Vitruvius Codifies Torsion Artillery and Siege Machines (Book 10)

When engineers talk about a machine’s “caliber,” they mean proportion. Vitruvius—architect, engineer, and veteran of Caesar’s world—committed those proportions to Latin. In Book 10 of De architectura he addressed the devices that had stalked walls from Sicily to Syria: scorpiones, ballistae, battering rams, helepoleis, and covered sheds. He wrote so a builder in Lugdunum could make a catapult to the same song as a builder in Antioch [5, 37].

The key was tying every dimension to a constant—the missile itself. He specifies, for example, that the circular hole for the skein box—the twisted sinew bundle that drives the arms—should be a fixed fraction of the bolt’s length, often a ninth. From that governing ratio flow the arm lengths, frame dimensions, and the spacing of metal fittings. This makes a field piece that can be scaled up or down without guesswork. A ballista designed for a 300‑mm bolt obeys the same geometry as one sized for 600‑mm bolts; only the parts change [5, 37].

Then comes tuning. Vitruvius advises that builders “wedge tight” the torsion bundles with an ear for pitch, matching the sound from left and right skeins so that the arms’ rebound forces remain symmetrical. You can hear the workshop in that line—the squeal of sinew as wedges turn, the tap of mallets, the shouted “enough” when both sides sing together. Symmetry by sound becomes accuracy in flight [37].

He also describes the lineage and function of other classic tools. The battering ram hangs like a balance pan from a frame, its iron head—Ammianus would later call it a “long, hard iron”—driven forward in rhythmic strokes to shake masonry keys loose. Towers overtop walls so that slingers and light scorpiones can scour parapets and screen sappers. Covered sheds shelter men digging at the foot of a wall or filling a ditch. Each device plays a part in a choreography that begins with approach and ends with breach [5, 7].

Vitruvius writes as a consolidator more than an inventor. He captures a tradition running back through Syracuse’s defenses and Macedonian siegecraft, then trims it into rules that any literate officer could consult. Where Archimedes matched engines to range at Syracuse by practice, Vitruvius gives the math that makes such matching replicable across legions [2, 5].

The text’s lasting power lies in how it turns vague art into checklist. Bring sinew, bronze, hardwood, iron fittings. Cut skein holes to one‑ninth bolt length. Tune by ear. Brace frames with diagonals. Keep spare arms and wedges. Write it down so the next cohort’s engineers can do it again on the Volturnus or the Orontes. The machines become a language that every Roman army learns to speak.