How did the hydraulis differ from later bellows-powered organs?

The primary difference between the hydraulis and later bellows-powered organs lies in their method of wind regulation and supply.

• Wind Regulation: The hydraulis, invented in the 3rd century BCE, utilized water pressure to regulate air flow. It featured a conical wind reservoir with an open bottom submerged in a container of water. As air was pumped into the reservoir, the water level inside would rise or fall to maintain steady air pressure for the pipes.12
• Wind Supply: While the hydraulis used piston pumps operated by levers to supply air to the reservoir, later organs transitioned to using bellows to provide wind.132
• Evolution: The bellows-powered organ did not appear until approximately 400 years after the invention of the hydraulis. By the 8th century, bellows-fed organs were being constructed in Europe, eventually becoming the standard for the instrument.3

While the hydraulis was a rudimentary engineering marvel often used for public entertainment, the fundamental principles of the organ—using a keyboard to admit pressurized air into pipes—have remained largely unchanged for over 2,000 years.13