Ancient Rome had eleven major aqueducts, built between 312 B.C. (Aqua Appia) and 226 A.D. (Aqua Alexandria); the longest (Anio Novus) was 59 miles long. It has been calculated that in imperial times, when the city's population was well over a million, the distribution system was able to provide over one cubic meter of water per day for each inhabitant: more than we use nowadays. For most of their length the early aqueducts were simply channels bored through the rock, from the water intake in the hills almost to the distribution cistern in Rome. The depth of the channel below ground varied so as to maintain a constant, very shallow gradient (less than 1/200) throughout the length of the aqueduct; vertical shafts were bored at intervals to provide ventilation and access. Only in the final stretches was the conduit raised on arches, to give a sufficient head for distribution of the water within the city. In order to keep the gradient constant, the aqueducts took a roundabout route, following the contours of the land and heading along spurs which led towards Rome. The most dramatic parts of a Roman aqueduct were—and still are—the bridges (also known as arcades) that carried the water over low spots in the terrain. Perhaps the best-known of these is the Pont du Gard, a part of the aqueduct that served the town in Nîmes in Gaul (today's France). The bridge stands on three tiers of arches and has a length of 274 m (900 ft). Built without the use of mortar, it stands 49 m (160 ft) above the Bornègre Ravine. Another famous Roman aqueduct is the one in Segovia, Spain; it still carries water, although it did need restoration in the 15th century. s time went on, Roman engineers became more daring in the construction of high arches to support the conduits across valleys and plains and some of the later aqueducts were as much as 27 meters (about 100 feet) above ground level in places. Closed pipes were occasionally used to cross valleys by the "inverted syphon" method: the pressure forced the water down and up again on the other side, to a level slightly lower than before. But this system was costly, as it required lead pipes (lead had to be imported from Spain or Great Britain) and it was difficult to make joints strong enough to withstand the pressure; so arches were far more common. For centuries, an army of laborers was constantly at work, under the supervision of the curator aquarum, extending and repairing the water system. But in the 6th century A.D., as the power of the Empire began to decline, the Goths besieged Rome and cut almost all the aqueducts leading into the city. (The only one that continued to function was that of the Aqua Virgo, which ran entirely underground.) One or two were later restored and were used during the Middle Ages, but most of the population had to resort to the Tiber as the only source of water: it is for this reason that the medieval buildings of Rome lie almost exclusively in the two great bends of the river, the Campo Marzio and Trastevere. It was not until Renaissance times that the Eternal City was once again provided with aqueducts and fountains.