Capacity of Field works.

The perimeter of a Field work, and the number of men to defend it, should bear a just proportion to each other, according to the nature, and object of the work: Linear measurement (on the crest of the parapet) 1 yard being allowed for each man, or for each file of men; and 5 or 6 yards for each gun: Superficial measurement (area within the banquette) 2 square yards for each man, and 36 square yards for each gun. Various authors, English and French, have published rules for determining the size of Field works, and there is a great discrepancy in their conclusions: the following rule (adopted at the Royal Military College) for computing the area, and perimeter of a square Redoubt will, however, meet the general requirements of these works.

Multiply the given number of men by 2, and the number of guns by 36, for the number of square yards which the work ought to contain within the foot of its banquette: the square root of the product will be the length, in yards, of the side of the square forming that area: add to this result, the breadth of the two interior slopes of the parapets, and of two banquettes, with their slopes (altogether about 7 yards), and the length of the side of the square, formed by the crest of the parapet, will thus be determined.

Rules.

1. To find the quantity of earth required for the Parapet, and Banquette of a field work, &c.

Divide the parapet, and banquette into trapezoids, and triangles; compute the contents of each separately (by the rules in MENSURATION OF PLANES), and the sum of them will be the superficial content of a section of the parapet, and banquette. Multiply this by the length of the perimeter, or periphery of the redoubt, battery, &c., for the solid content of the parapet, and banquette.

In square redoubts, or works having salient angles, if the areas of the sections of the parapet, and ditch, are made nearly equal, there will be too much earth, Bearing this in mind, previous to commencing the excavation of the ditch, an allowance must be made for the angles, to prevent any excess of earth for the parapet, and banquette.

2. To find, rapidly, the quantity of earth required for a Parapet, and Banquette.

Multiply the height of the crest of the parapet, into the sum of the bases of the superior, and exterior slopes; which will give the superficial content, very nearly.

3. To compute the superficial content of the Ditch.

Multiply the depth into the breadth at bottom, to which product add the areas of the escarp, and counterscarp, for the content required. 4. To find the breadth of the Ditch, of the usual form.

Divide the area of the section of the parapet by the intended depth of the ditch, and the quotient will be the mean breadth of the ditch;

to this add half the sum of the bases of the slopes of the escarp, and counterscarp, for the breadth at top, and deduct the same for the breadth at bottom.

5. To find the breadth of the Ditch, having a triangular section. Divide the area of the section of the parapet by half the given depth of the ditch, and the quotient will be the required breadth at the top.

Construction of Field works.-Vide Plate.

Fig. 1. The redan.

Draw a base line, 60 yards from the centre of which erect a perpendicular, 40 yards; join the terminations of the base, and perpendicular, which will form the crest of the parapet of the work.

Fig. 2. The lunette.

Construct a redan (vide No. 1), base 80 yards, perpendicular 50 yards: make the faces of the lunette 45 yards in length, and draw the flanks to points on the base line, 30 yards, from the perpen

dicular.

Fig. 3. The square redoubt.

Construct a square, each side 40 yards, (vide PRACTICAL GEOMETRY). To form additional faces when required, bisect the side of the square, draw perpendiculars inwards equal to th of the side, and join the termination of the perpendiculars, and the sides of the square, thus forming a double number of faces.

Fig. 4. The pentagonal redoubt.

Describe a circle, radius 30 yards, and construct a pentagon in the circle (vide PRACTICAL GEOMETRY), thus forming the crest of the parapet of the redoubt.

Fig. 5. The hexagonal redoubt.

Describe a circle, radius 30 yards, and construct a hexagon within it (vide PRACTICAL GEOMETRY); the sides of which form the crest of the parapet of the work.

Fig. 6. The circular redoubt.

Describe a circle, radius 30 yards, which will form the crest of the parapet of the redoubt.

Fig. 7. The star fort, with six points.

Construct an equilateral triangle, and divide each side, 75 yards, into three equal parts: form also an equilateral triangle on the central portion of each side, 25 yards, and the crest of the parapet of the fort will be traced.

Fig. 8. The star fort, with eight points.

Construct a square: divide each side, 60 yards, into three equal parts, and on the central portion, 20 yards, describe an equilateral triangle the periphery of the fort will thus be obtained.

Fig. 9. The field fort, with bastions, and half bastions.

Construct a square; from the centre of each side, 120 yards, drop a perpendicular equal to one-eighth of the side, through the extremity of which, from the angles of the square, draw the lines of defence; make the faces of the bastions, and half bastions, two-sevenths of the exterior side, and draw the flanks perpendicular to their respective lines of defence.

Fig. 10. The bridge head, or tête du pont.

Construct a redan, base 50 yards, perpendicular 30 yards, at an appropriate distance from the bridge, 50 yards; draw flanks, 20 yards, perpendicular to the faces, and from their termination draw lines to the river parallel to the capital of the work. To strengthen the interior defence of the tête du pont, construct a flêche, faces 20 yards each, and 10 yards in front of the bridge, which is 4 yards wide.

In the construction of bridge heads, the foregoing Figures may be employed when expedient; the simplest form, the redan, being for light bridges; and the more perfect defence, the bastioned front, or fronts, for bridges of material consequence.

Figs. 11, 12, 13. Lines.- Vide Plate.

Fig. 11. Construct a redan, base 40 yards, perpendicular 30 yards; which join by a curtain, 100 yards, to a queue d'aronde.

Fig. 12. Side of square, 35 yards, and lines drawn from summit to points on the curtains 10 yards. To increase the defence of the next curtain, 100 yards, bisect it by a perpendicular, 15 yards, and draw the two faces. Lengthen the lines by cremaillères.

Fig. 13. Base 100 yards; crochet, base 5 yards; perpendicular 20 yards.

Lines, continuous, are formed by a modification of redans, lunettes, curtains, &c., dependent on the nature of the ground, and the means of defence.

Lines, with intervals, are formed by detached redans, lunettes, &c., within range of each other; the rear works flanking those in front.

Bridges, and passages into field works are from 6 feet to 12 feet wide, according to the requirements.*

Traverses are placed about 9 feet from the slope of the banquette, their length being so regulated as to exclude from the view of the enemy the interior of the field work, through the bridge, &c.*

The nature, and form of the field work, or lines, required for the defence of a post, &c., &c., having been determined, the perimeter may be laid down, in conformity to the construction detailed in the foregoing figures: after which the requisite dimensions of the parapet, ditch, &c., (dependent, of course, on the nature of the enemy's ordnance) must be taken into consideration, and the quantity of earthwork com