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DREDGE AND DREDGING
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DREDGE AND DREDGING . The word " dredge " is used in two senses. (r) From See also:Mid. Eng. dragie, through Fr. dragee, from Gr. rpayiivara, sweetmeats, it means a confection of See also:sugar formed with seeds, bits of spice or medicinal agents. The word in this sense is obsolete, but survives in " dredger," a See also:box with a perforated See also:top used for sprinkling such a sugar-mixture, See also:flour or other powdered substance. " Dredge " is also a See also:local See also:term for a mixed See also:crop of oats and See also:barley sown together (" maslin " or " meslin," cf. Fr. dragee), and in See also:mining is applied to ore of a mixed value. (2) Connected with "See also:drag," or at least derived from the same See also:root, dredge or dredger is a See also:mechanical appliance for See also:collecting together and See also:drawing to the See also:surface (" dredging ") See also:objects and material from the beds of See also:rivers or the bottom of the See also:sea. In the following See also:account the operations of dredging in this sense are discussed (I) as involved in See also:hydraulic See also:engineering, (2) in connexion with the See also:work of the naturalist in marine See also:biology. 1. HYDRAULIC ENGINEERING Dredging is the name given by See also:engineers to the See also:process of excavating materials under See also:water, raising them to the surface and depositing them in See also:barges, or delivering them through a shoot, a See also:longitudinal conveyor, or pipes, to the See also:place where it is desired to See also:deposit them. It has See also:long been useful in See also:works of marine and hydraulic engineering, and has been brought in See also:modern times to a See also:state of high perfection. The employment of dredging plant and the selection of See also:special appliances to be used in different localities and in varying circumstances require the exercise of See also:sound See also:judgment on the See also:part of the engineer. In rivers and estuaries where the bottom is composed of See also:light soils, and where the scour of the See also:tide can be governed by training walls and other works constructed at reasonable expense, so as to keep the channel clear without dredging, it is See also:manifest that dredging machinery with its large cost for working expenses and for See also:annual upkeep should be as far as possible avoided. On the other See also:hand, where the bottom consists of See also:clay, See also:rock or other hard substances, dredging must, in the first instance at any See also:rate, be employed to deepen and widen the channel which it is sought to improve. In some instances, such as the See also:river See also:Mississippi, a deep channel has for many years been maintained by jetties, with occasional resort to dredging to preserve the required channel See also:section and to hasten its enlargement. The See also:bar of the river See also:Mersey is 11 m. from See also:land, and the cost of training works would be so See also:great as to forbid their construction; but, by a See also:capital See also:expenditure of £120,000 and an annual expense of £20,000 for three years, the See also:depth of water over the bar at See also:low tide has been increased by dredging from II ft. to 27 ft., the channel being 1500 ft. wide.
" Bag and See also:Spoon" Dredger.—The first employment of machinery for dredging is, like the See also:discovery of the See also:canal See also:lock, claimed by See also: This See also:system is still in use, but is only adaptable to a limited depth of water and a soft bottom; it has been largely used in canals and frequently in the See also:Thames. At the Fosdyke Canal in See also:Lincolnshire 135,000 tons were raised in the manner described. According to J. J. See also:Webster (Proc. Inst. C. E. vol. 89), the first application of See also:steam See also:power for dredging operations was to a " spoon & bag" dredger for cleansing See also:Sunderland See also:harbour, the See also:engine being made by Messrs See also:Boulton & See also:Watt of Soho, See also:Birmingham. Dredging by Bucket between Two Lighters.—Another See also:plan of dredging, practised at an early period in rivers of considerable breadth, was to See also:moor two barges, one on each side of the river. Between them was slung an iron dredging bucket, which was attached to both barges by chains See also:wound on the barrels of a crab winch worked by six men in one barge and See also:round a See also:simple windlass worked by two men in the other barge: The bucket, being lowered at the side of the barge carrying the windlass, was See also:drawn across the bottom of the river by the crab winch on the other barge; and having been raised and emptied, it was hauled across by the opposite windlass for repetition of the process. This process was in use in the River See also:Tay until 1833. Bucket See also:Ladder Dredgers.—The earliest See also:record of a bucket ladder dredger is contained in the first See also:paper of the first See also:volume (1836) of the Transactions of the Institution of See also:Civil Engineers. This See also:machine was brought into use at the See also:Hull Docks about 1782. The bucket See also:chain was driven by two horses working a See also:horse-See also:gear on the See also:deck of the See also:vessel. The buckets were constructed of s in. bars of iron spaced in. apart, and were 4 ft. long, 13 in. deep, 12 in. wide at the mouth and about 6 in. wide at the bottom. This dredger raised about 30 tons per See also:hour at the cost of 20. per ton, which covered the See also:wages of three men working the dredger, eight men working the lighters and the keep of three horses. A dredger of this See also:kind and power would only work in See also:ballast, mud or other soft material, but the machine was gradually improved and increased in capacity and power by different manufacturers until it became a very efficient machine in skilful hands, excavating and raising material from depths of 5 ft. to 6o ft. of water at a cost not very different from, and in many cases less than, that at which the same work could be performed on land. With the powerful dredgers now constructed, almost all materials, except solid rock or very large boulders, can be dredged with ease. Loose See also:gravel is perhaps the most favourable material to work in, but a powerful dredger will readily break up and raise indurated beds of gravel, clay and boulders, and has even found its way through the surface of soft rock, though it will not penetrate very far into it. In some cases See also:steel diggers alternating with the buckets on the bucket See also:frame have been successfully employed. The construction of large steam dredgers is now carried on by many engineering firms. The See also:main feature of the machine is the bucket ladder which is hung at the top end by See also:eye straps to the frame of the vessel, and at the See also:lower end by a chain reived in See also:purchase blocks and connected to the hoisting gear, so that the ladder may be raised and lowered to suit the varying depths of water in which the dredger works. The upper See also:tumbler for working the bucket chp.in is generally square or pentagonal in See also:form and made of steel with loose steel wearing pieces securely bolted to it. The tumbler is securely keyed to the steel See also:shaft which is connected by gearing and shafting to the steam engine, a See also:friction See also:block being inserted at a convenient point to prevent breakage should any hidden obstacle causing unusual See also:strain be met with in the path of the buckets. The lower tumbler is similar in construction to the upper tumbler, but is usually pentagonal or hexagonal in shape. The buckets are generally made with steel backs to which the plating of the buckets is riveted; the cutting edge of the buckets consists of a strong steel bar suitably shaped and riveted to the See also:body. The intermediate links are made of hammered iron or steel with removable steel bushes to take the See also:wear of the connecting pins, which are also of steel. The hoisting gear may be driven either from the main engine by frictional gearing or by an See also:independent set of engines. Six anchors and chains worked by powerful steam crabs are provided for regulating the position of the dredger in regard to its work. Barge-loading Dredgers used formerly to be provided with two ladders, one on each side of the vessel, or contained in See also:wells formed in the vessel near each side. Two ladders were adopted, partly to permit the dredger to excavate the material See also:close to aquay or See also:wall, and partly to enable one ladder to work while the other was being repaired. Bucket ladder dredgers are now, however, generally constructed with one central ladder working in a well; frequently the bucket ladder projects at either the See also:head or stern of the vessel, to enable it to cut its own way through a shoal or See also:bank, a construction which has been found very useful. In one modification of this method the bucket ladder is supported upon a traversing frame which slides along the fixed framing of the dredger and moves the bucket ladder forward as soon as it has been sufficiently lowered to clear the end of the well. In places where a large quantity of dredging has to be done, a stationary dredger with three or four large hopper barges proves generally to be the most economical kind of plant. It has, however, the disadvantage of requiring large capital expenditure, while the dredger and its attendant barges take up an amount of space which is sometimes inconvenient where See also:traffic is large and the navigable width narrow. The See also:principal improvements made in barge-loading dredgers have been the increase in the See also:size of the buckets and the strength of the dredging gear, the application of more economical engines for working the machinery, and the use of frictional gearing for See also:driving the ladder-hoisting gear. It is very important that the main drive be fitted with the friction blocks or clutches before alluded to.
