The attack began on September 13, as the British fleet of some nineteen ships began pounding the fort with Congreve rockets (from rocket vessel HMS "Erebus") and mortar shells (from bomb vessels "Terror", "Volcano", "Meteor", "Devastation", and "Aetna"). After an initial exchange of fire, the British fleet withdrew to just beyond the range of Fort McHenry's cannons and continued to bombard the American redoubts for the next 25 hours. Although 1,500 to 1,800 cannonballs were launched at the fort, damage was light due to recent fortification that had been completed prior to the battle.