Fortification Friday: Wheeler vs. Mahan, comparing barbettes

Last week, I compared Junius B. Wheeler’s post-war field fortification instructions to the pre-war writings of Dennis H. Mahan, specific to the classes of interior arrangements.  The take away there was Wheeler giving the classification more thought and refinement, which no doubt was based on wartime experience.  More of that experience worked into Wheeler’s instructions as the lesson went into specifics about each class.

The first of those classes was on the parapet.  Mahan, of course, narrowed the definition to just that of the batteries.  Wheeler, on the other hand, asked the cadets to consider all type of firepower used in defense of the works:

Defense. – The work may be defended by musketry alone, or it may be defended by artillery combined with musketry.

The arrangements of the parapet for musketry are completed when the banquette and the revetment of the interior slope are finished.

The work, in this condition, does not admit of the use of artillery.  Some additional arrangements must be provided, if artillery is to be employed. The fire of artillery is either over the parapet or through it…..

And with that, Wheeler’s path merged back with that of Mahan leading into the discussion of barbette and embrasure batteries.  Last August when discussion the construction of barbettes, I briefly compared Wheeler’s instructions with Mahan’s.  Wheeler opted for a “least common denominator” planning factors.  Otherwise, the process was generally the same.  I would say that Wheeler’s instructions are easier for me (schooled in the 20th century) to follow. But that’s always a subjective measure.  Still, to be direct with the comparison, here are Mahan’s planning factors, for field guns:

  • Mound of earth 2 feet 9 inches high.
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Depth of 24 feet (atop the tread of the banquette).
  • Ramp behind the mound at least 10 feet wide, sloped at 1:6 ratio.

And here are Wheeler’s (again for field guns):

  • Mound of earth 2 feet 9 inches high (which Wheeler said was optional)
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Mound depth of 20 feet (this could include a platform built just for the cannon).
  • Mound width of 10 to 15 feet (again, this could be the platform built for the cannon)
  • Ramp behind the mound at least 9 feet wide, sloped at 1:6 ratio.

Wheeler offered this illustration to support the instructions:

WheelerFig36

I’m not too concerned with the variation in the dimensions.  If we really need a “culprit” to point towards, I would mention that Mahan was writing at a time when Alfred Mordecai had just introduced revised carriages for field artillery.  But we would be quibbling over the difference in inches within the “instructed” dimensions for something being built out in the field where general measurements would be the rule.  I think Wheeler was just giving us a least common denominator response.

However, since Wheeler gives us a detailed diagram, let us give his instructions a close look.  He set the major line A-B as the interior crest of the parapet.  Eleven inches back of that is line a-b (lower case), where the mound (platform for me) touched the parapet.  The width of the mound’s surface was then set across the line a-b, which is specified as 15 feet in the diagram.  From there perpendiculars extend back twenty feet (a-c and b-d).  That gives us a fifteen by twenty foot surface of the mound (again, I prefer to call this the platform) on which the gun can be worked, allowing for recoil.  From there, Wheeler specified the earth set on the natural slope to support what I call the platform.

As for the ramp, the setup remained the same, though one foot narrower, as that prescribed by Mahan.  Note that Wheeler left the rest of the banquette as configured for musketry, meaning shallow depth.

What we don’t see described here is a battery configured with several guns in barbette along the parapet.  While that could be done, if the need arose, Wheeler agreed with Mahan that barbettes were more likely to be used on the salients.  However, while Mahan gave us very detailed instructions for the construction of such barbettes, Wheeler made short work of this.  After describing the need (and particulars of) the pan-coupé, he waved his hand through the rest:

The construction of the plan differs from the one described only in the form of the supper surface.  In this case, the upper surface is pentagonal in form, care being taken to make it large enough to allow the gun to be fired over the faces of the salient, as well as along the capital.

He even recycled Mahan’s diagram:

WheelerFig37

From there, Wheeler simply added that more guns could be added along the sides of the salient… avoiding the lengthy instructions given by Mahan in that regard.   Sort of leaves me thinking Wheeler didn’t like barbettes.

