I am wondering about the firing and loading mechanism of a Roman Ballista?

Here's an article in Encarta:

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 ******. 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

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The wheel must have an "escapement" like this:

.lock will press down→ █

..........►►►►►►►►►→crank move this way

crank can go only to one direction. The lock once pressed down will stop backward motion, but allow forward motion.

You can see this link:

http://en.wikipedia.org/wiki/Ballista

To reload, the wheel and axle you see in the picture, pulls down the yoke that cradles the spear or stone, putting the levers attached to the twined ropes in a drawn position again. There is a latch that holds the cradle, and this latch could be released instantly if desired thus firing the ballista.

Ballista Trigger