Fortification Friday: Complex “Intrenchments” and complexity fortification history

Earlier in this series, we looked at a class of fortification that Mahan labeled “simple fortifications.”  These included some well recognized – and some often misidentified – fortification layouts… “the Right Line; the Redan; the Lunette, or Detached Bastion; the Crémaillère, or Indented Line; the Priest-Cap, or Swallow-Tail; the Redoubt; the Star Fort; and Bastion Fort.”  And these were, for emphasis, “simple” in relative terms in regard to the level of effort required for the engineer to design the works.  Briefly, the simple entrenchments (or, if you prefer, fortifications) allowed the engineer to focus simply on a single point to be defended or single intended purpose… simply… for sake of redundancy.

But if there are simple entrenchments, there must be complex ones also, right?  Indeed.  And how do we define those complex entrenchments… or as they were called in Mahan’s day “intrenchments?  Well… as with many other things, Mahan avoided a specific definition.  Almost as if he wanted the cadets to focus more on the practical application than the theory.  But we can derive a general definition of “complex” from the description of the forms constituting these arrangements:

Lines or Complex Intrenchments.

Dispositions made to cover extended positions, and which present a front in one direction to the enemy, are termed Lines.  There are two classes of lines – Continued Lines, and Lines with Intervals.  Continued lines present no openings through which the enemy can penetrate except the ordinary outlets.  Lines with intervals consist of detached works, which are enclosed partly, or entirely, throughout their perimeters, arranged in defensive relations with each other; and presenting wide intervals between them defended only by their fire.

Implied here are three notions.  First, that complex entrenchments are actually a super-set of simple works arranged to present a front.  Second, that complex entrenchments are, as an aggregate, not just defending a salient point of importance, but rather the whole of an area.  Lastly, complex entrenchments, as the title “lines” implies, are erected to bar an adversary from approaching along a broad front.  These presumptions are reinforced by the second introductory paragraph from Mahan:

The same general principles apply to lines as to other intrenchments; but, from their great extent, they usually receive a slight relief, and the simplest angular figures are adopted for their plan.  In laying them out, the engineer should avail himself of all the natural obstacles presented by the position, so as to diminish the labor of erecting artificial ones.

See how this becomes counter-intuitive?  Complex entrenchments should, according to Mahan, be refined with simple angles of defense and fewer ancillary obstacles.  And from the perspective of the historian studying fortifications of the Civil War to latch onto.  Mahan feared exponential growth of labor requirements that might come from very complex arrangements.   And to that point, let me skip ahead to the last passage from the chapter on complex entrenchments:

Besides, a very capital objection in war, the time and labor required to throw up so many works are altogether beyond what can be disposed of in the ordinary circumstances of an army.

The emphasis is in the original –  “a very capital objection in war.”

You see, this is the point at which we must start reconciling what we know of field fortifications as a function of military science with what we know of the same fortifications as a described from wartime correspondence and observed with the surviving works.  Indeed, if one studies the accounts and walks along the surviving works, there are serious questions about how much of Mahan’s teaching shows up on the battlefield.  Sure, we see star forts, bastions, and redans at some places.  But how do we account for the entrenchments scattered about Virginia, particularly from the Overland Campaign?  (And mention Northern Georgia and a plethora of other places, but I’m selecting a handy point of reference here.)  The works at the Wilderness, Spotslyvania, and North Anna seem to be contradictory, in some places by small increments but at others with a wide gap, to Mahan’s teaching.

And this requires us, as historians, to recognize military science is not a fixed field of study.  Like any other profession, military science evolves, changes, and re-invents.  It is ever changing, even while reliant on core principles.  And with that in mind, we must lay Mahan’s pre-war teachings beside his post-war revisions, to the same manual of study.  Furthermore, we must also pull in the post-war manuals, such as Wheeler’s, which I much prefer, as additional perspectives.  It is vital to understand not only that military science was applied on the battlefield, but it also evolved due to its application on the battlefield.  We’ll see, as I progress along this thread of posts, how Mahan’s construct of “simple and complex” field fortifications would transition into Wheeler’s “simple, complex, and hasty” fortifications…. and that’s being overly simplistic.

There in lies the complexity when studying these complex entrenchments.  There is much a “point in time” to consider.  Not so much that manuals needed to be re-written, but that the re-writing was derived from experience derived from the very occurrences being studied.

