Fan

This is a curious machine intended for compressing air and force it out of a duct. This device could be used either for keeping the air in a room cool or, more probably, for driving air into a furnace. It is made up of a wooden drum fitted with propulsion fins. It can be either water- or hand-operated. The inside of the drum is divided into four sectors communicating with one another through valved outlets. A certain amount of water is circulated throughout the inside of the drum. The drum, rotating, helps water flow from one sector to the next, thus compressing the air and "forcing" it out of the central shaft which, in turn, is fitted with a valve.
Codex Atlanticus, sheet 754 r.