We have already seen that when the resultant of all the forces that act upon any point in a liquid is zero, there will be a condition of equilibrium, and the liquid will be at rest .
In order that the surface of a liquid may be at rest, it must be horizontal. Suppose that the surface is not horizontal, as in Fig. 1.
The force of gravity, AB, which acts upon any molecule upon the surface, as A, may be resolved into two forces, one of which, AC, is perpendicular, and the other, AD, parallel to the surface. Now the first, AC, will be opposed by the resistance of the liquid, while the other, AD, will move the molecule to a lower level. When the surface is horizontal, the action of gravity is perpendicular to it, and if we try to resolve this force into two components as before, we find that the component perpendicular to the surface is equal to the force, and the horizontal component is zero. Hence no movement will take place, and the liquid will remain at rest.
