The Romans used torsion machines, meaning they were powered by twisted cords. Sinew was the preferred cord material, but horse hair (and reportedly the long hair of women) was also used. When triggered, a considerable amount of energy was released and directed toward launching the projectile. There were two generla artillery types: stone-firing and arrow-firing. Unfortunately, the Latin terms used to describe the different siege machines are not fully understood. In the Republic and early Empire catapultae referred to bolt-shooters (arrow-firing). Scorpiones were the smaller mobile catapultae. Ballistae were strictly stone-shooters. Yet, in the 2nd century AD the term, ballistae, was applied to both arrow throwing and stone throwers. In the 4th century AD it appears that the arrow throwing machines were differentiated from stone throwing catapults by having the term ballistae apply only to them.The most common artillery piece was a catapult that could fire a bolt 3-hand spans long. The scorpio (circa 50BC) threw a bolt 27 inches long (67cm).
Although the wooden bolt shafts have long since rotted, quite alot of the iron points have survived to this day. Stabilizing flights may have been made of feathers, leather, or wood. The iron points were pyramid shaped. The hand cranks at the rear were used to winch the arrow, or bolt, back to the firing position. The largest artillery filed pieces could lob a 78kg stone ball!
Both types of artillery were manned by men called ballistarii. It is not known exactly how many each legion had. Each cohort may have had only 1, or as many as 6. Auxiliary units typically did not have artillery, although it appears there were exceptions to this rule. If these non-Roman citizens should turn against the legions, the Romans did not want them to have the added firepower of artillery. In the 2nd century AD the metal-frame carroballista was introduced. It had more power and better aim than the older wooden-frame scorpion. See above illustration. It's lighter weight was an improvement over the older wooden-frame catapults by enabling it to be mounted on a type of cart.
By the 4th century AD the legions no longer had a compliment of artillery. Artillery was reserved, instead, for a few specialist artillery legions and for defending fixed positions. Artillery was also used on warships. They were often mounted on wooden towers, giving them a more advantageous angle of fire.
The Onager was named after a wild ass because of the way it kicked back upon firing. although it was powered by torsion, it was an artillery piece apart from catapultae and ballistae. It was first mentioned in writing in the 200s BC. It was most common in the armies in the mid 4th century AD. Since they were so large and slow to move the onagri were most suited for sieges. The length of the sling on the end of the arm could be adjusted to affect the trajectory of the stone.
Two excellent Roman artillery books were coincidentally both hpublished in 2003. They are both available inexpensively as used books:
1. Greek and Roman Artillery, By Duncan Campbell.
2. Roman Artillery, By Duncan Campbell.