ght 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 gu