Well the alternative, as we have seen, for guns in barbette are those firing through embasures.  We’ll discuss Wheeler’s notions about those next week.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 115-20.)

 

Fortification Friday: Wheeler and the evolution of interior arrangements during and post-war

I’ve mentioned on several occasions how the lessons on fortifications, used to teach cadets, demonstrate the evolution of doctrine from pre-war thinking into the post-war era.  We see some of this evolution when comparing pre-war and post-war versions of Mahan’s treatise.  But where the changes really come into play is when comparing Wheeler’s  1880 textbook.

An example of these changes is how Wheeler chose to relate details of interior arrangements.  Even when classifying these arrangements, Wheeler offered a shift, sometimes subtle, change of focus.  Recall Mahan’s pre-war text described the classes of interior arrangements as such:

The class of constructions required for the above purposes, are batteries; powder magazines; traverses; shelters; enclosures for gorges and outlets; interior safety-redoubt, or keep; and bridges of communication.

Somewhat brief, and from there Mahan proceeded to detail how batteries were placed on the parapet… with emphasis on the employment of artillery.

Wheeler offered a similar set of classifications, but abstracted those a bit with a mind to functional requirements:

Classes. – The earth work for the parapet being completed, and the revetments of the interior slope constructed, attention is then paid to the interior of the work.  Certain arrangements have to be made in the interior, to add to the efficiency of the defense, and to provide for the comfort of the troops who have to occupy the work.  These interior arrangements are divided into classes, according to the object to be attained by them.

The divisions may be classified as follows:

  1. The arrangements of, and along a parapet, intended to add to the efficiency of the defense;

  2. The arrangements within the area enclosed by the parapet, to shelter the men and matèriel from the fire of the enemy;

  3. The arrangements made to allow egress and ingress of the troops; including those made to guard the outlets against surprise; and

  4. The arrangements which may be made to provide for the comfort and welfare of the garrison when occupying the interior of the work for some time.

While Mahan named specific structures that would be constructed within the interior, Wheeler’s classifications come across is more so proper doctrine.  In other words, Wheeler put the reason before the task.  Such is a more formal approach to doctrine, as opposed to simply providing a list of structures, and  their specifications, to be used.

And with that approach, abstracted from naming particular types of structures, Wheeler was able to identify some of the needs, beyond the basics, of a garrison occupying a fortification.  You know… like those “comfort and welfare” things he mentions.  But not to be overlooked, shelter from enemy fire and ease of entry or exit.  These were all things Mahan addressed, but gave limited treatment. Did Mahan simply not care about the troops?  No.  But Mahan’s text was rooted in some pre-war concepts which, though we have discussed before, should be repeated for clarity.

Mahan’s instruction about fortification borrowed heavily from the European experience.  It was a textbook on field fortifications which would complement other instruction relying heavily, as is often the case with military science, on the “last war.”  Many scholars have debated the influence of Antoine-Henri Jomini on Civil War generalship.  But I don’t think we can dispute a “what would Napoleon do?” approach prevailed.

That in mind, the military minds came to make several operational assumptions.  With respect to fortifications, the assumption was two types would be employed.  Fixed, permanent fortifications were constructed to defend vital areas.  For the Americans, these were most often seacoast fortifications, as most potential adversaries would need to gain lodgement at some port (the exception, prior to the 1840s, being the northern border, but even there the great lakes presented a seacoast-like need).

The other form of defense assumed was temporary or field fortifications for use by an army on campaign.  And those were intended to work within the tactical framework handed down by the European experience on the Napoleonic battlefield.  These fortifications were employed to protect important areas related to the army’s campaign objectives.  And those objectives were subject to change.  The design of the works was more so to deter direct attack.  In that way the temporary works would deter direct attack, requiring a deliberate effort (i.e. a siege or other significant commitment of resources).  The temporary fortification was not designed for prolonged occupation or lengthy defense.  Above all, the temporary fortification was always a function of the campaign being undertaken at that moment in time.

