Hydraulic machines are machinery and tools that use liquid fluid power to do simple work. Heavy equipment is a common example.
The popularity of hydraulic machinery is due to the very large amount of power that can be transferred through small tubes and flexible hoses, and the high power density and wide array of actuators that can make use of this power. - https://en.wikipedia.org/wiki/Hydraulic_machinery
A fundamental feature of hydraulic systems is the ability to apply force or torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers.
There are some simple interactive examples here: https://science.howstuffworks.com/transport/engines-equipment/hydraulic1.htm
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