Starting from Laplace's equation in Cartesian coordinates, assume that the boundary and free-surface conditions are all equal to zero and that the solution is separable. Let be the height of the liquid when it is still. The solution at time is:
where is the frequency and the gravitational acceleration.
 P. Hagedorn and A. DasGupta, Vibrations and Waves in Continuous Mechanical Systems, Chichester, UK: John Wiley & Sons, 2007.