A general, continuous model for two dimensional solids and liquids on a substrate is studied by means of Monte Carlo simulation. The results can be applied to the case of adsorbed atoms or molecules on surfaces as well as intercalated compounds. We have focused on the study of the melting of a commensurate onroof-radical-sign 3 x onroof-radical-sign 3 structure on a triangular lattice with 1/3 coverage. We have in particular investigated the contribution from the two dimensional liquid to the Bragg peaks corresponding to the substrate structure. Reiter and Moss et al. have demonstrated that this gives valuable information about the substrate potential. A universal dependence is found between the intensity and the particle fluctuations around the substrate potential wells. This dependence may be useful for an experimental determination of the magnitude of the substrate potential from scattering experiments, in particular for weak potentials and large atomic mean square displacements. New results for large potentials are also presented and possible relations to the Potts lattice gas description studied.