products, we must proceed as
follows, if we wish our work to be finished without flaws. Let the stone
be taken from the quarry two years before building is to begin, and not
in winter but in summer. Then let it lie exposed in an open place. Such
stone as has been damaged by the two years of exposure should be used in
the foundations. The rest, which remains unhurt, has passed the test of
nature and will endure in those parts of the building which are above
ground. This precaution should be observed, not only with dimension
stone, but also with the rubble which is to be used in walls.

[Illustration: Photo. Moscioni

EXAMPLE OF OPUS INCERTUM. THE CIRCULAR TEMPLE AT TIVOLI]

CHAPTER VIII

METHODS OF BUILDING WALLS

1. There are two styles of walls: "opus reticulatum," now used by
everybody, and the ancient style called "opus incertum." Of these, the
reticulatum looks better, but its construction makes it likely to crack,
because its beds and builds spread out in every direction. On the other
hand, in the opus incertum, the rubble, lying in courses and imbricated,
makes a wall which, though not beautiful, is stronger than the
reticulatum.

2. Both kinds should be constructed of the smallest stones, so that the
walls, being thoroughly puddled with the mortar, which is made of lime
and sand, may hold together longer. Since the stones used are soft and
porous, they are apt to suck the moisture out of the mortar and so to
dry it up. But when there is abundance of lime and sand, the wall,
containing more moisture, will not soon lose its strength, for they will
hold it together. But as soon as the moisture is sucked out of the
mortar by the porous rubble, and the lime and sand separate and
disunite, the rubble can no longer adhere to them and the wall will in
time become a ruin.

3. This we may learn from several monuments in the environs of the city,
which are built of marble or dimension stone, but on the inside packed
with masonry between the outer walls. In the course of time, the mortar
has lost its strength, which has been sucked out of it by the porousness
of the rubble; and so the monuments are tumbling down and going to
pieces, with their joints loosened by the settling of the material that
bound them together.

4. He who wishes to avoid such a disaster should leave a cavity behind
the facings, and on the inside build walls two feet thick, made of red
dimension stone or burnt brick or lava in courses,