Testo originale - Inglese - the theory of van der waals

we denote by 0 and 0' the points on bodies 1 and 2 for which the separation distance is least and let H be this distance of closest approach.An axis system is erected on 0 with the z axis along the inward normal to body 1 and the(x,y)plane as shown in Fig.4.The orientation of the xy axis system in this plane we will specify shortly.The bodies are assumed smooth and quadratically curved in the neighbourhood of 0 and 0'.