Category Archives: Engineers

Fortification Friday: The symbiotic relationship between attack and defense

As we leaf through Mahan’s Treatise on Field Fortifications, the lesson plan offered, after defining components of the profile and trace, an important perspective for planning and evaluating fortifications – a connection between the attack and defense of the works:

The attack and defense of intrenchements bear a necessary relation to each other; and it is upon a knowledge of the course pursued by the assailant, that the principles regulating the defense should be founded.

Grant me some license for an analogy here and call this a symbiotic relationship.  Symbiotic, that is, borrowed from biology and defining two organisms of different species that exhibit a long-term, close interaction.  In the case of military affairs, the only reason a place would be defended with earthworks is because the defender feels an attacker might wish to gain possession.  Furthermore, the defender would build the works specifically to counter (if not deter) the most likely form of attack.  Likewise, the attacker would plan the assault based on knowledge of the layout of the defenses.  In short, one plan will exist only because of the other, counter, plan.  Otherwise, there’s simply no reason for the planned action – be that placing a defensive work or organizing an assault of the position.

Mahan elaborated further, providing the students a generalized example of what an attack looked like:

 An attack is, generally, opened by a fire of the enemy’s artillery, whose objective is to silence the fire of the intenchments, and to drive the assailed from the parapet; when this object is attained, a storming party, which usually consists of a detachment of engineer troops, a column of attack, and a reserve, is sent forward, under the fire of the artillery, to the assault. The detachment of engineer troops proceeds the column of attack, and removes all obstacles that obstruct its passage into the ditch. The line of march is directed upon a salient, through a sector without fire, and on the prolongation of the capital, as this line is least exposed to the fire of the works.

Depicting that approach on Mahan’s figure:


See how this approach was designed to take advantage of an inherent flaw of the works? Mahan continued with more exploitation of the fortification flaws:

When the ditch is gained, the party makes its way to a re-entering angle, where, sheltered from the fire of the flanks, the work is entered by the column of attack, either by making a breach in the parapet, or else by means of ladders.  The reserve supports the column of attack in case of need; and if it is driven from the works, covers its retreat.

Again, as that would look on Mahan’s figure:


This approach allowed the attacker to pick apart the defense by working under the parapet within the ditch inside the dead space, avoiding the angles of defense.

So how does that look from the defenders side?

The manner of making the defense is with artillery, musketry, the bayonet, and sorties.  The enemy is attained at a distance by the fire of the artillery and musketry, whose effect will chiefly depend upon the length of time that he is kept exposed to it by the ditch, and the obstacles in front of it. The bayonet is resorted to, as soon as the enemy shows himself on the berm; and sorties are made, either when any irresolution or confusion is seen in the enemy’s ranks, or at the moment he is repulsed from the parapet.

Note that Mahan didn’t emphasize here the nature of the parapets, faces, and flanks in order to build the perfect defensive line.  That technical perspective he saved for a more detailed explanation.  Instead he focused on what the defender could do with their weapons.  Implied in the notion of the sortie is that the defender retained high motivation to conduct such a counter-attack.  And with that, Mahan is admitting that flaws would be present in any defense.  To mitigate those flaws, where existing, the defender applied cold steel, hot lead, flesh, and bone.

But you see here how the nature of attack and defense fit against each other.  The attack had to be planned with a mind to exploit the flaws of the defense.  The defense had to be planned to minimize those flaws.  After establishing that symbiotic relationship, Mahan proceeded to lay out nine principles of the defense – some technical, others tactical, and yet others addressing the “spirit” of the defenders.  We’ll take a look at those next week.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 5-6.)

Fortification Friday: Know your banquettes and slopes

Over the last couple of posts in this series, I’ve discussed parapets and their function.  Now let us turn to the parts of a parapet and look at those in detail.  As a refresher, this is Mahan’s profile of the parapet (highlighted line):


Mahan defined this profile as the lines between points A-B-C-D-E-F.   It’s important to note that each individual line (defined between the points) also defines a separate component of the parapet.

Between points A and B is the Banquette Slope:


Specifically, point A is the Foot of the Banquette Slope and B is the Crest of the Banquette Slope.

Wait… what is a Banquette?  Mahan described the Banquette as:

The banquette is a small terrace on which the soldier stands to deliver his fire ; the top of it is denominated the tread, and the inclined plane by which it is ascended the slope.

So this explains lines A-B and B-C.  The latter being the Tread of the Banquette, and including point C, the Foot of the Interior Slope:


From a functional standpoint, the Banquette had to be wide enough to allow a rank or ranks of soldiers to stand in formation and work their musket.  The measure would be different depending on the number of ranks that the defender planned to use.  One rank might get by with two feet of width.  Two ranks required four feet.  So something on the order of 4 ½ to 5 feet would be preferred to allow ease of movement.  The Banquette was also given a slight slope to the interior to allow for drainage.

