In Roman times long distance water transport was realized by means of aqueducts. Water was conveyed in mortared open channels with a downward slope from spring to destination. Also wooden channels and clay pipelines were applied. The Aqua Appia, the oldest aqueduct of Rome, was constructed in the third Century BCE. During the Pax Romana (second Century CE), a time of little political turmoil, prosperity greatly increased, almost every town acquiring one or more aqueducts to meet the rising demand from the growth of population, the increasing number of public and private bath buildings, and the higher luxury level in general. Until today over 1600 aqueducts have been described, Gallia (France) alone counting more than 300. Whenever a valley was judged to be too wide or too deep to be crossed by a bridge, pressure lines known as ‘inverted siphons’ or simply ‘siphons’ were employed. These closed conduits transported water across a valley according the principle of communicating vessels. About 80 classical siphons are presently known with one out of twenty aqueducts being equipped with a siphon. After an introductory note about aqueducts in general, this report treats the ancient pressure conduit systems with the technical problems encountered in design and function, the techniques that the ancient engineers applied to cope with these problems, and the texts of the Roman author Vitruvius on the subject. Reviewers noted that the report is rather long, and it is. Yet to understand the difficulties that the engineers of those days encountered in view of the materials available for their siphons (stone, ceramics, lead), many a hydraulic aspect will be discussed. Aspects that for the modern hydraulic engineer may be common knowledge and of minor importance when constructing pressure lines, in view of modern construction materials. It was different in Vitruvius’s days.
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