On Fri, 31 Jan 2003, Bill Marshall wrote:

> It was built up as 2 towers that curved in toward each other.  Where they
> met, there was a final 8' keystone to go between the two sides and link them.
>  The sides ended up only 3' apart, though, so they had to jack the sides
> apart to get the keystone in.

Not unexpected, considering how different the forces in an arch are with
and without the keystone.  The amazing feat of accuracy, I think, was
getting the two sides to line up horizontally.  If they hadn't lined up
there wouldn't have been much to do to fix it, and there were no laser
alignment tools back then.

Where I used to live there were a lot of abandoned factory smokestacks,
built mostly in the 1920s I believe, and made out of reinforced concrete.
Every one of them had a slight crook in it at some point partway up -- not
one was perfectly straight, even to the naked eye.

             _ _
  __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
 / _` | | | | | |  +-----------------------------------------------------
| (_| | |_| | | | @ cyberspace.org
 \__, |\__,_|_|_| "To the optimist, the glass is half full.  To the
 |___/             pessimist, the glass is half empty.  To the engineer,
                   the glass is twice as big as it needs to be."