A hydraulic system works on the principle that force applied at one point can be transmitted to another point through an incompressible fluid, usually some kind of oil. Hydraulic system manufacturers typically multiply force by manipulating the size and surface area of the system's internal parts.
One of the most basic laws of mechanics allows force to be traded for distance. Engineers use this concept when designing hydraulic systems by using pistons and cylinders of various sizes and using them to apply greater force than would otherwise be possible without the presence of an incompressible fluid. Typically, this means that the operator of a hydraulic device applies a smaller force over a greater distance to get the result of a large force over a small distance.
One of the simplest and most common hydraulic devices is the log splitter. It features an engine attached to a hydraulic oil pump, which runs to a valve actuating a hydraulic cylinder that splits logs. This device uses high-pressure oil to transfer the force created by the engine to the hydraulic cylinder, splitting the log and feeding the resulting low-pressure oil back into the tank. The operator can then pull back the handle, resetting the machine and putting the high-pressure oil back in its place.