In Fig. 25, circular construction is exhibited as applied to the apsidal end of a church, slabs similar to those shown in Fig. 21 being employed for that purpose, while Figs. 22, 23, and 24 show forms of slabs suitable for constructing cylinders with horizontal axes and domes. In Fig. 19, which is the upper part of Fig. 20, is shown a system of constructing floors of these slabs. It is only necessary to explain that the slabs are first keyed to the lower flange of the iron joist by means of a cement (bituminous preferred), and the combination is then fixed in position, the edges of the slabs adhering to, or rather supported by, the iron joist being rebated so as to receive and support intervening slabs, the heading joints of which are laid to break with those of the slabs supported by the joists. For double floors the iron joists are made with a double flange on their lower edge, and are fitted to iron girders, which cross in the opposite direction. This provision secures the covering of the cross girders on their undersides by the ceiling slabs. The concrete having been deposited upon the slabs, its upper surface may be finished off in any of the usual ways, while the ceiling may be treated in any of the ways described for the walls. This system does not exclude the ordinary methods of constructing floors and roofs, although it supplies a fireproof system. Where required, bricks, stone, and, in fact, any other building material, may be used in conjunction with the slabs. The system of building construction is intended, as in the case with all concrete, to supersede brickwork and masonry in the various uses to which they have been applied, and, at the same time, to offer a more perfect system of building in concrete. Hitherto slab concrete work has never been erected in a perfectly finished state (i.e., with mouldings, etc., complete), but has either been left in a rough state or without ornament, or else has been constructed so as never to be capable of receiving good ornamental treatment. Hitherto the great difficulty in constructing concrete walls of concrete and other slabs has been to prevent the slabs from being forced outward or from toppling over by the pressure of the plastic filling-in material from the time of its deposition between the slabs until it has become hard enough to form, with the slabs, a solid wall. Besides the system of forming the slabs of L (vertical or horizontal) section, or with a kind of internal buttress and shoring them up from the outside, or of supporting the slabs upon framing fixed against the faces of the wall, several devices have been used to obviate this difficulty.