Chapter XIII: Part 13

_Dredging on the River Scheldt below Antwerp._--This dredging took place at Krankeloon and the Belgian Sluis under the direction of L. Van Gansberghe. At Melsele there is a pronounced bend in the river, causing a bar at the Pass of Port Philip, and just below the pass of Lillo there is a cross-over in the current, making a neutral point and forming a shoal. After dredging to 8 metres (26.24 ft.) below low tide, in clay containing stone and ferruginous matter, a sandstone formation was encountered, which was very compact and difficult to raise. A suction dredger being unsuited to the work, a bucket-ladder dredger was employed. The dredging was commenced at Krankeloon in September 1894 and continued to the end of 1897. A depth of 6 metres (19.68 ft.) was excavated at first, but was afterwards increased to 8 metres (26.24 ft.). The place of deposit was at first on lands acquired by the State, 2.17 m. above Krankeloon, and placed at the disposal of the contractor. The dredgings excavated by the bucket-ladder dredger were deposited in scows, which were towed to the front of the deposit ground and discharged by a suction pump fixed in a special boat, moored close to the bank of the river. The material brought by the suction dredger in its own hull was discharged by a plant fixed upon the dredger itself. In both instances the material was deposited at a distance of 1640 ft. from the river, the spoil bank varying in depth from 2 to 7 metres. The water thrown out behind the dyke with the excavated material returned to the river, after settlement, by a special discharge lock built under the dyke. After 1896 the material was delivered into an abandoned pass by means of barges with bottom hopper doors or by the suction dredger. One suction dredger and three bucket-ladder dredgers were employed upon the work, and a vessel called "Scheldt I." used for discharging the material from the scows. Four tugboats and twenty scows were also employed.

The largest dredger, "Scheldt III.," was 147.63 ft. long by 22.96 ft.
wide by 10.98 ft. deep, and had buckets of 21.18 cub. ft. capacity.
The output per hour was 10,594 cub. ft. This dredger had also a
complete installation as a suction dredger, the suction pipe being 2
ft. diameter. The fan of the centrifugal pump was 5.25 ft. diameter,
and was driven by the motor of the bucket ladder. The three bucket
dredgers worked with head to the ebb tide. They could also work with
head to the flood tide, but it took so long a time to turn them about
that it was impracticable. The work was for from 13 to 14 hours a day
on the ebb tide. The effective daily excavation averaged 4839 cub.
yds. Each dredger was fitted with six anchors. The excavated cut was
164 ft. wide by 6.56 ft. deep. "Scheldt III." was capable of lifting a
mass 9.84 ft. thick. The suction dredger "Scheldt II." was of the
multiple type, and is stated to be unique in construction. It can
discharge material from a scow alongside, fill its own hopper with
excavations, discharge its own load upon the bank or into a scow by
different pipes provided for the purpose, and discharge its own load
through hopper doors. The machinery is driven by a triple expansion
engine of 300 i.h.p. working the propeller by a clutch. Owing to the
rise and fall in the tide of 23 ft. the suction pipe is fitted with
spherical joints and a telescopic arrangement. The vessel is 157.5 ft.
by 28.2 ft. by 12.8 ft. The diameter of the pump is 5.25 ft. The wings
of the pump are curved, the surface being in the form of a cylinder
parallel to the axis of rotation, the directrix of which is an arc of
a circle of 2.62 ft. radius with the straight part beyond. The suction
and discharge pipes are 2 ft. diameter. A centrifugal pump is provided
for throwing water into the scows to liquefy the material during
discharge. The dredger, which is fitted with electric lights for work
at night, is held by two anchors, to prevent lurching backwards and
forwards; it can work on the flood as well as on the ebb tide, and can
excavate to a depth of 42.65 ft., the output depending upon the nature
of the material. With good material it can fill its tanks in thirty
minutes. To empty the tanks by suction and discharge upon the bank
over the dyke takes about fifty minutes, depending upon the height and
distance to which the material requires to be delivered. The daily
work has averaged eighteen hours, ten trips being made when the
distance from the dredging ground to the point of delivery is about 1
m. When the dredged material is discharged into the Scheldt, a
quantity of 5886 cub. yds. has been raised and deposited in a day, the
mean quantity being 4700 cub. yds. When the distance of transportation
is increased to 2-1/2 m., six voyages were made in a day, and the day's
work amounted to 3530 cub. yds.