Up to the See also:year 1877 dredgers were seldom made with buckets of a capacity exceeding 9 cub. ft., but since that See also:time they have been' gradually increased in capacity. In the dredger " See also:Melbourne," constructed by Messrs See also: The efficiency of the machine, or the See also:net work in raising materials compared with the power exerted in the cylinders, is about 25 %. The dredged material is delivered into barges moored alongside. Contrasting favourably with former experience, the " Melbourne " worked for the first six months without a single breakage. She is fitted with very powerful mooring winches, a detail which is of great importance to ensure efficiency in working. The " St Austell " (See also:Plate I. fig. 3), a powerful barge-loading dredger 195 ft. long by 35 ft. 6 in. See also:beam by 13 ft. deep, fitted with twin-screw See also:compound surface-condensing propelling engines of moo i.h.p., either set of engines being available for dredging, was constructed for H.M. Dockyard, See also:Devonport, by Messrs Wm. Simons & Co. in 1896. This dredger loaded See also:thirty-five Soo-ton hopper barges in the See also:week ending See also:April 2, 1898, dredging 17,500 tons of material in the working time of 29 See also:hours 5 minutes. An instance of a still larger and more powerful dredger is the "Develant," constructed by Messrs Wm. Simons & Co., for Nicolaiev, See also:South See also:Russia. She is a See also:bow-well, barge-loading, bucket ladder dredger, with a length of 186 ft., a breadth, moulded, of 36 ft., and a depth, moulded, of 13 ft. The bucket ladder is of sufficient length to dredge 36 ft. below the water level. The buckets are exceptionally large, each having a capacity of 36 cub. ft., or fully two tons See also:weight of material, giving a lifting capacity of 1890 tons per hour. At the dredging trials 2000 tons of spoil were lifted in one hour with an expenditure of 250 i.h.p. The propelling power is supplied by one pair of compound surface-condensing marine engines of 85o i.h.p., having two cylindrical boilers constructed for a working pressure of 120 lb per sq. in. Each boiler is capable of supplying steam to either the propelling or dredging machinery, thus allowing the vessel to always have a boiler in reserve. On the trials a speed of 8a knots was obtained. The bucket ladder, which weighs over too tons, exclusive of dredgings, is raised and lowered by a set of independent engines. For manoeuvring, powerful winches driven by independent engines are placed at the bow and stern. The vessel is fitted throughout with electric light, arc lamps being provided above the deck to enable dredging to be carried on at See also:night. Steam steering gear, a repairing See also:shop, a three-ton See also:crane, and all the latest appliances are installed on See also:board. The Derocheuse " (Plate II. fig. 12), constructed by Messrs Lobnitz & Co., is a See also:good example of the dredger fitted with their patent rock cutters, as used on the See also:Suez Canal. These rock cutters consist of stamps passing down through the bottom of the dredger, slightly in advance of the bucket chain, and are employed for breakingup rock in front of the bucket ladder so that it may be raised by 564 buckets afterwards. This system of subaqueous rock cutting plant, on Messrs Lobnitz's patent system, was effectively employed in deepening the See also:Manchester See also:Ship Canal, and removed a considerable length of rock, increasing the depth of water from 26 ft. to 28 ft. at a cost of about 9d. per cub. yd. A full and illustrated description of this plant, and of a similar plant supplied to the See also:Argentine See also:Government, was published in Engineering of See also:August 17, 1906. An See also:illustration of a bucket of 54 cub. ft. capacity constructed by Messrs Lobnitz & Co. is given (Plate II. fig. x1), from which some See also:idea of the size of dredging machinery as See also:developed in See also:recent practice may be obtained. In regard to the depth of water that can be obtained by dredging, it is interesting to See also:note that the dredger " See also:Diver," constructed by Messrs. See also:Hunter & See also:English for Mr See also:Samuel See also:Williams of See also:London, is capable of working in 6o ft. of water. In this vessel an ingenious arrangement was devised by Mr Williams, by which part of the weight of the dredger was balanced while the ladder itself could be drawn up through the bucket well and placed upon the deck, enabling a long ladder to be used for a comparatively See also:short vessel. The " Tilbury " dredger, also constructed by Messrs Hunter & English, was able to dredge to a depth of 45 ft. below the surface of the water. Hopper Barges.—To receive the materials excavated by barge-loading dredgers, steam hopper barges are now generally employed, capable of carrying 500 tons or more of excavation and of steaming loaded at a speed of about 9 M. per hour. These hopper barges are made with hinged flaps in their bottoms, which can be opened when the place of deposit is reached and the dredgings easily and quickly discharged. Good examples of these vessels are the two steam hopper barges built for the Conservators of the river Thames in 1898. The dimensions are: length Igo ft., breadth 30 ft., depth 13 ft. 3 in., hopper capacity 900 tons. They are propelled by a set of triple expansion engines of 1200 i.h.p., with two return-See also:tube boilers having a working pressure of 16o lb. Special appliances are provided to work the hopper doors by steam power from independent engines placed at the forward end of the hopper. A steam windlass is fixed forward and a steam See also:capstan aft. The vessels are fitted with cabins for the See also:officers and See also:crew. On their trial trip, the hoppers having their full load, a speed of 11 knots was obtained, the See also:coal See also:consumption being 1.44 lb per i.h.p. Methods of Dredging.—In river dredging two systems are pursued. One plan consists in excavating a See also:series of longitudinal furrows parallel to the See also:axis of the stream; the other in dredging See also:cross furrows from side to side of the river. It is found that inequalities are See also:left between the longitudinal furrows when that system is practised, which do not occur, to the same extent, in side or cross dredging; and cross dredging leaves a more See also:uniform bottom. In either See also:case the dredger is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredgers are wound round windlasses worked by the engine, so that the vessel can be moved ahead or astern by simply throwing them into or out of gear. In longitudinal dredging the vessel is worked forward by the head chain, while the buckets are at the same time performing the excavation, so that a longitudinal See also:trench is made in the bottom of the river. After proceeding a certain length, the dredger is stopped and permitted to drop down and commence a new longitudinal furrow, parallel to the first one. In cross dredging, on the other hand, the vessel is supplied with four additional moorings, two on each side, and these chains are, like the head and stern'chains, wound round barrels worked by steam power. In cross dredging we may suppose the vessel to be moored at one side of the channel to be excavated. The bucket frame is set in See also:motion, but instead of the dredger being drawn forward by the head chain, she is drawn across the river by the starboard chains, and, having reached the extent of her work in that direction, she is then drawn a few feet forward by the head chain, and the bucket frame being still in motion the vessel is hauled across by the See also:port chains to the side whence she started. By means of this transverse motion of the dredger a series of cross cuts is made; the dredger takes out the whole excavation from side to side to a uniform depth and leaves no protuberances such as are found to exist between the furrows in longitudinal dredging, even when it is executed with great care. The two systems will be understood by reference to fig. 1, where A and B are thehead and stern moorings, and C, D, E and F the side moorings. The arc e f represents the course of the vessel in cross dredging; e... ,:fin%yP~rl? yeugtt:, .as while in longitudinal dredging, as already explained, she is drawn forward towards A, and again dropped down to commence a new longitudinal furrow. Hopper Dredgers.