Leonardo da Vinci (1452-1519) wrote a treatise, the Codex on the Flight of Birds, which put forth the first scientific observations on the subject of flight. He discovered the vortices that are produced off the wings, and observed the alulae, or "thumbs" of the wings. He was concerned with the center of gravity, stability, and maneuverability.
Leonardo sketched several types of flying machines: helical wing, beating wings, parachute, and bat's wings. Through real life trial and error Da Vinci learned the difficulty of realizing his great dream of flying in a machine powered by human propulsion, and turned his talents toward the problem of gliding flight. In the glider drawing below, the flyer's position is studied at the point where he is balanced through movements of the lower part of the body. The wings, modelled upon those of bats and birds of large wingspans, are fixed on the inboard portion (next to the flyer), and mobile at the external portion. This part of the wing in fact can be moved by the flyer by a control cable connected to handles. Leonardo arrived at this solution by studying the wing structure of birds and observing that the inboard part of their wings move more slowly than the outboard, and that therefore serve to thus sustain themselves and produce forward thrust.