 Stevin's work in the "waterstaet" involved improvements to the sluices and spillways to control flooding, exercises in hydraulic engineering.Windmills were already in use to pump the water out but in "Van de Molens" ("On mills"), he suggested improvements including ideas that the wheels should move slowly with a better system for meshing of the gear teeth.These improved the efficiency of the windmills used in pumping water out of the polders by three times.