Catenary Evolute

$\begin{figure}\begin{center}\BoxedEPSF{CatenaryEvolute.epsf scaled 800}\end{center}\end{figure}$

$\displaystyle x$	$\textstyle =$	$\displaystyle a[x-{\textstyle{1\over 2}}\sinh(2t)]$
$\displaystyle y$	$\textstyle =$	$\displaystyle 2a\cosh t.$