Damiaen van Doorninck, a Dutch officer fluent in Cosmography and advanced mathematics, and he lectured interested Dutch and British prisoners on both. He also had a natural ability when it came to wrist watch repair, and his customers included German guards as well as prisoners. He invented a device which, when attached to a micrometer, could obtain measurements accurate to within a tenth of a millimetre of any cruciform lock, and he was therefore able to manufacture a key to fit any such lock in Colditz. He lectured other prisoners on how to use this device correctly, a course that lasted six months. Van Doorninck later successfully escaped from Colditz and made his way to Switzerland with Bill Fowler, an Australian officer.