Houghton Mill is an 18th-century water mill, full of impressive machinery and, last weekend, actually grinding flour by the power of the river Great Ouse. Although I am not knowledgeable about these kinds of buildings or this technology I found myself spellbound by a small, but crucial, component in the milling process, the slipper.
The slipper is a kind of hopper that feeds grain into the millstones for grinding, Here’s a short film I took of it in operation when I was there with some friends and our kids:
It has a system of its own, and also it is intimately connected to other systems.
It has inputs: a gravity feed brings grain into the top of the slipper; energy is supplied by the vertical axle which is in turn driven indirectly from the water wheel.
It has outputs: grain is dropped into the centre of the millstones immediately below it.
It is self-regulating: as the flow of the river varies, the speed of the wheel varies, the rotation of the axle varies, and the extent to which the slipper is agitated varies. Slower water means less grain supplied to the millstones, which is what is required, as they are also grinding more slowly. A second form of self-regulation occurs with the flow of grain into the slipper.
It has balance: there is a cam mechanism on the axle which pushes the slipper to the left, and a taut string which pulls it back to the right, providing the motion that encourages grain to move.
It can be tuned: the strings that you can see at the front provide ways to alter the angle of the slipper, and the tension of the string to the right can be adjusted to change the balance.
Tuning is important. If properties of the grain change (such as size, or stickiness, or texture, …) then the action of the slipper may need to change in step. If the properties of the millstones change (e.g. they are adjusted to grind more coarsely or finely, or they are replaced for cleaning, or the surface roughness alters as they age, …) then the rate of delivery of grain will need to adjust too.
Although the system is self-regulating, these are examples of variables that it does not self-control for. It has no inherent feedback mechanism for them, and so requires external action to change its behaviour.
Further, beyond the skilled eye and ear (and fingers, which are used to judge the quality of the flour) of the miller, I could see no means of alerting that a change might even be required. In a mill running at full tilt, with multiple sets of stones grinding in parallel, with the noise, and dust, and cramped conditions, this must have been a challenge.
Another challenge would be in setting the system up at optimum balance for the conditions that existed at that point. I found no evidence of gauges, or markers, or scales that might indicate standard settings. I noted that the tuning is analogue, there are infinite fine variations that can be made, and the ways in which the system can be tuned no doubt interact.
The simplicity of the self-regulation is beautiful to me. But I wondered why not regulate the power coming from the water wheel instead and so potentially simplify all other systems that rely on it. There are technologies designed to implement this kind of behaviour, such as governors and flywheels.
I wondered also about the operational range of the self-regulation. At what speeds would it become untenable: too slow to shake any grain, or too fast to be stable? There didn’t seem to be scope for an automatic cut-out.
So that was an enjoyable ten minutes – while the kids were playing with a model mill – to practice observation, and thought experiments, and reasoning in a world unfamiliar to me.
I doubt you’ll find it difficult to make an analogy to systems that you operate within and with, so I won’t labour any points here. But I will mention the continual delight I find in those trains of thought in one domain that provoke trains of thought in another.