9The length of the spur is one twelfth and three quarters; its width at bottom one sixteenth, and its width at top a quarter and one sixteenth. The base, which is called εσχάρα, is a ninth of a hole long; the piece in front of the base (antibasis) four holes and one ninth; the width and thickness of each are to be the ninth of a hole. The half column is a quarter of a hole high, and its width and thickness half a hole; as to its height, that need not be proportioned to the hole, but made, however, of such size as may be fit for the purpose. Of the arm the length will be six holes, its thickness at bottom half a hole; at the bottom one twelfth of a hole. I have now given those proportions of the catapultæ and balistæ, which I consider most useful; I shall not, however, omit to describe, as well as I can by writing, the manner of preparing them with cords twisted of guts and hair.
9The stays are . . . holes in length, one quarter of a hole in breadth at the bottom, and one sixth in thickness at the top. The base, termed ἑσχἁρα, has the length of . . . holes, and the anti-base of four holes; each is one hole in thickness and breadth. A supporter is jointed on, halfway up, one and one half holes in breadth and thickness. Its height bears no relation to the hole, but will be such as to be serviceable. The length of an arm is six holes, its thickness at the base two thirds of a hole, and at the end one half a hole.
I have now given those symmetrical proportions of ballistae and catapults which I thought most useful. But I shall not omit, so far as I can express it in writing, the method of stretching and tuning their strings of twisted sinew or hair.