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 general defect in this class of dredger. It is also capable of lifting large loose pieces of rock weighing from 1 to 2 tons. A dredger of this type, with grab holding 1 ton of mud, dredged during six days, in 19 ft. of water, an average of 52½ tons and a maximum of 68½ 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 1.63d. 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 “Miles K. Burton” (Plate I. fig. 9), belonging to the Mersey Docks and Harbour Board. It is equipped with 5 grabs on 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 1s. 5d. per cub. yd. of spoil, including the working expenses for wages of crew, 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 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 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 20th 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 cargo. The first quadruple dredger used by the 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 100 cub. ft. respectively. In discharging coal at Southampton, Havre, Erith, as well as at the coaling station at Purfleet on the Thames, grabs having a capacity of about 80 cub. ft. are in 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 150 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 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 wire ropes. Grabs worked with two chains or ropes have many advantages, and are therefore adopted for large undertakings.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 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.
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 general defect in this class of dredger. It is also capable of lifting large loose pieces of rock weighing from 1 to 2 tons. A dredger of this type, with grab holding 1 ton of mud, dredged during six days, in 19 ft. of water, an average of 52½ tons and a maximum of 68½ 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 1.63d. 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 “Miles K. Burton” (Plate I. fig. 9), belonging to the Mersey Docks and Harbour Board. It is equipped with 5 grabs on 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 1s. 5d. per cub. yd. of spoil, including the working expenses for wages of crew, 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 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 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 20th 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 cargo. The first quadruple dredger used by the 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 100 cub. ft. respectively. In discharging coal at Southampton, Havre, Erith, as well as at the coaling station at Purfleet on the Thames, grabs having a capacity of about 80 cub. ft. are in 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 150 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 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 wire ropes. Grabs worked with two chains or ropes have many advantages, and are therefore adopted for large undertakings.
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 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.
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 Stour, consisted of a boat with a broad rake fitted to the bow, capable of 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 accumulation of the dredgings, operations were begun at the mouth of the river and carried on backwards. The apparatus was very 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 gunboat 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 bevel gear from a cross shaft which derived its motion by belting from the 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 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 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 80 cub. yds. per hour can be raised by this dredger.The “Aquamotrice,” designed by Popie, and used on the Garonne at Agen, appears to be a modification of the old bag and spoon arrangement. A flat-bottomed boat 51½ ft. long by 6½ 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 pulley on uprights and under a 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 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 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 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 cast iron 12 in. deep. This rake was hung from the bow of a steamer 180 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 harrow and high pressure water jets, arranged by B. Tydeman, was found very efficacious in removing a largequantity of mud which accumulated in the Tilbury Dock basin, which has an area of about 17 acres, with a depth of 26 ft. at low-water 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 80 lb pressure per sq. in. at the bottom of the dock.Ive’s excavator consists of a long weighted spear, with a sort of 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 rod. When these eight spades are drawn up by means of chains, they form one flat table or 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 by releasing the spade. This apparatus has been extensively used both in Great Britain and in India for excavating in bridge cylinders.The clam 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. 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.Bruce & Batho’s dredger, when closed, is of hemispherical form, the bucket being composed of three or four 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 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 ℔ per sq. in., and the pumps are used in connexion with a steam accumulator.An unloading apparatus was designed by Mr A. Manning for the East & West India Dock Co. for unloading the dredged materials out of barges and delivering it on the marsh at the back of the bank of the river Thames at Crossness, Kent. A 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 1 d. per cub. yd.
Kingfoot’s dredger, used for cleansing the river Stour, consisted of a boat with a broad rake fitted to the bow, capable of 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 accumulation of the dredgings, operations were begun at the mouth of the river and carried on backwards. The apparatus was very 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 gunboat 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 bevel gear from a cross shaft which derived its motion by belting from the 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 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 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 80 cub. yds. per hour can be raised by this dredger.
The “Aquamotrice,” designed by Popie, and used on the Garonne at Agen, appears to be a modification of the old bag and spoon arrangement. A flat-bottomed boat 51½ ft. long by 6½ 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 pulley on uprights and under a 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 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 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 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 cast iron 12 in. deep. This rake was hung from the bow of a steamer 180 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 harrow and high pressure water jets, arranged by B. Tydeman, was found very efficacious in removing a largequantity of mud which accumulated in the Tilbury Dock basin, which has an area of about 17 acres, with a depth of 26 ft. at low-water 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 80 lb pressure per sq. in. at the bottom of the dock.
Ive’s excavator consists of a long weighted spear, with a sort of 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 rod. When these eight spades are drawn up by means of chains, they form one flat table or 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 by releasing the spade. This apparatus has been extensively used both in Great Britain and in India for excavating in bridge cylinders.
The clam 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. 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.
Bruce & Batho’s dredger, when closed, is of hemispherical form, the bucket being composed of three or four 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 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 ℔ per sq. in., and the pumps are used in connexion with a steam accumulator.
An unloading apparatus was designed by Mr A. Manning for the East & West India Dock Co. for unloading the dredged materials out of barges and delivering it on the marsh at the back of the bank of the river Thames at Crossness, Kent. A 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 1 d. per cub. yd.
