The main galleries (those running out in a straight line from the counterscarp gallery _e_ to three of the points _a_) fall gently to the front to a depth of 30 or 40 ft. below the surface--the deeper they are the less they will suffer from the enemy's mines. Branch galleries (marked _c b + d c_) run obliquely upward from them to right and to left, leading to the mines, which are placed at various depths, according to circumstances. Two main points must be observed in any system of countermines: the branch galleries must run obliquely forward, so as not to present their sides to the action of the enemy's mines; and the distance between the ends of the branches from adjacent main galleries should be such that the enemy cannot pass between them unheard. This distance will vary with the nature of the soil, but may be taken roughly as 20 yds. A convenient size for main galleries is 6 ft. high by 3 ft. wide: branch galleries may be 5 ft. by 3 ft. When the enemy is approaching, other branch galleries, called _listeners_, will be pushed out from main and branch galleries. The section to fig. 1 of fig. 72 shows openings left for the purpose. Another use of mines in defence is in connexion with breaches. A permanent arrangement for this purpose, by General Dufour, is shown in fig. 72. Yet another use, on which much ingenuity was expended in the 18th century, is to extemporize retrenchments. Different kinds of mines. The charges of mines depend of course upon the effect which is desired. When the charge is strong enough to produce a crater, the radius of the circular opening on the surface of the ground is called the _radius of the crater_. The line drawn from the centre of the charge to the nearest surface, which is expressed in feet, is called the _line of least resistance_ (L.L.R.). When a mine produces a crater the diameter of which is equal to the line of least resistance, it is called a one-lined crater; when the diameter is double the L.L.R., a _two-lined crater_ and so on. _Common mines_ are those which produce a two-lined crater. _Over-charged mines_ produce craters greater than two-lined, and _undercharged mines_ less. A _camouflet_ does not produce a crater; it is used when the object is to destroy an enemy's gallery without breaking the surface. Fig. 73 shows sections of the different kinds of mines, with their craters and the effect they will produce downwards and horizontally in ordinary earth. [Illustration: Action of a Common Mine Probable spheroids of rupture for overcharged Mines From _Instructions in Military Engineering_, by permission of the Controller of H.M. Stationery Office. FIG. 73.--Mines.] Consideration of this figure will show that it is possible to place a long charge at such a depth below the surface that it will destroy all galleries of the enemy within a considerable radius, without much disturbing the surface of the ground.