Working model after Francesco di Giorgio, Ms. 197.b.21 (BML), fol. 33r
The bridge is operated by means of a crank that meshes with a toothed wheel. The wheel moves a toothed rack on the underside of the bridge, which travels between two rollers. The wheel motions are coordinated with those of a pair of ropes fastened to a chains-and-shaft frame. The bridge is fitted with a raisable protection shield.