Fortification Friday: As Wheeler would say, “Loopholes are not just for blockhouses”

As we continue to compare and contrast the pre-war writings of Dennis H. Mahan with the post-war instructions of Junius B. Wheeler, let’s move from arrangements for the artillery to that of the infantry on the parapet.  Mahan, like his contemporaries, left the infantry on a bare parapet, with a firing step on the banquette that would allow them to step-up to the crest and fire over.  Little else was deemed necessary.  But experience of the Civil War indicated something more was indeed necessary.   Wheeler discussed this under the heading of “loopholes”:

Loop-holes.  – Troops on the banquette, when in the act of firing their pieces, are frequently exposed to the fire of the enemy’s sharp-shooters.  Under these circumstances, expedients must be devised to protect the men, without interfering with their fire.  The expedient which is most generally used, is that of an improvised loop-hole.  The loop-hole is made, in this case, by arranging two or more rows of sand bags, placed upon the parapet and filled with earth, so that the top row will be higher than the men’s heads, and so as to leave intervals between the bags in the lower rows, through which the men can aim and fire their pieces.

Figure 28 illustrated this arrangement:


Let’s walk through this passage, as it offers another glimpse into the changing doctrine applied to the battlefield.  Right from the start, we see something “non-Mahan” as a condition.  In those pre-war days where the Napoleonic battlefield framework was in play, musketry was generally used in mass.  Volley fire, by attacker and defender, was the expected means of delivering those lead projectiles.  In that framework, working a musket into a gap in sandbags would slow down the delivery of a volley.

But Wheeler alluded to a change in how musketry was used.  Instead of massing fires in volleys, the Civil War armies employed much more individual fires.  Skirmishing, of course, took on greater importance.  And in these field fortifications, that translated to sharpshooting.  More likely the attacker would employ this means of attriting the defender, instead of attempting a rush of the works.

This is not to say nobody ever thought of putting sandbags on the parapet before the Civil War (or headlogs, which we’ll circle back to).  But this is to say changes in the way musketry was delivered brought out a need to employ this feature (loopholes) as a standard fit on the parapet.  Wheeler and his contemporaries didn’t invent the sandbag loophole.  They simply introduced it to meet an evolving requirement.  Yes, “innovation” does not always mean “invention.”

There were other ways to setup a loophole on the parapet of course:

Gabions are also used for a similar purpose.  The gabions are placed in pairs upon the parapet and filled with earth, each pair being separated from the adjacent pair by an interval of about two inches.

And… field experience gave us even more options:

A contrivance adopted in the war of 1861-5, was quite effective for the same purpose.  Skids were placed upon the parapet, with notches cut in them.  A heavy log was placed on the skids, occupying a position parallel to the interior crest and just in contact with the superior slope.  Notches were cut in the underside of this horizontal log and these were used as loop-holes.  The openings to the exterior were made as small as possible, and in some cases were protected by small patches of boiler iron spiked upon the log.  When exposed to artillery fire, earth was banked against the log.

We often hear this or similar arrangements called a “head log” in the writings of veterans.  I am most curious that Wheeler didn’t use the term.  And even more curious why Wheeler didn’t include an illustration!  At any rate, he continued with this description, naming an “innovator” from the late war:

A wooden loop-hole was devised by Lieut. King (now Major) of the United States Engineers, which was used in 1864.  It was practically a wooden hopper made of boards, placed upon the superior slope of the parapet, and covered with earth.  The splay of the sole and the angle of the cheeks were made to suit the field of fire required.

The officer mentioned was Lieutenant William R. King.  Brett Schulte has King’s report on Beyond the Crater, and a detailed report it is.  The accompanying illustration matches to Wheeler’s description.  For brevity, I’ll refer you to Brett’s excellent site.

Wheeler continued, with detailed requirements for these loopholes:

The exterior orifice of a loop-hole for musketry should be made as small as possible.  A width of two inches and a height of five, is sufficiently large for ordinary purposes.  The sides are sloped, and an inclination given to the bottom and top, according to the field of fire which is to be swept.

