A trebuchet, also sometimes called a trebucket is a medieval siege engine, a weapon employed either to batter masonry or to throw projectiles over walls. The invention of the trebuchet derives, no doubt, from the ancient sling. A variant of it, the staff sling, involved using a short piece of wood to extend the arm and provide greater leverage. A traction trebuchet functions in the same way as a counterweight trebuchet, except that instead of a hoisted weight, the hurling arm is powered by a crew of men, pulling on ropes attached to the short lever arm. A counterweight trebuchet is powered by a very heavy counterweight, acting on a lever arm. The fulcrum of the lever (usually an axle) is supported by a high frame, and the counterweight is suspended from the short arm of the lever. The sling is attached to the end of the long arm of the lever. One end of the sling is captive, while the other end is hooked to the long arm in such a way as to release when the arm and sling reach the optimal hurling angles. The trebuchet is energized by lowering the long arm and raising the weighted short arm, usually with a winch, and is locked into the charged state by a trigger mechanism. With the long arm lowered near ground level, the sling is loaded with the projectile, and laid out on the ground, with the captive and hooked ends away from the target, and the load and pouch laid on the ground toward the target. When the trigger is released, the weighted short arm is driven by gravity into an accelerating pendulum motion, causing the lighter, long arm of the lever to revolve around the fulcrum at the opposite arc, which in turn, pulls the sling and its contents into a whipping motion at the end of the long arm. As the arm continues to swing past the vertical position, the counterweight rises causing the lever motion to begin to slow down, while the sling continues to whip forward around the end of the long arm. When the sling reaches its launch angle, one end slips from its hook, releasing the projectile toward the target.