Reading: Diagram Label Completion: Question 3

Long before steam or electricity became available, watermills used the force of flowing water to grind grain, and the most efficient design for doing so was the overshot wheel. Unlike simpler wheels that are pushed round by the current striking their lower edge, an overshot wheel is turned by the weight of water falling onto it from above, a difference that allows it to extract far more energy from the same stream.

Water bound for the wheel first passes through a headrace, an artificial channel that carries it from a point upstream, often diverted from the river by a low dam called a weir. Where the headrace reaches the wheel, a hinged wooden or metal gate known as the sluice controls how much water is allowed through; opening the sluice further increases the flow reaching the wheel and therefore its turning speed. Just beyond the sluice, the water enters a narrow wooden trough called the launder, which is angled so that it delivers the water directly into a series of buckets fixed around the wheel's outer rim rather than letting it spill onto the wheel's flat face.

As each bucket fills, its weight pulls that side of the wheel downward, and the wheel rotates continuously as successive buckets fill on one side and empty on the other. At the centre of the wheel runs a solid horizontal axle, which turns together with the wheel and carries the rotational motion into the mill building, where a system of wooden gears steps the axle's slow, powerful turning up to the much faster speed needed to drive a millstone. Once the water in each bucket has emptied out, it falls into a channel below called the tailrace, which carries the spent water back to rejoin the main river some distance downstream of the mill.

The diagram below shows a side-on cross-section of an overshot water wheel and its water supply, viewed from upstream on the left to downstream on the right.

Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. the low dam across the river that diverts water into the channel supplying the wheel
2. the artificial channel that carries water from upstream to the wheel
3. the hinged gate that controls how much water reaches the wheel
4. the narrow angled trough that delivers water into the wheel's buckets
5. the containers fixed around the outer rim of the wheel that fill with falling water
6. the solid horizontal bar running through the centre of the wheel
7. the channel below the wheel that carries the spent water back to the river