Hydraulics are a type of industrial equipment that use the power of pressurized liquid to produce force. These machines do not utilize traditional mechanical levers and gears to function. Hydraulic systems were used all the way back in ancient Egypt. It was used to create irrigation systems. Grain mills also utilized water wheels to turn heavy grinding stones. When we think of hydraulics today we generally think of heavy equipment. There are two types of hydraulic circuits open and closed. Open hydraulic circuits use pumps supplying a continuous flow of fluid. The fluid is returned to the tank via the control valves open center. This means the fluid is not highly pressurized until it is actuated. Then the valve closes sending the fluid to the actuator tank rising to meet desired resistance. If the pressure gets to high the pressure relief valve opens returning fluid to the tank. Closed circuits are always at full pressure whether the valves are actuated or not. The pumps control small amounts of fluid at various pressures until the machine operator actuates a valve. This circuit does not need an open return path to the main tank. Some advantages of hydraulic systems are capable of moving large loads, provide large amounts of force, and there is no need to bleed off unused pressure.