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D R E D G I N G also capable of lifting large loose pieces of rock weighing from one over a pulley on uprights and under a roller, and a beam was to two tons. A dredger of this type, with grab holding one ton of attached to the chain 14 ft. 8 in. long, passing through a hole in mud, dredged during six days, in 19 ft. of water, an average of 52| the deck. At the end of the beam was an iron scoop 2 ft. wide tons and a maximum of 68£ tons per hour, and during 12 days, in and 2 ft. 6 in. deep. When the tide was strong enough it drew 16 ft. of water, an average of 48 tons and a maximum of 58 tons the scoop along by means of the paddles and chains, and the scoop per hour, at a cost of 1 ’eSd. per ton, excluding interest on the when filled was discharged by means of a lever opening it. About capital and depreciation. The largest dredger to which this 65 cubic yards of'gravel can be raised by the apparatus in 12 hours. apparatus has been applied is the grab bucket hopper dredger When the tide failed, the apparatus was worked by men. The Danube Steam Navigation Co. removed the shingle in the Miles K. Burton (see Tig. 6), belonging to the Mersey Docks and Harbour Board. It is equipped with five grabs on Morgan’s 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 patent system, which is a modification of Priestman’s, the grabs being worked by five hydraulic cranes. It raised and deposited, cast-iron 12 in. deep. This rake was hung from the bow of a 12 to 15 miles out at sea, 11 loads of about 1450 tons each with a steamer 180 ft. long by 21 ft. beam, and dragged across the double shift of hands, at a cost of about I'Sd. per cubic yard of shallows, increasing the depth of water in one instance from spoil, including the working expenses for wages of crew, fuel, and 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 stores. Mr Marillier of Hull has stated that ‘ ‘ the efficiency of these grabs is not at all dependent upon the force of the blow in by Mr Tydeman, was found very efficacious in removing a large falling for the penetration and grip in the material, as they do quantity of mud which accumulated in the Tilbury Dock basin, their work very satisfactorily even when lowered quite gently on to which has an area of about 17 acres, with a depth of 26 ft. at lowthe material to be cut out, the jaws being so framed as to draw water spring tides. In the first instance chain harrows merely were down and penetrate the material as soon as the upward strain is put used, but the addition of the water jets added materially to the on the lifting chain. Even in hard material the jaws penetrate so success of the operation. The system accomplished in six tides more thoroughly as to cause the bucket to be well filled. The grab is than was done in twelve tides without theif aid. The water jets found to work successfully in excavating hard clay from its natural worked at about 80 lb pressure per square in., attaining an effective bed on dry land.” It is claimed on behalf of grabs that they lift pressure of about 60 lb per square in. at the bottom of the dock. Ives’s excavator consists of a long weighted spear, with a sort of a smaller proportion of water than any other class of dredger. Wild’s single chain half-tine grab works entirely with a single large spade at the end of it. The spade is hinged at the top, and chain, and has been found very useful in excavating the cylinders is capable of being turned at right angles to the spear by a chain in Castries harbour. Upon experimenting with an ordinary grab attached to the end of the spear. The spade is driven into the a rather curious condition of things was observed with respect to ground and, after releasing the catch which holds it in position sinking. On penetrating the soil to a certain depth the ground during its descent, it is drawn up at right angles to the spear by was found, as it were, nested, and nothing would induce the grab the chain, carrying the material along with it. Milroy’s excavator to sink lower. Mr Matthews suggested that an additional set of is similar, but instead of having only one spade it generally has external tines might possibly get over this difficulty. A new grab eight, united to the periphery of an octagonal iron frame fixed to having been made with this modification, and also with a large a central vertical rod. When these eight spades are drawn up by increase of weight—all the parts being steel—it descended to any means of chains, they form one flat table or tray at right angles to required depth with ease, the outside tines loosening the ground the central rod. In operation the spades hang vertically, and are effectually, whilst the inside bucket or tines picked up the material. dropped into the material to be excavated ; the chains are then drawn up, and the table thus formed holds the material on the top, Miscellaneous Appliances.