The vessels, when received into service, were named after cities standing upon the banks of the rivers which they were to defend--Cairo, Carondelet, Cincinnati, Louisville, Mound City, Pittsburg, St. Louis. They, with the Benton, formed the backbone of the river fleet throughout the war. Other more pretentious, and apparently more formidable, vessels, were built; but from thorough bad workmanship, or appearing too late on the scene, they bore no proportionate share in the fighting. The eight may be fairly called the ships of the line of battle on the western waters. The Benton was of the same general type as the others, but was purchased by, not built for, the Government. She was originally a snag-boat, and so constructed with special view to strength. Her size was 1,000 tons, double that of the seven; length, 202 feet; extreme breadth, 72 feet. The forward plating was 3 inches of iron, backed by 30 inches of oak; at the stern, and abreast the engines, there was 21/2-inch iron, backed by 12 inches of oak; the rest of the sides of the casemates was covered with 5/8-inch iron. With guns and stores on board, she drew nine feet. Her first armament was two IX-inch shell-guns, seven rifled 42s, and seven 32-pounders of 43 cwt.; total, sixteen guns. It will be seen, therefore, that she differed from the others simply in being larger and stronger; she was, indeed, the most powerful fighting-machine in the squadron, but her speed was only five knots an hour through the water, and her engines so little commensurate with her weight that Flag-Officer Foote hesitated long to receive her. The slowness was forgiven for her fitness for battle, and she went by the name of the old war-horse. There was one other vessel of size equal to the Benton, which, being commanded by a son of Commodore Porter, of the war of 1812, got the name Essex. After bearing a creditable part in the battle of Fort Henry, she became separated by the batteries of Vicksburg from the upper squadron, and is less identified with its history. Her armament was three IX-inch, one X-inch, and one 32-pounder. On the 6th of September Commander Rodgers was relieved by Captain A.H. Foote, whose name is most prominently associated with the equipment and early operations of the Mississippi flotilla. At that time he reported to the Secretary that there were three wooden gunboats in commission, nine ironclads and thirty-eight mortar-boats building. The mortar-boats were rafts or blocks of solid timber, carrying one XIII-inch mortar.