The American experience, even before the war, offered a slight twist to the paradigm.  With a significant commitment to the frontier, structures such as blockhouses became important.  Yet the US Army did not give much instruction to that part.  In my opinion there were two significant reasons for this.  First, perhaps foremost, the methods for establishing frontier garrisons were judged as intuitive.  Mahan’s “Outpost” manual covered some of this.  Secondly, much of the responsibility for these frontier garrisons was given to territorial, state, or local authorities.  In short, it was not a major mission in the broader sense.  Rather, the US Army was supposed to give Congress detailed plans for defending the coast.  Those commitments out west were not the “big show”.

Civil War experience demonstrated the “two types of forts” assumption to be incorrect, to say the least.  Operational requirements demonstrated there was a third type which fell directly between the chairs.  That being a semi-permanent fortifications which were not directly tied to ongoing campaigns.  The best example of that would be the Washington defenses.  Lesser so the Richmond defenses, but of course those became the focus of a campaign later in the war.  Other examples, which we’ve mentioned in relation to blockhouses, were those works constructed to protect communications and supply lines. All of which supported the army in the field, but were not a direct function of that army’s operation… i.e. the campaign.   And as a function of those requirements, the engineers had to address other needs within the fortifications, to include “comfort and welfare.”

What I like about the passage from Wheeler enumerating the classes of interior arrangements is that he removed a lot of the Napoleonic baggage from the discussion.  Sure, forts were still built in the manner employed in the first quarter of the 19th century.  Men used shovels to dig, then pile the earth.  Fortifications still required parapets and ditches to be effective means of resistance.  The “physics” of the matter did not change, other perhaps than the need for additional thickness to resist rifled projectiles.  But it was the requirement those fortifications were filling that shifted over time.   That’s what I see reflected in the approach Wheeler gave to these particulars.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 51-2; Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, page 114. )

Fortification Friday: Wheeler’s take on blockhouses

As we have often seen, when writing instructions for cadets in the 1880s, Civil War veteran Junius B. Wheeler put a twist or two on Mahan’s pre-war, and post-war, teaching about fortifications.  The blockhouse is yet another example of this.  For starters, Wheeler introduced the notions about using blockhouses with a different set of terms and classifications.  Instead of “safety redoubts”, Wheeler stated with…

Secondary interior arrangements. – Besides the interior arrangements which have been described, or mentioned, there are others which are secondary in their nature.  These are the arrangements which are to be used under certain contingencies, or in cases of emergency.  An example, would be  defense placed within a field work, which defense can be used only when the main work is no longer defensible, etc.

This is certainly keeping in line with the concepts we saw Mahan teaching, but is giving a broader sweep to things.  In the next paragraph, Wheeler jumped right to the blockhouse… but with some fresh considerations:

Block-houses. – It is frequently the case that a separate fortification is constructed, laying entirely within a work exterior to it, into which a garrison can retire and protract their resistance, even after the outer fortification has been taken, or has been made unfit for further defense.

If this interior work is a line of earthen parapet, it is called a retrenchment; if it is a defensible building, it is termed a keep.

The term, keep, is also applied to a work which is entirely separate and distinct from the work exterior to it, whatever may be the material used in its construction. In a field work, the keep is built of timber, and is called a block-house.

There you have it – in fortifications the engineer will consider “secondary interior arrangements” to include retrenchments and keeps.  And if the latter of which is made of timber, it is called a blockhouse.  It’s the same basic thing Mahan wrote, just using different terms.  But, as we all know, terms have meaning and are selected for reasons. In this case, I think Wheeler was simply saying there were many different options for these interior works, and the blockhouse was favored.

Regardless of the emphasis, Wheeler’s blockhouse keep differed little from Mahan’s in form and function.  Wheeler directed the blockhouse have “good command over all of the interior space” and “that all parts of the exterior work can be seen from it.” Wheeler’s blockhouse could be square, rectangular, and “even cruciform” in plan, just like Mahan’s.

