What is "Fire-proof Construction?" is a question which has given rise to a great deal of discussion, simply, as it appears to me, because the size of the buildings, and the quantity and description of the contents, have not always been taken into account. That which may be perfectly fireproof in a dwelling house, may be the weakest in a large warehouse. Suppose an average-sized dwelling-house 20 x 40 x 50 = 40,000 cubic feet, built with brick partitions, stone or slate stairs, wrought-iron joists filled in with concrete, and the whole well plastered. Such a house will be practically fire-proof, because there is no probability that the furniture and flooring in any one room, would make fire enough to communicate to another. But suppose a warehouse equal to twenty such houses, with floors completely open, supported by cast-iron pillars, and each floor communicating with the others by open staircases and wells; suppose, further, that it is half filled with combustible goods, and perhaps the walls and ceilings lined with timber. Now, if a fire takes place below, the moment it bursts through the upper windows or skylights, the whole place becomes an immense blast furnace; the iron is melted, and in a comparatively short time the building is in ruins, and, it may be, the half of the neighbourhood destroyed. The real fire-proof construction for such buildings is groined brick arches, supported on brick pillars only. This mode of building, however, involves so much expense, and occupies so much space, that it cannot be used with advantage. The next best plan is to build the warehouses in compartments of moderate size, divided by party-walls and double wrought-iron doors, so that if one of these compartments takes fire, there may be a reasonable prospect of confining the fire to that compartment only. Again, cast iron gives way from so many different causes, that it is impossible to calculate when it will give way. The castings may have flaws in them; or they may be too weak for the weight they have to support, being sometimes within 10 per cent., or less, of the breaking weight. The expansion of the girders may thrust out the side walls. For instance, in a warehouse 120 feet x 75 feet x 80 feet, there are three continuous rows of girders on each floor, with butt joints; the expansion in this case may be twelve inches. The tie rods to take the strain of the flat arches must expand and become useless, and the whole of the lateral strain be thrown on the girders and side walls, perhaps weak enough already. Again, throwing cold water on the heated iron may cause an immediate fracture. For these and similar reasons, the firemen are not permitted to go into warehouses supported by iron, _when once fairly on fire_.