Sometimes, shafts are sunk from the ends of these listeners, and by connecting these shafts, a second envellope formed. Behind the escarps of the different works, galleries are likewise made, about the level of the bottom of the ditch; from whence branches may be run out into or under the foundations of the walls; and if the ditch be dry, galleries of communication may be made from these to the magistral gallery; and from which communications branches may be run out for chambers to annoy the besiegers in their passage of the ditch. The entrances to the escarpe galleries are by means of posterns, which descend from behind the interior slope of the rampart. If a place be not countermined before hand, a great deal may be done, even after the investment of the place, to prolong the siege by countermines. In this case, the first thing to be done immediately that the place is invested, to sink a shaft in each of the places of arms of the covert way; one in each branch of the covert way, opposite that part of the bastion where the breach will most probably be made; and one in the flanked angle of each bastion. Those on the covert way will be on the banquette, and sunk to about 18 inches below the bottom of the ditch. Those in the bastions to about 12 feet below the bottom of the ditch. Thus prepared, the moment the side on which the attack is to be made can be ascertained, galleries must be carried on from these shafts on the side attacked along the capitals, in the form of tressles; or double T; and advanced as far into the country as the time will admit. Communication galleries may likewise be driven between these different works on the covert way, and from them to the work in the bastion; which will prevent the enemy gaining possession of their entrances. All these works may be carried on after the investment of the place; and be in sufficient forwardness by the time the enemy gains the third parallel. The following rules are given by Vauban for fougasses, or small mines, having the diameter of the excavation equal to double the line of least resistance: The side of the chamber must be exactly a sixth part of the depth of the shaft. The side of the box to hold the powder exactly a ninth part of the depth of the shaft. These remarks respecting mines are principally extracted from the General Essay on Fortification before mentioned, written in French, and published at Berlin, 1799. _MORTARS._—_Weight and Dimensions of English Mortars._