Chap. VIII.

lUtlTAIN.

4G5

The piers of the centre arch are 9 feet in thickness, the next 8 feet 6 inches, and the other8 feet; the water-way in the clear is 389 feet, the thickness of the piers 51 feet, and theclear width between the abutments 440 feet.

The clear width between the parapets of the bridge is 58 feet, and measured on thesoffite of the arches 60 feet.

The subjoined specification shows the manner in which the works were executed, andrequires nothing further to convey a clear notion of its construction.

“ Cofferdams .— Before any works can be proceeded with, it is necessary that all the stonesthat lie upon the bed of the river, where the cofferdams are to be constructed, should becleared away; and afterwards the foundation deepened as far as practicable with safety, soas not to alter the level of the water above bridge by too much lowering the bed.

“ The cofferdams are to be formed by driving two rows of gauged piles at a parallel dis-tance of 5 or 6 feet apart; the area comprised within the inner row being not only sufficientto allow the foundations of the pier to be laid, but also a width of a 3 or 4 feet clear spaceentirely around it, for the convenience of the masons and other workmen employed.

“ The timber employed may be either Dantzic, Memel, or red American pine, care beingtaken to select that which is perfectly sound ; these gauge piles, when about 30 feet inlength, are to be made of whole timber, about 12 inches square, pointed and shod with iron,each shoe weighing about 12 pounds; the heads also to be hooped with the best scrap iron,not less than 3 inches in breadth, and | inch in thickness, to prevent their splitting whenunder the weight of the pile-driving machine. A bench mark is established, to thelevel of which all the piles are to bo driven, and which serves as a guide to the work-men.

“ That each row of gauge piles are to be closely wedged together, and driven 6 feetapart from centre to centre; care being taken that they are quite perpendicular, andtruly range with each other. Double waling pieces, 12 by 9 inches, are to be laid hori-zontally on each side, and secured to the heads of the gauge piles by ;j-i»eh iron screwbolts; and at 8 or 9 feet below them to be attached another horizontal row, forming adouble groove to receive the sheeting piles, which are to be wedged in closely between thegauge piles.

“ The dam sheeting piles, 12 by 6 inches, are also to be shod with wrought-iron shoes, ofabout 9 pounds weight, and to be hooped with the best scrap iron, 2£ inches broad, and| inch thick. These sheeting piles are to be driven down to the level of the gauge piles,and in a manner that the last firmly wedges all the rest in the bay, and makes them joinclosely together.

“ The engines employed to drive the gauge piles are to carry a ram of not less than 12ewt., and for the dam sheeting pile 2 cwt. less; and it is necessary to construct scaffoldingand proper stages for the performance of these works in the most convenient situations.

“ After the dam is completed, the soil is then to be taken out between the two rows ofpiles to the depth determined by the engineer, and a sluice trough introduced, for thepurpose of letting water into the clam, when it rises on the outside to a height whichwould endanger its stability ; in rivers subject to floods this is of the highest importance,otherwise the dam might at such times be liable to be blown.

“ After the soil has been removed between the two rows of piles, the gauge piles of therespective rows are then to be connected by round iron screw bolts, made to pass throughthem, as well as through the centres of the two rows of waling pieces. Each of thesescrew holts is 1^ inch diameter, and is provided with a wrought-iron washer | inch thick,and 4 inches square, placed under the head of the nutt, to prevent the wood of the walingyielding to the strain.

“ When this is performed, pounded clay well puddled is to be introduced between thetwo rows of piling, and the whole worked together, until the dam has become perfectlywater-tight; then the pumping out of the water to be commenced by means of a steam-engine.

“ In pumping out the water, and excavating the soil from the interior of the dam, it isnecessary to guard against and prevent the sand from blowing up through the bottom :this may sometimes be done by driving the foundation piles, which consolidates the sandand allows of its being taken out to the required depth ; at other times it may be requisiteto drive extra piles all around, before it would be safe to take out all the earth to the re-quired depth.

“ As the works of excavation proceed, the dam will require bracing, otherwise theoutward pressure would force it inwards, which is to be done according to the directionsgiven by the engineer.

“ Piling for the foundations. — The outside rows of foundation piles are to he eitherMemel, Dantzic, or sound American red pine; and the gauge piles 12 by 12 or more,as the length exceeds 30 feet, to be driven 6 feet from centre to centre. Waling piecesare to be bolted to them as before directed, for keeping the intermediate sheet pilingbetween them in a regular line. After these sheet pilings have been driven to the required

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