A rhumb line appears as a straight line on a Mercator projection map. These charts showed coastline with rhumb lines connecting ports. From that corner, emanate a series of compass rhumb lines. The course of the rhumb line is found merely by using the ellipsoidal isometric latitude. A rhumb line is not a spherical spiral. When limited to small seas, planispheres approximately follow both rhumb lines and great circles. The final part of the work contained a treatise inspired by Harriot on rhumb lines.