Caltech engineer Mory Gharib was poring over the digitized notebooks of Leonardo da Vinci one day, looking for sketches of flow visualization to share with his graduate students for inspiration. That’s when he noticed several small sketches of triangles, whose geometry seemed to be determined by grains of sand poured out from a jar. Further investigation revealed that Leonardo was attempting to study the nature of gravity, and the little triangles were his attempt to draw an equivalence between gravity and acceleration—well before Isaac Newton came up with his laws of motion, and centuries before Albert Einstein would demonstrate the equivalence principle with his general theory of relativity. [Edited for clarity.] Gharib was even able to re-create a modern version of the experiment.
Source: Leonardo noted link between gravity and acceleration centuries before Einstein | Ars Technica
Caltech engineers even re-created his experiment with a modern apparatus.