Two of the ironclad gunboats that attacked Fort Henry on February 6, 1862, were among several designed by Samuel S. Pook for the U.S. War Department in 1861. Called "Pook Turtles" because their sloping rectangular sides, or casemates, made them look like mud turtles, these ironclads proved invaluable weapons in Union operations on western waters from their first use at Fort Henry to the Red River Expedition of 1864. Built between August 1861 and January 1862 by James B. Eads at an average cost of $101,808 each, four were constructed at Carondelet Marine Ways near St. Louis, Missouri, and three at Mound City, Illinois. Named for towns along the Ohio and Upper Mississippi rivers—Cairo, Carondelet, Cincinnati, Louisville, Mound City, Pittsburgh, and St. Louis—they were the first ironclad warships built in the Western Hemisphere. At Fort Henry on February 6, 1862, the Union gunboats Carondelet, Cincinnati, St. Louis, and Essex exchanged fire with the fort in their first test in battle. Their armament included six 32-pounder smoothbore cannon, four 42-pounder rifled cannon, and three 8-inch Dahlgren rifled cannon, and they carried two and a half inches of iron armor on the forward and central slopes of the casemate, but they were very slow, had a large turning radius, left the upper deck vulnerable to plunging fire, and had an unarmored aft portion of the casemate. The ironclads Carondelet and St. Louis, which took part in the attack on Fort Henry, were built at Carondelet Marine Ways. James Eads, a retired engineer who later became one of America's most famous bridge builders, financed construction of the gunboats and was later reimbursed by the U.S. Government. The ironclad USS Essex, not one of the Eads gunboats, was disabled in the attack on Fort Henry when a Confederate shot ruptured its boiler.