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:

PlateIVFig32W

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:
PlateIVFig32A1

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:

PlateIVFig32A2

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

PlateIVFig32A3

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

PlateIVFig32A4

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

PlateIVFig32A5

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:

PlateIVFig32A6

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

PlateIVFig32A7

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:

PlateIVFig32A8

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:

PlateIVFig32A9

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.)

Fortification Friday: Fire over the parapet, build some barbettes

In opening the discussion of interior arrangements for field fortifications, Mahan impressed upon his students that artillery placement was of great importance.  Poorly placed artillery allowed the enemy to become contemptuous of the defenses.  That, of course, would turn the attacker’s conversation from “those are mean defenses” to “we can do this.”  And the defender never wants to concede such, even if it be purely psychological ground.

So were do we place the cannons in our fort?  Simple answer – we put them in batteries:

Batteries. The term battery is usually applied to a collection of several guns; it is also used in speaking of the arrangements made of a parapet to enable the guns to fire over it, or through the openings in it; as a barbette battery, and embrasure battery, &c. Two kinds of batteries are used in the defense of intrenchments, the barbette battery and the embrasure battery.

Readers are probably familiar with the terms barbette and embrasure from discussions of key fortifications made in sesquicentennial posts.  But, as a reminder, this is a barbette as employed on a fixed, permanent fortification:

Fort Moultrie 3 May 2010 510

There are some refinements seen with there at Fort Moultrie which are not necessary with field fortifications.  And of course field fortifications are more apt to use field artillery.  But let us focus here on the basics – the gun is elevated to give a clear line of sight over the parapet.  As Mahan wrote, “The barbette is a construction by means of which a piece can fire over a parapet.”

But recall that with field fortifications, the parapet was designed to afford protection for musket-firing infantry.  Thus the interior stood a bit higher from the tread of the banquette, compared to what we see at Fort Moultrie.  An important consideration when planning a barbette in those field fortifications, as Mahan continued:

[The barbette] consists of a mound of earth, thrown up against the interior slope; the upper surface of which is level, and two feet nine inches below the interior crest for guns of small caliber, and four feet for heavy guns.  If the barbette is raised behind a face, its length should be sufficient to allow sixteen-and-a-half to eighteen feet along the interior crest for each gun; and its depth, or perpendicular distance from the foot of the interior slope to the rear, should be twenty-four feet, for the service of the guns.

Consider the suggested dimensions and what governs those. We have to first consider the line of the bore above the ground, as mounted on a carriage, above the ground.  In his post-war update to the instructions, Junius Wheeler cited this as 43 inches, close to the 43.1 inches for carriages used with 6-pdr field guns or 12-pdr field howitzers. For a 24-pdr field howitzer, the height increased to 44.8-inches. And for a 12-pdr Napoleon (or 32-pdr field howitzer) the height was 45.2.  So we see Wheeler was offering a “least common denominator” planning factor. (For those with a soft spot for little cannon, the mountain howitzer on prairie carriage was 30.5 inches from ground to the line of the bore.)

But… 43 inches is good only if we intend to fire the gun at zero elevation.  We’ll want to depress those muzzles to best cover the ground in front of the fort.  Thus, the mound of earth specified will need to be a little higher.  Mahan and Wheeler came to the number of 33 inches (2 feet, 9 inches).  Of course, siege carriages (NOTE: these were the “larger” field carriages and not the fixed-fortification barbettes, seen in Fort Moultrie) were larger, starting with the bore some 52-53 inches above the ground, then given 48 inches (four feet) above the parapet.  Siege guns only depressed 4º where the field guns could depress as much as 8º.  So the clearance was halved.

Next consider the horizontal space for the gun and crew servicing the piece.  Pack them in too tightly, and efficiency drops (not to mention giving the enemy a dense target to fire upon).  Spread them out too much, and parapet space is wasted – firepower per foot drops below acceptable levels.  Tactical practice, in the field, was to provide for 42 feet (fourteen yards) between pieces.  But within the fort, that factor was reduced to almost a third.

Out on the field, the gun was usually allocated eleven yards (thirty-three feet) of “depth” – broken down with fifteen feet for the gun and space for recoil, then eighteen feet back to the limber (and team).  That would allow ample space for recoil and avoid placing the horses and ammunition chests to closely (but still within easy reach for the “number five” guy bringing up the rounds.  But in the fortification, where other ammunition storage arrangements were in place, that could be reduced to just twenty-four feet (eight yards).  Such allowed room for recoil on the banquette and room for the crew servicing the piece.

So we see some “form follows function” reasoning within the suggested dimensions.

But there were some other arrangements needed. Particularly how the gun was worked up to the “mound of earth” that constituted the barbette (I’ll avoid for the moment calling this a “platform” to avoid confusion later):

The earth of the barbette at the rear and sides receives the natural slope. To ascend the barbette, a construction termed a ramp, is made; this is an inclined plane of earth, which connects to the top of the barbette with the terre-plein. The ramp is ten feet wide at top, and its slope is six base to one perpendicular. The earth at the side receives the natural slope. The ramp should be at some convenient point in the rear, and take up as little room as possible.

