Of the important bridges built on the great military roads of the Empire — the Appia, the Aurelia, the Flaminia, the Via Salaria or Ponte Salaro, the Cassia, the Valeria, the Latina and the /Emilia, those built during the Augustan period were the most remarkable. The Flaminia commenced at the Mulvian Bridge and ended at the Ariminium (Rimini) Bridge. The latter, which consists of five great arches with a total length of 236 feet, is the best pre served of them all; but vestiges of a great many others still remain, the most important being those near Narni and Borghetto, the great one of Vincenza, rebuilt at Verona, and those near Aosta and Calzi.
With the expansion of the empire many bridges were built upon the fine macadamized roads which connected the various provinces with the Eternal City, by the Roman legions under the direction of their skilful military engineers. Of these, the greatest was the bridge across the Danube, built in 103 A.D., by the engineer Apollodorus, to enable the Em peror Trajan to conquer Dacia. It consisted of 20 massive stone piers, 60 feet square, con nected together by a superstructure of wood with arches of 170 feet span. It had a height of 150 feet and a total length of 4,770 feet. It was subsequently destroyed by Hadrian, but the great piers are still standing. They were built in caissons in 18 feet of water, and illus trate effectively the consummate daring of the Roman engineer.
Prior to its construction, intervals of such enormous length had been spanned by tem porary bridges of boats, a practice handed down from a period reaching farther back than that of Xerxes, Darius, Hytarpus or Cyrus in the 6th century a.c. It is interesting to note in this connection, however, that the most cele brated bridges built by the Romans were not generally distinguished for the great span of their arches, their great total length or the peculiar lightness of their piers, but for their excellence of construction, and durability. The span of their arches seldom exceeded 70 or 80 feet, and their height was nearly half the span, thus giving the arches a semi-circular form, or, more exactly, a segment of that form.
In all the countries which formed the provinces of the Roman empire, with the ex ception of the Spanish Peninsula, most of the bridges built by the Romans were allowed to fall to pieces during the Dark Ages, so that very few of them have survived. The ruins and foundations of some still exist near Vaison, and Chateau Neuf, in France, but are not of sufficient size to convey much of an idea of the special characteristics of the struc tures. In Spain, however, the high culture of the Moors ensured the preservation of these superb examples of Roman engineering. Of these, the best examples are the bridges at Cuenca, Evora, Martorell, Merida, Chaves, Al conetar, Orense, 011oniego, Almazan, Ona and Salamanca. The greater number of them were built by Trajan, and that of Salamanca is the most magnificent of all. Many of these struc tures were adorned by triumphal and memorial arches at each end, or in the centre, which, while adding greatly to their architectural ef fectiveness, also served the purpose of a toll gate, or a fortification. Interesting examples
of these are the bridges at Saint Chamas, in France, and the Ponte Salaro at Rome.
The Moors not only preserved the Roman structures, but imitated them and built many bridges fully their equals in size and elaborate ness of design. The bridge at Cordova, across the Guadalquivir, is of 16 spans and is one of the most notable examples of their work, and also that of Alcantara, across the Tagus, built by Lacer during the reign of Trajan. It con sists of six granite arches, the largest, 115 feet span. It has a total length of 600 feet, with a width of 26 feet, and carries the roadway at a height of 45 feet above the level of the river. The early history of Oriental bridge-build ing appears to extend back to the days and works of Semiramis, who is credited by Dio dorus Siculus with the construction of a bridge across the Euphrates at Babylon about 776 B.C., and is described by him as a movable draw bridge, 30 feet wide, supported by stone piers. The earliest existing examples of Oriental work, however, appear to be the Persian bridges at Dizfel 1,250 feet long, and at Shus ter 1,700 feet long, which were probably built during the rule of the later Achmminid Kings over Iran, about 350 a.c. Their design em bodies the flat-pointed arch characteristic of later Mohammedan architecture, and they are still in a fair state of preservation.
It is noteworthy, that, while the culture and continuity of the Byzantine and Mohammedan civilizations maintained in good condition the bridges already built, and created new ones of equal usefulness and magnitude, the science and art of bridge-building was practically lost in the European countries during the six centuries (500-1100 A.D.) which comprised the Dark Ages. In the 12th century, however, the archi tectural and engineering sciences felt the awakening impulses which culminated in the magnificent structural creations of the Renais sance. The revival was first experienced in Italy and France. Bridges were built not only by the governing authorities but also by the churches—the building of a bridge being con sidered in the nature of a pious undertaking by a religious order of the Benedictines, known as the "Brethren of the Bridge." Bridges were built at Tours, Orleans and Vienna, and many other large cities, and the Rhone was spanned at Lyons and Avignon by two fine bridges, the last named being about 3,000 feet in length. It was built during the years 1178-88, by Benezet. It is remarkable on account of the elliptical curvature of the arches, the radius of which is shorter at the crown than at the haunch, and, therefore, conforms more nearly to the linear equilibrated arch, than the modern elliptical arch which has the longer radius at the crown. It consisted of 19 arches. The span of the largest arch was 110 feet 9 inches with a height of 45 feet 10 inches. Four of the arches still remain.