Another incident, not only picturesque in itself but of great importance in the history of naval war, marks the opening months of 1862. After the failure of the first attempt to take Richmond by a _coup de main_ the war became in its essence a siege of the Confederacy. To give it this character, however, one thing was essential--the control of the sea by the Union forces. The regular United States navy--unlike the regular army, which was divided--was fully under the control of the Federal Government, and was able to blockade the Southern ports. Davis had attempted to meet this menace by issuing letters of marque to privateers; but this could be little more than an irritant to the dominant power. It so happened, however, that a discovery had recently been made which was destined to revolutionize the whole character of naval war. Experiments in the steel-plating of ships had already been made in England and in France, but the first war vessel so fitted for practical use was produced by the Southern Confederacy--the celebrated _Merrimac_. One fine day she steamed into Hampton Roads under the guns of the United States fleet and proceeded to sink ship after ship, the heavy round shot leaping off her like peas. It was a perilous moment, but the Union Government had only been a day behind in perfecting the same experiment. Next day the _Monitor_ arrived on the scene, and the famous duel between the first two ironclads ever constructed commenced. Each proved invulnerable to the other, for neither side had yet constructed pieces capable of piercing protection, but the victory was so far with the North that the hope that the Confederacy might obtain, by one bold and inventive stroke, the mastery of the sea was for the moment at an end.