cles. These
listening posts are rifle pits with over head cover, fully protected
from fire from the rear as well as front, and loop holes for
observation and fire. They are connected with the fire trenches by
means of a covered communication or even tunnels in some cases and are
provided with some form of prompt communication with the firing
trenches by telephone, bell or other means. The communicating trench
or tunnel is provided with a strong door which may be closed to
prevent an enemy from securing access to the fire trench, in case the
lookout is surprised. Pits with trap doors are also used to prevent an
enemy from creeping up the tunnel to the fire trench.

These lookouts can give early warning of the approach of an enemy,
either for the purpose of assault or cutting through the obstacles. In
many instances they have detected mining operations of the enemy by
hearing the blows of picks under ground.

=1192. Automatic alarms.= Many automatic alarms have been used to give
warning of attack on the obstacles. These vary from the simple setting
of a pistol or rifle, which is fired when the enemy attempts to cut
through the entanglement, to intricate electrical alarms.

=1193. Searchlights.= Searchlights have been provided so that, the
instant an alarm is given the obstacles are flooded by a brilliant
light and the enemy exposed to fire.

CHAPTER XII

TRENCH AND MINE WARFARE[14]

=1194. Asphyxiating gases.= The asphyxiating gases employed may be
divided into three general classes, viz:

_Suffocating gases_, the most common of which are carbonic and
nitrogen.

_Poisonous gases_, under which head come carbon monoxide and cyanogen.

_Gases which affect the throat and bronchial tubes_, such as chlorine
and bromine. The latter class is most commonly employed.

The methods usually employed for liberating these gases are to have a
plant some distance in rear of the trenches where the gas is stored
under pressure and carried to the trenches through pipes, where it can
be liberated towards the enemy's trenches when there is a favorable
wind to carry it along; or, the gas may be carried in cylinders or
other containers and liberated at the desired points. Hand grenades or
bombs are also employed which, upon bursting, liberate the gas or in
some cases scatter acids or caustic soda. Some of these bombs contain
a chemical which when liberated affects the eyes, causing impaired
vision. The Germans employ several