The upper fluid, , will be referred to as region I,the middle fluid, 0 < y < h, will be referred to as region II, while the lower fluid, y < 0, will be referred to as region III. The potential in the upper fluid will be denoted by ϕI and that in the middle and lower fluids by ϕII, ϕIII respectively. ϕI, ϕII and ϕIII satisfied Laplace's equation