Captain Field’s Improved Parallel Ruler, Circa 1870.
Dimensions: 45.3 cm long by 7.3 cm wide by 2 cm high.
Navigators use parallel rulers for laying off a specific course on a chart. They were first invented in 1584 by a Frenchman named Mordente, but only became common in navigation in the 18th century. Captain Feild’s (or Field’s) design had one of its outer edges divided into degrees like a protractor, with the opposite edge divided into points, half-points and quarter-points (of the compass). This made it easier to read off the bearing of a line than with a plain parallel rule, which had to be moved across the chart to the nearest compass diagram.