We have now arrived at the arch of Dolabella, built when he was consul, A.D. 10, as an entrance to that part of the Cœlian Hill which had been the keep when it was a separate fortress, and was afterwards made the Claudium. This arch is of very simple construction, of travertine, the same construction as the early part of the Basilica Julia, built by Julius Cæsar and Augustus. It was used by the engineers of Nero as a foundation for their great reservoir, or rather for one corner of it, as we clearly see in 72. The work of the time of Nero terminated here, but the design was carried on by his immediate successors. The water was then divided into three branches; the one on the right hand, to the north, went to the great reservoir between the Cœlian and the Esquiline, called the _Stagna Neronis_, and was used for the sham naval battles. Around these Stagna the Colosseum was afterwards built. A portion of the arcade on which the _specus_ was carried is visible in the garden of the monks called the Passionists of SS. John and Paul, and is well shewn in No. 1773, looking towards the east, with the ruins of the reservoir over the arch of Dolabella, and the church of S. Stefano Rotondo in the distance. There are remains of a reservoir for this, of the third century, in the garden of the monks on the terrace of the Claudium, opposite to the Palatine, shewn in No. 1765. The ruins of the fine arcade of the Claudium stand on this terrace; the _specus_ also is in a wall here, behind the arcade. A portion of another reservoir or _piscina_ for them can be seen at the foot of the cliff of the Claudium, at its north-east corner, opposite that part of the Colosseum near the Meta Sudans; this is also shewn in No. 1743, with _opus reticulatum_ of the time of Nero. This branch continued in use in the third century; for there are remains of another reservoir for it, again at the foot of the Claudium opposite to the Colosseum, but more to the east, near the north-east corner of the Claudium, and near to the eastern end of the Colosseum. The remains of this reservoir are shewn in No. 1735. A colonnade carrying the _specus_ from this reservoir to the second story of the Colosseum, is shewn on one of the coins of Septimius Severus, who probably built this reservoir. In the corridors of the Colosseum are open stone troughs lined with the cement for water. These carried water from this aqueduct in a constant running stream to cool the air. They are work of the third century, with old inscriptions on some of them, shewing that they were made of old materials.