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BOOK IX. 



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. 



A FURNACES. B FOREHEARTH. C DOOR. D WATER TANK. E STONE WHICH 

 COVERS 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 wall, thirteen 

 feet 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 consequent 

 repetition of " beam " (trabs), " timber " (tignum), " billet " (tigillum), " pole " (asser), 

 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. 



