Hydraulic jacks and many other technological advancements such as automobile brakes and dental chairs work on the basis of Pascal's Principle
Basically, the principle states that the pressure in a closed container is the same at all points.
Hydraulic systems use a incompressible fluid, such as oil or water, to transmit forces from one location to another within the fluid. Most aircraft use hydraulics in the braking systems and landing gear. Pneumatic systems use compressible fluid, such as air, in their operation. Some aircraft utilize pneumatic systems for their brakes, landing gear and movement of flaps.
Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.
Sources : http://www.physlink.com/education/askexperts/ae526...
Hope this helps!