The MacTutor History of Mathematics Archive

The winding and setting stem of a calendar watch has a pushed-in winding position and a pulled-out time setting position. In the pushed-in position, rotation of the stem is transmitted by a crown wheel which moves for one direction of rotation into a barrel-winding position and for the opposite direction of rotation into a position in which it drives a heart to advance a date-ring step-by-step. A sprung hammer tends to keep the heart in a disengaged position and prevents unwanted setting of the date.