region and/or the cooling length of the shock may exceed the physical extent of the pre-shocked cloud. In Figure \[fig5\] we show the cooling length of the shock to 1000K, the depth of the photoionised precursor to the point where hydrogen is only 1% ionised, and the fraction of the shock H$\beta$ emission which arises from the precursor. The cooling length and precursor length are given for a ram-pressure of $P_{\mathrm{ram}} = 1.0\times 10^{-7}$dynes cm$^{-2}$. The shock cooling length scales inversely as the ram pressure, and the precursor length remains approximately constant. At the distance of the LMC 0.1pc corresponds to 0.45arc sec.

![The computed cooling length of the shock to $T_e = 1000$K (thick line), the thickness of the photoionised precursor to the point where hydrogen is only 1% ionised (thin line), and the fraction of the shock H$\beta$ emission which arises from the precursor (dotted line). These are all given for a ram pressure of $P_{\mathrm{ram}} = 1.0\times 10^{-7}$dynes cm$^{-2}$. At the densities of those models, the shock cooling length scales inversely as the ram pressure, while the precursor length is almost unaffected by changes in the ram pressure. []{data-label="fig5"}](fig5.pdf)

Shock Velocity Diagnostics
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Here we investigate potentially 