This old-fashioned map distance measuring tool is a gem. It uses the fact that the same angular displacement produces a different linear displacement. The linear displacement is proportional to the radial distance.
I read the radial distances as 6mm, 8.5mm, 10.75mm, 12.5mm. The ratios almost work. 5/4 = 1.25 but 12.5/10.75 = 1.16. 4/3 = 1.33 but 10.75/8.5 = 1.26. 3/2 = 1.5 but 8.5/6 = 1.41.
