rhysimeter
/rɪˈsɪmɪtə/rare
noun
An instrument for measuring the speed of flow of water or the speed of a ship through water, comprising an open-ended tube positioned in opposition to the flow and connected to a column of mercury or other liquid, which is raised by the pressure of the water.
Origin
Late 19th century. From ancient Greek ῥύσις flow, stream + -meter.