During both the eighth and ninth centuries, indeed, various repairs for maintaining the aqueduct of Trajan [X.], which carried water from the lake Sabatinus (di Bracciano, Acqua Paola), are mentioned by Anastasius; and in 786 the “Jovia” aqueduct was restored by Hadrian I. In the twelfth century, the Aqua Lateranensis is mentioned as still in use, being that part of the Claudian aqueduct carried upon the arcade called the Arches of Nero near the Lateran, where the _specus_ is very conspicuous. The aqueduct of Trajan [X.], and that of Agrippa [VI.], continued to supply water, though but scantily, for about two centuries later. The Appia [I.] has long been supposed to have been gradually stopped up by the deposit of clay, and thus rendered useless[164]; and so total was the failure of all these waters in the fourteenth century, that the population (said to have been not more than 17,000 during the absence of the Papal Court at Avignon) were without any supply of running water, except from the Tiber[165]. The first restoration, in the fifteenth century, was that of Agrippa [VI.], by Nicholas V., whose aims and efforts were renewed by Sixtus IV., and by later Pontiffs. Sixtus V. [Felice Peretti] determined to restore to Rome the water then supposed to be the Marcia, but really of Hadrian [XIII.] The arcade of the Claudian [VIII.] and Anio Novus [IX.], was the most magnificent of all, and conveyed its several streams along a distance of forty-six miles. This had been restored by Vespasian and Titus, by Trajan, Septimius Severus and Constantine; but, in the greater part, especially within the first few miles from Rome, the existing stone arcade is of the time of Claudius, with repairs in the brickwork of the Flavian emperors. At the beginning of the ninth century, it was certainly still serviceable, and was known by the name Forma Claudiana. The piers of this arcade were used by the engineers of Felice to carry that _specus_ on the part near Rome; but the greater part of the Marcian arcade, and considerable parts of the Claudian, near Rome, were used as a stone-quarry by them. The channels through which the Anio and the other streams flowed, were the first, 9 ft. in height and nearly 3 in breadth, the second 6 ft. by 3, as seen in their ruins at the Porta Maggiore. The more ancient portion is not in brickwork, but of enormous squared blocks of tufa and peperino. The popular notion that all these aqueducts were a mere waste of money, arising from the ignorance of the ancient Romans of the simple fact that water will rise to its level, is altogether erroneous. Vitruvius gives directions for taking the levels for carrying water through valleys, where, he says, there should be standing or upright pipes (_columnaria_,), now called respirators, to let off the confined air (_spiritus_[166]), and explains how to bring the water on by earthenware, lead, and even leather pipes.