Askezc, or askew arches, whose faces are oblique to their axes, present considerable difficulties in construction, especially when the material employed for the purpose is hewn stone, because of the com plicated shapes of stone required. They demand, therefore, special treat ment. One form of askew arch is shown on Plate 41 (fig. 9).
Apeducis ana' o-reatest elevation above the surface level is exhibited in the ancient aqueduct of Spoleto (fig. 13), built twelve hundred years ago and still in an excellent state of preservation. The centre-pieces of this vast structure, which rest in the bed of a mountain stream, have a height of 656.6 feet. Among the masterpieces of construc tion may be mentioned, likewise, the Roquefayour Aqueduct (fig. 12), near Aix, 129o.3 feet long and 265.9 feet high. The Goeltzschtlial and Elsterthal viaducts, on the line of the Saxon-Bavarian State Railroad, are notable examples of recent construction.
The Goeltzschthal Iiira'zict (pi. 41, fig. lo), completed in 1851, is built in its main portion of four tiers of masonry arches, the central portion being formed of two large superposed arches. The total length of the viaduct is 1900.5 feet; elevation above water-surface, 263.6 feet., span of the lower central arch, 94.25 feet; height, 136.2 feet; span of upper centre arch, 101.74 feet; height from the deck-plane of the lower one, 104.5 feet. The smaller arches are built open, and the disposition of the material employed is snch that the structure combines strength with lightness.
The Elslerthal iadzia (fig. t i) is a two-story masonry structure. The lower story is formed of two double piers pierced with small arches, two large arches, and two retaining-walls. The length of the lower tier is 550.5 feet, and its height 110.32 feet. The central arches have a span of 95.16 feet. The upper tier has two double piers and six large cylindrical arches, each of 91.5 feet span. The total leng,th of the upper tier is 918 feet and its height is 113.8 feet, making the total height of the structure above the water-surface 224.15 feet. The viaduct of Chaumont has a
height of 164.16 feet and a length of 1936.6 feet.
In France and England the custom has been followed largely of em ploying for important works of this nature unbroken or continuous piers and of more or less slender proportions, copying after the pattern of the Spoleto Viaduct, above referred to. In Germany the preference has been given generally to the plan of building important structures in tiers or stories, one tier of arches being carried above another, after the pattern of the Roman aqueducts; witness the examples above noted. The lowest tier of arches is occasionally carried through only a portion of the wall, and not through its entire width (fig. It, cross-section). The plan of building in tiers has been modified in the case of the Pont du Jour, over the Seine, in Paris (figs. 6-8). Here a double-track railway viaduct is carried on the back of a broader road-bridge, passenger travel being accom modated on a lower roadway carried through apertures pierced in the piers of the upper story of arches. The Calvine Viaduct (fig. 4), in Perthshire, Scotland, exhibits a form of construction in which a railway bridge crosses a highway bridge, running obliquely to its axis and at a lower level. In England, on account of the excellent quality of the cement available for building- purposes, the practice of leaving large vacant spaces inside the masonry of the piers is extensively followed. On the Continent, on the other hand, the general disposition has been to give sncli members a compact construction.
With the founding- of the corps of engineers in France, in 1720, bridge constrnction received a fresh impulse. Perronet was the great master of the art in the eighteenth century. At present large bridges of stone are rarely built, on account of their excessive cost as compared with that of another type of structure, the iron bridge, which has been perfected to a remarkable deg,ree within recent years, and which will be considered in the following section.