Portuguese Compass Box

This case contains a range of instruments used for cartography and mathematical calculations. They include a ruling pen with spares, 2 pencil cases, a variety of spares for both reduction and clamp compasses, and a flat scale with a graduated ruler on the front for cross sections. On the back it has an equivalency scale reminiscent of a Gunter's sector, used for trigonometric and logarithmic calculations. There is also another Gunter's sector, on the front of which are string lines, solid lines, bullets, and metals (lead, silver, iron, tin, and leather) with their corresponding signs. It would have originally had a graduated semi-circle but this has not been preserved.

All these instruments allowed the navigator to make the calculations required to determine their route on the nautical chart and do topographical calculations.