The Aqua Alexandrina must also have entered the city here, though its channel, which lay at some depth below ground, has not been discovered. Considerable remains of its brick aqueducts exist in the district between the Via Praenestina and the Via Labicana.
Of the two aqueducts on the right bank of the Tiber, the Alsietina, as we have said, has no remains at all, while those of the Traiana are not of great importance. The line of the aqueducts was marked by _cippi_, inscribed (in the case of the Anio Vetus, Marcia, Tepula, Julia and Virgo--those of the Claudia and Anio Novus are uninscribed, and those of the Traiana are differently worded) with the name of the aqueduct, the distance from the next _cippus_ (generally 240 ft.) and the number, counting from Rome (not from the springs). These boundary stones were erected in pairs, to mark off the strip of land 30 ft. in width reserved for the aqueduct, and for the road or path which generally followed it. The shafts (_putei_) often stood, but not necessarily, at the same points as the _cippi_.
To these nine must be added the two following, constructed after Frontinus's time: (10) AQUA TRAIANA, from springs to the north-west of the Lacus Sabatinus (Lago di Bracciano), constructed by Trajan in A.D. 109, about 36½ English miles in length. It was restored by Paul V. in 1611, who made use of and largely transformed the remains of the ancient aqueduct; he allowed some of the inferior water of the lake to flow into the channel, and it is thus no longer used for drinking. (11) AQUA ALEXANDRINA, rising about 14 English miles from Rome, between the Via Praenestina and the Via Labicana, the work of Alexander Severus (A.D. 226). The springs now supply the modern Acqua Felice, constructed by Sixtus V. in 1585, but the course of the latter is mainly subterranean and not identical with that of the former.
_Photo, Neurdein._ PONT DU CARD, NÎMES (NEMAUSUS).]
_Photo, Laureat y Cia._ ROMAN AQUEDUCT AT SEGOVIA.
AQUEDUCT OF ROQUEFAVOUR, MARSEILLES. Early nineteenth century.
_Photo, Brogi._ PISCINA MIRABILIS AT BAIAE.
_Photo, Dr T. Ashby._ AQUA MARCIA, ROME.]
It is agreed that these eleven are all that were constructed. Procopius speaks of fourteen (and the Regionary catalogues mention others), but this number includes branch conduits. All the aqueducts ended in the city in huge _castella_ or reservoirs for the purpose of distribution. Vitruvius recommends the division of these into three parts--one for the supply of fountains, &c., one for the public baths and one for private consumers. In the Piazza Vittorio Emmanuele at Rome there are still to be seen the remains of a large ornamental fountain built probably for the Aqua Julia by Domitian or Alexander Severus (Jordan-Hülsen, _Topographie_, i. 3350). Besides these main _castella_ there were also many minor _castella_ in various parts of the city for sub-distribution. To allow the water to purify itself before being distributed in the city, filtering and settling tanks (_piscinae limariae_) were built outside the walls. These _piscinae_ were covered in with a vaulted roof, and were sometimes on a very large scale, as in the example still preserved at Fermo, which consists of two stories, each having three oblong basins communicating with each other; or the Piscina Mirabilis at Baiae, which is covered in by a vaulted roof, supported on forty-eight pillars and perforated to permit the escape of foul air. Two stairs lead by forty steps to the bottom of the reservoir. In the middle of the basin is a sinking to collect the deposit of the water. The walls and pillars are coated with a stucco so hard as to resist a tool.