Chap. VIII

BRITAIN.

499

river; they are 36 feet in width at the base, and 24 feet at the point where the verticalpart of the piers commences ; the whole rests upon ten rows of piles.

At high tides the lowest course of masonry is 36feet below the surface of the water; considerable carewas consequently requisite in the construction of thecofferdam. It was formed of three rows of. piles repre-senting on the plan three oblong octagons, one within -r

the other ; the clear width of that on the inside was 60 Sis IP

feet, and the length nearly double; the thickness wasabout 6 feet. The piles were 13 or 14 inches square,and 50 feet in length; of this 15 feet 8 inches weredriven into the bed of the river, 28 feet allowed for hightides, and 6 feet above high water. Traverses werefixed inside the dam 60 feet in length, pressing at eachend against longitudinal timbers, which prevented therows of piles from being forced in by the pressure of thewater from without. On the inside of the dam, aroundthe space left for the pier, a range of planks was driven,

6 or 8 inches in width, united closely together withouteither nails or grooves, in order to prevent the groundbeing washed away between the piles. The piles of theabutments were driven 3 feet 4 inches distant from eachother, and covered with a strong frame of timberplanked all over. Below were sixteen or eighteenisolated piles, to protect the dams against the craftnavigating the river in the service of the works. Thefirst piles were driven to form the figure of the threeoctagons, and others were then forced into the intervals,so as to close them, and when the three rows werethus completed, the gravelly earth to form the damwas thrown in. A steam-engine of fourteen horse-power, erected on the banks of the river, gave motionto the several pumps, and by means of scaffoldingreaching from the shore to the cofferdams the alternatemovements were transmitted.

To build the abutments a dam was constructed infront of them, and when the water was pumped out theground was prepared, at a slope of two in fifteen towardsthe bed of the river; the piles were then driven perpen-dicular to this inclination. A timber floor rested onthe head of these piles, like that of the piers, and onthis was laid the first course of Portland stone, eachcourse being on aplane more and more inclined towardsthe river; the stones were prevented from sliding bydice and the power of their own weight; by thismeans the plane of the upper course was made exactlyperpendicular to the springing of the arch, and capableof more effectually resisting the pressure of the firstrib. The total width of each abutment is 80 feet.

Each arch is composed of eight ribs of the same formand dimension ; each rib is divided into two parts; thatwhich forms the lowest contour of the arch is massive,and the other, which rests upon it, consists of open work.

These iron voussoirs are in thickness 2| inches, but ad-ditional strength is given to them by a rim of 4 inchesin width, which enables the joints to bear greater ver-tical pressure. Transverse plates of iron of the samebreadth and thickness as the voussoirs, and isolatingthem from each other, traverse all the ribs at rightangles, in numbers equal to the joints between the vous-soirs of each rib; and on the abutments, where thelowest voussoirs rest, are similar plates to carry thearch. Thus the whole becomes a centre to support firstthe spandrills, and afterwards the roadway. The span-drills are formed of open work resting vertically uponthe voussoirs, and those on the outside have a lozenge

k k 2

Fig. 495.