The ancient engines were distinct from those of a later age in depending for their efficacy upon the forces of tension and torsion as compared with that of counterpoise in the middle ages. The art of preparing the sinews of animals so as to preserve their elastic powers was known to the ancients, and great bundles so treated were utilised in different ways in the various engines. Experiments on sinews, ropes of hair, and other materials at the present day have proved that loss of elasticity soon occurs, whereas we learn that such was not the fact in classical times with their special method of preparation. By fixing an endless skein in a suitable frame, stretching it tightly and then twisting the skein in the centre by means of a beam of wood, the necessary torsion was obtained; if a missile were placed upon the beam when drawn back and the beam released, the projectile would be hurled to a distance proportionate to the velocity of the arm and the weight of the missile. The principle may readily be gleaned from the accompanying diagram which exemplifies the two vertical skeins used in a portable balista for throwing arrows; by being fixed in a suitable frame an action like that of the bow could be obtained. By using immense coils of twisted sinew the nations of antiquity, and especially the Greeks, threw stones weighing 50 lbs. or more to a distance of from 400 to 500 yards, and as a general rule with marvellous accuracy, while lighter missiles are stated to have been hurled to between 700 and 800 yards. These engines received the general name of "catapults," although the Greeks generally referred to them under the term "tormentum," in reference to the twisted sinews, thongs, and hair, of which the skeins were made. Broadly speaking, catapults shot darts, arrows, and the falarica,--a long iron-headed pole; balistas projected stones or similar missiles, though the names are often interchanged by the chroniclers. Some time after the fall of the Roman empire the secret of preparing the sinews was lost. [Illustration: DIAGRAM ILLUSTRATING THE PRINCIPLE OF CONSTRUCTION IN CLASSICAL ENGINES.]