_Gold Dredgers._--Dredgers for excavating from river beds soil containing gold are generally fitted with a screen and elevator.

They have been extensively designed and built by Messrs Lobnitz & Co. (fig. 2) and also by Messrs Hunter & English.

The writer is indebted to the _Proceedings_ of the Institution of
Civil Engineers, and especially to the paper of Mr J. J. Webster
(_Proc. Inst. C.E._ vol. 89), for much valuable information upon the
subject treated. He is also indebted to many manufacturers who have
furnished him with particulars and photographs of dredging plant.
(W. H.*)

Plate I.

Plate II.

The latter, the largest ever made, were for the hopper-dredger "David Dale" (Plate I. fig. 10), built by Lobnitz & Co.]

Built for special work on the Suez Canal by Lobnitz & Co. Length 180 ft., breadth 40 ft., depth 12 ft.]

2. MARINE BIOLOGY

The naturalist's dredge is an instrument consisting essentially of a net or bag attached to a framework of iron which forms the mouth of the net. When in use as the apparatus is drawn over the sea-bottom mouth forwards, some part of the framework passes beneath objects which it meets and so causes them to enter the net. It is intended for the collection of animals and plants living on or near the sea-bottom, or sometimes of specimens of the sea-bottom itself, for scientific purposes.

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.

_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 2-1/2 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 Muller 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 Muller 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."

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 Muller'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 Muller's,
especially in later dredges. The short edges are of round iron bar.

Like Muller'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 deg. 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 1-1/2 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 Farue 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.

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
(1/9) 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 manoeuvre 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 | Speed of ship |Length of|Angle of dredge|Angle of dredge|
| water. | while shooting | rope | rope while | rope while |
| |dredge or trawl.|required.|lowering trawl.|dragging trawl.|
+----------+----------------+---------+---------------+---------------+
| Fathoms. | Knots. | Fathoms.| | |
| 100 | 3 | 200 | 60 | 55 |
| 200 | 3 | 400 | 60 | 55 |
| 400 | 3 | 700 | 60 | 52 |
| 600 | 2-3/4 | 1000 | 55 | 50 |
| 800 | 2-1/2 | 1200 | 50 | 44 |
| 1000 | 2-1/2 | 1500 | 50 | 40 |
| 1500 | 2-1/4 | 2166 | 50 | 40 |
| 2000 | 2 | 2670 | 45 | 35 |
| 3000 | 2 | 4000 | 40 | 35 |
+----------+----------------+---------+---------------+---------------+

The speed of towing, always slow, may be assumed to be approximately correct if the appropriate angle is maintained. Hauling should at first be slow from great depths, but may increase in speed as the gear rises.

For further details of deep-sea dredging, especially of the hauling
machinery and management of the gear, the special reports of the
various expeditions must be consulted. Commander Tanner, U.S.N., has
given in _Deep Sea Exploration_ (1897) a very full and good account of
the equipment of an exploring ship; and to this book the present
article is much indebted.

_Modifications and Additions to the Dredge._--From 1818, when Sir John Ross brought up a fine Astrophyton from over 800 fathoms on a sounding line in Baffin's Bay, instances gradually accumulated of specimens being obtained from great depths without nets or traps. The naturalists of the "Porcupine" and other expeditions found that echinoderms, corals and sponges were often carried up adhering to the outer surface of the dredge and the last few fathoms of dredge rope. In order to increase the effectiveness of this method of capture a bar was fastened to the bottom of the dredge, to which bunches of teased-out hemp were tied. In this way specimens of the greatest interest, and frequently of equal importance with those in the dredge bag, were obtained. The tangle bar was at first attached to the back of the net. From the "Challenger" expedition onward it has been fixed behind the net by iron bars stretching back from the short sides of the dredge frame which pass through eyes in their first ends (fig. 15). The swabs are thus unable to fold over the mouth of the dredge. Rope lashings to the lips of the dredge are sometimes added, and a weight is tied to the larger bar to keep it down.

Occasionally the tangle bar is used alone (Agassiz), and one form (Tanner) has two bars, stretching back like the side strokes of the letter A from a strong steel spring in the form of an almost complete circle. The whole is pulled forward from a spherical sinker fastened in front of the spring apex; and should the apex enter a crevice between rock masses, the side bars are closed by the pressure instead of catching and bringing up. This is said to be a very useful instrument among corals.