However, there was some difference in the internal dimensions.  Rooms inside the blockhouse were to provide six feet of height, though eight or nine were suggested for ventilation.  And Wheeler suggested an interior width of nine feet, “as this is the least distance which can be used and give room for a passageway and a row of bunks.”  Recall, Mahan specified at minimum a nine foot height and a width of up to twenty feet.  But keep in mind Mahan’s dimensions were governed by the need for handling muskets within the blockhouse.  By the time Wheeler was writing, those Springfields had been converted to trap-door models.  Wheeler indicated the overall length of the blockhouse, including all rooms and spaces, would “depend upon the number of men it has to accommodate, after the width has been assumed.”

As to the thickness of walls, Wheeler gave no specification for single or double thickness of timbers.  Rather, he simply indicated, “Block-houses must be made strong enough to resist the projectiles which may strike them and should be proof against fire and splinters.”  Elaborating further, Wheeler said, “The conditions given for a bomb-proof are applicable to the block-house, with the additional one of arranging its walls for defense.”  And by defense, he meant loopholes.   To illustrate this, Wheeler recycled one of Mahan’s drawings:

WheelerFig53_54_55

So in form and function, Mahan’s post-war blockhouse remained the standard.

But what of Mahan’s “American blockhouse” for use as a stand-along fortification?  Well, Wheeler had a different label:

Isolated block-houses. – Timber blockhouses were used frequently in the war of 1861-5 in isolated spots, as independent works.

In these places, they were, as a rule, exposed to attack only from infantry or cavalry, or a few pieces of field artillery.

The construction shown in Figures 53, 54, and 55 is a type of these isolated block-houses.

It was found from experience that it required a thickness of forty inches of solid timber to resist the projectiles of field-guns.

These isolated block-houses were frequently built two stories high.  The upper story was usually placed so as to have its sides make an angle with the sides of the lower story. By this arrangement, the corners of the upper story projected over the sides of the lower.  This arrangement of the upper story removed the dead space near the sides of the lower story, and the sector without fire in front of the angles.  Block-houses exposed to artillery should not have a second story.

Again, very close to the words Mahan wrote, but mixing the pre-war with a bit of wartime experience.  There are a few points which followed in Wheeler’s text that we shall return to in time.  But for the most part, we see the “American blockhouse”, with a few new terms to describe its classification within the fortification form.

Wheeler’s manual was aimed at a generation of cadets some twenty years removed from the Civil War.  That generation would include men like John J. Pershing.  While some, as was the case with Pershing, would serve on the frontier where there was use for the old blockhouses.  And around that time, our romantic notions of the frontier posts took root.  So I close this discussion of blockhouses coming full circle to those movie props and toys which come to mind when that sort of fortification is mentioned.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 156-60.)

Fortification Friday: “Arrangements Intended for the Comfort and Health of a Garrison”

All well and good to build up a fort, complete with obstacles and defilements for to make the attacker’s job that much harder. Better still to have the interior arrangements for proper placement, and protection, of batteries, with bombproof magazines filled with ammunition at the ready.  And maybe even offer some bombproofs just so the garrison can seek shelter from enemy fire.  But we must remember that for most of the fort’s existence, it will be more “home” for the garrison than defensive structure.

In short, a fort has to be “livable”, otherwise the garrison will go sour.  And by sour, we are referring to failing health and morale.  Disease and malaise have often felled more than bullets and shells.  Writing before the war, Mahan gave only general instructions in his treatise on fortification. But for the post-war instruction, Wheeler set aside a section specifically to discuss what he classified as, “Arrangements Intended for the Comfort and Health of a Garrison.”

Nature of the arrangements. – A garrison compelled to live within an enclosed space like a field work, should be provided with all the arrangements which are necessary for the health and for the comfort of the men, consistent with surrounding circumstances.

The arrangements essential to the health and comfort of the men include those intended to protect them from the weather, to provide for their support, and to supply their necessities.

Necessities?  Yes, we are talking, for the most part, about facilities for personal hygiene, food preparation, and berthing. Hurrah for the Sanitary Commission!  Of course, under the “consistent with surrounding circumstances” the commander might bring in other amenities such as a hospital or sutler store. But, all depended upon circumstances.

