But Belisarius placed upon the towers engines which they call "ballistae". Now these engines have the form of a bow, but on the under side of them a grooved wooden shaft projects; this shaft is so fitted to the bow that it is free to move, and rests upon a straight iron bed. So when men wish to shoot at the enemy with this, they make the parts of the bow which form the ends bend toward one another by means of a short rope fastened to them, and they place in the grooved shaft the arrow, which is about one half the length of the ordinary missiles which they shoot from bows, but about four times as wide...but the missile is discharged from the shaft, and with such force that it attains the distance of not less than two bow-shots, and that, when it hits a tree or a rock, it pierces it easily. Such is the engine which bears this name, being so called because it shoots with very great force..."