THE PRINCIPLES OF BRIDGES.

TRACT I.

execution. With respect to the form; strength, utility, andbeauty ought to he regarded and united ; the chief part ofwhich lies in the arches. The form of the arches will dependon their height and span ; and the height on that of the water,the navigation, and the adjacent hanks. They ought to bemade so high, as that they may easily transmit the water atits greatest height, either from tides or floods; and theirheight and figure ought also to be such as will easily allow ofa convenient passage of the craft through them. This, andthe disposition of the bridge above, so as to render the passageover it also convenient, make up its utility.—Having fixed theheights of the arches, their spans are still necessary for deter-mining their figure. Their spans will be known by dividingthe whole breadth of the river into a convenient number ofarches and piers, allowing at least the necessary thickness ofthe piers out of the whole. In fixing on the number ofarches, let an odd number always be taken; and few andlarge ones, rather than man}’ and smaller, if convenient: Forthus we shall have not only fewer foundations and piers tomake, but fewer arches and centres, which will producegreat savings in the expense; and besides, the arches them-selves will also require much less materials and workmanship,and allow of more and better passage for the water and craftthrough them; and will appear at the same time more nobleand graceful, especially if constructed in elliptical, or in cy-cloidal forms; for the truth of which, it may be sufficient torefer to that noble and.elegant bridge lately built at Black-friars, London, by Mr. Mylne ; which might perhaps be ac-counted incomparable, at least in England, if the piers wereof equal excellence: but these are too thick, and clumsy, andtheir appearance is made still less graceful by the double co-lumns placed before them. So that Blackfriar’s arches andthe Westminster’s piers united, would be preferable to eitherbridge separately.

If the top of the bridge be a straight horizontal line, thearches may be made all of a size; if it be a little lower atthe ends than the middle, the arches must proportionally dc-