In chapter 4 the fluid density was defined 
.
In chapter 5 gravity was applied and the density became a function
of depth, The gradient 
was found to be proportional to the gravitational
strength 
in fluid i. This density gradient it produced solely
by gravity so the potential density is constant
in each fluid. In chapter 6 the equations describing interfacial
wave motion were in terms of f, the ratio of the potential densities.
If we consider two fluids with the same density, 
,
and apply
gravity to fluid 1 with strength 
and to fluid 2 with strength 
,
then the ratio of the body forces, 
, is 
. This
is the same as the ratio for two fluids with densities 
and 
when gravity act equally, with strength g, in each.
Thus when we apply gravity
with strength in fluid 1 and in fluid 2 it is equivalent to
simulating two fluids with relative densities 
in a constant
gravitational field.
When the interface occurs close to the
centre of the grid g is taken to be 
.