Fortification Friday: “Sufficient strength to resist” artillery taken on cavalry raids

Last week, we discussed Mahan’s American blockhouse and how that form of fortification became important during the Civil War.  During the war, the blockhouse became a common feature along railroads, roads, rivers, and other key points in the rear areas.  I don’t think this reflected a “brand new” use of the blockhouse fortifications, but rather one of greater significance as result of the needs of the war.

Pre-war thinking on such matters focused on a conflict against European powers, in a “War of 1812” scenario.  As such, the rear areas would be somewhat secure behind the Atlantic Ocean (with Mexico and the British in Canada assessed as more defensively oriented).  Only on the frontier would there be great need for blockhouses to secure supply and communication lines.  But the American Civil War upset that line of thinking.  With extended lines across half the continent, the armies could not expect to guard every quarter. This gave an opening for leaders with names like Stuart, Forrest, Wheeler, Mosby, and Morgan.  Yes, those glorious raiders riding about disrupting Yankee operations…

I would offer the Confederate raider threat reached its peak during the Atlanta Campaign.  Not to downplay other sectors, but the spring-summer of 1864 was somewhat a “point of no return” in many regards.  As a counter to the raiders, Major-General William T. Sherman directed the fortification of numerous posts along his supply and communication lines.  One of those we saw last week:

02151r

And as I pointed out last week, the blockhouse in the photo compares well to the figures offered by Mahan in his post-war manual:

MahanPage65Fig1_2

Assuming Mahan’s figures are indeed close matches, we can project all sorts of details not visible in the photo – such as internal arrangements.  And thinking of those, we have the other half of Mahan’s illustrations to consider:

MahanPage65Fig4_5_6_7_8

Now we might say a picture is worth a thousand words.  If so, I’d offer these detailed figures are worth a couple thousand more. Figures 4, 5, and 6 give us a measure of the loop-holes on the lower story.  Figures 7 and 8 provide the same for the upper story.  One might dismiss the details as simply “common sense.”  But my counter would be that “common sense” is usually derived from experience.  And in this case, the manual attempts to impart some experience onto inexperienced cadets… who definitely needed sage advice based on wartime experience.  Besides, as I like to say on such matters, “it goes to show us how THEY did it.”

The caption provided for this page of figures further solidifies the linkage to wartime experiences:

Figs 1, 2, 3, etc., represent the chief details of the two-story block-houses that have been adopted for the defense of railroad stations, bridges, etc., along the line of communications of General Sherman’s Army.  From experiments made upon them, the lower story, with its double row of heavy logs from three and a half to four feet in thickness, is regarded of sufficient strength to resist the field artillery usually taken with cavalry on their raids.

And that was the goal – a “keep” for an outpost garrison that would afford protection against the raider’s weapons.

Now some will point out that Forrest and Wheeler captured their fair share of blockhouses while out raiding.  I would offer that in most cases those captures involved a pause of action under a flag of truce.  Words like “… to prevent further effusion of blood” were mentioned.  So we might contend the blockhouse did indeed serve the “keep” function even if the garrison were captured.

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 65.)

Fortification Friday: Mahan’s suggestions to improve blockhouses

When it came to fortifications, Professor Mahan held to the “you can never do enough” principle. As we’ve discussed in preceding posts, in a field fortification the blockhouse was the keep, or last point of defense for the garrison.  It was to be outfitted in such a way that the garrison could cover the parapet with fires.  And interior arrangements would grant ample space for handling weapons.  But more could be done to improve the blockhouse.  First and foremost, one should surround the structure with a ditch:

The block-house is surrounded by a ditch, similar to one used for a defensive stoccade. A strong door is made in one of the re-entering angles, and a slight bridge leads from it across the ditch.

We saw this arrangement along a profile line in the illustrations:

PlateVIFig43

Somewhat analogous to the ditch-parapet in profile, with the blockhouse structure itself being the parapet.  Notice the glacis in front of the ditch.  And also the palisade in the ditch to the left. And we have those heavy doorways, conforming to the structure prescribed for outlets.  It’s all coming together here for the keep.

