The larger and more powerful warships are in these times covered with a strong armor made of thick plates of steel, in some cases from 16 to more than 20 inches thick. The steel is often alloyed with nickel to make it harder, and its surface is specially hardened, so as to enable it to resist the blows of the great shot from modern rifledcannon. The armor-plate is carried below the water-line, and covers the gun turrets, while thinner plate is laid on the decks. Not only battle ships, but forts, are now made strong by armor-plate, which shields the men, and behind which the guns are drawn down after firing.