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.