_The Blake Dredge._--In the soft ooze which forms the bottom of deep
seas the common dredge sinks and digs much too deeply for its ordinary
purpose, owing partly to its chief weight bearing on the frame only,
partly to its everted lips. To obviate these defects Lieutenant
Commander Sigsbee of the "Blake" devised the Blake dredge. Its novel
features were the frame and lips. The former was in the form of a
skeleton box; that is, a rectangle of iron bars was placed at the back
as well as the front or mouth of the net and four more iron bars
connected the two rectangles. The lips instead of being everted were
in parallel planes--those, namely, of the top and bottom of the net.
The effect of this was to minimize digging and somewhat spread the
incidences of the weight. Another advantage was that the net being
constantly distended by its frame, and, moreover, protected top and
bottom by an external shield of canvas, quite delicate specimens
reached the surface uninjured. The dredge weighed 80 lb. and was 4
ft. square and 9 in. deep.

_Rake Dredges._--These are devices for collecting burrowing creatures
without filling the dredge with the soil in which they live. Holt
used, at Plymouth, a dredge whose side bars and lower lip were of
iron, the latter armed with forward and downward pointing teeth which
stirred up the sand and its denizens in front of the dredge mouth. The
upper lip of the dredge was replaced by a bar of wood. The bag was of
cheese-cloth or light open canvas, and the whole was of light
construction. The apparatus was very useful in capturing small
burrowing crustacea. The Chester rake dredge is a Blake dredge in
front of which is secured a heavy iron rectangle with teeth placed
almost at right angles to its long sides and in the plane of the
rectangle. Each of these instruments has a width along the scraping
edge of about 3 ft.

_Triangular and Conical Dredges._--Two other dredges are worthy of
mention. The triangular dredge, much resembling Muller's but with a
triangular mouth, and hung by chains from its angles, is an old
fashion now not in general use. It is, however, very useful for rocky
ground. At the Plymouth marine laboratory was also devised the conical
dredge (1901), the circular form being the suggestion of Garstang.
This dredge (fig. 16) was intended for digging deeply. It is of
wrought iron, and of the following dimensions: diameter of mouth 16
in., length 33 in., depth of ring at mouth 9 in. Its weight is 67
lb. As at first used the spaces between the bars are closed by wire
netting; if used for collecting bottom samples it is furnished with a
lining of strong sail-cloth.

Its weight and the small length of edge in contact with the ground
cause this dredge to dig well, and enable the user to obtain many
objects which though quite common are of rare occurrence in an
ordinary dredge. Thus on the Brown Ridges, a fishing-ground west of
Holland, although _Donax vittalus_ is known from examination of fish
stomachs to be abundant, it is rarely taken except in the conical
dredge: the same is true of _Echinocyamus pusillus_, which is in many
parts of the North Sea abundant in bottom samples and in no ordinary
dredgings. With the sail-cloth lining the conical dredge fills in
about 10 minutes on most ground, and no material washing out of fine
sediment occurs on hauling. In shallow seas such as the North Sea
commercial beam and other trawls are now used as quantitative
instruments in the estimation of the fish population, especially of
the _Pleuronectidae_.

_Use of Small Trawls for Dredging._--Although these trawls do not here
concern us, certain adaptations of small beam trawls for biological
exploration are of such identical use with the dredge, and differ from
it so little in structure and size, that they may be here described.

A small beam trawl was first used from the "Challenger" (fig. 17). It
was sent down in 600 fathoms off Cape St Vincent, the reason for its
use being the frequency with which the dredge sank into the sea-bottom
and there remained until hauling. The experiment was entirely
successful. The sinking of the net was avoided, the net had a much
greater spread than the dredge, and in addition to invertebrates it
captured several fish. After this the trawl was frequently used
instead of the dredge. Indeed tangle bar, dredge and trawl form a
series which are fitted for use on the roughest, moderately rough and
fairly firm, and the softest ground respectively, although the dredge
can be used almost anywhere.

FIG. 17.--Trawl of the "Challenger."]

