a one-armed stone-throwing engine, also known as onager, while ballista means a two-armed piece which shoots bolts only. The authors of Greek treatises classified artillery pieces into two categories: euthytones and palintones. Hero writes that euthytones were arrow-throwing, while palintones usually stone throwing, but sometimes could also shoot arrows or both. The precise meaning of these terms is disputed. According to Schramm, Marsden and their followers, this distinction reflects the difference in the shape of the detail of the frame which is called "hole carrier". According to the so-called "French school", the arms of a euthytone extended outside the frame, while the arms of a palintone moved inside the frame. The problem arises because the ancient descriptions that we have do not contain original pictures, and the meaning of certain technical terms is unclear. Dimensions, materials and performance Much research was done by Hellenistic Greek scientists and craftsmen on the design of artillery pieces. The main parameter that determines the sizes of all parts of the machine is the weight of the projectile or the length of the bolt (arrow). The fundamental size characteristic is called the hole diameter; it is the same as the diameter of the spring (which is a bunch of sinew rope). Vitruvius gives the following formulas for the hole diameter: it is the length of the bolt divided by 9 for the bolt-throwing machines, or : for the stone-throwing machines, where W is the weight of the projectile in Attic minas (1 mina = ), and the hole diameter is measured in dactyls (1 dactyl = ). Then the dimensions of all parts are defined as fixed multiples of the hole diameter, for example the length of the arm is 7 hole diameters. Most of the frame detail were made in the Hellenistic period of wood. Hero of Alexandria proposed metal frame and metal cylinders to enclose the springs. Depictions on Trajan's column and archaeological evidence indicate that these metal artillery pieces were widely used by the Romans. An important detail, washer used in the stretching of the spring was always made of metal, and these washers are the only pieces of Hellenistic artillery, besides the stone balls and arrowheads which are found by archaeologists. The preferred material for the springs was sinew and human hair. Horse hair was considered an inferior substitute. In 250 BC, Rhodes sent to Sinope for her war with Mitridates