As for particulars:

The principal arrangements are the tents, huts, or shelters in which the men are sheltered; the guard-houses, and rooms for those on duty; the kitchens and bake-ovens in which the food is prepared; the sinks or privies, and the places provided for the men for washing themselves and their clothing; the hospitals for the sick; etc. Wells, or means of providing the garrison with a supply of good drinking water, form no unimportant part of the arrangements necessary for the comfort as well as the health of a garrison.

But for all that importance, Wheeler lacked the space necessary to best cover this subject:

The limits of this book will not admit of a discussion, nor even a reference to the various divisions, of this important section of the interior arrangements of a field work.

Those arrangements are second only to those required for actual defense, and in many cases they are equal, as the defense of the work in a great measure depends upon them.

The only rule that can be laid down is to make all these arrangements of a temporary character, and to place them so that they can be removed, at a moment’s notice, out of the way of any interference with an active defense of the fortification.

Certainly, Wheeler’s words on this subject reflected extensive experience from the Civil War. We can find evidence attesting to this at numerous surviving sites.  In addition to the works, we see company streets laid out with tent pads or huts provided; kitchens in close, but safe, proximity; sinks, not so close in proximity; and always attention paid to water.  Not just clean water for drinking and cleaning, but also ensuring any standing, stagnant water was drained away.

With respect to the limited space allocated within the fortification manuals, there were other classes at West Point and other manuals of instruction. Perhaps best serving that point, we consult Major General Daniel Butterfield’s Camp and Outpost Duty for Infantry, published in 1863:

In camp, the best water will be pointed out before the men are dismissed… The places for cooking and sinks must be pointed out to the orderly sergeants of companies… The cooking places must be chosen with a view to avoid danger of fire.

It must be explained to the men, as a standing order, that when no regular sinks are made, nor any particular spot pointed out, they are to go to the rear, at least 200 yards beyond the sentries of the rear guard.  All men disobeying this order must be punished.

Punished for the sake of their own health and comfort, mind you!

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 155-6;  Daniel Butterfield, Camp and Outpost Duty for Infantry, New York: Harper & Brothers, 1863, page 53.)

Fortification Friday: “a redan or a lunette, is thrown up on the exterior to cover the outlet”

Before we close the discussion of openings for forts (see what I did there?), let me circle back to compare Mahan and Wheeler in regard to one of the fine points considered.  That being the use of a detached redan or lunette in advance of the outlet.  Recall that in pre-war writing, Mahan suggested:

In very frequented passages, a redan or a lunette, is thrown up on the exterior to cover the outlet, and thus ensure its safety in case of surprise.

And Wheeler, in the post-war, mentioned a similar arrangement, but perhaps narrowed the application to those larger outlets, for sorties, where simple interior traverses would not be practical.

Mahan offered two figures that illustrated the redan to the front of an outlet:

PlateVIIFig48_49

Figure 48 offers a wide redan in front of an outlet, which is further covered and flanked by by the “horns” of the larger work.  A very well protected outlet, we might say.  Mahan considered this a Redan Line.

On Figure 49, we see much more complexity.  Particularly with the defensive lines of fire.  The outlet is nested within a redan of a larger line.  On both sides are faces within redans of differing angles. This is considered a Tenaille Line – a proper definition we will discuss later.  But the point being the covering redan, to the front of the outlet, was absolutely necessary here in order to protect that weak spot.  The covering redan is somewhat off center of the outlet, perhaps to limit exposure at the expense of accessibility.

Wheeler, as you may recall, gave us only a simple rendition of the covering redan:

WheelerFig51

The question I have in regard to these advanced, detached “parts” covering openings is… just how often were these employed during the Civil War?

When examining surviving earthworks, we often find the area around the outlets obliterated.  Sometimes, due to necessity, that is done to facilitate visitor access.  But more often, just a case where the structures around the outlets were the most susceptible to erosion.

