It was a repeating straight-spring catapult which had the possibility of automatically launching arrows in succession and constituted the leading achievement of ancient Greek catapult engineering. The catapult was realised for the Rhodians. It was equipped with a turning roller that had two grooves (one length-wise and one helical) and a wooden case that held the launch arrows. Also at both sides of its case it had two pairs of pentagonal sprockets (gears) that were connected with an iron-wooden chain. A pin on each chain was connected at the same point with the slider of the catapult. The slider had a bent axle with its end entering the helical groove of the roller above. With the right rotation (by the operator of the weapon) of the handspikes at the rear sprockets the slider moved automatically forwards, the roller turned left automatically until the lengthwise groove was aligned with the corresponding opening of the arrow case and then an arrow fell into the groove of the roller. At the same time the string entered automatically into the claw of the slider and a stable pin pushed the trigger automatically and locked the claw. With the left rotation of the sprockets the slider moved automatically backwards, the roller turned right automatically until the lengthwise groove was aligned with the receiver of the slider and the arrow fell automatically into this. At the same time a stable pin pressed the trigger automatically and the claw was lifted. Then the string was released automatically and the arrow was launched. With the continuous backward and forward movement of the handspikes in this way and in minimal time the operator launched in succession the all arrows of the magazine.

SOURCES: Philon of Byzantium, Belopoietica.