730 



DREDGING. 



dwd, still used In Holland and Flanders, in deepen 

 ing the extensive tracts of canals. The excavatec 

 matters are generally of a mossy description, which 

 being compressed in moulds and dried, are used as 

 turf-fuel. On the Thames, this operation is con- 

 ducted on a large scale, under the immediate direc- 

 tion of the Trinity board ; and the stuff dredged 

 from the bottom, consisting chiefly of gravel, is sold 

 at the rate of about one shilling a ton, for ballast 

 particularly to the colliers ; and, to such an exten 

 is this process carried on, that the ballast hills of 

 Shields and Newcastle, which are curious from their 

 great extent, have been chiefly raised by the dis- 

 clmrge from the vessels which have brought grave 1 

 in ballast from the Thames. 



The spoon apparatus consists of a strong ring or 

 hoop of malleable iron, about six or seven feet in cir- 

 cumference, properly formed for making an impres- 

 sion upon the soft and muddy ground. To this ring 

 is strongly attached a large bag of bullock's hide or 

 tanned leather, perforated with a number of small 

 holes, with a capacity of four or five cubic feet. 

 A long pole or handle is attached to the spoon, 

 and a rope to the bottom of the bag, for directing 

 their position at the commencement of each opera- 

 tion. The pole or handle varies in length and thick- 

 ness, according to the depth of water, from fifteen to 

 tliirty feet. This apparatus is generally worked with 

 a wheel and pinion, or winch ; and the chain or rope 

 is brought from the spoon to the winch, through a 

 block suspended from a small crane, for bearing the 

 spoon and its contents to the side of the boat. The 

 purchase-rope is led upon deck by a snatch-block, in 

 the proper direction for the barrel of the winch. In 

 situations where the command of head-water is con- 

 siderable, it is retained in a scouring basin, which is 

 a water-tight compartment of a harbour, furnished 

 with sluices to run off the water as required. All 

 harbours left dry every tide at low water, wherein 

 the deposition of mud is most apt to take place, ought, 

 if possible, to be furnished with a sconring basin. 



For clearing the bottom and bar of a harbour, in 

 conjunction with that mode of dredging which simply 

 loosens the stuff, the use of the scouring basin is most 

 effectual. The harbour of Montrose is a striking 

 instance in point, where the great natural basin con- 

 nected with that port is covered every tide, by which, 

 it has been computed, about fifty-five millions of 

 cubic yards of back water are obtained, which pro- 

 duce so great a current that the shifting sand-bank 

 off the coast, called the Annet, is prevented from 

 being thrown across the mouth or entrance of that 

 harbour, in gales of wind from the eastward ; and 

 the navigation is kept open and preserved of consi- 

 derable depth, even, at the lowest ebbs. 



The same remarks are applicable to the entrance 

 of all great rivers, in which the navigation can only 

 be preserved by a strong current of water. The 

 most eminent engineers in Europe, in accordance 

 with this idea, have introduced scouring basins into 

 their designs of tide-harbours. Of these, the sluices 

 at Ostend and Ramsgate harbours are particular ex- 

 amples, where the silt in the outer harbours is dredged 

 and loosened, and raked into the tracks or courses of 

 the water issuing from the scouring basins. To 

 effect this, the dredging-harrow, consisting of a frame 

 of timber and plate iron, is used ; the common har- 

 row, the ordinary plough, and even large rakes, have 

 been employed with good effect in many places, par- 

 ticularly in Holland, upon the extensive flats at the 

 entrance of some of the large rivers. 



In wet docks connected with each other, much 

 use may be made of this mode of scouring or floating 

 away mud by opening numerous sluices from one 

 dock into another. This has been done at Liver- 



pool, Leith, and Bristol, with good effect. But in 

 the improvement of navigable rivers, many of there 

 modes of dredging and scouring have been laid aside, 

 and the operation of narrowing the channel and con- 

 fining the current has been adopted. By this sys- 

 tem, the bed of the river Clyde has been deepened 

 from five to nine feet, to the great advantage of the 

 trade and commerce of Glasgow. In like manner 

 the opening of the Eau Brink Cut, a little above 

 Lynn-Regis, has produced the most salutary effects 

 in clearing away the obstructions in the river Ouse, 

 below Ely ; and the depositions in front of the town 

 of Lynn will be scoured away so soon as a proper 

 direction has been given to the current. 



The bucket dredging machine has been generally 

 supposed to be of British origin ; and it was certainly 

 first used in England, by the late Mr Rennie, at Hull. 

 It is probable that steam was not applied to the 

 bucket dredging apparatus prior to the commence- 

 ment of the present century, nor brought into gen- 

 eral use sooner than ten or twelve years after that 

 period. At the present day, whenever a continued 

 necessity exists for dredging, the steam apparatus is 

 always employed. 



In plate XXVIII., we have given an elevation and 

 ground plan of one of the dredging machines em- 

 ployed on the river Clyde. A short description will 

 be sufficient to render the construction of this ma- 

 chine understood. A is the room in which the steam 

 engine and boilers are contained, the engine being 

 similar in construction to those employed in steam 

 packets. It is furnished with a fly-wheel for the 

 purpose of regulating the motion. There is a spur- 

 wheel, B, with wooden cogs, fixed on the end of the 

 lying shaft, C, C, which receives motion from a 

 wheel on the end of the crank shaft of the engine ; 

 but as the lying shaft gives motion to the buckets, and 

 i they may meet with some sudden obstruction, and 

 the machinery thereby receive injury, this wheel is 

 constructed after a peculiar manner to obviate 

 such accidents. The wheel, B, has a large eye ac- 

 curately turned, and made to receive a nave, also 

 accurately turned, and firmly fastened on the lying 

 shaft. The wheel is furnished with pinching plates, 

 by which it may be fastened to the nave, and made 

 tighter or slacker at pleasure, in consequence of 

 which arrangement, when the buckets meet with 

 any sudden obstruction, the nave of the lying shaft 

 will stop, while the cog-wheel revolves, and thus the 

 machinery receives no strain. The lying shaft, C, C, 

 rives motion to the tumbler block, E, by means of the 

 oevil wheel, D. The form of the tumbler block will 

 be easily understood from the enlarged elevation, 

 Fig. 3, and the ground plan of the tumbler block 

 lead, Fig. 4. The bucket frame, G, G, is movable 

 on an axis at the head, and suspended by chains, 

 O, O,at the lower end, which pass round the barrel, 

 M, seen in the ground plan, Fig. 2 ; by turning 

 which, the lower end, where the ouckets meet the 

 jed of the river, may be raised or depressed as the 

 depth of the water may require. The buckets are 

 made of iron, pointed with, steel on the cutting edges, 

 and are fastened to two strong iron endless chains 

 which pass round the tumbling blocks at the top and 

 )ottom of the bucket frame, G, G. The tumbler at the 

 ;op is four-sided, that at the bottom has five sides, 

 is a barrel, round which the chain attached to the 

 anchor is wound. The punt, H, on which the mud, 

 ravel, &c. fells, is seen at the stern of the vessel. 

 S T ine men are required to work one of these dredging 

 machines, and it is stated that those machines will 

 raise, on an average, about 80 tons in an hour. 

 This will vary witu the nature of the bed of tlie 

 river, there being more or less according as the bot- 

 tom consists of gravel or clay. 



