A large and a very remarkable reservoir of the time of Trajan or Hadrian remains a little to the west of the mausoleum, and on rather higher ground. This was probably to receive the water for a branch to the imperial villa, afterwards taken possession of by the Gordiani, who built at that place their great family mausoleum, on which a tall medieval tower was afterwards erected; of this, very picturesque ruins remain, under the title of Torre de’ Schiavi. There are at the same place other large reservoirs of the time of the Gordiani, shewing that the great imperial villa was still supplied with water from the Marcian aqueduct in the third century. No water under the name of Hadrian is mentioned in the Regionary Catalogue of the fourth century, it is therefore evident that this great aqueduct was not called then after that emperor, although we are told in his life by Spartianus, that many were made in his time, and were at first called after him (as we have said[178]). At the first reservoir, the starting-point, there was an inscription of Hadrian, and the other reservoirs belonging to it are also constructions of the same period. It may be that this water did not come into Rome at all, but that the aqueduct was made to supply the great villa in the place now called “Cento-Celle,” to which it leads in a direct line, and where it appears to terminate. If any alterations were made, these may have been done in the time of Alexander Severus, and this may now represent the so-called Aqua Alexandrina, as Fabretti thought, although he was certainly mistaken in one part of his account of it. The branch which went by the Mausoleum of S. Helena, and which apparently ran underground to the Villa of the Gordiani, was not a branch of the aqueduct to convey water into Rome, but a branch from the great aqueduct for the villa or villas, as it now does from the Marrana at a lower level. This short arcade is of the time of Constantine. The great and long one that leads to the Cento-Celle is of two periods, the earlier part of the time of Hadrian, the latter of the third century. It is quite possible that the original _specus_ had become choked up with stalactite in the course of a century, and that it was restored to use for a time by Alexander Severus, and so called after him. THE BRANCH OF TRAJAN ON THE AVENTINE.