Oh, hydrology.
This is a device to measure the speed of water flow, as described in A practical treatise on hydraulic and water-supply engineering: relating to the hydrology, hydrodynamics, and practical construction of water works, in North America (which I found on archive.org).
The most successful of all the simple mechanical hydrometers, not requiring the assistance of an electric battery, has been the revolving mill introduced by Woltmann in 1790, and known as “Woltmann’s Tachometer,” or moulinet. This current meter has from two to five blades, either flat or like marine propeller blades, set upon a horizontal shaft as shown….
There are some instructions on adjusting it and getting numerous measurements at different depths for greater accuracy.
It does seem wonderfully straightforward.