It will be seen that this engine is almost identical in construction with the one last described. ([Fig. 13], p. 21.)
The difference is that it propelled a stone ball instead of a large arrow.
The ball was driven along a square wooden trough, one-third of the diameter of the ball being enclosed by the sides of the trough so as to keep the missile in a true direction after the bow-string was released.
The bow-string was in the form of a broad band, with an enlargement at its centre against which the ball rested.
The description given of the mechanism and management of the engine for throwing arrows can be applied to the construction and manipulation of this form of balista, which was also made of large and small dimensions.
Small engines, with arms about 2 ft. in length and skeins of cord about 4 in. in diameter, such as those I have built for experiment, will send a stone ball, 1 lb. in weight, from 300 to 350 yards.
There is little doubt that the large stone-throwing balista of the Greeks and Romans was able to project a circular stone, of 6 to 8 lbs. weight, to a distance of from 450 to 500 yards.[11]
[11] The balls used by the ancients in their catapults and balistas were often formed of heavy pebbles inclosed in baked clay, the reason being that balls made in this way shattered on falling and hence could not be shot back by the engines of the enemy. The balistas for throwing arrows, and those employed for casting stones, were fitted with axles and wheels when constructed for use in field warfare.
Fig. 16.—The Sliding Trough of the Stone-throwing Balista.