The Falkirk Wheel is a rotating boat lift in Scotland, UK. The main project architect was Tony Kettle. Bachy/Soletanche and Morrison Construction Joint Venture won the contract to design the wheel and receiving basin, a new section of canal, a tunnel beneath the Antonine wall and a section of aqueduct. In turn the Joint Venture appointed Butterley Engineering to design and construct the wheel. Butterley undertook all construction work for the wheel and set up its own team to carry out the design work. This team comprised Tony Gee and Partners, to undertake the structural design responsibilities and M G Bennett & Associates to design the mechanical and electrical equipment for the wheel. The wheel has an overall diameter of 35 metres (115 ft) and consists of two opposing arms which extend 15 metres beyond the central axle and take the shape of a Celtic-inspired, double-headed axe. Two sets of these axe-shaped arms are attached about 35 metres (115 ft) apart to a 3.5 metres (11 ft) diameter axle. Two diametrically opposed water-filled caissons, each with a capacity of 80,000 imperial gallons (360,000 l; 96,000 US gal), are fitted between the ends of the arms. These caissons (also known as gondolas) always weigh the same whether or not they are carrying their combined capacity of 600 tonnes (590 long tons; 660 short tons) of floating canal barges as, according to Archimedes’ principle, floating objects displace their own weight in water, so when the boat enters, the amount of water leaving the caisson weighs exactly the same as the boat. This keeps the wheel balanced. Despite its enormous mass, it rotates through 180° in five-and-a-half minutes using very little power. It takes just 22.5 kilowatts (30.2 hp) to power the electric motors, which consume just 1.5 kilowatt-hours (5.4 MJ) of energy in four minutes, roughly the same as boiling eight kettles of water. The wheel is the only rotating boat lift of its kind in the world, and is regarded as a Scottish engineering landmark. The United Kingdom has one other boat lift: the Anderton boat lift in Cheshire. The Falkirk Wheel is an improvement on the Anderton boat lift and makes use of the same original principle: two balanced tanks, one going up and the other going down. But the rotational mechanism is unique to the Falkirk Wheel.