While the mortars were pounding away through those six long days and nights, Farragut was getting ready to take his ships past the forts. Union scouts and spies had travelled over every foot of land and water about the forts; and the exact strength of the Confederates, and the difficulties to be overcome, were clearly known to the Federal admiral. One of the chief obstructions was a chain of rafts and old hulks that stretched across the channel by which the fleet would be obliged to ascend the river. Under cover of a tremendous fire from all the mortars, two gunboats were sent up to remove this obstruction. The night was dark and favorable to the enterprise, and the vessels reached the chain before they were discovered. Then, under a fierce cannonade from the forts, Lieut. Caldwell put off in a row-boat from his vessel, boarded one of the hulks, and managed to break the chain. The string of hulks was quickly swept ashore by the swift current, and the channel was open for the ascent of the Union fleet. [Illustration: Breaking the Chain.] On the 23d of April, Farragut determined that his fleet should make the attempt to get past the forts the following day. He knew that the enemy must be exhausted with the terrible strain of Porter's bombardment, and he felt that the opportunity had arrived for him to make a successful dash for the upper river. The fleet was all prepared for a desperate struggle. Many of the captains had daubed the sides of their vessels with the river mud, that they might be less prominent marks for the Confederate gunners. The chain cables of all the vessels were coiled about vulnerable parts, or draped over the sides amidships to protect the boilers. Knowing that it was to be a night action, the gun-decks had been whitewashed; so that even by the dim, uncertain light of the battle lanterns, the gunners could see plainly all objects about them. Hammocks and nettings were stretched above the decks to catch flying splinters from the spars overhead. Late at night the admiral in his longboat was pulled from ship to ship to view the preparations made, and see that each captain fully understood his orders. It was two o'clock on the morning of the 24th of April, when the Confederates on the parapets of their forts might have heard the shrill notes of fifes, the steady tramp of men, the sharp clicking of capstans, and the grating of chain cables passing through the hawse-holes on the ships below. Indeed, it is probable that these sounds were heard at the forts, and were understood, for the Confederates were on the alert when the ships came steaming up the river.