Near the sources this arcade is low and much damaged, and the _specus_ where it remains is nearly filled up with stalactite (1541, 1542). In some parts of the line the stalactite has all the appearance of a petrified cascade, and is evidently formed by the water oozing out and dripping and petrifying as it fell (1436). Further on there is a fine arcade for it across the country in the direction of Cento-Celle; and in some places the arcade is double to raise the _specus_ to the necessary level, as in the Arches of Nero (1428, 1429). A portion of the arcade, where it is broken off, is seen in 1427, with the tower of Cento-Celle in the distance. Another portion of this fine arcade is shewn in 1640. It is of two periods; the upper part is of the third century, and may have been rebuilt by Alexander Severus (as has been said). At Cento-Celle the ground is high, and the aqueduct passes underground for some distance, along the side of the road towards Rome. About a mile nearer to Rome, there is a branch aqueduct from the foot of the Marcian arcade, in the direction of the Mausoleum of S. Helena, which Fabretti considers as part of the same aqueduct; but it is difficult to see upon what grounds. There is a fine arcade here also for about a quarter of a mile; but it is of the time of Constantine, and I have not been able to trace any connection between this and the other. This arcade has been originally double, and the lower one only now remains, with a modern _specus_ made upon it (555 and 556); but the water now flows _from_ the Marrana, at the foot of the arcade of the great aqueducts, which were here on higher ground, and _runs down_ upon this arcade to the garden and small monastery of S. Peter and Marcellinus, at the Mausoleum of S. Helena, called the Torre Pignattara, from the earthenware pots of which the vault was made. XII. AURELIA, A.D. 185, AND XIII. SEVERIANA, A.D. 190. These two aqueducts were made to convey water to the Thermæ of Commodus and Severus in Regio I., of which the remains were found in the excavations of 1870, just within the Porta Latina. The first part was originally made by Marcus Aurelius, for the use of his great villa on the Via Appia, called the Villa dei Quintilii, and the great reservoir and thermæ connected with it remain (2346, 2349, 2350, 2351, 2352). From thence it was brought into Rome by his successor, Ælius Aurelius Commodus. The water came from the Alban hills, near Marino (2358, 2359, 2360, 2361, 2362, 2363), at first underground, and then on an arcade, of which there are considerable remains near the Torre di Mezza Via di Albano (1626 and 1627).