The greater part of the furnaces in our country still retain the ancient and imperfect form of bellows, hence their height is restricted to the limits of from eighteen to twenty-four feet, and rarely or never reaches thirty. But when the apparatus is such as to supply a proper quantity of air, it has been found that even with light and porous charcoal, such as is given by white pine, the height ought not to be less than thirty feet, and when hard woods are used should be as great as thirty-six feet. Furnaces of even forty feet have been found to answer an excellent purpose, where the charcoal was prepared from oak. When coke is used, furnaces have been made as high as fifty, or even as seventy feet; but experience in England has shown, that from forty-five to forty-eight feet is the proper limit. This height is not at present exceeded in that country, even when the furnace has the greatest dimensions in other respects, and has been found efficacious, even when the vast quantity of eighteen tons has been furnished daily by a single furnace. The force of the blast will depend upon the nature of the fuel, the volume of air, the quantity of mixed material the furnace holds; and thus furnaces in which coke is used, will require the most powerful blast, whether we have regard to the volume or the intensity. The latter may be measured by a column of mercury adapted in a syphon tube to the air pipes, exactly as the gauge is adapted to the pipes of the steam engine. The reduction and liquefaction of the metal take place progressively, as the charges descend in the furnace. The separation of the oxygen is due to the presence of carbonaceous matter at high temperatures, begins at the surface of the pieces of ore, and proceeds gradually inwards; the earthy parts of the ore, of the fuel employed, and the flux, unite and melt; they are thus separated, and being sooner fused than the metal, make their way through the charcoal, and descend first to the hearth. The reduced metal, continuing in contact with the burning carbon, acquires a greater or less portion of that substance, becomes fusible, melts, and follows the liquified earths. Dropping into the hearth that already contains the liquid vitrified earths, it passes by its superior gravity to the bottom, and is protected by them from the blast. Even at the bottom of the hearth, the heat is sufficient to retain the carbureted metal in a liquid state, and this is permitted gradually to accumulate, until it rises nearly to the level of the dam.