Who invented the hidraulis, the first water organ?

Ctesibius or Ktesibios or Tesibius (fl. 285–222 BC), Greek physicist and inventor, the first great figure of the ancient engineering tradition of Alexandria, Egypt. He wrote the first treatises on the science of compressed air and its uses in pumps (and even in a kind of cannon).

The discovery of the elasticity of air is attributed to Ctesibius, as is the invention of several devices using compressed air, including force pumps and an air-powered catapult. His most famous invention, however, was an improvement of the clepsydra, or water clock, in which water dripping at a constant rate raised a float that held a pointer to mark the passage of the hours.

Another notable invention was a hydraulis, or water organ, in which air was forced through the organ pipes by the weight of water rather than by falling lead weights. It has a reservoir of air which is inserted into a cistern of water. The air is pushed into the reservoir with hand pumps, and exits the reservoir as pressurized air to blow through the pipes. The reservoir is open on the bottom, allowing water to maintain the pressure on the air as the air supply fluctuates from either the pumps pushing more air in, or the pipes letting air out.

Ctesibius’ writings have not survived, and his inventions are known only from references to them by Vitruvius and Hero of Alexandria, but he laid the foundations for the engineering tradition that culminated in the works of Hero of Alexandria and of Philo of Byzantium.