“The object that strikes the air develops a force equal to that of the air that strikes the object. Observe how the beating of its wings against the air suffices to bear up the weight of the eagle in the highest and thinnest air. Observe too that, conversely, the air fills the sails and pushes the heavily laden ship. From this you can understand that man, with great wings, acting against the resistance of air, might overcome it victoriously and rise above it,”
Leonardo da Vinci