Copyright
Egbert L. (Egbert Ludovicus) Viele.

Hand-book of field fortifications and artillery; also manual for light and heavy artillery online

. (page 2 of 12)
Online LibraryEgbert L. (Egbert Ludovicus) VieleHand-book of field fortifications and artillery; also manual for light and heavy artillery → online text (page 2 of 12)
Font size
QR-code for this ebook


because it is of simple construction; the ditches are more easily
flanked, and there are not so many points of attack as in a redoubt
of a greater number of angles. Redoubts, being closed works, are
better calculated to stand detached than redans or lunettes, and are,
therefore, constructed when a small work is required without any
immediate protection from the gorge — the armed party being strong



FIELD FORTIFICATIONS. 13

enough to complete and man a four-sided redoubt, eacli side of
which is not less than 15 yards long.

The size depends on the number of men who are to garrison -it,
and upon the number of guns which it is to contain; also upon the
length of time during which it is to be occupied: this may be for a
few hours only, (as on a field of battle,) or for a period of weeks or
months.

If wanted only for a few hours, it will be sufficient to allow 3 feet
in length of parapet for every man of the detachment; or for every
two men, if they are to be formed in double rank. If guns are to
be placed in the work, 15 feet of parapet must be given to each, in
order that the gunners may have sufficient room on each side to
work it.

But when the redoubt is destined to contain a body of men for a
constderable length of time, it becomes necessary to have room for
them to lie down within the banquette with their arms and packs;
supposing one-third to be on guard, patrolling, &c., two square yards,
in addition to the slope of the banquette, are sufficient for each
man, and 36 square yards for each gun with its appointments.

The rule, consequently, for a square redoubt is: to multiply the
given number of men by 2, and number of guns by 36, for the
numbei' 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; adding
to this result the breadth of two interior slopes, and of 2 banquettes
with their slopes, (about 7 yards altogether,) we shall have the side
of the square formed by the crest of the parapet.

A square redoubt ought not to be traced with less extent of side
than 15 yards; for, by employing the calculation explained above,
it will be found that such a work is only just sufficient to contain
the men necessary for its defence : on the other hand, it is unusual
to make a square redoubt with a longer side than 40 yards, because
it would require a garrison more suitable to u work of a stronger
outline.

The imperfections of redoubts are, that they are entirely without
a flanking fire for the defence of the ground in front of their faces,
also that their ditches and the sectoral spaces before the angles are
without any fire whatever for their defence.

A flanking defence for the ditches may be obtained by placing
palisade or stockade caponnieres in them, either at the angles or in
the middle of the faces; by tambours in a like position, or by loop-
holed galleries under the counterscarp at the salients of the work.



14 FIELD FORTIFICATIONS.

The want of a fire in the directions of the capitals may be reme-
died, as in the redan, by cutting off an angle by a short face, by
making it curved, or by tracing a portion of the line of parapet en
cremailUre; viz, : by disposing it in a succession of salient and re-
entering angles, the sides of which are alternately parallel to the
capital : this construction is, however, very difficult, and causes in-
convenient variations in the thickness and height of the parapet.

A ditch caponniere is an oblong structure formed with palisades,
or with stockade work, loop-holed, and roofed over with planks and
earth to secure^ the men from the effects of shell, and a plunging
fire from the counterscarp. It ought, if possible, to be flanked with
musketry, to prevent an enemy from closing on it, and getting un-
der cover.

The best position for a caponniere in the ditch of a redoubt is at
the salient angle, as then one caponniere flanks two branches of the
ditch. It should be separated from the counterscarp by an enlarge-
ment of the ditch, to prevent an enemy from using it as a bridge to
cross the ditch, and it ought to have a wicket to allow of sallies into
the ditch.

The bottom or sole of the caponniere may, sometimes with advan-
tage, be sunk 3 or 4 feet below the bottom of the ditch, in order
that the fire from the loopholes may graze along it, and prevent an
enemy from closing on them; by this construction; and by making
the roof convex, it becomes more difficult for him to use the capon-
niere as a bridge to pass the ditch.

To lessen the destructive eff'ect of shell, traverses should be placed
in all closed works when those missiles are likely to be employed
against them.

