problem in air navigation, one requiring basic vector mathematics. Wimperis was very familiar with these techniques, and would go on to write a seminal introductory text on the topic. The same calculations would work just as well for bomb trajectories, with some minor adjustments to account for the changing velocities as the bombs fell. Even as the Drift Sight was being introduced, Wimperis was working on a new bombsight that helped solve these calculations and allow the effects of wind to be considered no matter the direction of the wind or the bomb run. The result was the Course Setting Bomb Sight (CSBS), called "the most important bomb sight of the war". Dialling in the values for altitude, airspeed and the speed and direction of the wind rotated and slid various mechanical devices that solved the vector problem. Once set up, the bomb aimer would watch objects on the ground and compare their path to thin wires on either side of the sight. If there was any sideways motion, the pilot could slip-turn to a new heading in an effort to cancel out the drift. A few attempts were typically all that was needed, at which point the aircraft was flying in the right direction to take it directly over the drop point, with zero sideways velocity. The bomb aimer (or pilot in some aircraft) then sighted through the attached iron sights to time the drop. The CSBS was introduced into service in 1917 and quickly replaced earlier sights on aircraft that had enough room – the CSBS was fairly large. Versions for different speeds, altitudes and bomb types were introduced as the war progressed. After the war, the CSBS continued to be the main bombsight in British use. Thousands were sold to foreign air forces and numerous versions were created for production around the world. A number of experimental devices based on a variation of the CSBS were also developed, notably the US's Estoppey D-1 sight, developed shortly after the war, and similar versions from many other nations. These "vector bombsights" all shared the basic vector calculator system and drift wires, differing primarily in form and optics. As bombers grew and multi-place aircraft became common, it was no longer possible for the pilot and bombardier to share the same instrument, and hand signals were no longer visible if the bombardier was below the pilot in the nose. A variety