named from its shape, is a machine about which there is but little reliable information; but what there is indicates it to have been a huge crossbow, the bowstring being bent on the cog principle. THE CATAPULTA AND BALLISTA. Their prototype was the “tormentum” of the Romans.[42] The two machines are often confounded with each other. The catapulta was used for throwing heavy darts, while the ancient ballista threw stones only, but the mediæval variety was often arranged for both quarrels and rocks. Some ballistæ threw stones three hundred pounds in weight. The difference in the construction of these military engines from those made on the ordinary principle of the bow consisted in the addition of a mechanical force. There were also small catapultæ used like hand-guns. Remains of ballistæ were found among the _débris_ of the castle of Russikon in Switzerland, which was burnt down in the thirteenth century. Vitruvius and other writers give a full account of these machines, but the copyist, as has usually happened in all ages, made such mistakes as to render the descriptions well-nigh unintelligible, so there is still a good deal of uncertainty concerning them. In chronicles of the twelfth century crossbows are always termed “ballistæ.” The principle applied in the ballista was that of the bow, but instead of the usual arc, with its simple directive force, a kind of double action was achieved by providing the machine with a strong rectangular frame of wood, constructed in three compartments, firmly fixed on to a stand, which was made of strong and hard wood, consisting of two uprights connected horizontally by a double crossbeam. Instead then of applying the entire arc, as in the crossbow or scorpion, and assuming such arc or bow to have been divided into four quarters, only the two end quarters were used; and in each of the outer compartments of the frame two very strong strands of twisted sinews were fixed, and through these the inner ends of the two pieces were firmly held, the bending of which gave much more elasticity and propulsive force, thus producing a recoil strong and forcible enough to project heavy missiles to a distance of as far as 250 yards. The engine was fitted with an iron groove. In sighting the machine for the discharge of a heavy stone, pieces of clay were used to keep the projectile at the necessary angle before discharge. There are four stone shot at Woolwich 15, 16, and 18 inches in diameter, supposed to be catapult balls. The above explanation will make apparent how very difficult it is to describe even the simplest machine in mere language; besides, you have the difficulties of translation to contend with. Fig. 48, from a MS. in the National Library of Paris, No. 17,339, explains the principle at a glance.