said course rule being adapted to bear against a chart placed on said plate, said parallel racks being spaced above said plate with the teeth of said racks facing said plate, a spring force acting between the carrying device and the course rule so as to urge the course rule towards the ch...
gear wheels provided at the opposite ends of said guide shaft which are positioned to mesh with and engage the underside of said parallel racks, a carrying device slideably mounted on said guide shaft and moveable along said shaft and associated with a course rule that is rotatable about a ve...