From the interior to the exterior of closed works there must be a
passage through the parapet, protected by a traverse or by stockade
work, and the traverse should extend far enough on each side of
the passage to intercept shot which might enter it obliquely. The
ditch is crossed by a bridge which is conveniently formed of loose
planks and beams, because, in case of attack, they can be quickly
taken up and used to barricade the passage. When the ditch is
more than twelve feet wide, a trestle must be placed in the middle
to support the beams or sleepers.

When rough timber only can be procured, stout straight limbs of
trees must be. selected for the sleepers, which may be covered with
strong hurdles, (or brushwood,) over which a layer of sods and then
a small quantity of gravel may be laid.

A STAR FORT is a closed work, the parapet of which forms seve-



FIELD FORTIFICATIONS. 15

ral acute salient angles and obtuse re-entering angles, giving it a
form like the usual representation of a star.

It has been seen that redoubts are defended only by direct fire,
and that without some contrivance for affording flanking fire, the
sectors at the salients, as well as the ditches, are absolutely unde-
fended. Star forts, consisting of re-entering as well as salient an-
gles, are intended to obviate that defect in some degree. They may
be constructed either upon an exterior or interior polygon. [f
ground is to be fortified which does not admit the possibility of work-
ing outwards, as an island for instance, a polygon is traced to suit
the form of the ground; the sides of the polygon are bisected by
perpendiculars drawn inwards, and the fac3s of the star fort are
drawn from the angles of the polygon to the inner extremities of
the perpendiculars: this is called fortifying upon the exterior poly-
gon.

The length of the perpendicular* in a square, pentagon, hexa-
gon, and octagon, should be respectively one-eighth, one-fifth,
one-fourth, and one-third of the side, in order that the flank-
ing angles may approach as near as possible to right angles, without
making the salient angles less than 60°. "When the polygon is ir-
regular, the length of the perpendiculars must be determined by
the an<z:les nearest to them.

Again, it may be required to surround a building witl\,a star fort
in such a position that the work could not be traced inwards • then,
a polygon surrounding the building must be laid down, and on each
of its' sides an equilateral triangle must be formed towards the ex-
terior.

If this construction be applied to a dodecagon, it will be found
that the re-entering angles are exactly right angles; in an octa-
eonal fort the re-enterino; or flankinq; an";les are each equal to.
105°. _

The necessity for employing a polygon superior to an octagon will
rarely occur ; yet with irregular figures it may happen that some of
the angles, are equal, or nearly so, to those of regular polygons of
more than twelve sides.

It is necessary to fix some limit as a minimum to the length of
face for these works; this depends on the distance at which a shot
fired from the parapet of a face would reach the level of the •
ground; for it is evident that if the face be made less than that dis-

* The lengths given are merely approximations in the form of the near-
est simple fraction of the side.



16 FIELD FORTIFICATIONS.

tance, the enemy, arrived at the rounding of the counterscarp, will
be more or less secure from the fire of the adjoining face. If we
suppose a man to fire along the superior slope of a parapet with a
plongee of one-sixth, and that the work has a command of seven
and a half feet, we have by similar triangles,

1 : 6 : : 7i (the command) : 45;

the distance, in feet, at which the shot would reach the level of the
ground.

The face should, therefore, not be less than 45 feet, viz., 15
yards. If the bottom of the ditch on any face is to be defended
by the fire of the next face, a still greater length is required: to
find this, we have the proportion 1:6:: the relief (the height of
the crest of the parapet above the bottom of the ditch) : the dis-
tance at which a shot would reach the bottom of the ditch; this
distance is 30 yards when the command of the work and depth of
its ditch are both seven and a half feet. The length of the faces
depends also upon the number of guns to be placed behind the
parapet, and upon the strength of the garrison; but 35 yards may
be considered as the greatest length of face, for the troops required
to defend a star fort having a longer face would be sufficient to con-
struct and defend a fort of a better tracing.

The construction of star forts is attended with some trouble, par-
ticularly if the ground is uneven : such works present at their acute
salient angels numerous points of attack: the faces and sahents are
without flanking defence when the polygon is inferior to an octagon,
and even in this case such defence is imperfect; the ditches are un-
defended, unless the faces are made unreasonably long; the line of
parapet to be manned is very great, when compared with the inte-
rior space, and is exposed to be enfiladed in all directions.

