Fortification Friday: Wheeler vs. Mahan, comparing barbettes

Last week, I compared Junius B. Wheeler’s post-war field fortification instructions to the pre-war writings of Dennis H. Mahan, specific to the classes of interior arrangements.  The take away there was Wheeler giving the classification more thought and refinement, which no doubt was based on wartime experience.  More of that experience worked into Wheeler’s instructions as the lesson went into specifics about each class.

The first of those classes was on the parapet.  Mahan, of course, narrowed the definition to just that of the batteries.  Wheeler, on the other hand, asked the cadets to consider all type of firepower used in defense of the works:

Defense. – The work may be defended by musketry alone, or it may be defended by artillery combined with musketry.

The arrangements of the parapet for musketry are completed when the banquette and the revetment of the interior slope are finished.

The work, in this condition, does not admit of the use of artillery.  Some additional arrangements must be provided, if artillery is to be employed. The fire of artillery is either over the parapet or through it…..

And with that, Wheeler’s path merged back with that of Mahan leading into the discussion of barbette and embrasure batteries.  Last August when discussion the construction of barbettes, I briefly compared Wheeler’s instructions with Mahan’s.  Wheeler opted for a “least common denominator” planning factors.  Otherwise, the process was generally the same.  I would say that Wheeler’s instructions are easier for me (schooled in the 20th century) to follow. But that’s always a subjective measure.  Still, to be direct with the comparison, here are Mahan’s planning factors, for field guns:

  • Mound of earth 2 feet 9 inches high.
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Depth of 24 feet (atop the tread of the banquette).
  • Ramp behind the mound at least 10 feet wide, sloped at 1:6 ratio.

And here are Wheeler’s (again for field guns):

  • Mound of earth 2 feet 9 inches high (which Wheeler said was optional)
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Mound depth of 20 feet (this could include a platform built just for the cannon).
  • Mound width of 10 to 15 feet (again, this could be the platform built for the cannon)
  • Ramp behind the mound at least 9 feet wide, sloped at 1:6 ratio.

Wheeler offered this illustration to support the instructions:


I’m not too concerned with the variation in the dimensions.  If we really need a “culprit” to point towards, I would mention that Mahan was writing at a time when Alfred Mordecai had just introduced revised carriages for field artillery.  But we would be quibbling over the difference in inches within the “instructed” dimensions for something being built out in the field where general measurements would be the rule.  I think Wheeler was just giving us a least common denominator response.

However, since Wheeler gives us a detailed diagram, let us give his instructions a close look.  He set the major line A-B as the interior crest of the parapet.  Eleven inches back of that is line a-b (lower case), where the mound (platform for me) touched the parapet.  The width of the mound’s surface was then set across the line a-b, which is specified as 15 feet in the diagram.  From there perpendiculars extend back twenty feet (a-c and b-d).  That gives us a fifteen by twenty foot surface of the mound (again, I prefer to call this the platform) on which the gun can be worked, allowing for recoil.  From there, Wheeler specified the earth set on the natural slope to support what I call the platform.

As for the ramp, the setup remained the same, though one foot narrower, as that prescribed by Mahan.  Note that Wheeler left the rest of the banquette as configured for musketry, meaning shallow depth.

What we don’t see described here is a battery configured with several guns in barbette along the parapet.  While that could be done, if the need arose, Wheeler agreed with Mahan that barbettes were more likely to be used on the salients.  However, while Mahan gave us very detailed instructions for the construction of such barbettes, Wheeler made short work of this.  After describing the need (and particulars of) the pan-coupé, he waved his hand through the rest:

The construction of the plan differs from the one described only in the form of the supper surface.  In this case, the upper surface is pentagonal in form, care being taken to make it large enough to allow the gun to be fired over the faces of the salient, as well as along the capital.

He even recycled Mahan’s diagram:


From there, Wheeler simply added that more guns could be added along the sides of the salient… avoiding the lengthy instructions given by Mahan in that regard.   Sort of leaves me thinking Wheeler didn’t like barbettes.

Well the alternative, as we have seen, for guns in barbette are those firing through embasures.  We’ll discuss Wheeler’s notions about those next week.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 115-20.)


Fortification Friday: Let’s put more guns in barbette batteries on the bastion

Last Friday we walked through the process of placing a gun, in a barbette battery, to cover the capital of a bastion (in other words… at the point of the salient).  Such is a significant improvement of a defense, as it allows the defender to put firepower on the “sector without fire” and address one of the inherent flaws of the bastion.  If setup by the numbers, the emplacement looked as such (ignore “Figure E” to the left for the moment):


Such is good news for the defender.  But the artillerist is quick to point out, that’s a rather exposed position.  To cover that gun on the capital, the defender would want to place additional guns in the bastion, on the faces.  This would not only afford counter-battery fire on any attacker cannon aimed at the point, such would also bolster the firepower of the faces of the salient.

With that expressed need in mind, consider the next paragraph from Mahan:

If three more more guns are placed in the salient, a pan-coupé is formed as in the last case, and twenty-four feet are in like manner set off on the capital but instead of proceeding as in the last case, a perpendicular is drawn from this point to each face, and the pentagonal space, thus enclosed, will be taken for the gun in the salient; from the perpendiculars last set off, as many times sixteen-and-a-half feet will be set off on the interior crest of each face, as there are guns required:  this will give the length of the barbette along each face; the depth will be made twenty-four feet, and the two will be united in the salient.  One of more ramps may be made as most convenient.

