In the siege of any place the mining operations of the besiegers are directed to the discovery and destruction of the galleries of counter mines ; to the blowing up of any advanced works belonging to the garrison ; to the demolition of the wall of the counterscarp, in order that the descent into the ditches may be facilitated ; and, occasionally, to the formation of breaches in the principal ramparts. On the other baud, the countermines are employed by the defenders to destroy the trenches and batteries of the besiegers beyond the foot of and upon the glacis, the galleries made by the besiegers, and also the lodgments which may be made on the breaches or withim the works. It is easy to perceive, therefore, that a system of countermines must add greatly to the strength of a place, by obliging the besieger to proceed with circumspection in his approaches above ground, in order to avoid the risk of being blown up at every step ; and, according to Bousmard (• Essal eneral de Fortification'), if the glacia of a fortress be counter mined, the duration of the siege, which otherwise would have extended to one month only, may be prolonged to six weeks.
By means of mines in the attack of a fortress, the besieger has a sure and certain method of gradually, it may be slowly or it may be rapidly, according to the ground, appliances, and activity of the enemy, but still a certain and irresistible means of reaching and destroying the defences of the besieged. And this with the loss of comparatively few men. The successful explosion of a mine by the enemy may destroy a few miners, while an assault costs hundreds or thousands of men. But yet almost every exploded mine is a step in advance for tho besiegsr. Being superior in the field he seizes the crater formed by the explosion, incorporates it in his works, and pushes on from it by fresh galleries The galleries of both besieger and besieged have been destroyed in the neighbourhood of the crater by the explosion of the mine, whether thew were exploded by besieger or besieged. The besieged then pushing on again from his nearest undostroyed gallery, meets the besieger half way, unless, as occasionally happens by skill ho circumvents him, but this half way has been a clear loss to the besieged and gain to the besieger. Hence no system of countermines can render a work impregnable; but as a means of delaying the fall of a work, in fact of restoring as far as science can the equilibrium of attack and defence, it exceeds all the other means at the disposal of the besieged : it to a great extent renders the possession of superior numbers, position and material, on the part of the besieger, nugatory. The onward and irresistible advance of the besieger, rapid above ground, from the nature of the work is, though certain, slow in these under ground opemtiona. And he cannot neglect them. If the besieged have
countermines the besieger cannot refuse to meet him with the same weapons. As the French engineers Gumpertz and Le Brim say, with respect to this, "The loss of a great number of men, and the dis couragement of the',whole besieging foree,rperhaps even their defection, would inevitably fellow such a resolution. When, therefore, the front of attack is countermined, the besieger must call the miners of his army to his aid?' The great difference between civil and 'military mining is this. In civil mining the operations generally carried on at a great depth arc in solid rock, whereas in military mining they are near the surface in soft ground, which requires supporting to prevent its falling in and crushing or stiffing the miner. The principal portion of the art of the military miner consists thus in the management of these supports or linings. In permanent systems of countermines, that is, in fortresses furnished with defensive mines as a portion of their defence, the galleries, being made to last for years, are lined with masonry.
Many different systems or plans for the arrangement of countermines have been suggested by the different engineers who have written on the subject. These, though varying much in many details having the same object in view, are to a great extent similar. They are disposed over the exposed portions of the front of fortification, and where an enemy would be likely to form his trenches, lodgments, and breaching batteries, that is, round the ravelins under the Oasis, and in front of the bastions in the main ditch and also in the rampart. The object to be kept in view is to expose as little as possible of the galleries to the destructive effect of the enemy's mines ; hence they should never, if possible, present their sides to the enemy, as the effect of a mine would be not only to destroy the gallery in its neighbourhood, but perhaps to cut off a large portion of uninjured gallery and render it useless for further defence. The galleries must therefore be disposed so as to present their ends to and lie in the direction of the enemy's approach.
At the same time they should not be so far apart that the enemy might pass between them without being heard. As the sound of a pickaxe cannot be heard beyond 60 feet underground, from 100 to 120 feet is the greatest distance they should be asunder. And on the other hand they should he close enough for the mines fired in them to cover the intermediate space with their destructive effects without injuring one another. Assuming then that the galleries are 15 feet under ground, which is considered the best depth for defensive purposes, they should for reasons which will appear from the action of the explosion explained further on, be at central intervals of 48 feet.