ms, it is obvious that we must proceed as
follows. If there is no river in the neighbourhood, but if there can be
a roadstead on one side, then, let the advances be made from the other
side by means of walls or embankments, and let the enclosing harbour be
thus formed. Walls which are to be under water should be constructed as
follows. Take the powder which comes from the country extending from
Cumae to the promontory of Minerva, and mix it in the mortar trough in
the proportion of two to one.

3. Then, in the place previously determined, a cofferdam, with its sides
formed of oaken stakes with ties between them, is to be driven down into
the water and firmly propped there; then, the lower surface inside,
under the water, must be levelled off and dredged, working from beams
laid across; and finally, concrete from the mortar trough--the stuff
having been mixed as prescribed above--must be heaped up until the empty
space which was within the cofferdam is filled up by the wall. This,
however, is possessed as a gift of nature by such places as have been
described above.

But if by reason of currents or the assaults of the open sea the props
cannot hold the cofferdam together, then, let a platform of the greatest
possible strength be constructed, beginning on the ground itself or on a
substructure; and let the platform be constructed with a level surface
for less than half its extent, while the rest, which is close to the
beach, slopes down and out.

4. Then, on the water's edge and at the sides of the platform, let
marginal walls be constructed, about one and one half feet thick and
brought up to a level with the surface above mentioned; next, let the
sloping part be filled in with sand and levelled off with the marginal
wall and the surface of the platform. Then, upon this level surface
construct a block as large as is required, and when it is finished,
leave it for not less than two months to dry. Then, cut away the
marginal wall which supports the sand. Thus, the sand will be undermined
by the waves, and this will cause the block to fall into the sea. By
this method, repeated as often as necessary, an advance into the water
can be made.

5. But in places where this powder is not found, the following method
must be employed. A cofferdam with double sides, composed of charred
stakes fastened together with ties, should be constructed in the
appointed place, and clay in wicker baskets made of swamp rushes should
be packe