By the 14th century, several thinkers had independently concluded, based on Aristotelian physics, that the sublunar airspace must be navigable by vessels. During the 15th century, the principle of aeronautics was sometimes replaced by attempts to achieve flight by imitating birds (see Aviatics, with fig.). At the beginning of the 16th century, in his manuscript Sul volo degli uccelli (1505; “On the Flight of Birds”), Leonardo da Vinci observed …