form to make it smaller as well. The compass, no longer containing pointers and dials, was replaced with a smaller unit. The result was a version of the CSBS that was much smaller than earlier versions. About 5,000 of the new Mk. X were built and awaiting fitting to aircraft at the opening stages of the war. After the disastrous raid on Wilhelmshaven in 1939, the RAF was forced to abandon daylight attacks and move to night bombing. The Mk. X proved to have very poor visibility at night, and it would be difficult to modify it to correct this problem. The Mk. X had to be abandoned, and Mk. VII's and Mk. IX's hurriedly re-fit to aircraft. Thus the older versions of the CSBS soldiered on long after they were due to be replaced, and remained the primary British bombsights into 1942. The Mk. VII was widely found on slower aircraft and training schools, while the Mk. IX was used in higher speed aircraft. Mk. XI Another problem with all of the existing CSBS designs was that it could only be read correctly with the aircraft absolutely level. This was true especially during the run-up to the drop point, when the sight was used to correct the direction of flight through the use of the drift wires. The biplane bombers the CSBS had been developed for had the ability to slip-turn using the rudder only, which made it simple for the pilot to adjust their heading without affecting the aim too much. Modern monoplanes were subject to an effect known as dutch roll that makes them oscillate for a time after turning to a new heading. During this time the drift wires were difficult to use, so the entire process of correcting the flight path was greatly extended. In the aftermath of the Wilhelmshaven raid on 3 September 1939, it was found that the lengthy setup and bomb run demanded by the CSBS made its aircraft extremely vulnerable to fighters and anti-aircraft artillery. At a pre-arranged meeting on 22 December 1939, Air Chief Marshal Sir Edgar Ludlow-Hewitt made a request for a new bombsight that did not require such a long run into the target, and which would allow the aircraft to manoeuvre throughout the bomb run. The solution to this problem was well understood within the industry: use gyroscopes to provide a level platform to mount the bombsight