The Springal or Espringal was a mechanical artillery device used for throwing large bolts.
There is a diagram of a springal in an 11th-century Byzantine manuscript. In Western Europe, springals became popular during the 12th and early 13th centuries.
Because springals used a torsion device with twisted skeins of silk or sinew to power two bow-arms, springals were also known as “skein-bows”. Rectangular springals on wheels were better for besieging fortifications, while moujnted ones were used for defending castles as they could aim down from turrets and bastions.
The medieval springal was constructed similarly to the Greek or Roman ballista, with the difference springals had inward swinging arms (and ballistae have outward-facing bow arms). These smaller arms faced forward when not bearing tension. When tension accumulates, they bend towards the operator using the windlass.
There is a large model of a springal at the Tower of London. The only full-size reproduction (8 feet long and capable of hurling a 2.4 kg bolt over 55 meters) can be found in the Royal Armouries Museum at Fort Nelson, Portsmouth.