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.
10I shall next explain the symmetrical principles on which scorpiones and ballistae may be constructed, inventions devised for defence against danger, and in the interest of self-preservation.
The proportions of these engines are all computed from the given length of the arrow which the engine is intended to throw, and the size of the holes in the capitals, through which the twisted sinews that hold the arms are stretched, is one ninth of that length.