Fortification Friday: Square, rectangle, or even a cross – blockhouse forms

Last week, we introduced the blockhouse as an interior structure, perhaps better classified as a facility, within a field fortification.  Allow me to stress again, the context of Mahan’s writings in “Field Fortifications” about blockhouses was scoped to discuss the use of those sort of structures in conjunction with larger works.  It was not to say blockhouses would always be used as such, nor to dismiss other sorts of employment of that fortification type. This particular Mahan lesson (of which there were many, across several manuals, as we must recall) was focused on building a “keep” so the defenders might “keep” something valuable… their lives in the event all was lost.

Having discussed the concept and general layout of the blockhouse, Mahan turned to particulars:

With regard to the details of the construction, the timber for the sides should be twelve inches thick, to resist an attack of musketry, and to resist field-pieces, two feet, in which case the sides are formed of two thicknesses of twelve-inch timber. If the timber is placed upright, each piece should be let into a mortise in the cap-sill; and every fourth piece of the top, at least, should be notched on the cap-sill, to prevent the sides from spreading out.

This would form, in essence, the walls of the blockhouse.  Notice the prescribed thickness, in regard to the expected threat – be that musketry or artillery.  I would add that with the introduction of rifled artillery, the two foot thickness was insufficient.  But there begins a point of diminishing return. How much more timber should one add to the blockhouse, thus subtracting usable interior space, in order to defend against an Ordnance or Parrott rifle?  Ah… a question best addressed when we consider the post-war manuals!  So let’s hold that thought.

I do wish Mahan had included a good illustration of the proposed arrangement of timbers. And I’ve not located any other contemporary illustration to serve.  But the general idea is apparent… perhaps for generations who suffered the splinters from Lincoln Logs, if not so much for those of more recent times and their Lego bricks.  We will revisit the arrangement of timbers in the walls for the post-war manuals.

Moving forward, we need to consider the layout of those walls and how best to arrange the blockhouse in order to meet requirements:

The plan of the block-house must conform to its object generally; it may be square or rectangular.  If flank defenses are required, its play may be that of a cross. The interior height should not be less than nine feet, to allow ample room for loading the musket; this height will require that the timber of the sides shall be twelve feet long, in order to firmly set in the earth.  Sometimes a ground sill is placed under the uprights, but this is seldom necessary.  The width may be only twelve feet in some cases, but it is better to allow twenty feet; this will admit of a camp bed of boards on each side, six-and-a-half feet wide, and free space of seven feet….

So the layout, as seen from above, could be the square form familiar to us from the playsets of yore.  Or could be extended or expanded to use other layouts as tactical needs demanded.  The layout tended to employ right angles, however.  We look back at Figure 44, which is somewhat a cross, in plan:


Notice how the dimensions are governed somewhat by the need to provide space for handling muskets.  Form will follow function.  The most important quality of the blockhouse, as a keep, is to allow the garrison to create a pause in the action, should the parapet be lost.

But “camp bed”?  Yes, that implies a place to sleep. But it was also a defensive arrangement.  “The camp bed serves also as a banquette; it is placed four feet three inches below the loop-hole, and has a slight slope of about eight inches inwards.”  Notice how the interior arrangement is to provide, in terms of wall to wall floor space, for a 6 ½ foot wide camp bed on each side with open space for seven feet between.

Now everything thus far has implied the garrison would only have muskets in the blockhouse.  Let us make arrangements, then, for artillery:

If cannon is to be used for the defense, the width must be at least twenty-four feet; this will allow eighteen feet for the service of the gun, which is generally ample, and six feet for a defense of musketry on the opposite side.  A greater width than twenty-four feet cannot well be allowed, because the bearing would be too great between the sides for twelve inch timber; and even for a width of sixteen feet it would be well to support the top pieces, by placing a girder under them on the shores.

Basically, bigger guns require more space.  So we adjust the arrangements.  But there is a physical limit as to how much more space is allocated.  If a really large blockhouse were built, it would require substantial structural reinforcement.  Better to stick with a single cannon per side, if used at all.

Since these arrangements place emphasis on affording space to handle weapons, be that musket or cannon, we need to discuss the loopholes in detail.  We’ll turn to that in the next installment.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 63-4.)

Fortification Friday: Blockhouses as Safety Redoubts in the Fort

When I say “blockhouse” many of you might be thinking about favorite childhood playsets:


Certainly suitable for the defense of the bedroom in the face of the elite Confederate Plastic Brigade, or perhaps the indigenous Plastikawi tribe.  But… something that could not hold against the Green Army Men armed with bazookas and flamethrowers.

