_Floating_-BRIDGE, a bridge made use of in form of a work in fortification called a redoubt; consisting of two boats, covered with planks, which are solidly framed, so as to bear either horse or artillery. Bridges of this kind are frequently used. Floating bridges made of large logs of light timber bound together with a floor along them are common in the United States. _Ponton_-BRIDGE, a number of tin or copper boats placed at the distance of 7 or 8 feet asunder, each fastened with an anchor, or a strong rope that goes across the river, running through the rings of the pontons. They are covered with baulks, and then with chests or planks, for the army to march over. See PONTON. _Cask_, or _Barrel_ BRIDGE, a number of empty casks that support baulks and planks, made as above into a bridge, where pontons, &c. are wanting. Experience has taught us that 5 ton of empty casks will support above water 9000 pounds: hence any calculation may be made. BRIDGES are made of carpentry or masonry. The number of arches of a bridge is generally made odd; either that the middle of the stream or chief current may flow freely without interruption of a pier; or that the two halves of the bridge, by gradually rising from the ends to the middle, may there meet in the highest and largest arch; or else, for the sake of grace, that by being open in the middle, the eye in viewing it may look directly through there as we always expect to do in looking at it, and without which opening we generally feel a disappointment in viewing it. If the bridge be equally high throughout, the arches, being all of a height, are made all of a size, which causes a great saving of centering. If the bridge be higher in the middle than at the ends, let the arches decrease from the middle towards each end, but so that each half have the arches exactly alike, and that they decrease in span proportionally to their height, so as to be always the same kind of figure. Bridges should rather be of few and large arches, than of many and small ones, if the height and situation will allow of it. Names of all the terms, peculiar to BRIDGES, &c. _Abutment._ See _Butments_. _Arch_, an opening of a bridge, through or under which the water, &c. passes, and which is supported by piers or butments. Arches are denominated circular, elliptical, cycloidal, caternarian, equilibrial, gothic, &c. according to their figure or curve. _Archivolt_, the curve or line formed by the upper sides of the voussoirs or arch-stones. It is parallel to the intrados or under side of the arch when the voussoirs are all of the same length; otherwise not. By the archivolt is also sometimes understood the whole set of voussoirs.