~ On The Trace Of Field Fortifications ~
~ Definition and Importance of the Trace ~ All field fortifications consisted of straight or curved lines of parapet, trench, or wooden wall that extended across the ground and joined at more or less well defined points to produce angles. These lines and angles laid out on the ground were referred to as a field work's trace or outline. Along with the general profile of the primary protective mass a field work's trace determined how well a work's garrison could defend ground immediately in front of the work with their fire and repel an assault. As a general rule defensive fire became more effective when it could be projected across the same ground from two or more directions at once; its power to stop an assault was increased as the direction of the defenders' fire became more oblique to an attacking body of troops' line of march as it approached a field work. A garrison's ability to reach an attacking body with fire from two or more directions or, at least, to cover as much ground as possible in front of a work with fire, was a basic function of a field work's trace.
It was a principle of fortification that a good defense required
that one part of a field work be able to participate in the defense of another
part, that is, that a work be laid out in such a way that troops lining one
Underlying this principle of mutual defense and the flanked
disposition was the constructive assumption that each soldier standing on
a field work's banquette would deliver his fire in a direction that was
perpendicular to the line of the interior crest. By extension a group of
soldiers manning a section of parapet would project a column of musket fire
that was as wide as the section of parapet was long and extended outward
from the interior crest of the parapet to the effective range of their muskets.
When two adjoining sections of parapet
Through the preceding three centuries European engineers developed a repertoire of well defined standard traces that were designed to produce specific relationships between the cover offered by the parapet and the columns of fire projected from sections of the parapet. The properties of these standard traces were well known by the middle period of the nineteenth centuries; their properties viewed in terms of advantages and disadvantages offered by established combinations of sections of parapet and the situations in which each standard trace could be applied with effect were well understood. Engineers tended to use these standard traces as archetypical models that could be applied to specific sites in pure form or modified to suit specific locations within the context of a proposed system of defense. Under the influence of the natural sciences these standard traces were categorized according to certain obvious common features exhibited by the traces that imparted similar adaptive characteristics.
~ The Repertoire of Received Forms ~ All standard field work traces were divided into two primary categories: detached (also called isolated) works and lines of works. Detached works could be applied to specific sites without reference to a system or other field works; lines of field works were closely related or directly connected works that covered extended positions. These two general categories were further subdivided into several classes: detached works were classified by the enclosure formed by the parapet into open works that were not completed enclosed by the parapet and enclosed works that were completely enclosed by the parapet. Enclosed works included redoubts, star forts, field forts, and blockhouses; priest-caps (swallow's tails), lunettes, and redans (fléches) were classified as open works. Lines were divided into two categories: continuous lines and lines with intervals. Continuous lines consisted of a series of connected sections of parapet that included redan lines, tenaille lines, Crémaillère (indented) lines, and bastion lines. Lines with intervals were composed of detached works positioned to defend each other, but with more or less wide intervals between the works that allowed the defender to pass through the line to conduct counterattacks. These types of lines could employ lunettes or redoubts (and bastions as in the line of General Rogniat) arranged in one or two rows along with redans in an advanced row or in positions where it was important to compel an attacking body to assault a succession of works before engaging the main works. It is important that these received forms were little more than models that informed and guided engineers in the development of the traces of individual works and lines; application of a form did not imply adherence to specific characteristics of each trace as precisely described in engineering manuals. It was more important to grasp the basic defensive characteristics that certain combinations of lines and angles produced and the general situations in which open or enclosed could be productively applied or the various types of lines employed within the context of a defensive scheme. To accomplish in a simple and straightforward way each standard trace was defined both in terms of its lines and angles and according to the advantages and disadvantages inherent to the trace.
Detached Works, Open at the Gorge
Detached Works, Enclosed
Enclosed works included redoubts, star (tenaille) forts, and bastion
field forts. All had the distinct advantage over open works of being capable
of an independent defense against attacks delivered from any direction. Redoubts
were generally defined as any enclosed works that did not include re-entering
angles in their traces; most were traced as quadrilaterals (four sides or
faces and four salients) and pentagons (five faces and five
Star forts were traced with alternating salient and re-entering
angles and were defined by the number of salients included in their traces.
