Air and sea navigation instrument - includes protractor scale with graduated sector pivoted about centre of graduated semicircle

摘要

The navigation instrument comprises a rectangular transparent plate on which two sets (1, 2) of angular graduations are marked. The graduations are centered on a common centre point (6) near one of the long edges of the plate. A movable sector is attached by a pivot at the centre point so as to move over the sets of graduations. In addition to a needle (7) marking the centre of the sector, there are further graduations on the sector indicating up to 20 degrees either side of the needle. A stop limits the movement of the sector to 90 degrees either side of the midpoint. The instrument may be used for measuring angles in air or sea navigation and allows for an angular correction to be made in readings taken. This allows for differences between true north and magnetic north or the north direction as indicated by any compass used.