—Under this head may be which is lifted and discharged by releasing the spade. This classed several machines or appliances which perhaps can apparatus has been extensively used both in this country and in hardly be called dredgers, although they are used for India for excavating in bridge cylinders. The clam-shell dredger consists of two hinged buckets, whicli, cleansing and deepening rivers and harbours. when closed, form one semi-cylindrical bucket. The buckets are Kingsfoot’s dredger, used for cleansing the river Stour, consisted held open by chains attached to the top of a crosshead, and the of a boat with a broad rake fitted to the bow, capable of adjust- machine is dropped on to the top of the material to be dredged. ment to different depths. At the sides of the boat were hinged The chains holding the buckets open are then released, while the two wings of the same depth as the rake and in a line with it. spears are held firmly in position, the buckets being closed by When the rake was dropped to the bottom of the river and the another chain. Bull’s dredger, Gatmell’s excavator, and Fouracres’s wings extended to the side, they formed a sort of temporary dam, dredger are modifications with improvements of the clam-shell and the water began to rise gradually. As soon as a sufficient head dredger, and have all been used successfully upon various works. Bruce and Batho’s dredger, when closed, is of hemispherical was raised, varying from 6 to 12 in., the whole machine was driven forward by the pressure, and the rake carried the mud along with form, the bucket being composed of three or four blades. It can it. A progress of about 3 miles an hour was made in this manner, be worked either by a single chain or by means of a spear, the and to prevent the accumulation of the dredgings, operations were latter being generally used for stiff material. The advantage of begun at the mouth of the river and carried on backwards. The this form of dredger bucket is that the steel points of the blades apparatus was very effective and the river was cleansed thoroughly, are well adapted for penetrating hard material. Messrs Bruce and Batho have also designed a novel dredger consisting of one of but the distance travelled by the dredger must have been great. In 1876 J. J. Rietschoten designed a “propeller dredger” for these buckets, but worked entirely by hydraulic power. This was removing the shoals of the river Maas. It consisted of an old gun- made for working on the Tyne. The excavator or dredger is fixed boat fitted with a pair of trussed beams—one at each side—each of to the end of a beam which is actuated by two hydraulic cylinders, which carried a steel shaft and was capable of being lowered or one being used for raising the bucket and the other for lowering raised by means of a crab. An ordinary propeller 3 ft. 6 in. in it; the hydraulic power is supplied by the pumps in the enginediameter was fixed to the lower end of each shaft, and driven by r-oom. The novelty in the design is the ingenious way in which bevel-gear from a cross shaft which derived its motion by belting the lever in ascending draws the shoot under the bucket to receive from the fly-wheel of a 12 H.P. portable engine. The propellers its contents, and draws it away again as the bucket descends. were lowered until they nearly reached the shoals, and were then The hydraulic cylinder at the end of the beam is carried on worked at 150 revolutions per minute. This operation scoured gimballs to allow for the irregularities of the surface being away the shoal effectively, for in about 40 minutes it had been dredged. The hydraulic pressure is 700 lb per square in., and lowered about 3 ft. for a space 150 yards long by 8 yards wide. the pumps are used in connexion with a steam accumulator. An unloading apparatus was designed by Mr A. Manning for the Mr Lavalley in 1877 designed an arrangement for the harbour of
- Dunkirk to overcome the difficulty of working an ordinary bucket- East and West India Dock Company for unloading the dredged
ladder dredger when there is even a small swell. A pump materials out of barges and delivering it on the marsh at the back injects water into the sand down a pipe terminating in three of the bank of the river Thames at Crossness, Kent. A stage nozzles to stir up the sand, and another centrifugal pump draws up constructed of wooden piles commanded a series of barge beds, the the mixed sand and water and discharges it into a hopper, the unloading dredger running from end to end of the stage, lifting pumps and all machinery being on board the hopper. To allow for and delivering the materials on to the marsh behind the river the rising and falling of the vessel—either by the action of the tide wall at the cost of Id. per cubic yard. or by the swell—the ends of the pipes are made flexible. The Dredging combined with Blasting.—In the Encyclopedia hopper has a capacity of 190 cubic yards, and is propelled and the Britannica, vol. vii. p. 466, a description was given of pumps worked by an engine of 150 I.H.P. From 50 to 80 cubic blasting and dredging upon the river Tees. This may yards per hour can be raised by this dredger. The “ Aquamotrice,” designed by . Mr Popie and used on the profitably be supplemented by an account of similar operaGaronne at Agen, appears to be a modification of the old bag and tions in Blyth harbour (cp. Proc. Inst. C. E. vol. Ixxxi. spoon arrangement. A flat-bottomed boat 51J ft. long by 6^ ft. p. 302). Here it became necessary to provide extensive wide was fitted at the bow with paddles, which were actuated by the tide. Connected with the paddles was a long chain, passing deep-water berths, and the site determined upon was on