I’ll go one further.  It is not enough that we recognize that military science evolved, offering a new approach to the application of field fortifications.  We should also seek out the drivers that caused that evolution.  Now we might lay this at the alter of the rifled musket.  But I’d challenge that long held belief a bit.  If this was all due to accurate, high-velocity projectiles, wouldn’t the natural evolution be towards thicker, stouter, more resilient works?

I say there’s more to it than just the mine ball and Hotchkiss shells.  And that something had to do with the point Mahan cited – time and labor.  In my opinion, the more important driver on military science were the advancements of technology that translated to better logistics and communications.  Those, by application, translated to operational mobility which far surpassed that of previous generations of warfighters. Then by extension, that gave the battlefield commander more resources to apply to the situation. You see, in order to add more complexity to those complex entrenchments, one must afford more time and labor.

OK…. I’m tipping my hand to a conclusion for which I’ve not presented the evidence.  To do that, we must first explore Mahan’s complex entrenchments, or lines, in detail.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 69 and 75.)

Fortification Friday: As Wheeler would say, “Loopholes are not just for blockhouses”

As we continue to compare and contrast the pre-war writings of Dennis H. Mahan with the post-war instructions of Junius B. Wheeler, let’s move from arrangements for the artillery to that of the infantry on the parapet.  Mahan, like his contemporaries, left the infantry on a bare parapet, with a firing step on the banquette that would allow them to step-up to the crest and fire over.  Little else was deemed necessary.  But experience of the Civil War indicated something more was indeed necessary.   Wheeler discussed this under the heading of “loopholes”:

Loop-holes.  – Troops on the banquette, when in the act of firing their pieces, are frequently exposed to the fire of the enemy’s sharp-shooters.  Under these circumstances, expedients must be devised to protect the men, without interfering with their fire.  The expedient which is most generally used, is that of an improvised loop-hole.  The loop-hole is made, in this case, by arranging two or more rows of sand bags, placed upon the parapet and filled with earth, so that the top row will be higher than the men’s heads, and so as to leave intervals between the bags in the lower rows, through which the men can aim and fire their pieces.

Figure 28 illustrated this arrangement:


Let’s walk through this passage, as it offers another glimpse into the changing doctrine applied to the battlefield.  Right from the start, we see something “non-Mahan” as a condition.  In those pre-war days where the Napoleonic battlefield framework was in play, musketry was generally used in mass.  Volley fire, by attacker and defender, was the expected means of delivering those lead projectiles.  In that framework, working a musket into a gap in sandbags would slow down the delivery of a volley.

But Wheeler alluded to a change in how musketry was used.  Instead of massing fires in volleys, the Civil War armies employed much more individual fires.  Skirmishing, of course, took on greater importance.  And in these field fortifications, that translated to sharpshooting.  More likely the attacker would employ this means of attriting the defender, instead of attempting a rush of the works.

This is not to say nobody ever thought of putting sandbags on the parapet before the Civil War (or headlogs, which we’ll circle back to).  But this is to say changes in the way musketry was delivered brought out a need to employ this feature (loopholes) as a standard fit on the parapet.  Wheeler and his contemporaries didn’t invent the sandbag loophole.  They simply introduced it to meet an evolving requirement.  Yes, “innovation” does not always mean “invention.”

There were other ways to setup a loophole on the parapet of course:

Gabions are also used for a similar purpose.  The gabions are placed in pairs upon the parapet and filled with earth, each pair being separated from the adjacent pair by an interval of about two inches.

And… field experience gave us even more options:

A contrivance adopted in the war of 1861-5, was quite effective for the same purpose.  Skids were placed upon the parapet, with notches cut in them.  A heavy log was placed on the skids, occupying a position parallel to the interior crest and just in contact with the superior slope.  Notches were cut in the underside of this horizontal log and these were used as loop-holes.  The openings to the exterior were made as small as possible, and in some cases were protected by small patches of boiler iron spiked upon the log.  When exposed to artillery fire, earth was banked against the log.

