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I was puzzling about Hydra- vs Hydro- the diagrams don't claim that water is involved, but I wondered if there was some sort of ingenious multi linkage gearing involved that might merit Hydra. Still really neat.

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It's a portmanteau of hydraulic and automatic. Of course the former was originally only referring to water, but later became used to refer to any system based on fluid pressure.

(Fun fact: "hydraulic elevator" in the late 19th and early 20th century often referred to one that actually used water.)

Right, and the "-a-" following "hydr-" is the initial letter of the word meaning "pipe" in Greek, so at its origins hydraulics was about water pipes, but now "hydraulic" is used for any devices that transmit forces through liquids, while "pneumatic" (from the word meaning "blowing") is used for those that transmit forces through gases.

I think that it is referring to a hydraulic mechanism, not the mythological Hydra.

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