Hero of Alexandria (c.10–70 AD) describes in his Pneumatica the aeolipile: a hollow sphere mounted on an axle that spins by expelling steam through two opposed curved nozzles, converting thermal energy into rotary motion. It is the first documented device to convert steam into mechanical work through reaction — the principle of the modern reaction turbine. Hero presents it as a mechanical curiosity, not as a source of practical power, and there is no evidence it was ever applied to useful work in antiquity. The aeolipile remained overlooked as a technical antecedent until 19th-century historians identified it as a conceptual precursor of the steam engine.