Falkirk Wheel, Scotland, United Kingdom

• How the caissons are kept level
The caissons need to rotate at the same speed as the wheel but in the opposite direction to keep them level and to ensure that the load of boats and water does not tip out when the wheel turns.
Each end of each caisson is supported on small wheels which run on the inside face of the eight-metre-diameter holes at the ends of the arms, allowing the caissons to rotate. The rotation is controlled by means of a train of gears: an alternating pattern of three eight-metre-diameter ring gears and two smaller idler gears, all with external teeth. The central large gear acts as a stationary sun gear. It is fitted loosely over the axle at its machine-room end and fixed to a plinth to prevent it from rotating. The two, smaller, idler gears are fixed to each of the arms of the wheel at its machine-room end and act as planetary gears. When the motors rotate the wheel, the arms swing and the planetary gears engage the sun gear, which results in the planet gears rotating at a higher speed than the wheel but in the same direction. The planetary gears engage the large ring gears at the end of the caissons, driving them at the same speed as the wheel but in the opposite direction. This cancels the rotation due to the arms and keeps the caissons stable and perfectly level.