The Slope of the Banquette (A-B) was structured as the hypotenuse of a right triangle.  The slope would be a compromise providing support for the Tread while offering the lowest slope for the troops to climb.

One other functional requirement to consider about the Banquette is its height above the tere-plein (natural surface level), or interior, and the height of the parapet.  The troops had to be able to stand on the Banquette and shoot with most of their body protected by the other parts of the Parapet.  This governed the overall height of the Parapet somewhat, given average height of soldiers and such.

Moving further down the profile of the Parapet, we come to the Slope portion of the Parapet.  By adding point D, the Interior Crest, we have line C-D, known as the Interior Slope:


As with the height of the Tread, this line’s length was governed by the need to allow soldiers to fire over the Parapet.  A sharp incline of this line allowed the troops space to move while keeping the mass close to their bodies.  But not being the most efficient structural support angle, that incline required careful maintenance.

Line D-E, with E being the Exterior Crest, is called the Superior Slope:


The Superior Slope declined outward (towards point E).  This allowed soldiers to depress their weapons to engage targets directly in front of the works.  This also ensure any fires hitting the front of the fort would glance upward and away from the defenders (hopefully).  The angle of the Superior Slope was also a compromise.  Too shallow and the Parapet might be excessive and perhaps not allow enough declination for the muskets.  Too deep and the Parapet’s strength is compromised.

Continuing the same convention, the Exterior Slope is line E-F, where point F is the Foot of the Exterior Slope:


This portion of the Parapet had the important mission of stopping projectiles.  The preferred angle was 45º, or the natural slope at which loose dirt will pile.  Structurally, that was the best support angle for the Parapet.  Furthermore, when under fire, any dirt thrown up from the Exterior Slope would naturally fall back to that angle… one would expect.

The Exterior Slope completed the profile of the Parapet.  But there is one other part to consider, although it is not part of the defined parapet – line F-G:


Point G is the crest of the Scarp, part of the Ditch.  Mahan called this the Berm.  The Berm connected the Parapet to the Ditch.

I have a problem with the choice of words here.  In modern context, berm is often a raised mound, almost a Parapet itself.  We spoke of “crossing the berm” in the Gulf Wars as noting a passage through defensive works thrown up in the desert.  Likewise, berms are tall, lengthy mounds built between roads and subdivisions to block noise.  And let’s not forget berms put up in front of raising flood waters.  So you see, a “berm” means some other shape to most modern readers.

But for Mahan, the Berm was a construct that allowed the weight of the Parapet to stand on something other than the back edge of the ditch (the Scarp, which we will discuss later).  Frankly, he wrote:

The berm is a defect in field works, because it yields the enemy a foot-hold to breathe a moment before attempting to ascend the exterior slope. It is useful in the construction of the work for the workmen to stand on; and it throws the weight of the parapet back from the scarp, which might be crushed out by this pressure. In firm soils, the berm may be only from eighteen inches to two feet wide; in other cases, as in marshy soils, it may require a width of six feet. In all cases, it should be six feet below the exterior crest, to prevent the enemy, should he form on it, from firing on the troops on the banquette.

Thus the Berm was a necessary evil.  It was a risk that need mitigation during construction.

These terms become very important when considering the engineering involved to build a fort.  Each component had a function. Those functions determined the measures of the line.  Engineers, being engineers, would compute those measures based on formulas provided by Mahan and others.  In short, the troops didn’t just throw this sort of thing up randomly:

They ENGINEERED it.  And that engineering involved careful study of the task using some of those terms presented above.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 1-3, 22.)

“I decidedly prefer the rope mantlets.”: Hunt and Abbot prepare for a siege

Yesterday, I made short reference to Colonel Henry L. Abbot’s suggestion to Brigadier-General Henry Hunt in regard to mantlets for the siege train at Petersburg.  To properly set this part of the story, let me step back to June 14, 1864 and a response from Major-General Delafield, the Army’s Chief Engineer (“Army” as in “all the Army” sense), to Brigadier-General John G. Barnard, Chief Engineer, Army of the Potomac.  The correspondence involved materials forwarded from Washington to Fort Monroe.  And one of the items discussed was mantlets:

Some further information is desired as to the armament for which these mantlets are intended. They were originally adopted for embrasures cut down into a parapet to suit guns mounted on ship truck carriages, which left a large opening to be covered over the gun. Now, in our siege batteries from the top of the 32-pounder, when in battery, to the crest of the parapet is only one foot nine inches to be covered by the mantlet, and with the siege 4½-inch ordnance gun, one foot four inches. This small space over the gun closed by rope gives but very little strength, in addition to which the rope is not musket or rifle proof at 200 yards. These considerations induced me to send you at Yorktown in 1862 wood and boiler-iron mantlets, with a box of chisels to cut the iron to suit your guns. A part of these old ones have lately been found by Stewart at Suffolk and sent forward; that may suffice for some fifty or sixty guns, giving me time to learn your wishes in regard to others to be forwarded and to learn the size of the guns. If made of rope you cannot alter them to suit guns of different exterior diameters, but it made of wood and iron you can enlarge the opening at pleasure. The splinters from the wood and iron are objectionable produced by artillery. During such a fire they would probably be withdrawn and used to guard against infantry fire only. While the rope would not give splinters, yet at the same time would not be proof against the rifle musket-balls. Advise me from Old Point by telegram; say “rope” or “iron and wood” and I will understand you. Also give diameters of guns, exterior.

