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