Catapults ran on one of three ideas or principals. The first was torsion. Twisting heavy cords created torsion. The cords were usually made of animal tendons, hair, and rope. A log or a spear or a rock was placed between the cords, and the cords were released, sending the object spinning at the unfortunate soldier(s) or wall(s). Using a huge bow mounted on a base, (not a ballista, but just a huge bow) they created tension by winding up or pulling back the ropes on the bow. They wound up the bow’s strings by using a windlass. The Romans fired the bow using a trigger. The last idea that catapults ran on was counterweight. The war weapon that used this idea was called the petrary. The petrary had a log attached to it, and the wood was shaped like a very big spoon. This spoon could hold boulders and balls of fire. It had a whip effect, and that made the counterweight. Soldiers would hold the spoon shaped side of the weapon down, and another soldier would put a boulder in it. The rest of the machine would be up, so it would make an arch. The weight on both ends would counter each other, and the “spoon” would whip forward, sending the boulder in it flying.
Catapults were not only used for siege when the Romans used them. The Romans, before the battle, fired their deadly contraptions at enemy soldiers to cause enough confusion to let them strike more effectively. After the Romans, however, nobody used the machines for anything except siege, and then they relied heavily upon them. They also used the catapults (mostly the ballistae) for defense. They would station the weapons on hills or behind the walls, and then they would eliminate as many of the enemy as possible before they got into the city.
Catapults were improved and changed by the Romans. Ideas from the bow and the catapult were put together to form the ballista. A rope would be put in the frame (taken from the bow) that was angled to form a “V” shape. This made the ballista more powerful than the catapult or the bow. Unfortunately for the Romans, this stronger and sturdier frame made movement a great deal harder and added weight. From the ballista came the Scorpio. The Scorpio was made out of wood and fired a 27” bolt. Because it was made out of wood, the arms of the Scorpio were easily curved, which gave a greater twist of the rope, which gave it a greater range than the ballista. The Scorpio was improved to make the Cheiroballistra. The Cheiroballistra was sturdier than the ballista and the Scorpio, but was lighter than both of them. It covered even more ground than the Scorpio with it’s shot. It was not made out of wood but of metal, so the rope springs were not as close together, thus resulting in better vision and more power. The rope springs were protected from the weather by cyndrilical sleeves. One of the best things about all of these was that they could all be constructed fast, and could be handled by just two men!