The Course Setting Bomb Sight (CSBS) is the canonical vector bombsight, the first practical system for properly accounting for the effects of wind when dropping bombs. It is also widely referred to as the Wimperis sight after its inventor, Harry Wimperis. The CSBS was developed for the Royal Naval Air Service (RNAS) in order to attack submarines and ships. It was introduced in 1917, and was such a great advance over earlier designs that it was quickly adopted by the Royal Flying Corps, and the Independent Air Force. It has been called "the most important bomb sight of the war". After the war the design found widespread use around the world. A US version of the CSBS was used by Billy Mitchell on his famous attack on the Ostfriesland in 1921. The basic design was adapted by almost all air forces and used well into World War II. It was eventually replaced in British service by more advanced designs like the Mark XIV bomb sight and the Stabilized Automatic Bomb Sight. Other services used vector bombsights throughout the war. History Early bombsights Prior to the introduction of the CSBS, bombsights were generally very simple systems of limited accuracy suitable only for low-level use. The primary pre-war device in RNAS service was the Lever Sight, which the pilot had to hold out of the cockpit in one hand while flying the aircraft with the other. The Central Flying School Sight replaced this in 1915, but was difficult to install in the cockpit. The CFS was in turn replaced by the Equal Distance Sight (EDS) designed in 1916 by F. W. Scarff, better known for the development of the Scarff ring. The EDS allowed the bomb-run parameters to be entered once and then left the pilot free to fly the plane. None of these sights had a way to calculate drift, the sideways motion of the bombs due to wind. This meant the aircraft had to attack their targets directly along the wind line. Even in this direction, the wind would cause the bombs to fall long or short. To correct for this, the bomb aimer would first measure their speed over the ground using a stopwatch. They would next look up the time it would take the bombs to reach the ground from their current altitude using a pre-computed table. Then, using both values, they would look up the proper angle