  We have simulated the classical Heisenberg antiferromagnet on a triangular
lattice using a local Monte Carlo algorithm. The behavior of the correlation
length $\xi$, the susceptibility at the ordering wavevector $\chi(\bf Q)$, and
the spin stiffness $\rho$ clearly reflects the existence of two temperature
regimes -- a high temperature regime $T > T_{th}$, in which the disordering
effect of vortices is dominant, and a low temperature regime $T < T_{th}$,
where correlations are controlled by small amplitude spin fluctuations. As has
previously been shown, in the last regime, the behavior of the above quantities
agrees well with the predictions of a renormalization group treatment of the
appropriate nonlinear sigma model. For $T > T_{th}$, a satisfactory fit of the
data is achieved, if the temperature dependence of $\xi$ and $\chi(\bf Q)$ is
assumed to be of the form predicted by the Kosterlitz--Thouless theory.
Surprisingly, the crossover between the two regimes appears to happen in a very
narrow temperature interval around $T_{th} \simeq 0.28$.
