474

HISTORY OF ENGINEERING.

Book I.

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where the arehivolt is made to partake of a different sweep to that of the soffite, in orderto produce a greater lightness inthe elevation, or the sofhtes ofthe arches are shaped to suit thecontracted vein of water asformed in the entrance or exit ofpipes. The roadway over thisbridge is maintained throughoutat a perfect level, and there is nobreak or distortion in the corniceor the balustrade : we have seenin some of those constructed byMr. Rennie how much attentionwas paid to have the lines per-fectly horizontal, and althoughin France the slopes to the sidesof bridges had been long aban-doned, it was some time beforethe-system underwent a changein England: in the bridge atLimerick the approaches arewell managed, and the effect ofthe structure is much to be ad-mired. The piers are elegantlycapped and proportioned, and forthe omission of columns, or of anyuseless ornament, the engineerdeserves great praise: simplicityand proportion, coupled withgood construction, are all thathave been aimed at, and in thisexample we find nothing extra-neous. The section through thearch and pier shows the con-struction, and that every meanshas been adopted to lighten theweight, and to strengthen theparts which sustain it: the cur-vature of the intrados meetswith a gentle inclination on thesides of the pier, and the alter-ation from the perpendicularline is a great improvement, notaffecting the width of water-way,but adding to the strength of thefootings of the piers.

It is not possible to enumerateall the stonp and brick bridgesthat have been constructed with-in the last half century, — thosefor the canals and the railroadsalone amount to several thou-sands : it is enough, perhaps, forour purpose to have describedexamples of each variety, andthus exhibited the differentphases of the science.

Every county in the king-dom has had its civil engineer,and some of the works executedare highly creditable, althoughthey do not generally show anacquaintance with the higherprinciples of construction. Tothe late Mr. Rennie we standindebted for our most beautifulbridges, and for the introduction of scientific principles: that eminent engineer had

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FitMIL WELLESLEY BRIDGE, NEAR LIMERICK