Chap. VIII.

BRITAIN.

429

The bridge consists of nine elliptical arches; the middle one is 100 feet span, and the othersdecrease gradually to 98 feet, 93 feet, 83 feet, and the two outer ones each 70 feet, leavinga clear water-way of 788 feet. The breadth across the bridge is 43 feet 6 inches, and thetotal length from wharf to wharf is 995 feet. The breadth of the carriage-way was 28 feet,and that of the footpath on each side 7 feet; tins, however, has recently undergone con-siderable alteration, and the effect of the original design is entirely destroyed. The pierswere built in caissoons, like those at Westminster, but previously to their being mooredand placed in their respective situations, the whole of the space to be occupied by them wasclosely piled over. They are smaller in proportion to the arches than those of Westminsterbridge, and the whole work shows that considerable advances had been made in buildingsof this character. The Ionic columns resting on the piers, which diminish in height, taking thecurvature of the roadway, have been censured, but there is much to admire, if we take intoconsideration the period of its erection. The caissoons were made of fir timbers, as wereall the piles ; the sides and bottom of the former were constructed like those at Westmin-ster, but their form was rectangular ; their length was 86 feet, their breadth 33 feet, andtheir entire height 29 feet. The sides were secured to the bottom by strong iron straps,six on each side, and three at each end, 20 feet in length. At one end a portion about 10feet was hung on short hinges, and a floor at 16 feet from the bottom served to brace andsecure the sides, and to receive the machinery which worked a chain pump. Level withthe top was a similar floor, on which was placed the capstan for lifting the stone; there wasalso a triangle for the same purpose, and a windlass for lifting the mortar. Four uprightpieces of timber were secured to each side of the caissoon, which, with the assistance ofbarges, enabled them to be removed. When the stone-work was built up to the level oflow water, a barge was laid alongside, and fixed to the upright pieces, so that when the tideraised the barges they lifted up the caissoon, care being taken to disengage the long ironstraps and the movable piece at one end; and when the caissoon was sufficiently raised toclear the bed of the river, it was floated off. The height of the caissoon rendered bargesnecessary, particularly as all the machines were attached to the upper floor.

Many stone bridges were erected in England during the eighteenth century, some ofwhich exhibited considerable talent, particularly those executed under the directions ofMr. Gwynn, a native of Shrewsbury. Kew, Maidenhead , Henley, and Oxford had stonebridges over the Thames , but one of the boldest designs was by Sir Thomas Robinson,who, in 1762, constructed a single arch at Winstone, over the Tees, with a span of 108feet 9 inches; the versed sine being 45 feet, the diameter of the circle of curvature at thevertex 112 feet, and the height of the key-stone 3 feet.

The bridges built by John Smeaton do not display any boldness of design, nor remark-able science in their construction ; we have already noticed much that he performed forthe improvement of our harbours, and also his great undertaking and masterpiece, theEddystone lighthouse . This celebrated man, and perhaps first practical civil engineer inEngland, was born at Austhorpe, near Leeds , on the 28th of May, 1729; he very early dis-played a fondness for mechanics, and at twenty-one we find him employed as a mathematicalinstrument maker. About 1750 he transmitted to the Royal Society an account of theimprovements in the mariner’s compass by Dr. Knight, and two years afterwards hefurnished the same society with those he had made on the air-pump, and an engine forraising water by fire, a subject upon which the French philosophers were then busily en-gaged : he was soon afterwards admitted as a member, when he introduced his ideas uponthe pyrometer, and several other subjects. About this period he was induced to travelinto Holland and the Netherlands , where he had the opportunity of examining the systemof draining and management of locks on the canals, and several important engineeringworks then progressing under the direction of very able men : and on his return to Englandhe occupied himself in inquiries relative to the powers of water and wind, on their appli-cation to machinery, where circular motion was required; and after great patience andlabour he was enabled to exhibit their results to the learned men of the society in severalmodels of undershot, breast, and overshot wheels, which he accompanied with calculations oftheir relative value : the novelty as well as utility of his observations obtained for him theCopley gold medal, and, what was far more important, he was selected as the builder of theEddystone lighthouse by the Earl of Macclesfield, who was then president. We havealready seen with what diligence he set about preparing himself for that great work, andhow ably he accomplished the very arduous and difficult task : after its completion, thepublic seem not to have encouraged his talent as he merited, for we do not find him em-ployed again until 1761, when he was associated with Mr. James Brindley in surveying forseveral canals in Staffordshire . Among his reports dated after this year are some oncanals, improvements of rivers and harbours, draining, and a description of many usefulinventions and applications of foreign methods for constructions in water. His reports areexceedingly valuable, and show that he was admirably qualified for all he undertook. Hewas a great observer of nature, and applied his mind diligently to examine her works;