Kidding aside, the playset fort is pattered after real structures from American history.  The blockhouse was not unique to America, as it was a form brought over by Europeans.  However, the blockhouse became the preferred fortification on the North American continent from colonial times right up to the 20th century.  Blockhouses work well in situations where the enemy is unlikely to possess anything larger than light artillery.  The interior of the blockhouse was easily adapted into living quarters.  Conversely, living quarters (houses) might be easily adapted into a blockhouse.  Those, and other qualities, made that form of fortification popular on the frontier.

The popular image of a blockhouse is something made of wood.  But stone, or even adobe, might be used instead.  Since wood was in abundance on the early American frontier, we tend to see a lot of structures like this one:

Ft King George 3 Aug 11 1273

This is recreation of Fort King George, Darien, Georgia (a place with many, many layers of history).  In this particular case, the blockhouse served several roles – a high observation platform over the marsh, a platform for covering fire to protect approaches to the fort, and, in the event the works were overwhelmed, a final defense for the fort’s garrison.

It is that last function that Mahan had in mind when considering interior arrangements for field fortifications.  Blockhouses were a structure that could be used for what he called “safety redoubts”:

Safety Redoubt.  In enclosed works a place of retreat, into which the troops may retire in safety after a vigorous defense of the main work, will remove the fears of the garrison for the consequences of a successful attack of the enemy, and will inspire them with confidence to hold out to the last moment.

This interior work, which may be very properly be termed the keep, can only be applied to works of large interior capacity.  It may be formed of earth, or consist simply of a space enclosed by a defensive stoccade, or palisading.  In either case it should be about four feet higher than the main work, to prevent the enemy from obtaining a plunging fire in it from the parapet of the main work.

Let us pause here before going to Mahan’s formal introduction of the blockhouse.  This “hold out to the last” is a notion steeped in 19th century presumptions about how a siege would play out.  A garrison “holding out” would force the enemy to make a direct attack on the parapet… in other words, to get up close, personal, and… well… very violent with the defender.  And in that violent melee, the defender was not exactly in a position to call a “time out”.

The safety redoubt, or keep, was a place to retreat and, more importantly, force a pause in the action.  And from the keep, within that pause, the defender might negotiate a cessation of the fight, with honor.  Thus we see how that might allay fears of “consequences” for the garrison.

That in mind, Mahan offered his preference for the keep:

The best arrangement for the keep is the construction termed the block-house. This work is made of heavy timber, either squared on two sides or four; the pieces which form the sides of the block-house are either laid horizontally, and halved together at the ends, like an ordinary log-house, or else they are placed vertically, side by side, and connected at the top by a cap-sill. The sides are arranged with loop-hole defenses; and the top is formed by laying heavy logs, side by side, of the same thickness as those used for the sides, and covering them with earth to the depth of three feet.

Mahan offered this figure as an example of a blockhouse:


This perspective is looking at the blockhouse along with a cross section of adjacent works and structures.  Rather busy.  This section is along the line a-b from Figure 44:


The combined caption reads:

Figs. 43,44. Shows the plan and section of a block-house of upright timber.  The plan is made to exhibit a portion of the top complete; the timber covering the top; the arrangement of the cap pieces; a plan of the loop-holes; and a plan of the camp-bed. Fig. 43 exhibits, in a like manner, a cross section of the block-house and ditch; with interior and exterior elevation.

We will go into the particulars for construction in later posts.  What is important to identify here is the functional nature of this blockhouse.  Just as with the colonial-era Fort King George, we see a blockhouse adjacent to a ditch and other defensive structures.  One might say the blockhouse filled up the fort’s interior.

For an attacker, this presents a serious tactical problem.  One might defeat the defender on the parapet.  But the parapet would be a dangerous place to make a living with the blockhouse overlooking all. So you see where a “pause” would be in order.

Keep in mind, within this discussion of keeps, Mahan was not stating or suggesting that blockhouses only be constructed within and in conjunction with elaborate field works.  Rather that he offered that a blockhouse was a structure that served well as a keep inside a larger set of works.  We see that usage applied by his students during the Civil War.  Looking back again to Fortress Rosecrans:


We see Redoubts Schofield, Brannan, T.J. Wood, and Johnson within the interior.  One wartime report described the arrangement as, “… strong against attack, being defended by large keeps, which deliver their fire upon every part of the interior.”  I would further add that most of the lunettes on the perimeter of this vast fortress included blockhouses.  So there were multiple “keeps” within a depth of the defense.  Keep in mind the scale of this fortress.  The safety redoubts, named above, were armed with 30-pdr Parrotts and 8-inch siege howitzers.  The Confederates would need to bring a large amount of iron in order to suppress the fort’s garrison.