But we were still not finished making the blockhouse unassailable. There was more that could be done.  Perhaps a structure on top from which the garrison could fire down onto the attackers?

It has been proposed to place a slight parapet of earth on top of the block-house.  It is thought that this accumulation of earth would be too heavy for the timbers, independently of leaving but little space for the defense.  Perhaps a better arrangement might be made on top, similar to a defensive stoccade, the uprights being secured at bottom, between two pieces resting on the top pieces, and held firmly by an arrangement of riband pieces and braces.

Mahan’s method would provide a lightweight structure, sufficient to stop musketry.  Artillery, though, might turn that blockhouse parapet to splinters. So, any suggestions to counter that?

It has also been proposed to place the the interior and exterior rows of uprights three feet apart, and fill in between them with closely packed earth, for a defense against artillery.  This method has been tried, and was found to be less solid than the one here laid down, independently of being more difficult to construct.

Recall, the guidance stated earlier was to use two thicknesses of twelve inch timbers for defense against field artillery. This was seen as more sustainable, with less physical footprint, as what would amount to packed earth at steeper than a natural slope.

Other techniques to improve the blockhouse included methods to make the enemy’s closure even more difficult.  “The top pieces should in no case project more than twelve inches beyond the sides, to admit of logs, &c., being rolled over on the enemy.”

And if that was not enough, one could stack up a second floor (somewhat as we saw from the colonial era Fort King George):

The block-house is sometimes arranged with two stories, the corners or the sides of the upper story projecting over the sides of the lower. Either of these methods is sufficient for the defense of the lower story; but the first is the best to procure a fire in the direction of the angles.  It can only be used, however, as a defense against infantry.

When artillery cannot be brought to bear against the top of the block-house, it may be constructed like an ordinary floor, and be covered with nine or twelve inches of earth to guard against fire.

Of course, where artillery might be brought to bear against the blockhouse… well let us just say a lower profile was preferred.

But, Mahan was tapping on this point about “places where artillery will not be” for a purpose.  He was looking toward the “west” of that era:

The application of wood to the purposes of defense is one of paramount importance in our country.  A block-house, surrounded by a defensive stoccade, is impregnable to the attack of infantry if properly defended, and is therefore peculiarly suitable to either wooded or mountainous positions, where a train of artillery cannot be taken without great labor, owing to the impediments that may be thrown in its way, by rendering the roads impassible from obstructions easily obtained.

Of course, just couple of decades after Mahan’s writing armies backed with steam power (rail and river) were able to overcome many of the natural impediments of the Western Theater.  Still, the suggestion held some merit further to the west, where most potential adversary possessed only light artillery. Oh, and the mountains were much higher.

And Mahan also saw an application for the blockhouse in the east, where pre-war thinking was focused on attackers that arrived by ship:

In positions covered by extensive earthen works, such as those that would be required for the defence of the towns on our sea-board, and which would be occupied during a war, a defensive arrangement of the barracks for the troops, so that they might serve, in case of the main works being force, as rallying points, under cover of which the main body of troops may retreat with safety, is a subject that commends itself to serious attention of the engineer.  From the details already entered into, an efficient combination for this purpose will suggest itself to the reader, without entering farther into particulars.

Such arrangements might have been of use for Confederates defending the Carolinas, Georgia, Florida, and the Gulf Coast.  We might contend the inner defensive lines at Charleston served the purposes of a keep in this regard.  But here again technology had rendered the blockhouse less useful.  By 1863, Federals could bring to bear rifled artillery of the largest caliber wherever they might encounter a blockhouse.

Yet, while we can say rifled artillery and means to transport such weapons rendered the blockhouse less desirable, we still can point to widespread use of such structures throughout the Civil War.  In particular along lines of communication.  In those rear areas, raiders were not likely to bring more than a handful of artillery pieces.  And that experience lead to some shifts in the instruction about blockhouses as field fortifications… which we shall discuss next.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 64-6.)

