which is sloped a little, not perpendicular to the plane of the triangle; an iron bar parallel to the base joins the arms. The net is fastened to the parallel bars and the portion of the arms between them, and consists of two parts: that attached to the shere is of round iron rings linked together by smaller ones of wire lashings, that attached to the upper bar is of ordinary network. Where these two portions of the bag meet a wooden beam is fastened. In use the frame is towed forward by its apex: the shere passes below oysters, &c., which pass back on to the iron netting. The length of each side of the triangular frame is about 6 ft., the width of the shere 3 in. and the height of the mouth just under a foot. The rings vary in size, but are usually some 2½ in. in diameter. The weight is about 60 lb. This dredge was soon abandoned: its weight was prohibitive for small boats, from which the naturalist usually worked, its wide rings allowed precious specimens to fall through, and its shallow net favoured the washing out of light objects on hauling through the moving water of the surface. Moreover, it sometimes fell on its back and was then useless, although when the apex or towing point was weighted no great skill is needed to avoid this.

Otho Müller used a dredge (fig. 13) consisting of a net with a square iron mouth, each of whose sides was furnished with a thin edge turned slightly away from the dredge's centre. As any one of these everted lips could act as a scraper it was a matter of indifference which struck the bottom when the dredge was lowered. The chief defect of the instrument was the ease with which light objects could be washed out on hauling, owing to the size of the mouth. However, with this instrument Müller obtained from the often stormy Scandinavian seas all the material for his celebrated _Zoologia Danica_, a description of the marine fauna of Denmark and Norway which was published with excellent coloured plates in 1778; and historical interest attaches to the dredge as the first made specially for scientific work.

_Ball's Dredge._--About 1838 a dredge devised by Dr Ball of Dublin was introduced. It has been used all over the world, and is so apt for its purpose that it has suffered very little modification during its 70 years of life. It is known as Ball's dredge or more generally simply "the dredge."

[Illustration: FIG. 14.--Ball's Naturalist's Dredge.]

Ball's dredge (fig. 14) consists of a rectangular net attached to a rectangular frame much longer than high, and furnished with rods stretching from the four corners to meet at a point where they are attached to the dredge rope. It differs from Müller's dredge in the slit-like shape of the opening, which prevents much of the "washing out" suffered by the earlier pattern, and in the edges. The long edges only are fashioned as scrapers, being wider and heavier than Müller's, especially in later dredges. The short edges are of round iron bar.

Like Müller's form, Ball's dredge will act whichever side touches the bottom first, as its frame will not remain on its short edge, and either of the long edges acts as a scraper. The scraping lips thicken gradually from free edge to net; they are set at 110° to the plane of the mouth, and in some later patterns curve outwards instead of merely sloping. All dredge frames are of 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 inch 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 and to the rings the mouth of the dredge-bag is securely attached by stout cord or strong copper wire. Various materials have been used for the bag, the chief of which are hide, canvas and netting. The hide was recommended by its strength, but it is now abandoned. Canvas bags fill quickly with mud or sand and then cease to operate: on the other hand wide mesh net fails to retain small specimens. 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, with an open canvas or "bread-bag" lining to the last 6 in. of the net. A return to canvas covering has latterly occurred in the small dredge called the mud-bag, trailed behind the trawl of the "Albatross" for obtaining a sample of the bottom, and in the conical dredge.

The dimensions of the first dredges were as follows: Frame about 12 in. by about 4 in.; scraping lips about 2 in. wide; all other iron parts of round iron bar 5/8 in. diameter; bag rather more than 1 ft. long. These small dredges were used from rowing boats. Larger dredges were subsequently made for use from yawls or cutters. The mouth of these was 18 by 5 in., the scraping lips about 2 in. wide and bag 2 ft. deep; such a dredge weighs about 20 lb. The dredge of the "Challenger" had a frame 4 ft. 6 in. by 1 ft. 3 in. and the bag had a length of 4 ft. 6 in.; the "Porcupine" used a dredge of the same size weighing 225 lb. Doubtless the size of Ball's dredge would have grown still more had it not been proved by the "Challenger" expedition that for many purposes trawls could be used advantageously instead of dredges.

