Part 160

3d. Works thrown up for the defence of a defile, should always be within musquet shot of it, which must not be more than 200 yards.

4th. The best defence in works that are flanked, or where one side is defended by the fire of another, is that formed by right angles.

5th. A saliant angle should never be less than 60, and a re-entering angle than 90 degrees; nor greater than 120 degrees.

6th. The entrance to the work should always be made in the part least exposed to attack, and if possible in a re-entering angle.

7th. Endeavor to present, if possible, a larger front to the enemy than he can occupy in making the attack.

8th. Avoid all ground commanded by an eminence, either in front, flank, or rear.

9th. Never leave the rear of a work so exposed that the enemy may turn it.

10th. Always make the angles of a work in the directions least exposed to attacks, and consequently always present a front to the most exposed.

11th. The garrison should never be drawn up more than two deep; and an ordinary pace of two feet is usually allowed for each file, and from 6 to 8 paces from each piece of ordnance.

12th. If a work is so large as to be defended by a battalion or two, a reserve should be allowed of about one sixth of the number.

13th. The space within a work should always be sufficient for the men to move and lie down. Every soldier will require at least 18 square feet, and every field gun at least 216 square feet.

14th. Provided the line is not made too extensive, the more inward space there is the better.

15th. A parapet to resist cannon shot should never be less than 12 feet thick; and for musquet shot not less than 6 feet.

16th. The height of the parapet must be regulated by the situation of the work, and of the adjoining ground; with this consideration, that its height above the banquette does not exceed 4¹⁄₂ feet.

17th. The depth and breadth of the ditch must be regulated by the quantity of earth required for the parapet and banquette.

18th. A tête de pont, or work to cover the embarkation of troops, or the passage of a river, should, if possible, be made where the line of the river or coast forms a kind of re-entering angle; that the flanks of the corps, as well as those of the works, may be covered.

To carry on the work.--The number of workmen must be proportioned to the time allotted for carrying on the work, the quantity of labor, and the number of hands capable of being employed at the same time. When the ditches are broad, the workmen may be posted in two rows; but if narrow, only in one. In the first case, the earth will be thrown by those who are on the outward edge of the ditch to the second row, and by them upon the parapet; for which reason the second row, to keep pace with the first, ought to be twice as numerous. The workmen should never be placed nearer than 2 paces, or 4 feet, from each other; and two men with shovels should be preceded by one with a pickaxe. If more than usual expedition be required, one man with a wheelbarrow, or basket, may be added to six or eight with shovels. Another row of workmen should also be placed upon the parapet, to spread the earth and beat it down, as it is thrown up.

In fixing the fascines, three men will be sufficient for every 24 feet of the work, who should be provided with mallets, a saw, and a handbil, or hatchet.

In order to form some idea of the time in which a field work may be completed, compute the number of cubic feet of earth to be excavated, thus; multiply half the sum of the breadth of the ditch at top and at bottom, by the depth, for the number of square feet in the profile; and this multiplied by the distance between the workmen in feet will give the number of cubic feet each man has to dig: or being multiplied by the length of the ditch, gives the cubic contents of the ditch. Now one man is supposed to be able to move 216 cubic feet of earth in a day, during the summer; but this is not always the case. If a field work be completed in 24 hours, it will be as much as the most diligent workmen are capable of. This time is generally allowed for the formation of a weak profile; 48 hours for that of a stronger, with a revetement of fascines; and 72 for the strongest.

The different slopes for the works must depend upon the nature of the soil, and the materials of which the work is composed. The interior slope of the parapet, though it be fascined, should be ¹⁄₆ of its height; exterior about ²⁄₃ its height. The slope of the banquette equal to its height. The slope of the scarp or counterscarp of the ditch, should be from half its height to its full height, according to the soil. The superior slope of the parapets must entirely depend upon the situation of the work, and that of the surrounding country. The interior slope of the parapet is generally lined with fascines, to keep up the earth; but it is not absolutely necessary to fascine the exterior slope, if the soil be pretty stiff. The embrasures are generally made 20 inches wide on the inside, and 9 feet on the outside; they must always be lined with something to retain the earth; turf is generally preferred, as fascines are so apt to take fire.

The manner of making the materials for field works, may be seen under the heads Fascines, Gabions, Hurdles, &c. and the manner of estimating the quantity of materials for works of this kind, may be seen under the word Battery. See Am. Mil. Lib.

FORTIFICATION....Permanent.

A parapet, to resist cannon should never be less than 18 feet thick in earth, and 8 or 9 in masonry. A wall need only be two feet thick in masonry to resist musquetry. The parapet should always be 4¹⁄₂ feet above the banquette, and 7¹⁄₂ or 8 feet above the rampart, or terre-plein.

The Rampart should always be sufficiently wide to allow for the platform, and for two carriages passing each other; about 9 fathoms at top. A parapet of earth, though it takes more room, is always preferable to one of masonry, when it can be raised; though the only objection to the masonry, is the number of splinters it produces.

Entire Revetements of masonry are not advantageous for the same reason. The masonry of revetments should not be so high as to be seen or battered from a distance; earth parapets are battered in vain, as the earth forms a natural slope.

The best Scarp is made of masonry, either in wet or dry ditches, be the earthen one ever so well fraized or palisaded. The earthen one may be stormed without making a breach. The scarp should be 30 or 35 feet high.

The Counterscarp should also be of masonry, and not less than 12 feet high. The inconveniences of an earth or low counterscarp, are the impossibility of defending to the last the covert way; as the enemy may descend into the ditch, and again mount the covert way, and so get in the rear of the traverses. The enemy may find his way along the natural slope of an earth counterscarp, and is not delayed by a tedious operation of getting into the ditch. Besides the natural slope of the end of an earth traverse prevents its effectually covering the covert way.

Ditches are generally 15 or 18 toises wide. Dry ditches are always preferable to wet ones, on account of the shelter they afford the troops, and the ready communication with the outworks, without the constant trouble and danger of bridges.