Engraved image of a pneumatic organ on the tomb of Rusticus, Rome, 4th century AD. The organ was a popular musical instrument in Ancient Rome and the hydraulis, or water organ, was the original version. Air was displaced by water that was pumped into a jar to sound the variously pitched pipes. The hydraulis was gradually replaced by the pneumatic or air-powered organ, in which the hydraulic mechanism was changed to a bellows system operated by hand or feet.