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From: bob@vaxwaller.UUCP (Bob Palin)
Newsgroups: net.jokes,net.puzzle
Subject: Re: Manhole covers (interview question)
Message-ID: <237@vaxwaller.UUCP>
Date: Tue, 26-Mar-85 16:37:37 EST
Article-I.D.: vaxwalle.237
Posted: Tue Mar 26 16:37:37 1985
Date-Received: Fri, 29-Mar-85 02:51:49 EST
References: <462@nbs-amrf.UUCP>, <7074@watdaisy.UUCP> <474@nbs-amrf.UUCP>
Organization: Varian, Walnut Creek, CA
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Xref: watmath net.jokes:11577 net.puzzle:659

> 
> > > > 	Why are manhole covers round?
> > > I can think of at least three answers:
> > > 	1.  Because they aren't any other shape.
> > > 	2.  Because manholes are round.
> > > 	3.  So that manholes can be round.
> 
> As a further point of information, what is this valid answer that was
> posted to the net?  If it is that a round manhole is the only shape that
> will prevent it from falling into the hole, it has also been pointed out
> on the net that this is false.  There is at least one infinite set of
> simple closed curves that have this property - the curves of constant
> diameter.  An example of a curve of constant diameter that is not a
> circle is the rotor of a Wankel engine.
> 
> Also, why doesn't a manhole cover (of any shape, but let's stick to round
> ones) fall into the manhole? It is because there is either a ledge on which
> it rests or the opening is beveled.  In either case, I claim that all
> regular polygons of sufficiently high order approximate a circle closely
> enough that they will not fall into the hole, either.  Obviously, not
> falling into the hole cannot be why (i.e., the cause that) manholes are
> round, only an effect of them being round.
> 
Man holes and their covers are indeed round to prevent the cover falling
into the hole,  this may be, as pointed out, not the only shape they could be
but is the most obvious shape to just about anyone.  As for other
'high order regular polygons' working just as well I suppose it depends
on what you mean by high order.  Certainly no four sided cover will work
and it seems likely that you would have to approximate a circle so closely 
as to be pointless.  Was the original question theoretical or practical ?

Bob Palin,  zehntel!varian!bob