Now what is good for the musket should also be good for the cannons, right?  Of course:

Embrasures are sometimes protected in a manner similar to this arrangement for loop-holes.  Timbers are laid across the embrasure, covering the throat, leaving only room for the muzzle of the piece.  These timbers are then covered by sand bags, by fascines, etc., to make them shot-proof.  Sometimes the embrasure is filled with sand bags or fascines to mask it, these things being quickly removed when the embrasure is needed for use.

Thick wooden shutters, made bullet-proof, and placed on vertical axes, and iron shutters swung on horizontal axes, have both been used to close the throat of the embrasure.

In some cases, timber supports were extended back from the parapet and a covering of timber and earth placed upon them, protecting the gun from vertical and plunging fire.  A gun thus sheltered is said to be case-mated.

Again we see the factor driving all this “innovation” and change – different types of fire were employed.   Individual musketry … sharpshooting as it may be called… brought out the need for protection on the parapet.  Vertical and plunging fire, which I have written about before, brought out the need for overhead protection.  No new inventions are introduced here, rather the innovation lay in the way existing practices were employed.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 126-8.)


Fortification Friday: Wheeler vs. Mahan, Embrasures and Bonnettes

Last week we gave time to Junius B. Wheeler’s instructions about barbette batteries.  Now let us turn to his thoughts on embrasures, which were the alternative siting of artillery in a field fortification.  Wheeler offered this drawing of an embrasure for reference:


Perhaps a cleaner diagram than Mahan used, either in his pre-war or post-war texts, but generally the same features. The art and science of making an embrasure changed little.  For reference, here are the labels and specifications Wheeler gave:

  • The Sole was the bottom of the embrasure: G-E-F-H in the figure.  This was inclined outward, usually at the same rate as the superior slope of the parapet.
  • The Throat was the opening on the interior: a-b-G-H in the figure. Normally 18 to 24 inches wide.
  • The Mouth was the exterior opening: C-E-F-D.
  • The Splay described the widening of the embrasure towards the exterior.
  • The Cheeks were sides of the embrasure: a-CE and b-F-D.
  • Directly bisecting the sole between the cheeks is the Directrix: M-N.  This determined the base orientation of the cannon in the embrasure.
  • The Genouillere was the slope between the throat and the banquette (or raised mound for the gun’s platform).
  • The Merlon was the section of parapet between embrasures on the parapet.

Wheeler indicated that embrasures were best cut out after the parapet was completed, adding “the exterior openings are masked until the moment to use them arrives, to prevent their position from being discovered by reconnoitering parties of the enemy.”  In terms of labor estimates, Wheeler indicated, “a detail of six men should be able to cut an embrasure in the parapet of a field work and finish it in eight hours.”

But before those six men could take shovel in hand, the engineer had to trace the embrasure.  Wheeler offered detailed instructions.  More detailed than Mahan’s but not significantly different.  The process started by drawing the directrix.  From there the throat was defined.  From there the sole, mouth, and cheeks were drawn out.  But the key to all those elements was the slope of the sole and the angles of the splay.  And those elements defined the angles at which the gun could be trained to fire.  Thus very important things to consider:

The splay of the sole is usually determined, in plan, by giving to E F some definite length, and then joining its extremities with the lower line of the throat.  A throat twenty inches wide will have a horizontal field of fire of twenty-two degrees, when E F is equal to one half the thickness of the parapet; a fire of thirty-one degrees, when the E F is equal to two-thirds of the thickness; a fire of forty-eight degrees, when this line is equal to the thickness of the parapet.

Mahan had offered a similar rule, but I tend to like Wheeler’s explanation better.  Just seems clearer and fine to the point.   From there, Wheeler discussed how to lay out the cheeks and complete the embrasure.  Like Mahan, Wheeler suggested revetting the embrasure to prevent damage when firing the cannon.  Gabions were preferred, though sod was also suggested.

Since more than one gun would be placed on the parapet:

Consecutive embrasures should not be nearer to each other than fifteen feet from center to center, to prevent crowding of the guns and to prevent the merlon, M, from being too weak.  A merlon which measures less than six feet on the exterior crest should not be allowed, as it would make the parapet too weak.

