  We use a Cartesian grid to simulate the flow of gas in a barred Galactic
potential and investigate the effects of varying the sound speed in the gas and
the resolution of the grid. For all sound speeds and resolutions, streamlines
closely follow closed orbits at large and small radii. At intermediate radii
shocks arise and the streamlines shift between two families of closed orbits.
The point at which the shocks appear and the streamlines shift between orbit
families depends strongly on sound speed and resolution. For sufficiently large
values of these two parameters, the transfer happens at the cusped orbit as
hypothesised by Binney et al. over two decades ago. For sufficiently high
resolutions the flow downstream of the shocks becomes unsteady. If this
unsteadiness is physical, as appears to be the case, it provides a promising
explanation for the asymmetry in the observed distribution of CO.
