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Ctesibius or Ktesibios or Tesibius (Greek Κτησίβιος) (flourished 285–222 BC) was a Greek^[1] or Egyptian^[2] inventor and mathematician in Alexandria, Ptolemaic Egypt. He wrote the first treatises on the science of compressed air and its uses in pumps (and even a cannon). This, in combination with his work on the elasticity of air On pneumatics, earned him the title of "father of pneumatics." None of his written work has survived, including his Memorabilia, a compilation of his research that was cited by Athenaeus.

Ctesibius was probably the first head of the Museum of Alexandria. Very little is known of his life and work. It is said (possibly by Diogenes Laertius) that his first career was as a barber. During his time as a barber, he invented a clever counterweight-adjustable mirror. His other inventions include the hydraulis, a water organ that is considered the precursor of the modern pipe organ, and an improved water clock called a clepsydra. The clepsydra kept more accurate time than any clock invented until the Dutch physicist Christiaan Huygens detailed the use of a pendulum to regulate a clock in the 17th century. The principle of the siphon has also been attributed to him.

According to Diogenes Laertius, Ctesibius was miserably poor. Laertius details this by recounting the following concerning the philosopher Arcesilaus:

"when he had gone to visit Ctesibius who was ill, seeing him in great distress from want, he secretly slipped his purse under his pillow; and when Ctesibius found it, 'This,' said he, 'is the amusement of Arcesilaus.'" [1]

Ctesibius's work is chronicled by Vitruvius, Athenaeus, and Philo of Byzantium who repeatedly mentions him, adding that the first mechanicians such as Ctesibius had the advantage of being under kings who loved fame and supported the arts. Proclus (the commentator on Euclid) and Hero of Alexandria (the last of the engineers of antiquity) also mention him.

• Ancient Greek Scientists: Ctesibius