_Elevation_, the orthographic projection of the front of a bridge, on the vertical plane, parallel to its length. This is necessary to shew the form and dimensions of the arches and other parts, as to height and breadth, and therefore has a plain scale annexed to it, to measure the parts by. It also shews the manner of working up and decorating the fronts of the bridge. _Extrados_, the exterior curvature or line of an arch. In the propositions of the second section in Professor Hutton’s _Principles of Bridges_, it is the outer or upper line of the wall above the arch; but it often means only the upper or exterior curve of the voussoirs. _Foundations_, the bottoms of the piers, &c. or the bases on which they are built. These bottoms are always to be made with projections, greater or less, according to the spaces on which they are built: and according to the nature of the ground, depth and velocity of water, &c. the foundations are laid and the piers built after different manners, either in caissons, in battardeaux, on stilts with sterlings, &c. for the particular method of doing which, see each under its respective term. The most obvious and simple method of laying the foundations and raising the piers up to the water-mark, is to turn the river out of its course above the place or the bridge, into a new channel cut for it near the place where it makes an elbow or turn; then the piers are built on dry ground, and the water turned into its old course again; the new one being securely banked up. This is certainly the best method, when the new channel can be easily and conveniently made. This, however, is seldom or never the case. Another method is, to lay only the space of each pier dry till it be built, by surrounding it with piles and planks driven down into the bed of the river, so close together as to exclude the water from coming in; then the water is pumped out of the inclosed space, the pier built in it, and lastly the piles and planks drawn up. This is coffer-dam work, but evidently cannot be practised if the bottom be of a loose consistence, admitting the water to ooze and spring up through it. When neither the whole nor part of the river can be easily laid dry as above, other methods are to be used; such as to build either in caissons or on stilts, both which methods are described under their proper words; or yet by another method, which hath, though seldom, been sometimes used, without laying the bottom dry, and which is thus: the pier is built upon strong rafts or gratings of timber, well bound together, and buoyed up on the surface of the water by strong cables, fixed to the other floats or machines, till the pier is built; the whole is then gently let down to the bottom, which must be made level for the purpose; but of these methods, that of building in caissons is best.