In the remains of these ancient Aqueducts, some are elevated above the ground upon a solid mass of stone work, or upon arches continued and raised one above the other; other portions are subterraneous, passing through deep excavations, and in many instances piercing through mountains of rock; such is that seen at Vicovaro beyond Tivoli, where a _tunnel_ of about five feet deep and four broad, pierces a rock for a distance of more than a mile. These Aqueducts were generally built of stone and covered by arches or large flat stones. At certain distances vents were provided to discharge the water from the channel-way; and cavities were formed, into which the water was precipitated, and where it remained till its mud was deposited, and ponds in which it might purify itself. One of these Aqueducts was formed with two channels, one above the other: they were, however, constructed at different periods; the most elevated was supplied by the waters of the Tiverone, _Anio novus_, and the lower one by the _Claudian_ water. It is represented by Pliny, as the most beautiful of all that had been built for the use of Rome. It was begun by Caligula, and finished by Claudius, who brought its waters from two springs called Cœruleus and Curtius. Vespian, Titus, Marcus-Aurelius, and Antonius Pius, repaired and extended it; it is now called _Aqua Felice_. The Aqueduct that conveyed the Aqua Neroniana to Rome, was built of brick; this, as well as the former, was in some instances 70 Roman feet high. The Aqueduct that brought the _Aqua Marcia_ into the city was repaired by Agrippa, who laid pipes from it to several parts of the city. The _Aqua Marcia_, _Aqua Julia_, _Aqua Tepula_, entered Rome in one and the same Aqueduct, divided into three ranges or stories; in the uppermost of which flowed the _Aqua Julia_, in the second the _Aqua Tepula_, and in the lowest the _Aqua Marcia_. This accounts for the extraordinary height of this Aqueduct, which far surpassed that of any other in Rome. From the ruins of this fabric, which are still seen, and are called “_Il castel del Acqua Marcia_,” it appears to have been a very superb structure. The Aqueducts were under the care and direction, first of the censors and œdiles, and afterwards, of particular magistrates called “Curatores Aquarum,” instituted by Agrippa, to whom the Aqueducts of Rome were objects of particular attention. Messala was one of these curatores in the reign of Augustus, and Frontinus held the same office in that of Nerva. Augustus caused all of them to be repaired. Procopius reckons only fourteen Aqueducts in ancient Rome; but Victor has enlarged the number to twenty.