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In all bridges the piers should be built up solid to about one-third of the rise of the arch, extending to the ends of the archstones. A wall should then be built upon the centre of the pier,to near the levelling for the road, and the space betwixt thatwall and the top of the arches formed into void spaces, by wallsfrom 18 inches to 2 feet thick, built longitudinally from the piersto the arch, at from 4 to 6 feet distance, and either arched withhigh pointed arches to carry the road, or covered from wall towall with long flat stones. In this manner the arches will not beoverloaded, and by avoiding all filling up with loose materials■within the building, it will not be liable to the accidents whichhave occasioned the ruin of many such structures.
The arch of the largest span in Britain is the Ponty Pridd overthe Taafe in Glamorganshire , built in 1755, being 140 feet with35 feet of rise ; but it is very narrow, being only 11 feet. Thegreatest bridge of one arch is that lately built at Aberdeen, whosespan is 130 feet, the rise 29 feet, and breadth within the para-pets 40 feet.