424

HISTORY OF ENGINEERING.

Rook I.

out it, the wedges being drawn up, liberty was given to clear the straps from the mortises,when the sides by their own buoyancy quitted the grating under the foundation of the pier.

The sides of the caissoons were prevented from being pressed inwards by means of aground timber or ribbon, 14 inches wide, and 7 inches thick, pinned upon the upper row oftimber of the grating, which exactly filled the space contained between the sides of thecaissoon and the first course or lower plinth .of the stone pier; and the top of the sides wassecured by a number of beams laid across, which also served to support a floor, on whichthe masons and labourers stood to hoist the stones out of the lighters, and lower them intothe caissoon.

In October the ballast men commenced making the excavation for the westernmost ofthe two piers; when they dragged and excavated a*pit 6 feet in depth, of the shape of the

Fig. 429.

WESTMINSTER BRIDGE.

caissoon, and 5 feet wider all around, with a slope of such a form that the ground wouldnot fall into it; short grooved piles, reaching 4 feet above low water mark, were also drivenbefore the two ends, and partly along the sides of the intended piers, parallel to the fenders

WESTMINSTER BRIDGE.

Fig. 430.

or guard piles, and at a distance of 15 feet from the sides of the caissoon, to prevent anysilting. Two rows of boards let into grooves were also applied between these piles.

Gauges, formed of flat stones, 15 inches square, and 3 inches thick, with a wooden pole