Turning to a consideration from a similar point of view of Ericsson’s services in connection with warship design and construction, note may be first taken of the condition of the art of naval warfare in the years 1840-50, or when Ericsson first began his labors in this field.

The material used was wood, the means of propulsion sails, with some thought of steam-engines and paddle-wheels; the means of offence were cast-iron guns large in number but small in size, the largest being 9 or 11 inches in diameter and throwing a shell of some 75 or 130 pounds weight, while the means of defence consisted solely in the “wooden walls,” and modern ideas regarding armor had not even appeared above the horizon.

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.

The “Monitor,” however, was Ericsson’s great contribution to the art of naval war, and with it his name will always be associated.  It broke with the past in every way.  It reduced the number of guns from many to few, two or at most four; it reduced the freeboard from the lofty topsides of the old ship-of-the-line to an insignificant two or three feet, and thus made of the target a circular fort and a low-lying strip of armor.  It placed the guns in this circular fort and covered it with armor thick enough to insure safety against any guns then afloat, and thus, as perfectly as the engineering means of the day would permit, insured the combination of offensive and defensive features in maximum degree.