Chap. VIII.
BRITAIN.
463
height. On these were laid another tier of beams, lengthways to the centre, one undereach rib; upon these beams the wedges were fixed, which were of three thicknesses, thebottom one being bolted down to the beams; the tongue or driving piece in the middlewas of oak, well hooped at the driving end ; the top side of the upper piece was laidperfectly level and straight, both transverse and longitudinally. The wedges were rubbedwith soft soap and black lead before they were laid upon each other.
Each rib of the centre was then brought and put together upon a scaffold made uponthe top of the wedge pieces, and lifted up whole, by means of two barges on the river andtwo cranes on shore. 'Hie scaffold was extended 30 feet beyond the striking end of thewedges, to lay the last ribs upon, previously to raising, and for the workmen to stand uponfor finally striking. After the ribs were properly braced, they were covered with the4-inch sheeting piles which had been used in the cofferdams.
This centre was so well formed, that when the arch was keyed, its sinking was not morethan an inch, and it was struck in the short space of three hours. This was performed byplacing beams upon the top of the work directly over the ends of the wedges ; to thesebeams was fixed a tackle, and at its lower end was slung a heavy ram of 12 cwt., withwhich the piles were driven; this ram was swung to and fro, so as to strike the driving endof the tongue-piece of the wedge. This operation required eight men to pull it back, andtwo men to bring it forward ; after twenty or thirty blows the wedges started, they thenslid easily, and pieces were put in to stop their going further than was required. Thecovering was then taken off, and the ribs were let down, in the same order in which theywere put up ; and when taken to pieces were carried on shore. The bearing piles werethen drawn by two 42 feet levers and strong chains.
Bewdley Bridge , over the Severn, in Worcestershire , being injured by the great flood of1795, an act of parliament was obtained to raise money, and levy a toll, that a new bridgemight be constructed j and Mr. Telford furnished the design for one of stone, of three
BBWDLEY BRIDGE.
arches, the centre having a span of 60 feet, and the two outer 52 feet each ; the breadth,measured across the soffite, is 28 feet. The versed sine of the centre arch is 18 feet, andthat of the two others, 16 feet 9 inches each. This bridge was completed in 1798, at anexpense of 9264/.
Bridge at Tongudand , near Kircudbright in Scotland , over the river Dec, has but onearch with a span of 112 feet; the width measured across the soffite is 24 feet. The depthof water at ordinary spring tides is 30 feet, and it required considerable skill to support thecentre to an arch of such magnitude. In order that the arch should not be unnecessarily
TONGUELAND BRIDGE.
loaded, a number of longitudinal walls were carried up to the level of the road, where theywere covered with flat stones, by which means the state of the arch may be at any timeexamined ; this system is a considerable improvement upon the usual practice of filling inthe spandrills with loose earth, which frequently produced an external pressure and destroyedthe work.
The foundations of this bridge were laid in March, 1805, and completed in November,1806, at the cost of 7710/.
Glasgow Bridge, built by Mr. Telford, consists of seven segmental arches, diminishingin their span towards the abutments ; that in the centre has an opening of 58 feet 6 inches,and a versed sine of 10 feet 9 inches ; the two adjoining are 57 feet 9 inches, with versed