156 TRANSACTIONS OF THE [mAT 23, 



afford an easy passage for the barbarians into the Empire. Some 

 of the piers are still to be seen, however, near the town of 

 Walkel, Hungary. 



The next considerable work of this kind built by the Romans 

 is the Pont du Garde, Avhich is still standing. 



It served the double purpose of a highway bridge over the 

 Gardon and an aqueduct for supplying with water the town of 

 Nismes in the south of France. The bridge is a triple arcade, 

 the lower tier of arches having a total length of 660 feet, and 

 supporting a second tier of eleven arches of 780 feet, and on 

 these is supported a third tier 850 feet in length. This brings 

 the structure up to the level of the aqueduct, which rests on the 

 third tier of arches. This extraordinary structure is built of 

 very large stones put together without cement and held together 

 by iron clamps. The whole height is 190 feet above the river. 



Another Roman bridge over the Tajo at Valenza, about 25 

 miles from Madrid, 679 feet in length, and consisting of only 

 six arches, was built at the time of Trajan. It is now standing, 

 but not used. 



A single arch near the old town of Brionde, in France, having 

 a span of 181 feet, and which is said to be still standing, is at- 

 tributed to the Romans. 



Another bridge, 2,400 feet in length, near Lyons, in the south 

 of France, was also erected by the Romans. 



Among the later Roman arched bridges may be mentioned the 

 Devil's Bridge over the Serchio, Italy, which is, in some 

 respects, the most remarkable in the whole history of stone 

 bridges, being only twelve feet wide between the parapets, and 

 sj^anning a stream in which the floods rise sometimes nearly to 

 the crowns of the arches, and yet, while every other structure 

 on the turbulent Serchio has been swept away, this bridge has 

 withstood the floods of nearly nine centuries. 



The bridge of Trezzo, built in 1380 by Bernabo Visconti, 

 Duke of Milan, consisted of a single arch of granite of 251 feet 

 span, the largest stone arch probably ever erected. It was de- 

 stroyed by Carmagnola 120 years after its erection. 



The aqueduct bridge of Alcantara, near Lisbon, begun 1713, 



