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.)

3 thoughts on “Fortification Friday: Fire over the parapet, build some barbettes

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