Chapter XXX: Part IX: Batteries, and Fortification (2)

Gorge of the full bastion.—From the angle of the interior slope, set off 16 yards on each side, from which points draw lines 42 yards in length diagonally along the interior slope; draw parallels to these at the distance of 5 yards, for the breadth of the ramps; erect perpendiculars at their head, from the intersection of which as a centre, with radius of the distance between them, describe an arc, parallel to which, with radius 6 yards less, describe another arc, to which draw tangents, completing the interior slope of the ramps.

Ravelin.—From the angle of the interior slope, set off 12 yards on each side; from these points draw lines 30 yards in length, diagonally along the interior slope, for the length of the ramps; to which draw parallels 4 yards distant for their breadth; erect perpendiculars at the commencement of the ramps, and from their intersection, as a centre, with radius of the distance from the ramps, describe an arc joining the two ramps; also from the junction of the perpendiculars draw lines to the termination of the ramps, for their slopes.

_Caponniere._

Make the passage of this work 30 feet wide, including the banquette on each side; the superior slope of each parapet terminates at 20 yards’ distance.

_Bridges, and communications._

These are about 14 feet wide.

_Stairs, or Pas de souris._

These steps of masonry are made at the gorges of the several works, and at the salient, and re-entering angles of the counterscarp. Those at the salients are generally 24 feet long, and at the re-entering angles 30 feet; they are 5 feet wide, and their steps 1 foot distant from each other.

_Sally-ports._

These passages, cut through the glacis, are about 12 feet wide, and 18 feet long.

_J. W. Lowry, sc._]

PROFILE, OR SECTION OF VAUBAN’S FIRST SYSTEM.

_Construction._

The interior slope of the rampart has a base of 18 feet, and a perpendicular height of 17 feet 6 inches.

The terreplein has a breadth of 25½ feet, its height being 18 feet sloping to 17 feet 6 inches, the height of the interior slope.

The banquette is 3 feet in height, the tread 4 feet wide, and the slope 5 feet wide.

The parapet is 4 feet 6 inches higher than the banquette, its interior slope is 18 inches, its thickness 18 feet, and its superior slope has a declivity of 3 feet; the revetment is 3 feet thick.

The escarp has a perpendicular height of 36 feet, measuring from the cordon to the bottom of the ditch.

The tablette, or coping-stone, at the top of the revetment, has a projection of 6 inches square.

The cordon is semicircular, its radius being 6 inches.

The slope of the escarp is 6 feet, the thickness of its revetment at the top 5 feet, and at the bottom 11 feet.

The counterfort joins the escarp; it is 1 foot lower than the top of it, 9 feet wide, and it extends to the bottom of the foundation, which is 3 feet below the bottom of the escarp; the retreat, or lessening, has a width of 1 foot.

The ditch is 38 yards wide, from the salient angle of the bastion.

The counterscarp is 17 feet 6 inches in perpendicular height, its slope being 3 feet, and its thickness at top 3 feet, the bottom, therefore, having a thickness of 6 feet; the foundation is 3 feet; and the retreat 1 foot.

The terreplein of the covered way is 20½ feet wide, and its slope is 6 inches. The banquette is 3 feet high, its tread 5 feet, and its slope 6 feet. The parapet is 4 feet 6 inches above the banquette, and its interior slope is 18 inches. The glacis, which forms the superior slope of the parapet of the covered way, is 50 yards in breadth.

The counterfort of the counterscarp is 5 feet in thickness, being 1 foot lower than the top of the counterscarp, and extending as low as the foundation of it.

The counterforts of the escarp, and counterscarp are 15 feet distant from centre to centre of each other, those of the escarp being at the end adjoining it 5 feet 6 inches, and at the termination 3 feet 8 inches thick; those of the counterscarp being in thickness at the larger part 3 feet 6 inches, and at the smaller 2 feet 4 inches.

_Vide Plate._

MODERN SYSTEM.

A Interior slope.
T Terreplein of rampart.
R Parapet of rampart.
A T R Rampart.
a a a a Escarp, or exterior slope of rampart.
M Full bastion.
L Empty bastion.
p q Face of bastion.
q G Flank of bastion.
K o p q G Outline of bastion.
G H Curtain.
t Ramps.
B Ditch.
D Tenaille.
Y Caponniere.
e Batardeau.
F F F Ravelin.
S Redoubt in ravelin.
b Counterscarp.
n Traverses in covered way.
c Re-entering places of arms.
W Redoubt in ditto.
P Salient places of arms.
V Covered way.
X Glacis.

Modern System.]

FIELD FORTIFICATION.

REMARKS, AND GENERAL RULES.

