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.