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I've got a small manual lathe that I bought second hand. My father taught me the technique of taking up backlash. When you need to move the top slide out, to increase the diameter of the cut, move it too far, then screw it in to the required setting. This ensures that every setting is set pushing the top slide in. This technique works, but is annoying. On a worn old lathe it is essential.

But how does modern CNC machinery work? It seems to zip back and forth by turning the lead screw, with out regard to backlash. I think there was a short era of using two nuts, held apart by a strong spring that both takes out the backlash and creates lots of friction. Then came recirculating ball screws. The rolling of the ball bearings solves the problem of friction. Accurate manufacturing of the ball race helps, and finally backlash is eliminated by having the nut done up tight.

Then I saw this video, which looks like it is the next stage, with all the baby thread rollers rotating around the lead screw. They claim repeatability of 0.006mm. I guess this is the possible error between screwing in and screwing out to your setting. I'm not involved in this technology, I just like to watch and keep up to keep in touch with how things work.

Peter Jacobsen's article is not exclusively *bio*realist. It is packed with uncomfortable truth bombs. For example


> The paper found that countries which were ethnically heterogeneous tended to have higher fertility rates. The reason? Higher population is important for different ethnicities in these countries competing for government resources.

Secondary school mathematics emphasizes method. Here is *the* way to do it. This spills over into proof. We ask "what is *the* proof". I like this video because the problem is not too hard, there seems to be little room for alternative approaches. Yet the video solves the problem in three *distinctly* different ways. That pushes back against the error of thinking that there is only one method and only one proof.

I also liked this video because the problem comes up later in trying to compute pi. One idea for computing pi is that tan(pi/4) = 1. That suggests a plan: find a power series for arctan and use it to evaluate pi = 4 arctan(1).

Since arctan is the integral of 1(1 + x^2) we can use the power series 1/(1 + x^2) = 1 - x^2 + x^4 - x^6 + x^8 - ...

Integrate term by term to get a power series for arctan. Evaluate at 1. Notice that 1 is the radius of convergence of the power series. It converges at 1, but slowly. The plan hasn't really worked.

But, spoiler ahead, what if we had some clever identity such as pi/4 = arctan(1/2) + arctan(1/3)? We would have to evaluate the power series twice, for 1/2 and for 1/3, but both converge geometrically; that makes a much better plan, and many more significant digits for our labor. The video doesn't go there, but the problem it solves is the problem of the clever identity that you will want later for computing pi

I think that the credit to Paul McNulty is for building a copy of a 1795 Walter Fortepiano, rather than doing a restoration. Jerassimos Coidan Takes advantage of the light action to give an exciting fast performance. The instrument has a sharp attack and quick decay. The Alberti bass throbs. The semiquavers rocket skywards. The passages, where repeated quavers get hammered, work. The crazy rhythms at the end are clear.

Did Beethoven write to take advantage of the sharp attack and quick decay? Did Beethoven write to mitigate the disadvantages of the sharp attack and quick decay?

Will the population explode, fulfilling the fears of that Ehrlich spread in his 1968 book *The Population Bomb*? Or is the more recent fear of population collapse the real danger. Eugene Kusmiak runs the numbers showing that everything depends on the heritability of fertility.

Then he anticipates looks at Israel, to see the "population rebound" already underway. Are the consequences eugenic or dysgenic? They seem inevitable enough to shrug off classification.

Most mathematics video present the results of deep researches. Results only. How does one come up with a result like that? Silence. Not even a hint of the first step along the path.

This video caught my eye because it walks through the thought process of coming up with a proof. The result is not deep, and the proof is not hard, but topic "how to get started on coming up with ones own proofs" is rarely tackled.

Thought about posting to "history", but this video has fascinating technical detail, maybe a better fit for "technology" or "retro technology", or even just /v/guns

Only 4'4" so don't be put off by Simon Webb being a leftist. He crisply makes the point that the European Convention on Human Rights is OK, it is just the judicial interpretation that is at fault.

I draw a different lesson from the facts presented. Laws written as vague principles hand over power to judges. Come back thirty years later and the judiciary is staffed by different people. Previously pedants, happy to follow strict rules. Now, power hungry activists, eager to use the power unwisely delegated to the courts. The European Convention on Human Rights has been a disaster because it has changed the job of a judge, and changed who wants to do it.

Bashar al-Assad won the Syrian Civil War. But the USA won't give up. There are 1000 US troops in Syria, and vicious economic sanctions to hinder Syrians from rebuilding their country. I found this video refreshing because it goes over the situation and recent developments, without toeing the Washington line.

I enjoyed her straight talk.

I try to look after a communal grassy area (drying green) for a block of flats. I fork the soil and often end up digging out fist sized rocks. A few years ago I stuck my fork in where the grass wasn't growing and found the top of a rock. If it is an erratic boulder, it is a few feet in diameter, much like the rocks that he splits in the video. I've been wondering how to get rid of it. I've started a compost heap, to have soil to fill the hole, should I eventually succeed. I've tried using a diamond cutting disk with an angle grinder, and made disappointing progress.

If a stone mason wants to break rocks using feather wedges, he does a skilled job, to get a flat break. What the video makes clear, is that just breaking the rock, so that you can get rid of the top six inches, is a low skill task. I should buy some feather wedges and give it a try :-)