A ten foot wide ramp allowed room to maneuver a six-foot wide field carriage, allowing ample foot-space for the men.   The slope of that ramp was a gentle one foot elevation for six feet of length.  Natural slope, recall, was a one to one ratio.

With that lengthy description in mind, Mahan offered Figure 32 as an illustration of a proper barbette placed on a salient:

PlateIVFig32W

As we’ve seen with other components of fortifications, simply having the diagram is one thing … building it to specifications is another.  Next week we’ll look at how these barbettes were built, so that the enemy would not gain contempt for our works!

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 52-3.)

Fortification Friday: Interior Arrangements, starting with armaments

The next aspect of field fortifications to consider are the interior arrangements.  Thus far most of our focus has been towards the exterior, with the exception of the traverses, and what could be done to block or stop the attacker.  With the interior arrangements, the engineer would consider what could make the defenders’ job easier and, shall we say, more comfortable.  Mahan prefaced his lesson on interior arrangements by calling attention to such factors:

Under the [heading] of interior arrangements is comprised all the means resorted to within the work to procure an efficient defense; to preserve the troops and the material from the destructive effects of the enemy’s fire; and to prevent a surprise.

You are probably thinking, “protect the troops?  Isn’t that what the parapet does?  Doesn’t the ditch prevent surprise?”  Well… yes… you might look at it from that standpoint.  But what Mahan was calling attention to here were the structures and features which were internal to the works and designed to improve the nature of the defense.  As such “within the work” is the important phrase to consider.  But, keep your questions in mind as we work through this topic, as we will revisit shortly.

Mahan continued to offer a list of classes of these interior arrangements:

The class of constructions required for the above purposes, are batteries; powder magazines; traverses; shelters; enclosures for gorges and outlets; interior safety-redoubt, or keep; and bridges of communication.

From that we have a subdivision:

All arrangements made for the defense, with musketry and artillery, belong to what is termed the armament.

So we have a name for structures to support things that shoot.  Armaments.  Just for the context of these field fortification discussions, OK?

The armament with musketry is complete when the banquette and the interior and superior slopes are properly arranged, to enable the soldier to deliver his fire with effect; and to mount on the parapet to meet the enemy with the bayonet.  For this last purpose stout pickets may be driven into the interior slope, about midway from the bottom and three feet apart. The armament with artillery is, in a like manner, complete, when suitable means are taken to allow the guns to fire over the parapet, or through openings made in it; and when all the required accessories are provided for the service of the guns.

So… yes the parapet’s design can be considered part of the interior arrangements.

Mahan continues with this profound statement:

The armament with artillery is a subject of great importance….

You got me at “great importance.”

Oh, wait, I cut the professor off.  He has more on this ….

The armament with artillery is a subject of great importance, because it is not equally adapted to all classes of works.  Experience has demonstrated that the most efficient way of employing artillery, is in protecting the collateral salients by a well directed flank and cross fire, which shall not leave untouched a single foot of ground within its range, over which the enemy must approach.  It has moreover shown, that a work with a weak profile affords but little security to artillery within it; for artillery cannot defend itself, and such a work can be too easily carried by assault to offer any hope of keeping the enemy at a distance long enough to allow the artillery to produce its full effect.

The logic here is “form should follow function.”  If the intent is to have artillery fire on the enemy in order to break up the attack, then a flank fire is recommended.  And that artillery should blanket the approaches with fire… “shall not leave untouched a single foot….”  Artillery sits at the top of the list when making decisions about weapon placement.  It is the most effective, man per man, weapon for influencing the battlefield Not necessarily saying “killing” or producing causalities, but influencing the other side’s actions.  Yet, artillery’s influence is best gained over longer ranges.  Thus the need to form works that not only provide the artillery a measure of protection but also keep the enemy at greater than small arms length (range).

The best position for artillery is on the flanks and salients of a work; because from these points the salients are best protected, and the approaches best swept; and the guns should be collected at these points in batteries of several pieces; for experience has likewise shown, that it is only by opening a heavy, well-sustained fire on the enemy’s columns, that an efficient check can be [given] to them.  If only a few files are taken off, or the shot passes over the men, it rather inspires the enemy with confidence in his safety, and with contempt for the defenses.

Sun Tzu should have said it!  Don’t let the enemy become contemptuous of your defenses!

Consider the “best practice” offered by Mahan.  By placing artillery on the salients, the guns were out of the direct line of the attacker’s fires while being placed behind the various, and likely complex, defensive works on the “horns” of the bastion.  And artillery shouldn’t be parceled out as singles, but rather massed and inter-operated to multiply the effect.

All this is great theoretical talk.  Everyone would agree massing artillery is best.  But now we have a practical problem on the parapet.  With infantry, the parapet works fine to protect most of the body, provide cover to crouch behind when reloading, and, if the fight is close, an orientation for the bayonets.  But artillerymen cannot “crouch” an artillery piece.  And when servicing the weapon, they are exposed. Furthermore, there are all sorts of problems bringing 12 pound or 24 pound or larger projectiles up to the gun.  So to make the big guns work best, one must make arrangements.. in the interior…. And those arrangements Mahan identified under the classification of “batteries.” We’ll look at those next.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 51-2.)