military

Ammianus Describes the Onager

Date

380

Part of

military

Ammianus Marcellinus contrasts two-armed ballistae with the single-armed stone-thrower that “modern times” call onager—the wild ass—for its violent kick [3]. He notes cushions, eight winders, and a gunner. It’s the late empire’s sound: one savage crack, one flying stone.

What Happened

Ammianus wrote as a soldier who read and a reader who had soldiered. In Book 23 he pauses to describe engines: the twin-armed ballista and the one-armed stone-thrower “which in modern times has been given the new name onager, for when it is fired it kicks back with a violent recoil like a wild ass” [3]. The simile is memorable because it is accurate.

He gives details that make late Roman stone-throwing sound and feel real. An onager’s frame sits braced; its single arm is drawn down by multiple men—“eight winders” in his account—while a gunner oversees aim and timing. Cushions absorb the recoil, a necessary concession to the physics of a heavy arm snapping upward [3]. The sound is a singular crack rather than the paired snap of twin arms.

Set this engine behind a line in Thrace or Syria. The arm comes down against a padded bed; the sling, loaded with a stone the color of baked clay, hums. When the pin releases, the arm lashes up; the sling opens; the stone arcs over the rear ranks and into a dense knot of enemies with a bone-deep thud. Dust rises bronze in the sun. In the quiet that follows, you hear men exhale.

Ammianus’ contrast with ballistae matters. The twin-armed torsion shooters, designed to hurl bolts or stones with precision from standardized frames—think Vitruvius’ one-ninth rule and peritreti, capitals, sideposts—belong to an earlier complexity [1]. The onager, by contrast, simplifies. It tolerates roughness and produces shock. Vegetius’ per-cohort allocation turns that quality into doctrine [4].

His account sits within a long Roman line. At Jerusalem, Josephus wrote of stones so visible that watchmen could shout their coming [2]. In Dacia, Trajan’s reliefs show frames rolling toward firing positions [9][11]. Ammianus’ “modern” onager is the late accent in a language long spoken—still concerned with arcs and pressure, now in a form the army could maintain when supply lines and skills faltered.

The places that frame his writing—Nisibis, Amida, the eastern marches—felt the need for engines that could travel and work from camp to wall. His prose, cool and exact, gives future readers numbers and roles. Eight winders. One gunner. Cushions to tame a wild ass. It is a manual in a sentence [3].

Why This Matters

Ammianus gives the onager its late Roman identity: a violent, one-armed thrower served by a defined crew and tamed by cushions. He helps explain why the machine gained ground at the expense of complex ballistae: it delivered shock reliably under rough conditions [3].

The description embodies the theme of late antique simplification. The army traded delicate skeins and precise symmetry for a rugged frame, a sling, and a manageable crew. Vitruvius’ ratios remained relevant but less decisive; the field wanted stones that flew on command [1].

In the larger story, Ammianus is the narrative partner to Vegetius’ tables. Where Vegetius counts onagers per cohort, Ammianus tells us how they worked and what they sounded like. Together they fill out the late empire’s artillery silhouette: mobile, cart-borne, behind the line, kicking like an ass and hitting hard [4].

Historians prize this passage because it humanizes a machine. You can see the eight men straining, hear the thump, and understand why late Roman commanders accepted the kick for the hit.