Dredging on the River Scheldt below Antwerp.—This dredging took place at Krankeloon and the Belgian Sluis under the direction of L. Van Gansberghe. At Melsele there is a pronounced bend in the river, causing a bar at the Pass of Port Philip, and just below the pass of Lillo there is a cross-over in the current, making a neutral point and forming a shoal. After dredging to 8 metres (26.24 ft.) below low tide, in clay containing stone and ferruginous matter, a sandstone formation was encountered, which was very compact and difficult to raise. A suction dredger being unsuited to the work, a bucket-ladder dredger was employed. The dredging was commenced at Krankeloon in September 1894 and continued to the end of 1897. A depth of 6 metres (19.68 ft.) was excavated at first, but was afterwards increased to 8 metres (26.24 ft.). The place of deposit was at first on lands acquired by the State, 2.17 m. above Krankeloon, and placed at the disposal of the contractor. The dredgings excavated by the bucket-ladder dredger were deposited in scows, which were towed to the front of the deposit ground and discharged by a suction pump fixed in a special boat, moored close to the bank of the river. The material brought by the suction dredger in its own hull was discharged by a plant fixed upon the dredger itself. In both instances the material was deposited at a distance of 1640 ft. from the river, the spoil bank varying in depth from 2 to 7 metres. The water thrown out behind the dyke with the excavated material returned to the river, after settlement, by a special discharge lock built under the dyke. After 1896 the material was delivered into an abandoned pass by means of barges with bottom hopper doors or by the suction dredger. One suction dredger and three bucket-ladder dredgers were employed upon the work, and a vessel called “Scheldt I.” used for discharging the material from the scows. Four tugboats and twenty scows were also employed.
The largest dredger, “Scheldt III.,” was 147.63 ft. long by 22.96 ft. wide by 10.98 ft. deep, and had buckets of 21.18 cub. ft. capacity. The output per hour was 10,594 cub. ft. This dredger had also a complete installation as a suction dredger, the suction pipe being 2 ft. diameter. The fan of the centrifugal pump was 5.25 ft. diameter, and was driven by the motor of the bucket ladder. The three bucket dredgers worked with head to the ebb tide. They could also work with head to the flood tide, but it took so long a time to turn them about that it was impracticable. The work was for from 13 to 14 hours a day on the ebb tide. The effective daily excavation averaged 4839 cub. yds. Each dredger was fitted with six anchors. The excavated cut was 164 ft. wide by 6.56 ft. deep. “Scheldt III.” was capable of lifting a mass 9.84 ft. thick. The suction dredger “Scheldt II.” was of the multiple type, and is stated to be unique in construction. It can discharge material from a scow alongside, fill its own hopper with excavations, discharge its own load upon the bank or into a scow by different pipes provided for the purpose, and discharge its own load through hopper doors. The machinery is driven by a triple expansion engine of 300 i.h.p. working the propeller by a clutch. Owing to the rise and fall in the tide of 23 ft. the suction pipe is fitted with spherical joints and a telescopic arrangement. The vessel is 157.5 ft. by 28.2 ft. by 12.8 ft. The diameter of the pump is 5.25 ft. The wings of the pump are curved, the surface being in the form of a cylinder parallel to the axis of rotation, the directrix of which is an arc of a circle of 2.62 ft. radius with the straight part beyond. The suction and discharge pipes are 2 ft. diameter. A centrifugal pump is provided for throwing water into the scows to liquefy the material during discharge. The dredger, which is fitted with electric lights for work at night, is held by two anchors, to prevent lurching backwards and forwards; it can work on the flood as well as on the ebb tide, and can excavate to a depth of 42.65 ft., the output depending upon the nature of the material. With good material it can fill its tanks in thirty minutes. To empty the tanks by suction and discharge upon the bank over the dyke takes about fifty minutes, depending upon the height and distance to which the material requires to be delivered. The daily work has averaged eighteen hours, ten trips being made when the distance from the dredging ground to the point of delivery is about 1 m. When the dredged material is discharged into the Scheldt, a quantity of 5886 cub. yds. has been raised and deposited in a day, the mean quantity being 4700 cub. yds. When the distance of transportation is increased to 2½ m., six voyages were made in a day, and the day’s work amounted to 3530 cub. yds.
