The cheiroballistra probably appeared in the late first century ad and was the most sophisticated Roman two-armed siege and field weapon. Its torsion springs were supported by all-metal frames and often were protected from the weather by bronze casings. The frames themselves were secured by metal components, with some possibly constructed with all-iron frames and two front-mounted wheels for rapid deployment.
Ranged by adjusting a ratchet controlling the firing cord tension, the cheiroballistra was highly accurate, being aimed with a simple rear sight aligned with the bolt tip. Capable of three to four shots a minute, it was particularly suited as a rapid-fire field piece, the majority of its up to ten-man crew serving as ammunition handlers.
Archeologists have excavated a relatively large number of metal cheiroballistra components and bolts; the finds have also been supported by a number of surviving artistic representations. Three scenes from Trajan ‘s Column (A.D. 106-113) show what appear to be larger types of cheiroballistrae used as defensive weapons in fortifications. Two scenes show smaller cheiroballistrae transported by wagon-a mode of transportation supported by Vegitius (fl. ca. A.D. 390), who reported that they were pulled by mule teams. The so-called Cupid Gem, a late Hellenistic or possibly early Augustine ornament, features a cheiroballistra in a rather humorous light, as it portrays Cupid aiming the weapon, in favor of his traditional bow, presumably at a particularly difficult and desirable object of affection .
As the size of siege engines increased, ancient artillery designers soon realized the limitations of bow-powered weapons. A later form of the oxybeles, appearing sometime before 340 bc, introduced the torsion principle for launching missiles. Torsion engines replaced the bow with powerful, vertically mounted twin springs on either side of the slider bed. These were composed of tightly wound skeins of sinew or hair wrapped around a wooden axis. The base of a wooden arm, approximately 2 to 3 feet in length, was mounted into each spring with the exposed end providing the attaching point for the bowstring. The introduction of the torsion principle quickly rendered the earlier tension-powered engines obsolete and opened new possibilities for huge weapons capable of projecting much more destructive projectiles at significantly greater distances.
Torsion-powered engines were, along with spring-powered and counterweight- powered engines, one of the three basic types of pre-gunpowder artillery. The first mechanical artillery was spring-powered, but steady increases in size of siege engines made its limitations all too apparent.
Sometime before 340 BCE, a form of the spring-powered oxybeles appeared, utilizing the torsion principle of elasticity of a tightly wound skein of hair or cord wrapped many times around two wood axles. The base of the wooden throwing arm, approximately two to three feet in length, was placed in the center of the skein. Winches and ratchets were then used to twist the ends of the skein. When released, the arm sprang forward and launched the projectile.
Torsion-powered engines could fire projectiles with higher velocity and greater range and accuracy than any other mechanical system and soon rendered spring – or tension – powered engines obsolete. They did, however, require heavy frames to withstand the stresses from the greater energy released.
Two early types of torsion weapons were the Greek euthytone and palintone. They derived their names from comparisons to hand bows. Euthytone means “straight spring” (as in a straight bow), while palintone means “V spring” (for the curved composite bow). The palintone appeared later and was the more powerful. It was used to hurl heavy stone shot, while the euthytone usually fired bolts against personnel.
In order to knock down a wall, artillerists would have to score a number of hits in the same spot. Optimum accuracy required standardized projectiles. Masons therefore shaped stone into equal-sized round shot, the shape of which enhanced its flight characteristics.
The Romans acquired significant quantities of artillery in their defeat of the Greeks, and Roman artillery was simply a refinement of earlier Greek forms. Thus, the Greek euthytone became the catapulta, and the palintone became the ballista. The ballista was a two-armed torsion machine used as an antipersonnel weapon in sieges and operated by a two-man firing crew. Julius Caesar equipped each of his legions with 30 ballistae. For portability, many ballistae were relatively small. Arm lengths varied from 2 to 4 feet. The larger stone-throwing weapons could hurl projectiles weighing up to 60 pounds as far as 550 yards. Bolt-firing ballistae could hurl 26-inch- to 3-foot-long projectiles 300 yards.
The onager (meaning “wild ass”) was perhaps the best known of late Roman torsion-powered engines. It was also the simplest of Roman siege engines. Its name came from the powerful kick upon discharge when the arm reached the end of its travel against padded boards at its front. The onager was in common use in the fourth century CE. It had a leather sling attached to the opposite end of the arm to accept projectiles. Once it was loaded and ready to fire, a mallet was used to strike the retaining pin and release its projectile. The onager could be manned by a crew of five to six men, although a larger number improved efficiency. The largest onagers could hurl stone shot of up to 180 pounds. At a weight of as heavy as 6 tons apiece, however, these engines were difficult to transport even disassembled. In consequence, they were most often a defensive, garrison weapon.
The scorpio (“scorpion”) was a relatively lightweight and mobile torsion – powered engine. Appearing in the mid-first century BCE, it had curved, tapered arms as with a recurved bow to increase its strength. It utilized metal on surfaces subject to wear. The scorpio could hurl a 7- to 10-pound shot 300 yards, while the bolt-firing scorpio usually fired a 27-inch arrow with a pyramidal iron head and three wood fletches.
The most sophisticated Roman two-armed torsion engine was the cheiroballistra. Appearing in the late first century CE, its torsion springs were supported by all-metal frames. It had two front-mounted wheels for rapid deployment. A well- trained 10-man crew, the majority of whom were ammunition handlers, could fire it three or four times a minute.
Further Reading The Diagram Group. Weapons: An International Encyclopedia from 5000 BC to 2000 AD. New York: St. Martin’s, 1990. Kinard, Jeff. Artillery: An Illustrated History of Its Impact. Santa Barbara, CA: ABC- CLIO, 2007. Marsden, E. W. Greek and Roman Artillery: Historical Development. Oxford, UK: Sand- piper, 1999.
A Multidisciplinary Re-evaluation of the Fabrication and Operation of the
4th Century CE Roman Artillery Engine known as the Onager PDF