An aeolipile is a device whose essential feature is that vapour confined in it under pressure can issue from it only through a small aperture. Although the Oxford English Dictionary agrees with the French Academy in deriving ‘aeolipile’ from a combination of the name of the god of the winds, Aeolus, and the Latin pylae or the Greek Πύλαι, implying thus that originally it meant something like ‘doorway of Aeolus’ or ‘gates of the winds’, I am inclined to accept rather its derivation from ‘Aeolus’ and the Latin pila, a ball, and an original meaning ‘ball of Aeolus’ or ‘of the winds’. Vitruvius, writing during the reign of Augustus, says, in a passage concerned with the winds, ‘id autem verum esse ex aeolipilis aereis licet aspicere … fiunt enim aeolipilae aereae cavae. hae habent punctum angustissimum quo aquae infunduntur conlocanturque ad ignem, et antequam calescent non habent ullum spiritum, simul autem ut fervere coeperint, efficiunt ad ignem vehementem flatum’. This seems to show pretty clearly, by both the form of the word and the description of the object to which Vitruvius applies it, that the word was based on pila and not on pylae. Conceivably it was the spherical shape of early aeolipilic fire-blowers which suggested the trunnioned steam-filled ball provided with projecting L-shaped tubes from which the steam could issue and, by its reaction, cause the ball to revolve, which Hero of Alexandria describes in his Pneumatika, and which is still used to demonstrate to students a jet of steam's inherent reactive force. Hero's text does not, however, include any word relatable to the term Vitruvius applied to his fire-blowing spheres.