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of the Spanish broom.
On the upper side of the arch a
composition of lime and sand is to be laid, so that if any
water fall from the floor above or from the roof, it may
not penetrate. If there be no supply of Greek reeds,
the common slender marsh-reeds may be substituted.
tied together with string in bundles of appropriate length.
but of equal thickness, taking care that the distance
from one ligature to another be not more than two feet.
These are bound with cord to the ribs, as above directed,
and made fast with wooden pins. All the remaining
work is to be performed as above described. The arches
being prepared and interwoven with the reeds, a coat is
to be laid on the underside. The sand is afterwards in-
troduced on it, and it is then polished with chalk or
marble. After polishing, the cornices are to be run
along the springing: they are to be as slender and light
as possible; for, when large, they settle by their own
weight, and are incapable of sustaining themselves. But
little plaster should be used in them, and the stuff
should be of uniform quality, such as marble-dust; for
the former, by setting quickly, does not allow the work
to dry of one consistence. The practice of the ancients,
in arched ceilings, is also to be avoided ; for their cor-
nices are dangerous, from their great projection and
consequent weight. Some cornices are of plain, others
of carved, work. In small private rooms, or where fire
or many lights are used, they should be plain, to allow
of being more easily cleaned; in summer rooms, and
exedræ, where the smoke is in such small quantity that
it can do no injury, carved cornices may be used; for
white works, from the delicacy of their colour, are always
soiled, not only with the smoke of the house itself, but
also with that of the neighbouring buildings. The cor-
nices being completed, the first coat of the walls is to be
laid on as roughly as possible, and, while drying, the
sand coat thereon; setting it out, in the direction of the
length, by the rule and square; in that of the height,