The usefulness of such a system had been noted earlier, and experimental designs with raisable platforms or eccentric wheels, with built-in counterweights, were built or proposed. Some used paired guns, in which one cannon acted as the other's counterweight, or counterpoise. An unsuccessful attempt at a disappearing carriage was King's Depression Carriage, designed by William Rice King of the United States Army Corps of Engineers in the late 1860s. This used a counterweight to allow a 15-inch (381 mm) Rodman gun to be moved up and down a swiveling ramp, so the weapon could be reloaded, elevated, and traversed behind cover. The carriage was subjected to six trials in 1869–1873. It was not adopted; an 1881 letter to the Chief of Engineers by Lt. Col. Quincy A. Gillmore stated that it "still leaves a great deal of heavy work to the slow and uncertain process of manual labor". Part of a test installation at Fort Foote, Maryland remains. King's design was better suited for breech-loaders; had the US not had a plethora of new muzzle-loaders just after the Civil War it may have seen wider use.