The theory of motion of a horizontally stratified fluid under gravity is extended to include three linear gradients from the surface to the bottom. Tho resulting equation is then simplified to give tho special cases In which first the upper layer and then tho upper and lower layers aro of constant density. The latter case is then shown by further simplification to load to Stokes's theory of two layers of constant density separated by a sharp discontinuity. The wave lengths, as calculated by Stokes's theory and its extension to linear gradients, agree with previously measured values as well as is to be expected.