VIADUCT. A bridge erected over a valley, for the purpose of avoiding the necessity for carrying a roadway either by long inclines, by zigzags, or by precipitous descents, from high level on one side of the hills bounding the valley, to a corresponding height on the other side. The conditions under which it is advisable to incur the expense of such a work, are principally when the annual cost of the traction upon the additional length of the inclined roads would exceed the interest upon the capital invested in the construction of the viaduct, added to the cost of its repairs; or when the rate of inclination of the roads is such as to render economical traction impossible. In roads designed to accommodate rapid traffic, for instance, Inclines of 1 in 12 ars inadmissible under any circumstances, and inclines of I in 80 are objectionable ; but in either case the construction of a viaduct for the purpose of obviating the necessity for the incline must depend upon the number of the carriages likely to resort to it. In railway structures viaducts are more commonly employed than in common roads, on ac count of the greater influence of the inclinations of the roadway upon the traction ; but even in them the use of viaducts can only be justified by considerations of economy.
In the article BRIDGES will be found a summary description of the dimensions of the most celebrated viaducts hitherto constructed. The principles upon which they are built have been discussed either in that article, or under ARCHES. It may suffice, therefore, here to add, that the skill of the engineer may be as usefully displayed in the execution of an embankment for the purposes of the roadway as in the construc tion of a monumental viaduct ; and that the repairs of the former would be in all probability much less than those of the latter. The 1Iighgate Archway, the North Bridge at Edinburgh, the Dee Viaduct near Cliirk, the Crumlin Viaduct, the Aricia, Barentin, Chaumont, Dinting Vale, Elsterthal, Goeltzchthal, Malaunay, Ouse Valley, Port age, Tyne, viaducts, may be cited as the most remarkable works of this description.