armament; their function in war is to capture the commerce-destroyers of the enemy, to act as commerce-destroyers themselves, and to convoy and protect fleets of large and fast merchant vessels. To accomplish these purposes great speed is necessary, either to overtake or convoy swift merchantmen; great endurance or coal supply, to enable them to keep the sea for long periods on the 
Fig. 6.—Brooklyn. Midship Section.
A, air duct; CE, cellulose; C, C, C, coal; P, P, P, passages. path usually frequented by merchant vessels; and offensive and defensive power sufficient to enable them to successfully resist the attacks of vessels of their own class.
The type next in importance and general usefulness is styled the protected cruiser, or those whose only protection against injury consists of sloping armor decks of varying thickness, in combination with coffer dams filled with water-excluding material and closely divided compartments in the region of the line of flotation. The characteristics of this type are not capable of being so clearly defined as those already considered, as they include such widely differing vessels as the triple-screw Columbia of 7,350 tons displacement, the Olympia of 5,500 tons, down to cruisers of 2,000 and 3,000 tons displacement as represented by the Cincinnati and Detroit. Their function is a varied though exceedingly useful one; some are designed particularly for preying upon the commerce of an enemy so as to cripple its resources, the most notable examples being the Columbia of our own and the Blake of the British Navy.
Speed and endurance are the features emphasized, combined with guns of light caliber for dealing with unarmed vessels;
