Cim\ XIII.

DREDGING.

1057

Dredying forms a very important part of the work of the civil engineer, and is effected invarious ways ; either by drags or scoops, or rakes, or machines. There are two sorts ofhand-drags, one for raising mud, the other sand ; the first consists of an iron box piercedwith holes, open in front as well as at the top ; to this is attached a slightly flexible handle,of a length proportionate to the depth it is to work in : when this is made use of, the menin a boat make the iron box enter the sand, sustaining the handle on the shoulder, andwhen it is filled they raise it, and if there be any large stones they are disengaged bymeans of hooks : a man will raise in this manner, where the depth is not more than 4 or5 feet, a cube yard in the course of a day, and sometimes more.

The Dray for Mud is differently formed: it is an iron ring, to tvhich a canvass bag isattached, by passing a cord through holes made in the ring purposely to receive it: thatpoint of the iron rim which is intended to touch the ground and enter the mud must besufficiently strong: two men in a boat or punt are required to manoeuvre it, and inthe course of a day they will raise from 12 to 14 cube yards, if the depth does notexceed 6 feet: when the boat is made use of, it is first moored in such a manner that itcannot drift, and in this way the canals at Venice are still dragged; such a drag allows thewater to flow out of it, and retains only the solid matter.

The Louchette , a kind of spade, or a collection of them, is used for cutting or extractingturf under water, without the necessity of first pumping it dry : this consists of a lightiron frame, which is armed all round with a cutting blade, in length about 3 feet ; the partbetween it and the handle is open, being formed of four horizontal rods, and two verticalones ; these receive the turf after it is cut and detached, and enable the workmen, bymeans of a rope and windlass, to pull it up ; these cutting instruments have a varietyof forms given them to adapt them to the peculiar work they may have to perform.

The Box Shovel consists of an open box fixed at the end of a long handle, usually madeof iron; the cutter traverses in a groove, and is worked by another handle; by this theturf is cut and detached, and each successive piece falls into the box : as many as four turfsmay be thus drawn up at one time.

The machine at Venice probably first suggested our modern dredger ; it consisted ofan oblong pontoon, covered in, on the deck of which the machine was worked ; itslength was nearly 50 feet, and its breadth a little less than half; below the deck the spacewas used by the workmen as a lodging place. The mechanism attached was a largewooden beam, which moved on a pivot in its centre : this beam was formed of taperingpieces, about 50 feet in length, and together 3 feet in thickness; there were live rows,bound and hooped round with iron ; this strongly constructed beam was furnished at oneend with two nuts, into which worked a perpendicular screw of beech, upwards of 30 feetin height, and about 14 or 15 inches in diameter. This screw worked in a socket or plate,fixed at the bottom of the pontoon, and this being provided with a windlass below the deck,the screw was made to elevate or depress the huge wooden beam, which usually lay in ahorizontal position.

At the other end of the beam, a large iron spoon was worked, which had by means ofpulleys an alternate motion of rotation given to it by two vertical cylinders placed withintlie pontoon : the iron spoons were opened and shut by ropes and pulleys, and the dredgingaction was commenced by slightly depressing, then raising the beam ; by the first move-ment the cover was moved, by the second the spoon was opened ; after it had filled, it wasagain covered and lifted out of the water: such a heavy complicated apparatus consumedmuch labour, and required to be moored very strongly to the place where it was worked,and the manoeuvring of the perpendicular screw, which gave movement to the weightytimber beam, was attended with painful and laborious exertions.

Eight men were required to work it, and they generally raised in a day about sixty cubeyards from a depth which did not exceed 14 or 15 feet: when a greater number of menwere employed, the quantity raised was proportionably greater, and twelve, which woreas many as well could work at one time, have been known to lift more than a hundredcube yards. The spoon or drag contained two and a half cubic yards, and the mean timefor lifting it was a quarter of an hour; thus with forty extractions in a day considerablespace was cleared: such a machine was said to have cost upwards of 800 pounds.

The Chapelet was composed of three rollers, two of which touched the ground, and theother was placed above a timber scaffold, on which the soil dredged was to be raised:round these rollers worked an endless chain formed of large links, alternately flat andsquare : to this was attached four or more scoops or scuttles, placed at regular distances,made of sheet-iron : these were pierced with holes, and provided with a strong, projectingbeak, which not only entered, but cut its way into the mud or earth below. The cylinderswere armed with iron spikes which entered the square links of the chain, and turnedround on pivots, working within a frame made for the purpose, of timbers sufficientlystrong to retain them, and put together so that they could be raised or lowered as the depthto l)e dredged required. The whole was*put in motion by a wheel and winch, placedparallel to the cylinders, and bv which tliev were turned; the axis of the winch working

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