A navigational instrument, rarely seen nowadays, used in coastal waters by which a ship's position can be plotted on the chart from horizontal sextant angles. In its simplest form it consists of a circular Perspex protractor about 15 centimetres (6 in.) in diameter fitted with three radial arms, each with a bevelled edge. The central arm is fixed at 0°, the other two pivoted at the axis so that each can be set to any angle relative to the fixed central arm. The horizontal sextant angles between three fixed points marked on the chart are set on the instrument's movable arms and the pointer laid on the chart so that the bevelled edges of the three arms correspond with the three fixed points on the chart, the ship's position being at the pointer's axis. It was of most use in hydrographic surveying. A similar procedure can be achieved by using tracing paper.
Subjects: Maritime History — Warfare and Defence.