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 272 BRIDGE ton aqueduct, over the Harlem river at New York ; its total length is 1,460 ft., and the top of the parapet is 116 ft. above high water ; there are in all 15 semicircular arches, 8 of which are of 80 ft. span, and 7 of 50 ft. (See AQUEDUCT.) The arches of bridges are of three principal kinds: 1. The semicircular; these were an- ciently most in use, and have the advantage of being easy to construct, and forming a solid structure ; their span is, however, restricted, on account of the great relative height of this form of arch, and as they are usually made of moderate size, they have the inconvenience of considerably obstructing the passage of the water. 2. Arches of a flat vault, either form- ing portions of an ellipse, or else described by several arcs of circles of different radii. Ellip- tical arches are pleasing to the eye, but trouble- some to construct, on account of the continual change in the form of the successive voussoirs ; hence it is usual to employ curves composed of a certain number of arcs of circles, varying generally from three to eleven. The use of flat- vaulted arches was introduced into France about the close of the 17th century, and their adoption was due to the necessity of affording a wider discharge without considerably aug- menting the height of the arches. This form not only answers this object effectually, but, when the two diameters are not very unequal, presents as much solidity and facility in con- struction as the semicircular arch. 3. Arches formed from an arc of a circle, which are of two principal kinds: first, those in which the springing planes are under water, examples of which are seen in the bridge of Saint Es- prit, and the ancient bridge of Avignon ; this form has the disadvantage of greatly reducing the discharge. In the second kind the spring- ing planes are on a level with the highest water of the river, as in the bridge de la Concorde at Paris. In this case the arc is necessarily very low, and the lateral pressure of the voussoirs so considerable as to require great care in the construction. Besides the three of which we have spoken, there is the Gothic arch, compos- ed of two arcs of a circle, sometimes though rarely employed; it has the fault of greatly reducing the outlet. Though the size of the arches is usually determined by local circum- stances, yet there are a few general principles to be considered. Small arches are best adapt- ed to quiet rivers, whose waters do not rise to any considerable height, while large arches are best suited to torrents, where it is difficult to lay the foundations, and where the piers are exposed to damage by obstacles brought down against them by the current. Arches of stone cannot be applied, however, to the wide rivers of Europe or America. In fixing the width of arches two plans are pursued : in one, the aper- tures are all equal, giving the tops of the vaults the same elevation above the water, and en- abling the constructor to use the same center- ing for all the arches. The economy of this arrangement may, however, be counterbalanced by the necessity of forming considerable em- bankments at the termini of the structure. In the other plan, the diameters of the arches are unequal, allowing a reduction of the embank- ments, thus diminishing the obstacles to the approaches. The advantages of both systems are sometimes combined by forming the arches of the same width, and placing the springing planes at heights decreasing from the centre to either extremity of the bridge. The breadth of the bridge depends wholly on the locality, and should be proportioned to the importance of the road on which it is built. For country roads a width of 14 to 16 ft. will be sufficient, particularly if the bridge be a short one. On what are termed roads of the second class, 20 to 25 ft. should be allowed, which will afford sufficient room for two carriages to pass at once, besides a space for foot passengers. On roads of the first class, 30 to 35 ft. is considered a fair allowance, while in the interior of cities from 80 to 60 ft. will be required. The Pont Neuf at Paris, which is perhaps one of the greatest thoroughfares in the world, has a width of about 70 ft. between the parapets. Timber, as a material for bridges, is much less costly and more easily worked than stone ; but all such structures lack the advantage of dura- bility, and are more troublesome to keep in re- pair. The oldest wooden bridge of which we have any account is the Pons Sublicius already mentioned ; it is supposed that no iron whatever was used in its construction. Casar's bridge was also of wood, and so was Trajan's across the Danube, though it is probable that the piers of the latter were of stone. One of the most famous wooden bridges on record is that of Johann Ulrich Grubenmann, an uneducated carpenter of Switzerland ; it was built at Schaff- hausen in 1757, and was composed of two wooden arches with the respective spans of 193 and 172 ft., supported at either terminus by abutments, and at their junction by a stone pier. After Grubenmann's death the bridge began to settle, as the oak beams, which had been placed too low, and not sufficiently ex- posed to the air, rotted at their points of con- tact with the stone abutments. Owing to the peculiar arrangement of the structure, by which the principal supports were so intimately con- nected together, it became necessary to sup- port the whole bridge before a single part could be removed ; this was performed by means of screw jacks, and the decayed timbers replaced. No other repairs were ever required, and the bridge excited much attention as a remarkable specimen of carpentry. It was burnt by the French hi 1799, having lasted 42 years. In modern times, the wooden bridges of Germany and France have taken high rank from their scientific combination in arrangement, but du- ring the last few years the United States have justly claimed the precedence for simplicity and. boldness of design. The upper Schuylkill bridge at Philadelphia, which was burned in 1838, had the remarkable span of 340 ft. It was de-