 

Fortification Friday: Square, rectangle, or even a cross – blockhouse forms

Last week, we introduced the blockhouse as an interior structure, perhaps better classified as a facility, within a field fortification.  Allow me to stress again, the context of Mahan’s writings in “Field Fortifications” about blockhouses was scoped to discuss the use of those sort of structures in conjunction with larger works.  It was not to say blockhouses would always be used as such, nor to dismiss other sorts of employment of that fortification type. This particular Mahan lesson (of which there were many, across several manuals, as we must recall) was focused on building a “keep” so the defenders might “keep” something valuable… their lives in the event all was lost.

Having discussed the concept and general layout of the blockhouse, Mahan turned to particulars:

With regard to the details of the construction, the timber for the sides should be twelve inches thick, to resist an attack of musketry, and to resist field-pieces, two feet, in which case the sides are formed of two thicknesses of twelve-inch timber. If the timber is placed upright, each piece should be let into a mortise in the cap-sill; and every fourth piece of the top, at least, should be notched on the cap-sill, to prevent the sides from spreading out.

This would form, in essence, the walls of the blockhouse.  Notice the prescribed thickness, in regard to the expected threat – be that musketry or artillery.  I would add that with the introduction of rifled artillery, the two foot thickness was insufficient.  But there begins a point of diminishing return. How much more timber should one add to the blockhouse, thus subtracting usable interior space, in order to defend against an Ordnance or Parrott rifle?  Ah… a question best addressed when we consider the post-war manuals!  So let’s hold that thought.

I do wish Mahan had included a good illustration of the proposed arrangement of timbers. And I’ve not located any other contemporary illustration to serve.  But the general idea is apparent… perhaps for generations who suffered the splinters from Lincoln Logs, if not so much for those of more recent times and their Lego bricks.  We will revisit the arrangement of timbers in the walls for the post-war manuals.

Moving forward, we need to consider the layout of those walls and how best to arrange the blockhouse in order to meet requirements:

The plan of the block-house must conform to its object generally; it may be square or rectangular.  If flank defenses are required, its play may be that of a cross. The interior height should not be less than nine feet, to allow ample room for loading the musket; this height will require that the timber of the sides shall be twelve feet long, in order to firmly set in the earth.  Sometimes a ground sill is placed under the uprights, but this is seldom necessary.  The width may be only twelve feet in some cases, but it is better to allow twenty feet; this will admit of a camp bed of boards on each side, six-and-a-half feet wide, and free space of seven feet….

So the layout, as seen from above, could be the square form familiar to us from the playsets of yore.  Or could be extended or expanded to use other layouts as tactical needs demanded.  The layout tended to employ right angles, however.  We look back at Figure 44, which is somewhat a cross, in plan:

PlateVIFig44

Notice how the dimensions are governed somewhat by the need to provide space for handling muskets.  Form will follow function.  The most important quality of the blockhouse, as a keep, is to allow the garrison to create a pause in the action, should the parapet be lost.

But “camp bed”?  Yes, that implies a place to sleep. But it was also a defensive arrangement.  “The camp bed serves also as a banquette; it is placed four feet three inches below the loop-hole, and has a slight slope of about eight inches inwards.”  Notice how the interior arrangement is to provide, in terms of wall to wall floor space, for a 6 ½ foot wide camp bed on each side with open space for seven feet between.

Now everything thus far has implied the garrison would only have muskets in the blockhouse.  Let us make arrangements, then, for artillery:

If cannon is to be used for the defense, the width must be at least twenty-four feet; this will allow eighteen feet for the service of the gun, which is generally ample, and six feet for a defense of musketry on the opposite side.  A greater width than twenty-four feet cannot well be allowed, because the bearing would be too great between the sides for twelve inch timber; and even for a width of sixteen feet it would be well to support the top pieces, by placing a girder under them on the shores.