The largest dredger, “Scheldt III.,” was 147.63 ft. long by 22.96 ft. wide by 10.98 ft. deep, and had buckets of 21.18 cub. ft. capacity. The output per hour was 10,594 cub. ft. This dredger had also a complete installation as a suction dredger, the suction pipe being 2 ft. diameter. The fan of the centrifugal pump was 5.25 ft. diameter, and was driven by the motor of the bucket ladder. The three bucket dredgers worked with head to the ebb tide. They could also work with head to the flood tide, but it took so long a time to turn them about that it was impracticable. The work was for from 13 to 14 hours a day on the ebb tide. The effective daily excavation averaged 4839 cub. yds. Each dredger was fitted with six anchors. The excavated cut was 164 ft. wide by 6.56 ft. deep. “Scheldt III.” was capable of lifting a mass 9.84 ft. thick. The suction dredger “Scheldt II.” was of the multiple type, and is stated to be unique in construction. It can discharge material from a scow alongside, fill its own hopper with excavations, discharge its own load upon the bank or into a scow by different pipes provided for the purpose, and discharge its own load through hopper doors. The machinery is driven by a triple expansion engine of 300 i.h.p. working the propeller by a clutch. Owing to the rise and fall in the tide of 23 ft. the suction pipe is fitted with spherical joints and a telescopic arrangement. The vessel is 157.5 ft. by 28.2 ft. by 12.8 ft. The diameter of the pump is 5.25 ft. The wings of the pump are curved, the surface being in the form of a cylinder parallel to the axis of rotation, the directrix of which is an arc of a circle of 2.62 ft. radius with the straight part beyond. The suction and discharge pipes are 2 ft. diameter. A centrifugal pump is provided for throwing water into the scows to liquefy the material during discharge. The dredger, which is fitted with electric lights for work at night, is held by two anchors, to prevent lurching backwards and forwards; it can work on the flood as well as on the ebb tide, and can excavate to a depth of 42.65 ft., the output depending upon the nature of the material. With good material it can fill its tanks in thirty minutes. To empty the tanks by suction and discharge upon the bank over the dyke takes about fifty minutes, depending upon the height and distance to which the material requires to be delivered. The daily work has averaged eighteen hours, ten trips being made when the distance from the dredging ground to the point of delivery is about 1 m. When the dredged material is discharged into the Scheldt, a quantity of 5886 cub. yds. has been raised and deposited in a day, the mean quantity being 4700 cub. yds. When the distance of transportation is increased to 2½ m., six voyages were made in a day, and the day’s work amounted to 3530 cub. yds.
Gold Dredgers.—Dredgers for excavating from river beds soil containing gold are generally fitted with a screen and elevator. They have been extensively designed and built by Messrs Lobnitz & Co. (fig. 2) and also by Messrs Hunter & English.
The writer is indebted to theProceedingsof the Institution of Civil Engineers, and especially to the paper of Mr J. J. Webster (Proc. Inst. C.E.vol. 89), for much valuable information upon the subject treated. He is also indebted to many manufacturers who have furnished him with particulars and photographs of dredging plant.
The writer is indebted to theProceedingsof the Institution of Civil Engineers, and especially to the paper of Mr J. J. Webster (Proc. Inst. C.E.vol. 89), for much valuable information upon the subject treated. He is also indebted to many manufacturers who have furnished him with particulars and photographs of dredging plant.
(W. H.*)
Plate I.
Plate II.
2. Marine Biology
The naturalist’s dredge is an instrument consisting essentially of a net or bag attached to a framework of iron which forms the mouth of the net. When in use as the apparatus is drawn over the sea-bottom mouth forwards, some part of the framework passes beneath objects which it meets and so causes them toenter the net. It is intended for the collection of animals and plants living on or near the sea-bottom, or sometimes of specimens of the sea-bottom itself, for scientific purposes.
Until the middle of the 18th century, naturalists who studied the marine fauna and flora relied for their materials on shore collection and the examination of the catches of fishing boats. Their knowledge of creatures living below the level of low spring tides was thus gained only from specimens cast up in storms, or caught by fishing gear designed for the capture of certain edible species only. The first effort made to free marine biology from these limitations was the use of the dredge, which was built much on the plan of the oyster dredge.
The Oyster Dredge.—At first naturalists made use of the ordinary oyster dredge, which is constructed as follows. The frame is an iron triangle, the sides being the round iron “arms” of the dredge, the base a flat bar called the shere or lip, which is sloped a little, not perpendicular to the plane of the triangle; an iron bar parallel to the base joins the arms. The net is fastened to the parallel bars and the portion of the arms between them, and consists of two parts: that attached to the shere is of round iron rings linked together by smaller ones of wire lashings, that attached to the upper bar is of ordinary network. Where these two portions of the bag meet a wooden beam is fastened. In use the frame is towed forward by its apex: the shere passes below oysters, &c., which pass back on to the iron netting. The length of each side of the triangular frame is about 6 ft., the width of the shere 3 in. and the height of the mouth just under a foot. The rings vary in size, but are usually some 2½ in. in diameter. The weight is about 60 ℔. This dredge was soon abandoned: its weight was prohibitive for small boats, from which the naturalist usually worked, its wide rings allowed precious specimens to fall through, and its shallow net favoured the washing out of light objects on hauling through the moving water of the surface. Moreover, it sometimes fell on its back and was then useless, although when the apex or towing point was weighted no great skill is needed to avoid this.Otho Müller used a dredge (fig. 13) consisting of a net with a square iron mouth, each of whose sides was furnished with a thin edge turned slightly away from the dredge’s centre. As any one of these everted lips could act as a scraper it was a matter of indifference which struck the bottom when the dredge was lowered. The chief defect of the instrument was the ease with which light objects could be washed out on hauling, owing to the size of the mouth. However, with this instrument Müller obtained from the often stormy Scandinavian seas all the material for his celebratedZoologia Danica, a description of the marine fauna of Denmark and Norway which was published with excellent coloured plates in 1778; and historical interest attaches to the dredge as the first made specially for scientific work.Ball’s Dredge.