earliest known mechanical pipe organ. It was invented in the 3rd century BC by Ctesibius of Alexandria, culminating prior attempts to apply a mechanical wind supply to a large set of panpipes. Its pipes stood on top of a wind chest that was connected to a conical wind reservoir. The reservoir was supplied with air by one or two pumps. For the pipes to sound evenly, the wind chest needed steady air pressure. The open bottom of the cone was set in a tall outer container half filled with water. When air pressure in the cone was low, the water level rose inside it, compressing the air and restoring the former air pressure. The player operated keys or, on some instruments, sliders that let air into the pipes.

The hydraulis was used at outdoor public entertainments; its sound was loud and penetrating. Its use declined in the West by the 5th century AD, although Arab writers of the 9th century refer to it. Later medieval writers thought the hydraulis was a steam-whistle organ such as the calliope.