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.)