—In places where barge-loading dredgers are inconvenient, owing to confined space and interference with See also:navigation, and where it is necessary to curtail capital expenditure, hopper dredgers are convenient and economical. These dredgers were first constructed by Messrs. Wm. Simons & Co. of See also:Renfrew, who patented and constructed what they See also:call the " Hopper Dredger," combining in itself the advantages of a dredger for raising material and a scow hopper vessel for conveying it to the place of See also:discharge, both of which services are performed by the same engines and the same crew. The vessel for this type of dredger is made of sufficient length and floating capacity to contain its own dredgings, which it carries out to the depositing ground as soon as its hopper is full. Considerable time is of course occupied in slipping and recovering moorings, and conveying material to the depositing ground, but these disadvantages are in many instances counterbalanced by the fact that less capital is required for plant and that less See also:room is taken up by the dredger. If the depositing ground is far away, the time available for dredging is much curtailed, but the four-screw hopper dredger constructed by Messrs Wm. Simons & Co. for See also:Bristol has done good work at the cost of 5d. per ton, including wages, See also:repairs, coals, grease, sundries and See also:interest on the first cost of the plant, notwithstanding that the material has to be taken 10 m. from the Bristol See also:Dock. She can lift 400 tons of stiff clay per hour from a depth of 36 ft. below the water See also:line, and the power required varies from 120 i.h.p. to 150 i.h.p., according to the nature of the material. The speed is 9 knots, and 4 propellers are provided, two at the head and two at the stern, to enable the vessel to steam equally well either way, as the river See also:Avon is too narrow to permit her to be turned round. The hopper dredger "La Puissante " (Plate I. fig. 4), constructed by Messrs Wm. Simons & Co. for the Suez Canal Co. for the improvement of Port Said Roads, is a See also:fine example of this class of dredger. She is 275 ft. long by 47 ft. beam by 19 ft. deep. The hopper capacity is 2000 tons, and the draught loaded 16 ft. 5 in. The maximum dredging depth is 40 ft., and the minimum dredging depth is only limited by the vessel's draught, she being able to cut her own way. The bucket ladder works through the well in the stern and weighs with buckets 120 tons. The buckets have each a capacity of 30 cub. ft. and raised on trial ',Soo tons per hour. The dredger is propelled by two sets of independent triple expansion surface-condensing engines of 1800 i.h.p. combined, working with steam at 160 lb pressure, supplied by two mild steel multitubular boilers. Each set of engines is capable of driving the buckets independently at speeds of 16 and 20 buckets per minute. The bucket ladder is fitted with buffer springs at its upper end to lessen the See also:shock when working in a sea-way. The dredger can deliver the dredged material either into its own hopper or into barges lying on either side. The vessel obtained a speed of 91 knots per hour on trial. The coal consumption durin 6 hours' steaming trial was 1.66 lb per i.h.p. hour. Fig. 9 (Plate I.I shows a still larger hopper dredger by the same constructors. Dredgers fitted with Long Shoot or See also:Shore Delivering A pparatus.—The first instance of dredgers being fitted with long shoots was in the Suez Canal. The See also:soil in the lakes was very variable, the surface being generally loose mud which See also:lay in some places in the See also:sand, but frequently more or less on hard clay. Resort was had to shoots 230 ft. long, supported on pontoons connected with the hull of the dredger. The sand flowed away with a moderate supply of water to the shoots when they were fixed at an inclination of about 1 in 20, but when the sand was mixed with shells these formed a coating which prevented the stream of water from washing out the shoot, and even with an inclination of r in 10 material could not be delivered. A pair of endless chains working down the long shoot overcame the difficulty, and also enabled hard clay in lumps to be dealt with. One dredger turned out about 2000 cub. yds. of thick clay in 15 hours, and when the clay was not hard it could deliver 150,000 cub. yds. in a See also:month for several consecutive months. Shore delivery has been successfully effected by raising the material by buckets in the See also:ordinary way and delivering it into a See also:vertical See also:cylinder connected with floating jointed pipes through which the dredgings pass to the shore. This, of course, can only be done where the place of deposit is near the spot where the material is dredged. Two plans have been satisfactorily employed for this operation. At the See also:Amsterdam Canal the stuff was discharged from the buckets into a vertical cylinder, and after being mingled with water by a revolving See also:Woodford See also:pump was sent off under a head of pressure of 4 or 5 ft. to the place of deposit in a semi-fluid state through pipes made of See also:timber, hooped with iron. These wooden pipes were made in lengths of about 15 ft., connected with leather See also:joints, and floated on the surface of the water. A somewhat similar process was also employed on the Suez Canal. A dredger (Plate I. fig. 5), constructed by Messrs Hunter & English for reclamation works on See also:Lake Copais in See also:Greece was fitted with de-See also:livery belts See also:running on rollers in steel lattice frames on each side of the vessel supported by masts and See also:ropes. It could deliver too cub. metres per hour at 85 ft. from the centre of the dredger, at a cost of 1•82d. per cub. ,See also:metre for working expenses, with coal at 45s. per ton, including o•66d. per cub. metre for renewal of belts, upon which the wear and See also:tear was heavy. Another instance of the successful application of shore delivery apparatus is that of a dredger for Lake Titicaca, See also:Peru, constructed by Messrs Hunter & English, which was fitted with long shoots on both sides, conveying the dredged material about too ft. from the centre of the dredger upon either side. The shoots were supported by shear-legs and ropes, and were supplied with water from a centrifugal pump in the engine room. This dredger could excavate and deliver 120 cub. yds. per hour at a cost of 1.725d. per cub. yd. with coal costing 4os. per ton. If coal had been available at the ordinary rate in See also:England of 2os. per ton, the cost of the dredging and delivery would have been o•82d. per cub. yd. for wages, coal, oil, &c., but not including the See also:salary of the See also:superintendent. An interesting example of a shore delivering dredger is a light draught dredger constructed by Messrs Hunter & English for the Lakes of Albufera at the mouth of the river See also:Ebro in See also:Spain (Plate I. fig. 6). The conditions laid down for this dredger were that it should See also:float in 18 in. of water and deliver the dredged material at 90 ft. from the centre of its own hull. In See also:order to meet these requirements the vessel was made of steel plates S in. thick, and longitudinal girders from end to end of the vessel, the upward strain of flotation being conveyed to them from the skin plating by transverse bulk-heads at short intervals. The dredger was 94 ft. long, 25 ft. wide, and 3 ft. deep, and the height of the top tumbler above the water was 25 ft. When completed the dredger See also:drew 17 in. of water. The dredgings were delivered by the buckets upon an endless See also:belt, driven from the main compound surface-condensing engine, which ran over pulleys supported upon a steel lattice girder, the See also:outer end of which rested upon an independent See also:pontoon. This belt delivered the dredgings at 90 ft. from the centre of the dredger round an arc of 1800. The dredger delivered 125 cub. yds. per hour of compact clay at a cost of 1.16d. per cub. yd. or o•86d. per ton for wages, coal and stores. Another method of delivering dredgings is that of pneumatic delivery, introduced by Mr F. E. Duckham, of the Millwall Dock Co., by which the dredgings are delivered into cylindrical tanks in the dredger, closed by See also:air-tight doors, and are expelled by compressed air either into the sea or through long pipes to the land. The Millwall Dock dredger is 113 ft. long, with a beam of 17 ft. and a depth of 12 ft. The draught loaded is 8 ft. It contains two cylindrical tanks, having a combined capacity of 240 cub. yds., and is fitted with compound engines of about 200 i.h.p., with a 20 in. air-compressing cylinder. The discharge See also:pipe is 15 in. diameter by 150 yds. long. The nozzles of the air-injection pipes must not be too small, otherwise the compressed air, instead of driving out the material, simply pierces holes through it and escapes through the discharging pipe, carrying with it all the liquid and thin material in the tanks. The cost of working the Millwall Dock dredger is given by Mr Duckham at 1.75d. per cub. yd. of mud lifted, conveyed and deposited on land 450 ft. from the water-side, for working expenses only. This dredger is believed to be the first machine constructed with a traversing ladder, as suggested by See also:Captain See also:Gibson when dock-See also:master of the Millwall Docks.
