LEFTOVERS I --- Apocrypha Topologica III

Have you contemplated lately that marvelous invention, the revolving door?

The usual application of a revolving door is to compromise between the extremes of an open window and a wall. Specifically, the Revolving Door is a device which can be set in motion by an intelligent force (say, of animal intelligence at least) but not by an unintelligent force (as, say, a differential in air pressure). In the particular case of pressurized buildings, a good revolving door is not driven by the pressure inside, though air will tend to exit the building as the door is driven by people.

Anyways, as several of the other artifacts already mentioned in this very-occasional series, a revolving door exemplifies mechanical separation within topological connectedness. How strange, that! Anyways, the actual story of the moment is how, some time after the invention of anchored submarine mines, first that a clever engineer found a way to dredge harbours for them by towing weighted cables between separated vessels, and secondly that another clever engineer partially foiled that scheme by inventing a Revolving Door For Cables, In a Cable. I do rather wish that there were happier circumstances around this Intrinsically Delightful Invention, but, well, there we are.

Now, I should also tell you that I first heard about the cable-passing cable-joint from Richard Feynman, who likes to sprinkle his works with ... incorrections... just so that intrepid readers don't take his word for everything, and also to ensure that those inclined to calculation have extra fun checking his facts. As I write elsewhere (no, I shan't link, here) sometimes a condensor should be an inductance, sometimes 2:1 should be 1:2. Is a "shaft-passer" or a "cable-passer" actually a thing?

As it turns out, you can build them semi-automatically. Actually, that there is one of two possible implementations. They can also be built with a fixed axle connected to one side, but the principal of the thing is that the Revolving Door Shape itself serves as the guide for the round "walls" of the revolving door's frame, in consequence of the side-angle-side theorem, or some such.

I am dubious whether these things were any sure foil against the undermining of... under... water... mines... (no, originally, "undermining" was part of Castle-Siege warfare)... because of the fiddly coincidences needed to pass the dredging cable from the a suitable angle at the correct depth... building a chain of these links would be a fiddly mess indeed. But perhaps it was effective enough that you could never feel sure that this-or-that harbour was really safe.

Which is to say, we're also discussing an invisible, and nonmechanical means of inducing a local separation: the threat of a perfectly unintelligent, violent force.