strike both the lines alike and at the same time, they show
that the instrument stands level.

2. But if the wind interposes, and constant motion prevents any definite
indication by the lines, then have a groove on the upper side, five feet
long, one digit wide, and a digit and a half deep, and pour water into
it. If the water comes up uniformly to the rims of the groove, it will
be known that the instrument is level. When the level is thus found by
means of the chorobates, the amount of fall will also be known.

3. Perhaps some reader of the works of Archimedes will say that there
can be no true levelling by means of water, because he holds that water
has not a level surface, but is of a spherical form, having its centre
at the centre of the earth. Still, whether water is plane or spherical,
it necessarily follows that when the straightedge is level, it will
support the water evenly at its extremities on the right and left, but
that if it slopes down at one end, the water at the higher end will not
reach the rim of the groove in the straightedge. For though the water,
wherever poured in, must have a swelling and curvature in the centre,
yet the extremities on the right and left must be on a level with each
other. A picture of the chorobates will be found drawn at the end of the
book. If there is to be a considerable fall, the conducting of the water
will be comparatively easy. But if the course is broken by depressions,
we must have recourse to substructures.

CHAPTER VI

AQUEDUCTS, WELLS, AND CISTERNS

1. There are three methods of conducting water, in channels through
masonry conduits, or in lead pipes, or in pipes of baked clay. If in
conduits, let the masonry be as solid as possible, and let the bed of
the channel have a gradient of not less than a quarter of an inch for
every hundred feet, and let the masonry structure be arched over, so
that the sun may not strike the water at all. When it has reached the
city, build a reservoir with a distribution tank in three compartments
connected with the reservoir to receive the water, and let the reservoir
have three pipes, one for each of the connecting tanks, so that when the
water runs over from the tanks at the ends, it may run into the one
between them.

2. From this central tank, pipes will be laid to all the basins and
fountains; from the second tank, to baths, so that they may yield an
annual income to the state; and from the third, to private houses,