This graphic explains how the concept of the disappearing gun works. According to the National Park Service, "the disappearing carriage is a unique characteristic of Endicott period [starting in 1885] fortifications. Many larger caliber guns used this carriage. First invented in Europe, it was perfected in America with the addition of a counterweight system. A pair of massive steel arms connect the barrel and a lead counterweight. In loading or 'service' position, the gun is held down behind a protective parapet and the counterweight is suspended above a well. When the gun is released, the counterweight falls into the well, swinging the gun up into firing position. The recoil from firing then pushes the gun back into service position."