REPORT OF MR. NELSON L. DERBY.
383
strength is required, beams are rivetted together from bar and angle iron. These forms are all well known in America, but are not in general use there for the construction of floors.
The first variety of floor I shall describe, is that most common on the continent, and, without doubt, known in special cases at home. Girders are laid parallel, from wall to wall, at intervals of about six feet, and upon them flat segmental vaults of hollow brick are sprung. Where additional security
Section across vaults and girders, a Filling, b Hollow bricks.
is desired, the ends of these girders are anchored fast in the walls, or where adjacent rooms occur, having this form of floor,
the girders may be tied together through the middle wall. In some cases, the thrust of the vaults is taken up by rods of iron tying the girders to one another transversely. To prevent the protrusion of these rods, it is becoming usual, in England, to
Section of the wall, showing the ends of girders. paSS them through the
interior of the bricks, which requires them to be slightly curved, but not sufficiently so to produce any undue pressure. The lower surface of the vaults receives a coating of plaster, aud is, in Vienna, generally decorated with frescoes, as may be seen in many of the new coffee-houses. Above, the vaults are covered with a filling, upon which come the joists and flooring, as before described. This form of floor is more thoroughly fire-proof than either of the others, and is required law in Vienna, in most cases, in the basement story of buildings. To secure the greatest solidity, a stone is built