We now wish to treat flows with very small viscosity or very high Reynolds numbers. An important contribution to the science of fluid motion was made in 1904 by L. Prandtl (1904). Prandtl showed the manner in which the viscosity has its effect for high Reynolds number flows and how the Navier–Stokes differential equations can be simplified to yield approximate solutions for this limiting case. We shall now derive the simplifications which arise for the Navier–Stokes equations in the case of very small friction forces in a physically illustrative manner.