So to add those additional guns, we start with the barbette on the capital as established before:


Note here the ramp is removed from the first barbette, so we have a pentagon to work with, instead of a hexagon.

Next step is to allocate space to the sides of that pentagon on a parallel of the parapet:


This is 24 feet back of the parapet, to confirm to the depth of a standard barbette. Note this is the “mound” or built up area prescribed for a barbette, and should bring elevation up to at least “two feet nine inches below the interior crest for guns of small caliber, and four feet for heavy guns.”

Next the engineer would allocate a frontage of 16 ½ feet (what Mahan wrote) or 18 feet (what Mahan put on his diagram) for each gun:


Confusion over the correct dimensions?  Not really.  Recall 16 ½ feet was for field guns and 18 feet was for siege guns. Both requirements appear to be offered interchangeably in the instruction.  In those frontages, we see the platforms (C) for the guns.

With those defined, in the case of our diagram, there are two barbette positions on each face:


But… how do we get the guns up there?  Oh… the ramps:


These would be ten feet wide and with a 1:6 slope.  Granted, if it were me those would be dressed a lot cleaner to avoid twists and turns.  Anyone who’s moved a couch into a small apartment door might relate.

Great!  We have a battery with five guns in barbette on the salient angle of the bastion. But everything has pros and cons.  So let us assess:

The advantages of the barbette consist in the commanding position given the guns, and in a very wide field of fire; on these accounts the salients are best positions for them.  Their defects are, that they expose the guns and men to the enemy’s artillery and sharpshooters.

Light guns, particularly howitzers, are the best for arming barbettes; because the hollow projectile of the latter is very formidable, both to the enemy’s columns and to his cavalry; and when his batteries are opened against the salients, the light pieces can be readily withdrawn.

See, it would be nice to have the ramps setup for easy handling.

But let’s close with focus on those advantages and disadvantages.  The barbette allowed the artillery to fire over the parapet and afforded a wide field of fire.  But unlike infantry, the artillery could not simply “duck down” behind the parapet.  And thus were exposed.  What we will look at next is an arrangement that traded field of fire for protection against enemy fire – the embrasure.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 53-4.)

Fortification Friday: Building a barbette in a bastion

Last week we discussed placement of artillery in barbettes to form batteries in a fortification defense.  Such an arrangement allowed the cannon to fire over the parapet, even with allowance for declination, at an attacker. Mahan’s description included details about spacing to allow for handling of the gun:

  • Mound of earth 2 feet 9 inches high.
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Depth of 24 feet (atop the tread of the banquette).
  • Ramp behind the mound at least 10 feet wide, sloped at 1:6 ratio.

Those figures were a rule of thumb to be adjusted to the situation.  And that rule of thumb best fit a situation were several guns were placed on a face, flank, or curtain wall.  In other words, a straight line of the defensive works.

But to illustrate the barbette, Mahan offered this illustration:


A barbette on a bastion’s salient angle.  Mahan observed:

As barbettes are usually placed in the salients, an arrangement is made for the guns to fire in the direction of the capital.  The construction in this case is somewhat different from the preceding. A pan-coupé of eleven feet is first made; from the foot of the interior slope at the pan-coupé, a distance of twenty-four feet is set off along the capital; at the extremity of this line a perpendicular is drawn to the capital; and five feet are set off on this perpendicular on each side of the capital; from these points, on the perpendicular, a line is drawn perpendicular to each face respectively; the hexagonal figure, thus laid out, is the surface of the barbette for one gun. The ramp in this case is made along the capitol [sic].

Let’s walk through this one step at a time, using Mahan’s illustration.  First we want to setup that pan-coupé within the salient angle:

As per Mahan’s guidance, this was eleven feet (indicated in red) perpendicular to the line of the capital (blue line included for reference).

From there, a distance of 24 feet – the depth prescribed for a barbette battery – was walked back towards the gorge:


Next, a width of five feet on either side of the capital was set aside within the depth:


With the depth and width established, this creates platform for the barbette, marked C on Mahan’s diagram:


From there, perpendiculars off each face of the bastion were defined:


Now the ramp was defined and laid out.  This would be ten feet wide, 1:6 slope, and along the line of the capital.  This is indicated as “B” on Mahan’s diagram:


With the ramp established, all sides of surface of the barbette are defined.  This being a hexagonal shape, indicated as “A” on the diagram:


One last bit of work to mention here. The parapet at the salient angle required adjustment to allow the cannon to depress.  Part of the parapet was cut down, indicated as “D” on the diagram:


This would cover much of the “sector without fire” at the capital. And it is called the superior slope of the pan-coupé.

Consider these features, the surface of the barbette, ramp, platform, and superior slope of the pan-coupé when seen on the horizontal:


This in place, the fortification had a position for one cannon on the bastion’s salient angle. And such could go a long way to reduce that sector without fire.

But one gun?  Just dangling out there over the parapet for the enemy to shoot away?  That won’t do!  So now we should look at arrangements made for several guns within the bastion.  That’s for next week….

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 53.)