Note the location of the merlon, M, on the figure:


Consider the rule of thumb regarding the size of the mouth (that E-F measure) when applied here.  Let’s say our parapet is five feet thick, and you want to allow a 48º traverse.  So the E-F line must be five feet on the exterior crest.  But the distance between “F” on the left side embrasure and the “E” on the right side embrasure must be at least fifteen feet.  Furthermore the distance between the left side “D” and the right side’s “C” must be at least six feet.  Adding all those together, we find a total front needed of twenty-five feet of parapet face, at minimum, if we want two cannon with 48º traverse.  All well and good if you have room. But we might want to reduce the traverse to avoid unnecessary work.

Like Mahan, Wheeler considered both direct and oblique embrasures.  Regarding the latter, Wheeler offered the limitations up front:

Oblique embrasures do not admit of the muzzle of the gun being inserted so far as the direct ones, and they weaken the parapets more.

Oblique embrasures are not used, as a rule, if the directrix makes with the normal to the crest an angle exceeding ten degrees.  In case the angle is greater, the embrasure is provided for, in field works, by modifying the interior crest by means of the method known as “indenting.”

This method consists of making a crest a crémaillère line, instead of a right line, with the short branches perpendicular to the direction of fire, and in those short branches constructing direct embrasures.

Or, simply put, if you need a larger angle than ten degrees off the dirextrix, then build a small redan or other extension out from the parapet.  Such implies a better trance should be considered to start with.

Overall, comparing barbettes to embrasures, Wheeler considered the former as offering wide fields of fire without weakening the parapet.  But the barbette exposed the gun crew to enemy fire.  While the embrasure protected the guns and crew, there were limitations to the field of fire and weaknesses along the parapet.  Furthermore, Wheeler warned that embrasures made a good mark for enemy fires against the fortification.  Recall during the war Federals were very proud of the 3-inch Ordnance Rifles’ ability to put rounds through Confederate embrasures at range.

To mitigate the exposure of the guns and crew from enemy fire, Wheeler offered an additional structure, calling them Bonnettes:

It is frequently desirable that the height of the parapet, at certain points, should be increased for a short distance.  This increase is generally obtained by making use of the constructions known as bonnettes.  A bonnette extends but a short distance along the parapet, is make of earth, and is used generally to give greater protection to the men standing on the banquette against a slant or an enfilading fire of the enemy.

Bonnettes are placed usually on the salilents; they are sometimes placed on the parapet between guns “en barbette.”

They may be constructed during the progress of the work, or after the work has been finished.  In the former case, their construction is, to all intents and purposes, similar to that of the parapet. In the latter case, they are constructed generally in haste, and sand bags or gabions filled with earth are used to build them.

Note, bonnettes are not traverses, as they stand directly on the parapet.  Rather these were structures placed to the sides of the barbette (or embrasure if needed).  While I can find references to bonnettes going back to the previous century, Mahan seems to have disregarded them.  The reason may lay in the disadvantage of the bonnette.  In effect, the structure raises the parapet’s interior crest relative to the banquette, thus preventing musketry from that section of the parapet.  In Mahan’s framework, musketry was considered important to the fort’s defense.  However, by Wheeler’s time canister fire seemed to be more desirable.  That would reduce manpower requirements, foot for foot, on the parapet.

Comparing Wheeler with Mahan, in regard to arrangements for batteries, there is not much difference in terms of form or even implementation.  But we do see some variance in the function.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 120-6.)

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:


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:


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: Wheeler and the evolution of interior arrangements during and post-war

I’ve mentioned on several occasions how the lessons on fortifications, used to teach cadets, demonstrate the evolution of doctrine from pre-war thinking into the post-war era.  We see some of this evolution when comparing pre-war and post-war versions of Mahan’s treatise.  But where the changes really come into play is when comparing Wheeler’s  1880 textbook.

An example of these changes is how Wheeler chose to relate details of interior arrangements.  Even when classifying these arrangements, Wheeler offered a shift, sometimes subtle, change of focus.  Recall Mahan’s pre-war text described the classes of interior arrangements as such:

The class of constructions required for the above purposes, are batteries; powder magazines; traverses; shelters; enclosures for gorges and outlets; interior safety-redoubt, or keep; and bridges of communication.