To answer this question, Barnard turned to Hunt.  And Hunt consulted Abbot.

First off, you might ask what are mantlets?  Well let us start with what an embrasure is – “an opening made in the parapet for a gun to fire through.”


The definition and illustration come from John Tidball’s 1891 Manual of Heavy Artillery.  Looking at Figure 4 – the right side and upper center – we see the cut through the parapet allowing the muzzle of the cannon an unobstructed line to fire out.  And we see Tidball offered views of direct, oblique, and high angle embrasures.  But, there are two tactical problems here.  First is maintenance.  The force and shock of firing would in time erode and enlarge the opening.  Since the embrasure faced the enemy, simply spading more dirt into place was not practical.  So common practice was to place raw-hide, gabions, or other reinforcement around the embrasure.  Tidball preferred iron plates to form a lining of the embrasure, as depicted in Figure 5 (to the left).

But that leaves us with the other problem – if the gun could fire out, the enemy could fire in!  In Figure 5, Tidball demonstrated the use of an iron plate as a mantlet, which he defined as “a shield placed over the mouth of an embrasure to prevent musketry bullets and fragments of shells from flying through and injuring those serving the piece.” (And recall the batteries on Morris Island used similar iron linings and mantlets, formed from the boilers of the blockade runner Ruby in 1863.) A door in the center, roughly a foot high and six inches wide, allowed the crew to pass a rammer through when loading, sight down the piece, and project the muzzle.  The preference was to make that opening as small as practical, still allowing for the service of the piece.

As alluded by Delafield, iron was not the only material that could serve as a mantlet.  Wood and iron, though resistant to blast damage and musket fire, had the disadvantage of producing splinters.  Rope, as Delafield stated, didn’t have the splinter problem, but was not considered stiff enough to resist musketry.  But what Delafield apparently didn’t take into account were experiments by Abbot to devise a better rope mantlet, as he advised Barnard about responding to Delafield:

I decidedly prefer the rope mantlets. I find by trial at twenty paces that the penetration of our Springfield rifle, elongated bullet, is between two and two and five-tenths inches. The mantlets are six inches thick and they are thus perfectly rifle-proof. Their dimensions are the following, which are very convenient in practice:


The opening can readily be cut larger if necessary. We have done so at least in one instance, to enlarge the traverse of the gun in an oblique embrasure. The men are afraid of splinters from a cannon-ball-and I think justly so–with the wood and iron mantlets. Moreover, the blast of a light 12-pounder has already rendered unserviceable one of the iron mantlets of this pattern.


I therefore entirely agree with yourself and General Hunt in thinking that only rope should be ordered. I think the dimensions cannot be improved. As to number required, my train proper, which is entirely distinct from my present guns, consists of forty-six guns requiring mantlets, and ten 8-inch siege howitzers which I think can hardly be used with them. I have here seventeen rope mantlets and twenty-three wood and iron, one of the latter unserviceable. As they are very liable to be destroyed, and moreover are quite useful even for light guns when sharpshooters are as troublesome as they have been here at times (I have had two men killed besides some wounded in my own regiment by them already), I think that about 100 could be safely ordered (besides those I have on hand). They should be made of tarred rope, like the old ones.

So with a thicker set of ropes, the gunners were better protected.

Turning back to Tidball’s post-war manual, he offered two illustrations of rope mantlets:


Figure 2 above matches somewhat to the second illustration offered by Abbot in 1864.  The use of rope afforded some flexibility for those working at the muzzle with rammers and sponges. Tidball stated those mantlets weighed 400 pounds.  Figure 3 appears to be a more refined fitting, which I would question in regard to ease servicing the weapon.

Without mantlets, sharpshooters could reduce the efficiency of the guns, if not silence them completely.  Thus these relatively minor devices became rather important as the army transitioned into siege operations.  In fact, I dare say any work discussing the fortifications at Petersburg, at a tactical level, should include a proper discussion of mantlets… lest the author be accused of dealing with the subject lightly.

(Citations from OR, Series I, Volume 40, Part II, Serial 81, pages 21, 223-4;  John C. Tidball, Manual of Heavy Artillery Service: For the Use of the Army and Militia of the United States, Washington, D.C.: James J. Chapman, 1891, pages 385-6, 399, and Plates 61 and 68.)