The prime cause of the approaching change was the appearance of a queer-looking little craft in the Civil War in America between 1861 and 1864. The United States Government had a fine fleet of wooden steamships at the outbreak of hostilities, but the naval authorities of the seceding Southern States, having raised the _Merrimac_, a 40-gun frigate which had been sunk at the Norfolk navy yard, cut her down, built a battery amidships armoured with two or three thicknesses of railway iron, and attacked the Federal fleet. The _Merrimac_ had it all her own way, rammed and sank the frigate _Cumberland_, set the bigger _Congress_ on fire and compelled her to surrender, and withdrew with all the honours of war. But she was yet to meet her match. John Ericsson, a Swedish engineer, was commissioned by the United States Government to construct a small ironclad of his own designing. While the _Merrimac_ was engaged in defeating the wooden ships of the Federals in Hampton Roads, the _Monitor_, as the new vessel was called, was on her way south from New York. She joined the Federal fleet the very night before the _Merrimac_ made a second sortie. On this occasion, as she came out into the Roads and opened up the fleet she intended to attack, the _Merrimac_ spotted what someone described as looking "like a cheese-box on a raft". It was an excellent description of the little _Monitor_, which was built with a very low freeboard and had nothing on her deck but a cylindrical revolving turret containing a couple of guns, no masts, and but the merest apology for a funnel. Yet she proved one too many for the _Merrimac_ with her more numerous battery of guns. She was unable actually to pierce her sides, as her commander had received the most peremptory orders not to use more than 15 pounds of powder to load his guns, but the _Merrimac_ got so "rattled" that she had to sheer off. [Illustration: The _Monitor_, the famous little ship that revolutionized warship design The upper figure is a broadside view, the lower one a transverse section amidships. The upper portion of the hull was very like a raft, and was heavily armoured all over, as was the turret and the little pilot-box forward.]