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:

WheelerFig36

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:

WheelerFig37

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):

PlateIVFig32W

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:

PlateIVFig33_A

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:

PlateIVFig33_B

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:

PlateIVFig33_C

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:

PlateIVFig33_D

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

PlateIVFig33_E

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:

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.)

Barbette or Casemate?

On several occasions I’ve mentioned barbette and casemate carriages.  In the 19th century, seacoast fortifications used both mountings for their main armaments.   To maximize the firepower on each facing of the fort, engineers stacked tiers, each supported by strong arch structures.   This cutaway model display at Fort Pulaski provides a good visual reference.

Savannah 5 May 10 183
Cutaway of Fort Pulaski

On top, the model has pads and fixtures for front pivot guns.  Manuals of the period identified these as “barbette”. The guns mounted there fired over the parapet, mounted on tall barbette carriages.   A good example of such supports a 32-pdr seacoast gun at Fort Moultrie, South Carolina today.

Fort Moultrie 3 May 2010 510
32-pdr Seacoast Gun on Barbette Carriage, Fort Moultrie

The gun sits on a truck, formed by an angle brace, sort of a triangle, which slides upon a bed.  The bed consisted of two side rails and a center rail.  Most pre-war (1840s or earlier) featured a large “wheel” for handling.  This allowed the crew to roll the truck forward on the supporting Later examples, particularly for the columbiads, used handspikes for running the gun out.

“Eccentric” axles, which were slightly off true center axis, baffled the recoil when the gun was fired.  As anyone who has experienced an out of balance tire on their car knows, such off-center wheels tend to reduce speeds.  When running the truck forward, a leaver lifted the gun and truck up to “true center” axis, so the crew didn’t have to work as hard.  But crews had to engage the eccentrics before the gun was fired, else the recoil was dangerous.  (And some will recall that failure to engage the eccentrics caused some damage to guns at Fort Sumter in April 1861.)

By the late 1850s, wrought iron replaced wood in many situations (although iron did not replace wood during the Civil War).  Located a few yards away from the 32-pdr at Fort Moultrie is a pair of 15-inch Rodmans on front pintle iron barbette mounts.

Fort Moultrie 3 May 2010 484
15-inch Rodman on Barbette Carriage, Fort Moultrie

Generally similar in layout with the wooden carriages, the iron versions relied on two side rails and dispensed with the center rail.  Regardless of wood or iron, the design of barbette carriages allowed the guns to stand over the parapet, and also afforded higher angles of elevation.  Because of this, the heaviest, longest-range guns in a seacoast fort usually sat on the barbette tier.  Of course, the height of the carriage profiled the gun and crew who had no overhead cover.

Looking back to the Fort Pulaski model, below the covered way are a series of arches, between which are more pads and fixtures for front pivot guns.  These form a gallery of casemates.  The casemate guns fired through an embrasure (seen to the right of the model).  An example of a gun on a casemate carriage greets visitors to Fort Sumter today.

Ft Sumter 3 May 2010 306
42-pdr Seacoast Gun on Casemate Carriage, Fort Sumter

Ordnance officers designed the casemate carriage with the cramped space of the gallery in mind.  Instead of a tall triangle truck, the casemate guns sat on a truck very similar in profile to navy carriages.  The casemate crews used handspikes to work a small wheel when running the gun out.

Ft Sumter 3 May 2010 312
Banded and Rifled 42-pdr, Fort Sumter

Like the barbette carriage, the casemates relied upon an eccentric to reduce recoil force.  One of those eccentric rollers is at the back of the truck in the view above.   Also in this view, note the slots for the handspikes in the wheel on the left of the carriage.

As with the barbette carriages, the Army began using wrought iron carriages just before the Civil War.  Several excellent examples of such carriages sit just a few yards away from the 42-pdrs at Fort Sumter.

Ft Sumter 3 May 2010 219
6.4-inch Parrott on Iron Casemate Carriage, Fort Sumter

Following similar evolutions to the barbette carriages, the iron casemates featured only two side rails.  Ordnance officers did not intend the casemate guns to fire at elevation, as the embrasure limited the angle of fire.  But the gunners were protected behind the fort’s structure.  Shutters (in some cases spring-loaded iron types) closed the embrasure when the gun ran back.  This gave the casemate gun crew much more protection than those exposed on the barbette.

Keep in mind that carriages allowed mounting even the largest guns on either the barbette or casemate tiers.  In operation, the Army left the employment to the opinions of the fort’s commander.  During peaceful times, many commanders kept the guns and equipment in the casemates to reduce exposure to the elements.

Not all American seacoast forts featured casemate tiers, but the major forts did.  Fort Sumter had two casemate tiers in addition to the barbette level.  In April 1861, Major Robert Anderson, considering the risks of manning the barbette tier of Fort Sumter, confined his crews to the casemate guns.  While better protecting the men, this also meant Fort Sumter’s guns were unable to effectively reply to the Confederate bombardment.  Yet, a few years later Confederate gunners sat in the relative safety of the casemates while responding, and damaging, Federal ironclads attacking Fort Sumter.

Each mounting had advantages and disadvantages, which the commander on the ground had to consider against the tactical situation.