Fortification Friday: Embrasures to protect the gunners

Over the last few weeks, we’ve looked at the placement of artillery in the field fortification, in what is called batteries.  The first option explored was a barbette mounting, where the cannon fired over the parapet.  Such an arrangement offered a wide field of fire.  But it also exposed the cannon crew to enemy fire.  Another alternative was to provide openings in the parapet through which the cannon could fire.  Thus allowing the gunners to remain behind the parapet as they worked the cannon. So let’s circle back to Mahan, who defined these openings along with introducing terms to describe the components:

The embrasure is an opening made in the parapet for a gun to fire through.  The bottom of the embrasure, termed the sole, is two feet nine inches, or four feet above the ground, on which the wheels of the carriage rest, according to the size of the gun; it slopes outward to allow the gun to be fired under an inclination, the base of this slope should never be less than six times the altitude….

I hate to interrupt the professor mid-sentence, but it is a run-on.  And we need to consider these components of the embrasure in turn.  Allow me to use the diagram from Mahan’s post-war instruction (which is somewhat cleaner for purposes of highlighting these parts of the embrasure):


What we see here is two sub-diagrams depicting a section of the parapet on which an embrasure is placed.  On the right is the plan, or the fortification as seen from directly above, laid out across the horizontal plane.  On the left is the elevation, or vertical.  I’ll put a solid blue line between the two sub-diagrams for clarity, and indicate where the sole is located:


On the right, the sole is C-D-I-K.  On the left, the sole is line a-b.  In simple terms, we might consider the sole to be the floor of the embrasure.  So the cannon will be firing over the sole.  Thus we have two properties to consider.

First is the height of the sole respective to what would be the tread of the banquette, or interior behind the parapet.  Mahan tells us “two feet, nine inches or four feet” depending on the size of the cannon. Recall that is the same measure specified when considering the height of the mound behind the parapet for barbette positions.  Yes, properties of the cannon and carriage remain the same, but we are applying those measures in different ways.  For a barbette, that specified the measure from the interior crest down to the mound of earth (and thus determined how high the mound of earth needed to be).  For a parapet, that specified the measure from the interior edge of the sole down to the tread. Another way to look at it, when building an embrasure the measure of two feet, nine inches (or four feet for the bigger gun) determines how deep the embrasure is cut into the parapet.  Regardless of barbette or embrasure, the cannon’s muzzle needed to have clearance to fire some 33 inches above the flat ground on which the carriage (wheels) sat. Got that?

The second property to consider is the horizontal lay of the sole as it extends through the parapet.  To allow declination, the sole had to slope down from the interior edge. So we have to think about how that opening is defined and regulated, as Mahan continued:

… the interior opening, termed the mouth, is from eighteen inches to two feet wide, according to the caliber of the gun, and is of a rectangular form….

Turning back to our diagram, here is the mouth:


On the right, that’s under I-K-O-P; to the left a triangle from a-d-unlabeled point.  This mouth was a foot-and-a-half to two feet wide.  The “gun books” tell us 6-pdr field gun muzzle swells are 8.25 inches in diameter.  The muzzle band on the 32-pdr field howitzer is 11 inches in diameter. Later models of the 24-pdr siege gun had a muzzle swell out to 15.5 inches. Going to the extreme end, 32-pdr and 42-pdr seacoast guns have muzzle swells to 15.5 inches and 17 inches, respectively.  So you can see where 24 inches (two feet) would give adequate clearance for the largest guns that could possibly be used in a field fortification… at the time Mahan was writing that is, as the big Parrotts and Rodmans were not yet in service.

From the mouth, the sole will decline outward as we noted above.  Furthermore, it should also expand wider to allow some traverse for the gun.  And the professor had a word for that widening of the sole:

… the embrasure widens toward the exterior, which widening is term the splay….

The splay is not necessarily a “part” but more so a specification applied to the embrasure.  But here’s where that specification would play out on the diagram:


If I had fancy 3-D modeling, you’d have a cool animation spinning about to show how this splay opens outward.  Sorry, I don’t so you’ll have to go visit a fort for that.  But you get the idea of where the splay is employed.  But as with all parts of a fortification, the splay had natural limits, lest it be too great or too small… or grossly impact other components:

… the manner in which the splay is regulated, is by producing the sole to the exterior slope of the parapet, and making this exterior line measured on the sole, equal to half the distance between the inner and outer lines of the sole. This construction makes the sole a trapezoidal figure, the side of the trapezoid, on the interior being eighteen inches, or two feet; the opposite side being equal to or half the perpendicular distance between the two sides.

We fixed the mouth at between 18 and 24 inches.  To set the exterior opening’s size, take a measure from the mouth to the front (line C-D in the diagram, but not necessarily correlating to the front of the parapet mind you).  Half of that measure will give you the necessary width of the exterior opening.

So it is important to have a line defined that is perpendicular to the mouth, running directly out the embrasure.  What do we call that?

The line which bisects the sole is termed the directrix of the embrasure….

And we see that on the diagram:


Since this is a 3-dimensional feature, the embrasure must have sides for the trapezoidal sole.  What are those called?

… the sides of the embrasure, termed the cheeks, are laid out by setting off two points on the exterior crest of the parapet, one on the right, the other on the left of the sole, so that the horizontal distance of these points from the sole shall be equal to one-third their height above it.  Lines are then drawn on the exterior slope, from these points to the exterior points of the sole; lines are in like manner drawn from the same points, on the superior slope to the upper points of the mouth, on the interior crest.  These four lines form the boundaries of the two cheeks on the superior and exterior slopes.

So let me highlight the cheeks on the diagram:


Note how these cheeks are also sloped… maybe we include that as part of the splay… to allow better clearance.  What we are defining here with the embrasure is not only the opening that the cannon’s muzzle will use, but also the opening that the projectile will exit the fortification.  Thus it is rather important to provide ample clearance!

Having identified the components and some of the properties of embrasures, let’s take a step back and consider how these were used.  Yes, the gunners now had a parapet between them and the attackers. But as laid out between the cheeks and sole, the splay also limited the field of fire.  Thus embrasure placement was very important.  One might set the directrix to be perpendicular to the line of the parapet.  Or perhaps the situation required an off-set angle, or oblique, to best cover a particular corner of the works.  Thus embrasure construction was not simply digging out some “windows” for the fort.  As these embrasures would define what sectors the big guns could dominate, the engineer and artillerist had to work together in order to get the most out of the emplacement.  We’ll look at that in the next installment.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 54-5.)

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

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