Mines, MILITARY, are underground passages by means of which explosives are lodged in such a position as to destroy the enemy's works or to gain cover for lodgments from which the besieger may continue his advance. They are generally but a few feet below the surface, never driven through rock for any distance, and require lining with wooden cases.
Before the invention of gunpowder, mines were constantly used to effect an entrance into a besieged place or to breach its walls by underpinning them with timber, which was then set on fire. By means of them Alexander the Great breached the walls of Gaza. Cæsar found the Gauls skilled in their use, and even able to arrest the progress of his own miners. Powder mines began to be largely used early in the 16th century. At the siege of Padua in 1509 the breaches were ruined and the mines exploded under the feet of the storming parties. Countermines, too, behind the walls near their base, and then long galleries from them under the ditch with transversals and listeners were added to the permanent fortifications of the 17th century. These enabled the besieged to ascertain the direction of the besiegers' mining approach and to destroy his galleries by timely explosions. Thus at the siege of Candia by the Turks, which lasted two years and a half (1667-69), more than 1300 mines were exploded by one side or the other. Systems of countermines became very elaborate during the 18th century, radiating from the salients sometimes in several tiers; the length of the galleries for a single front in some cases amounted to 3 miles or more. Modern systems are perhaps less elaborate, but their importance is fully recognised.
When the besieger can no longer advance by surface approaches he has recourse to mining, sinking a vertical shaft ( inside measurement) or an inclined gallery ( or ), generally from the third parallel, and from it working his way forwards. Branch-galleries () are broken out wherever necessary, and specially constructed frames fitted throughout as the work progresses. In such a confined space only very small tools can be used, and only one man can work at the head of a gallery at one time, doing about 12 inches an hour. Small trucks and bellows, or other ventilating appliances, are also necessary. At the head of the gallery a chamber is constructed to hold the charge, which is then tamped—that is to say, the gallery in rear is filled with earth, for a distance greater than the length of the line of least resistance, or distance to the nearest surface, and fired by electricity, powder hose, or Beckford's fuse. If the circular opening made by the explosion has a diameter equal to the line of least resistance it is called a one-lined crater; if double that line, a two-lined crater, and so on. Mines producing two-lined craters are called common mines; those so lightly charged as to produce no craters are called camoufls. The latter are used by the defender to destroy the assailant's mines without forming craters which he might utilise as lodgments. Gunpowder is preferred to other explosives if the gallery is to be used again, as the fumes are not so noxious.
Mining tactics require very great coolness, judgment, and resolution, especially on the part of the besieged. He must from his listening galleries estimate the distance of the enemy and avoid exploding his countermines too soon or he will only injure his own galleries. If he ceases to hear the miner's truck running in the enemy's gallery he will know that tamping has commenced and that, if within range, the time has come to explode his camoufl. The defender is restricted to small charges for fear of making craters, unless his countermine galleries are very deep, while the besieger can advantageously use very large ones. Thus the former may be said to fight with a short-range weapon against an adversary using one which is effective at a greater distance. The chief point in his favour is that he can prepare beforehand a network of galleries, and by using boring tools he can place charges some distance in advance of their heads; but the besieger will ascertain their disposition and extent by means of plans or spies, and will place heavily charged mines on a line as nearly as possible parallel to the ends of the countermines, but not nearer than 14 yards, the distance at which work is audible to the enemy. These are fired and lodgments formed in their craters from which the same tactics are repeated; the countermines when broken into are occupied, and thus ground is gradually gained, and the defender driven back step by step until the counterscarp is reached. This wall is broken through by a mine, the ditch crossed, the breach reached and occupied. Under the breach the defender will have placed mines which he will spring at the moment of assault.
Somewhat akin to these latter are the ground torpedoes placed in front of a work close under the surface of the ground over which the enemy must pass to the assault, and fired by the pressure of his weight upon them. These are chiefly of use against savages, and were largely employed by General Gordon in the defence of Khartoum.
Another similar form of defence called a fougasse is an excavation in the form of the frustum of a cone with its axis inclined at about with the horizon. The charge is placed in a recess at the bottom, covered with a strong wooden platform on which rough stones, bricks, or shells are placed. On being fired these are projected forward and cover a large surface of the ground in front. Eighty pounds of powder would throw 5 tons of bricks and stones over a space 160 yards long and 120 broad. A shell fougasse is simply a box buried in the ground, the lower part filled with powder and the upper with shells. It is generally self-exploding, like the ground torpedo.
Submarine mines are charges of explosive material (usually gun-cotton or dynamite) sunk in rivers, estuaries, or roadsteads to prevent the passage of hostile ships. They are either observation mines, fired by electricity from an observing station; controlled electro-contact mines, fired by the defender when a vessel striking them gives notice of its being over them; uncontrolled mines, mechanical, electro-mechanical, or chemical, which are exploded when struck with adequate force by friend or foe; or dormant mines, which, sunk at first, can at any time be caused to rise and obstruct the passage.
Submarine mines are usually placed chequerwise in several rows and groups and often at varying depths. Observation mines cannot be relied upon at greater distances than one sea mile in foggy weather, and at night must be watched by electric search-lights. With contact mines there is the danger of friendly ships mistaking the channel. All mine fields must be defended by heavy guns placed in batteries secure against landing parties, otherwise the enemy's boats will creep for the mines or clear a passage through them by exploding countermines. It is so difficult to arrange a system of mines which will not interfere with the passage of friendly ships and yet can be relied upon to prevent that of an enemy's vessels at all times and in all weathers, however disguised, that they are only unobjectionable when used for the absolute closing of comparatively narrow channels to all traffic.
Some sea-forts have torpedo passages from which Whitehead fish torpedoes and similar projectiles can be discharged; but these can hardly be classed as submarine mines.