The steel columns reach from the foundation to the top, riveted together by plates and may be extended to an indefinite height. In fact there is no engineering limit to the height. The outside walls of the sky-scraper vary in thickness with the height of the building and also vary in accordance with the particular kind of construction, whether cage or skeleton. If of the cage variety, the walls, as has been said, act as curtains and consequently they are thinner than in the skeleton type of construction. In the latter case the walls have to resist the wind pressure unsupported by the steel frame and therefore they must be of a sufficient width. Brick and terra-cotta blocks are used for construction generally. Terra-cotta blocks are also much used in the flooring, and for this purpose have several advantages over other materials; they are absolutely fire-proof, they weigh less per cubic foot than any other kind of fire-proof flooring and they are almost sound-proof. They do equally well for flat and arched floors. It is of the utmost importance that the sky-scraper be absolutely fire-proof from bottom to top. These great buzzing hives of industry house at one time several thousand human beings and a panic would entail a fearful calamity, and, moreover, their height places the upper stories beyond reach of a water-tower and the pumping engines of the street. The sky-scrapers of to-day are as fireproof as human ingenuity and skill can make them, and this is saying much; in fact, it means that they cannot burn. Of course fires can break out in rooms and apartments in the manufacturing of chemicals or testing experiments, etc., but these are easily confined to narrow limits and readily extinguished with the apparatus at hand. Steel columns will not burn, but if exposed to heat of sufficient degree they will warp and bend and probably collapse, therefore they should be protected by heat resisting agents. Nothing can be better than terra-cotta and concrete for this purpose. When terra-cotta blocks are used they should be at least 2 inches thick with an air space running through them. Columns are also fire-proofed by wrapping expanded metal or other metal lathing around them and plastering. Then a furring system is put on and another layer of metal, lathing and plastering. This if well done is probably safer than the layer of hollow tile. The floor beams should be entirely covered with terra-cotta blocks or concrete, so that no part of them is left exposed. As most office trimmings are of wood care should be taken that all electric wires are well insulated. Faulty installation of dynamos, motors and other apparatus is frequently the cause of office fires.