But the size of this work was perhaps its weak point.  After the Army of the Cumberland moved further south, through the summer of 1863, there the need to keep this fortification in order was taxing, in terms of manpower. An 1865 report suggested all be reduced to simple blockhouses covering the bridge and depots.

That circles back to the point about blockhouse usage.  As said before, Mahan was not suggesting the only place to use a blockhouse was as a fort’s keep. But as his text was focused on field fortifications, the focus was on that function.  We will see blockhouses enter the conversation in regard to detached defenses in particular.  Furthermore, the post-war instructions would place more emphasis on the detached, singular blockhouse.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 62-3; OR, Series I, Volume XLIX, Part 2, Serial 104, page 502.)

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:


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:


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:


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:


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:


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


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


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:


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:


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:


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.)

Fortification Friday: Drawbridges? For field fortifications?

Last week we looked at the use of outlets in the field fortifications.  Certainly it was impractical to insist the garrison stockpile sufficient food and supplies for the anticipated duration of the conflict at hand… not to mention cross their legs the whole time.  Some sort of gate or entrance was required.  However, that presented a weak point that an attacker might exploit.  Thus Mahan urged his students to build additional structures, namely traverses, to cover those entrances.  That reduced the risk to a degree.

But where the point where the outlet crossed the ditch (if it had to cross a ditch, that is) presented a greater problem.  As we’ve seen in detail, one of the important properties of the ditch was serving as an obstacle to any attack.  But the outlet would require a six to ten foot wide path across the ditch.  Thus granting an attacker a potential highway across.  Common sense response to that problem was to simply build some sort of retractable bridge.  But pre-war, Mahan did not offer that as a direct solution (indeed, avoiding mention of the problem altogether).  But in his post-war edition, Mahan offered:

Draw-Bridge. For the usually narrow ditches of field works, either a light rolling bridge may be used for a communication, from the outlet, across the ditch; or else an ordinary wooden draw-bridge.  A very simple one, and of easy construction, was proposed by Colonel Bergère of the French engineers.

I believe this refers to Colonel Pierre Bergère (1785-1868), but I don’t know of a specific work to reference in regard to the bridges in question here.  I find it interesting the more detailed discussion of bridges enters Mahan’s post-war edition with Bergère serving as a reference and introduction.  Either as if the 1861-65 experience simply followed that of the French engineer, or could not be applied directly.  I have my thoughts, but let’s save that for later.

Mahan offered Figure 45, bis to illustrate the proposed bridge:


Technical details followed:

The bridge is a light platform a,a’, of joists and boards, long enough to span the ditch D, and so arranged as to turn around an axle at A, the crest of the scarp.  At the point B, on each side of the platform, an iron gudgeon is firmly attached to it and turns in the eye of a socket at the end of lever C B.  This lever is formed of two pieces of scantling of some tough flexible wood, each about four inches square. The lever has an eye, at the middle point O, which receives a strong iron bolt that connects two ordinary gun carriage wheels.  The two pieces which form the lever are firmly fastened together, as shown in the figure; a weight, consisting of shells filled with sand, or shot, being fastened at the end c, and serving as a counterpoise to the bridge.  Two rails A, of heavy scantling are laid for the wheels to run upon in maneuvering the bridge; which is done simply by one or two men taking hold of the spokes of the wheels, and so, by turning them, causing them to run backwards or forwards, and thus raise or lower the bridge.

The arrangement is a clean piece of engineering.  But nothing novel or particularly advanced, technically speaking.  Noteworthy is the re-purposing of common equipment, such as carriage wheels and projectiles, on hand with the field army.

But I have to throw a flag out here.  If those building the fortification had ample time to build fancy drawbridges, they were more so working as garrison troops and probably not actively campaigning.  The premise behind field fortifications was those would be temporary structures established as part of a field army’s operations.  Not structures to be garrisoned, which should shade more to the permanent fortifications with far more elaborate arrangements all around.  But experience in the Civil War took the American officers away from some pre-war assumptions about how fortifications would be use.  And more importantly how to classify them.  We’ve already noted that Junius Wheeler began classifying semi-permanent and temporary garrison fortifications within the discussion of field fortifications.

Now this is not to say no self-respecting American engineer in 1861 would consider the drawbridge when planning the outlet to a fort. Rather that in 1861, the use of drawbridges were not emphasized in the classroom.  But by 1870, that changed to provide a couple of paragraphs and a nice illustration based on a French officer’s recommendations.  As we will see by looking at Wheeler’s text, by the next decade, those instructions evolved even further.  Pages, mind you, dedicated to the discussion of small bridges!

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 57-8.)