Basically, bigger guns require more space.  So we adjust the arrangements.  But there is a physical limit as to how much more space is allocated.  If a really large blockhouse were built, it would require substantial structural reinforcement.  Better to stick with a single cannon per side, if used at all.

Since these arrangements place emphasis on affording space to handle weapons, be that musket or cannon, we need to discuss the loopholes in detail.  We’ll turn to that in the next installment.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 63-4.)

Fortification Friday: Blockhouses as Safety Redoubts in the Fort

When I say “blockhouse” many of you might be thinking about favorite childhood playsets:

OLYMPUS DIGITAL CAMERA

Certainly suitable for the defense of the bedroom in the face of the elite Confederate Plastic Brigade, or perhaps the indigenous Plastikawi tribe.  But… something that could not hold against the Green Army Men armed with bazookas and flamethrowers.

Kidding aside, the playset fort is pattered after real structures from American history.  The blockhouse was not unique to America, as it was a form brought over by Europeans.  However, the blockhouse became the preferred fortification on the North American continent from colonial times right up to the 20th century.  Blockhouses work well in situations where the enemy is unlikely to possess anything larger than light artillery.  The interior of the blockhouse was easily adapted into living quarters.  Conversely, living quarters (houses) might be easily adapted into a blockhouse.  Those, and other qualities, made that form of fortification popular on the frontier.

The popular image of a blockhouse is something made of wood.  But stone, or even adobe, might be used instead.  Since wood was in abundance on the early American frontier, we tend to see a lot of structures like this one:

Ft King George 3 Aug 11 1273

This is recreation of Fort King George, Darien, Georgia (a place with many, many layers of history).  In this particular case, the blockhouse served several roles – a high observation platform over the marsh, a platform for covering fire to protect approaches to the fort, and, in the event the works were overwhelmed, a final defense for the fort’s garrison.

It is that last function that Mahan had in mind when considering interior arrangements for field fortifications.  Blockhouses were a structure that could be used for what he called “safety redoubts”:

Safety Redoubt.  In enclosed works a place of retreat, into which the troops may retire in safety after a vigorous defense of the main work, will remove the fears of the garrison for the consequences of a successful attack of the enemy, and will inspire them with confidence to hold out to the last moment.

This interior work, which may be very properly be termed the keep, can only be applied to works of large interior capacity.  It may be formed of earth, or consist simply of a space enclosed by a defensive stoccade, or palisading.  In either case it should be about four feet higher than the main work, to prevent the enemy from obtaining a plunging fire in it from the parapet of the main work.

Let us pause here before going to Mahan’s formal introduction of the blockhouse.  This “hold out to the last” is a notion steeped in 19th century presumptions about how a siege would play out.  A garrison “holding out” would force the enemy to make a direct attack on the parapet… in other words, to get up close, personal, and… well… very violent with the defender.  And in that violent melee, the defender was not exactly in a position to call a “time out”.

The safety redoubt, or keep, was a place to retreat and, more importantly, force a pause in the action.  And from the keep, within that pause, the defender might negotiate a cessation of the fight, with honor.  Thus we see how that might allay fears of “consequences” for the garrison.

That in mind, Mahan offered his preference for the keep:

The best arrangement for the keep is the construction termed the block-house. This work is made of heavy timber, either squared on two sides or four; the pieces which form the sides of the block-house are either laid horizontally, and halved together at the ends, like an ordinary log-house, or else they are placed vertically, side by side, and connected at the top by a cap-sill. The sides are arranged with loop-hole defenses; and the top is formed by laying heavy logs, side by side, of the same thickness as those used for the sides, and covering them with earth to the depth of three feet.

