Navigational sailing equipment on a nautical chart. At left are some dividers, a pair of compasses for measuring distances on the map. In the centre is a Silva compass, for use in conjunction with the map for finding directions. It consists of a fluid-damped, magnetised needle fixed to a plastic ruler base. The needle becomes aligned to the local component of the Earth's magnetic field, resting North to South. At right is a parallel ruler, an instrument used, again in conjunction with the chart, to calculate courses.