Somewhat brief, and from there Mahan proceeded to detail how batteries were placed on the parapet… with emphasis on the employment of artillery.

Wheeler offered a similar set of classifications, but abstracted those a bit with a mind to functional requirements:

Classes. – The earth work for the parapet being completed, and the revetments of the interior slope constructed, attention is then paid to the interior of the work.  Certain arrangements have to be made in the interior, to add to the efficiency of the defense, and to provide for the comfort of the troops who have to occupy the work.  These interior arrangements are divided into classes, according to the object to be attained by them.

The divisions may be classified as follows:

  1. The arrangements of, and along a parapet, intended to add to the efficiency of the defense;

  2. The arrangements within the area enclosed by the parapet, to shelter the men and matèriel from the fire of the enemy;

  3. The arrangements made to allow egress and ingress of the troops; including those made to guard the outlets against surprise; and

  4. The arrangements which may be made to provide for the comfort and welfare of the garrison when occupying the interior of the work for some time.

While Mahan named specific structures that would be constructed within the interior, Wheeler’s classifications come across is more so proper doctrine.  In other words, Wheeler put the reason before the task.  Such is a more formal approach to doctrine, as opposed to simply providing a list of structures, and  their specifications, to be used.

And with that approach, abstracted from naming particular types of structures, Wheeler was able to identify some of the needs, beyond the basics, of a garrison occupying a fortification.  You know… like those “comfort and welfare” things he mentions.  But not to be overlooked, shelter from enemy fire and ease of entry or exit.  These were all things Mahan addressed, but gave limited treatment. Did Mahan simply not care about the troops?  No.  But Mahan’s text was rooted in some pre-war concepts which, though we have discussed before, should be repeated for clarity.

Mahan’s instruction about fortification borrowed heavily from the European experience.  It was a textbook on field fortifications which would complement other instruction relying heavily, as is often the case with military science, on the “last war.”  Many scholars have debated the influence of Antoine-Henri Jomini on Civil War generalship.  But I don’t think we can dispute a “what would Napoleon do?” approach prevailed.

That in mind, the military minds came to make several operational assumptions.  With respect to fortifications, the assumption was two types would be employed.  Fixed, permanent fortifications were constructed to defend vital areas.  For the Americans, these were most often seacoast fortifications, as most potential adversaries would need to gain lodgement at some port (the exception, prior to the 1840s, being the northern border, but even there the great lakes presented a seacoast-like need).

The other form of defense assumed was temporary or field fortifications for use by an army on campaign.  And those were intended to work within the tactical framework handed down by the European experience on the Napoleonic battlefield.  These fortifications were employed to protect important areas related to the army’s campaign objectives.  And those objectives were subject to change.  The design of the works was more so to deter direct attack.  In that way the temporary works would deter direct attack, requiring a deliberate effort (i.e. a siege or other significant commitment of resources).  The temporary fortification was not designed for prolonged occupation or lengthy defense.  Above all, the temporary fortification was always a function of the campaign being undertaken at that moment in time.

The American experience, even before the war, offered a slight twist to the paradigm.  With a significant commitment to the frontier, structures such as blockhouses became important.  Yet the US Army did not give much instruction to that part.  In my opinion there were two significant reasons for this.  First, perhaps foremost, the methods for establishing frontier garrisons were judged as intuitive.  Mahan’s “Outpost” manual covered some of this.  Secondly, much of the responsibility for these frontier garrisons was given to territorial, state, or local authorities.  In short, it was not a major mission in the broader sense.  Rather, the US Army was supposed to give Congress detailed plans for defending the coast.  Those commitments out west were not the “big show”.

Civil War experience demonstrated the “two types of forts” assumption to be incorrect, to say the least.  Operational requirements demonstrated there was a third type which fell directly between the chairs.  That being a semi-permanent fortifications which were not directly tied to ongoing campaigns.  The best example of that would be the Washington defenses.  Lesser so the Richmond defenses, but of course those became the focus of a campaign later in the war.  Other examples, which we’ve mentioned in relation to blockhouses, were those works constructed to protect communications and supply lines. All of which supported the army in the field, but were not a direct function of that army’s operation… i.e. the campaign.   And as a function of those requirements, the engineers had to address other needs within the fortifications, to include “comfort and welfare.”

