[Illustration: BESSEMER CONVERTER BLOWING AIR THROUGH A MASS OF MOLTEN IRON] [Illustration: A STEEL BEAM PASSING THROUGH THE FINISHING ROLLS OF A STEEL MILL] [Illustration: FIG. 74.--SECTIONAL VIEW OF A REGENERATIVE OPEN-HEARTH FURNACE] While the Bessemer converter provides a very economical and expeditious method of converting cast iron into steel, it is difficult to regulate the carbon content with great accuracy and hence the use of the open-hearth furnaces which furnish a slower method of burning out the carbon. Figure 74 is a diagrammatic representation of such a furnace. Below the hearth of the furnace there are two pairs of chambers, _A_, _B_ and _C_, _D_, filled with a checkerwork of bricks. Gas is passed through one chamber _A_, and air through the other _B_, and they combine to form a very intense flame above the hearth _E_ in which the metal is placed. The burnt gases pass over and through the other pair of chambers, _C_, _D_, on their way to the stack. By this means the bricks in the latter chambers are raised to a white heat. Then the process is reversed; air flows through the hot checkerwork of bricks in the chamber _C_ and gas through the hot checkerwork in chamber _D_, and after combustion in the furnace the burnt gases are drawn through the bricks of the first pair of chambers. By alternating the direction of flow the air and gas fed to the furnace are always preheated by the stored-up heat of the previously burned gases. While it takes but a few minutes to convert cast iron into steel in the Bessemer converter, the open-hearth process occupies from eight to twelve hours. The steel produced in the open-hearth furnaces is poured into ingot molds. These are approximately rectangular in section and slightly larger at the bottom than at the top. They are open at the top and bottom, but at the bottom rest upon a base plate. As soon as the steel has hardened the plunger of a stripping machine holds down the glowing ingot while a pair of hooks lift off the mold, leaving the ingot resting on the base plate. ROLLING INGOTS INTO RAILS