Brian Clegg:
In his letter De mirabili potestate artis et naturae (c1250), Bacon set out a veritable catalogue of wonders. Because of his constant concern to separate the realities of nature and the fictions of magic, he began by establishing a very clear context for these inventions. These are marvels wrought through the agency of Art and Nature... In these there is no magic whatsoever because, as has been said, all magical power is inferior to these works and incompetent to achieve them.
Bacon then launched into an incredible list of modern concepts. He made the extraordinary claim that these devices had been constructed in antiquity and in his own time. He claimed to be acquainted with them explicitly, except with the instrument of flying [an ornithopter] which I have not seen. And I know no one who has seen it. But I know a wise man who has thought out the artifice.It is possible that a device for flying shall be made such that a man sitting in the middle of it and turning a crank shall cause artificial wings the beat the air after the manner of a bird's flight.