Bish Bash Bosh – Ancient Artillery Part Two

 
These angry little fellas are making use of a simple tension catapult  in order to propel themselves at their smug green nemesis. As we saw in Part One however, engineers in the classical world had devised a means of harnessing the energy held in tightly coiled bundles of rope made from hair or sinew in order to increase the power of a catapult significantly. These torsion catapults had reached the pinnacle of their development by the end of the First Century AD and the armies of Trajan would have terrorised the defenders of Sarmizegethusa and Ctesiphon with the ultimate in torsion artillery technology. The cheiroballista was built on the same lines as the earlier examples but its frame was constructed from iron and the torsion bundles or skeins were protected from the elements by being housed in metal cannisters. This more robust weapon was also smaller and more mobile than the equivalent wooden examples. We can see here depicted on Trajan’s column a cheiroballista being transported on a cart.

 
Optimising the design of such machines was dependant on a thorough understanding of the mathematical relationship between the diameter of the torsion spring, the length of the bow arms and the weight or length of the projectile. Men such as Trajan’s chief engineer Apollodorus of Damascus, who famously constructed a magnificent stone bridge across the Danube, were well versed in such reckoning. By the late Roman Empire however, such knowledge was beginning to be lost and masterpieces of the siege engineer’s craft such as the cheiroballista were disappearing from the battlefield.
Instead the torsion artillery of choice for the Roman army was a brute of a siege engine known as the onager; ‘wild ass’, so named for its vicious kick.
The onager was a simpler device possessing just a single arm as opposed to the two arms of the ballistae. The throwing arm was held upright in a skein stretched between two beams, the tension of which could be adjusted with a crank mechanism. A second crank mechanism allowed the throwing arm to be pulled back and a slip-hook mechanism then allowed the missile to be released from a sling suspended from the end of the throwing arm.
 
Nevertheless there is evidence for the continuing use and continuing deadly accuracy of bolt throwing ballistae in the late Roman and early Byzantine world. Ammianus Marcellinus; writing in the Fourth Century AD describes both one and two armed torsion catapults being used in the defence of Amida on the Tigris and two hundred years later Procopius describes ballistae being used to defend Rome from the Goths.
In the late Sixth Century the Avars; terrifying horsemen from the Eastern Steppe, began raiding across the Danube into the Balkans. These invaders brought with them a new type of siege engine originally developed in China. The trebuchet had arrived. These early examples were traction trebuchets, powered by men heaving in unison on ropes to pull the throwing arm of the machine up and over the top of the frame and release the missile from the sling. At the siege of Thessalonika in 597 AD these machines, which he refers to as petroboles; stone throwers, are described in some detail by the Archbishop John who was an eye witness.
These petroboles were tetragonal and rested  on broader bases, tapering to narrow extremities. Attached to them were thick cylinders well clad in iron at the ends, and there were nailed to them timbers like beams from a large house. These timbers had the slings from the back and from the front strong ropes, by which, pulling down and releasing  the sling, they propel the stones up high and with a loud noise. And on being  fired they sent up many great stones so that neither earth nor human constructions could bear the impacts. They also covered those tetragonal petroboles with boards  on three sides only, so that those inside firing  them might not be wounded with arrows by those on the walls. And since one of these, with its boards, had been burned to a char by a flaming arrow, they returned, carrying away the machines. On the following day they again brought these Petroboles covered with freshly skinned hides and with the boards, and placing them closer to the walls, shooting, they hurled mountains and hills against us. For what else might one term these extremely large stones?
 
Medieval Weapons: An Illustrated History of their Impact By Kelly De Vries & Robert D. Smith Copyright 2007 by ABC-CLIO, Inc.
http://www.scribd.com/doc/15778880/Medieval-Weapons-An-Illustrated-History-of-Their-Impacttqwdarksiderg
Smashing the Bridge between Roman and Medieval Artillery: The Onager by Brian Pangburnhttp://bit.csc.lsu.edu/~pangburn/onager.pdfAmmianus Marcellinus on Roman artilleryhttp://penelope.uchicago.edu/Thayer/E/Roman/Texts/Ammian/23*.htmlAmmianus Marcellinus on the siege of Amida
http://www.tertullian.org/fathers/ammianus_19_book19.htm
More on catapults
http://www.mlahanas.de/Greeks/war/CatapultTypes.htm