This page created 11 January 2003, and last modified: 10 October 2015 (Maier reference numbers added)
Above is a picture from the De Rebus Bellicis, an originally 4th (or possibly 5th) century work; the pictures of which are found sandwiched between the two sets of Notitia pictures in the Munich manuscript. The De Rebus Bellicis illustrates many rather fanciful implements of war that the author hopes will be adopted by the emperor.
Seven units of balistarii are noted in the Notitia (the numbers in parentheses refer to Ingo Maier's numbering scheme):
Balistarii (102/5.149), a unit in the Magister Equitum's Gallic command (likely with pseudocomitatenses status)Of these, the first two are likely one and the same unit.
Milites balistarii (156/8.10), a limitanei unit under the Dux Mogontiacensis
Balistarii seniores (15.20), a comitatenses legion under the Magister Militum per Orientem
Balistarii iuniores (18.25), a comitatenses legion under the Magister Militum per Thracias
Balistarii Dafnenses (18.24), another comitatenses legion under the Magister Militum per Thracias
Balistarii Theodosiaci (15.34), a pseudocomitatenses unit under the Magister Militum per Orientem
Balistarii Theodosiani iuniores (21.31), a pseudocomitatenses unit under the Magister Militum per Illyricum
Balistarii should probably be read as "artillery", but some doubt exists due to the existence of the manuballista, or "hand ballista" (i.e. crossbow), whose usage by Roman light infantry is described by Vegetius (2.15, and also 2.3, where he calls it simply a ballista). However, most such light infantrymen would likely not form separate regiments, but be integral to other legionary formations, and the number of units recorded above seems inordinately excessive for such a rare troop type (cf. the single unit named Funditores (i.e. "slingers") recorded in the Notitia, when Vegetius implies slingers should be much more numerous that manuballistarii: also note there are only three units of Exculcatores in the Notitia, which Vegetius implies is the word used to describe all contemporary light infantry formations).
Of course, artillery was also an integral part of legionary units, at least in the past when legionary units were larger. By the 4th century, this was probably no longer the case; the legions seem to have operated as rather small units that would have been incapable of fielding many pieces individually, and maintenance issues likely required that any such pieces that were routinely taken into the field were looked after by dedicated units, which would appear to be the balistarii units listed above.
It should be noted that Ammianus for instance describes (16.2.5) Julian in the mid-4th century marching through wooded terrain accompanied by only catafractarii and balistarii, and that they were unsuitable as a bodyguard in such terrain. This implies that at this time the balistarii were separate from his (other) legionary units as they are in the Notitia, and also that they were artillery: if they were crossbow-armed light infantry, they would appear to have been an especially suitable escort in the circumstances!
Assuming the first two units above are indeed one and the same, it can be seen that the western empire has just one dedicated unit of field artillery compared to five in the east. This highly skewed ratio reflects the rarity of siege warfare in the west (where barbarians attackers proclaimed 'they were at peace with walls') compared to that in the east where the Persians were fully capable of reducing the strongest cities.
Indeed, "field artillery" is likely a misnomer. In earlier times, light boltshooters had been mounted on carts for field use, as depicted on Trajan's column and described by Arrian in his tactical treatise regarding action against the Alans. The anonymous inventor of the De Rebus Bellicis, probably writing during Valentinian I's time, was advocating the usage of similar weapons in the 4th century, but apart from this suggested use, the (admittedly scant) literary sources to not record the actual usage of any artillery in the field in the 4th or 5th centuries unless we include Vegetius as such a source (or look outise the empire, to the Chersonessus). Even if ballistae were occasionally employed as field artillery, balistarii units most likely saw the bulk of their action in the attack and defence of fortified positions.
There is, as far as I can ascertain, no evidence available whatsoever as to the strength of these balistarii units, whether in terms of men or machines. It seems reasonable to assume that a balistarii unit would have appromimately the same number of men (including men responsible for looking after the draught animals, maintenance etc., as well as loaders and engineers) as a normal legion, it ought to be possible to estimate how many artillery pieces were employed by a balistarii unit, although this would require a knowledge of how many men (for all tasks) were required per machine, and this information seems to be unavailable even for the much better documented legions of the Principate.
However, Vegetius does state (2.25) that each century in his 'ancient legion' had 11 men to arm and load its carroballista (cart-mounted ballista), and as this would seem to be an excessive number of men to operate a single light ballista in the field, it may be taken as the number of men required to service it in addition. If we assume that a balistarii legion kept the same ratio of heavy to light pieces as before, approximately one heavy piece per 6 light pieces, and that a heavy piece required (based on no evidence whatsoever!) twice as many men to look after it as a light piece, a balistarii unit of say 1000 men should be able to field a dozen heavy onagers and nearly 70 light scorpions, which would be not that much more than the combined artillery park of a single legion of the first or second centuries AD.
With only seven units of balistarii in the entire empire (at least, as recorded in the Notitia), this would imply that the amount of artillery available to the legions of the Notitia was considerably less than in the Principate era, unless they either still retained some of their own, beside those incorprated into the balistarii units, or, as seems much more likely, most artillery of the era was routinely kept in cities and other fortified places rather than with field army units; as is indeed implied by Ammianus' description of the siege of Amida in 359 AD, and is backed up by archaeological finds (e.g. see this paper on Danubian forts, such as garrisoned by the Balistarii Dafnenses).
Shown above are the shield patterns for the four units that have them recorded, from the Bodleian (O), the Parisian (P) and the two Munich (M, W) manuscripts, as well as the Froben edition (B). A visual relationship between the Balistarii seniores and the Balistarii Theodosiaci, both under the Magister Militum per Orientem, is immediately apparent (see the discussion here). Further, since the Magister Militum per Illyricum's balistarii unit is the Balistarii Theodosiani iuniores, it is entirely possible that the Balistarii Theodosiaci is a seniores unit, assuming a confusion of Theodosiani and Theodosiaci. The two units under the Magister Militum per Thracias, the Balistarii Dafnenses and the Balistarii iuniores, share no such resemblance.
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