A Late-6th Century description of Traction Trebuchets by John, Archbishop of Thessaloniki
The Byzantine historian Theophylactus Simocatta wrote that the Avar people learned about siege engines from a captured Byzantine soldier - and then used the machines on their teacher's cities.
Theophylactus names this Byzantine as Bousas and says that he, in exchange for his life, constructed for the Avars a siege engine called a "helepolis" or "City Taker" (a heavy trestle-framed traction trebuchet).
In 587 the Avars, using this new technology, besieged and took the fortress of Appiareia in what is now northern Bulgaria. Here is part of Theophylactus' account:
"Bousas taught the Avars to construct a certain siege machine, for they [the Avars] happened to be most ignorant of such machines, and he built the helepolis to hurl missiles.
Soon thereafter the fortress was leveled, and Bousas collected judgment for their inhumanity, having taught the barbarians something frightful, the technology of besieging.
Thence the enemy captured effortlessly a great many of the Roman cities by making use of this original device."
Ten years later, in 597, the Byzantine centre of Thessaloniki was besieged by Avaro-Slavic forces. The attacking army didn't fool around - they attacked with fifty large traction trebuchets that could hurl "great stones" into the town. In his description of the siege John, Archbishop of Thessaloniki, calls the big traction trebuchets "petroboles" (Greek for "rock thrower"). Smaller machines were called "lithoboles" or "stone throwers".
"These petroboles were tetragonal and rested on broader bases, tapering to narrower 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 side and from the front strong ropes, by which, pulling down and releasing the sling, they propel the stones up high and with a great 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 from those on the walls. And since one of these, with its boards, was burned to a char by a flaming arrow, they returned, carrying away the machines.
On the following day they again brought up 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 immensely large stones?"
We are somewhat at the mercy of the translator here (or at least, I am, having no knowledge of Greek nor access to the original document..). This is the best I can manage for now:
I can only assume that this refers to the axles of the machines as these cylinders rest on the machines' bases and have the throwing arms attach to them. The comment about "well clad in iron at the ends" is interesting. Following along on the axle idea, this may refer to a bearing surface (either on the axle or the stationary frame timbers) or to a re-enforcement driven onto the end of the axle timber (perhaps to stop it splitting).
Basically, the machines are described as having bases that resemble a pyramid with its top removed, as in the illustration I have used on this page - although it is possible that the front and rear faces could be rectangular, with only the sides tapering. I have seen this interpretation in a few modern drawings.
Oddly enough, this page's illustration hints at an arrangement the other way around - with rectangular faces on the sides.
This is a bit of a puzzle to a modern reader. The noises made by a "Frankish" traction trebuchet (where the beam can swing unimpeded between the supporting timbers) are generally just the creak of flexing timbers and axle bearings, a whip-like crack from the suddenly unfurled sling and the buzz of the rapidly spinning projectile through the air.
Unless we assume that 6th Century ears were enormously more delicate than our own noise-shattered ones, we are left to contemplate what else this loud noise was. - One possibility is that the crew was making the noise, possibly chanting to keep time and bellowing during the strenuous pull. On the other hand, you might expect this to be described as a loud cry or shout. - Another more likely possibility is that the base of the beam struck the frame. Many machines illustrated on the Grey Co Trebuchet page seem to be drawn as if they do this. This practice would work, although at the cost of shorter machine life as the trebuchet hammered away at itself. On the other hand, in these machines the crew is standing behind the protective front framework, so anything that would catch the beam end would also be in the way of the pulling crew's ropes. - Finally, the noise may refer not to the flight of the projectile but to its impact (although the text as it stands here doesn't seem to support this one)
"so that neither earth nor human constructions could bear the impacts".. Generally, the traction trebuchet is considered to be a relatively light weapon, throwing stones of a only few (say, 10) kilos in weight - making it more an anti-personnel weapon able to destroy only light structures. Either John's comments are in awe of the new weapon (he felt the need to describe what one looks like), contain wild exaggeration or, just possibly, are an accurate description (in which case we would need to revise our ideas on how big traction trebuchets could be - even very early ones such as these).
Alternatively, if the human construction being discussed here is a tiled roof, then perhaps it is entirely believable, even on a modest scale. If you are not looking at massive stonework then even a stone of a few kilos, flying at perhaps 160 km/hr (100mph) or dropping from a great height, could do significant injury or damage to civilian-style buildings.
John describes the throwing arm as being as large as a beam from a large house. This might give an idea of size - if we learn what is meant by a "large" house...
The above quote is taken from "The Traction Trebuchet: A Reconstruction of an Early Medieval Siege Engine" by W.T.S. Tarver's who, in his footnotes, credits the source and translation as
'Chevedden, "Artillery Revolution", after Speros Veryonis jr "The Evolution of Slavic Society and the Slavic Invasions in Greece: The First Major Slavic Attack on Thessaloniki, AD 597", Hesperia 50.'
"The Invention of the Counterweight Trebuchet: A Study in Cultural Diffusion" by Paul E. Chevedden
Tarver notes that Chevedden uses the original word "petrobole" (ie "rock thrower") rather than Veryonis' translation to "ballistrae", with its implications of torsion power.