The hydraulic press is composed of two hydraulic cylinders of different sizes, which are filled with water or oil. Water filling is called "hydraulic press"; Oil filled is called "oil press"

There is a sliding piston in each of the two hydraulic cylinders. If a certain value of pressure is added to the small piston, according to Pascal's law, the small piston transmits this pressure to the large piston through the pressure of the liquid and pushes the large piston up. 

Let the cross-sectional area of the small piston be S1 and the downward pressure on the small piston be F1. Therefore, the small piston pair can be transmitted in all directions by the liquid with constant size ". The pressure on the large piston must also be equal to P. 

If the cross-sectional area of the large piston is S2, the upward pressure cross-sectional area generated by pressure P on the large piston is a multiple of the cross-sectional area of the small piston. From the above formula, if a small force is added to the small piston, a large force will be obtained on the large piston. 

In practice, a hydraulic press can be used to press plywood, extract oil, extract heavy objects, forge steel, etc.