groups within the RAF. Description The following description is based on the Mk. IX as described in A.P.1730A, but will be separated into sections on the basic operation and the later additions. Crosswind bombing The bombsight problem is the need to determine the exact spot in the air where the bombs should be dropped to hit a target on the ground. Due to the acceleration of gravity, bombs follow a roughly parabolic path, the steepness being defined by the forward speed of the aircraft at the instant of release. The distance the bombs travel between being dropped and hitting the ground is known as the range, it is a function of the speed and the time to fall, the latter a function of altitude. The bomber attempts to manoeuvre along a line toward the target and then drop the bombs at the instant they are that distance, the range, from the target. The location at that instant is known as the drop point or point of release. Simple trigonometry can calculate the angle that the target would appear when the aircraft was at the drop point. This is known as the range angle or drop angle, and was typically looked up from a set of pre-computed tables or using a simple mechanical calculator. The bombsight is then set to that angle, and the bomb aimer drops the bombs when the target passes through the sights. In the presence of a cross-wind, as the aircraft flies forward the wind will push it sideways, away from the drop point. The solution is to calculate the angle that the aircraft should fly in order to cancel out this drift, the difference between the course and the heading. Calculating the proper drift angle is a simple task of basic vector addition, and is commonly carried out on a circular slide rule like the E6B. This is a somewhat time-consuming process. The CSBS solved this problem by reproducing the basic vector math in a mechanical system. The vectors that would normally be drawn by hand were duplicated in a series of screws, gears and sliding components. By dialling in the four inputs, altitude, airspeed, wind speed and wind direction, the mechanism moved the aiming pippers so they directly represented the required heading and range angle for the current airspeed and altitude. The wind will also have an effect on the bomb after it leaves the