The height of the interior crest above the ground within the work must not be less than 6 ft. 6 in.We assume it ordinarily to be 8 ft., and limit its greatest height to 12 ft., owing to the difficulty of throwing up a work with this relief in a reasonable length of time, with the ordinary means at hand. Works of a greater command than this have been constructed; the length of time consumed in building these, their importance, and the many interior arrangements devised for the safety and comfort of the troops using them, cause them to approach more nearly in detail works of a permanent character; and they have been called for these reasons semipermanent works. The ditch that surrounds the work affords the earth for the parapet, and should be wide and deep enough (not less than 12 and 6 ft.) to form a considerable obstacle to the assailants. The slopes are the same as given for the permanent work, viz.: superior slope, 1/1 and exterior slope, -}-; and interior slope, 3/1. The sides of the ditch are not ordinarily revetted; if so, however, it is by some material of a perishable nature, as plank, timber, etc. The interior slope is revetted by sods, or some material hastily gathered for the purpose, as logs, boards, fascines, gabions, etc.

The banquette is placed 4 ft. 3 in. below the interior crest, and is from 2 to 4 ft. wide. It is connected with the ground by a slope of 1/2, which is sometimes replaced when interior space is needed by steps whose tread is 12 in. and rise 9 in. In laying out the plan of the work the line of the interior crest is adopted as the directing line. The plans of these works vary according to their object and site. Supposing the site level or approximately so, the relief is generally uniform throughout. Such works may be classed into three kinds: 1, those that are arranged to sweep only the ground in their front; 2, those that do the same for their flanks in addition; 3, those that are arranged to tire on all sides. A work consisting of a simple straight line, or of two faces, the rear, called the gorge, being open, is an example of the first class. This is called a redan, and is used in front of a defile, at the exit from a bridge, or for the defence of an outpost, where the attack is expected only from the front. A redan with flanks, called a lunette, is an example of the second class, and is hardly ever used alone. In this case the attack is not expected from the rear, but an assault made on the flanks would be liable to work around and carry it at the rear.

Both in this case and the first, when they are used alone, the gorge is closed by palisading or a stockade. A work of a polygonal figure, without reentering angles, in plan as a square or rectangle, arranged to fire in all directions, is used when liable to be attacked on all sides. It is called a redoubt. The ditch of a redoubt cannot be swept by the fire from the work itself; therefore the bastioned trace has been sometimes used, but the great length of time required to construct it prevents its general adoption. When these works are joined together or placed along a given front with intervals, they form either a continued line or a line with intervals. A number of redans joined by curtains is a simple case of a continued line. If the line be formed entirely of redans, it is called a tenaille line. The usual plan of a line with intervals is to place lunettes and simple redoubts, or either, on a given front at distances apart from 250 to 500 yards, and in rear of them, opposite the intervals, redans or other field works, arranged to sweep by their fire the ground in front of the salients of the first line. In field works all accessory defences, both artificial and natural, should be freely used.

On irregular sites it will not bo possible to preserve the relief uniform, and the same problems for defilement will be met with as in permanent works. The only general rules that can be laid down are to lay out the principal lines so as to obtain a direct and cross fire on the approaches of the enemy, and to place them as nearly as practicable parallel to the general crests of the commanding heights, so that the enemy will have only a direct fire on them. Where the defilement would involve considerable labor in raising the interior crest to intercept the fire, traverses should be resorted to in preference.-Field works require to be simple and easy of construction. All lines and fronts should have their flanks protected by natural obstacles if possible. The relief of a work should be obtained by its position and not by the construction of embankments, and if possible the necessity of defilement should be avoided. The class of works already named will require several days and often weeks to construct. In an active campaign they are freely used, especially in the defence of defiles, the crossing of rivers, depots of supplies, intrenched camps, etc.

Anticipating the movements of the enemy, time enough may be had to finish them, or at least to put them in such condition as to render them useful for defence if an attack be made. When, however, an engagement is imminent, or has begun, in the absence of such works, resort is had to what may be called hasty in-trenchments for opposing the enemy's advance, sheltering troops, or strengthening the weak parts of a line. These are shelters that may be executed in a short time, say within an hour, certainly not more than three hours. They are not limited to defensive positions, but are used on the battle field itself. By excavating a trench 2 ft. wide and 1 1/4 ft. deep, throwing the earth to the front, building up the slope next to the trench as steep as possible by using clods of earth, sods, fallen trees, fence rails, etc, shelter will be given to two ranks, one kneeling in the trench, the other lying flat in rear of it. By placing a man at every 4 ft., the trench can be dug in 20 minutes. This trench can be widened to 4 ft. in 20 minutes more, when both ranks can enter it; in 20 minutes more it may be widened to 7 ft., increasing the thickness of the mound of earth between the men and the enemy, and allowing freedom of movement to the limbs of the soldiers.

Thus it is seen that in an hour good shelter can be obtained by active troops. These are known as shelter trenches, and upon being further strengthened form what we call rifle trenches or pits. In these the trench at bottom is 5 ft. wide and 3 ft. deep; the earth is thrown forward far enough to allow the natural surface of the ground to act as a banquette, the embankment being 4 ft. 6 in. high. These expedients have entered largely into the operations of recent wars, but in no country and at no time have they been used with greater success or more generally than in the late civil war in the United States. Whenever an army halted in the presence of the enemy they immediately began to intrench themselves, before eating or resting. The difficulty was to get them to wait until a proper line was selected. This custom has not been confined to modern times. The Roman legions intrenched their camp every night, and remains of many of their intrenched camps are yet to be seen.-The use of field works runs back to the remotest antiquity, and has ever played a most important part in the operations of war.

Prominent examples are Frederick the Great's camp at Bunzelwitz, Wellington's lines at Torres Vedras, the French lines at Weissenburg, the Austrian intrenchments in front of Vienna in 1848, and the lines of field works around Vicksburg, Nashville, Petersburg, Richmond, and, most prominent of all, the city of Washington.-For the offensive works used in siege operations, see Siege.-There are numerous works on fortification, but full information may be obtained from those of Prof. D. H. Mahan,"Military Engineering: Part I., Field Fortifications, etc.; Part II., Permanent Fortifications" (2 vols. 8vo., New York, 1865-'7), and Gen. A. von Zas-trow, Geschichte der bestandigen Befestlgung (8vo, Leipsic, 1854; translated into French, Histoire de la fortification permanente, 2 vols. 8vo, Paris, 1856).