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:

PlateIVFig33bisDirectObliqe

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:

PlateIVFig33bisObliqe

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.)

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