Engineering Wonders of the World
Volume I

288 ENGINEERING WONDERS OF THE WORLD. Fig. 2.—AN ANCIENT TRANSPORTER BRIDGE. wanderer’s assistance, and a fallen tree enabled him to continue his journey ; and from this fallen tree, or a plank thrown across a stream, have been evolved, by most interesting and instructive stages, the gigantic structures now to be seen in all parts of the world. The primary object of any bridge is to enable road or railway traffic to be carried between two points across some intervening obstruction ; and considering the numerous types of bridges now available, the question to be decided is, which particular type is best adapted for certain conditions of site and traffic ? One of the problems which has very often to be dealt with is the best method of providing facilities for taking the traffic across a navigable river. A fixed bridge which, under ordinary circum- sary to adopt some form of movable bridge ; and to within comparatively recent years this form has been either a swing, draw, or a bascule bridge. These three types of bridges answer their purpose as far as the accommodation of the cross-road traffic and the free passage of the vessels are concerned, but the opening span has to be in a definite position and of somewhat limited dimensions. In many important estu- aries and rivers the vessels go up and down the waters under their own sail, and to be able to make for a definite opening requires skilled navigation not unattended by great risks, especially if there be a strong tide running, as often happens. It would therefore appear that, for any river or estuary navigated by large vessels to be crossed satisfactorily by a bridge, this last should be of a type which does not impede navigation at all, and gives ample clear- ance for the masts. A high-level bridge can be adopted only where the banks on either side of the water are of sufficient height to reduce the length and consequent cost of the ap- proaches. The Clifton Suspension Bridge over the Avon at Bristol is a good illustration. Such physical conditions obtain very seldom, however. All these difficulties of site and navigation are overcome by the adoption of a Transporter stances, is the best form to adopt, would be quite out of the question for a navigable channel; for it would have to be of such a height to clear the ships’ masts that, in most cases, the cost of construction and the pur- chase of land for the long approaches would quite pre- clude its adoption, unless there were high banks on both sides of the channel. Fig. 3.—DESIGN FOR MIDDLESBOROUGH BRIDGE.' It would therefore be neces-