_Piers_, the walls built for the support of the arches, and from which they spring as their bases. They should be built of large blocks of stone, solid throughout, and cramped together with iron, which will make the whole as one solid stone. Their faces or ends, from the base up to high-water-mark, should project sharp out with a salient angle, to divide the stream: or, perhaps the bottom of the pier should be built flat or square up to about half the height of low-water-mark, to allow a lodgement against it for the sand and mud, to go over the foundation; lest, by being kept bare, the water should in time undermine, and so ruin or injure it. The best form of the projection for dividing the stream, is the triangle; and the longer it is, or the more acute the salient angle, the better it will divide it, and the less will the force of the water be against the pier; but it may be sufficient to make that angle a right one, as it will make the work stronger; and in that case the perpendicular projection will be equal to half the breadth or thickness of the pier. In rivers, on which large heavy craft navigate and pass the arches, it may, perhaps, be better to make the ends semicircular: for, although it does not divide the water so well as the triangle, it will both better turn off and bear the shock of the craft. The thickness of the piers should be such as will make them of weight or strength sufficient to support their interjacent arch independent of any other arches; and then, if the middle of the pier be run up to its full height, the centering may be struck to be used in another arch before the hanches are filled up. The whole theory of the piers may be seen in the third section of Professor Hutton’s _Principles of Bridges_. They should be made with a broad bottom on the foundation, and gradually diminishing in thickness by off-sets up to low-water-mark. _Piles_, are timbers driven into the bed of the river for various purposes, and are either round, square, or flat like planks. They may be of any wood which will not rot under water; but oak and fir are mostly used, especially the latter, on account of its length, straightness, and cheapness. They are shod with a pointed iron at the bottom, the better to penetrate into the ground, and are bound with a strong iron band or ring at top, to prevent them from being split by the violent strokes of the ram by which they are driven down.