Oil is then forced into it on top of the piston. As the pressure above it increases, the piston is forced down, squeezing the air into a smaller space. When oil pressure is reduced, large quantities of oil under working pressure are instantly available to operate hydraulic rams or motors any place on the submarine. The Navy uses many devices whose operation depends on the hydrostatic principle. Hydrostatic pressure refers to pressure at any depth in a liquid that is not flowing.
Pressure depends upon both depth and density.
The working principle of all hydraulic mechanisms is simple enough. Machines that operate liquids under pressure are called hydraulic machines. Liquids are incompressible. They cannot be squeezed into spaces smaller than they originally occupied. A force applied on any area of a confined liquid transmits equally to every part of that liquid.