See also:Blasting combined with Dredging.—In some cases it has been found that the bottom is too hard to be dredged until it has been to some extent loosened and broken up. Thus at See also:Newry, See also: The cost of blasting and dredging the See also:marl beds is given as being 4S. per cub. yd. A See also:combination of blasting and dredging was employed in 1875 by John See also:Fowler of See also:Stockton at the river See also:Tees. The See also:chief novelty was in the barge upon which the machinery was fixed. It was 58 ft. by 28 ft. by 4 ft., and had eight legs which were let down when the barge was in position. The legs were then fixed to the barge, so that on the tide falling it became a fixed See also:platform from which the drilling was done. Holes were bored and charged, and when the tide See also:rose the legs were heaved up and the barge removed, after which the shots were discharged. There were 24 See also:boring tubes on the barge, and that was the limit which could at any time be done in one tide. The See also:area over which the blasting was done measured 500 yds. in length by 200 in breadth, a small part being uncovered at low water. The depth obtained in mid-channel was 14 ft. at low water, the See also:average depth of rock blasted being about 4 ft. 6 in. The holes, which were bored with the See also:diamond See also:drill, varied in depth from 7 to 9 ft., the distance between them being 10 ft. See also:Dynamite in See also:tin canisters fired by patent fuse was used as the explosive, the charges being 2 lb and under. The rock is oolite shale of variable hardness, and the average time occupied in drilling holes 5 ft. deep was 12 minutes. The dredger raised the blasted rock. The cost for blasting, lifting and discharging at sea was about 4s. per cub. yd., including interest on dredging and other plant employed. The dredger sometimes worked a See also:face of blasted material of from 7 to 8 ft. The quantity blasted was 110,000 cub. yds., and the See also:contract for blasting so as to be lifted by the dredger was 3s. Id. per cub, yd. A similar plan was adopted at See also:Blyth Harbour (see Proc. Inst. C.E. vol. 81, p. 302). The cost of the See also:explosives per cub. yd. was Is. 4d., of boring Is. 9d. per cub. yd., and of dredging 3S. per cub. yd., including repairs, but nothing for the use of plant. The whole cost worked out at 6s. Id. per cub. yd. on the average. Sand-pump Dredgers.—Perhaps the most important development which has taken place in dredging during recent years has been the employment of sand-pump dredgers, which are very useful for removing sandy bars where the particular See also:object is to remove quickly a large quantity of sand or other soft material. They are, however, See also:apt to make large holes, and are therefore not fitted for positions where it is necessary to finish off the dredging work to a uniform See also:flat bottom, for which purpose bucket dredgers are better adapted. Pump dredgers are, how ever, admirable and economical See also:machines for carrying out the work for which they are specially suited. In the discussion upon Mr J. J. Webster's paper upon " Dredging-Appliances " (Prot. Inst. C.E. vol. 89) at the Institution of Civil Engineers in 1886, Sir John Coode stated that he had first seen sand-pump dredgers at the mouth of the See also:Maas in Holland. The centrifugal pump was placed against the bulkheads in the after part of the vessel, and the sand and water were delivered into a See also:horizontal breeches-piece leading into two pipes running along the full length of the hopper. The difficulty of preventing the sand from running overboard was entirely obviated by its being propelled by the pump through these pipes, the bottoms of which were perforated by a series of holes. In addition, there were a few small flap-doors fixed at intervals, by means of which the men were able to regulate the discharge. On being tested, the See also:craft pumped into its hopper 400 tons of sand in 22 minutes. The coamings round the well of the hoppers were constructed with a See also:dip, and when the hopper was full the water ran over in a steady stream on either side. The proportion of sand delivered into the hopper was about 20 % of the See also:total capacity of the pump. The dredger was constructed by Messrs 566 Smit of Kinderdijk, near See also:Rotterdam. In the same discussion Mr A. A. See also:Langley, then engineer to the Great Eastern railway, gave particulars of a sand pump upon the See also:Bazin system, which had been used successfully at See also:Lowestoft. The See also:boat was 6o ft. long by 20 ft. wide, and the pump was 2 ft. in diameter, with a two-bladed disk. The discharge pipe was 12 in. in diameter. The pump raised 400 tons of sand, gravel and stones per hour as a maximum quantity, the average quantity being about 200 tons per hour. The depth dredged was from 7 ft. to 25 ft. The pump was driven by a See also:double-cylinder engine, having cylinders of 9 in. diameter by io in. stroke, and making 120 revolutions per minute. An important improvement was made by fitting the working faces of the pump with See also:india-See also:rubber, which was very successful and largely reduced the wear and tear. The cost of the dredging at Lowestoft was given by Mr Langley at 2d. per ton, including delivery 2 M. out at sea. The quantity dredged was about 200,000 tons per annum. One of the earliest pumps to be applied to dredging purposes was the Woodford, which consisted of a horizontal disk with two or more arms working in a case somewhat similar to the ordinary centrifugal pump. The disk was keyed to a vertical shaft which was driven from above by means of belts or other gear coupled to an ordinary portable engine. The pump within rested on the ground ; the suction pipe was so arranged that water was drawn in with the sand or mud, the proportions being regulated to suit the quality of the material. The discharge pipe was rectangular and carried a vertical shaft, the whole apparatus being adjustable to suit different depths of water. This arrangement was very effective, and has been used on many works. Burt & See also:Freeman's sand pump, a modification of the Woodford pump, was used in the construction of the Amster-See also:dam Ship