Springald

A Springald, or espringal, is mechanical artillery device for throwing large bolts and less commonly stones or Greek fire. It is depicted in diagram in an 11th-century Byzantine manuscript, but in Western Europe is more evident in the late 12th century and early 13th century. It was constructed on the same principles as a Greek or Roman ballista, but with inward swinging arms. It was also known as a 'skein-bow', and was a torsion device using twisted skeins of silk or sinew to power two bow-arms.