The frame of the "Challenger" trawl consisted of a 15 ft. wooden beam
which in use was drawn over the sea-bed on two runners resembling
those of a sledge, by means of two ropes or bridles attached to eyes
in the front of the runners or "trawl heads." A net 30 ft. long was
suspended by one side to the beam by half-a-dozen stops. The remainder
of the net's mouth was of much greater length than the beam, and was
weighted with close-set rolls of sheet lead; it thus dragged along the
bottom in a curve approximately to a semicircle, behind the beam. The
net tapers towards the hinder end, and contains a second net with open
bottom, which, reaching about three-quarters of the way down the main
net, acts as a valve or pocket. Both heels (or hinder ends) of the
trawl heads and the tail of the net were weighted to assist the net in
digging sufficiently and to maintain its balance--an important point,
since if the trawl lands on its beam the net's mouth remains closed,
and nothing is caught.

The main differences of this trawl from the dredge are the replacement
of scraping lip by ground rope, the position of this ground rope and
the greater size of the mouth. The absence of a lip makes it less
effective for burrowing and sessile creatures, but the weighted ground
rope nevertheless secures them to a very surprising extent. The
position of the ground rope is an important feature, as any free
swimming creature not disturbed until the arrival of the ground rope
cannot escape by simply rising or "striking" up. This and the greater
spread make the trawl especially suitable for the collection of fishes
and other swiftly moving animals. The first haul of the "Challenger"
trawl brought up fishes, and most of our knowledge of fish of the
greatest depths is due to it.

A tendency to return to the use of the small beam trawl for deep-sea
work has lately shown itself. That used by Tanner on the "Albatross"
has runners more heart-shaped than the "Challenger's" instrument; the
net is fastened to the downward and backward sloping edge of the
runner as well as to the beam, being thus fixed on three sides instead
of one; and a Norwegian glass float is fastened in a network cover to
that part of the net which is above and in front of the ground rope in
use, to assist in keeping the opening clear. These floats can stand
the pressure at great depths, and do not become waterlogged as do cork
floats. The largest "Albatross" trawl has a beam 11 ft. long, runners
2 ft. 5 in. high, and its frame weighs 275 lb.

_Agassiz or Blake Trawl._--This is generally considered to possess
advantages over the preceding, and is decidedly better for those not
experts in trawling. Its frame (fig. 18) consists of two iron runners
each the shape of a capital letter D, joined by iron rods or pipes
which connect the middle of each stroke with the corresponding point
on the other letter. The net is a tapering one, its mouth being a
strong rope bound with finer rope for protection till the whole
reaches a thickness of some 2 in. It is fastened to the frame at four
points only, the ends of the curved rods, and thus has a rectangular
opening.

FIG. 18.--Agassiz or Blake Trawl.]

The chief advantage of this frame is that it does not matter in the
least which side lands first on the bottom; it is to the other trawls
what Ball's dredge is to an oyster dredge. The course can also be
altered during shooting or towing the Blake trawl with far greater
ease than is the case with others. An Agassiz trawl very successful in
the North Sea has the following dimensions: length of the connecting
rods and therefore of the mouth 8 ft., height of runners and of mouth
1 ft. 9 in., extreme length of runners 2 ft., length of net 11 ft. 3
in., weight of whole trawl 94 lb., 63 of which are due to the frame.

It is instructive to note how closely our knowledge of bottom-living forms has been associated with the instruments of capture in use. As long as small vessels were used in dredging, the belief that life was limited to the regions accessible to them was widely spread. The first known denizens of great depths were the foraminifera and few echinoderms brought up by various sounding apparatus. Next with the dredge and tangles the number of groups obtained was much greater. As soon as trawls were adopted fish began to make their appearance. The greatest gaps in our knowledge still probably occur in the large and swiftly moving forms, such as fish and cephalopods. As we can hardly hope to move apparatus swiftly over the bottom in great depths, the way in which improvement is possible probably is that of increasing the spread of the nets; and a start in this direction appears to have been made by Dr Petersen, who has devised a modified otter sieve which catches fish at all events very well, and has been operated already at considerable depths.

Of the economy of quite shallow seas, however, we are still largely ignorant. Much as has been learnt of the bionomics of the sea, it is but a commencement; and this is of course especially true of deep seas. The dredge and its kindred have, however, in less than a century enabled naturalists to compile an immense mass of knowledge of the structure, development, affinities and distribution of the animals of the sea-bed, and in the most accessible seas to produce enumerations and morphological accounts of them of some approach to completeness. (J. O. B.)

