The word “hydraulics” generally refers to power produced by moving liquids. Modern hydraulics is defined as the use of confined liquid to transmit power, multiply force, or produce motion.
Though hydraulic power in the form of water wheels and other simple devices has been in use for centuries, the principles of hydraulics weren’t formulated into scientific law until the 17th century. It was then that French philosopher Blaise Pascal discovered that liquids cannot be compressed. He discovered a law which states: Pressure applied on a confined fluid is transmitted in all directions with equal force on equal areas.
To better understand Pascal’s Law, lets use a bottle full of liquid as an example. Let’s say the bottle has a 1 square inch opening. If we were to apply 10 pounds of force on a cork at the opening, 10 pounds of force would be applied equally to all sides of the bottle. This is expressed as 10 psi or 10 pounds of force per square inch. 10 psi represents the fluid pressure of the system.