—About 1838 a dredge devised by Dr Ball of Dublin was introduced. It has been used all over the world, and is so apt for its purpose that it has suffered very little modification during its 70 years of life. It is known as Ball’s dredge or more generally simply “the dredge.”Fig. 14.—Ball’s Naturalist’s Dredge.Ball’s dredge (fig. 14) consists of a rectangular net attached to a rectangular frame much longer than high, and furnished with rods stretching from the four corners to meet at a point where they are attached to the dredge rope. It differs from Müller’s dredge in the slit-like shape of the opening, which prevents much of the “washing out” suffered by the earlier pattern, and in the edges. The long edges only are fashioned as scrapers, being wider and heavier than Müller’s, especially in later dredges. The short edges are of round iron bar.Like Müller’s form, Ball’s dredge will act whichever side touches the bottom first, as its frame will not remain on its short edge, and either of the long edges acts as a scraper. The scraping lips thicken gradually from free edge to net; they are set at 110° to the plane of the mouth, and in some later patterns curve outwards instead of merely sloping. All dredge frames are of wrought iron.The thick inner edges of the scrapers are perforated by round holes at distances of about an inch, and through these strong iron rings about an inch in diameter are passed, and two or three similar rings run on the short rods which form the ends of the dredge-frame. A light iron rod, bent to the form of the dredge opening, usually runs through these rings, and to this rod and to the rings the mouth of the dredge-bag is securely attached by stout cord or strong copper wire. Various materials have been used for the bag, the chief of which are hide, canvas and netting. The hide was recommended by its strength, but it is now abandoned. Canvas bags fill quickly with mud or sand and then cease to operate: on the other hand wide mesh net fails to retain small specimens. Probably the most suitable material is hand-made netting of very strong twine, the meshes half an inch to the side, the inter-spaces contracting to a third of an inch across when the twine is thoroughly soaked, with an open canvas or “bread-bag” lining to the last 6 in. of the net. A return to canvas covering has latterly occurred in the small dredge called the mud-bag, trailed behind the trawl of the “Albatross” for obtaining a sample of the bottom, and in the conical dredge.The dimensions of the first dredges were as follows: Frame about 12 in. by about 4 in.; scraping lips about 2 in. wide; all other iron parts of round iron bar5⁄8in. diameter; bag rather more than 1 ft. long. These small dredges were used from rowing boats. Larger dredges were subsequently made for use from yawls or cutters. The mouth of these was 18 by 5 in., the scraping lips about 2 in. wide and bag 2 ft. deep; such a dredge weighs about 20 ℔. The dredge of the “Challenger” had a frame 4 ft. 6 in. by 1 ft. 3 in. and the bag had a length of 4 ft. 6 in.; the “Porcupine” used a dredge of the same size weighing 225 ℔. 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.
The Oyster Dredge.—At first naturalists made use of the ordinary oyster dredge, which is constructed as follows. The frame is an iron triangle, the sides being the round iron “arms” of the dredge, the base a flat bar called the shere or lip, which is sloped a little, not perpendicular to the plane of the triangle; an iron bar parallel to the base joins the arms. The net is fastened to the parallel bars and the portion of the arms between them, and consists of two parts: that attached to the shere is of round iron rings linked together by smaller ones of wire lashings, that attached to the upper bar is of ordinary network. Where these two portions of the bag meet a wooden beam is fastened. In use the frame is towed forward by its apex: the shere passes below oysters, &c., which pass back on to the iron netting. The length of each side of the triangular frame is about 6 ft., the width of the shere 3 in. and the height of the mouth just under a foot. The rings vary in size, but are usually some 2½ in. in diameter. The weight is about 60 ℔. This dredge was soon abandoned: its weight was prohibitive for small boats, from which the naturalist usually worked, its wide rings allowed precious specimens to fall through, and its shallow net favoured the washing out of light objects on hauling through the moving water of the surface. Moreover, it sometimes fell on its back and was then useless, although when the apex or towing point was weighted no great skill is needed to avoid this.
Otho Müller used a dredge (fig. 13) consisting of a net with a square iron mouth, each of whose sides was furnished with a thin edge turned slightly away from the dredge’s centre. As any one of these everted lips could act as a scraper it was a matter of indifference which struck the bottom when the dredge was lowered. The chief defect of the instrument was the ease with which light objects could be washed out on hauling, owing to the size of the mouth. However, with this instrument Müller obtained from the often stormy Scandinavian seas all the material for his celebratedZoologia Danica, a description of the marine fauna of Denmark and Norway which was published with excellent coloured plates in 1778; and historical interest attaches to the dredge as the first made specially for scientific work.
Ball’s Dredge.—About 1838 a dredge devised by Dr Ball of Dublin was introduced. It has been used all over the world, and is so apt for its purpose that it has suffered very little modification during its 70 years of life. It is known as Ball’s dredge or more generally simply “the dredge.”
Ball’s dredge (fig. 14) consists of a rectangular net attached to a rectangular frame much longer than high, and furnished with rods stretching from the four corners to meet at a point where they are attached to the dredge rope. It differs from Müller’s dredge in the slit-like shape of the opening, which prevents much of the “washing out” suffered by the earlier pattern, and in the edges. The long edges only are fashioned as scrapers, being wider and heavier than Müller’s, especially in later dredges. The short edges are of round iron bar.
