mechanism for forcefully propelling stones, spears, or other projectiles, in use mainly as a military weapon since ancient times. The ancient Greeks and Romans used a heavy crossbowlike weapon known as a ballista to shoot arrows and darts as well as stones at enemy soldiers. The term catapult too can refer to these weapons, but more often it designates a larger engine that is used to hurl stones from a single long arm swinging through the vertical plane. Nearly all catapults employed in ancient and medieval artillery operated by a sudden release of tension on bent wooden beams or of torsion in twisted cords of horsehair, gut, sinew, or other fibres. An exception was the medieval trebuchet, powered by gravity. In this formidable weapon, the long end of an arm on a pivot was hauled or winched down and then released, allowing a heavy counterweight at the short opposite end of the arm to drop and swing the long end upward through a vertical arc. Modern mechanisms using hydraulic pressure, tension, or other force to launch gliders, aircraft, or missiles are also called catapults.