It is difficult to determine the exact number of field artillery pieces would accompany a Legion, but Vegetius Renatus in his Epitoma Rei Militaris, from the time of Valentinian, makes some suggestions that are accurate enough for this article.
His writings make it safe enough, without being horribly inaccurate, to assume that each centuria of the legion would be provided with a carroballistae.
An additional 10 onagri would be assigned, 1 to each cohort. There is some argument that the 10 onagri would simply replace 10 of the carroballistae for a total of 59 artillery pieces in the Legion but again, the difference regarding this particular outline is irrelevant. The pieces would be drawn, mounted on a carriage, by either oxen or mules and each required 10 artilleryman (Libritors) to operate. Each piece would apparently fall under the command of the centurion within each century that it was assigned to.
Some general descriptions of a couple of common Roman Artillery pieces:
The ballista was essentially a giant crossbow and worked on the same principle by firing iron tipped bolts towards the enemy positions. It was smaller weapon than the catapult (onager) and was used to kill and injure the enemy soldiers advancing or those within a fort, trying to sustain a siege.
The ballista came in a range of sizes with varying ranges. A ballistae bolt could be fired anywhere from 300 yards (275m) to 550 yards (500m). It was loaded with a 3 ft (100cm) bolt that could be fired at up to 115 mph (184 kph). The effect of impact could be devastating.
The Roman catapult, the onager, Latin for wild ass, was a very large and cumbersome piece of equipment. It could fire rocks of up to 150 lbs (70 kgs) to be used to smash through walls and fortifications. It could also be loaded with the equivalent mass of smaller stones or fiery pitch to use against enemy troops or to bombard the inside of a fort. Its range was much shorter than that of the ballistae and could be in danger of attack by enemy bowman during a siege.
Did you know...?
Ancient artillery can be divided into two primary groups: torsion and non-torsion.