Ericsson's contributions to the art of naval warfare are embodied in the "Princeton," the "Monitor" and its class, and the "Destroyer." In the "Princeton" the material used was wood, and in the "Monitor" and "Destroyer" iron, following simply the developments of the age. In the three the means of propulsion was by screw-propeller. In the "Princeton" the means of offence were two 12-inch wrought-iron guns, as already noted. In the "Monitor" and its type the means of offence were two 11-inch smooth-bore cast-iron guns, followed later by larger guns of 13 and 15 inches of similar type. In the double-turreted monitors four such guns were of course installed. In the "Destroyer" the means of offence was a single gun for discharging a torpedo under water at the bow. On the "Princeton" the means of defence consisted still in wooden walls, while in the "Monitor" and its class the change was profound and complete. The essential idea of the "Monitor" was low freeboard and thus small exposed surface to the ship herself, combined with the mounting of guns in circular revolving turrets, thus giving an all-around fire and on the whole making possible an adequate protection of the exposed parts of the ship and providing for the combination in maximum proportions of armored protection and heavy guns for offence. On the "Destroyer" the means of defence consisted simply in a light deflecting deck armor forward, the vessel being intended to fight bows on and depending on her means of offence rather than defence, which were made quite secondary in character.