eys, nor any
substructure built on a level, but merely an elbow, the water will break
out, and burst the joints of the pipes. And in the venter, water
cushions must be constructed to relieve the pressure of the air. Thus,
those who have to conduct water through lead pipes will do it most
successfully on these principles, because its descents, circuits,
venters, and risings can be managed in this way, when the level of the
fall from the sources to the city is once obtained.

7. It is also not ineffectual to build reservoirs at intervals of 24,000
feet, so that if a break occurs anywhere, it will not completely ruin
the whole work, and the place where it has occurred can easily be
found; but such reservoirs should not be built at a descent, nor in the
plane of a venter, nor at risings, nor anywhere in valleys, but only
where there is an unbroken level.

8. But if we wish to spend less money, we must proceed as follows. Clay
pipes with a skin at least two digits thick should be made, but these
pipes should be tongued at one end so that they can fit into and join
one another. Their joints must be coated with quicklime mixed with oil,
and at the angles of the level of the venter a piece of red tufa stone,
with a hole bored through it, must be placed right at the elbow, so that
the last length of pipe used in the descent is jointed into the stone,
and also the first length of the level of the venter; similarly at the
hill on the opposite side the last length of the level of the venter
should stick into the hole in the red tufa, and the first of the rise
should be similarly jointed into it.

9. The level of the pipes being thus adjusted, they will not be sprung
out of place by the force generated at the descent and at the rising.
For a strong current of air is generated in an aqueduct which bursts its
way even through stones unless the water is let in slowly and sparingly
from the source at first, and checked at the elbows or turns by bands,
or by the weight of sand ballast. All the other arrangements should be
made as in the case of lead pipes. And ashes are to be put in beforehand
when the water is let in from the source for the first time, so that if
any of the joints have not been sufficiently coated, they may be coated
with ashes.

10. Clay pipes for conducting water have the following advantages. In
the first place, in construction:--if anything happens to them, anybody
can repair the damage. Secondly, water from cla