REPORT OF MR. NELSON L. DERBY.
381
these details still more minute, and taking it for granted that great interest is felt at home in these matters, I will state further that as regard the walls upon which the beams rest, the exterior one is of such thickness that it can generally be diminished six inches at each story at the point where these rest, thus giving them a free support, and diminishing the danger of injury to the wall by the vibration of the floors ;• where it is impossible to make the middle wall of such thickness as to admit of a diminution of six inches on each side in every story, the ends of the beams, before being built in, are covered with zinc to prevent the corrosive effect ot the wet mortar upon the wood. These matters are not connected with fire, but I mention them here incidentally as of value and perhaps not generally known at home.
A building-law of Vienna requires that all woodwork should be separated from the interior surface of flues by a thickness of at least six inches of brick. It further insists, that in the interior of the flue, an earthen pipe shall be inserted in every story, extending at least from the ceiling to the surface of the flooring above. Flues occur frequentty here in the middle
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Manner of guarding floors from flues, a Flues.
wall, which supports the ends of the beams for floors, and often several flues occur side by side, so that the end of one of the beams might lie directly upon their opening, if all are supported directly by the wall. To avoid this, a simple contrivance is employed, known, without doubt, in America, and easily understood from inspection of the accompanying drawing. Upon the two adjacent beams a cross-timber is supported, which receives, at its middle point, the end of the beam in question. The flues are in general six inches in diameter, and the wall necessarily at least eighteen inches