  We answer the question: who first proved that $C/d$ is a constant? We argue
that Archimedes proved that the ratio of the circumference of a circle to its
diameter is a constant independent of the circle and that the circumference
constant equals the area constant ($C/d=A/r^{2}$). He stated neither result
explicitly, but both are implied by his work. His proof required the addition
of two axioms beyond those in Euclid's \emph{Elements}; this was the first step
toward a rigorous theory of arc length. We also discuss how Archimedes's work
coexisted with the 2000-year belief -- championed by scholars from Aristotle to
Descartes -- that it is impossible to find the ratio of a curved line to a
straight line.
