Positive Feedback and Ewe

A mountain walk

We often take our walks along the slopes of Benevenagh. Pleasant paths lead past sheep infested pastures, cool forests and green fields. There are some great views and fascinating wild flowers. Mind you, I forgot to mention howling storms, lashing rain and meter deep snowdrifts. But don't get frightened away: This tale happened on a pleasant sunny day. We drove up from the valley and even from a distance we could see that there was something wrong with the sheep on one of the fields.

Around here, when a farmer puts his ewes into a field he first has a quiet word with them: "Eat grass - or else!" The sheep take this advice very seriously and generally spread out evenly all over the width of field and eat their way on a united front from one side to the other. This only stops when it gets dark, though on moonlit nights I have seen them chomping grass till well past midnight.

We noticed that the sheep on that particular field behaved very strangely indeed: they were all bunched up on one side of the field and none of them was eating. At first we thought that a dog might be holding them, but as we got closer we saw no dog except ours - and he watched in as much amazement as we did.

There were thirty sheep in the field. Twenty-nine of them formed a flock to one side; they were all watching a single sheep about ten or so yards away. All thirty circulated around the field in an anti-clockwise direction, and slowly increased their speed. Now! Sheep are fairly silly animals at the best of times, but I have never seen them behave in this strange manner before. We watched for about 15 minutes, in which time the animals had circled the large field two or three times, when suddenly I noticed that there was something strange about that lone ewe - and suddenly I understood what was happening.

The land on Benevenagh lacks certain minerals which the farmers supply in large buckets containing a substance which the sheep lick with great gusto. Often these buckets get left behind or blow in from other fields.

This is what must have happened.

Ewe Number Thirty spotted an empty bucket and thought to herself: "Yummy!" She stuck her head into the bucket but found that the container was empty. Frustrated in her search for rare minerals she raised her head and looked for some grass instead.

Unfortunately her horns had passed underneath the thin metal handle and as she straightened up, she noticed that the bucket followed her and hung around her neck. This was not only frightening, but stopped her from eating, because every time she tried to bend down to reach the grass, the bucket got in the way.

There is a phenomenon in electronics known as 'positive feedback'. If you hold a microphone too close to a loudspeaker it will pick up the sound, this gets amplified and the sound gets louder, the louder sound gets picked up and is amplified again until the system oscillates in a loud and penetrating screech (often more musical than the impending performance). I used to teach electronics and am therefore familiar with the principle, but I never knew that farmers also have to know all about positive feedback, because this was the explanation of the strange circular dance.

Some strange sheep

What was Number Thirty to do? She was in great distress and decided to join the others. After all, when you are a sheep you know that there is comfort in numbers. Number One through to Number Twenty nine weren't so sure and didn't like the looks of Number Thirty - because of that great big dirty bucket dangling from her neck. They all decided instantly to get out of the way of this dangerous looking apparition as fast and as far as possible. When Number Thirty saw them all move away she baahed: "Wait for me!" and followed them sightly faster in order to catch up. The others now felt even more threatened and moved away from our lone ewe at a slightly increased speed. This of course forced Number Thirty to move faster still, which forced the others to speed up, which caused Number Thirty to speed up - and so on and on.

This process can't continue indefinitely, of course. If it did, many of our fields would be filled with sheep rotating at a speed approaching the velocity of light.

After some time natural damping occurs and the animals settle on a rotational speed which presumably is based on the natural harmonic frequency of ewes, the roughness of the ground, the colour of the bucket and the level of fear obtainable on a sunny day in Ulster, possibly many additional factors. I would never have believed that farming is so complicated!

None of the animals got anything to eat and the scientist in me was toying with the idea of watching them for a few days to see if the inevitable weight loss would increase or decrease the rate of rotation. Tudor, at any rate, had never seen anything like it and watched with great interest and possibly an even greater understanding.

However; it all ended happily. I was persuaded that to wait any longer would be cruel and so we went off to tell the farmer what was happening. I had great difficulty explaining all this and I don't think he believed me at first because he gave me a very old-fashioned look, but he went up on to the mountain just the same and removed the bucket - and the resultant forced damping effect brought it all to a natural halt.

And they all ate grass happily ever after.



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