Mahan offered this figure as an example of a blockhouse:

PlateVIFig43

This perspective is looking at the blockhouse along with a cross section of adjacent works and structures.  Rather busy.  This section is along the line a-b from Figure 44:

PlateVIFig44

The combined caption reads:

Figs. 43,44. Shows the plan and section of a block-house of upright timber.  The plan is made to exhibit a portion of the top complete; the timber covering the top; the arrangement of the cap pieces; a plan of the loop-holes; and a plan of the camp-bed. Fig. 43 exhibits, in a like manner, a cross section of the block-house and ditch; with interior and exterior elevation.

We will go into the particulars for construction in later posts.  What is important to identify here is the functional nature of this blockhouse.  Just as with the colonial-era Fort King George, we see a blockhouse adjacent to a ditch and other defensive structures.  One might say the blockhouse filled up the fort’s interior.

For an attacker, this presents a serious tactical problem.  One might defeat the defender on the parapet.  But the parapet would be a dangerous place to make a living with the blockhouse overlooking all. So you see where a “pause” would be in order.

Keep in mind, within this discussion of keeps, Mahan was not stating or suggesting that blockhouses only be constructed within and in conjunction with elaborate field works.  Rather that he offered that a blockhouse was a structure that served well as a keep inside a larger set of works.  We see that usage applied by his students during the Civil War.  Looking back again to Fortress Rosecrans:

FortressRosecrans

We see Redoubts Schofield, Brannan, T.J. Wood, and Johnson within the interior.  One wartime report described the arrangement as, “… strong against attack, being defended by large keeps, which deliver their fire upon every part of the interior.”  I would further add that most of the lunettes on the perimeter of this vast fortress included blockhouses.  So there were multiple “keeps” within a depth of the defense.  Keep in mind the scale of this fortress.  The safety redoubts, named above, were armed with 30-pdr Parrotts and 8-inch siege howitzers.  The Confederates would need to bring a large amount of iron in order to suppress the fort’s garrison.

But the size of this work was perhaps its weak point.  After the Army of the Cumberland moved further south, through the summer of 1863, there the need to keep this fortification in order was taxing, in terms of manpower. An 1865 report suggested all be reduced to simple blockhouses covering the bridge and depots.

That circles back to the point about blockhouse usage.  As said before, Mahan was not suggesting the only place to use a blockhouse was as a fort’s keep. But as his text was focused on field fortifications, the focus was on that function.  We will see blockhouses enter the conversation in regard to detached defenses in particular.  Furthermore, the post-war instructions would place more emphasis on the detached, singular blockhouse.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 62-3; OR, Series I, Volume XLIX, Part 2, Serial 104, page 502.)

Fortification Friday: Barriers, Bridges, and Ramps … the fort “communications” infrastructure

Writing the instructions for cadets, almost two decades after the Civil War, Junius B. Wheeler focused more on the functional requirements of outlets in the fortifications, as opposed to the important details of construction.  The nuances here are, I think, important.  Mahan presented the outlet had important operational uses within fortifications, but proceeded to discuss the particulars assuming the student would understand what those uses were. Wheeler focused on the uses up front, citing communications as the need and outlets as the remedy. And we must broaden “communications” a bit, perhaps our 21st century writers might say “traffic” and refer to movement of personnel, supplies, as well as messengers carrying communication.

Wheeler added to Mahan’s instructions with mention of “turn back” traverses at the mouth of outlets (to reduce the area under enemy lines of fire) and wider outlets for sorties.  But for the most part, the construction techniques remained the same.  Likewise, Wheeler identified the same supplemental structures as Mahan – barriers and bridges – while adding a few embellishments.

First off, we have the barriers. Which were… well… gates:

Barriers. – The outlets are usually arranged so that they can be quickly closed, to guard against surprise. The means used is a gate, technically termed a barrier.

The gate is made with two leaves, hanging on posts by hinges, and made to open inward.

The frame of each leaf is composed of two uprights, called stiles; two cross pieces, one at the other at the bottom, called rails; and a diagonal brace, called a swinging bar.

