Noting that warm air moved up a chimney (which were another Medieval innovation), larger Medieval kitchens had fans installed in the chimney to automatically turn spits by use of a gearing system.

FORBES: What Are Examples of Things in the "Common Knowledge" about History That Historians Almost Universally Consider Incorrect?

His scheme replaces the chimney with a tornado-like vortex of spinning air, which could extend several kilometres into the atmosphere.

ECONOMIST: Alternative energy: The power of spin | The