The mine had this advantage over the sap--that the besieger, being out of sight while engaged in the former method of subterranean work, had every chance of surprising the besieged. In order to effect this, an underground gallery was dug as noiselessly as possible, and carried beneath the foundations of the ramparts. When the mine had reached the walls, these were propped up with pieces of timber, and the earth was dug away until they were supported entirely by this artificial method. Dry vine wood and other inflammable materials were then piled round the props and set on fire, so that when the timber was consumed the walls crumbled down and opened a large breach to the besiegers. Nothing then was left to the garrison but to surrender, in order to avoid the horrors of an assault and the sack of the town (Figs. 60 and 61). [Illustration: Fig. 62.--Watch-tower, lighted up with beacons and protected by dogs.--Fac-simile of a Miniature of the Fifteenth Century, from a drawing by M. Prosper Mérimée.] The only remedy possessed by a garrison against this last method of attack was to keep a good watch and to endeavour to discover the whereabouts of the mine, and neutralise it by a countermine. At the siege of Rennes, in 1356, the governor of the town ordered basins of copper, each containing several globes of the same metal, to be placed all about the ramparts; when these globes were seen to vibrate and tremble at each stroke of the hidden pickaxe, it was easy to guess that the mine was not far off. There was also a body of night watchmen, who carefully noted the enemy’s movements, and who rang the alarm-bell at the slightest noise. These watchmen were often replaced by dogs, whose barks, in case of a surprise, gave notice to the garrison (Fig. 62). [Illustration: Fig. 63.--Machine intended to break the ranks of the enemy and to crush his soldiers.--Végèce, “L’Art Militaire:” 1532.] [Illustration: Fig. 64.--Machine to shoot arrows, and to assist in approaching a besieged town.--Robert Valturin, “La Discipline Militaire:” 1555.] The slow and laborious work of the miner was often advantageously replaced by the more powerful action of certain machines, which may be divided into two distinct classes. The first, intended to be used at close quarters and to make a breach in the wall, comprised several varieties of the ancient battering-ram; the second, employed at a distance, were termed _pierriers_, _mangonneaux_, _espringales_, &c. (Figs. 63 and 64).