KK. The hollows for the lower tenons of the two sloping supports which prevent the uprights, and the cross-beam between them, from giving way when the arm recoils. (Fig. 6, p. 12.) [Illustration: FIG. 9.--ONE OF THE PAIR OF WINCHES OF A CATAPULT. Scale: 1/16 in. = 1 in.] I. Surface view of one of the winches and of the thick iron plate in which the socket of the large winding wheel of the winch revolves. II. View of a winch (from above) as fitted into one of the sides of the frame of the catapult. One end of the twisted skein may be seen turned round the cross-bar of the large wheel. III. Side view of the large wheel of a winch. IV. The cross-bar of one of the large wheels. These pieces fit like wedges into tapering slots cut down the barrels, or inside surfaces, of their respective wheels. V. Perspective view of the wheels of a winch. The winches are the vital parts of the catapult as they generate its projectile power. [Page 18] They are employed to twist tightly the skein of cord between which the butt-end of the arm of the engine is placed. The cord composing the skein is stretched to and fro across and through the sides of the catapult, and alternately through the insides of the large wheels and over their cross-bars; as shown in fig. 8, p. 16. FIG. 10. THE IRON SLIP-HOOK. [Illustration: FIG. 10.] This simple contrivance not only pulled down the arm of a catapult but was also the means of setting it free. However great the strain on the slip-hook, it will, if properly shaped, easily effect the release of the arm. The trajectory of the missile can be regulated by this form of release, as the longer the distance the arm is pulled down the higher the angle at which the projectile is thrown. On the other hand, the shorter the distance the arm is drawn back, the lower the trajectory of its missile. The slip-hook will release the arm of the engine at any moment, whether it is fully or only partially wound down by the windlass. The slip-hook of the large catapult shown in fig. 6, p. 12, has a handle, _i.e._ lever, 10 inches long, the point of the hook, which passes through the eye-bolt secured to the arm, being one inch in diameter. [Illustration: FIG. 11.--A SPRING ENGINE WITH A SLING ATTACHED TO ITS ARM, WHICH CAST TWO STONES AT THE SAME TIME. _From ‘Il Codice Atlantico,’ Leonardo da Vinci._ 1445–1520.] [Page 19] [Illustration: FIG. 12.--THE SKEIN OF CORD.] A. The skein as first wound over the cross-bars of the large wheels (shown in section) of the winches. B. The skein with the butt-end of the arm (shown in section) placed between its halves. C. The skein as it appears when tightly twisted up by the winches. Compare with AA, fig. 8, p. 16.