10I shall now proceed to an explanation of those instruments which have been invented for defence from danger, and for the purposes of self-preservation; I mean the construction of scorpions, catapultæ, and balistæ, and their proportions. And first of catapultæ and scorpions. Their proportions depend on the length of the arrow which the instrument is to throw, a ninth part of whose length is assigned for the sizes of the holes in the capitals through which the cords are stretched, that retain the arms of the catapultæ.
2The height and width of the holes in the capital are thus fashioned. The plates (tabulæ) which are at the top and bottom of the capital, and which are called parallels (paralleli) are equal in thickness to one hole, in width to one and three quarters, and at their extremities to one hole and a half. The side posts (parastatæ) right and left, exclusive of the tenons four holes high and five thick, the tenons three quarters of a hole. From the hole to the middle post also three quarters of a hole, the width of the middle post one hole and a quarter, its thickness one hole.
3The space wherein the arrow is placed in the middle of the post, the fourth part of a hole. The four angle pieces which appear on the sides and front, are strengthened with iron hoops fastened with copper or iron nails. The length of the channel which is called στρὶξ in Greek, is nineteen holes. That of the slips (regulæ) which lie on the right and left of the channel, and which some persons call buckle, is also nineteen holes, their height and width half a hole. Two other slips are fixed for attaching the windlass, three holes long and half a hole wide. The thickness of a slip is called camillum, or according to others the dove-tailed box, and is of the dimension of one hole, its height half a hole. The length of the windlass is eight holes and an eighth. The roller nine holes wide.
4The length of the epitoxis is three quarters of a hole, and its thickness one quarter. The chelo or manucla is three holes long, its length and thickness three quarters of a hole. The length of the bottom of the channel sixteen holes, its width and thickness each three quarters of a hole. The small column (columella) with its base near the ground eight holes, the breadth of the plinth in which the small column is fixed three quarters of a hole, its thickness three twelfths. The length of the small column up to the tenon twelve holes; three quarters of a hole wide, and five-sixths of a hole thick. The three braces are nine holes long, half a hole wide, and a sixth of a hole thick; the length of the tenon one hole. The length of the head of the small column is one hole and three quarters. The width of the fore-piece (antefixa) is three eighths of a hole, its thickness one hole.
5The smaller back column, which in Greek is called ἀντίβασις, is eight holes long, one hole and a half wide, and three twelfths of a hole thick. The base (subjectio) is twelve holes, and its breadth and thickness the same as that of the smaller column. The chelonium or pillow as it is called, over the smaller column, two holes and a half; also two holes and a half high, and one hole and three quarters wide. The mortices (carchesia) in the axles are two holes and a half; their thickness also two holes and a half, and their width one hole and a half. The length of the transverse pieces with the tenons is ten holes, their width one hole and a half, their thickness ten holes. The length of the arm is seven holes, its thickness at bottom three twelfths, and at top half a hole. The curve part eight holes.
6All these proportions are appropriate; some, however, add to them, and some diminish them; for if the capitals are higher than the width, in which case they are called anatona, the arms are shortened: so that the tone being weakened by the height of the capital, the shortness of the arm may make the stroke more powerful. If the height of the capital be less, in which case it is called catatonum, the arms must be longer, that they may be the more easily drawn to, on account of the greater purchase; for as a lever four feet long raises a weight by the assistance of four men, if it be eight feet long, two men will raise the weight; in like manner arms that are longer are more easily drawn to than those that are shorter.