Summary: We study the small oscillations of a pendulum containing a viscous liquid. Using the principle of virtual work, we derive the equations of motions and variational formulation of the problem. Renouncing a little long method of Russian scientists, we obtain directly from the variational equation an operator equation in Hilbert space. The corresponding bundle of operators belongs to a known class, so that we can study the spectrum of the problem and specify the set of eigenvalues according to the coefficient of viscosity.