_Operation of the Dredge from Small Vessels._ For work round the coasts of Europe, at depths attainable from a row-boat or yawl, probably the best kind of line is bolt-rope of the best Russian hemp, not less than 1½ in. in circumference, containing 18 to 20 yarns in 3 strands. Each yarn should be nearly a hundredweight, 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 subsequent 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 lb. to half a hundredweight, 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 rowlock-pin. The boat should move very slowly, probably not faster than a mile an hour. In still 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 commanded. 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 yawl or yacht, and for depths over 50 fathoms, a winch is a great assistance. The rope takes a 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.

It is easier to operate a dredge from a steam vessel than a sailing boat, but if the steamer is of any size great care should be taken that the dredge does not move too rapidly.

Two ingenious cases of dredging under unusual conditions are worthy of mention, one case from shore, one from ice. In the Trondligem Fjord, Canon A. M. Norman in 1890 worked by hauling the dredge up the precipitous shores of the fjord. The dredge was shot from a boat close to the shore, to which after paying out some hundreds of fathoms of line it returned. The dredge was then hauled from the top of the cliffs up whose side it scraped. Hitches against projecting rocks were frequent and were overcome by suddenly paying out line for a time. The dredge was lifted into a boat when it reached the surface of the sea. The other case occurred during the Antarctic expedition of the "Discovery." Hodgson dropped loops of line along cracks which occasionally formed in the ice. The ice always joined up again, but with the line below it; and a hole being cleared at each place at which the end of the line emerged, the dredge could be worked between them.

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 contents of the dredge, but it does well enough to capsize it on an old piece of tarpaulin. 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 contrivance 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 introduces 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 unavoidably subjected to a good deal of rough usage, and the less friction among the stones the better.

_Examination of the Catch. Sifting._--The sorting of the catch is facilitated by sifting. The sieves used in early English expeditions were of various sizes and meshes, each sieve having a finer mesh than the sieve smaller than itself. In use the whole were put together in the form of a nest, the smallest one with the coarsest mesh being on top. A little of the dredge's contents were then put in the top sieve, and the whole set moved gently up and down in a tub of sea water by handles attached to the bottom one. Objects of different sizes are thus left in different sieves. A simple but effective plan is to let the sieves of various sized mesh fit accurately on each other like lids, the coarsest on top, and to pour water upon material placed on the top one. In the United States Bureau of Fisheries ship "Albatross" these sieves are raised to form a table and the water is led on them from a hose: the very finest objects or sediments are retained by the waste water escaping from a catchment tub by muslin bags let into its sides. Any of these methods are preferable to sifting by the agitation of a sieve hung over the side, as in the last anything passing through the sieve is gone past recall.

_Preservation of Specimens._--The preservation of specimens will of course depend on the purpose for which they are intended. For microscopic observation formaldehyde has some advantages. It can be stored in 40% solution and used in 2%, thus saving space, and it preserves many animals in their colours for a time: formalin preparations do not, however, last as well as do those in spirit. The suitable fluids for various histological inquiries are beyond the scope of the present article; but for general marine histology Bles' fluid is useful, being simple to prepare and not necessitating the removal of the specimen to another fluid. It is composed of 70% alcohol 90 parts, glacial acetic acid 7 parts, 4% formaldehyde 7 parts.

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 circumstances of the dredging--the position, the depth, the nature of the ground, the date, the bottom-temperature, &c.--are recorded. In the British Marine Biological Association's work in the North Sea, a separate sheet of a printed book with carbon paper and duplicate sheets (which remain always on the ship) is used for the record of the

## particulars of each haul; depth, gear, &c., being filled into spaces

indicated in the form. This use of previously prepared forms has been found to be a great saving of time and avoids risk of omission. Whether labelled externally or not, all bottles should contain parchment or good paper labels written with a soft pencil. These cannot be lost. The more fully details of reference number of station, gear, date, &c., are given the better, as should a mistake be made in one particular it can frequently be traced and rectified by means of the rest.

_Growth of Scope of Operations._--At the Birmingham meeting of the British Association in 1839 an important 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 Pliocene period." Of this committee Edward Forbes was the ruling spirit, and under the genial influence of his contagious enthusiasm 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 who 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 qualifications 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 the Aegean 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 experience to maintain, with Sir James Clark Ross, that "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." 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 instrument 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 microscopists; 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 "Bulldog" 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 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 lb. 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.