  We present the results of simulations of the development of the
Kelvin-Helmholtz (KH) instability in a cooled, slab symmetric system. The
parameters were chosen to approximate the physical conditions typically found
in jets from young stellar objects (YSOs). The effect of different methods of
maintaining the initial equilibrium were examined for varying density. In
addition, the effect of adjusting the width of the shear layer between the jet
and ambient material was studied and found not to have significant long-term
effects on the development of the instability.
  We find that, in general, cooling acts to: increase the level of mixing
between jet and ambient material through the `breaking' of KH induced waves on
the surface of the jet; increase the amount of momentum transferred from jet
material to ambient material; increase the time taken for shocks to develop in
the flow; reduce the strength of these shocks; reduce the rate of decollimation
of momentum flux. The first and second of these results appear to contradict
the conclusions of Rossi et al. (1997) who carried out a similar study to ours
but in cylindrical symmetry. It is found, however, that the differences between
slab and cylindrical symmetry, while insignificant in the linear regime,
explain the apparent discrepancy between our results and those of Rossi et al.\
(1997) in the non-linear regime.
