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From: colonel@gloria.UUCP (Col. G. L. Sicherman)
Newsgroups: net.puzzle
Subject: Re: manholes
Message-ID: <1015@gloria.UUCP>
Date: Thu, 14-Mar-85 13:37:08 EST
Article-I.D.: gloria.1015
Posted: Thu Mar 14 13:37:08 1985
Date-Received: Sun, 17-Mar-85 01:46:26 EST
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Distribution: net
Organization: The Church of Artificial Intelligence
Lines: 14

> In response to the question, "Why are manhole covers round?" :  I learned
> that it was because the circle is the only two-dimensional shape that has
> the same diameter no matter how you turn it.

Mathematicians know better.  They call such curves "curves of constant
breadth," and there are an infinity of them.  Martin Gardner did a column
on them once.

The simplest example (besides a circle) is constructed thus: let ABC be
an equilateral triangle.  Draw arcs AB with C as center, AC with B as center,
BC with A as center.  The three arcs form a curve of constant breadth.
-- 
Col. G. L. Sicherman
...{rocksvax|decvax}!sunybcs!colonel