Wang's theory for determining the approximate configurational partition function of the adsorbed layer is modified in two different ways. One is to assume that the configurational energy should be corrected; the other to advocate that the deficiency due to a wrong expression for the a priori probability of the central site is more significant. The configurational partition function is evaluated in both methods and the adsorption isotherm and the heat of adsorption computed for the case of quadratic lattice with dipole interaction. Values for the last two quantities when a uniform continuous distribution of the distant adsorbed particles is assumed are further given for comparison. The second method, which surpasses the first, is compared with Kirkwood's method in the case of hexagonal lattice with neighbour interaction. Numerical work is also carried out in this case.