There is no doubt that the engines made and used by the Romans after their conquest of Greece (B.C. 146), in the course of two or three centuries became inferior to the original machines previously constructed by the Greek artificers. Their efficiency chiefly suffered because the art of manufacturing their important parts was gradually neglected and allowed to become lost. [Page 6] [Illustration: FIG. 2.--A SIEGE. _Criticism._--The picture is open to the spectator in order that he may see both defenders and besiegers at work. The besieged have just cast a stone from a catapult. The stone is falling on the movable tower belonging to the attacking side. _From Polybius. Edition 1727._] [Page 7] For instance, how to make the skein of sinew that bestowed the very life and existence on every projectile-casting engine of the ancients. The tendons of which the sinew was composed, the animals from which it was taken, and the manner in which it was prepared, we can never learn now. Every kind of sinew, or hair or rope, with which I have experimented, either breaks or loses its elasticity in a comparatively short time, if great pressure is applied. It has then to be renewed at no small outlay of expense and trouble. Rope skeins, with which we are obliged to fit our models, cannot possibly equal in strength and above all in elasticity, skeins of animal sinew or even of hair. The formation of the arm or arms of an engine, whether it is a catapult with its single upright arm or a balista with its pair of lateral ones, is another difficulty which cannot now be overcome, for we have no idea how these arms were made to sustain the great strain they had to endure. We know that the arm of a large engine was composed of several spars of wood and lengths of thick sinew fitted longitudinally, and then bound round with broad strips of raw hide which would afterwards set nearly as hard and tight as a sheath of metal. We know this, but we do not know the secret of making a light and flexible arm of sufficient strength to bear such a strain as was formerly applied to it in a catapult or a balista. Certainly, by shaping an arm of great thickness we can produce one that will not fracture, but substance implies weight, and undue weight prevents the arm from acting with the speed requisite to cast its projectile with good effect. A heavy and ponderous arm of solid wood cannot, of course, rival in lightness and effectiveness a composite one of wood, sinew and hide. The former is necessarily inert and slow in its action of slinging a stone, while the latter would, in comparison, be as quick and lively as a steel spring. When the art of producing the perfected machines of the Greeks was lost, they were replaced by less effective contrivances.