It was the first worldwide keyboard instrument, that was invented by Ktesibios in the 3rd century B.C.

It consisted of two pumps that supplied the air (of piston pump type), the “pnigeus” for the regulating of constant air pressure, the keyboard, and the musical pipes (auloi). The pumps were placed at both sides of the “pnigeus” and allocated non-return valves that were controlled automatically by two bronze dolphins, while their pistons reciprocated with the help of hand levers. The “pnigeus” consisted of a cylindrical container with water, which had a sunken inverted cone-shaped funnel fitted on holders at a small distance from its bottom. Two air supply pipes converged at the top of the funnel while another pipe led the air with constant pressure to the collector of the keyboard. The stability of the air pressure was achieved due to the escaping of the redundant air from the bottom of the funnel and thus the stability of musical notes was ensured henceforth only depending on the length of the musical pipes.
The keyboard of the reconstructed hydraulis consists of 24 keys that control the equal in number bronze air supply valves to 24 unequals in length musical pipes (like the hydraulis of the ancient Dion), which produced two complete octaves. The keys-valves spring back with the help of small flexible wooden boards from maple tree.
The hydraulis constitutes the forerunner of the contemporary church organ.

SOURCES: Vitruvius, On Architecture,  Heron of Alexandria, Pneumatics.