ROUGH ROADS MAY EXPLAIN why bicycle-like vehicles were not invented until early in the nineteenth century, rather than in the time of the ancient Greeks. Yet this lack of creativity ran deeper, drawing on a widespread difficulty in making pertinent mental leaps even when opportunities were there to be seen. The advent of the hot-air balloon gives an illustration. Smoke rises; this has been known since the early use of fire, hundreds of thousands of years ago. Yet it was not until 1782 that anyone thought to trap warm smoke within a lightweight enclosure, thereby inventing the balloon.
Leonardo da Vinci might have done it. He held an ongoing fascination with aviation and with the flight of birds. He also sought to take advantage of the power of a rising column of smoke within a chimney, by having this smoke spin a windmill. This whirling propeller then was to provide power to turn a large joint of meat as it