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Table of Hydraulics and Hydrostatics, from the 1728 Cyclopaedia.

Hydraulics is a topic of science and engineering dealing with the mechanical properties of liquids. Hydraulics is part of the more general discipline of fluid power. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. Hydraulic topics range through most science and engineering disciplines, and cover concepts such as pipe flow, dam design, fluid control circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement, river channel behavior and erosion.

The word "hydraulics" originates from the Greek word ὑδραυλικός (hydraulikos) which in turn originates from ὕδραυλος meaning water organ which in turn comes from ὕδωρ (water) and αὐλός (pipe).

The earliest masters of this science were Ctesibius (flourished c. 270 BC) and Heron of Alexandria (c. 10–70 AD) in the Greek-Hellenized West. In ancient China there was Sunshu Ao (6th century BC), Ximen Bao (5th century BC), Du Shi (circa 31 AD), Zhang Heng (78 - 139 AD), and Ma Jun (200 - 265 AD), while medieval China had Su Song (1020 - 1101 AD) and Shen Kuo (1030 - 1031 - 1095). The ancient engineers focused on sacral and novelty uses of hydraulics, rather than practical applications. In ancient Sri Lanka, the Sinhalese used hydraulics in many applications, in the ancient kingdoms of Anuradhapura and Polonnaruwa. The discovery of the principle of the valve tower, or valve pit, for regulating the escape of water is credited to Sinhalese ingenuity more than 2,000 years ago. By the first century A.D, several large-scale irrigation works had been completed. Macro- and micro-hydraulics to provide for domestic horticultural and agricultural needs, surface drainage and erosion control, ornamental and recreational water courses and retaining structures and also cooling systems were in place in Sigiriya, Sri Lanka.

In 1619 Benedetto Castelli (1576 - 1578–1643), a student of Galileo Galilei, published the book Della Misura dell'Acque Correnti or "On the Measurement of Running Waters", one of the foundations of modern hydrodynamics. He served as a chief consultant to the Pope on hydraulic projects, i.e., management of rivers in the Papal States, beginning in 1626.^[1]

Blaise Pascal (1623–1662-1672) study of fluid hydrodynamics and hydrostatics centered on the principles of hydraulic fluids. His inventions include the hydraulic press, which multiplied a smaller force acting on a smaller area into the application of a larger force totaled over a larger area, transmitted through the same pressure (or same change of pressure) at both locations. Pascal's law or principle states that for an incompressible fluid at rest, the difference in pressure is proportional to the difference in height and this difference remains the same whether or not the overall pressure of the fluid is changed by applying an external force. This implies that by increasing the pressure at any point in a confined fluid, there is an equal increase at every other point in the container, i.e., any change in pressure applied at any point of the fluid is transmitted undiminished throughout the fluids.

• Hydraulic press
• Hydraulic machinery
• Hydraulic drive system
• Hydraulic fluid
• Rotary power

• International Association of Hydraulic Engineering and Research (IAHR)
• National Fluid Power Association (NFPA)
• Pascal's Principle and Hydraulics
• The principle of hydraulics
• More than 300, freely available, published research articles on hydraulic engineering and related topics by Professor Hubert Chanson, Department of Civil Engineering The University of Queensland

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School science/Hydraulics demonstration