The Greeks and Romans both made extensive use of artillery for shooting large arrows, bolts or spherical stones or metal balls. Occasionally they also used ranged early thermal weapons. There was heavy siege artillery, but more mobile and lighter field artillery was already known and used in pitched battles, especially in Roman imperial period. The technology was developed quite rapidly, from the earliest gastraphetes in about 399 BC to the most advanced torsion artillery in about 300 BC at the time of Demetrius Polyiorcetes. No improvement, except in details, was ever made upon the catapults of Demetrius. The Romans obtained their knowledge from the Greeks, and employed the Greek specialists. Five Greek and Roman sources have survived: two treatises by Heron of Alexandria, Belopoeika and Cheiroballistra; and the books by Biton of Pergamon, Philo of Byzantium and Vitruvius Energy storage The earliest artillery pieces, like gastraphetes, were driven by large composite bows. According to Marsden's analysis of ancient sources, they were invented in Syracuse in 399 BC, when tyrant Dionysius I gathered there an assembly of expert craftsmen to conduct a research on new armament. Diodorus XIV.41.3, says that these were the first catapults, and describes the impression new weapons made during the siege of Motya by Dionysius. Torsion siege engine pieces were probably invented in Macedonia, shortly before the times of Alexander III. These were driven by the torsion of a spring made of an appropriate organic material, usually sinew or hair, human or horse. Stone-throwing torsion-powered machines had their first recorded use in 332 BC at the siege of Tyre by Alexander. Although other power systems such as metal springs and pneumatically powered machines were experimented with by Ctesibius – according to Philo – there is no record of their actual use. Metal springs were not sufficiently resilient at that time, and both the traction and counterweight trebuchet were unknown to the Greeks and Romans. Nomenclature The names of the artillery pieces changed with time. Though all inventions in the field of artillery were made by the Greeks, the best known are the Latin names, catapulta and ballista. Originally, catapulta (καταπέλτης ὀξυβελής) meant an arrow- or bolt-throwing engine, and a ballista (καταπελτης λιτοβολος or πετροβολος) was a more powerful machine primarily designed for throwing stones. At some time between 100 CE and 300 CE a change occurred in the nomenclature. Thus, in the 4th century CE catapulta indicates