Orleans. In ensuing weeks a mighty fleet assembled for the campaign. In mid-March Commander David Dixon Porter's flotilla of mortar schooners arrived towed by steam gunboats. The next task was to get Farragut's ships across the bar, a constantly shifting mud bank at the mouth of each pass entering the Mississippi. Farragut managed to get all of his ships but across the bar and into the river where Forts St. Philip and Jackson challenged further advance. A line of hulks connected by strong barrier chains, six ships of the Confederate Navy—including ironclad and unfinished but potentially deadly ironclad , two ships of the Louisiana Navy, a group of converted river steamers called the Confederate River Defense Fleet, and a number of fire rafts also stood between Farragut and the great Southern metropolis. On 16 April, the Union ships moved up the river to a position below the forts, and David Dixon Porter's gunboats first exchanged fire with the Southern guns. Two days later his mortar schooners opened a heavy barrage which continued for six days. On the 21st, the squadron's Fleet Captain, Henry H. Bell, led a daring expedition up river and, despite a tremendous fire on him, cut the chain across the river. In the early hours of 24 April, a red lantern on Hartfords mizzen peak signaled the fleet to get underway and steam through the breach in the obstructions. As the ships closed the forts their broadsides answered a fire from the Confederate guns. Porter's mortar schooners and gunboats remained at their stations below the southern fortifications covering the movement with rapid fire. Hartford dodged a run by ironclad ram Manassas; then, while attempting to avoid a fireraft, grounded in the swift current near Fort St. Philip. When the burning barge was shoved alongside the flagship, only Farragut's leadership and the training of the crew saved Hartford from being destroyed by flames which at one point engulfed a large portion of the ship. Meanwhile, the sloop's gunners never slackened the pace at which they fired into the forts. As her firefighters snuffed out the flames, the flagship backed free of the bank. When Farragut's ships had run the gantlet and passed out of range of the fort's guns, the Confederate River Defense Fleet attempted to stop their progress. In the ensuing melee, they managed to sink converted merchantman , the only Union ship lost during the night.