Lecture Two The Shape of An Idea

Abstract

Socrates said those who come at the second night are truly lovers of wisdom. It isn’t certain that Socrates ever said that, but it is quite clear that it is the sort of thing that he might have said, and if he had I would have quoted him tonight. Last night we considered the discovery of the air, and what this meant for the History of aerodynamic theory and the development of the concept of flight. This evening I want to discuss the shaping of an idea. The idea is this: flight is a subject matter which can be treated objectively and scientifically and consideration of this idea might sooner or later actually lead to the construction of a proper flying machine. Erwin Schrödinger once said “Nature will tell you a direct lie if she possibly can”. Nature told such a lie to Daedalus and to Icarus; nature told the same lie to the Chinese Emperor Shun and to the Saracen of Constantinople. And nature also told that lie to Leonardo Da Vinci. The lie was this: all birds flap their wings; therefore flapping wings are somehow essential to flight. Further, any theory of flying through air requires the idea of flapping as a primary premise in the argument. In that collection of notes and random jottings that remain of the literary works of Leonardo, we find something resembling a monograph on the flight of birds, a tract written in 1505.¹ Here, Leonardo was quite sympathetic to the Aristotelian doctrine of antiperistasis which we considered last night. It is the doctrine that a body moving through air is assisted in its forward progress by the circulation of those particles separated by the arrowhead which come around behind and impinge on the aft section of the arrow. But after 1505 Leonardo lost his sympathy for this theory. From that point on he saw air as fundamentally a factor of resistance, something that tended to slow moving objects down, and thought that this was due to a property of air which Galileo and others later referred to as condensibility. Today we would call it compressibility.