Piles are either used to build the foundations on, or they are driven about the pier as a border of defence, or to support the centres on; and in this case, when the centreing is removed, they must either be drawn up, or sawed off very low under water; but it is better to saw them off and leave them sticking in the bottom, lest the drawing of them out should loosen the ground about the foundation of the pier. Those to build on, are either such as are cut off by the bottom of the water, or rather a few feet within the bed of the river; or else such as are cut off at low-water mark, and then they are called stilts. Those to form borders of defence, are rows driven in close by the frame of a foundation, to keep it firm, or else they are to form a case or jettée about the stilts, to keep the stones within it, that are thrown in to fill it up: in this case, the piles are grooved, driven at a little distance from each other, and _plank-piles_ let into the grooves between them, and driven down also, till the whole space is surrounded. Besides using this for stilts, it is sometimes necessary to surround a stone pier with a sterling, or jettée, and fill it up with stones to secure an injured pier from being still more damaged, and the whole bridge ruined. The piles to support the centres may also serve as a border of piling to secure the foundation, cutting them off low enough after the centre is removed. _Pile-driver_, an engine for driving down the piles. It consists of a large ram or iron sliding perpendicularly down between two guide posts; which being lifted up to the top of them, and there let fall from a great height, comes down upon the top of the pile with a violent blow. It is worked either with men or horses, and either with or without wheel-work. The bridge on Schuylkill, Philadelphia, is a master-piece of workmanship; and the new bridge at Trenton, over the Delaware, is equally bold and ingenious in its plan--in the latter the floor is suspended from the voussoirs of the arches, by stirrups of iron. _Pitch_, of an arch, the perpendicular height from the spring or impost to the key stone. _Plan_, of any part, as of the foundations, or piers, or superstructure, is the orthographic projection of it on a plane parallel to the horizon. _Push_, of an arch. See DRIFT. _Salient angle_, of a pier, the projection of the end against the stream, to divide itself. The right-lined angle best divides the stream, and the more acute, the better for that purpose; but the right angle is generally used, as making the best masonry. A semicircular end, though it does not divide the stream so well, is sometimes better in large navigable rivers, as it carries the craft the better off, or bears their shocks the better. _Shoot_, of an arch. See DRIFT. _Springers_, are the first or lowest stones of an arch, being those at its feet, and bearing immediately on the impost.