Leonardo’s studies for a flying machine are based on the imitation of the organic devices created for birds by nature. In the Codex on the Flight of Birds, compiled in Florence in 1505-1506, studies on the manoeuvres of birds in flight alternate with projects for mechanical contrivances that attempt to reproduce those natural operations. Leonardo is convinced that the similarities between human anatomy and that of birds are sufficient to allow man, with the aid of mechanical instruments (of which he analyses the laws in the Codex), to take off and fly. He thus compares men and animals not only as regards physiognomy (as in his studies for Herculean figures) but also in his studies on flight. These researches, strengthening his conviction that it is possible for man to fly, induced Leonardo to perform in these years daring but ill-fated experiments of human flight.