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 moveable counterpoise attached to the beam, turning upon an axis. This variety was by the Romans named _biffa_. The third kind, which is called _tripantum_, has two weights: one fixed to the beam and the other moveable around 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[409]." The trebuchet arranged with cords is represented in the treatise _Du feu grégeois_ noticed above, and in the _Études sur l'Artillerie_, vol. ii. Pl. III. Those familiar with the sights of the Thames will not fail to be struck with the curious resemblance between this ancient engine of warfare and the apparatus by which a gang of colliers raise the cargo from the hold of their ships. Matthew Paris mentions the plying by day and by night of the terrible trebuchet. Under 1246, he gives us the letter of Master Walter de Ocra, a clerk of the Emperor, to the king of England, recounting the events of the Italian campaign: "About eight days before the end of last July, my Lord laid siege to the Castle of Capaccio, in which were (certain knights) traitors to him, and who had attempted his life, with a hundred and fifty others, including knights, cross-bowmen, and other friends of theirs; all of whom my said Lord, by uninterrupted discharges of missiles, day and night, from seven well-ordered Trebuchets, and by vigorous and unceasing assaults, also made night and day, reduced to such a helpless state that they could not assist one another[410]." The castle was finally taken and destroyed, the garrison punished by loss of eye-sight and other mutilations; and the six leaders who had attempted the life of the Emperor, having partaken the punishment of their comrades, were by the imperial order "sent to all the kings and princes throughout the various countries of the world, with the impression of the papal bull, which was found there, stamped on their foreheads, to give public notice of their treachery."