Like Müller’s form, Ball’s dredge will act whichever side touches the bottom first, as its frame will not remain on its short edge, and either of the long edges acts as a scraper. The scraping lips thicken gradually from free edge to net; they are set at 110° to the plane of the mouth, and in some later patterns curve outwards instead of merely sloping. All dredge frames are of wrought iron.
The thick inner edges of the scrapers are perforated by round holes at distances of about an inch, and through these strong iron rings about an inch in diameter are passed, and two or three similar rings run on the short rods which form the ends of the dredge-frame. A light iron rod, bent to the form of the dredge opening, usually runs through these rings, and to this rod and to the rings the mouth of the dredge-bag is securely attached by stout cord or strong copper wire. Various materials have been used for the bag, the chief of which are hide, canvas and netting. The hide was recommended by its strength, but it is now abandoned. Canvas bags fill quickly with mud or sand and then cease to operate: on the other hand wide mesh net fails to retain small specimens. Probably the most suitable material is hand-made netting of very strong twine, the meshes half an inch to the side, the inter-spaces contracting to a third of an inch across when the twine is thoroughly soaked, with an open canvas or “bread-bag” lining to the last 6 in. of the net. A return to canvas covering has latterly occurred in the small dredge called the mud-bag, trailed behind the trawl of the “Albatross” for obtaining a sample of the bottom, and in the conical dredge.
The dimensions of the first dredges were as follows: Frame about 12 in. by about 4 in.; scraping lips about 2 in. wide; all other iron parts of round iron bar5⁄8in. diameter; bag rather more than 1 ft. long. These small dredges were used from rowing boats. Larger dredges were subsequently made for use from yawls or cutters. The mouth of these was 18 by 5 in., the scraping lips about 2 in. wide and bag 2 ft. deep; such a dredge weighs about 20 ℔. The dredge of the “Challenger” had a frame 4 ft. 6 in. by 1 ft. 3 in. and the bag had a length of 4 ft. 6 in.; the “Porcupine” used a dredge of the same size weighing 225 ℔. Doubtless the size of Ball’s dredge would have grown still more had it not been proved by the “Challenger” expedition that for many purposes trawls could be used advantageously instead of dredges.
Operation of the Dredge from Small Vessels.For work round the coasts of Europe, at depths attainable from a row-boat or yawl, probably the best kind of line is bolt-rope of the best Russian hemp, not less than 1½ in. in circumference, containing 18 to 20 yarns in 3 strands. Each yarn should be nearly a hundredweight, so that the breaking strain of such a rope ought to be about a ton. Of course it is never voluntarily exposed to such a strain, but in shallow water the dredge is often caught among rocks or coral, and the rope should be strong enough in such a case to bring up the boat, even if there were some little way on. It is always well, when dredging, to ascertain the approximate depth with the lead before casting the dredge; and the lead ought always to be accompanied by a registering thermometer, for the subsequent haul of the dredge will gain greatly in value as an observation in geographical distribution, if it be accompanied by an accurate note of the bottom temperature. For depths under 100 fathoms the amount of rope paid out should be at least double the depth; under 30 fathoms, where one usually works more rapidly, it should be more nearly three times; this gives a good deal of slack before the dredge if the boat be moving very slowly, and keeps the lip of the dredge well down. When there is anything of a current, from whatever cause, it is usually convenient to attach a weight, varying from 14 ℔ to half a hundredweight, to the rope 3 or 4 fathoms in front of the dredge. This prevents in some degree the lifting of the mouth of the dredge; if the weight be attached nearer the dredge it is apt to injure delicate objects passing in.
In dredging in sand or mud, the dredge-rope may simply be passed through the double eye formed by the ends of the two arms of the dredge-frame; but in rocky or unknown ground it is better to fasten the rope to the eye of one of the arms only, and to tie the two eyes together with three or four turns of rope-yarn. This stop breaks much more readily than the dredge-rope, so that if the dredge get caught it is the first thing to give way under the strain, and in doing so it often alters the position of the dredge so as to allow of its extrication.
The dredge is slipped gently over the side, either from the bow or from the stern—in a small boat more usually the latter—while there is a little way on, and the direction which the rope takes indicates roughly whether the dredge is going down properly. When it reaches the ground and begins to scrape, an experienced hand upon the rope can usually detect at once a tremor given to the dredge by the scraper passing over the irregularities of the bottom. The due amount of rope is then paid out, and the rope hitched to a bench or rowlock-pin. The boat should move very slowly, probably not faster than a mile an hour. In still water or with a very slight current the dredge of course anchors the boat, and oars or sails are necessary; but if the boat be moving at all it is all that is required. It is perhapsmost pleasant to dredge with a close-reefed sail before a light wind, with weights, against a very slight tide or current; but these are conditions which cannot be commanded. The dredge may remain down from a quarter of an hour to twenty minutes, by which time, if things go well, it ought to be fairly filled. In dredging from a small boat the simplest plan is for two or three men to haul in, hand over hand, and coil in the bottom of the boat. For a large yawl or yacht, and for depths over 50 fathoms, a winch is a great assistance. The rope takes a couple of turns round the winch, which is worked by two men, while a third hand takes it from the winch and coils it down.