The leaf of the barrier may be open, by spiking stout upright pieces, with intervals between them, to the pieces of the frame; or it may be made solid, forming what is known as a bullet-proof gate.

Yes, something you might purchase, pre-fabricated, at the home improvement store.  But note that Wheeler offered two versions – a light “open” leaf version with spaces between the uprights AND a heavy “bullet-proof” version with no spaces.  The latter was illustrated in Figure 52 of Wheeler’s book:

WheelerFig52

Wheeler added some practical observations about the construction of these heavy gates:

Since the gate must be strong, the leaves of it are necessarily very heavy.  The leaves must be hung upon stout posts, firmly braced into the ground, to sustain the great weight of the gate.

The top rails of all barriers should not be less than six feet above the ground.

In the barriers with open leaves, the vertical pieces are usually extended from eighteen inches to two feet above the top rails, and their upper ends sharpened.

In those which are solid, it is usual to arrange some obstruction upon the top rail, such as sharp pointed spikes, broken glass, etc., to interfere with persons climbing over the top. It is usual to provide apertures in the leaves, through which the men can fire upon the ground outside.

Gotta love those details.  From having a “peephole” to shoot out from to having broken glass atop the gate.

Bridges were another supplement to the outlet’s composition:

Bridges. – When the ditch has been completed along that part of the work in front of the outlet, it is usual to carry the roadway across the ditch by means of a bridge.

The ditches of field works are, as a rule, quite narrow, and the bridges used to span them are very simple constructions.

A common method of building the bridge is to lay three or more sleepers across the ditch, and cover them with planks laid transversely.  If the span is sufficient to require intermediate supports, these are obtained by using trestles placed in the ditch.

A bridge built in this way can be quickly removed and speedily re-built, if there be any necessity for it.

We might consider this a temporary bridge without any means for retracting or removing, save dismantling.  Thus it was kept to the bare minimum arrangements.

Wheeler mentioned that other “hand-books on military engineering” described the use of draw bridges or rolling bridges in fortifications. He only briefly discussed the former, being basically as that detailed by Mahan in earlier texts.  The latter were designed to be “… pushed out from the work, and drawn back into it.” Both sort of bridges were,

… known as movable bridges, are useful to guard against surprise, to prevent stragglers from entering, and to keep the garrison in the work.  As a defense against an assault of a field work, they are of but little value.

To belabor that point:

The best method, is to have no ditch in front of an outlet, but let the roadway be on the natural surface of the ground.

Of course, operational needs might vary. But you get the point.

Wheeler offered one more supplemental structure under his heading of communications, but not one that was directly associated with outlets.  This was the ramp.  While all writers on fortifications discussed ramps in some regard, Wheeler saw fit to highlight the structure and its role for interior communications (and again, broaden that definition as mentioned above):

Ramps. – The short roads used in fortifications to ascend from one level to another, are termed ramps.

The width of a ramp depends upon its use, following the rule laid down for the width of passages.  A width of six feet for infantry, and of ten feet for artillery, are the widths generally used.

The inclination of the ramp may be as great as one on six, and as little as one on fifteen, depending upon the difference of level between the top and bottom. The side slopes of earth with its natural slope.

The ramps in a work should be placed in positions where they will not be in the way, nor occupy room which may be required for other purposes.

Steps or stairways are sometimes used instead of ramps. The rule for them is that the breadth of each step, called the tread, shall be at least twelve inches, and the height of the step, known as the rise, shall be about eight inches.

They are substituted for ramps in those places where there is not sufficient room for the ramp.

Nothing new or advanced here.  Ramps and stairs were part of fortifications dating back to the earliest times.  What is noteworthy here is that Wheeler considered them part of the fort’s communication infrastructure… again communication in the sense of how one moves into, out of, and inside of the fort.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 151-5.)

 

Fortification Friday: Drawbridges? For field fortifications?

