_The Siemens Regenerative Furnace_, in which this process is carried out, is seen in Fig. 256. Four chambers, C, E, E′, C′, are filled with fire brick loosely stacked with spaces between, in checker-work style. Gas is forced in the bottom of chamber C, and air in bottom of chamber E, and they pass up separate flues, G, on the left, and being ignited in chamber D above, impinge in a flame on the metal in hearth H, the hot gases passing out flues F on the right, and percolating through and highly heating the checker-work bricks in chambers E′ and C′. As soon as these are hot, gas and air are shut off by valves from chambers C and E, and gas and air admitted to the bottoms of the now hot chambers C′ and E′. The gas and air now passing up through these chambers C′, E′, become highly heated, and when burned above the melted iron on hearth H produce an intense heat. The waste gases now pass down flues G, and impart their heat to the checker-work bricks in chambers C and E. When the bricks in E′ C′ become cooled by the passage of gas and air, the valves are again adjusted to reverse the currents of gas and air, sending them now through chambers C and E again. In this way the heat escaping to the smoke stack is stored up in the bricks and utilized to heat the incoming fuel gases before burning them, thus greatly increasing the effective energy of the furnace, saving fuel, and keeping the smoke stack relatively cool. _Armor Plate._--In these late days of struggle for supremacy between the power of the projectile and the resistance of the battleship, the production of armor plate has become an interesting and important industry. Three methods are employed. One is to roll the massive ingots directly into plates between tremendous rolls, a single pair of which, such as used in the Krupp works, are said to weigh in the rough as much as 100,000 pounds. Usually there are three great rollers arranged one above the other, and automatic tables are provided for raising and lowering the plates in their passage from one set of rolls to the other. The man in charge uses a whistle in giving the signals which direct these movements, and without the help of tongs and levers the glowing blocks move easily back and forth between the rollers. The men standing on both sides of the rollers have only to wipe off the plates with brooms and occasionally turn the plates. [Illustration: FIG. 257.--14,000-TON HYDRAULIC PRESS FORGING AN ARMOR PLATE.]