Hot air-powered cooking spit. Historical artwork of a propeller (at upper centre) used to power a cooking spit. The warm air from the fire (at bottom) rises up the flue, causing the propeller, or airscrew, to turn and rotate the hook over the fire. This rotary movement also turns a spit (in the fire) through a set of gears (at lower centre) and a belt drive (at lower right). This drawing is by Leonardo da Vinci (1452-1519).