II.

Siege Engines

The catapult was a special type of siege engine, a large device used to attack a fortress or a city. Catapults were used in ancient and medieval warfare until the introduction of the gunpowder cannon in the 14th century. A catapult could hurl large stones, spears, or other projectiles at an enemy, but was difficult to aim. A catapult was sometimes mounted on a wheeled cart, and as an army changed positions, the catapult could be moved accordingly. Some catapults were mounted permanently within fortresses and used for defense against attackers. Many different types of catapults were invented and used. Three of the most common were the ballista, the mangonel, and the trebuchet. See also Fortification and Siege Warfare.

The ballista and the mangonel used the energy created by twisting ropes to create torsion. The sudden release of this energy was used to launch large rocks, arrows, or flaming balls at an enemy. The earliest catapult was the ballista. It resembled a crossbow and originated in Greece in the 3rd century bc. Engineers working for Philip II of Macedonia, father of Alexander the Great, most likely invented the ballista. A ballista used two vertical groups of ropes that were twisted over and over again to create torsion. Wooden arms were inserted horizontally into the twisted ropes. A single cord attached to both arms pulled them back against the force of the twisted ropes, much like a crossbow as it is being cocked. A spear was placed in a pocket on the cord. When the cord was released, the twisted ropes pulled the wooden arms forward, thrusting the spear toward the enemy. The distance varied depending on how much and how tightly the ropes had been twisted.

The Roman mangonel also used twisted ropes to provide energy, but the mangonel had only one arm. The twisted ropes were stretched horizontally, and the arm was inserted vertically into the ropes. A scoop fitted on the end of the arm held a rock. The arm was pulled down against the force of the twisted ropes, loaded with a rock, and released. The arm flung forward and as it hit a wooden barrier on the mangonel, the rock would fly toward the enemy. The mangonel was less complicated than the ballista, but much energy was wasted as the arm collided with the wooden barrier.

Trebuchets, unlike mangonels and ballistas, did not derive their power from torsion. Trebuchets relied either on muscle power or gravity to hurl objects. A trebuchet was essentially a long wooden arm resting on a pivot point, and acted as a large class 1 lever. A stone would be placed on one end of the trebuchet, and soldiers would pull on ropes attached to the other end to swing the arm around and hurl the stone. Later designs used a counterweight on one end, rather than muscle power, to provide energy. Soldiers would pull the launching end down against the weight of the counterweight, load the projectile, and release the arm. The counterweight would pivot the arm around to launch the projectile.