Roman Aqueducts: Inverted Siphons, Frontinus and the Water Thieves of Rome

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Rome drew water from the Tiber, springs and cisterns until 312 BC, when the censor Appius Claudius Caecus commissioned the Aqua Appia, an almost entirely underground channel built during the Samnite Wars so enemies could not cut it. By the third century AD eleven state-funded aqueducts carried hundreds of millions of gallons a day to a city of over a million people, moved only by gravity on gradients Vitruvius set at no less than 1 in 4,800. Surveyors leveled routes with the 20-foot chorobates, the groma and the dioptra, and where valleys were too deep for arches, inverted siphons dropped water through pressurized lead or stone pipes and pushed it up the far side.

Aqueducts needed a 15-foot clear corridor marked by boundary stones, landowners sued for compensation, and the Decemviri cited the Sibylline Books to stall the Aqua Marcia. In 97 AD the water commissioner Frontinus found water billed by pipe size, illegal taps on rural estates and bribed workers fitting oversized pipes with fake seals. Aqueducts also powered the 16-wheel flour mill at Barbegal in Gaul and hushing at Las Médulas and Dolaucothi. When the Ostrogoths cut the lines in 537, Belisarius sealed the conduits and floated mills on the Tiber, and once crews went unpaid, calcium deposits slowly choked the channels.

• Rome's hard water coated pipes with calcium carbonate, a barrier that undercuts the empire-wide lead poisoning myth.
• Excavations at Vindolanda in Roman Britain uncovered water pipes made from hollowed alder and pine trunks.
• Water went first to public fountains, then to bathhouses, then to private households paying a premium.
• Polluting the supply or tampering with the mains carried a fine of 100,000 sesterces.
• By the Middle Ages only the underground Aqua Virgo still delivered water to Rome reliably.