Duccius[?] - inscriptions, notably a tombstone of Lucius Duccius Rufinus at York[2] (http://museums.ncl.ac.uk/archive/arma/contents/iconog/tombston/cent1/uk/york/duccius.htm)
... simple. It can easily be understood with a little knowledge of gyroscopes. Their striking property is that the axis of a spinning gyroscope will resist a force ...