Category Archives: Engineers

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):

Parapet

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:

Parapet_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:

Parapet_Banquette_Thread

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:

Parapet_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:

Parapet_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:

Parapet_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:

Parapet_Berm

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.”

embrasure

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:

Abbot_Mantlet1

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.

Abbot_Mantlet2

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:

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.)

Crossing the Pontoons: Forbes recalls his crossing of the James

Edwin Forbes was among the best known illustrators of the Civil War.  Working for Frank Leslie’s Magazine, Forbes produced some vivid drawings depicting the action… and inaction… of life with the armies.  Forbes also provided a great narrative of his wartime experiences in Thirty Years After: An Artist’s Memoir of the Civil War.  Here’s his account of crossing the James River, 150 years ago:

A most interesting movement in my army experience was the crossing of the James River during the advance of the Army of the Potomac on Petersburg. The longest pontoon bridge that was build during the war was here made use of, and was crossed by the left wing of the army, while the right wing made use of steamboats further up the river near City Point.

I took position near the head of the bridge, and watched the column in its course. Regiments of infantry came surging along at route step, ragged and footsore, their faces much discolored with powder-stain and dust.  Officers were scarcely discernible from privates.  The latter were laden with all kinds of traps and plunder, and the pack mules and horses had more than the usual burden.  Everything bore the terrible imprint of a month’s hard fighting, and made sad contrast to the hopeful men who in new uniforms had so recently started out from the winter camp near Culpepper, under the great commander, Grant.  The forces had been terribly reduced in numbers, but with the same determined spirit that always pervaded the troops, even in defeat, the column marched promptly and cheerfully forward. Out from the shore was a fleet of war vessels to protect the bridge, and upon one of them was a group of newly and jauntily dressed sailors, who had evidently experienced no hard service. With open-eyed wonder they watched the progress of the column, and laughed loudly at the grotesqueness that misfortune had given to many groups of soldiers. There was indeed great contrast in the two arms of the service, but my heart went out in sympathy to the poor fellows with tattered clothing and blackened faces. All day the throng poured over the bridge, and with each moment came changing scenes. This was heightened by the many steamboats and sailing crafts anchored in the river, and on the steep bluff of the farther bank arose Fort Powhatan, an abandoned earthwork, which had been thrown up by the Rebs to dispute the ascent of the stream.  Low muttering like distant thunder was after a time heard from the southwest; the advance had arrived at Petersburg.

But before long the sun sank into the rest, and evening brought a beautiful picture. Hundreds of bright camp fires on the river bank lit up the bridge, which, with the colored lights flung out from the vessels, looked like fairyland. The trains clattered noisily along, and the many sounds of a moving army rose in the still night air, and were accompanied by the boom of distant cannon – that solemn suggestion of deadly conflict. Long into the night I watched this moving panorama, until from sheer exhaustion I sought repose.

One view of the crossing of the James, 150 years ago.

(Citation from Forbes, Edwin. Thirty Years After: An Artist’s Memoir of the Civil War. Baton Rouge: Louisiana State UP, 1993, pages 189-90.)