Last week we looked at the use of outlets in the field fortifications.  Certainly it was impractical to insist the garrison stockpile sufficient food and supplies for the anticipated duration of the conflict at hand… not to mention cross their legs the whole time.  Some sort of gate or entrance was required.  However, that presented a weak point that an attacker might exploit.  Thus Mahan urged his students to build additional structures, namely traverses, to cover those entrances.  That reduced the risk to a degree.

But where the point where the outlet crossed the ditch (if it had to cross a ditch, that is) presented a greater problem.  As we’ve seen in detail, one of the important properties of the ditch was serving as an obstacle to any attack.  But the outlet would require a six to ten foot wide path across the ditch.  Thus granting an attacker a potential highway across.  Common sense response to that problem was to simply build some sort of retractable bridge.  But pre-war, Mahan did not offer that as a direct solution (indeed, avoiding mention of the problem altogether).  But in his post-war edition, Mahan offered:

Draw-Bridge. For the usually narrow ditches of field works, either a light rolling bridge may be used for a communication, from the outlet, across the ditch; or else an ordinary wooden draw-bridge.  A very simple one, and of easy construction, was proposed by Colonel Bergère of the French engineers.

I believe this refers to Colonel Pierre Bergère (1785-1868), but I don’t know of a specific work to reference in regard to the bridges in question here.  I find it interesting the more detailed discussion of bridges enters Mahan’s post-war edition with Bergère serving as a reference and introduction.  Either as if the 1861-65 experience simply followed that of the French engineer, or could not be applied directly.  I have my thoughts, but let’s save that for later.

Mahan offered Figure 45, bis to illustrate the proposed bridge:

MahanFig45bis

Technical details followed:

The bridge is a light platform a,a’, of joists and boards, long enough to span the ditch D, and so arranged as to turn around an axle at A, the crest of the scarp.  At the point B, on each side of the platform, an iron gudgeon is firmly attached to it and turns in the eye of a socket at the end of lever C B.  This lever is formed of two pieces of scantling of some tough flexible wood, each about four inches square. The lever has an eye, at the middle point O, which receives a strong iron bolt that connects two ordinary gun carriage wheels.  The two pieces which form the lever are firmly fastened together, as shown in the figure; a weight, consisting of shells filled with sand, or shot, being fastened at the end c, and serving as a counterpoise to the bridge.  Two rails A, of heavy scantling are laid for the wheels to run upon in maneuvering the bridge; which is done simply by one or two men taking hold of the spokes of the wheels, and so, by turning them, causing them to run backwards or forwards, and thus raise or lower the bridge.

The arrangement is a clean piece of engineering.  But nothing novel or particularly advanced, technically speaking.  Noteworthy is the re-purposing of common equipment, such as carriage wheels and projectiles, on hand with the field army.

But I have to throw a flag out here.  If those building the fortification had ample time to build fancy drawbridges, they were more so working as garrison troops and probably not actively campaigning.  The premise behind field fortifications was those would be temporary structures established as part of a field army’s operations.  Not structures to be garrisoned, which should shade more to the permanent fortifications with far more elaborate arrangements all around.  But experience in the Civil War took the American officers away from some pre-war assumptions about how fortifications would be use.  And more importantly how to classify them.  We’ve already noted that Junius Wheeler began classifying semi-permanent and temporary garrison fortifications within the discussion of field fortifications.

Now this is not to say no self-respecting American engineer in 1861 would consider the drawbridge when planning the outlet to a fort. Rather that in 1861, the use of drawbridges were not emphasized in the classroom.  But by 1870, that changed to provide a couple of paragraphs and a nice illustration based on a French officer’s recommendations.  As we will see by looking at Wheeler’s text, by the next decade, those instructions evolved even further.  Pages, mind you, dedicated to the discussion of small bridges!

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 57-8.)

Fortification Friday: Don’t for get the door! Gotta get in and out of the fort

So you’ve built a strong fortification in accordance with Mahan’s instructions.  The fortification has wonderful fields of fire, properly sized parapets, protected batteries, and bombproofs for everything to include the commander’s liquor stash.  Great!

