These 12-inch mortars were manufactured at the Watervliet Arsenal, Watervliet, New York in 1901. The carriages are the Model 1896 and were manufactured by the American Hoist and Derrick Company, St. Paul, Minnesota.
The fortification you are standing in is known as Battery Laidley, in honor of Colonel Theodore T.S. Laidley, who had served in the Mexican and Civil Wars and died in Florida. In August, 1902, eight mortars were mounted. However, the weapons were not test-fired until November 19 & 20, 1903, (five years after the fort's foundations were laid). The mortars stood ready to defend the harbor of Tampa Bay but never fired a shot at an enemy. The only time they were fired was during practice.
In 1917, four of the eight mortars were dismounted and shipped to Fort Rosecrans, San Diego, California for coastal defense. After World War I, many weapons such as these were melted down to be converted to other types of weaponry. Today Fort De Soto has the only 12-inch seacoast mortars, Model 1890-M1, in the continental United States. The only other ones in the world are on the island of Corregidor in the Philippines.
Length, overall: 161 inches
Length, muzzle to breech face: 141 inches
Length of bore in calibers: 10
Maximum diameter of chamber: 12.5 inches
Total weight w /carriage: 138,000
Recoil mechanism, type: Hydraulic
Length of recoil: 23 inches
Recuperating mechanism, type: 5 columns of springs
Maximum elevation: 70 degrees
Minimum elevation: 45 degrees
Traverse: 360 degrees
Powder charge: Nitro cellulose
Maximum charge: 65 pounds
Maximum range: 6.82 miles
Projectile sizes: 800, 824 & 1046 pounds
Artillery spotters, in towers, directed the firing of the weapons. They relayed readings, by telephone, to the relocating room; then the information was transmitted to the data booths just behind the gun pits. The data was interpreted there and posted to the twelve-man mortar crews by slate boards. A floating target was pulled by a small ship during practice firings at Fort De Soto. When fired, the projectile could penetrate six inches of a ship's steel deck at a range of six miles.