Of machines formed on the sling principle, that called Onager may be regarded as typical of all the rest. Its force entirely depended upon the torsion of a short thick rope, acting upon a lever which described an arc of a vertical circle. The lever had attached to its free extremity a sling, or sometimes it merely terminated in a spoon-shaped cavity. When bent back, it was secured by a catch or trigger, and charged with a stone. On starting the catch by a blow with a mallet, the lever described its arc of a circle with great velocity, and projected the stone to a considerable distance.