used by pilots to calculate ground speed and direction from air and wind speeds.
Set the air speed (m.p.h. or knots) by moving the circular base up and down, 200 mph in this example.
Set the wind speed (distance of the peg from the centre of the table, 52 mph above)
and direction (turn the circle) on the circular table.
The ground speed can then be read on the plastic ruler (approx 244 mph),
and the directional drift (22 degrees starboard) at the top of the circle.
It uses the GCSE Applied Mathematics "triangle of velocities" principle.
One can only ponder how far the pilot would have flown while performing this complicated manual process!