And when examining wartime plans, we see some use of these redans… but more often not.  Consider Fortress Rosecrans outside Murfreesboro:

FortressRosecrans

This was, some have said, the largest fort built during the war.  And in this plan we see examples of many features suggested by Mahan.  Specific to the outlets, we see up near the top that Battery Cruft was a detached lunette (maybe a “half lunette”) covering an outlet.  Elsewhere, such as next to Lunette McCook at the bottom right, we see an outlet (an existing road) without a covering redan or traverse.  Though we do see obstacles erected to the right of Lunette McCook.  And certainly that named work was positioned to dominate the approaches to the outlet.  Furthermore, what you don’t see in my “snip” are works in advance of the fortress that covered the railroad and road.  Though those were oriented south and not regarded as covering the outlet in question.

Another plan to consider is from Virginia, at Deep Bottom:

DeepBottomSnip

Here we see five road crossings at the main line of the works.  One of those is blocked entirely by a redan.  The other four (including one that appears to be a path cut just to clear a redan) have no traverses or covering works.  Just obstacles placed in front.

If we are assessing the protection of outlets, with Mahan’s suggestions in mind, we find a mixed application of those covering redans.  Seems to me the use of that sort of feature was based on the engineering assessment of need.

Now considering such use under Wheeler’s suggested implementation, let’s look to the location of a few large scale sorties.  First, how about the works were the Crater assault was mounted:

CraterSector

And further around the lines, and further forward in the historical timeline, to the sector around Fort Mahone:

FortMahoneSector

And to the left of that sector near where the Federal Sixth Corps mounted their sortie:

FortWelchSector

Now the scale of these maps mean these are not so much “plans” as operational maps.  So we know there are structures that escaped the pen here.  But what stands out, with double underlines, is the use of something far more elaborate than Mahan and Wheeler discussed.  We see entire sections of works advanced in a manner to provide staging grounds for those formations preparing for the assaults. Major assaults, mind you, involving whole divisions.  These were, you see, works built for the offensive.  Grand offensives!  In that light, might we say the entire Federal line was one large “covering work” in front of an array of staging areas and supply depots?

Fortification Friday: Barriers, Bridges, and Ramps … the fort “communications” infrastructure

Writing the instructions for cadets, almost two decades after the Civil War, Junius B. Wheeler focused more on the functional requirements of outlets in the fortifications, as opposed to the important details of construction.  The nuances here are, I think, important.  Mahan presented the outlet had important operational uses within fortifications, but proceeded to discuss the particulars assuming the student would understand what those uses were. Wheeler focused on the uses up front, citing communications as the need and outlets as the remedy. And we must broaden “communications” a bit, perhaps our 21st century writers might say “traffic” and refer to movement of personnel, supplies, as well as messengers carrying communication.

Wheeler added to Mahan’s instructions with mention of “turn back” traverses at the mouth of outlets (to reduce the area under enemy lines of fire) and wider outlets for sorties.  But for the most part, the construction techniques remained the same.  Likewise, Wheeler identified the same supplemental structures as Mahan – barriers and bridges – while adding a few embellishments.

First off, we have the barriers. Which were… well… gates:

Barriers. – The outlets are usually arranged so that they can be quickly closed, to guard against surprise. The means used is a gate, technically termed a barrier.

The gate is made with two leaves, hanging on posts by hinges, and made to open inward.

The frame of each leaf is composed of two uprights, called stiles; two cross pieces, one at the other at the bottom, called rails; and a diagonal brace, called a swinging bar.

The leaf of the barrier may be open, by spiking stout upright pieces, with intervals between them, to the pieces of the frame; or it may be made solid, forming what is known as a bullet-proof gate.

Yes, something you might purchase, pre-fabricated, at the home improvement store.  But note that Wheeler offered two versions – a light “open” leaf version with spaces between the uprights AND a heavy “bullet-proof” version with no spaces.  The latter was illustrated in Figure 52 of Wheeler’s book:

WheelerFig52

Wheeler added some practical observations about the construction of these heavy gates:

Since the gate must be strong, the leaves of it are necessarily very heavy.  The leaves must be hung upon stout posts, firmly braced into the ground, to sustain the great weight of the gate.

The top rails of all barriers should not be less than six feet above the ground.

In the barriers with open leaves, the vertical pieces are usually extended from eighteen inches to two feet above the top rails, and their upper ends sharpened.

