At seven o'clock on the night of April 21, the United States steamer "Pawnee," which had been lying under the guns of Fortress Monroe, hoisted anchor, and headed up the bay, on an errand of destruction. It was too late to save the navy-yard with its precious stores. The only thing to be done was to burn, break, and destroy every thing that might be of service to an enemy. The decks of the "Pawnee" were black with men,--soldiers to guard the gates, and complete the work of destruction within the yard; blue-jacketed tars to do what might be done to drag the entrapped vessels from the snare set them by the Virginians. It was a bright moon-light night. The massive hull of the ship-of-war, black in the cold, white rays of the moon, passed rapidly up the Elizabeth River. The sunken wrecks were reached, and successfully avoided; and about nine o'clock the "Pawnee" steamed into the anchorage of the navy-yard, to be greeted with cheers from the tars of the "Cumberland" and "Pennsylvania," who expected her arrival. The townspeople seeing the war-vessel, with ports thrown open, and black muzzles of the guns protruding, took to their houses, fearing she would open fire on the town. Quickly the "Pawnee" steamed to her moorings. The marines were hurriedly disembarked, and hastened to guard the entrances to the navy-yard. Howitzers were planted so as to rake every street leading to the yard. Thus secure against attack, the work of the night began. Nearly two thousand willing hands were set hard at work, cannon were dismounted and spiked, rifles and muskets dashed to pieces; great quantities of combustibles were piled up in the mammoth buildings, ready to be fired at a given signal. In the mean time, the blue-jackets were not idle. It was quickly decided, that, of all the magnificent vessels anchored in the harbor, the "Cumberland" was the only one that could be towed past the obstructions in the river. All hands were set to work removing every thing of value from the doomed vessels to the "Cumberland." Gunpowder and combustibles were then arranged so as to completely destroy the vessels when ignited. When the moon went down at twelve o'clock, the preparations were complete. All the men were then taken on board the "Cumberland" and "Pawnee," save a few who were left to fire the trains. As the two vessels started from the moorings, the barracks were fired, the lurid light casting a fearful gleam upon the crowded yards and shrouds of the towering frigate. A little way out in the stream a rocket was sent up from the "Pawnee." This was the signal for the firing of the trains. The scene that followed is thus described by an eye-witness:--