Brass mariner's quadrant. Brass scale 0°-90° by 30'. Shadow square 0°-50°-0° by 1°. Two sights and plumb line, probably a later replacement. Radius 153 mm.

A polished brass quarter circle with two sights on the right-hand edge. The shadow square is used for taking altitudes in surveying. The back of the instrument is blank. The sights and plumb bob are probably later replacements.

The Mariner's Quadrant is a simple instrument that enabled navigators to measure altitude. An object is observed through the sights and the weighted line gives the altitude above the horizon in degrees along the curved edge. This example also has a shadow square set out inside the main scale. This can be used for making basic surveying measurements. The reverse is blank.