Even though it clearly has two wheels, this machine still counts as a unicycle. The air-screw propeller means that at high speeds, the front control wheel is lifted up, so you coast along on just the one big wheel. We weren't terribly impressed by the vehicle, since this design, by A.F. d'Harlingue, removed the only advantage the unicycle holds over the bicycle: the ability to turn just by leaning one way or the other. However, we thought that by augmenting the air-screw propeller with an honest-to-god engine, we could turn this bad boy into a "jolly sport machine."
Read the full story in Here's the Air-Propelled Unicycle