A book of bridges

by Walter Shaw Sparrow

THE HUGE DEFENSIVE BRIDGE AT CÓRDOVA IN SPAIN. ORIGINALLY ROMAN, BUT REMODELLED BY THE MOORS IN THE NINTH CENTURY, RECENTLY SO MUCH REPAIRED THAT LOOKS ALMOST NEW

III

A few remarks must be made on the technique of Roman bridges and aqueducts. Vitruvius mentions a method known as opus quadratum in which stones were put in regular courses of headers[74] and stretchers[75]; they were big stones, about two feet by four feet and two feet high, as in the Marcian Aqueduct dating from B.C. 145.[76] Each stone was bordered with a draft cut one and a half inches wide, and the middle surface was roughed with a pick. This technique may be studied in the aqueducts at Segovia and Tarragona. The arches were set back at their springing behind the imposts, leaving ledges upon which the scaffolds rested.

Not all the Roman aqueducts were of stone. The one named after Nero was in brickwork of the finest kind; and another, the Alexandrine, that brought water to the Thermæ of Alexander Severus, was faced with bricks over concrete. At Minturnæ, a town of the Volci, a decorative effect was given to the wall surfaces by means of coloured tufa arranged in geometrical patterns. This is enough to show that the virile conservatism of Rome did not stereotype building methods.

Many persons believe that the Romans built aqueducts because they were unacquainted with the hydraulic principle that water in a closed pipe finds its own level. Yet Vitruvius gives an account of the leaden pipes that distributed water in Roman towns; and Pliny says that this piping was used very often for rising mains to carry water to the upper floors of houses. But lead pipes might burst, and they were costly; it was cheaper to build aqueducts, for their materials belonged to the State and slave labour was in vogue.[77]

Finally, we should pay attention to the Roman aqueducts because they were an apprenticeship in the building of lofty and daring arches. In the Anio Vetus, for example, which dates from about the year B.C. 272, some of the arches rise to a height that exceeds ninety feet. And any architect who conceived and brought to completion a fine aqueduct, such as the Pont du Gard, or the wonderful structure at Segóvia, deserved to take rank with Caius Julius Lacer. No problem of bridge construction would have baffled his matured knowledge.

It is said that the earliest vaulted bridge of the Romans was erected under the elder Tarquin, about six hundred years before the Birth of Christ. Emiland Gauthey says, for example, “Pont Salaro, à Rome, sur le Teverone. Cet ouvrage, composé de trois arches en plein cintre, de 16,6 à 21 mètres, et de deux arches plus petites, de 6,8 mètres, fut élevé sous Tarquin l’ancien, six cents ans avant J. C.” Yet there is no evidence to justify this dogmatism. The bridge may have been a timber one, like the Pons Sublicius. It carried the Via Salaria over the Anio (Teverone) about two and a half miles from Rome, and was called usually the Pons Salarus. Livy speaks of it under another name, Pons Anienis, and makes it the theatre of an immortal fight, the one between Manlius and a gigantic Gaul, B.C. 361. In single combat Manlius killed the barbarian, and took a chain (torques) from the dead body, and put it around his own neck, as a proof of his victory, winning by this act the surname of Torquatus.

The Pons Salarus does not appear again in early history. By the year B.C. 361 it may have been made into an arched bridge of stone, though it was not till B.C. 313 that the first aqueduct to Rome was constructed. In any case, however, we learn from an inscription, which Sir William Smith accepted as authentic, that the Pons Salarus was rebuilt in the sixth century A.D., by Narses, general and statesman, in the reign of Justinian. If in this reconstruction any earlier work was preserved, we must look for it in the smallest arches described by Gauthey, for we find narrow spans in the earliest Roman aqueducts. Those of the Marcian are only eight metres. The Ponte Salaro existed till 1867, when it was blown up during a panic caused by Garibaldi’s march to Rome. A fortified castle stood above one side of the central arch, rising from the footway, whose width was more than eight metres. The bridge was about a hundred metres long, and its vaults were built with exceedingly heavy stones remarkable for their bossage work. A woodcut of this late Roman bridge is given by Professor Fleeming Jenkin, but it differs from the illustration in Emiland Gauthey’s “Traité de la Construction des Ponts,” Paris, 1809-16, Vol. I.