CONSTRUCTIONS.

The following are convenient methods of tracing on the ground
the most useful polygonal redoubts and star forts:

For a pentagonal redouht : on a base equal to one fifth of the
perimeter, or length of the parapet, form an isosceles triangle, of
which each of the equal sides is one-third of the perimeter, and on
each side of this triangle as a base form another isosceles triangle
with its (equal) side, each equal to the side of the pentagon, or first
base.



FIELD FORTIFICATIONS* 17

For a hexagonal redoubt : trace an equilateral triangle, the side
of which is in length three times that of the redoubt; trisect each
side, and join the nearest outer extremities of the centre portions :
these lines, with those which join their extremities, will constitute
the hexagonal redoubt required.

For a hexagonal star fort: trace an equilateral -triangle on a
base equal to one-fourth of the whole length of the parapet; trisect
each side, and form equilateral triangles on the three centre por-
tions. These will complete the figure.

For an octagonal redoubt: trace a square on aside equal to
three-tenths of the whole length of parapet of the redoubt ; and
from the angles of the square measure on each side half the diago-
nal; the points being joined, the magistral line is traced.
. For an octagonal star fort: trace a square on a base equal to
three-twentieths of the whole length of the parapet of the star fort;
with this square form an octagon as before, and on each of its sides
trace an equilateral triangle.

Forts with bastions are the most perfect of closed field works,
as it is evident that they possess all the advantages of mutual de-
fence afforded by the corresponding works in permanent fortifica-
tions; they are traced similarly to these last, although rarely on a
polygon superior to a pentagon; as, however, their defence mainly
depends on the fire of common muskets, their lines of defence must
not exceed the efi'ective range of such arms, or about 160 yards,
and therefore the side of the polygon on which they are constructed
must not exceed 200 yards in length.

On the other hand, the side of the polygon should not be less
than 120 yards in length; since, if it were so, the bastions would
be too small, and the flanks and curtain too short for the defence re-
quired from them.

Bastioned forts should have within them a good reduit, in order
to give confidence to the garrison, and secure its retreat: such a re-
duit should have a command of four or five feet over every part of
the main work, in order that the enemy, having gained the parapet
of the latter, may not fire from thence into the reduit.

The reduit may either conform to the outline of the fort, or it
may be a simple redoubt, a blockhouse, or a tower of brick or stone,
so traced that the defenders may fire into the bastions of the fort,
these being the points at which an enerfty is most likely to force an
entrance.

As "bastioned forts require a strong garrison, they are constructed
when it is intended to occupy a point of importance for a conside-



18 FIELD FORTIFICATIONS.

rable time, and, tlierefore, the reduit often forms^ at the same time,
the barrack of the o;arrison.

In order to throw an additional fire towards the direction of the
sahents, the curtain is sometimes broken in the prolongation of the
lines of defence; but in order that some fire may be directed imme-
diately in front, a portion may be formed in a line parallel to the
original curtain, and equal to about one-third of its length- the
two brisures should form with each other a re-entering rather than
a salient angle, in order that tliere may be no dead spaces in the
ditch.

The counterscarp of the ditch may be di\awn either to the shoulder
angles of the bastions, as in permanent fortification, or parallel to
the faces, flanks, and curtain; the latter method is generally prefer-
able, as it saves time and labor; in this case, however, the counter-
scarp of one flank would conceal the ditch of the nearest face from
the fire of the opposite flank; this counterscarp ought, therefore, in
part, to be cut away in an inclined plane, or ramp, parallel to, or
coincidina; with, the line of fire from that flank.

CD ^ •

Demi-bastioned forts, like those with bastionss, are traced by
letting fall a perpendicular from the middle of each exterior side,
and drawing lines of defence; but each front has only one flank,
every alternate face extending from the angle of Ihe polygon to
the inner extremity of that flank, and coinciding with the line of
defence throughout its entire length; such works have the defect of
aff'ording a regular flanking defence only to every alternate face; as
the short face of each front receives a very oblique and imperfect
flanking defence from the collateral long face.

Loopholes are narrow rectangular openings made in walls of
masonry or wood, through which to direct a fire of musketry. In
walls of two feet or two and a half feet thiok they are about nine
inches high by fifteen inches wide on the inside, and twenty inches
high by four inches wide on the outside. In timber six or eight
inches thick they are eight inches wide inside and three inches out-
side, the height being twelve inches.

