If an inert gas is used to force piston with a Pressure say P (Assume the volume of gas to be sufficiently large). I would like to know the following: (consider piston to be horizontal) 1) If say one puts a small volume of a hydraulic fluid in front of the piston. Keeping the same pressure of gas, will he be able to achieve more force off the piston. 2) Go on increasing the volume of fluid and decrease the volume of gas, but keep the pressure of gas constant. What will happen to the force in this case ? I feel it is the compressibility which should make a difference. But how should I quantify it? I understand the derivation of hydraulic, but th concept seems impoosible to imagine. How can the same pressure be transferred to the entire huge area generating a huge force differential? I would like to thank Physics Forums. Cheers!!