What I like about the passage from Wheeler enumerating the classes of interior arrangements is that he removed a lot of the Napoleonic baggage from the discussion.  Sure, forts were still built in the manner employed in the first quarter of the 19th century.  Men used shovels to dig, then pile the earth.  Fortifications still required parapets and ditches to be effective means of resistance.  The “physics” of the matter did not change, other perhaps than the need for additional thickness to resist rifled projectiles.  But it was the requirement those fortifications were filling that shifted over time.   That’s what I see reflected in the approach Wheeler gave to these particulars.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 51-2; Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, page 114. )

Fortification Friday: Wheeler’s take on blockhouses

As we have often seen, when writing instructions for cadets in the 1880s, Civil War veteran Junius B. Wheeler put a twist or two on Mahan’s pre-war, and post-war, teaching about fortifications.  The blockhouse is yet another example of this.  For starters, Wheeler introduced the notions about using blockhouses with a different set of terms and classifications.  Instead of “safety redoubts”, Wheeler stated with…

Secondary interior arrangements. – Besides the interior arrangements which have been described, or mentioned, there are others which are secondary in their nature.  These are the arrangements which are to be used under certain contingencies, or in cases of emergency.  An example, would be  defense placed within a field work, which defense can be used only when the main work is no longer defensible, etc.

This is certainly keeping in line with the concepts we saw Mahan teaching, but is giving a broader sweep to things.  In the next paragraph, Wheeler jumped right to the blockhouse… but with some fresh considerations:

Block-houses. – It is frequently the case that a separate fortification is constructed, laying entirely within a work exterior to it, into which a garrison can retire and protract their resistance, even after the outer fortification has been taken, or has been made unfit for further defense.

If this interior work is a line of earthen parapet, it is called a retrenchment; if it is a defensible building, it is termed a keep.

The term, keep, is also applied to a work which is entirely separate and distinct from the work exterior to it, whatever may be the material used in its construction. In a field work, the keep is built of timber, and is called a block-house.

There you have it – in fortifications the engineer will consider “secondary interior arrangements” to include retrenchments and keeps.  And if the latter of which is made of timber, it is called a blockhouse.  It’s the same basic thing Mahan wrote, just using different terms.  But, as we all know, terms have meaning and are selected for reasons. In this case, I think Wheeler was simply saying there were many different options for these interior works, and the blockhouse was favored.

Regardless of the emphasis, Wheeler’s blockhouse keep differed little from Mahan’s in form and function.  Wheeler directed the blockhouse have “good command over all of the interior space” and “that all parts of the exterior work can be seen from it.” Wheeler’s blockhouse could be square, rectangular, and “even cruciform” in plan, just like Mahan’s.

However, there was some difference in the internal dimensions.  Rooms inside the blockhouse were to provide six feet of height, though eight or nine were suggested for ventilation.  And Wheeler suggested an interior width of nine feet, “as this is the least distance which can be used and give room for a passageway and a row of bunks.”  Recall, Mahan specified at minimum a nine foot height and a width of up to twenty feet.  But keep in mind Mahan’s dimensions were governed by the need for handling muskets within the blockhouse.  By the time Wheeler was writing, those Springfields had been converted to trap-door models.  Wheeler indicated the overall length of the blockhouse, including all rooms and spaces, would “depend upon the number of men it has to accommodate, after the width has been assumed.”

As to the thickness of walls, Wheeler gave no specification for single or double thickness of timbers.  Rather, he simply indicated, “Block-houses must be made strong enough to resist the projectiles which may strike them and should be proof against fire and splinters.”  Elaborating further, Wheeler said, “The conditions given for a bomb-proof are applicable to the block-house, with the additional one of arranging its walls for defense.”  And by defense, he meant loopholes.   To illustrate this, Wheeler recycled one of Mahan’s drawings:


So in form and function, Mahan’s post-war blockhouse remained the standard.

But what of Mahan’s “American blockhouse” for use as a stand-along fortification?  Well, Wheeler had a different label:

Isolated block-houses. – Timber blockhouses were used frequently in the war of 1861-5 in isolated spots, as independent works.