Originally designed to alleviate some of the defects of unflanked redoubts
by breaking the sides of the fortified polygon into re-enterings that projected
crossing columns of fire over ground immediately in front of the work, star
forts had the great disadvantage of requiring an excessive length of
Field forts included one or more bastion fronts of fortification
in their traces. This type of enclosed work provided the best flanking defense
against close assaults, but like star forts, field forts required extra time
and labor to lay out and construct properly. Minimum and maximum conditions
necessary to produce an effective
Continuous Lines
Continuous lines of field works were used to cover a position
assumed by a field army or the intervals between enclosed works with long
unbroken lines of parapet. Most engineering manuals of the period warned
against their use, with the provision that no fortification form could be
abstractly excluded from consideration when it might meet the fortification
requirements of a specific site or situation. Continuous lines required a
great deal of time and labor to construct and were considered an encumbrance
to an effective defense since every point of the long line had to be equally
well guarded and defended. A breach at one point in any of the continuous
lines discussed in period manuals would generally lead to the loss of the
entire line since no point of these lines was enclosed and capable of a defense
independent of the remainder of the line. Crémaillère
Redan lines were composed of a series of redans sited on commanding
ground within the fortified position connected by more or less straight curtains.
While the redans produced crossing columns of fire in front of the curtains,
they also offered salients with wide sectors without fire that could only
be defended by oblique fire from the curtains. The ditch immediately in front
of the parapet could not be seen from any point of the line and could not
be defended with flanking fire delivered from the parapet. Tenaille lines
were developed to answer the difficulty involved in defending the ditch and
salients of redan lines. This type of line was broken into a series of salient
and re-entering angles; each face of the line could be traced to project
a column of fire that passed parallel to the adjacent face and reached ground
in front of and into the ditches of collateral salients.
Crémaillère (indented) lines were designed to cover
ground in front of the work with a network of columns of fire that crossed
at right angles so that an attacking body of troops would be hit by both
close slanting and enfilading musket. A crémaillère line was
itself composed of a series of branches or faces connected to short flanks
to form a series of right angles at both the salients and re-entering angles.
This was probably the best type of continuous for producing an effective
close defense of the parapet,
Lines With Intervals
All of the standard traces exhibit two fundamental features: parapets
laid out along right lines and sections of parapet that intersect to form
angles. Parapets' right lines produced columns of fire, as has already been
shown, that defended specific and well defined areas in front of a field
work; but a good defense required that these columns overlap or cross so
that the defenders' fire could be concentrated against an attacking body
of troops. Defensive fire tended to have greater impact when delivered at
an angle to an attacker's line of approach: not only did slanting and enfilading
fire tend to disconcert attacking troops, it also increased the defenders'
chances of striking enemy troops and putting them out of action before they
could reach the ditch
in
Three angles were important to the trace of field works: salient
angles, re-entering angles, and angles of defense. Salient angles were formed
by two sections of parapets joining in such a way that they pointed toward
the exterior
Salient Angles
Salient angles were generally the most advanced parts of a field
work's trace and were, consequently, the most obvious points of attack. When
two sections of parapet joined to form a salient, the angle at which they
Experience of the erosive effects of weather on sharply defined salient angles established that the stability of the exterior slope of the parapet and scarp of the ditch required a minimum angle of 60º. Lesser angles would erode away too rapidly and could be broken down by hostile artillery fire too quickly. Angles more acute than 60 degrees also tended to reduce the interior space to the point that troops defending one parapet would interfere with troops defending the other parapet; the reserve, called up when an enemy had mounted the superior slope, would not be able to reach the angle without being disrupted by troops retreating from the parapet. More acute angles also prevented the establishment of barbette batteries on the capital of the salient to cover the sector the sector without fire. Two principles of fortification were deduced from these conditions inherent to salients: 1) salient angles should never be made less than 60º to protect them from a too rapid destruction by enemy fire or the action of the weather and to reduce the sector without fire as much as possible without encumbering the interior of the work. 2) Salients should be made as obtuse as possible to prevent the interior of the parapet from being swept by enfilading and ricochet fire and to allow defense of the salient by oblique fire from the parapet.
Re-entering Angles
~ Conditions Governing the Selection of a Trace ~ Any number of conditions specific to a particular site and the circumstances of construction could influence the manner in which a field work was actually traced on the ground. These conditions were generally condensed into three categories that applied to the design and construction of all field works. First, the ground or site where the fortification would be laid out and constructed had to be considered along with ground within cannon range. Second, the number of men who would compose the garrison and the number of guns in its artillery armament determined the total length of parapet that could be laid out. Third, the time and labor necessary to construct the work had to be considered in relation to the importance of the position and the expected period that would elapse between beginning construction and the arrival of an enemy force.
