BOOK IX.

358

this vent slopes upward, and sooner or later penetrates through to the otherside of the wall, against which the furnace is built. At the end of this ventthere 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 makingthe tank and the vent is much the best. Another kind has a similar ventbut a different tank, for it does not he transversely under the forehearth,but lengthwise ; it is two feet and a palm long, and a foot and three palmswide, and a foot and a palm deep. This method of making tanks is notcondemned 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 maydischarge freely, are indeed to be condemned.

It

171X171

1 1 1

CCr.i.

cm

111 1 1.

mill

mm

S*Ti

■101

A— Furnaces. B —Forehearth. C — Door. D— Water tank. E— Stone whichcovers it. F— Material of the vent walls. G— Stone which covers it. H— Pipe

exhaling the vapour.

Fifteen feet behind the second wall is constructed the first wah, thirteenfeet high. In both of these are fixed roof beams 4 , which are a foot wide and

‘The paucity of terms in Latin for describing structural members, and the consequentrepetition of beam {trabs) timber (hgnum), “billet” (iigillum), “pole” (asser),with such modifications as small, large, and transverse, and with long explanatory clausesshowing 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