Addendum

When I said that a point at the top of a wheel moves faster with respect to the ground than any other point on the wheel, I

could have added that it moves exactly twice as fast as the center of the wheel. A. J. Knisely called attention to this in a short article, "The Rolling Wheel," in Scientific American, July 1891, and described a simple way of demonstrating it with a spool of thread.

George Lenfestey wrote to say that although he enjoyed my column on the wheel, it ruined his day:

The trouble is, I've been sitting here wasting the better part of the afternoon imagining that gorgeous blue-eyed blond girl of yours in the hip huggers and halter top, twirling that hula hoop around her perfectly-formed golden middle. Please try to be more considerate in the future.

In my column I spoke of how difficult it would be for evolution to introduce a wheel into living organisms. A few years later, to my amazement, I read in Scientific American ("How Bacteria Swim," by Howard C. Berg) about the discovery that bacteria rotate their flagella like tiny propellers! In his Oz books L. Frank Baum introduced the "Wheelers" who have four wheels instead of four feet, and a bird called the "Ork" that flies by means of a propeller on its tail. These creatures are of course as imaginary as Escher's rolling animal or the fabled "hoop snake" that is said to bite its tail and roll like a hoop. However, there are spiders in Africa that actually escape from predators by curling into a ball and rolling down a hill.

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