Severus (Jordan-Hulsen,
_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.

The oversight of aqueducts was placed, in the times of the republic,
under the aediles, who were not, however, the constructors of them; of
the four aqueducts built during this period, three are the work of
censors, one (the Marcia) of a praetor. Under the empire this task
devolved on special officials styled _Curatores Aquarum_, instituted by
Augustus, who, as he himself says, "rivos aquarum omnium refecit"
(inscription on the arch by which the Aqua Marcia crossed the Via
Tiburtina). (T. As.)

Among the aqueducts outside Italy, constructed in Roman times and
existing still, the most remarkable are: (1) the aqueduct at Nimes
(Nemausus), erected probably by Vipsanius Agrippa in the time of
Augustus, which rose to 160 ft. The Pont du Card, as this aqueduct is
now called, consists of three tiers of arches across the valley of the
river Gardon. In the lowest tier are six arches, of which one has a span
of 75 ft., the others each 60 ft. In the second tier are eleven arches,
each with a span of 75 ft. In the third tier are thirty-five smaller
arches which carried the _specus_. As a bridge, the Pont du Gard has no
rival for lightness and boldness of design among the existing remains of
works of this class carried out in Roman times. (2) The aqueduct bridges
at Segovia (Merckel, _Ingenieurtechnik_, pp. 566-568), Tarragona
(_ibid._ 565-566), and Merida in Spain, the former being 2400 ft. long,
with 109 arches of fine masonry, in two tiers, and reaching the height
of 102 ft. The bridge at Tarragona is 876 ft. lon