Fortified Site and Its Surroundings
A field work's trace had to be adapted to the topography of its
site to accomplish two basic objects: command all probable approaches to
the work and insure the security of the interior space of the work. The first
allowed a work's garrison to use their arms effectively against an attacking
body of troops and prevented an enemy from approaching a work without being
discovered. In general the faces of a field work had to be laid out perpendicular
to or at a slight angle to the avenues of approach that an enemy would follow
to attack the work. Whenever the ground permitted, flanks or adjacent faces
could be laid out to cover the avenues of approach with slanting or enfilading
fire. When a field work was commanded by higher ground within cannon
Garrison and Armament
A field work's trace had to be adapted to its garrison and artillery
armament; these two conditions determined the total length of parapet of
the work and the interior space that the work had to enclose to house its
garrison
Enclosed works were expected to be capable of housing their garrisons
for short periods of time. It was, therefore, necessary that the work enclose
sufficient space measured from the lines of the foot of the banquette slope
for this purpose. Each man was assumed to require one and one-half square
yards; each gun would
Temporal and Material Resources Field fortifications did not, of course, magically spring from the native earth at the theoretical beckoning of theoretical engineers. All field works, whether intended to shelter a single individual for a few hours or protect an important strategic point for years, were designed and constructed within the limits of the time and means available when the necessity for the work was fixed. Engineers rarely worked in an environment blessed with unlimited resources; the period between the realization of the necessity for a field work and the need for its shelter was usually short; manpower that could be committed to any particular construction was generally limited in both numbers and willingness to do hard labor; tools were often lacking in both quantity and quality, and materials necessary to complete a field work had to be collected and prepared before construction could proceed beyond a few scratches in the surface of the ground. In many cases officers serving as acting engineers were compelled to rely on inventive ingenuity rather than studied skill to design and oversee the construction of field fortifications. These conditions, taken separately or together, tended to limit the initial period of development of a field work's trace.
The first priority in the design and construction of a field work
was to raise the parapet (or excavate a trench) to the point that it was
capable of performing its most basic function of providing protection for
the individuals sheltered behind it. When time was short or resources lacking
the two most effective means for getting a parapet up as quickly as possible
were to reduce the profile and simplify the trace. Haste generally lead to
sacrifices in the defensive qualities of field works; a good defense required
combinations of angles that produced crossing columns of fire, but angles
imposed completion delays by requiring equalization of the deblai and remblai
at each salient and re-entering, which, of course, imposed either extra design
work to widen the ditch at the re-enterings and narrow it at the salients
or extra labor to move earth from the salients to
If the working detailed to complete the work was small, the time necessary to complete the work would increase. It was generally assumed that 7 or 8 men per 12 feet along the counterscarp would be sufficient to complete a work in a reasonable period of time. To complete a work as rapidly as possible the whole length of the parapet had to be raised at the same time. The hexagonal star fort would have a parapet 1260 feet long which would be divided into 105 individual 12 foot sections with each section requiring a minimum of 7 men. A minimum working party of 735 men would be required to raise the full length of the parapet at the same time. Several reliefs would be required to keep the work going until the parapet had been raised. Providing three reliefs would require a massive working party of 2205 men to complete just 420 yards of parapet in 2 or 3 days. From this it should be relatively easy to grasp the idea that economy of resources was an absolute necessity; even the slightest twist in a trace could lead to a wastefully heavy expenditure of time, manpower, and materials. This sort of wastefulness could prevent the work from being completed or the parapet from being sufficiently raised by the time it was needed.