It is easier to operate a dredge from a steam vessel than a sailing boat, but if the steamer is of any size great care should be taken that the dredge does not move too rapidly.
Two ingenious cases of dredging under unusual conditions are worthy of mention, one case from shore, one from ice. In the Trondligem Fjord, Canon A. M. Norman in 1890 worked by hauling the dredge up the precipitous shores of the fjord. The dredge was shot from a boat close to the shore, to which after paying out some hundreds of fathoms of line it returned. The dredge was then hauled from the top of the cliffs up whose side it scraped. Hitches against projecting rocks were frequent and were overcome by suddenly paying out line for a time. The dredge was lifted into a boat when it reached the surface of the sea. The other case occurred during the Antarctic expedition of the “Discovery.” Hodgson dropped loops of line along cracks which occasionally formed in the ice. The ice always joined up again, but with the line below it; and a hole being cleared at each place at which the end of the line emerged, the dredge could be worked between them.
The dredge comes up variously freighted according to the locality, and the next step is to examine its contents and to store the objects of search for future use. In a regularly organized dredging expedition a frame or platform is often erected with a ledge round it to receive the contents of the dredge, but it does well enough to capsize it on an old piece of tarpaulin. There are two ways of emptying the dredge; we may either turn it up and pour out its contents by the mouth, or we may have a contrivance by which the bottom of the bag is made to unlace. The first plan is the simpler and the one more usually adopted; the second has the advantage of letting the mass slide out more smoothly and easily, but the lacing introduces rather a damaging complication, as it is apt to loosen or give way. Any objects visible on the surface of the heap are now carefully removed, and placed for identification in jars or tubs of sea-water, of which there should be a number secured in some form of bottle basket, standing ready. The heap should not be much disturbed, for the delicate objects contained in it have already been unavoidably subjected to a good deal of rough usage, and the less friction among the stones the better.
Examination of the Catch. Sifting.—The sorting of the catch is facilitated by sifting. The sieves used in early English expeditions were of various sizes and meshes, each sieve having a finer mesh than the sieve smaller than itself. In use the whole were put together in the form of a nest, the smallest one with the coarsest mesh being on top. A little of the dredge’s contents were then put in the top sieve, and the whole set moved gently up and down in a tub of sea water by handles attached to the bottom one. Objects of different sizes are thus left in different sieves. A simple but effective plan is to let the sieves of various sized mesh fit accurately on each other like lids, the coarsest on top, and to pour water upon material placed on the top one. In the United States Bureau of Fisheries ship “Albatross” these sieves are raised to form a table and the water is led on them from a hose: the very finest objects or sediments are retained by the waste water escaping from a catchment tub by muslin bags let into its sides. Any of these methods are preferable to sifting by the agitation of a sieve hung over the side, as in the last anything passing through the sieve is gone past recall.
Preservation of Specimens.—The preservation of specimens will of course depend on the purpose for which they are intended. For microscopic observation formaldehyde has some advantages. It can be stored in 40% solution and used in 2%, thus saving space, and it preserves many animals in their colours for a time: formalin preparations do not, however, last as well as do those in spirit. The suitable fluids for various histological inquiries are beyond the scope of the present article; but for general marine histology Bles’ fluid is useful, being simple to prepare and not necessitating the removal of the specimen to another fluid. It is composed of 70% alcohol 90 parts, glacial acetic acid 7 parts, 4% formaldehyde 7 parts.
The scientific value of a dredging depends mainly upon two things, the care with which the objects procured are preserved and labelled for future identification and reference, and the accuracy with which all the circumstances of the dredging—the position, the depth, the nature of the ground, the date, the bottom-temperature, &c.—are recorded. In the British Marine Biological Association’s work in the North Sea, a separate sheet of a printed book with carbon paper and duplicate sheets (which remain always on the ship) is used for the record of the particulars of each haul; depth, gear, &c., being filled into spaces indicated in the form. This use of previously prepared forms has been found to be a great saving of time and avoids risk of omission. Whether labelled externally or not, all bottles should contain parchment or good paper labels written with a soft pencil. These cannot be lost. The more fully details of reference number of station, gear, date, &c., are given the better, as should a mistake be made in one particular it can frequently be traced and rectified by means of the rest.
Growth of Scope of Operations.—At the Birmingham meeting of the British Association in 1839 an important committee was appointed “for researches with the dredge with a view to the investigation of the marine zoology of Great Britain, theillustrationof the geographical distribution of marine animals, and the more accurate determination of the fossils of the Pliocene period.” Of this committee Edward Forbes was the ruling spirit, and under the genial influence of his contagious enthusiasm great progress was made during the next decade in the knowledge of the fauna of the British seas, and many wonderfully pleasant days were spent by the original committee and by many others who from year to year were “added to their number.” Every annual report of the British Association contains communications from the English, the Scottish, or the Irish branches of the committee; and in 1850 Edward Forbes submitted its first general report on British marine zoology. This report, as might have been anticipated from the eminent qualifications of the reporter, was of the highest value; and, taken along with his remarkable memoirs previously published, “On the Distribution of the Mollusca and Radiata of the Aegean Sea,” and “On the Zoological Relations of the existing Fauna and Flora of the British Isles,” may be said to mark an era in the progress of human thought.
