Water wheels were invented 2000 years ago by the Greeks although their wheels were set horizontally rather than vertically as at Sarehole. The sluice gate controls the flow of water from the pond onto the back of the wheel. The miller has to raise a lever that in turn raises the gate on a rack and pinion fixed back to it. This mechanism is very similar to that which is used to raise a paddle on a canal lock so that water can flow in. The wooden sides of the sluice are called the launder boards. The channel that connects the pond with the sluice gate is called a head race and the channel that takes the water away from underneath the water wheel to the river is called a tail race.