~ Interpreting the Trace By Reference to the Profile ~ It should be more than obvious that an ability to accurately interpret the horizontal lines used to represent the divisions between surfaces comprising a field work's parapet (and other structural features) on paper is absolutely necessary to gain an adequate understanding of the characteristics of a field work's trace. Complex plans that include many closely spaced parallel and diagonal lines can become quite confusing, but clarity is greatly assisted by keeping in mind that each line represents the intersection of two surface features or elements of a field work's profile. The space between lines represents the actual surface features. These lines, it is also good to remember, do not show the various features' points of intersection with the ground or plane of the site, rather, they show the lines of intersection of the features' intersection as if viewed from a point directly above a field work. But there is, as there must be, more than one type of line. Solid lines represent the intersections of surface features while dotted or dashed lines represent the location of a feature that is somewhere below or hidden by another feature above it or to its side. When a dotted line terminates in a cross or arrow it generally includes a reference that denotes the distance between two points and, sometimes, the generally direction of the slope of the plane of the site. This does not exhaust all of the possibilities, but is sufficient for interpreting most simple fortification drawings and plans.
When viewing the plan of a field fortification the most important
line to locate is the line of interior crest of the parapet or, if dealing
with a blockhouse or trench, the line showing the interior side of the primary
protective structure. In general, the line of the interior crest was made
a bit thicker than other lines in hand draw plans, but most printed plans
(such as those found in the Atlas to Accompany the Official Records) neglected
this easy method for identifying the interior crest. Either way, the line
of the interior crest was the single most important line of a field work's
trace. When a field works was actually laid out on the ground the line of
the
Interpreting a field fortification's trace on paper is a very
practical matter that requires something more than a theoretical appreciation
of the purpose of symbolic lines. In the example illustration of a parapet
prolonged and broken by a 60º angle (to the right) lines AB and BC (marked
in red) represent the crest of the interior slope. Lines de and df mark the
foot of the interior slope. MG and GN show the crest of the exterior slope
of the parapet while HK and KL mark the crest of the scarp. Line BG shows
the intersection of the superior slopes of the two faces of the angle, that
is, the line where the two sloping planes formed by the superior slope cut
each other. All of these lines are keyed by the profile of the ditch and
parapet at the bottom of the illustration which helps determine whether the
lines should be understood to indicate an ascending (elevated) feature, a
descending (sunken) feature such as the ditch, or a level feature. Notice
that the ditch in front of the salient angle is curved (on a radius equal
to the width of the ditch); the curved surface is not cut by any lines
representing the intersection of the curve with the sloping plane of the
counterscarp. This method of representation also holds true for the
intersection of surfaces that are more
Complexity and confusion can arise when more features are added to a fortification's plan. The second illustration (to the left) shows a trace with the same ditch and parapet profile and 60º angle as before, but with two embrasures, one on each face, and one gabionade traverse. In this case the embrasures are laid out according to Mahan's method with necks two feet wide on the interior slope with a width on the exterior slope equal to half the distance from the interior slope to the exterior slope taken on the sole of the embrasure. The sole has a slope of 1 : 6 that falls 1 foot for every 6 feet of the sole's length and is no more than 4 feet below the plane of the superior slope. To achieve this last prescribed requirement each gun platforms has been placed on raised mounds that cut the banquette and banquette tread while connecting with the interior slope. Cheeks of the embrasures have a slope at the line of the exterior crest of 1 foot of base for 3 of height. The gabionade has been placed two feet in rear of the foot of the interior crest and is 12 gabions long and 6 wide with a height of 8 feet. Again, the important thing to notice is that lines mark the intersections of surfaces that are not on the same plane and each intersection represents an element of a feature's profile. The lighter gray lines (which are dotted lines) that bisect the embrasures mark the directrices of the embrasures and do not represent actual surface features.
Hyde, John T. Elementary Principles of Fortification. London: Wm. H. Allen, 1860. Jebb, Joshua. Practical Treatise on Strengthening and Defending Outposts, Villages, Houses, Bridges, &c., in Reference to the Duties of Officers in Command of Picquets, as Laid Down in the Field Exercise and Evolutions of the Army. Fifth Edition. London: William Clowes and Sons, 1857. See pages 17-22 for estimates of the time required to raise a breastwork. Lendy, Captain. Treatise On Fortification. Or, Lectures Delivered to Officers reading for the Staff. London: W. Mitchell, Military Bookseller, 1862. Mahan, D. H. An Elementary Course of Military Engineering. Part I. Comprising Field Fortification, Military Mining, And Siege Operations. New York: John Wiley & Son, 1865. (Photocopy.) Mahan, D. H. A Treatise on Field Fortification, Containing Instruction On The Methods Of Laying out, Constructing, Defending, And Attacking Intrenchments, With The General Outlines Also Of The Arrangement, The Attack And Defence Of Permanent Fortifications. Third Edition, Revised and Enlarged. New York: John Wiley, 1861. Straith, Hector. Introductory Essay To The Study Of Fortification, For Young Officers Of The Army. New Edition. London: Wm. H. Allen, 1858.
