We prove that the exact non local equation derived by the present authors for
the temporal linear evolution of the surface of a viscous incompressible fluid
reduces asymptotically for high viscosity to a second order Mathieu type equa-
tion proposed recently by Cerda and Tirapegui. The equation describes a
strongly damped pendulum and the conditions of validity of the asymptotic
regime are given in terms of the relevant physical parameters.