One would not, perhaps, expect such a machine to possess any precision of action, but according to our author the case was far otherwise. "These machines," he continues, "anciently called _trabutium_, cast their missiles with the utmost exactness, because the weight acts in a uniform manner. Their aim is so sure, that one may, so to say, hit a needle. If the gyn carries too far, it must be drawn back or loaded with a heavier stone; if the contrary, then it must be advanced or a smaller stone supplied; for without attention to the weight of the stone, one cannot hope to reach the given mark." "Others of these machines have a movable counterpoise attached to the beam, turning upon an axis. This variety the Romans called _biffa_. The third kind, which is called _tripantum_, has two weights, one fixed to the beam and the other movable round it. By this means it throws with more exactness than the _biffa_, and to a greater distance than the trebuchet. The fourth sort, in lieu of weights fixed to the beam, has a number of ropes, and is discharged by means of men pulling simultaneously at the cords. This last kind does not cast such large stones as the others, but it has the advantage that it may be more rapidly loaded and discharged than they. In using the perriers by night it is necessary to attach a lighted body to the projectile. By this means one may discover the force of the machine, and regulate the weight of the stone accordingly."[380] This, then, is the engine which our captain, repulsed in his attempt to take the place by a _coup de main_, has ordered up, adjusting it, no doubt, like a good captain, with his own eye and hand, until he has got it, "so to say, to hit a needle," on the weak points of the place. It was usual in great sieges to have several of them, so that a whole battery might be set to work to overmaster the defence. [Illustration: _The Assault._] We must bear in mind that similar engines were, it is probable, usually mounted on the towers of the castle. We should judge from the roundness of the stones which the defenders in both the preceding woodcuts are throwing down by hand upon the enemy immediately beneath, that they are the stones provided for the military engines. We find that, as in modern times cannon is set to silence the cannon of the enemy, so that a battle becomes, for a time at least, an artillery duel, so engine was set to silence engine. In the account which Guillaume des Ormes gives of his defence of the French town of Carcasonne in 1240 A.D., he says: "They set up a mangonel before our barbican, when we lost no time in opposing to it from within an excellent Turkish petrary, which played upon the mangonel and those about it, so that when they essayed to cast upon us, and saw the beam of our petrary in motion, they fled, utterly abandoning their mangonel."