In those which are solid, it is usual to arrange some obstruction upon the top rail, such as sharp pointed spikes, broken glass, etc., to interfere with persons climbing over the top. It is usual to provide apertures in the leaves, through which the men can fire upon the ground outside.

Gotta love those details.  From having a “peephole” to shoot out from to having broken glass atop the gate.

Bridges were another supplement to the outlet’s composition:

Bridges. – When the ditch has been completed along that part of the work in front of the outlet, it is usual to carry the roadway across the ditch by means of a bridge.

The ditches of field works are, as a rule, quite narrow, and the bridges used to span them are very simple constructions.

A common method of building the bridge is to lay three or more sleepers across the ditch, and cover them with planks laid transversely.  If the span is sufficient to require intermediate supports, these are obtained by using trestles placed in the ditch.

A bridge built in this way can be quickly removed and speedily re-built, if there be any necessity for it.

We might consider this a temporary bridge without any means for retracting or removing, save dismantling.  Thus it was kept to the bare minimum arrangements.

Wheeler mentioned that other “hand-books on military engineering” described the use of draw bridges or rolling bridges in fortifications. He only briefly discussed the former, being basically as that detailed by Mahan in earlier texts.  The latter were designed to be “… pushed out from the work, and drawn back into it.” Both sort of bridges were,

… known as movable bridges, are useful to guard against surprise, to prevent stragglers from entering, and to keep the garrison in the work.  As a defense against an assault of a field work, they are of but little value.

To belabor that point:

The best method, is to have no ditch in front of an outlet, but let the roadway be on the natural surface of the ground.

Of course, operational needs might vary. But you get the point.

Wheeler offered one more supplemental structure under his heading of communications, but not one that was directly associated with outlets.  This was the ramp.  While all writers on fortifications discussed ramps in some regard, Wheeler saw fit to highlight the structure and its role for interior communications (and again, broaden that definition as mentioned above):

Ramps. – The short roads used in fortifications to ascend from one level to another, are termed ramps.

The width of a ramp depends upon its use, following the rule laid down for the width of passages.  A width of six feet for infantry, and of ten feet for artillery, are the widths generally used.

The inclination of the ramp may be as great as one on six, and as little as one on fifteen, depending upon the difference of level between the top and bottom. The side slopes of earth with its natural slope.

The ramps in a work should be placed in positions where they will not be in the way, nor occupy room which may be required for other purposes.

Steps or stairways are sometimes used instead of ramps. The rule for them is that the breadth of each step, called the tread, shall be at least twelve inches, and the height of the step, known as the rise, shall be about eight inches.

They are substituted for ramps in those places where there is not sufficient room for the ramp.

Nothing new or advanced here.  Ramps and stairs were part of fortifications dating back to the earliest times.  What is noteworthy here is that Wheeler considered them part of the fort’s communication infrastructure… again communication in the sense of how one moves into, out of, and inside of the fort.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 151-5.)

 

Fortification Friday: Outlets evolved in the post-war instructions

Earlier we considered the pre-war instructions offered by Mahan in regard to outlets, particularly how those would be constructed.  Training cadets before the war, Mahan covered the topic with a few paragraphs.  Later, after the war, he added a section about bridges associated with the outlets.  However, by the 1880s, Junius Wheeler’s instructions to cadets would sprawl over the better part of seven pages.  Maybe this is simply a case where pre-war there was a premium on copy space for printed manuals.  On the other hand, maybe Wheeler felt “his” cadets needed “special help” getting this particular aspect of fort-building in order.  Or perhaps, which I tend to think, wartime experiences prompted the extended emphasis.

The first thing we see with Wheeler’s treatment of the subject is a focus shift.  Before discussing the construction of these outlets, he steps back to discuss what these are used for.  Notably, the section is titled “Communications, barriers, etc.”:

Communications. – The defenders of a closed work must have arrangements made by means of which they can enter or go out of the work when necessary. In the case of continued lines, arrangements should be provided by means of which the defenders can make sorties.

