in zero wind speed and due north wind direction, which points the drift bar straight forward. With the bar in this position, the bomb aimer uses the drift wires to tune out any sideways drift and thereby find the exact wind direction. The bearing plate is rotated to the compass heading and locked, thereby recording the wind direction for future reference. The pilot then turned 90 degrees to one side or the other, placing the wind directly off the side of the aircraft. The bomb aimer then rotated the milled head to the same 90 degrees. At this point the wind speed knob is adjusted, pushing the drift bar sideways until objects on the ground could be seen moving directly along the drift wires. The wind speed is now known and set, and the aircraft can then manoeuvre as it wants with only the milled head needing adjustment. A later modification to the CSBS, and supplied with most of the Mk. VII and Mk. IX examples, was the auxiliary drift bar. This was attached at the front of the main drift bar and consisted of a single drift wire mounted on a rotating fixture. This allowed relative measurements of the drift relative to the aircraft without having to rotate the main drift bar and thereby possibly leave the sight in an incorrect setting. To use the system, the bomb aimer would lower the auxiliary bar and rotate it until objects were moving along its single wire. This provided a measurement relative to the current setting, say +10. The aimer could then update the wind bar to the correct setting. Next, the ground speed was measured by timing objects as they passed through any two sets of small beads on the main drift bar using a stopwatch. To calculate the resulting wind speed and direction, systems with the auxiliary bar were also equipped with the wind gauge bar. This was normally stowed folded against the back of the height bar, but could be rotated down and to the rear to lie over the compass. The top of the bar was indexed in seconds, corresponding to the measurement made of the times using the stopwatch. The cursor slid along the wind bar and was set to that measurement. A small scale on the cursor allowed for the conversion of indicated air speed to true air speed, which differs depending on altitude. A