In Chapters 1.22-4-11 of his History, Polybius describes this device as a bridge 1.2 m (4 ft) wide and 10.9 m (36 ft) long, with a small parapet on both sides. The engine was probably used in the prow of the ship, where a pole and a system of pulleys allowed the bridge to be raised and lowered. There was a heavy spike shaped as a bird's beak on the underside of the device. The spike was designed to pierce the enemy ship's deck when the boarding-bridge was lowered. This allowed a firm grip between the vessels and a route for the legionaries to cross to the other ship.