Hero produced several written works on geometry, giving formulae for the areas and volumes of polygons and conics. His formula for the area of a triangle was contained in Metrica (Measurement), a work that was lost until 1896. This book also describes a method for finding the square root of a number, a method now used in computers, but known to the Babylonians in 2000 bc. In another of Hero's books, Pneumatica (Pneumatics), he wrote on siphons, a coin-operated machine, and the aeolipile – a prototype steam-powered engine that he had built. The engine consisted of a globe with two nozzles positioned so that steam jets from the inside made it turn on its axis. Hero also wrote on land-surveying and he designed war engines based on the ideas of Ctesibius. Yet another of his works, Mechanica (Mechanics), was quoted by Pappus of Alexandria.