We often hear this or similar arrangements called a “head log” in the writings of veterans.  I am most curious that Wheeler didn’t use the term.  And even more curious why Wheeler didn’t include an illustration!  At any rate, he continued with this description, naming an “innovator” from the late war:

A wooden loop-hole was devised by Lieut. King (now Major) of the United States Engineers, which was used in 1864.  It was practically a wooden hopper made of boards, placed upon the superior slope of the parapet, and covered with earth.  The splay of the sole and the angle of the cheeks were made to suit the field of fire required.

The officer mentioned was Lieutenant William R. King.  Brett Schulte has King’s report on Beyond the Crater, and a detailed report it is.  The accompanying illustration matches to Wheeler’s description.  For brevity, I’ll refer you to Brett’s excellent site.

Wheeler continued, with detailed requirements for these loopholes:

The exterior orifice of a loop-hole for musketry should be made as small as possible.  A width of two inches and a height of five, is sufficiently large for ordinary purposes.  The sides are sloped, and an inclination given to the bottom and top, according to the field of fire which is to be swept.

Now what is good for the musket should also be good for the cannons, right?  Of course:

Embrasures are sometimes protected in a manner similar to this arrangement for loop-holes.  Timbers are laid across the embrasure, covering the throat, leaving only room for the muzzle of the piece.  These timbers are then covered by sand bags, by fascines, etc., to make them shot-proof.  Sometimes the embrasure is filled with sand bags or fascines to mask it, these things being quickly removed when the embrasure is needed for use.

Thick wooden shutters, made bullet-proof, and placed on vertical axes, and iron shutters swung on horizontal axes, have both been used to close the throat of the embrasure.

In some cases, timber supports were extended back from the parapet and a covering of timber and earth placed upon them, protecting the gun from vertical and plunging fire.  A gun thus sheltered is said to be case-mated.

Again we see the factor driving all this “innovation” and change – different types of fire were employed.   Individual musketry … sharpshooting as it may be called… brought out the need for protection on the parapet.  Vertical and plunging fire, which I have written about before, brought out the need for overhead protection.  No new inventions are introduced here, rather the innovation lay in the way existing practices were employed.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 126-8.)


Fortification Friday: Wheeler vs. Mahan, Embrasures and Bonnettes

Last week we gave time to Junius B. Wheeler’s instructions about barbette batteries.  Now let us turn to his thoughts on embrasures, which were the alternative siting of artillery in a field fortification.  Wheeler offered this drawing of an embrasure for reference:


Perhaps a cleaner diagram than Mahan used, either in his pre-war or post-war texts, but generally the same features. The art and science of making an embrasure changed little.  For reference, here are the labels and specifications Wheeler gave:

  • The Sole was the bottom of the embrasure: G-E-F-H in the figure.  This was inclined outward, usually at the same rate as the superior slope of the parapet.
  • The Throat was the opening on the interior: a-b-G-H in the figure. Normally 18 to 24 inches wide.
  • The Mouth was the exterior opening: C-E-F-D.
  • The Splay described the widening of the embrasure towards the exterior.
  • The Cheeks were sides of the embrasure: a-CE and b-F-D.
  • Directly bisecting the sole between the cheeks is the Directrix: M-N.  This determined the base orientation of the cannon in the embrasure.
  • The Genouillere was the slope between the throat and the banquette (or raised mound for the gun’s platform).
  • The Merlon was the section of parapet between embrasures on the parapet.

Wheeler indicated that embrasures were best cut out after the parapet was completed, adding “the exterior openings are masked until the moment to use them arrives, to prevent their position from being discovered by reconnoitering parties of the enemy.”  In terms of labor estimates, Wheeler indicated, “a detail of six men should be able to cut an embrasure in the parapet of a field work and finish it in eight hours.”

But before those six men could take shovel in hand, the engineer had to trace the embrasure.  Wheeler offered detailed instructions.  More detailed than Mahan’s but not significantly different.  The process started by drawing the directrix.  From there the throat was defined.  From there the sole, mouth, and cheeks were drawn out.  But the key to all those elements was the slope of the sole and the angles of the splay.  And those elements defined the angles at which the gun could be trained to fire.  Thus very important things to consider:

The splay of the sole is usually determined, in plan, by giving to E F some definite length, and then joining its extremities with the lower line of the throat.  A throat twenty inches wide will have a horizontal field of fire of twenty-two degrees, when E F is equal to one half the thickness of the parapet; a fire of thirty-one degrees, when the E F is equal to two-thirds of the thickness; a fire of forty-eight degrees, when this line is equal to the thickness of the parapet.

