A vertical section of all intrenchments is of the same general form; the dimensions will, of course, vary with the nature of the soil, and the time and means employed in their construction.  The minimum dimensions that can be used with any considerable advantage are given in Fig. 49.

In laying out field-works advantage should be taken of all available artificial obstacles, such as hedges, walls, houses, outbuildings, &c.  A thickset hedge may be rendered defensible by throwing up against it a slight parapet of earth.  Stone fences may be employed in the same way.  Walls of masonry may be pierced with loop-holes and arranged for one or two tiers of fire.  The walls of houses are pierced in the same manner, and a projecting wooden structure, termed a machicoulis gallery, is sometimes made from the floor of the second story, to enable the assailed to fire down upon their opponents.  This arrangement is frequently employed to advantage in wooden blockhouses against a savage foe; but it is of little avail when exposed to the fire of artillery.  Some have proposed galleries of this description in permanent works of masonry, but the project is too obviously absurd to merit discussion.

In addition to the parapet of an intrenchment, a good engineer will always find time and means for constructing other artificial obstacles, such as trous-de-loup, abattis, palisades, stockades, fraises, chevaux-de-frise, crows’-feet, mines, &c.

Trous-de-loup are pits dug in the earth in the form of an inverted truncated cone, some six feet in diameter, and about the same number of feet in depth.  They are usually placed a few yards in front of the ditch, and concealed by some slight covering.

Abattis are tops and large limbs of trees arranged along the glacis of a work; the ends of the branches are lopped off and sharpened.

Palisades are stakes some eight or ten feet long, with one end fastened in the ground and the other made sharp.  They are placed in juxtaposition and connected together by horizontal riband-pieces.  This arrangement is frequently placed at the foot of the counterscarp.  When the timbers are large and the work is intended as a part of a primary defence, it is called a stockade; when the stakes are placed at the foot of the scarp, either horizontally or inclined, they receive the name of fraises.

A cheval-de-frise consists of a horizontal piece of timber armed with wooden or iron lances, which project some eight or ten feet.  It is much employed against cavalry, and on rocky soils serves as a substitute for palisades.

Crows’-feet are small wooden or iron forms filled with sharp spikes.  They are thrown, with their points upward, on ground which is to be passed over by cavalry.

Mines are sometimes used in connection with intrenchments, but more commonly in the attack and defence of permanent works.  They will be noticed further on.