Once gunpowder came on the scene, a range of new weapons appeared. By the sixteenth century, catapults and ballistae had been entirely replaced by cannons. Like ballistae, these usually have to be pre-built if they are to be at all safe to the user.
The ballista is a rock-throwing artillery engine powered by two vertical bundles of horsehair or sinew rope which are under very high tension and are also twisted.
The two separate arms of the ballista are each suspended in the center of each cord bundle. The outer ends of each arm are connected to the end of a sling which is used to propel the stone down the channel and out the front of the engine. This arrangement looks similar in geometry to a crossbow.
The release mechanism consists of a single, pivoting hook which releases the cord ring pleated into the center of the sling when a lever is pulled from beneath the rear end of the hook. The slider houses the release mechanism, and forms the channel that the rock ride on when launched. It can slide forward to allow the mechanism to be locked onto the sling. When the release hook is secured to the sling, the slide is drawn back using a winch manned by two crewmen.
Pawls connected to each side of the slider prevent the slider from moving back if the tension on the winch is released. When the slider is pulled back far enough, the rock is placed in front of the sling to load the engine. The ballista is aimed entirely by sight by an experienced artilleryman.
The trigger line is pulled and the sling is released, propelling the rock. The ballista is designed to be portable, and can be quickly assembled and deployed. It is used as a countersiege weapon to destroy enemy siege engines. The average ballista threw a thirty pound stone and was used as far back as the time of Alexander the Great. The largest Roman ballistae threw rocks weighing as much as one hundred and seventy-two pounds and were used primarily for smashing fortress walls.