Canal, for which it was designed. The excavations from the canal had to be deposited on the See also:banks some distance away from the dredgers, and after being raised by the ordinary bucket dredger, instead of being discharged into the barges, they were led into a vertical chamber on the top side of the pump, suitable arrangements being made for regulating the delivery. The pump was 3z ft. in diameter, and made about 230 revolutions per minute. The water was drawn up on the bottom side and mixed with the descending mud on the top side, and the two were discharged into a pipe 15 in. in diameter. The discharge pipe was a special feature, and consisted of a series of wooden pipes jointed together with leather hinges and floated on buoys from the dredger to the bank. In some cases this pipe was 300 yds. long, and discharged the material 8 ft. above the water level. Each dredger and pump was capable of discharging an average of 1500 cub. yds. per See also:day Of 12 hours. See also:Schmidt's sand pump is claimed to be an improvement on the Burt & Freeman pump. It consists of a revolving See also:wheel 6 ft. in diameter, with cutters revolving under a See also:hood which just allows the water to pass underneath. To the top side of the hood a 20 in. suction pipe from an ordinary centrifugal pump is attached. The pump is driven by two • 16 in. by 20 in. cylinders, at 134 revolutions per minute, the boiler pressure being 95 lb per sq. in. This apparatus is capable of excavating sticky See also:blue clayey mud, and will deliver the material at 500 to 65o yds. distance. The best results are obtained when the mixture of mud and water is as 1 to 6.5. The average quantity excavated per diem by the apparatus is 1300 cub, yds., the maximum quantity being 2500 cub. yds. Kennard's sand pump is entirely different from the pumps already described, and is a See also:direct application of the ordinary lift pump. A wrought iron box has a suction pipe fitted at the bottom, rising about See also:half way up the inside of the box; on the top of the box is fitted the actual pump and the flap valves. The apparatus is lowered by chains, and the pump lowered from above. As soon as the box is filled with sand it is raised, the catches holding up the bottom released, and the contents discharged into a See also:punt. Sand-pump dredgers, designed and arranged by Mr Darnton See also:Hutton, were extensively used on the Amsterdam Ship Canal. A centrifugal pump with a See also:fan 4 ft. in diameter was employed, the suction and delivery pipes, each 18 in. in diameter, being attached to an open wrought-iron framework. The machine was suspended between guides fixed to the end of the vessel, which was fitted with tackle for raising, lowering and adjusting the machine. The vessel was fitted with a steam engine and boiler for working and manipulating the pumps and the heavy side chains for the guidance of the dredger. The engine was 70 h.p., and the total cost of one dredger was X8000. The number of hands required for working this sand-pump dredger was one captain, one engineer, one stoker and four sailors. Each machine was capable of raising about 1300 tons of material per day, the engines working at 6o and the pump at 18o revolutions per minute. The sand was delivered into barges along-side the dredger. The cost of raising the material and depositing it in barges was about Id. per ton when the sand pumps were working, but upon the year's work the cost was 2.4d. per cub. yd. for working expenses and repairs, and 1.24d. per cub. yd. for interest and depreciation at io% upon the cost of the plant, making a total cost for dredging of 3.64d. per cub. yd. The cost for transport was 3.588d. per cub. yd., making a total cost for dredging and transport of 7.234d. per cub. yd. Dredging and transport on the same works by an ordinary bucket dredger and barges cost 8.328d. per cub. yd. Two of the largest and most successful instances of sand-pump dredgers are the ` Brancker " and the " G. B. See also:Crow," belongingto the Mersey Docks and Harbour Board. Mr A. G. Lyster gave particulars of the work done by these dredgers in a paper read before the Engineering Congresein 1899. They.are each 320 ft. long, 47 ft. wide and 20.5 ft. deep, the draught loaded being 16 ft. They are fitted with two centrifugal pumps, each 6 ft. in diameter, with 36 in. suction and delivery pipes, See also:united into a 45 in. diameter pipe, hung by a See also:ball and socket See also:joint in a trunn' xi, so as to work safely in a sea-way when the waves are io ft. high. The suction pipe is 76 ft. long and will dredge in 53 ft. of water. The eight hoppers hold 3000 tons, See also:equivalent when solid to 2000 cub. yds. ; they can be filled in three-quarters of an hour and discharged in five minutes. Mr Lyster stated that up to May 1899, the quantity removed from bar and main-channel shoals amounted to 41,240,360 tons, giving awidth of channel of 1500 ft. through the bar, with a minimum depth of 27 ft. The cost of dredging on the bar by the " G. B. Crow " during 1898, when 4,309,350 tons of material were removed, was o•61d. per ton for wages, supplies and repair's. These figures include all direct working See also:costs and a proportion of the See also:charge for actual superintendence, but no See also:allowance for interest on capital cost or depreciation. On an average, 20% of the sand and mud that are raised escapes over the side of the vessel. Mr Lyster has, however. to a considerable extent overcome this difficulty by a special arrangement added to the hoppers (see Psnc. Inst. C.E. vol. 188).
At the Engineering See also:Conference, 1907, Mr Lyster read a note in which he stated that the total quantity of material removed from the bar of the Mersey, from the See also:Crosby channel, and from other points of the main channel by the " G. B. Crow " and " Brancker " suction dredgers amounted to 108,675,570 tons up to the 1st of May 1907. " In the note of 1899 (he added) it was pointed out that the Mersey was a striking instance of the improvement of a river by dredging rather than by permanent works, and the See also:economy of the system as well as the See also:advantage which its See also:elasticity and adaptability to varying circumstances permit, was pointed out. .