1. The size of a work depends in general upon the number of men who are to defend it. If labour is the sole object of attention, the advantage must necessarily be the greater in proportion as the size of the work is less; but if the accommodation of the troops is only to be considered, the advantage depends greatly upon occupying much ground.

2. The form of the work should be such as to contain the greatest surface with the least perimeter. By an adherence to this maxim, we obtain the greatest accommodation for the troops with the least labour. The form of a field work seldom depends upon choice, but generally upon the spot where it is to be raised, the purposes for which it is to be constructed, and the nature of the ground in the vicinity.

3. The interior of the work ought to be so covered by the parapet, that the men within, except when on the banquette, may not be seen from any part without, at the distance of cannon-shot.

4. The circumjacent ground (to as great a distance as possible) ought to be cleared, that the enemy may not conceal, or shelter himself against the fire from behind the parapet. The nearer to the work that the enemy can find cover, the more advantageously he can form his dispositions; and, as his attacks may consequently be made with greater vigour, and be more readily supported, the success will be the more probable.

5. The flanking parts ought to be sufficiently capacious to contain all the men required for the defence of the flanked portions of the work.

6. The flanking parts ought to have nearly a direct view of those flanked; that is, the defence should be nearly at right angles, the most advantageous angle being 100 degrees.

7. The parts flanked ought to be within musket fire of their flanking parts.

8. The fire ought to be equally distributed, that every part of the work may be equally defended.

9. The work ought to be equally strong in all its parts, that it may everywhere equally resist the assaults of the enemy; and the parapet should be thick enough to withstand the shot fired against it.

10. The dimensions of the parapet should not only be sufficient to secure, and cover the troops within the work, but ought also to be of such a form as to afford a full view of the enemy in his approach; and at the same time discover, as little as possible, the men employed for its defence.

_Capacity of Field works._

The perimeter of a work, and the number of men to defend it, should bear a just proportion to each other, one yard being allowed for each man, or for each file of men, according to the nature, and object of the work.

In calculating the area of enclosed field works, 10 square feet may generally be estimated for each man, and 324 square feet for each gun, and its stores.

_Content of Field works._

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; were the angles of the plan re-entering, instead of salient, the result would be reversed. Bearing this in mind, previous to commencing the excavation of the ditch, a correct calculation must be made to prevent any excess, or deficiency of earth for the parapet, and banquette.

_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 periphery of the redoubt, battery, &c., for the solid content of 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 perpendicular.

_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 ⅛th 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.

_J. W. Lowry, sc._]

_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 defence of the tête du pont, construct a flèche, faces 20 yards each, and 10 yards in front of the bridge, 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.

_Fig. 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.

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 computed by the _Rules_, page 245, or by those in PRACTICAL GEOMETRY. The following Table will, however, in many cases be found useful; and, by a judicious adaptation of it, much time may be saved in the computation, and construction of field works.

TABLE,

showing the dimensions, in feet; and the superficial content of earth of banquettes, parapets, and ditches, of field works.

Key:
A = Number.
B = Base of slope.
C = Tread.
D = Height.
E = Base.
F = Superficial content.
G = Berm.
H = Breadth at top.
I = Escarp slope.
J = Counterscarp slope.
K = Depth.

+----+-----------------+----------------------------------------+
| | BANQUETTE. | PARAPET. |
| +---+---+---+-----+---------+---------+---------+----------+
| | | | | | Interior| Superior| Exterior| |
| | | | | | slope. | slope. | slope. | |
| | | | | +----+----+----+----+----+----+ |
| A | B | C | D | F | D | E | D | E | D | E | F |
+------------+---+-----+----+----+----+----+----+----+----------+
| 1 | 6 | 5 | 3 | 24 | 7½ | 1½ | 7½ | 15 | 5½ | 5½ | 120½ |
| 2 | 6 | 4 | 3 | 21 | 7½ | 1½ | 7½ | 15 | 5 | 5 | 113⅞ |
| 3 | 6 | 5 | 3 | 24 | 7½ | 1½ | 7½ | 12 | 5½ | 5½ | 101 |
| 4 | 6 | 4 | 3 | 21 | 7½ | 1½ | 7½ | 12 | 5 | 5 | 98⅜ |
| 5 | 6 | 4 | 3 | 21 | 7½ | 1½ | 7½ | 9 | 5½ | 5½ | 81½ |
| 6 | 6 | 4 | 3 | 21 | 7½ | 1½ | 7½ | 9 | 5½ | 5½ | 81½ |
| 7 | 5 | 4 | 3 | 19½ | 7½ | 1½ | 7½ | 6 | 6 | 6 | 66⅜ |
| 8 | 5 | 4 | 3 | 19½ | 6 | 1 | 6 | 6 | 4½ | 4½ | 44-6/8 |
| 9 | | | | | 8 | 1½ | 8 | 12 | 5½ | 5½ | 102⅛ |
| 10 | | | | | 7½ | 1 | 7½ | 9 | 5½ | 5½ | 78 |
| 11 | | | | | 6 | 1 | 6 | 6 | 4½ | 4½ | 44⅝ |
| 12 | | | | | 6 | 1 | 6 | 3 | 5¼ | 5¼ | 33-15/16 |
| 13 | 4 | 4 | 3 | 18 | 7½ | 1 | 7½ | 4 | 5½ | 5½ | 44¾ |
+----+---+---+---+-----+----+----+----+----+----+----+----------+