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March, 2003 |
section of a
parapet would be able to project their fire across the front of another section
of parapet. As an attacking body of troops approached a field work it could
then be opposed by direct fire delivered from the parapet they were attacking
and by slanting or enfilading fire from another section of parapet that might
not be under attack. This condition of a field work's trace produced by the
orientation of faces and flanks (taken with the angles produced by their
intersections) was sometimes referred to as the flanked disposition and was
considered the most effective means of covering ground immediately in front
of a work with the defenders' small arms and artillery fire.
were laid out
on the ground the degree to which troops manning those two sections could
defend ground in front of the work and participate in each others' defense
could be predicted. From this it can be seen that a field work's trace indicated
both the area sheltered from hostile fire by the parapet and the areas where
the defenders' fire would be concentrated to stop an attack.
The
three standard types of detached works open at the gorge were the redan,
lunette, and priest-cap. Since their gorge was not covered by the parapet,
all were somewhat vulnerable to being taken by a determined attack from the
rear and all offered salient angles that were subject to enfilading ricochet
fire that could sweep their defenders from their banquettes. But this was
not necessarily a disadvantage if these open works were supported by a second
row of works that could throw their fire into their interiors and prevent
an enemy from actually occupying and gaining any advantage from a captured
work. Redans were composed of two faces, or lines of parapet, that joined
to form a salient angle. A lunette trace included four sections of parapet:
two that joined to form a
salient angle
and two that extended from the faces to form flanks. Both redan and lunette
traces produced sectors without fire in front of the salient and the ditch
could not be defended by direct fire from the parapet unless auxiliary flanks
were added. The lunette trace had three salients and three sectors without
fire, but allowed its garrison to defend ground to either side of the work
from its 
flanks.
A priest-cap also included four sections of parapet laid out to form two
salient angle and one re-entering angle on the capital. This trace was intended
to obviate the sector without fire in front of the salient of the redan and
lunette traces by crossing columns of fire projecting from the two central
faces, but this arrangement also produced two salient that could be enfiladed
by distant artillery fire and created a dead angle within the re-entering
that could not be alleviated by attaching auxiliary flanks to the two outside
flanks.
salients). Without
re-entering angles the ditches of redoubts could not be defended from their
parapets without some auxiliary means within their ditches such as counterscarp
galleries and caponnieres. There were as many sectors without fire as there
were salient angles, a defect that could be remedied by constructing pan
coupes in the salients to reduce the area in front of the work that could
not normally be defended by direct fire from the parapet. Redoubts had the
great advantage of being relatively easy to lay out on the ground and could
be constructed with much greater ease and swiftness than any of the other
types of enclosed works.
parapet compared
to the space actually enclosed and the lengths of the sides of the fortified
polygon. The faces of the re-enterings and the salient angles could only
be defended by oblique fire, which was a rather problematic affair in the
excitement of an assault. Both the re-entering and salient angles produced
dead ground in the ditch that could not be defended from the parapet. Finally
and decisively, star forts were the most difficult type of enclosed work
to actually lay out on the ground and construct. It was an axiom of construction
that the ditch in front of salients tended to produce too little soil to
construct the parapet while re-enterings did not produce enough soil; the
deblai of the salients had to be moved to complete the parapets at the
re-enterings or the width and depth of the ditches adjusted to produce just
the right amount of soil at both the salients and re-enterings. Whichever
solution might be preferred in any individual case, performing the solution
on the ground tended to extend the period of time required to complete a
star fort's parapet.
bastion trace
constrained the application of field forts to a limited number of critical
positions that required very strong defensive efforts. In general, the length
of a field fort's lines of defense could not be less than six times the relief;
the length of each front of fortification could generally not be less about
125 yards to produce flanks of sufficient length to allow a good flanking
defense of the bastion salients. These and other proportional requirements
of the bastion trace imposed a higher degree of design work prior to the
first stages of construction than would usually be acceptable for a temporary
work. This combined with the need to excavate excess soil (that is, deblai
not actually necessary to complete parapet) in front of the curtains to allow
fire from the flanks to reach the bottom of the ditches at the salients of
collateral bastions made the construction of field forts rather costly and
time consuming.
