construction of the Anio Vetus was undertaken--that is to say, the 608th
year after the foundation of the city--the increase of the city
necessitated a more ample supply of water, and it was determined to bring
it from a still greater distance. It was no longer considered necessary
to conceal the aqueduct underground during the whole of its course, and
so it was in part carried above ground on embankments or supported upon
arches of masonry. The water was brought from some pools in one of the
valleys on the eastern side of the Anio, some miles farther up than the
point from which the Anio Vetus was supplied; and the new aqueduct, which
was 54 miles in length, was called the Marcian, after the Prætor Marcius,
to whom the work was intrusted. Frontinus also tells us the history of
the other six aqueducts which were in existence in his time, viz., the
Tepulan, the Julian, the Virgo, the Alsietine or Augustan, the Claudian,
and the Anio Novus; the last two being commenced by the Emperor Caligula,
and finished by Claudius, because "seven aqueducts seemed scarcely
sufficient for public purposes and private amusements;" but it is not
necessary for our purpose to give any detailed account of the course of
these aqueducts; it is only necessary to mention one or two very
interesting points in connection with them. In order to allow of the
deposit of suspended matters, piscinæ, or settling reservoirs, were
constructed in a very ingenious manner. Each had four compartments, two
upper and two lower; the water was conducted into one of the upper
compartments, and from this passed, probably by what we should call a
standing waste or overflow pipe, into the one below; from this it passed
(probably through a grating) into the third compartment at the same
level, and thence rose through a hole in the roof of this compartment
into the fourth, which was above it, and in which the water, of course,
attained the same level as in the first compartment, thence passing on
along the aqueduct, having deposited a good deal of its suspended matter
in the two lower compartments of the piscinæ. Arrangements were made by