The masts of the mortar schooners were dressed off with bushes, to render them indistinguishable from the trees on shore near the forts. The schooners were then towed up to a point within range, and moored where the woods hid them, so that they could not be seen from the forts. Lieut. F. H. Gerdes of the Coast Survey had made a careful map of that part of the river and its banks, and elaborate calculations by which the mortars were to be fired with a computed aim, none of the gunners being able to see what they fired at. They opened fire on April {92} 18, and kept up the bombardment steadily for six days and nights. Six thousand enormous shells--eight hundred tons of iron--were thrown high into the air, and fell in and around the forts. For nearly a week the garrison saw one of Porter's aërolites dropping upon them every minute and a half. They demolished buildings, they tore up the ground, they cut the levee and let in water, and they killed and mangled men; but they did not render the forts untenable nor silence their guns. The return fire sank one of the mortar boats and disabled a steamer. Within the forts about fifty men were killed or wounded--one for every sixteen tons of iron thrown. [Illustration: SHIP ISLAND.] While the fleet was awaiting the progress of this bombardment, a new danger appeared. The Confederates had prepared several flat-boats loaded with dry wood smeared with tar and turpentine; and they now set fire to them one after another, and let them float down the stream. But Farragut sent out boats' crews to meet them, who grappled them with hooks, and either towed them ashore or conducted them past the fleet, and let them float down through the passes and out to sea.