It consisted of a wheeled platform and a fireproof double-pitched roof padded to absorb the blows from enemy missiles. On its floor, there was a grooved timber mounted on supporters (which had cylindrical rollers on the lower part), a horizontal winch behind and two pulleys in front. A wooden beam with a metal head was suitably fastened with ropes which started at the rear, ran around the pulleys, went around the drum of the winch and ended up at the front. The operators, positioned at both sides of the beam, pulled the ropes backwards and forwards causing the beam to strike the enemy wall with great force. Two more horizontal rollers at the front end of the groove insured the precise aim of the head of the reciprocating beam achieving an exceptional breach (as in modern percussion drills that operate in the same manner). If the head jammed on the enemy wall, they used an additional manually-operated winch for its detachment. The right side of the model has not been completed for better visual understanding.

SOURCES: Vitrouvius, On architecture, X, Athinaeus, On machines.