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, piscinae, 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 piscinae. Arrangements were made by
which these two lower compartments should be cleaned out from time to
time. The specus or channel itself was, of course, constructed of
masonry, generally of blocks of stone cemented together, and it was
frequently, though not, it would appear always, lined with cement inside.
It was roofed over, and ventilating shafts were constructed at intervals;
in order to encourage the aeration of the water, irregularities were
occasionally introduced in the bed of the channel. The water supplied by
the different aqueducts was of various qualities; thus, for instance,
that of the Alsietine, which was taken from a lake about 18 miles from
Rome, was of an inferior quality, and was chiefly used to supply a large
naumachia, or reservoir, in which imitation sea fights were performed;
while, on the other hand, the water of the Marcian was very clear and
good, and was therefore used for domestic purposes. Frontinus gives the
most accurate details as to the measurements of the amount of water
supplied by the various aqueducts, and the quantities used for different
purposes. From these details Mr. Parker computes the sectional area of
the water at about 120 square feet, and says: "We can form some opinion
of the vast quantity if we picture to ourselves a stream 20 ft. wide by 6
ft. deep c