The dredging operations of the British Association committee were carried on generally under the idea that at the 100-fathom line, by which amateur work in small boats was practically limited, the zero of animal life was approached—a notion which was destined to be gradually undermined, and finally overthrown. From time to time, however, there were not wanting men of great skill and experience to maintain, with Sir James Clark Ross, that “from however great a depth we may be enabled to bring up mud and stones of the bed of the ocean we shall find them teeming with animal life.” Samples of the sea-bottom procured with great difficulty and in small quantity from the first deep soundings in the Atlantic, chiefly by the use of Brooke’s sounding machine, an instrument which by a neat contrivance disengaged its weights when it reached the bottom, and thus allowed a tube, so arranged as to get filled with a sample of the bottom, to be recovered by the sounding line, were eagerly examined by microscopists; and the singular fact was established that these samples consisted over a large part of the bed of the Atlantic of the entire or broken shells of certain foraminifera. Dr Wallich, the naturalist to the “Bulldog” sounding expedition under Sir Leopold M’Clintock, reported that star-fishes, with their stomachs full of the deep-sea foraminifera, had comeup from a depth of 1200 fathoms on a sounding line; and doubts began to be entertained whether the bottom of the sea was in truth a desert, or whether it might not present a new zoological region open to investigation and discovery, and peopled by a peculiar fauna suited to its special conditions.
In the year 1867, while the question was still undecided, two testing investigations were undertaken independently. In America Count L. F. de Pourtales (1824-1880), an officer employed in the United States Coast Survey under Benjamin Peirce, commenced a series of deep dredgings across the Gulf Stream off the coast of Florida, which were continued in the following year, and were productive of most valuable results; and in Great Britain the Admiralty, on the representation of the Royal Society, placed the “Lightning,” a small gun-vessel, at the disposal of a small committee to sound and dredge in the North Atlantic between Shetland and the Faröe Islands.
In the “Lightning,” with the help of a donkey-engine for winding in, dredging was carried on with comparative ease at a depth of 600 fathoms, and at that depth animal life was found to be still abundant. The results of the “Lightning’s” dredgings were regarded of so great importance to science that the Royal Society pressed upon the Admiralty the advantage of continuing the researches, and accordingly, during the years 1869 and 1870, the gun-boat “Porcupine” was put under the orders of a committee consisting of Dr W. B. Carpenter, Dr Gwyn Jeffreys, and Professor (afterwards Sir Charles) Wyville Thomson, one or other of whom superintended the scientific work of a series of dredging trips in the North Atlantic to the north and west of the British Islands, which occupied two summers.
In the “Porcupine,” in the summer of 1869, dredging was carried down successfully to a depth of 2435 fathoms, upwards of two miles and a half, in the Bay of Biscay, and the dredge brought up well-developed representatives of all the classes of marine invertebrates. During the cruises of the “Porcupine” the fauna of the deep water off the western coasts of Great Britain and of Spain and Portugal was tolerably well ascertained, and it was found to differ greatly from the fauna of shallow water in the same region, to possess very special characters, and to show a very marked relation to the faunae of the earlier Tertiary and the later Cretaceous periods.
In the winter of 1872, as a sequel to the preliminary cruises of the “Lightning” and “Porcupine,” by far the most considerable expedition in which systematic dredging had ever been made a special object left Great Britain. H.M.S. “Challenger,” a corvette of 2306 tons, with auxiliary steam working to 1234 h.p., was despatched to investigate the physical and biological conditions of the great ocean basins.
The “Challenger” was provided with a most complete and liberal organization for the purpose; she had powerful deck engines for hauling in the dredge, workrooms, laboratories and libraries for investigating the results on the spot, and a staff of competent naturalists to undertake such investigations and to superintend the packing and preservation of the specimens reserved for future study. Since the “Challenger” expedition the use of wire rope has enabled far smaller vessels to undertake deep sea work. The “Challenger,” however, may be said to have established the practicability of dredging at any known depth.
Operating Dredges and Trawls in deep Seas.—Dredging operations from large vessels in deep seas present numerous difficulties. The great weight of the ship makes her motion, whether of progress or rolling, irresistible to the dredge. The latter tends to jump, therefore, which both lowers its efficiency and causes it to exert a sudden strain on the dredge rope.
The efficiency or evenness of dredging was secured, therefore, by the special device of fastening a heavy weight some 200 or 300 fathoms from the dredge end of the dredge rope. This was either lowered with the dredge or sent down after by means of a “messenger,” a ring of rope fixed round, but running freely on, the dredge rope. The latter plan was used on the “Challenger”; the weights were six 28 ℔ leads in canvas covers: their descent was arrested by a toggle or wooden cross-bar previously attached to the rope at the desired point. When, however, the rope used is of wire this front weight is unnecessary.
