Chap. VIII.

BRITAIN.

461

Fig.463. plan.

It must be admitted, after the description given of the bridges erected in England at thecommencement of the present century, that our engineers have exhibited great science, andintroduced such a method of construction as has greatly economised material. The thrustand pressure of the arch on its several points are better understood, and the nature of thematerials employed is also more thoroughly known : the qualities of all used in such con-structions have been tested by Mr. George Rennie upon a large scale, and their capacitiesof expansion and contraction under the various changes of temperature examined andreported upon most satisfactorily by that eminent engineer. He has also subjected themetals, stone, timber, and brick, to severe pressure, and ascertained their relative strengthsand fitness fbr construction.

Chester Bridge , for which an act was obtained in 1825, is one of the last designs of Mr.Harrison, an architect of that city ; it has one segmental arch of stone over the Dec 200feet span, the largest yet constructed; the key-stone is 54 feet above the level of low watermark, and the roadway is 33 feet in width.

This bridge is situated between the castle and the village of Overlegh, immediately at thehead of the harbour, where the tide rises 12 feet at ordinary springs. The abutments arcfounded on the solid rock, except for a small portion, where it was necessary to pile. The archis the segment of a circle whose radius is 140 feet, and the rise, or versed sine, 42 feet. Thevoussoirs at the crown are 4 feet deep, and increase towards the springing, where they are6 feet. The centre, executed by Mr. Trubshaw, the contractor, consisted of six ribs inwidth; the span of the arch was divided into four spaces by three piers, at regular dis-tances, built up in the river, from which the timbers spread like a fan towards the soffite,so that each timber received its weight in the direction of its length ; the lower ends ofthese radiating supports rested on cast-iron shoes, placed on the tops of the stone piers, andthe upper ends were bound together by two thicknesses of 4-inch plank, cut and arrangedto follow the form of the arch ; on these were laid the lagging or covering, 4^ inches thick,which was supported over each rib by a pair of folding wedges 16 inches long, and 1 footbroad, tapering about 1£ inches; each course of voussoirs had six pair of striking wedges.The horizontal timbers of the centre were 13 inches deep, and the six ribs were tied toge-ther transversely near the top by bolts of inch iron ; the timber used was fir, and thequantity required about 10,000 cube feet. When the centre was removed the crown sunkonly 2^ inches. The cost of the bridge was 42,400/., and the approaches 7500/., making atotal of 49,900/.

One of the chief bridge-builders at the end of the last and commencement of the presentcentury was Thomas Telford, who rendered his name celebrated throughout Europe bythe erection of the Menai suspension bridge ; he was born at VVestkirk, in the district ofEskdale, the 9th of August, 1757, and died the 2d of September, 1834, aged 77. Few menhave done more to advance the profession of the civil engineer: he commenced life as a mason,on the property of the Duke of Buccleuch, and afterwards worked at Somerset House, underSir William Chambers , about 1787 ; his talent and industry gained him the good opinionof Sir William Pultenay, for whom he had done some repairs at Shrewsbury Castle, and byhim he was appointed surveyor to the county of Salop . His first public work was abridge over the Severn , at Montford, four miles west of Shrewsbury ; it consists of threeelliptical arches, one of 58 feet, the others 55 feet span, and measuring 20 feet across thesoffite; here he made use of cofferdams for the construction of the piers, and the whole wassatisfactorily performed with the red sandstone of the country for 5,800/.

Buildwas Bridge , mentioned among those constructed of iron, was his next work, afterwhich he built upwards of forty, stone bridges in the county of Shropshire , the span of thearches varying : two were 85 feet span ; three of iron, 55 feet; one of stone, 50 feet; fourof stone, 40 feet; two of stone, 35 feet; one of iron, 27 feet; two of stone, 24 feet; nineof stone, 20 feet; and sixteen less than 24 feet span. For the last twenty-eight years ofhis life he was engineer under the commissioners for making the Highland roads,bridges, &c.