The most recent experience, which has resulted in the See also:adoption of the proposal to revet the See also: 7 in. Her dredging machinery consists of four centrifugal pumps driven by four sets of inverted triple expansion engines, and connected to four suction tubes 90 ft. long and 42 in. in See also:internal diameter. Her.propelling machinery, consisting of two sets of triple expansion engines, is capable of driving her at a speed of io knots. Another powerful and successful sand-pump dredger, " Kate " (Plate I. fig. 7), was built in 1897 by Messrs Wm. Simons & Co. Ltd. for the See also:East London Harbour Board, South See also:Africa. Its dimensions are: length 200 ft., breadth 39 ft., depth 14 ft. 6 in., hopper capacity moo tons. The pumping arrangements for filling the hopper with sand or discharging overboard consist of two centrifugal pumps, each driven from one of the propelling engines. The suction pipes are each 27 in. in diameter, and are so arranged that they may be used for pumping either forward or aft, as the state of the See also:weather may require. Four steam See also:cranes are provided for manipulating the suction pipes. Owing to the exceptional weather with which the vessel had to contend, special precautions were taken in designing the attachments of the suction pipes to the vessel. The See also:attachment is above deck and consists of a series of joints, which give a perfectly free and universal See also:movement to the upper ends of the pipes. The joints, on each side of the vessel, are attached to a See also:carriage, which is traversed laterally by hydraulic gear. By this means the pipes are pushed out well clear of the vessel's sides when pumping, and brought inboard when not in work. Hydraulic cushioning cylinders are provided to give any required resistance to the fore and aft movements of the pipes. When the vessel arrived at East London on the 18th of See also:July 1897, there was a depth of 14 ft. on the bar at high tide. On the loth of October, scarcely three months afterwards, there was a depth of 20 ft. on the bar at low water. Working 22 days in rough weather during the month of See also:November 1898, the " Kate " raised and deposited 21 M. at sea 60,000 tons of dredgings. Her best day's work (12 hours) was on the 7th of November, when she dredged and deposited 6440 tons. A large quantity of sand-pump dredging has been carried out at See also:Boulogne and See also:Calais by steam hopper pump dredgers, workable when the head waves are not more than 3 ft. high and the cross waves not more than 11 ft. high. The dredgings are taken 2 M. to sea, and the See also:price for dredging and depositing from 800,000 to 900,000 cub. metres in 5 or 6 years was 7.25d. per cub. yd. The contractor offered to do the work at 4.625d. per cub. yd. on See also:condition of being allowed to work either at Calais or Boulogne, as the weather might permit. Sand-pump dredging has also been extensively carried out at the mouth of the ports of Amsterdam, Rotterdam and on the See also:north See also:coast of See also:France by sand dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. The largest dredger, the Amsterdam," is 141 ft. by 27 ft. by to ft. 8 in., and has engines of 190 i.h.p. The hopper capacity is to,600 cub. ft., and the vessel can carry 600 tons of dredgings. The pump fan is 6 ft. 3 in. in diameter by to in. wide, the plates being of wrought iron, and makes 130 revolutions a minute. The pump can raise 230 cub. ft. a minute from a depth of 33 ft., which, taking the proportion of 1 of sand to 7 of water, gives a delivery of 29 cub. ft. of sand per minute. The hopper containing 10,600 cub. ft. was under favourable circumstances filled in 40 minutes. The vessels are excellent sea boats. Combined Bucket-Ladder and Sand-Pump Dredgers.—Bucket ladders and sand pumps have also been fitted to the same dredger. A successful example of this practice is furnished by the hopper dredger " See also:Percy See also:Sanderson " (Plate I. fig. 8), constructed under the direction of Sir C. A. See also:Hartley, engineer of the See also:Danube See also:Commission for the deepening of the river Danube and the See also:Sulina bar. This dredger is 220 ft. by 40 ft. by 17 ft. 2 in., and has a hopper capacity for 125o tons of dredgings. The buckets have each a capacity of 25 cub. ft., and are able to raise See also:I000 tons of ordinary material per hour. The suction pump, which is driven by an independent set of triple expansion engines, is capable of raising 700 tons of sand per hour, and of dredging to a depth of 35 ft. below the water-line. The lower end of the suction pipe is controlled by special steam appliances by which the pipe can be brought entirely inboard. The " Percy Sanderson " raises and deposits on an average 5000 tons of material per day. Grab Dredgers.—The grab dredger was stated by Sir See also:Benjamin See also:Baker (Prot. Inst. C.E. vol. 113, p. 38) to have been invented by Gouffe in 1703, and was worked by two ropes and a bar. Various kinds of apparatus have been designed in the shape of grabs or buckets for dredging purposes. These are usually worked by a steam crane, which lets the open grab down to the surface of the ground to be excavated and then closes it by a chain which forces the tines into the ground; the grab is then raised by the crane, which deposits the contents either into the hopper of the vessel upon which the crane is fixed or into another barge. The Priestman grab has perhaps been more extensively used than any other apparatus of this sort. It is very useful for excavating mud, gravel and soft sand, but is less effective with hard sand or stiff clay—a See also:general defect in this class of dredger. It is also capable of lifting large loose pieces of rock weighing from I to 2 tons. A dredger of this type, with grab holding t ton of mud, dredged during six days, in 19 ft. of water, an average of 521 tons and a maximum of 681 tons per hour, and during 12 days, in 16 ft. of water, an average of 48 tons and a maximum of 58 tons per hour, at a cost of I.63d. per ton, excluding interest on the capital and depreciation. The largest dredger to which this apparatus has been applied is the grab bucket hopper dredger " See also:Miles K. See also:Burton " (Plate I. fig. 9), belonging to the Mersey Docks and Harbour Board. It is equipped with 5 grabs on See also:Morgan's patent system, which is a modification of Priestman's, the grabs being worked by 5 hydraulic cranes. It raised and deposited, 12 to 15 M. at sea, 11 loads of about 1450 tons each with a double shift of hands, at a cost of about Is. 5d. per cub. yd. of spoil, including the working expenses for wages of crew, See also:fuel and stores. Mr R. A. Marillier of Hull has stated that "the efficiency of these grabs is not at all dependent upon the force of the See also:blow in falling for the penetration and grip in the material, as they do their work very satisfactorily even when lowered quite gently on to the material to be cut out, the jaws being so framed as to draw down and penetrate the material as soon as the upward strain is put on the lifting chain. Even in hard material the jaws penetrate. so thoroughly as to cause the bucket to be well filled. The grab is found to work successfully in excavating hard clay from its natural See also:bed on dry land." It is claimed on behalf of grabs that they lift a smaller proportion of water than any other class of dredger. Since the beginning of the loth See also:century considerable advance has been made in the use of Priestman grabs, not only for dredging and excavating (for which work they were originally designed), but also in discharging bulk See also:cargo. The first quadruple dredger used by the See also:Liverpool Docks Board had grabs of a capacity of 30 cub. ft., but subsequently second and third quadruple dredgers were put to work in the Liverpool Docks, with grabs having a capacity of 70 and too cub. ft. respectively. In discharging coal at See also:Southampton, See also:Havre, See also:Erith, as well as at the coaling station at Purfleet on the Thames, grabs having a capacity of about 8o cub. ft. are in See also:constant use. Perhaps the most difficult kind of bulk cargo to lift is " Narvick " iron ore, which sets into a semi-solid body in the holds of the vessels, and for this purpose one of.the largest grabs, having about 15o cub. ft. capacity and weighing about 8 tons, has been adopted. This grab was designed as a result of experiments extending over a long period in lifting iron ore. It is fitted with long, forged, interlocked steel See also:teeth for penetrating the compact material, which is very costly to remove by hand labour. The Priestman grab is made to work with either one or two chains or See also:wire ropes. Grabs worked with two chains or ropes have many advantages, and are therefore adopted for large undertakings. See also:Wild's single chain half-tine grab works entirely with a single chain, and has been found very useful in excavating the cylinders in Castries harbour. Upon experimenting with an ordinary grab a rather curious condition of things was observed with respect to sinking. On penetrating the soil to a certain depth the ground was found as it were nested, and nothing would induce the grab to sink lower. Sir W. Matthews suggested that a further set of See also:external tines might possibly get over this difficulty. A new grab having been made with this modification, and also with a large increase of weight—all the parts being of steel—it descended to any required depth with ease, the outside tines loosening the ground effectually whilst the inside bucket or tines picked up the material. See also:Miscellaneous Appliances.