An important advance in the art of gun laying came with the introduction of the first recoil mechanisms. The barrel recoil was absorbed by hydraulic cylinders and then the barrel was returned to its firing position by a spring that had stored some of the recoil energy. This meant the gun did not have to be repositioned after each time it was fired. An early prototype incorporating this design feature was built in 1872 by Russian engineer, Vladimir Stepanovich Baranovsky. His 2.5-inch rapid-firing gun was also equipped with a screw breech, a self-cocking firing mechanism and it fired a fixed round (shell and cartridge case together). The recoil mechanism was contained in the gun cradle. Despite this effort, nothing followed from it, and it was only with the introduction of the French 75 mm in 1897, that recoil systems started to become normal. The gun's barrel slid back on rollers, pushing a piston into an oil-filled cylinder. This action absorbed the recoil progressively as the internal air pressure rose and, at the end of recoil, generated a strong, but decreasing, back pressure that returned the gun forward to its original position. By this time smokeless powder had replaced gunpowder as the standard propellant. The first practical rangefinder was developed by Barr & Stroud a pioneering Scottish optical engineering firm. Archibald Barr and William Stroud became associated from 1888. In 1891 they were approached by the Admiralty to submit a design for a short-base rangefinder for trial, and in 1892 they were awarded with a contract for six of their rangefinders. The device, operated by one person, brought two images from a distance object into coincidence allowing the distance to be calculated from their relative motions. Now that the barrel remained aligned with the target after firing, the more primitive tangent sight was replaced with the rocking-bar sight for direct-fire sighting. These were installed on QF 4.7-inch Gun Mk I–IV quick firing gun from 1887. The rocking-bar (or 'bar and drum') sight had an elevation scale, could mount a telescope as well as the open sight, and provided a small amount of horizontal deflection. These provided 'independent line of sight' because they enabled data to be set on the mount and the telescope (or open sight) aimed at the target independent of the barrel elevation. A related problem, particularly for large and longer range guns, was that the wheels could be at different heights due to the slope of the ground, which caused inaccuracy.