concrete, bedded upon bar iron gratings inserted between the beams—are the only practical systems of fire-proof floor construction, now in use. The only attempt to lay the floor _on_ the beams, of which I have knowledge, is in the sugar house above mentioned. This has suggested to me several methods of laying rigid floors upon beams at considerable spaces (three to five feet) from one another. Preliminary to so doing, I have above suggested the revival of the deck beam, or the I beam with a better form for the bottom flange, and the adoption of cast-iron shoes for the bearings. The objections to the brick arches are that their great weight requires heavier beams than would otherwise be used, and that the form of their soffits is not beautiful; for they have the appearance of a long succession of little wagon vaults, requiring a resort to the doubtful expedient of furring the ceiling with iron lath. I think it might be objected to the French system of floors, that the expense would be too great, plaster being a dear article with us in comparison with its price in France, while our own cement has not the requisite properties to enable it to be substituted, besides being almost equally costly. The stone slabs, of Mr. Eidlitz, are the only rigid material thus far used successfully with iron beams, and could be used to better advantage if laid _on_ the beams rather than resting upon their lower flanges, as is done in the American Exchange Bank. They are doubtless the handsomest material that can be used for this purpose, but are open to the objection of being heavy and expensive—where expense is a question, and utility only is sought—requiring heavy beams and calling for elaborate cutting on the under side. It will be pertinent to our inquiry, therefore, to ask if there are any other rigid materials adaptable to this purpose, and possessing the desired quality of lightness and cheapness. A former draughtsman of mine, now a member of the Institute, first suggested the use of slabs of slate, about two inches in thickness, for spans of four feet, and thicker or thinner in proportion to the distance of the beams from centres. I give his suggestion for what it is worth. But it led me to believe that we would eventually come to cast-iron as the practicable material for this purpose, possessing the requisite qualities of lightness and cheapness and capable of being bolted to the beams, thus answering all the purposes of flooring and bridging. Cast-iron plates may be used for flooring in two ways; first, when deafening and finished floor covering are required; second, when neither is required, as in manufacturing buildings, wherein a reasonably smooth flooring is required, and a few planks, laid where workmen habitually stand, will answer the purpose of non-conductors of heat. Experiment must determine the minimum quantity of iron (in proportion to the strength required) to be used in the floor plates. In obtaining the proper form for strength, and