+---+-----------------------+
| | DITCH. |
+ +----+----+---+---+-----+
| | | | | | |
| | | | | | |
| | | | | | |
| G | H | I | J | K | F |
+---+----+----+---+---+-----+
| 3 | 26 | 3 | 3 | 6 | 138 |
| 3 | 20 | 4 | 4 | 8 | 128 |
| 3 | 23 | 3 | 3 | 6 | 120 |
| 3 | 17 | 4 | 4 | 8 | 104 |
| 3 | 19 | 3 | 3 | 6 | 96 |
| 3 | 16 | 4 | 4 | 8 | 96 |
| 2 | 18 | 3 | 3 | 6 | 90 |
| 2 | 15 | 10 | 5 | 8 | 60 |
| 3 | 19 | 3 | 3 | 6 | 96 |
| 2 | 16 | 4 | 4 | 6 | 72 |
| 2 | 12 | 4 | 3 | 6 | 51 |
| 2 | 9 | 6 | 3 | 8 | 36 |
| 2 | 15 | 10 | 5 | 8 | 60 |
+---+----+----+---+---+-----+

SIMPLE METHODS OF TRACING FIELD WORKS, ON THE GROUND.

_1. Square redoubt._

Place pickets in a line (in length conformable to the side of the intended work), at each end of which erect perpendiculars equal in length to the side first marked out, and join the termination of these lines; which will complete the perimeter of the redoubt.

_Note._—A perpendicular is raised on a given line, with a chain or cord, by forming a right-angled triangle from the numbers 3, 4, and 5, or any multiples thereof, and extending the cord, &c., so that the base may correspond with the base line of the pickets, and the perpendicular be in the direction of the side required.—_Vide_ PRACTICAL GEOMETRY.

_2. Pentagonal redoubt._

With a chain, tape, or cord, construct, and lay down with pickets five similar, and contiguous triangles, having their bases, which form the sides of the pentagon, in the proportion of 47 to the other two equal sides, the length of each of these being 40.

_3. Hexagonal redoubt._

From a central point with a chain, or line, construct, and lay down with pickets six equilateral, and contiguous triangles, the bases of which will form the required hexagon.

_4. Octagonal redoubt._

Construct a square (_vide_ No. 1), from the centre of each side of which erect perpendiculars outwards, in length proportional to the side as 13 to 60 (nearly 1 to 5); join the extremities, or termination of the perpendiculars, to the angles of the square, which will determine the sides of the octagon.

_Note 1._—The directions for the construction of the pentagonal, and hexagonal redoubts are on a small scale; but the redoubts may be increased by the equal extension of the interior sides of the triangles, until the bases are sufficiently long for the periphery of the work required.

_Note 2._—By means of the pocket sextant, prismatic compass, or reconnoitring protractor, the pentagonal, hexagonal, and octagonal redoubt may be thus traced on the ground. From a central point place pickets at the requisite distance from each other, and in the direction of lines drawn from the angle of the centre of the intended work. (_Vide_ PRACTICAL GEOMETRY. _To find the angles at the centre, and circumference of a polygon._) Extend these radii equally until the relative distances between them are of the length required to form the sides of the proposed equilateral redoubt.

_5. Front of fortification, for a Field fort._

Place pickets in a straight line, of the length required for the front of the proposed field work; from the centre of which drop a perpendicular inwards, making it for a square, pentagon, hexagon, or octagon, respectively one-eighth, one-fifth, one-fourth, or one-third of the exterior side. Direct the lines of defence from the termination of the exterior side to the end of the perpendicular, making the faces of the bastions two-sevenths of the exterior side, and constructing the flanks perpendicular to, and joining the lines of defence. Other fronts are traced by laying down the exterior sides, at the angle of the circumference of the intended polygon (_Vide_ PRACTICAL GEOMETRY) by means of the prismatic compass, &c., and then proceeding as directed for the former front.