When, at the outbreak of the rebellion, the navy-yard at Norfolk, Virginia, had to be abandoned to the enemy, the destruction at that time attempted by Commodore Paulding remained very incomplete. Among the vessels set on fire, the screw-frigate _Merrimac_, which had been scuttled, was burned only to the water's edge, leaving her hull and machinery entirely uninjured. In due time she was raised by the Confederates, covered with a sloping roof of railroad iron, provided with a huge wedge-shaped prow of cast iron, and armed with a formidable battery of ten guns. Secret information came to the Navy Department of the progress of this work, and such a possibility was kept in mind by the board of officers that decided upon the construction of the three experimental ironclads in September, 1861. The particular one of these three especially intended for this peculiar emergency was a ship of entirely novel design, made by the celebrated inventor John Ericsson, a Swede by birth, but American by adoption--a man who combined great original genius with long scientific study and experience. His invention may be most quickly described as having a small, very low hull, covered by a much longer and wider flat deck only a foot or two above the water-line, upon which was placed a revolving iron turret twenty feet in diameter, nine feet high, and eight inches thick, on the inside of which were two eleven-inch guns trained side by side and revolving with the turret. This unique naval structure was promptly nicknamed "a cheese-box on a raft," and the designation was not at all inapt. Naval experts at once recognized that her sea-going qualities were bad; but compensation was thought to exist in the belief that her iron turret would resist shot and shell, and that the thin edge of her flat deck would offer only a minimum mark to an enemy's guns: in other words, that she was no cruiser, but would prove a formidable floating battery; and this belief she abundantly justified.