Spatially resolved, long-slit echellograms at different position angles of the bright, low excitation planetary nebula NGC 40 indicate that, the higher is the gas excitation, the faster is the radial motion, thus confirming the overturn of the Wilson law already suggested by Sabbadin and Hamzaoglu (1982). New reduction procedures, giving the radial trends of the electron density and of the ionic and chemical abundances, were applied to NGC 40; they show that: - the radial matter distribution has a sharp ``bell'' profile with peaks up to 4000 cm-3; - the ionization structure is peculiar, indicating the presence of chemical composition gradients within the nebula: the innermost regions, hydrogen depleted, are essentially constituted of photospheric material ejected at high velocity by the WC8 nucleus. Moreover, detailed H+, O++ and N+ tomographic maps, giving the spatial ionic distributions at four position angles, are presented and discussed within the interacting winds evolutionary model.