(indented), tenaille,
and redan lines could all be employed to cover an extended position and all
had specific advantages and defects arising from their trace.
The re-entering
angles did produce dead ground within the ditch that could not be reached
by fire from the parapet, but these sections of ditch were fronted by the
best defended ground in front of the line where the columns of fire from
the faces crossed at the most favorable angles for stopping an attack.
but it still produced
a dead area in the ditch in front of the flanks that could be exploited by
a determined and well instructed attacking body of troops.
Lines
with intervals were composed of detached works arranged in two or more rows
that were oriented to defend each other and the ground between them with
their fire. The advanced line was composed of open works, either redans or
lunettes, while the second was composed of lunettes or redoubts that could
project fire across the front of works of the advanced line and sweep their
interiors to prevent hostile troops from occupying and using any works captured
during an attack. A third line consisting of epaulments to shelter cavalry
or infantry reserves was sometimes added. These types of lines allowed its
defenders to conduct an active defense to protect their position: when the
hostile force attacked and had lost its ensemble in an attempt to subdue
the works of the first line or pass through the intervals between them, the
defenders reserves could advance through the second line of works in order
of battle to complete the enemy's defeat. Lines with intervals required a
great deal of time and labor to lay out on the ground and construct; their
defense, when properly conducted, required a sizable reserve capable of taking
offensive action at the decisive moment when the attacker's assault faltered.
front of the parapet.
While the standard traces illustrated the consequences of various combinations
of lines and angles they were little more than models that could rarely be
applied in pure form. To be able to lay out an effective trace that gave
a field work's garrison every chance for defending their fortification it
was necessary to have a good grasp of the defensive properties that various
forms of angles produced.
side
of the parapet. Re-entering angles were formed when two sections of parapet
joined to point toward the interior side of the parapet. An angle of defense
was the angle at which fire from one section of parapet passed in front of
another section of parapet. When a field work was designed or laid out on
the ground the first two types of angles were manipulated to produce the
third.
intersected produced
a sector without fire immediately in front of the salient. This area along
the capital of a salient angle was defined by two lines drawn perpendicular
from the extremities of the interior crests of the two sections of parapet
forming the salient. Acute angles produced wide sectors without fire; obtuse
angles tended to reduce the area along the capital that could not be reached
by direct fire from the parapet. While acute angles could not be defended
by oblique fire from the parapet obtuse angles did allow troops to fire in
a direction oblique to the line of the interior crest (less than 60º)
to cover ground immediately in front of the salient. Since acute angles tended
to direct the line of the banquette toward ground that an enemy might be
expected to occupy they were vulnerable to enfilading or ricochet fire delivered
from batteries positioned at a distance from the work and within the work's
sector without fire. Obtuse angles could still be enfiladed, but compelled
an enemy to position his ricochet
batteries at widely
separated points that were relatively closer to the work itself and outside
the sector without fire.
Re-entering
angle posed a somewhat different set of difficulties since the two parapets
actually faced each other and were within the range of vision of each other.
This meant, basically, that one parapet forming a re-entering angle had to
be protected against the action of the other without reducing the effectiveness
of its fire on ground immediately in front of the work. Given the idea that
columns of fire were projected perpendicular to the lines of interior crests
of parapets, the minimum angle necessary to prevent troops lining one parapet
from firing into troops manning the other was 90º. This angle also allowed
defenders on each parapet to cross their fire in such a way that an attacking
body of troops entering the re-entering would be hit by slanting fire from
two directions, or by direct fire from the parapet being attacked and enfilading
fire the other. Angles over
120º tended
to produce columns of fire that did not effectively overlap or crossed at
too great a distance from the work to stop a determined close assault; after
passing through the area where the defenders' fire crossed, an attacking
body of troops would only be opposed by the fire of one of the parapets forming
the re-entering angle. This negated the principle of mutual defense; a trace
on a right line would be just as effective as a re-entering with an angle
greater than 120º and would require fewer troops to adequately man the
parapet.
