Before the troops were sent to the Peninsula several things were to be done. An expedition to restore communication westward by the Baltimore and Ohio Rail way involved bridging the Potomac with boats which were to be brought by canal. It collapsed because McClellan's boats were six inches too wide for the canal locks. Then Lincoln had insisted that the navigation of the lower Potomac should be made free from the menace of Confederate batteries which, if McClellan would have co-operated with the Navy Department, would have been cleared away long before. This was now done, and though a new peril to the transportation of McClellan's army suddenly and dramatically disclosed itself, it was as suddenly and dramatically removed. In the hasty abandonment of Norfolk harbour on the south of the James estuary by the North, a screw steamer called the _Merrimac_ had been partly burnt and scuttled by the North. On March 1 she steamed out of the harbour in sight of the North. The Confederates had raised her and converted her into an ironclad. Three wooden ships of the North gave gallant but useless fight to her and were destroyed that day; and the news spread consternation in every Northern port. On the very next morning there came into the mouth of the James the rival product of the Northern Navy Department and of the Swedish engineer Ericsson's invention. She was compared to a "cheesebox on a raft"; she was named the _Monitor_, and was the parent of a type of vessel so called which has been heard of much more recently. The _Merrimac_ and the _Monitor_ forthwith fought a three hours' duel; then each retired into harbour without fatal damage. But the _Merrimac_ never came out again; she was destroyed by the Confederates when McClellan had advanced some way up the Peninsula; and it will be unnecessary to speak of the several similar efforts of the South, which nearly but not quite achieved very important successes later.