, but in practice he appointed what he might call his
Minister of Water-supply, and gave him liberal means to provide a
large staff of engineers, surveyors, masons, pipelayers, inspectors,
and custodians. It is a common error to imagine that the Romans were
ignorant of the simple hydraulic law that water will find its own
level, and to suppose that their aqueducts were built in consequence
of that ignorance. In point of fact they knew the law as well as we
do. Their earlier aqueducts were conduits almost wholly underground;
their later were all on arches. When they wished to carry water to a
height within the city, up a watertower to a distributing cistern, or
to the top storey of a building, they did so by pipes, just as we
should; but when they brought water from forty miles away they
preferred to bring it in channels lined with impermeable cement and
carried upon arches, which wound across the country according to the
levels in order to avoid the excessive pressure of too steep a
gradient. The reasons for their choice are simple enough. Their chief
difficulty was in making pipes of iron of sufficient capacity. On the
other hand, it was easy to construct a cemented channel in masonry of
any size you desired. In the next place the water about Rome rapidly
lays a calcareous deposit, and it is much easier to clear this from a
readily accessible channel than from pipes buried in the ground. The
pipes which the Romans commonly made were of lead, bronze, or wood.
None of these could be made and cleared cheaply enough to serve for
the volume of water required for household use, the baths, and the
public fountains of Rome. Meanwhile slave labour was inexpensive, and
the cost of building an aqueduct of any length was of little account
to the Roman.

When the water reached the city it was conducted into settling and
distributing reservoirs and its flow regulated. Thence it was carried
by pipes, mostly of lead, wherever it was required. When Agrippa was
minister of water-supply he constructed in the city 700 public pools
or basins and 500 fountains, drawing their supply from 130 collecting
heads or reservoirs. And it is to the credit of Agrippa and of Rome
that all these pools, fountains, and reservoirs were made pleasant to
the eye with suitable adornment. There is mention of 400 marble
columns and 300 statues, but these are to be regarded as only chief
among the embellishments.

The streets of Rome were commonly paved with b