The possibility of sudden strain necessitates a constant watching of the dredge rope, as the ship’s engines may at any moment be needed to ease the tension by stopping the vessel’s way, and the hauling engines by paying out more rope. The use of accumulators both renders the strain more gradual and gives warning of an increase or decrease; indeed they can be calibrated and used as dynamometers to measure the strain. One of the best forms of accumulator consists of a pile of perforated rubber disks, which receive the strain and become compressed in doing so. The arrangement is in essence as follows. The disks form a column resting on a cross-bar or base, from which two rods pass up one on each side of the column. Another cross-bar rests on the top disk, and from it a rod passes freely down the centre perforation of disks and base. Eyes are attached to the lower end of this rod and to a yoke connecting the side rods at the top: a pull exerted on these eyes is thus modified by the elasticity of the dredge. In the “Porcupine” and other early expeditions the accumulator was hung from the main yard arm, and the block through which the dredge rope ran suspended from it. In more recent ships a special derrick boom is rigged for this block, and a second accumulator is sometimes inserted between the topping lift by which this is raised and the end of the boom.The margin of safety of steel wire rope is much larger than is that of hempen rope, a fact of importance both in towing in a rough sea and in hauling. Galvanized steel wire with a hempen core was first used by Agassiz on the “Blake.” He states that his wire weighed one pound per fathom, against two pounds per fathom of hempen rope, and had a breaking strain nearly twice that of hempen rope, which bore two tons. Thus in hauling the wire rope has both greater capability and less actual strain. It has also the advantages of occupying a mere fraction (1⁄9) of the storage space needed for rope, of lasting much longer, and its vibrations transmit much more rapid and minute indications of the conduct of the dredge.Wire rope is kept wound on reels supplied with efficient brakes to check or stop its progress, and an engine is often fitted for winding it in and veering it out. From the reel it passes to the drum of the hauling engine, round which it takes some few turns; care is taken by watching or by the use of an automatic regulator (Tanner) that it is taken at a rate equal to that at which it is moving over the side. From the hauling engine it passes over leading wheels (one of which should preferably be a registering wheel and indicate the amount of rope which has passed it), and so it reaches the end of the derrick boom.
The possibility of sudden strain necessitates a constant watching of the dredge rope, as the ship’s engines may at any moment be needed to ease the tension by stopping the vessel’s way, and the hauling engines by paying out more rope. The use of accumulators both renders the strain more gradual and gives warning of an increase or decrease; indeed they can be calibrated and used as dynamometers to measure the strain. One of the best forms of accumulator consists of a pile of perforated rubber disks, which receive the strain and become compressed in doing so. The arrangement is in essence as follows. The disks form a column resting on a cross-bar or base, from which two rods pass up one on each side of the column. Another cross-bar rests on the top disk, and from it a rod passes freely down the centre perforation of disks and base. Eyes are attached to the lower end of this rod and to a yoke connecting the side rods at the top: a pull exerted on these eyes is thus modified by the elasticity of the dredge. In the “Porcupine” and other early expeditions the accumulator was hung from the main yard arm, and the block through which the dredge rope ran suspended from it. In more recent ships a special derrick boom is rigged for this block, and a second accumulator is sometimes inserted between the topping lift by which this is raised and the end of the boom.
The margin of safety of steel wire rope is much larger than is that of hempen rope, a fact of importance both in towing in a rough sea and in hauling. Galvanized steel wire with a hempen core was first used by Agassiz on the “Blake.” He states that his wire weighed one pound per fathom, against two pounds per fathom of hempen rope, and had a breaking strain nearly twice that of hempen rope, which bore two tons. Thus in hauling the wire rope has both greater capability and less actual strain. It has also the advantages of occupying a mere fraction (1⁄9) of the storage space needed for rope, of lasting much longer, and its vibrations transmit much more rapid and minute indications of the conduct of the dredge.
Wire rope is kept wound on reels supplied with efficient brakes to check or stop its progress, and an engine is often fitted for winding it in and veering it out. From the reel it passes to the drum of the hauling engine, round which it takes some few turns; care is taken by watching or by the use of an automatic regulator (Tanner) that it is taken at a rate equal to that at which it is moving over the side. From the hauling engine it passes over leading wheels (one of which should preferably be a registering wheel and indicate the amount of rope which has passed it), and so it reaches the end of the derrick boom.
The dredge is lowered from the derrick boom, which has been previously trained over to windward so that its end is well clear of the ship, while the ship is slowly moving forward. The rope is checked until the net is seen to be towing clear, and then lowered rapidly. Where a weight is used in front of the trawl Captain Calver successfully adopted the plan of backing after sufficient line had been paid out: the part of the rope from weight to surface thus became more vertical, while the shorter remainder, previously in line with it, sank to the bottom without change of relative position of weight and dredge. The ship was then ready for towing. When no front weight is used the manœuvre is unnecessary.
There should be a relation maintained between speed of vessel onward and of rope downward, or a foul haul may result owing to the gear capsizing (in the case of a trawl), or getting the net over the mouth (in a dredge). The most satisfactory method of ensuring this relation seems to be so to manage the two speeds that the angle made by the dredge rope is fairly constant. This angle can be observed with a simple clinometer. The following table abridged from Tanner most usefully brings together the requisite angles with other useful quantities.
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.