fluid pressure

Consider two points 1 and 2 lie in the ends of fluid prism having a cross-sectional area dA and length L. The difference in elevation between these two points is h as shown in Figure 02 below. The fluid is at rest and its surface is free. The prism is therefore in equilibrium and all forces acting on it sums up to zero.
 

Note: FFS stands for Free Fluid Surface which refers to fluid surface subject to zero gauge pressure.