“The Anio Vetus takes its rise beyond Tibur (now called Tivoli), outside the gate, at the twentieth milestone, where it parts with some of its water for the use of Tibur. Its course has in length, in consequence of the difficulties caused by the levels, 43 miles. Of this for 42 miles and 779 paces, the stream is underground, and for 221 paces (about 350 yards) on a substructure above ground. “Near the fourth milestone within the new road (_infra novum_) the Anio Vetus crosses between the arches from the Via Latina into the Via Lavicana, and has (there?) a _reservoir_ (or _piscina_). Thence just within the second milestone it gives a part of its water into the _specus_, which is called the Octavian, and this comes in the direction of the New Road to the Asinian Gardens, whence it is distributed through that neighbourhood; but its direct channel, following the _specus_ (or conduit) coming within the Porta Esquilina, _along_ the _Spes (Specus?) Vetus_, is drawn off for distribution in the city, through the high channels[31]. “It held the sixth place in height, but would have been sufficient for even the higher parts of the city, if it had been carried across the valleys and the low ground, where requisite, upon substructure, arches, and buttresses[32].” Passing on to the second aqueduct, namely, the stream taken from the River ANIO (and which was called the ANIO VETUS to distinguish it from a later diversion from the same river), we find that Frontinus is also very distinct in many of his details; a few only he leaves to conjecture.