The hydraulis is a hand-powered organ. Just like a pipe organ in a modern church, the hydraulis makes sound by blowing air through pipes, and is played by pressing keys on a keyboard. This 2-minute video demonstrates what a Roman replica sounds like.
The word “hydraulis” literally means “water-pipe” because water (“hydra”) is used to deliver steady air pressure to a pipe (“aulos”).
This 2-minute video shows a hydraulis performance with a closer look at how the organ works.