In the most general sense, hydraulics is the branch of engineering and science that deals with the properties of fluids. It can also be considered pneumatics, but in liquid form. In its most common understanding, hydraulics involves the engineering properties of fluids. In this sense, they can be used for creating, controlling, or transmitting power using pressurized liquid. The earliest sources of hydraulic power were free surface hydraulics, which deal with free surface areas such as lakes, seas, and rivers. Because of this, machine equipment in St. Paul, for example, has been historically different from tools designed for use in arid environments.
The word itself comes from the Greek hydraulikos which combines hydro which means water and aulos which means pipe.
Some of the earliest uses of water power occurred in ancient Egypt and Mesopotamia. Irrigation was first used around 7,000 years ago. Water clocks were first used around 4,000 years ago. In ancient China as well as Persia, water systems were also in use.
The ancient Greek water system was fairly complex. Much of the machine equipment being built today is in a direct line from the ancient Greek devices such as the Perachora wheel. The most notable advancement might be Ctesibius’ construction of the first water-powered automata.
Ancient Romans used hydraulic pressure to create water pumps that could raise water from wells and force water for fire engines.
The Roman Republic and Empire both had very profound effects on modern machine equipment. One of the most significant advancements was the development of the aqueducts which formed one of the earliest public water systems. Water mills were also built from aqueducts, rivers, and streams. The force of the moving water turned wheels. The wheels powered machines. These watermills were common until the 20th century in the United States. In fact, they are still in use in certain areas.
Romans were also one of the first civilizations to use hydraulic properties to carry water across valleys and dry areas. Because water pressure can be altered by the size of its pipe, the Romans discovered that water could be transported long distances and even up hills by creating progressively smaller pipes. As the water moved from a large pipe to a smaller pipe, the decreasing size created more pressure, which carried the water longer distances. The pressurized water was also used for mining. In the first century BCE, Caesar Augustus conquered northern Spain. The Romans in this area used water to mine for gold. The streaming water eroded the soft dirt and exposed the gold buried there. This is basically the same system used for hydraulic fracturing.
This system and the use of watermills form the basis for pretty much all of modern hydraulics.