Mahan had offered a similar rule, but I tend to like Wheeler’s explanation better.  Just seems clearer and fine to the point.   From there, Wheeler discussed how to lay out the cheeks and complete the embrasure.  Like Mahan, Wheeler suggested revetting the embrasure to prevent damage when firing the cannon.  Gabions were preferred, though sod was also suggested.

Since more than one gun would be placed on the parapet:

Consecutive embrasures should not be nearer to each other than fifteen feet from center to center, to prevent crowding of the guns and to prevent the merlon, M, from being too weak.  A merlon which measures less than six feet on the exterior crest should not be allowed, as it would make the parapet too weak.

Note the location of the merlon, M, on the figure:


Consider the rule of thumb regarding the size of the mouth (that E-F measure) when applied here.  Let’s say our parapet is five feet thick, and you want to allow a 48º traverse.  So the E-F line must be five feet on the exterior crest.  But the distance between “F” on the left side embrasure and the “E” on the right side embrasure must be at least fifteen feet.  Furthermore the distance between the left side “D” and the right side’s “C” must be at least six feet.  Adding all those together, we find a total front needed of twenty-five feet of parapet face, at minimum, if we want two cannon with 48º traverse.  All well and good if you have room. But we might want to reduce the traverse to avoid unnecessary work.

Like Mahan, Wheeler considered both direct and oblique embrasures.  Regarding the latter, Wheeler offered the limitations up front:

Oblique embrasures do not admit of the muzzle of the gun being inserted so far as the direct ones, and they weaken the parapets more.

Oblique embrasures are not used, as a rule, if the directrix makes with the normal to the crest an angle exceeding ten degrees.  In case the angle is greater, the embrasure is provided for, in field works, by modifying the interior crest by means of the method known as “indenting.”

This method consists of making a crest a crémaillère line, instead of a right line, with the short branches perpendicular to the direction of fire, and in those short branches constructing direct embrasures.

Or, simply put, if you need a larger angle than ten degrees off the dirextrix, then build a small redan or other extension out from the parapet.  Such implies a better trance should be considered to start with.

Overall, comparing barbettes to embrasures, Wheeler considered the former as offering wide fields of fire without weakening the parapet.  But the barbette exposed the gun crew to enemy fire.  While the embrasure protected the guns and crew, there were limitations to the field of fire and weaknesses along the parapet.  Furthermore, Wheeler warned that embrasures made a good mark for enemy fires against the fortification.  Recall during the war Federals were very proud of the 3-inch Ordnance Rifles’ ability to put rounds through Confederate embrasures at range.

To mitigate the exposure of the guns and crew from enemy fire, Wheeler offered an additional structure, calling them Bonnettes:

It is frequently desirable that the height of the parapet, at certain points, should be increased for a short distance.  This increase is generally obtained by making use of the constructions known as bonnettes.  A bonnette extends but a short distance along the parapet, is make of earth, and is used generally to give greater protection to the men standing on the banquette against a slant or an enfilading fire of the enemy.

Bonnettes are placed usually on the salilents; they are sometimes placed on the parapet between guns “en barbette.”

They may be constructed during the progress of the work, or after the work has been finished.  In the former case, their construction is, to all intents and purposes, similar to that of the parapet. In the latter case, they are constructed generally in haste, and sand bags or gabions filled with earth are used to build them.

Note, bonnettes are not traverses, as they stand directly on the parapet.  Rather these were structures placed to the sides of the barbette (or embrasure if needed).  While I can find references to bonnettes going back to the previous century, Mahan seems to have disregarded them.  The reason may lay in the disadvantage of the bonnette.  In effect, the structure raises the parapet’s interior crest relative to the banquette, thus preventing musketry from that section of the parapet.  In Mahan’s framework, musketry was considered important to the fort’s defense.  However, by Wheeler’s time canister fire seemed to be more desirable.  That would reduce manpower requirements, foot for foot, on the parapet.

Comparing Wheeler with Mahan, in regard to arrangements for batteries, there is not much difference in terms of form or even implementation.  But we do see some variance in the function.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 120-6.)