The explosion of a mine produces a com pression of the surrounding soil which will blow in any gallery within a certain radius. It also causes a pit or crater to be excavated in the surrounding soil, provided the chamber is sufficiently near the surface. A mine which does not break the surface is known as a corn ouflet. In a normal mine, the depth of the charge beneath the surface is equal to the radius of the crater at the surface of the ground.
- Mines dug from the surface are known as land mines, or if the crater is filled with stones, etc., so as to form a crude mortar, as fou gasses. Land mines are only permissible in ground obviously prepared for defense. They may be set off by devices under the control of some operator, or automatically by triggers or other such means.
The tactics of mining depend on the opposi tion of mines by countermines. It is always necessary to supplement one's own effort to attain his objective by an effort to frustrate the plans of the enemy. The only way by which re liable information of the progress of the enemy can be attained is by listening either directly, or with the aid of microphones. Men are
trained for this purpose, and stationed in galleries called listeners, placed sufficiently near to one another to prevent a hostile gallery from being pushed between them without being heard.
It is generally better in mining operations to come into actual contact with the counter mines than to fire too soon. The best place to attack a hostile gallery is from the side, as then a greater length of gallery is destroyed.
Mines are sometimes used to form craters to take the place of trenches in an advance. A row of such craters serves not only as a parallel on the surface but as a starting-point for new mines.
Galleries are usually started as near the hostile lines as possible, i.e., from advanced saps. In fortresses concrete lined permanent galleries often serve as a beginning for counter mine galleries in case of siege. Consult 'Engi neer Field Manual' (Washington 1912).