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this vent slopes upward, and sooner or later penetrates through to the other
side of the wall, against which the furnace is built. At the end of this vent
there is an opening where the steam, into which the water has been converted,
is exhausted through a copper or iron tube or pipe. This method of making
the tank and the vent is much the best. Another kind has a similar vent
but a different tank, for it does not lie transversely under the forehearth,
but lengthwise ; it is two feet and a palm long, and a foot and three palms
wide, and a foot and a palm deep. This method of making tanks is not
condemned by us, as is the construction of those tanks without a vent ;
the latter, which have no opening into the air through which the vapour may
discharge freely, are indeed to be condemned.
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Fifteen feet behind the second wall is constructed the first wall, thirteen
feet high. In both of these are fixed roof beams4, which are a foot wide and
*The paucity of terms in Latin for describing structural members, and the consequent
repetition of " beam " (trabs), " timber " (tignum), " billet " (tigillum), " pole " (asset),
with such modifications as small, large, and transverse, and with long explanatory clauses
showing their location, renders the original very difficult to follow. We have, therefore,
introduced such terms as " posts," " tie-beams," " sweeps," " levers," " rafters," " sills,"
" moulding," " braces," " cleats," " supports," etc., as the context demands.
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