Steinhaus longimeter

The Steinhaus longimeter, patented by the professor Hugo Steinhaus, is an instrument used to measure the lengths of curves on maps. It is a transparent sheet of three grids, turned against each other by 30 degrees, each consisting of perpendicular lines spaced at equal distances 3.82 mm. The measurement is done by counting crossings of the curve with grid lines. The number of crossings is the approximate length of the curve in millimetres.