> <i>Hero</i> [Hero of Alexandria, 10-70 AD] <i>even suggested a mechanical use for the effect. By setting a fire on a hollow, airtight altar, the heated air within would flow down a tube into a sphere full of water, which in turn would be pushed up another tube into a hanging bucket. The bucket, when sufficiently heavy with water, would then pull on a rope to open some temple doors. Crucially, when the fire was extinguished, Hero noted that the cooling of the air in the altar would draw the water back into the sphere again, lighten the bucket, and so allow the doors to be closed by a counterweight.</i><p>Put the contraption on a chariot, use the ropes to turn the wheels instead of "temple doors" (!) and voilà: you have an auto-mobile.<p>Even if it's much less practical than using a horse, it's amazing that in almost 2000 years, nobody thought of making that for its sheer entertainment value -- or even to convince people that ghosts exist!