They are' made wider on the inside than on the outside, because,
thus formed, they afford better cover for the men behind them;
they are placed at not less than three feet asunder, that the wall
may not be too much weake'ned, and that the men firing through
them may not be crowded; they are made from four feet t9 four
and a quarter feet above the. banquette or ground on which the men
stand to fire through them.



y*



FIELD FORTIFICATIONS. 19

Stockade 'w6Rk is a wall composed of trunks of trees, or
rout;h pieces of timber placed upright in the ground; they are
made to touch each other, and loopholes are cut through them; if
composed of tre«s, they ought to be squared, that the parts in con-
tact may be of the same thickness as the rest of the "wall.

A TAMBOUR is an enclosure of palisadts or stockade work, some-
times with a ditch and banquette, and of any form that may be ne-
cessary to afford the defence required.

Blockhouses are covered field worl s, generally rectangular;
the walls are formed of trunks of trees, a:id above the timbers of
the roof there is, usually, a bed of earth, Aree or four feet thick.

In mountainous and well-wooded coui tries blockhouses are the
best description of 'field works, because ihe enemy cannot easily
bring cannon to destroy them. It is verj difiicult in mountainous
countries to find ground where works ma} be constructed free from
the defect of being commanded, and cons iquently open works are
there comparatively useless.

Blockhouses are of great advantage as . eduits in situations where
it is difficult to defilade the interiors of works from commanding
heights, more especially since they may serve as barracks for the
troops; in such a case the b'edsteads, arr mged on each side, are
used as banquettes, and the loopholes a -e made four feet above
them.

A blockhouse to resist musketry shou d be composed of trees,
squared so that the parts in contact may 1 e at least six inches thick,
that being the depth to which a musket ball will penetrate in fir.
In order to resist artillery, two rows of ti ees (or -of stockades) are
placed vertically in the ground, with an interval between them from
three to six feet wide, which is filled with earth well rammed. The
trees or logs should be eleven or twelve feet long, so that they may
be planted at least three feet in the ground, and allow the interior
of the blockhouse to be eight or nine fSot high; it should also be
from eighteen to twenty-four feet wide in the interior.

The earth used to render the covering ^ hell-proof may be shaped
like a small parapet, and from this an additional fire(of musketr3'^)may
be obtained; the access to this upper parapet is through a trap-door
in the roof To prevent the blockhouse from being set on fire, a
ditch should be dug round it, leaving a berme of eight or ten feet,
and on this the earth is piled up against the wood as high as the
loopholes.

Sometimes blockhouses are constructed in the form of a cross,
when the flanking fire thus obtained on their faces renders them



20 FIELD FORTIFICATIONS.

much more powerful; they are also, occasionally, built with an up-
per story, the angles of which should project over the sides of the
lower story; the foot of the lower walls may thus be defended by
the fire from above.

An ordinary dwelling-house, with thick masonry walls, may be
formed into a blockhouse by pulling down the upper stories, and
heaping a mass of materials, three or four feet in thickness, over
the ceiling of the lower rooms; earth or rubbish should also be
placed about the house as high as the loopholes.



DEFILADING OF FIELD WORKS.



To DEFILADE a work from a height is so to regulate the direction
and elevation of the parapets or covering masses, that its interior
may be screened from the view of an enemy on the heights.

A PLANE OF SIGHT is an imaginary plane supposed to pass
through the summit of the height from which the work is to be de-
filaded, and the terreplein of the work.

A PLANE OF DEFILADE is a plane supposed to pass through the
crest of the parapet of the work parallel to the plane of sight.

In many situations it is practicable (and then it is the easier
method) to defilade the i.ices or longest branches of a work by
the tracing; viz., by directing them on marshes, rivers, lakes, pre-
cipices, hollows, &c., where batteries cannot be erected, or at worst,
on points of the height not nearer than 8U0* yards to the work.
Also the choice of the outline of the work should be attended to ;
for among the dilFcrent tracings by which the same object may be
attained, some will be more easy to defilade than others.

