iron rims. These starts support 96 float boards each of which is by by , and is paired with a seal board bolted directly to the rim. The original timber selection for the wheel is not known, but since 1983 the starts have been made from Iroko due to its longevity even in conditions where it is cycled between submerged and dry due to intermittent running. The breastshot wheel is vertically mounted, and falling water strikes the blades. Breastshot wheels are less efficient than backshot or overshot wheels, but more efficient than undershot wheels. At full power the wheel uses 2 tons (2 tonnes) of water per second and rotates five times a minute. The water wheel drives a flexible coupling to a pit wheel with a diameter of , which has 408 (204 pairs) hand-fitted wooden teeth that mesh with a cast iron gear, increasing the speed to 16 rpm. From there, cranks drive vertical connecting rods that transfer the energy to two long cast iron rocking beams made by Boulton and Watt. Each beam drives an diameter lift pump, which also takes its supply from the mill leat. Each pump stroke raises 50 imperial gallons (230 litres) of water to the canal via of diameter cast iron pipe. With the sluice fully depressed the flow is 73 ft³/s and due to the build-up of spent water in the tail-race the working head is reduced to , giving a potential power of 34 hp. The output power, calculated from the water lifted into the canal per minute, is 24 hp representing an efficiency of 62%. History Construction The idea of an east to west waterway link across southern England was first mentioned in Elizabethan times, between 1558 and 1603, to take advantage of the proximity of the rivers Avon and Thames, only apart at their closest. Plans for a waterway were shelved until the early 18th century. In 1723 the Kennet Navigation through Reading opened. The Avon navigation from Bristol to Bath was opened in 1727. In 1788 the so-called "Western Canal" was proposed to improve trade and communication links to towns such as Hungerford, Marlborough, Wiltshire, Calne, Chippenham and Melksham. The following year the engineers Barns, Simcock and Weston submitted a proposed route for this canal, although there were doubts about the adequacy of the water supply. The name was changed from Western Canal to Kennet and Avon