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.)


Fortification Friday: Direct and Oblique Embrasures

Last Friday we introduced the embrasure and identified the components of that structure.  In brief, the embrasure allowed the defender to place artillery so as to fire through the parapet instead of over it, thus granting the gunners some protection.  However, the nature of the embrasure limited the field of fire for the cannon.  Thus the directrix, bisecting the sole and thereby setting the cannon’s orientation, becomes an important line when setting up embrasures.

The lay of the directrix would determine what the cannon could point at.  And it was possible that the directrix be arranged with off-set angles to allow the cannon to fire with the desired effects:

When the directrix is perpendicular to the direction of the parapet, the embrasure is termed direct; when the directrix makes an acute angle with it the embrasure is termed oblique.

Those two types were demonstrated by Mahan in Figure 33 Bis:


I’ve highlighted the directrix of the direct embrasure in blue.  That of the oblique embrasure is in red, on the right. Obviously, the change in the angle would bring about refinements to the manner of construction of the embrasure.  And Mahan detailed those:

The manner of laying out an oblique embrasure is similar to the direct; the mouth is of a rectangular form, but is made wider in proportion to the obliquity, in order that the pat of the embrasure, which corresponds to the muzzle of the gun may be nearly of the same width in both the direct and oblique embrasures.  The exterior width of the sole is made equal to one half the length of the directrix, measured on the sole.  The cheeks are laid out as in the last case.

Allow me to use all colors to best illustrate what the professor is calling for here:


A color explosion!  Yes, so we can call out specific lines for reference. First we have the mouth of the embrasure in green.  As suggested, this is nearly the same as on a direct embrasure.  But it is wider to the inside of the angle, to allow clearance of the muzzle.  And where it needs to be wider is on the exterior end of the box that is the mouth.

Take the measure of the directrix and apply half to the exterior end of the sole, the gold line above. As the angle of the dirextrix in this example is only slightly off perpendicular, we see the left side of the sole, and the corresponding cheek, are not far off the direct embrasure arrangements (outlined in yellow).

But it is on the right, on the inside of the angle, that adjustments are made. A line from the exterior of the sole back to the mouth has to cross outside the exterior of the mouth to reach the back of the interior (dashed blue line).  Thus the portion of what would have been the cheek (in red) must be cleared back.  And the right-side cheek must have a different interior point.  That adjustment is depicted with the solid blue line.

In short, a little geometry in order to ensure the projectile clears the embrasure without obstructions.

Such modifications, where applied, bring us to one limitation of the embrasure:

The muzzle of the gun should enter at least six inches into the embrasure, to prevent the blast from injuring the cheeks; this limits the obliquity of the directix to about 60° for long guns.

Thus, not only is the traverse of the gun limited by the embrasure, the line of fire possible from the gun has a limit.  No 59° oblique embrasures… definitely no 45° oblique embrasures.  For locations requiring fires at those angles from the parapet, a barbette was required.

And considering the force of the cannon’s discharge, another limiting factor comes into play:

The height of the cheeks must not be more than four feet, for the same reason; it will, therefore, in some cases, be necessary to raise the ground on which the wheels rest.

Common sense in play here – the deeper the sole of the embrasure is set, the more contained the force of the cannon’s blast.  And if contained too much, the force is apt to cause damage to the embrasure… or worse, to the parapet.  And to avoid a deep embrasure, Mahan suggested a mound or platform just as used with the barbette. Though obviously not as high.

These limitations defined, Mahan proceeded to introduce new terms defining components outside the embrasure itself:

The parapet of a battery is usually termed the epaulment. The interior face of the epaulment, and the cheeks of the embrasures, are [reveted] in the usual manner. That part of the interior face which lies below the chase of the gun is termed the genouillére. The mass of earth between two embrasures is termed a merlon.

The most important term to note here is epaulment, technically just indicating an altered parapet.  At times we see Civil War reports referring to epaulments and including parapets to the flanks of detached batteries. Even where one might think “traverse” would be an apt name to apply. In that general sense, epaulment was applied to the entire parapet constructed to protect the gun or guns.

As to constructing epaulments with embrasures?

The embrasures are generally cut out after the epaulment is thrown up. If their position is decided upon beforehand, they may be roughly formed at first, and be finished after the epaulment is made.

So you can just build a parapet, then clear out the embrasure later to create the epaulment.  Or one might save some earth moving and leave the intended embrasure clear while piling up the parapet/epaulment.  Results may vary.

One last word on embrasures – their advantages and best use:

The advantages of embrasures are, that the men and the guns are less exposed than in a barbette battery.  Their principal defects are, that they have a very limited field of fire; they weaken the parapet; and present openings through which the enemy may penetrate in an assault. Owing to their limited field of fire, they are chiefly used for the protection of particular points; as to flank a ditch, protect a salient, enfilade a road, &c. The most suitable position for them in a work is on the flanks.

This passage is important for historians to consider.  We see lots and lots of embrasures on surviving field fortifications.  Some of them, such as at Petersburg, are on besieging fortifications, where directrix would be defined so as to bring fire on specific points.  At the same time, Confederate fortifications at Petersburg have embrasures oriented so as to bring counter-fire on specific sections of the besieging lines.  Those pass a “form follows function” logic.

But on the other hand, there are many surviving defensive fortifications not associated with siege operations that don’t fall in line with Mahan’s suggested employment of embrasures.  Fort Evans, just a few blocks from where I’m writing this post, comes to mind.

Fort Evans 004

These embrasures were placed on a curtain wall, with dirextrix oriented across the face of the fortification.  Not a particular point, a salient, a flank, or to enfilade a road (though Edwards Ferry Road is nearby, the angle and separation work against that idea).

So why did the engineer make embrasures?

Well first off, the engineer most closely associated with the fort, John Morris Wampler, did not benefit from tutelage under Mahan at West Point.  Wampler was a trained topographical engineer, with experience surveying the coasts.  All indications are his military engineering skills were learned “on the job site.”  And this was his first major project. Not to detract from Wampler’s engineering skills (as he would later be involved with some rather good works around Charleston in particular).  But he built the works according to requirements set down by superiors.

Which brings us to another line of inquiry here – what did the Confederate commander want to do with those guns?  Well if one looks at the orientation of the embrasures, guns on that particular wall were oriented to fire over the Potomac River to the Maryland side of Edwards Ferry.  And embrasures on the north side of the fort were oriented at Ball’s Bluff, Harrison Island, and points beyond on the Maryland shore.

During the time Wampler and other Confederates built Fort Evans, their batteries faced off against Federal rifled batteries north of the Potomac.  While the Confederates had some rifled guns, the Federals at this time employed Parrott rifles (up to 30-pdrs).  Given the nature of the situation, embrasures were probably justified in order to best protect the gunners.  But, as with so much about Fort Evans, we don’t know that for sure.  Just offering some “form should follow function” logic.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 55-6.)