Word choice here.  Mahan’s writing pressed that outlets were important for servicing the works.  Left unstated, implied at best, were other uses for the outlets – communications, logistics, reconnaissance, or simply just getting out to stretch the garrison’s legs. On the other hand, by selecting this sub-heading, Wheeler set different functional requirements… although, much vaguer than Mahan’s.

Wheeler’s second paragraph discussed the basics.  The outlets had to be made through the parapet, and that created a weak point in the work.  To minimize the risk, he insisted the number of outlets be kept to a minimum and built where least exposed.  Specifically, he gave these suggested placements:

In redoubts, the outlets are on the sides least exposed to attack; in half-enclosed works, they are placed near the middle of the gorge; in forts, they are usually placed near the re-entrants.

And that matches well to Mahan’s suggestions from decades before.  But where we see variations is with the size of these outlets, which Wheeler relates based on functional requirements:

A passage for the use of infantry only should not, as a general thing, be less than six feet wide; for artillery, not less than ten feet wide; for sorties, the outlets in continued lines should be at least fifty yards wide.

See what happened here?  Wheeler took Mahan’s two size categories and then added a third, based on an additional use-case.  I think at this passage Wheeler is thinking back to works at Vicksburg or Petersburg or around Atlanta.  And in those campaigns, field works had become part of an offensive operation – be that a very deliberate siege at some levels, but more dynamic than an “According to Vauban” siege.  Again, not to say wide passages for sorties did not exist prior to 1861, but rather to say those took on different emphasis around about 1863-4… at least in the American context.

Demonstrating that the more things change, the more they tend to stay the same, Wheeler offered this diagram for an idea outlet through the parapet with masking traverse:

WheelerFig50

Yes, he ripped off Mahan.  But give him a little credit for adding more notations and giving us that nice profile on the right.  And he offered the lines (c and g, the “crossing” arrows through the middle) which were the enemy’s “extreme lines of fire”.  Those lines, Wheeler instructed, governed the length of the traverse (T).  Note how those lines are drawn off the corner of the superior slope of the parapet.

Toward that, Wheeler did add a lot more to the notion of masking the outlet:

The length of the traverse may be shortened by turning back the interior crest at right angles to its general direction, and extending it as far as the crest of the Banquette.

This “turn back” is indicated by “B” on the figure.  One on each side of the outlet.  Notice how that would send the lines of fire (again, lines c and g) into a tighter intersection, further back in the outlet, and thus reducing the area the enemy might fire upon. Such angles reduced, the traverse need not be so long.  All in all a nice functional flourish to the outlet design.

Beyond that, Wheeler offered other options that would reduce the vulnerability:

Instead of having a road along its entire front, the traverse is sometimes joined to the parapet on one side of the opening, as shown by the dotted lines b d and e f, in Fig. 50.

And such would mean only one “extreme line of fire” need be considered in regard to traverse length.  But that did mean traffic had a nasty chicane to deal with.

This is all good, but how about that fifty-yard wide outlet for sorties?

The method adopted to mask the interior of the work in this latter case, is to place the traverse opposite the outlet on the outside, and beyond the ditch….

The traverse in this case is usually broken, generally a redan in trace, with the profile of a parapet, but commanded by the parapet in rear.

Figure 51 illustrated this manner of masking:

WheelerFig51

Basically, if you have to provide a wide gap in the works, the only viable solution is to build a miniature work in front to cover it.  Keep in mind, such sortie outlets would be placed on less exposed sections of the line.  And of course, these would be covered by strong faces on either side.  Such would reduce the chance the enemy might target the outlet for their own “sortie”.  And the redan covering the front would further reduce the danger of direct artillery fire.  Note, however, you don’t see a traverse in the interior of the works to cover the sortie outlet.  The purpose of this outlet is to allow infantry in wide columns to emerge in quick order. Bad enough, though necessary, to have that redan in the way.  Putting a traverse on the inside would impose yet another choke point for maneuver… and yet another delay in an operation where time was critical.

Before leaving Wheeler’s discussion of outlets, we’ll also examine the evolution of barriers and bridges.  Furthermore, Wheeler introduced the notion of ramps within these outlets.  All interesting facets to consider with respect to the art of military fortifications.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 148-51.)