Chemical . . Magnetic . . Effort variables are ones for which there is a sense of pushing , while flow variables are ones for which there is a sense of motion . Hydraulic systems contain an operating fluid. We will assume that this operating fluid is incompressible .
Water is a nearly incompressible fluid.
So is hydraulic oil. Systems whose operating fluid is air are called pneumatic . Since our operating fluid is essentially incompressible, the volume of it that is within the system is a constant. The figure below shows a wide pipe connected to a narrower pipe. The cross sectional area of the smaller pipe is half that of the larger one.