if the load of the walls
or columns rests on the middle of spans, they can have no permanent
durability.

2. It will also do no harm to insert posts between lintels and sills
where there are piers or antae; for where the lintels and beams have
received the load of the walls, they may sag in the middle, and
gradually undermine and destroy the walls. But when there are posts set
up underneath and wedged in there, they prevent the beams from settling
and injuring such walls.

3. We must also manage to discharge the load of the walls by means of
archings composed of voussoirs with joints radiating to the centre. For
when arches with voussoirs are sprung from the ends of beams, or from
the bearings of lintels, in the first place they will discharge the load
and the wood will not sag; secondly, if in course of time the wood
becomes at all defective, it can easily be replaced without the
construction of shoring.

4. Likewise in houses where piers are used in the construction, when
there are arches composed of voussoirs with joints radiating to the
centre, the outermost piers at these points must be made broader than
the others, so that they may have the strength to resist when the
wedges, under the pressure of the load of the walls, begin to press
along their joints towards the centre, and thus to thrust out the
abutments. Hence, if the piers at the ends are of large dimensions, they
will hold the voussoirs together, and make such works durable.

5. Having taken heed in these matters to see that proper attention is
paid to them, we must also be equally careful that all walls are
perfectly vertical, and that they do not lean forward anywhere.
Particular pains, too, must be taken with substructures, for here an
endless amount of harm is usually done by the earth used as filling.
This cannot always remain of the same weight that it usually has in
summer, but in winter time it increases in weight and bulk by taking up
a great deal of rain water, and then it bursts its enclosing walls and
thrusts them out.

6. The following means must be taken to provide against such a defect.
First, let the walls be given a thickness proportionate to the amount of
filling; secondly, build counterforts or buttresses at the same time as
the wall, on the outer side, at distances from each other equivalent to
what is to be the height of the substructure and with the thickness of
the substructure. At the bottom let them run out to a distance
cor