water mill dreamstime
I’ve just returned from a quick flit to England where I had the pleasure of being able to take several walks in the countryside in some rather splendid weather. The particular region I was in has an abundance of brooks and streams, and these were boisterous and lively following a period of lengthy rain.

Back in the good ol’ days these streams powered all sorts of mills, providing the mechanical power to crush stone, grind corn, saw logs, etc., and the area was littered with either the remains of old mills or with ones converted into other uses (i.e. holiday homes). There would have been a number of mills on one stream, one after the other as the water flowed downhill, with the same water that powered the top mill being used for successive establishments downstream.

What a wonderful early example of a renewable energy source.

Between mills, the streams morphed into narrow raging cuts and broad, shallow areas like fords, where horses and carts would have once crossed and children now play and paddle.

Typically, where a mill was located on a section with a shallow incline, some of the water would have been diverted from the main stream and used to fill a mill pond that would act as a reservoir. The pond water would then be used to turn the wheel by pouring it over the top of the wheel, or somewhere above half-height, filling up individual buckets on the vanes in the process and letting the weight of the water and gravity do most of the work.

Where the stream narrowed and followed a steeper incline, a diverter channel (race) would often have been dug to direct some of the stream so that it ran underneath a water wheel, turning it by sheer force as the water hit the paddles.

The amount of energy produced in this undershot style wheel would have been the product of the amount of water hitting the vanes and the pressure behind that water. Having a shallow, narrow stream with a lot of pressure behind it would be capable of producing the same amount of energy as a wider, deeper stream but with less force, or pressure.

To some, this may be reminiscent of the power/pressure/flow relationship in electrical circuits (don’t tell me you didn’t see this coming …), so let’s explore that briefly.

The pressure of electricity can be considered to be volts, the flow rate as amps, and the power produced as watts. Considering that Watts = Volts x Amps, then the same amount of power might be produced with low flow under high pressure, as with high flow under low pressure, similar to the water powering our water wheel.

If the water pressure in the stream had been reduced due to an extended period of dry conditions (not normally a concern in England), then diverting more water into the race to increase the flow rate would have helped keep the wheel turning and a smile on the miller’s face.

Don’t confuse the “trickle-down” configuration of multiple mills on one stream with the much-discussed economic version. In the mill stream example, all entities (mills) have equal access to the same source of income. When the top mill is done with the water used to drive the wheel it sends it on to the next mill downstream free of charge together with the water it did not use.

Imagine an economic model based on that!

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