Re-entering
angles tended to produce an area of dead ground in the ditch that could not
be reached by direct fire from the parapet. Dead ground could be used as
shelter by attacking troops attempting to organize themselves to scale the
scarp and mount the parapet for a last rush over the superior slope. An angle
of 90º could obviate this difficulty if the defenders' fire could be
directed toward open ground in front of the work, but, in some cases, this
also exposed one parapet to the fire of another. But in most cases parapets
could not be made long enough to reach the bottom of the ditch at the re-entering
with fire and other means of defense had to be employed if time and resources
allowed. Scarp galleries and caponnieres could be placed at the re-entering,
or grenades prepared in advance and stored near the re-enterings to be thrown
over the parapet if enemy troops attempted to collect in the re-enterings.
Even a good scarp revetment, protected from distant artillery fire and given
a near vertical slope, could prevent enemy troops shelter by re-entering
angles from posing an immediate threat to the parapet.
range its trace
could be manipulated to prevent enemy fire from enfilading the parapet and
sweeping the defenders from the banquette. Faces could be laid out parallel
to the commanding ground and the lines of the interior crest directed onto
ground, such as steep slopes, that could not be exploited by an enemy to
enfilade the parapet. These considerations were particularly important when
the work was sited in irregular and undulating terrain; when sited on level
ground or on ground that had a general command over the approaches to the
work, other considerations, such as the close defense of the work and mutual
defense took precedence.
. Each
soldier was assumed to occupy one linear yard of the interior crest; each
gun occupied five or six linear yards. At least one-third of the garrison
was assumed to compose a reserve that would not occupy space on the parapet.
A detached work open at the gorge, for example, designed for a garrison of
250 men (in one rank on the banquette) and three guns would require a parapet
about 185 linear yards of interior crest. About 167 soldiers would man the
parapet, each man requiring one yard on the interior crest, with a reserve
of 83 men on the terre-parade; each of the three guns would require 6 yards
along the interior crest for a total of 18 linear yards. If two ranks
were considered necessary on the banquette (one file per yard), the linear
measure of the interior crest would be reduced to 83 yards for the garrison
and 45 for the artillery, giving a total length of 128 yards for the interior
crest. Given the total length of parapet, the work could be given the most
suitable trace for its site and importance.
require about
60 square yards. An enclosed work for 250 men and 3 guns would require a
minimum area inside the banquette slope of 555 square yards. Traverses,
bomb-proof shelters, and magazines would increase the area that needed to
be enclosed. The total length of parapet needed to enclose the minimum required
interior space could be determined more or less exactly by slow calculation;
it was much quicker and easier when hastily designing a redoubt without
re-entering angles or flanks to figure the length of parapet required for
the garrison and artillery armament and dividing the length of parapet by
the number of faces. Using this quick method a square redoubt for 250 men
a three guns would have faces about 46 yards (185 / 4) long on the interior
crest. The length of each face would be reduced by the bases of the interior
slope, banquette, and banquette tread (about 3 1/2 yards for each parapet)
and the enclosed interior space calculated. In this example the square redoubt
would enclose 1521 square yards within the area bounded by the foot of the
banquette slope (46 - 7 = 39; 39 x 39 = 1521); this would be more than enough
space to house the garrison (which required a minimum of 555 square yards)
and still have plenty of space remaining (966 square yards) for any traverses
or magazines that were found necessary.
the re-enterings
to complete the parapet. Angles also required an increase in the linear length
of parapet for any given length of ground by deviating from the right line
of the front of fortification. If haste was an absolute necessity, the simpler
a trace could be designed and laid out on the ground using as few angles
as possible, the faster the work could be completed and the fewer manpower
and material resources had to
be expended to
produce a parapet capable of providing shelter. A hexagonal star fort, for
example, with sides 60 yards long would require 70 yards of parapet for each
front or 420 total linear yards of parapet to fortify a hexagon with a perimeter
of 360 yards. With 12 angles, 6 salient and 6 re-entering, equalizing the
remblai and deblai would require an excessive amount of extra labor compared
to a simple six sided redoubt without re-enterings if it were necessary to
have the work finished in a matter of a few days.
interior
crest was marked first. All other lines were then marked on the ground and
the construction profiles set up in relation to the interior crest. Again,
the line of the interior crest was the line along which troops would stand
on the parapet to deliver their fire and it, basically, established the width
and direction of the columns of fire that could be produced from a well manned
parapet.
or less level,
such as the tread of the banquette and bottom of the ditch, and occur on
the same horizontal plane on both faces of the angle.