—There are several machines or appliances which perhaps can hardly be called dredgers, although they are used for cleansing and deepening rivers and harbours. Kingfoot's dredger, used for cleansing the river See also:Stour, consisted of a boat with a broad See also:rake fitted to the bow, capable of See also:adjustment to different depths. At the sides of the boat were hinged two wings of the same depth as the rake and in a line with it. When the rake was dropped to the bottom of the river and the wings extended to the side, they formed a sort of temporary dam, and the water began to rise gradually. As soon as a sufficient head was raised, varying from 6 to 12 in., the whole machine was driven forward by the pressure, and the rake carried the mud with it. Progress at the rate of about 3 M. an hour was made in this manner, and to prevent the See also:accumulation of the dredgings, operations were begun at the mouth of the river and carried on backwards. The apparatus was See also:vet.; effective and the river was cleansed thoroughly, but the distance travelled by the dredger must have been great. In 1876 J. J. Rietschoten designed a " propeller dredger " for removing the shoals of the river Maas. It consisted of an old See also:gun-boat fitted with a pair of trussed beams, one at each side, each of which carried a steel shaft and was capable of being lowered or raised by means of a crab. An ordinary propeller 3 ft. 6 in. in diameter was fixed to the lower end of the shaft, and driven by See also:bevel gear from a cross shaft which derived its motion by belting from the See also:fly-wheel of a 12 h.p. portable engine. The propellers were lowered until they nearly reached the shoals, and were then worked at 150 revolutions per minute. This operation scoured away the shoal effectively, for in about 40 minutes it had been lowered about 3 ft. for a space of 150 yds. long by 8 yds. wide. A. Lavalley in 1877 designed an arrangement for the harbour of See also:Dunkirk to overcome the difficulty of working an ordinary bucket-ladder dredger when there is even a small swell. A pump injects water into the sand down a pipe terminating in three nozzles to stir up the sand, and another centrifugal pump draws up the mixed sand and water and discharges it into a hopper, the pumps and all machinery being on board the hopper. To allow for the rising and falling of the vessel—either by the See also:action of the tide or by the swell —the ends of the pipes are made flexible. The hopper has a capacity of 190 cub. yds., and is propelled and the pumps worked by an engine of 150 i.h.p. From 50 to 8o cub. yds. per hour can be raised by this dredger. The " Aquamotrice," designed by Popie, and used on the See also:Garonne at See also:Agen, appears to be a modification of the old bag and spoon arrangement. A flat-bottomed boat 511 ft. long by 61 ft. wide was fitted at the bow with paddles, which were actuated by the tide. Connected with the paddles was a long chain, passing over a See also:pulley on uprights and under a See also:roller, and a beam was attached to the chain 14 ft. 8 in. long, passing through a hole in the deck. At the end of the beam was an iron See also:scoop 2 ft. wide and 2 ft. 6 in. deep. When the tide was strong enough it drew the scoop along by means of the paddles and chains, and the scoop when filled was opened by a See also:lever and discharged. About 65 cub. yds. of gravel could be raised by the apparatus in 12 hours. When the tide failed the apparatus was worked by men. The Danube Steam Navigation Co. removed the See also:shingle in the shallow parts of the river by means of a triangular rake with wrought-iron sides 18 ft. long, and fitted with 34 teeth of chilled See also:cast iron 12 in. deep. This rake was hung from the bow of a steamer 18o ft. long by 21 ft. beam, and dragged across the shallows, increasing the depth of water in one instance from 5 ft. 6 in. to 9 ft., after passing over the bank 355 times. A combination of a See also:harrow and high pressure water jets, arranged by B. Tydeman, was found very efficacious in removing a large quantity of mud which accumulated in the Tilbury Dock See also:basin, which has an area of about 17 acres, with a depth of 26 ft. at low-water See also:spring tides. In the first instance chain harrows merely were used, but the addition of the water jets added materially to the success of the operation. The system accomplished in six tides more than was done in twelve tides without the water jets which worked at about 8o lb pressure per sq. in. at the bottom of the dock. Ive's excavator consists of a long weighted See also:spear, with a sort of See also:spade at the end of it. The spade is hinged at the top, and is capable of being turned at right angles to the spear by a chain attached to the end of the spear. The spade is driven into the ground, and after releasing the catch which holds it in position during its descent, it is drawn up at right angles to the spear by the chain, carrying the material with it. Milroy's excavator is similar, but instead of having only one spade it generally has eight, united to the periphery of an octagonal iron frame fixed to a central vertical See also:rod. When these eight spades are drawn up by means of chains, they form one flat table or See also:tray at right angles to the central rod. In operation the spades hang vertically, and are dropped into the material to be excavated; the chains are then drawn up, and the table thus formed holds the material on the top, which is lifted and discharged byy releasing the spade. This apparatus has been extensively used both in Great Britain and in India for excavating in See also:bridge cylinders. The clam See also:shell dredger consists of two hinged buckets, which when closed form one semi-cylindrical bucket. The buckets are held open by chains attached to the top of a cross-head, and the machine is dropped on to the top of the material to be dredged. The chains holding the bucket open are then released, while the spears are held firmly in position, the buckets being closed by another chain. See also:Bull's dredger, Gatmell's excavator, and Fouracre's dredger are modifications with improvements of the clam shell dredger, and have all been used successfully upon various works. See also:Bruce & Batho's dredger, when closed, is of hemispherical form, the bucket being composed of three or four See also:blades. It can be worked by either a single chain or by means of a spear, the latter being generally used for stiff material. The advantage of this form of dredger bucket is that the steel points of the blades are well adapted for penetrating hard material. Messrs Bruce & Batho also designed a dredger consisting of one of these buckets, but worked entirely by hydraulic power. This was made for working on the See also:Tyne. The excavator or dredger is fixed to the end of a beam which is actuated by two hydraulic cylinders, one being used for raising the bucket and the other for lowering it; the hydraulic power is supplied by the pumps in the engine-room. The novelty in the design is the ingenious way in which the lever in ascending draws the shoot under the bucket to receive its contents, and draws away again as the bucket descends. The hydraulic cylinder at the end of the beam is carried on gimbals to allow for irregularities on the surface being dredged. The hydraulic pressure is 700 lb per sq. in., and the pumps are used in connexion with a steam See also:accumulator. An unloading apparatus was designed by Mr A. See also:Manning for the East & See also:West India Dock Co. for unloading the dredged materials out of barges and delivering it on the See also:marsh at the back of the bank of the river Thames at Crossness, See also:Kent. A See also:stage constructed of wooden piles commanded a series of barge beds, and the unloading dredger running from end to end of the stage, lifted and delivered the materials on the marsh behind the river wall at the cost of i d. per cub. yd. Dredging on the River See also:Scheldt below See also:Antwerp.—This dredging took place at Krankeloon and the Belgian Sluis under the direction of L. See also:Van Gansberghe. At Melsele there is a pronounced See also:bend in the river, causing a bar at the Pass of Port See also: 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 tug-boats 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 See also:complete See also: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 See also: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 See also: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. See also: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 See also: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 21 m., six voyages were made in a day, and the day's work amounted to 3530 cub. yds. See also:Gold Dredgers.—Dredgers for excavating from river beds soil containing gold are generally fitted with a See also: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 (Prot. Inst. C.E. vol. 89), for much valuable See also: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. ) 2. MARINE BIOLOGY The naturalist's dredge is an See also: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 See also:plants living on or near the sea-bottom, or sometimes of specimens of the sea-bottom itself, for scientific purposes. Until the See also:middle of the 18th century, naturalists who studied the marine See also:fauna and See also: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 See also:capture of certain edible See also: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 See also: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 See also:base a flat bar called the shere or See also:lip, which is sloped a little, not perpendicular to the See also: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 See also: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 See also:foot. The rings vary in size, but are usually some 21 in. in diameter. The weight is about 6o lb. This dredge was soon abandoned: its weight was prohibitive for small boats, from which the naturalist usually worked, its wide rings allowed See also: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. More-over, it sometimes See also: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.
