In the matter of attack and defence, vessels are distinguished by the number and weight of the guns they carry, and by the distribution and thickness of their armour. Protective armour is of two kinds, that which surrounds the guns, so as to protect them from the enemy’s fire, and that which protects the motive-power of the ship, so as to prevent the engines from being rendered useless. The maximum of guns and armour and the minimum of speed are to be found in the first-class battle-ship, which is simply a floating fortress, so constructed that she need never run away, but can stand up and fight as long as her gun turrets revolve. The general plan of construction in a battle-ship is to surround the engines, boilers, and magazines with a wall of Harveyized steel armour eighteen inches or so thick, and seven or eight feet high, which extends about four feet below the water-line and three feet above it. This armour belt is not only on the sides of the ship, but is carried across it fore and aft, immediately in front of and behind the space occupied by the engines and magazines, and the whole affair is covered with a solid steel roof three or four inches thick. Outside this central fortress, and extending from it clear to the bow and stern at each end, is a protective deck of steel, three inches thick, which is placed several feet below the water-line. Everything above this deck and outside this fortress might be shot away, and the vessel would still float and fight. On the roof of the fortress are placed the turrets containing the big guns. The largest of these guns, 13-inch calibre, weigh about sixty tons each, and will carry a shell weighing eleven hundred pounds about twelve miles. The turrets are circular, as a rule, large enough to hold two guns, and are made of face-hardened steel from fifteen to eighteen inches thick. They revolve within a barbette or ring of steel eighteen inches thick, which protects the machinery by which the guns are trained. Farther back on the roof of the fortress are other and lighter turrets made of 8-inch steel and carrying 8-inch guns, and at other places are stationed rapid-fire guns of lighter calibre, protected by thinner armour than that of the main belt. If all this secondary battery is stripped off, leaving nothing but the turrets with the big guns, and these are brought down close to the water, and the armour belt is reduced to seven or eight inches in thickness, the type of vessel known as the monitor is reached. It is simply a battle-ship on a reduced scale. Such vessels are very slow and cannot stand rough weather, on account of their low freeboard. The speed of the monitors is seldom more than twelve or fourteen miles an hour, and they are intended to act in coast defence, usually in connection with shore-batteries. The best types in the navy are the _Terror_ and the _Puritan_.