When a work is thrown up in front of a height, it is the more
difl&cult to defilade in proportion to its depth; it should, therefore,
have an oblong form, and its longest faces should be traced parallel
to the hieght. If, for instance, the work were a rectangular redoubt,
the long faces should be traced parallel to the height, and the short
ones be directed on it.

An open work will be defiladed when the plane of defilement
passes through a line 8 feet above the ground at its gorge, and at a

* ArtJUeiy on a heigl)t, even of 120 feet, at 800 yards distance from a
work, has no more advantage, in respect of a plunging fire, than if it were
on a level witlr the rock ; for in both cases it mi\st be elevated about 1^
degree to attain this range.



FIELD FORTIFICATIONS. ' 21

point 4 or 8* feet above the commanding hill, according as the
work is to be defiladed against artillery or musketry.

It is usual to defilade a work against musketry if there are
heights within 300 yards of it, and against artillery, when the
heights are not farther distant than 800 yards.

When the commanding ground is not occupied by the enemy,
the work may be defiladed in the following manner : stretch a rope
between two poles planted in the line of the gorge at 8 feet above
the ground ; direct visual rays from various points of this rope to
the top of a pole placed on the commanding hill, and 4 feet high if
the work is to be defiladed against artillery, but 8 feet if it is to be
defiladed against musketry; the intersection of the rays with poles
planted on the tracing of the intended parapet, will indicate the
height to which the parapet must be raised in order that its defend-
ers may be situated under the plane of defilade; and since these
visual rays represent lines of fire from the enemy's position on the
hill, it will be evident that a parapet whose height is thus deter-
mined will defilade the interior of the work.

When it is impossible to place the pole on the commanding
ground, the following method must be adopted : along the gorge of
the intended work stretch a rope, which is to be 4 feet above the
ground if the work is to be defiladed against artillery, and 1^ feet
if against musketry; in rear of this rope at any convenient distance
(about 5 yards) drive two pickets into the ground, and upon them
raise or lower a cord or a straight edge of wood, until it is in the
same plane with the rope at the gorge, and the top of the height
from which the work is to be defiladed; then look from the rear
cord or straight edge along that at the gorge, and observe where
the line of sight from thence cuts the poles raised on the tracing of
the intended parapet; these points of section (indicating the position
of the plane of sight) may be marked by one of the party ; lastly,
make the crest of the parapet 4 feet higher than the points thus
found if the work is to be defiladed against cannon, but 6j feet
higher if against musketry.

If it is found that, by this process, the parapet must have more
than 12 feet command in order to defilade the work; the parapet
must be raised to any convenient height, ^suppose 10 or 12 feet,)
and then, in order to defilade the part which is not protected by

*A field-gun stands about 3^ feet above tlie gioinid, and a man on
horseback can fire about 7^ feet above the ground ; therefore 4 and S are
taken as the nearest whole numbers to these commands respectively.



22 FIELD FORTIFICATIONS.

the parapet, a traverse must be erected, or the terreplein of the un-
protected part must be lowered, or both of these steps must be taken
conjointly.

In defilading a tete de pont, the plane of defilade should pass
8 feet above that part of the bridge which is most remote from the
height.

To defilade a closed work, (or one with a parapet both on the side
nearest to and on the f^ide furthest from the height,) unless the
crests on both sides are in a plane passing 8 feet above the ground
which the enemy may occupy, in front and in rear, a parados to
cover the defenders on the banquette of the side nearest to the
height from reverse fire, is indispensable; for it is clear that thq
higher the parapet nearest to a commanding ground is raised in or-
der to defilade a portion of the whole of the interior of tl»e work,
the more will the defenders standing on the banquette of that para-
pet be elevated above t.\e plane of defilement of the parapet fur-
thest from the height, (or the lower one;) they will, consequently,
become exposed to a reverse fire directed over the lower parapet.

In this case, therefore, make the parapet nearest to the command-
ing ground as high as convenient, and so as to defilade a portion (sup-
pose one-half) of the interior : at the extremity of this defiladed por-
tion, and (about) paraliel to the parapet, raise a parados high


2 4 5 6 7 8 9 10 11 12

Online LibraryEgbert L. (Egbert Ludovicus) VieleHand-book of field fortifications and artillery; also manual for light and heavy artillery → online text (page 2 of 12)