Workshops / Workshop 3
Water mills
A mill turns the push of a river into useful work. For centuries it was the most common machine in the countryside.
In brief
A water mill uses the weight or push of flowing water to turn a wheel, and gearing passes that turn to millstones or other machines. Mills ground grain for many centuries, then also sawed wood, pressed cloth, beat paper and drove early factories. Many river towns grew around them.
What it is
A water wheel is a large wheel with paddles or buckets, turned by falling or flowing water. In a corn mill the wheel’s turning is passed through gears to a pair of millstones. The upper stone, the runner, turns above a fixed lower stone, the bedstone. Grain trickles from a hopper into the gap, and the grooved faces of the stones grind it to flour.
To make a river work reliably a mill needs a few built features: a weir or dam to raise the water, a channel called a leat (or mill race) to bring it to the wheel, a millpond to store it and a sluice to control the flow. The old channels often survive in the landscape long after the wheel is gone.
Four kinds of wheel
| Type | How the water acts | Best for | Notes |
|---|---|---|---|
| Undershot | Water flows under the wheel and pushes the paddles | Fast, flat rivers with little drop | Simplest to build; lowest efficiency |
| Breastshot | Water enters about level with the axle, with push and weight | A moderate drop | A middle way |
| Overshot | Water is poured onto the top and its weight turns the wheel | A higher drop and a smaller flow | The most efficient of the traditional types |
| Horizontal | A wheel turns flat, on an upright shaft, driven by a jet of water | Fast small streams | Needs no gears: the shaft turns the upper millstone directly |
A Roman writer on building, Vitruvius, described a gear-driven vertical water wheel for grinding grain in the first century BC. Experiments by the English engineer John Smeaton in the 1750s showed that, for the same water, overshot wheels did far more work than undershot ones, which helped later builders choose designs.
Why mills mattered
- Food. Grinding by hand is slow, hard work. A mill ground much more grain with much less labour.
- Settlement. A mill gave people a reason to meet at one place. Farmers brought grain, and a track, a bridge or a market often followed.
- Money and power. In medieval Europe, lords often required tenants to use the lord’s mill and paid the miller with a share of the grain, called multure. Disputes over fair shares are common in records.
- Industry. Water power was adapted to many jobs. Water-driven hammers pressed and thickened woollen cloth (fulling) by the twelfth century in Europe; paper mills were at work in Europe by the thirteenth century; forges used water-driven bellows and hammers; and in the eighteenth century water-powered mills spun cotton on a large scale.
How much power is in a river?
The power of falling water depends on how much flows and how far it falls. In round terms, power in watts is about 1000 times 9.8 times the flow in cubic metres per second times the drop (called the head) in metres, before losses.
Worked example (for illustration). A stream delivering 1 cubic metre per second with a 3 metre drop holds about 29,000 watts, or 29 kilowatts, of power. A good overshot wheel might turn perhaps 60 per cent of it into useful work, about 18 kilowatts. A stream with half the flow gives half the power.
That is why mill builders raised weirs, dug leats to gain extra drop and valued sites on steep streams.
Effects and decline
- Landscape. Weirs, leats and mill ponds changed river levels and drainage. Fish such as eels and salmon had trouble getting past weirs.
- Conflict. Mills upstream and downstream, boatmen and fishermen could all be affected by one weir, so disputes over water rights were common.
- Decline. In the nineteenth century steam engines, and later turbines and electricity, let industries leave the river bank. Large roller mills replaced many village corn mills.
- Afterlife. Many mill buildings were turned into homes, workshops or museums. Their leats and ponds survive as waterways and wildlife habitats.
Try it
- On an old map, look for a weir or a mill symbol, a straight channel running beside the river and a pond.
- Compare it with a modern map. Is the building still there? What remains of the channels?
- Measure a small stream’s speed by timing a floating leaf over ten metres, and estimate the flow with an adult’s help. Never go near fast or deep water alone.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
What is a leat?
A channel that carries water from the river to the mill wheel.
Which wheel type needs no gears?
The horizontal wheel, because its upright shaft turns the upper millstone directly.
Why is the overshot wheel the most efficient traditional type?
It uses the weight of the water falling onto the wheel, so it can use a high drop and a small flow well.
What was multure?
The share of the grain that a miller kept as payment.
Name two jobs water power did besides grinding grain.
Any two of: fulling cloth, sawing wood, making paper, driving forge hammers and bellows, spinning cotton.
If a stream’s drop is doubled and its flow stays the same, what happens to the available power?
It doubles.
Basis of the facts
Where these facts come from. The mill parts and wheel types follow Reynolds (1983). Vitruvius describes the geared vertical water wheel in book X of his work on architecture (first century BC). Smeaton’s tests are in his 1759 paper in the Philosophical Transactions of the Royal Society; the unit says only that overshot wheels did far more work than undershot ones. Medieval mill rights and multure follow Holt (1988) and Langdon (2004). The power formula is the standard physics relation; the worked example is illustrative and its figures are rounded. See the sources list.