Notice that if the thickness dx is infinitesimal, x varies only infinitesimally on the slice. An affine -manifold is then a manifold which looks infinitesimally like -dimensional affine space. Such a theory confines the chemistry and diffusive transport processes to an infinitesimally thin zone. An ideal point source is an infinitesimally small point radiating sound. We will break the rotor area up into annular rings of infinitesimally small thickness. By saying these changes are infinitesimally small, the equation can be written in differential form. We will assume that the rotor is infinitesimally thin. "dx" between two infinitesimally close points should be zero. This is easiest to see by taking a path-integral over infinitesimally separated times. Heaviside (1881) represented the line as a network of infinitesimally small circuit elements. Let the infinitesimally small intercepts "PD" and "PE" be labelled, respectively, as "dx" and d"q". Consider again propagation of signals between infinitesimally close points "A" and "B" in Fig. 1. Consider two infinitesimally close points formula 59 and formula 60.