It is unnecessary to follow in detail, through its many tests in peace, the advance of iron armor. Its growth in strength, as the power of the gun developed, came almost solely from increase in thickness, the latter reaching its maximum with the British Inflexible, completed in 1876, which carries from 16 to 24 inches of iron on her belt and citadel. This plating, however, is divided and “sandwiched” with wood, there being, exterior to the skin, 6 inches of teak, then 12 inches iron, 11 inches teak, and an outer 12-inch plate. As armor, iron received its death-blow in the famous tests at Spezia, Italy, during the autumn of 1876, when the 100-ton gun, with a full charge, at a range of 100 yards, attacked solid and “sandwich” targets of iron and solid targets of steel—the single or aggregate thickness of metal in each case being 22 inches. These trials were undertaken through Italy’s desire to build, in the Duilio and Dandolo, the most formidable vessels afloat. Steel won the day, and the roar of that mighty gun, thundering from the Spezia firing ground, sounded the knell of iron armor, deprived the as yet unlaunched Inflexible of her crown of invulnerability, and demanded, with success, a revolution in the armor manufacture of Europe. As a compromise, compound armor, i.e., iron faced with steel, became popular for a time. As with steel, its beginnings were old, dating back at least to the year 1857. The first perfected compound plate, made by Cammel & Co., of England, was tested at Shoeburyness in 1877. It was composed of 5 inches of iron with a 4-inch face of steel; the iron being raised to a welding heat and the molten steel poured on its top. The great heat partially fused the contact face, the two metals were united, and the combination was assured by immediate rolling. Compound plates sprang in 1877 from obscurity to popularity; by 1879 iron armor had become obsolete with progressive naval powers, and, in 1880, both compound and steel plates had reached such development that they were close rivals, the leading competitors being Cammel in England and Schneider in France. Steel, however, slowly forged ahead during the next decade; and, at its close, compound armor was practically out of the race. In steel’s victory, its alloy with nickel, in minute proportions, has materially aided; the combination imparting hardness without decreasing the toughness of the plate. This material gave superior results from the beginning. Its first plate, tested in 1889, was 9⅓ inches thick; it was pierced by a Holtzer shell, whose body did not pass wholly through and whose energy was 1.6 times that just necessary to perforate a wrought-iron plate of the same thickness. To the increased strength given by nickel there has been added a further gain through the application of face-hardening processes—such as that of the American, Harvey—which produce superficial carbonization, transforming the surface into a high grade of very hard steel,