Shelters.  For the protection of men not actually on duty three forms of shelters are used.  The splinter-proof is a form of light shelter whose covering affords protection only against splinters.  These are usually on the reserve line.  About 12 inches to 20 inches of earth over a roof of logs or planks will afford protection from splinters and shrapnel.  Curved sheets of iron may also be used.  The deep shelter or bomb-proof is a chamber constructed by digging from the surface and constructing a roof.  To protect against eight-inch shells the top of the chamber should be twenty feet below the surface.  Heavy beams or sections of railroad iron are laid across the roof.  Above them is a layer of earth several feet thick; then another layer of timber extending to undisturbed ground on the sides with concrete, crushed stone, metal, etc., above to make a percussion surface for exploding projectiles that penetrate the upper layer of earth.  This layer fills in the rest of the space to the level of the ground.

[Illustration:  Plate No. 20.]

Shelters should not exceed six feet in width, but can be of any length.  This will prevent the crushing in of the roof timber by the explosion of a projectile buried in the upper layer of earth.  The principles of constructing shelters will be considered under “Dugouts,” as they are similar for the two types.

Dugouts are chambers tunnelled into the ground with twenty feet or more of undisturbed earth above them.  They are used in the cover trenches and sometimes in the first line.  Enough of them must be built to shelter the garrison of each sector, allowing one man per yard of front.  They must also be built for machine gun and trench mortar detachments.

Sentries must be stationed in observation posts which command the ground in front of each dugout.  They must be connected with the dugouts by telephone or speaking tube.

Position.—­Dugouts must always be on the side of the trench toward the enemy.  This prevents flying shells from falling into the entrances.  They should connect with lateral trenches, not with boyaux, as men at the entrance obstruct traffic through the boyaux.

Entrances and exits must be well concealed, with not less than five feet of head cover.  This should be provided with a bursting layer.  All dugouts must have at least two openings, one on the opposite side of the traverse or angle from the other.  It is well to have an exit behind the parados leading to a surprise position for a machine gun and bombers.  All openings must have a sill 6 inches to 8 inches high, to prevent water from entering the dugout.

Galleries leading to dugouts should be built at an incline of 45 degrees.  Their dimensions should be 2 feet 6 inches by 6 feet.  Frames are of squared timber.  The sill and two side posts should be not less than 6 inches square, and the frames in the passageways 2 feet 6 inches by 4 feet 6 inches.  They must be placed at right angles to the slope of the gallery, with distance pieces between uprights.  In treacherous soil the frames rest on sills.  Steps in the passageway are 1 foot broad and 1 foot high.