Stevens, in the course of his experiments, made the important discovery, that a single plate of boiler-iron, five-eighths of an inch in thickness, and weighing less than twenty-five pounds to the superficial foot[A], when nailed to the side of a ship, was impenetrable by shell and red-hot shot, the two missiles most dangerous to wooden walls.  When a solid shot strikes the side of a wooden ship, it passes in and usually stops before it reaches the opposite side.  The fibres of the wood yield and close up behind it, and it often happens, from the reunion of the fibres, that it is difficult to find the place perforated by the ball, and if found, it is often easy to remedy the injury by a simple plug.  But if a red-hot shot enter the ship, it may imbed itself in the wood or coils of cordage or sails, or reach the magazine, and thus destroy the whole structure, while the shell may explode within the ship and carry destruction to both men and vessel.  If, then, the iron-plate had answered no other purpose, the discovery by Stevens of its capacity to resist the two most formidable weapons of his day would alone have been of great value to the country; but he went farther, and demonstrated by actual construction the idea of Montgery, that successive plates of iron would resist the cold spherical shot thrown by the best artillery, and his floating battery or frigate is protected by plate within plate of iron armor.

[Footnote A:  Sheet-iron plates of one inch in thickness weigh forty pounds per superficial foot.]

While our Government slept upon its unfinished frigate, and forgot the honor and interest of the country in the lap of the siren of the South,—­of that South which sixty years since broke down the navy of John Adams, and left us to encounter the embargo and war with England without a navy, or, at most, with a few frigates which sufficed to show what the navy of Adams might have effected,—­the honor of launching the first iron-clad steamer, the Gloire, was resigned to the French.  The first Napoleon made the army of France the best in Europe, if not in the world; the third, while he maintains the standing of the army, aspires to give the same position to her navy.

In 1854, Napoleon, who had long studied the art of war, and during his stay in New York had doubtless seen or heard of the floating battery, determined to construct two such batteries, and accordingly built the Lave and Tonnerre.  With one of these, the Lave, during the Russian War, he assailed and destroyed in the brief space of one hour the strong fortress of Kinburn, near Sebastopol; and in striking contrast to this success, a large British steamship, heavily armed, but constructed of wood, was actually captured near Odessa by a small party of Russians with two or three thirty-two-pounders worked through a gap in an embankment.

The invulnerable battery of France anchored close under the fortress.  Before its cannon, granite walls are shivered into fragments most dangerous to the gunners, while the shells, burying themselves two or three feet deep in the brickwork, by their explosion shake the walls to pieces.  Iron, protected by iron, triumphed over both bricks and granite, which had defied the fleet of England.