DRELINCOURT, CHARLES (1595-1669), French Protestant divine, was born at Sedan on the 10th of July 1595. In 1618 he undertook the charge of the French Protestant church at Langres, but failed to receive the necessary royal sanction, and early in 1620 he removed to Paris, where he was nominated minister of the Reformed Church at Charenton. He was the author of a large number of works in devotional and polemical theology, several of which had great influence. His _Catechism_ (_Catechisme ou instruction familiere_, 1652) and his _Christian's Defense against the Fears of Death_ (_Consolations de l'ame fidele contre les frayeurs de la mort, 1651_) became well known in England by means of translations, which were very frequently reprinted. It has been said that Daniel Defoe wrote his fiction of Mrs Veal (_A True Relation of the Apparition of Mrs Veal_), who came from the other world to recommend the perusal of _Drelincourt on Death_, for the express purpose of promoting the sale of an English translation of the _Consolations_; Defoe's contribution is added to the fourth edition of the translation (1706). Another popular work of his was _Les Visites charitables pour toutes sortes de personnes affligees_ (1669). Drelincourt's controversial works were numerous. Directed entirely against Roman Catholicism, they did much to strengthen and consolidate the Protestant party in France. He died on the 3rd of November 1669.

Several of his sons were distinguished as theologians or physicians. Laurent (1626-1681) became a pastor, and was the author of _Sonnets chretiens sur divers sujets_ (1677); Charles (1633-1697) was professor of physic at the university of Leiden, and physician to the prince of Orange; Peter (1644-1722) was ordained a priest in the Church of England, and became dean of Armagh.

DRENTE, a province of Holland, bounded N. and N.E. by Groningen, S.E. by the Prussian province of Hanover, S. and S.W. by Overysel, and N.W. by Friesland; area, 1128 sq. m.; pop. (1900) 149,551. The province of Drente is a sandy plateau forming the kernel of the surrounding provinces. The soil consists almost entirely of sand and gravel, and is covered with bleak moorland, patches of wood, and fen. This is only varied by the strip of fertile clay and grass-land which is found along the banks of the rivers, and by the areas of high fen in the south-eastern corner and on the western borders near Assen. The surface of the province is a gentle slope from the south-west towards the north-east, where it terminates in the long ridge of hills known as the Hondsrug (Dog's Back) extending along the eastern border into Groningen. The watershed of the province runs from east to west across the middle of the province, along the line of the Orange canal. The southern streams are all collected at two points on the southern borders, namely, at Meppel and Koevorden, whence they communicate with the Zwarte Water and the Vecht respectively by means of the Meppeler Diep and the Koevorden canal. The Steenwyker Aa, however, enters the Zuider Zee independently. The northern rivers all flow into Groningen. The piles of granite rocks somewhat in the shape of cromlechs which are found scattered about this province, and especially along the western edge of the Hondsrug, have long been named _Hunebedden_, from a popular superstition that they were "Huns' beds." Possibly the word originally meant "beds of the dead," or tombs.

Two industries have for centuries been associated with the barren heaths and sodden fens so usually found together on the sand-grounds, namely, the cultivation of buckwheat and peat-digging. The work is conducted on a regular system of fen colonization, the first operation being directed towards the drainage of the country. This is effected by means of drainage canals cut at regular intervals and connected by means of cross ditches. These draining ditches all have their issue in a main drainage canal, along which the transport of the peat and peat-litter takes place and the houses of the colonists are built. The heathlands when sufficiently drained are prepared for cultivation by being cut into sods and burnt. This system appears to have been practised already at the end of the 17th century. After eight years, however, the soil becomes exhausted, and twenty to thirty years are required for its refertilization. The cultivation of buckwheat on these grounds has decreased, and large areas which were formerly thus treated now lie waste. Potatoes, rye, oats, beans and peas are also largely cultivated. In connexion with the cultivation of potatoes, factories are established for making spirits, treacle, potato-meal, and straw-paper. Furthermore, agriculture is everywhere accompanied on the sand-grounds by the rearing of sheep and cattle, which assist in fertilizing the soil. Owing to the meagreness of their food these animals are usually thin and small, but are quickly restored when placed on richer grounds. The breeding of pigs is also widely practised on the sand-grounds, as well as forest culture. Of the fen-colonies in Drente the best known are those of Frederiksoord and Veenhuizen.