112

’/HE PRINCIPLES OF BRIDGES.

TRACT 1 .

injure it. The best form of the protection for dividing thestream, is the triangle; and the longer it is, or the more acutethe salient angle, the better it will divide it, and the less willthe force of the water be against the pier ; but it may be suf-ficient to make that angle a right one, as it will make thework stronger, and in that case the perpendicular projectionwill be equal to half the breadth or thickness of the pier. Inrivers on which large heavy craft navigate, and pass thearches, it mav perhaps be better to make the ends semicircu-lar ; for though it docs not divide the water so well as thetriangle, it will both better turn off and bear the shock of thecraft.

The thickness of the piers ought to be such, as will makethem of weight or strength sufficient to support their interja-cent aixh, independent of any other arches. The thickness,in most cases of practice, may be made about i of the span ofthe arch. And then, if the middle of the pier be run up to itsfull height, the centring may be struck, in order to be used inanother arch, before the hanchcs are filled up. The wholetheory of the piers may be seen in the third section. Theyought to be made with a broad bottom on thefoundation, andgradually diminished in thickness bv offsets, up to low-watermark. The methods of laying their foundations, and build-ing them up to the surface of the water, are given under theword Foundation.

Piles, arc timbers driven into the bed of the river for va-rious purposes, and are either round, square, or flat likeplanks. They may be of any wood which will not rot underwater, but elm, oak, and fir are mostly used, especially thelatter, on account of its length, straightness r and cheapness.They are shqd with a pointed iron at the bottom, the betterto penetrate into the ground ; and are bound with a strongiron band or ring at top, to prevent them from being splitby the violent strokes of the ram by which they are drivendown. It is said, that the stilts, or piles, under London-bridge, are of elm, which lasts a long time in the water.

Piles are either used to build the foundations on, or are