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HISTORY OF ENGINEERING.

Book I.

are surmounted by an entablature. The cornice, level over the centre arch, and inclinedover the two others, is sustained by strong modillions. The breadth is 55 feet 9 inches.

Ancient bridge at Vicenza , has been described by Palladio: the centre arch, 34 feet8 inches span, is very ancient; the two others are modern, their span is 25 feet 11inches. The width of the piers is 5 feet 9 inches; that of the bridge, 27 feet 7 inches.The versed sine of the arcs of which the arches consist is two-thirds of their diameter.They are adorned with archivolts : the cornice is sustained by modillions.

Pilantio Bridge , over the Teverone, near Rome , on the road to Tivoli, is composed ofthree arches, arcs of circles. The thickness of the piers is one-fourth the span of thearches, they have no starlings. It is constructed with very large stones; the total lengthis 170 feet 8 inches.

Bridge and Aqueduct of Spoleto, was built near the town which bears that name, in a. d.741, by Theodoric king of the Goths . It consists of ten large Gothic arches, each 70 feet3 inches in span, and sustained by piers 11 feet 10 inches thick. The centre arches overthe river Moragia are above 328 feet in height. The others are much lower, the twoslopes on which they are built being very steep. At the upper side of the bridge, thirtysmall Gothic arcades sustain an aqueduct, serving to convey water into the town. Thismonument, of a very bold execution, and built of small hard stones, remains entire, andstill conveys water to Spoleto. The total length is 810 feet 5 inches, the breadth 42 feet

8 inches.

Bridge and Aqueduct of Civita Castellano. —This work is part of a causeway, constructedabout 400 years b. c., to approach Castellana , 820 feet long, 33 feet wide, and 128 feethigh, pierced in the centre by nine great arches loaded with about 13 feet of earth.The three centre arches are 87 feet 3 inches in span, the others 63 feet 11 inches. Some ofthe piers are strengthened by counterforts, and others by flying buttresses with a detachedbase.

Bridge on the Cremera, at Civita Castellana . This bridge, celebrated as the place wherethe Veii gained an advantage over the Fabii , b. c. 447, is constructed of brick, stone, andmarble. It consists of three arches, the centre is 74 feet 5 inches span, and the two others50 feet 2 inches ; the breadth is 34 feet 2 inches. The foundation is established on a radieror timber platform, on accopnt of the instability of the soil, on which are inverted arches ofthe same span as those of the bridge.

Trajan's Bridge over the Danube. This colossal work, the most magnificent in Europe ,was built under Trajan by Apollodorus, his architect , about A.n. 120. The rapidity anddepth of the current in the place where it is situated added to the difficulties of the work :a general foundation was constructed by means of large barges filled with stones, liine, andsand, sunk at the bottom of the river ; sacks of different sizes were thrown into theinterstices, filled with the same materials. On this base the piers were established. Thebridge consisted of twenty semicircular arches, 180 feet 5 inches span. Their springingswere raised 46 feet above the general level of the river; the thickness of the piers was64 feet, they were 85 feet 3 inches wide ; the stones used were enormous, but it was de-stroyed a short time after its construction. Some of the piers are still to be seen with thespringings of the arches which they supported.

M. de Marsigli, in his work on the Danube, charges Dion Cassius with having assertedthat the arches of Trajan’s bridge were of stone, and says that they arc representea as woodon the bas-reliefs of Trajan’s Column .

Bridge near Temi , on the Nera, whose ruins still exist, consisted of seventeen arches,131 feet 3 inches span ; its piers were 27 feet 6 inches thick, and 111 feet 6 inches to thespringing : its total length was 2592 feet, and its width 32 feet. It was constructed oflarge blocks of stone, and the piers had no starlings ; the foundation of intermediate piersmay be seen, which divided the opening of each arch into three parts, intendedprobably to support the centres during the construction of the vault, and afterwardsdemolished. The bridge has no parapets, and in their place are white marble blocks,between which chains are suspended as guards.

Bndge of Capo Dorso , believed to have been built in Sicily by the Romans, consists ofone semicircular arch 96 feet span. Its small width may cause us to doubt its Romanconstruction, being only 17 feet.

The Bridge de Boisseron, upon the Domitian Way, near Nismes, said to have been con-structed by C. Domitius iEnobarbus, has five semicircular arches; the centre spans 30feet

9 inches, the two contiguous 28 feet 9 inches, and the others at the extremities 19 feet 6inches ; the piers are 9 feet 4 inches ; the entire width of the bridge, 11 feet 6 inches.

All the arches spring from the same level, consequently the roadway and parapets inclinefrom the centre. The piers between the springing of the arches are perforated, to affordmore water-way, and to prevent too great a pressure at the time of floods.

In this bridge the projecting voussoirs, on which the timbers, struts, or centres, weresupported at the time of its construction still remain ; in .almost all the examples wherehard stone was used for the turning an arch, these projecting blocks are to be seen, as in the