See also:Otho See also: As any one of these everted lips could See also: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 See also:Denmark and See also:Norway which was published with excellent !coloured plates in 1778; and See also: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 See also:Dublin was introduced. It has been used all over the See also:world, and is so apt for its purpose that it has suffered very little modification during its 70 years of See also: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 dredge in the slit-like shape of the opening, which prevents much of the " washing out " suffered by the earlier See also: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 See also:Ito to the plane of the mouth, and in some later patterns See also: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 See also: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, See also: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 See also:cord or strong See also:copper wire. Various materials have been used for the bag, the chief of which are hide, See also: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 " See also: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 " See also:Albatross " for obtaining a See also: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 See also:bare in. diameter; bag rather more than t ft. long. These small dredges were used from See also: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 i ft. 3 in. and the bag had a length of 4 ft. 6 in.; the " See also: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 See also:Europe, at depths attainable from a See also:row-boat or See also:yawl, probably the best kind of line is See also:bolt-rope of the best See also:Russian See also:hemp, not less than 11 in. in circumference, containing 18 to 20 yarns in 3 strands. Each See also: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 See also: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 See also: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 See also:geographical See also:distribution, if it be accompanied by an accurate note of the bottom temperature. For depths under too 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 See also:bench or See also:rowlock-See also: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 See also:sail before a light See also: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 See also: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 See also:ice. In the Trondligem See also:Fjord, See also:Canon A. M. See also: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 See also:Antarctic expedition of the " Discovery." See also: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 See also:store the objects of See also: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 See also: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 See also:identification in jars or tubs of sea-water, of which there should be a number secured in some form of See also:bottle See also:basket, See also: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 See also:sieves used in early English expeditions were of various sizes and meshes, each See also:sieve having a finer mesh than the sieve smaller than itself. In use the whole were put together in the form of a See also: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 See also: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 See also:Bureau of See also:Fisheries ship " Albatross " these sieves are raised to form a table and the water is led on them from a See also:hose: the very finest objects or sediments are retained by the See also:waste water escaping from a catchment tub by See also: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% See also:solution and used in 2%, thus saving space, and it preserves many animals in their See also:colours for a time: See also:formalin preparations do not, however, last as well as do those in spirit. The suitable fluids for various histological inquiries are beyond the See also:scope of the See also:present See also:article; but for general marine See also: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% See also:alcohol 90 parts, glacial acetic See also:acid q 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 See also:British Marine Biological Association's work in the North Sea, a See also:separate See also:sheet of a printed See also:book with See also: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 See also:risk of omission. Whether labelled externally or not, all bottles should contain See also:parchment or good paper labels written with a soft See also:pencil. These cannot be lost. The more fully details of reference number of station, gear, date, &c., are given the better, as should a See also:mistake be made in one particular it can frequently be traced and rectified by means of the See also:rest.
Growth of Scope of Operations.—At the Birmingham See also:meeting of the British Association in 1839 an important See also:committee was appointed " for researches with the dredge with a view to the investigation of the marine See also:zoology of Great Britain, the illlustra tion of the geographical distribution of marine animals, and the more accurate determination of the fossils of the See also:Pliocene period." Of this committee See also:Edward See also:Forbes was the ruling spirit, and under the genial See also:influence of his contagious See also:enthusiasm great progress was made during the next See also:decade in the knowledge of the fauna of the British seas, and many wonderfully pleasant days were spent by the See also:original committee and by many others who from year to year were " added to their number." Every annual See also:report of the British Association contains communications from the English, the Scottish, or the Irish branches of the committee; and in 185o 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 See also:memoirs previously published, " On the Distribution of the See also:Mollusca and See also:Radiata of the See also:Aegean Sea," and " On the Zoological Relations of the existing Fauna and Flora of the British Isles," may be said to See also: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 See also:loo-See also:fathom line, by which See also:amateur work in small boats was practically limited, the zero of See also:animal life was approached—a notion which was destined to be gradually undermined, and finally over-thrown. From time to time, however, there were not wanting men of great skill and experience to maintain, with Sir See also: See also:Carpenter, Dr See also:Gwyn See also:Jeffreys, and See also:Professor (afterwards Sir See also: The efficiency or evenness of dredging was secured, therefore, by the special See also: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 canvascovers: their descent was arrested by a toggle or wooden cross-bar previously attached to the rope at the desired point. When, how-ever, 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 See also: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 See also: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 See also: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 See also:arm, and the block through which the dredge rope ran suspended from it. In more recent See also:ships a special See also:derrick See also: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 See also:Agassiz on the " See also:Blake." He states that his wire weighed one See also:pound per fathom, against two pounds per fathom of hempen rope, and had a breaking strain nearly twice that of hempen rope, which See also: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 See also:mere fraction (1) 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 See also:reel it passes to the See also: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 (See also: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 See also:remainder, previously in line with it, sank to the bottom without See also: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 See also:manage the two speeds that the See also: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 water. Speed of ship Length of Angle of dredge Angle of dredge while See also:shooting rope rope while rope while dredge or trawl. required. lowering trawl. dragging trawl. Fathoms. Knots. Fathoms. 6o 55 loo 3 200 200 3 400 6o 55 400 3 700 6o 52 600 2; lo00 55 5o Boo 21 1200 50 44 lo00 21 1500 5o 40 1500 2} 2166 50 40 2000 2 2670 45 35 :woo 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. See also: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 Boo fathoms on a sounding line in See also: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, See also:corals and See also: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 See also: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 See also: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 Blake Dredge.—In the soft See also:ooze which forms the bottom of deep seas the See also: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 See also: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 See also: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 See also:shield of canvas, quite delicate specimens reached the surface uninjured. The dredge weighed 8o 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. See also:Holt used, at See also: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 See also:wood. The bag was of See also:cheese-See also:cloth or light open canvas, and the whole was of light construction. The apparatus was very useful in capturing small burrowing See also:crustacea. The See also: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 See also: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 See also:Miller's but with a triangular mouth, and hung by chains from its angles, is anold See also: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 See also: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 manq objects which though quite common are of rare occurrence in an ordinary dredge. Thus on the See also: 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 See also:valve or See also: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 See also:differences of this trawl from the dredge are the replacement of scraping lip by ground rope, the position of this ground rope From Sir Charles Wyville Thomson's Voyage aj the "Challenger."By permission of See also:Macmillan & Co., Ltd. " Challenger." and the greater size of the mouth. The See also: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 See also:swimming creature not disturbed until the arrival of the ground rope cannot See also: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 know-ledge 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 See also: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 See also:Norwegian See also:glass float is fastened in a network See also: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 See also: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 pre-ceding, and is decidedly better for those not experts in See also: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 See also:bound with finer rope for See also: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. 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 II 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 See also:groups obtained was much greater. As soon as trawls were adopted fish began to make their See also: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 See also: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 See also: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, See also: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. 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