But sooner or later the fort’s garrison has to wonder outside the works.  Say for maintenance of the works, or patrolling the outskirts, or simply for resupply.  So there needs to be a passage to the exterior somewhere in the plan.  Mahan called these outlets:

Outlets are passages made through a parapet, or an enclosure of a gorge, for the service of the work.  They should, in all cases, be made in the least exposed part of the work.  Their width need not be more than six and a half feet, when used only for the service of the work; but when they serve as a common passage for wagons, &c., in the case of the intrenchment crossing a road, they should be at least ten feet wide.

Clear definition.  And some clear specifications with the idea width established at either 6 ½ feet, for “walk out” outlets, or ten feet, for “ride out” outlets.  That established as a rule of thumb, we turn to the structural components and advice for building these outlets:

When cut through the parapet, the sides receive a slope of three perpendicular to one base, and are riveted with sods, &c.

A gate, termed a barrier, serves as an enclosure to the outlet.  The framework of the barrier is made like an ordinary gate, consisting of two uprights, or stiles, a cross-piece, or rail, at the top and bottom, and a swinging bar, or a diagonal brace. Upright palisades, about seven feet long and four inches thick, are spiked to the frame about four inches apart; they are finished at top with spikes. A barrier, thus constructed, will not offer a shelter to the enemy should he attempt to cut it away.  The barrier is hung on hinges like an ordinary gate.  The posts of the framework should be very solidly braced to support the weight of the barrier.

Figure 41 illustrates such a barrier:

PlateVIFig41

Yes, they sell these as pre-fabricated products at Home Depot and Lowes.  Well… not exactly to military specifications.  But you get the point, the base form is a simple gate. For the annotations, Mahan offered:

  • A A – posts to which the gate is swung.
  • B B – the uprights of the gate.
  • c c’ – the upper and lower cross pieces. (c’ seems to have been left off).
  • D – the diagonal brace.
  • E – the bar of the gate.

Note also the mention of spikes on the gate. Specifically palisades.  These were not intended to impale an attacker, but rather to keep the attacker’s reach away from the structure of the gate.  So drop all those Medieval notions there.

Continuing, Mahan offered,  “A cheval-de-frise is sometimes used for a barrier, it presents but a trifling obstacle.”  So let us relegate that to the level of lazy engineering.

But just keeping the enemy at arms length was not enough.  One also had to protect the outlet from cannon fire. Toward that end:

The outlet should be covered by a mask, thrown up either on the interior, or on the exterior, to prevent the enemy from firing through it into the work.  A traverse is thrown across it, if placed on the interior.  Sufficient space should be left between the traverse and the parapet for the passage of a gun.  The length of the traverse is arranged to prevent the enemy from firing into the work, by an oblique fire through the outlet.  The traverse may be of earth or of wood; in either case it should be arranged for defense to enfilade the outlet. In some cases, and it would generally be safest, a barrier is erected between the parapet and the traverse, on each side of the outlet.

Figure 42 provides us Mahan’s suggested layout of the masking traverse:

PlateVIFig42

From the key for this figure:

  • O – the passage or outlet through the parapet.
  • P – (to the left of “Fig. 42.”) the passage between the parapet and the traverse
  • T – the traverse

Note Mahan gave us 6 ½ feet between the parapet and traverse for “P”.  We also see the prescribed cross fires built across the passage.

Mahan went on to suggest more elaborate defensive arrangements to protect important passages.  “In very frequented passages, a redan or a lunette, is thrown up on the exterior to cover the outlet, and thus ensure its safety in case of surprise.”  These arrangements followed the standard configurations for faces and flanks.  No doubt, such added more work for the defender’s labor force, as they would be clearing and leveling both front and rear.

The important take away with respect to passages is how a necessary weak spot in the defenses would be turned into a strong point by way of barriers, traverses, and other cover.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 61-2.)