In these places, they were, as a rule, exposed to attack only from infantry or cavalry, or a few pieces of field artillery.

The construction shown in Figures 53, 54, and 55 is a type of these isolated block-houses.

It was found from experience that it required a thickness of forty inches of solid timber to resist the projectiles of field-guns.

These isolated block-houses were frequently built two stories high.  The upper story was usually placed so as to have its sides make an angle with the sides of the lower story. By this arrangement, the corners of the upper story projected over the sides of the lower.  This arrangement of the upper story removed the dead space near the sides of the lower story, and the sector without fire in front of the angles.  Block-houses exposed to artillery should not have a second story.

Again, very close to the words Mahan wrote, but mixing the pre-war with a bit of wartime experience.  There are a few points which followed in Wheeler’s text that we shall return to in time.  But for the most part, we see the “American blockhouse”, with a few new terms to describe its classification within the fortification form.

Wheeler’s manual was aimed at a generation of cadets some twenty years removed from the Civil War.  That generation would include men like John J. Pershing.  While some, as was the case with Pershing, would serve on the frontier where there was use for the old blockhouses.  And around that time, our romantic notions of the frontier posts took root.  So I close this discussion of blockhouses coming full circle to those movie props and toys which come to mind when that sort of fortification is mentioned.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 156-60.)

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:


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:


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:


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 American Blockhouse

One of the themes I’ve worked in this series of posts is a comparison between pre-war and post-war manuals. In small ways, the comparison points to the influence of wartime experience in the practice of military science… at least in the American army.  Another such example is the use of blockhouses.  Pre-war, Mahan discussed blockhouses in relation to field fortifications as a safety redoubt or keep.  That was not, of course, to say such was the exclusive use of blockhouses.  But Mahan simply offered less thought about the use of stand-alone blockhouses. Rather the emphasis was upon structures which would be used in conjunction with standard, conventional field fortifications to meet the traditional needs of an army on campaign. Again, this is not to say Mahan didn’t agree with stand alone blockhouses, but rather that in instructions to his students he put all the emphasis on blockhouses used as keeps within field fortifications.  So this is a “how they were taught” consideration instead of a “this is the only way they would use it” declaration. Keep that fine point in mind.

On the other hand, the American military experience, drawn out even more so by the Civil War, included the need for fortifications guarding rear areas.  In particular, protection of railroad lines, bridges, and other such infrastructure was rather important. And, as we know from ample examples from official records, photographs, and other sources, the blockhouse became the preferred fortification for that need.  And Mahan identified that in his post-war writings, declaring an American-ism in such employment:

American Block-House. In the more recent block-houses erected in our service for the protection of bridges, railroad stations, etc, the sides and roof … are constructed with a double thickness of logs eighteen inches in diameter, hewn to a face of eight inches where they are in contact.  The inner logs are placed upright, the outward horizontal. A space is left in the outward casing sufficient for the fire from the loop-holes made through the inner. The horizontal logs above the loop-holes are held up by short uprights, mortised into them and into those just below. The ceiling is covered with earth, as shown in the section, three feet thick at the ridge and sloping towards the eaves to about six or nine inches, where it is confined by a pole plate. The earth is protected from the weather by a board roofing.  Tin or sheet iron ventilators are made through the roofing and ceiling, and a brick flue to receive the pipe of the stove used in cold weather.

The loop-holes are nearly of the same form and dimensions of those already given.

Mahan gave us two sets of illustrations supporting this passage.  The first demonstrated the blockhouse particulars mentioned above:


We see a double layer of timbers – the inner most placed vertical and the outer laid horizontally. In both cases, the logs are flattened on the sides in contact to close up any gaps.

Also notice the provisions for loopholes.  The dimensions remained the same, generally. But instead of simply carving out loopholes, Mahan suggested a more elaborate arrangement.  I’d describe this as a set of small columns between a gap in the outer, horizontal timbers.  Traditional loophole cuts are made in the vertical timbers of the interior. So there is still the double row of protection and the minimum opening required for the musket barrel.

I say a picture is worth a thousand words:


This is a blockhouse protecting the railroad between Nashville and Chattanooga in Tennessee.

Continuing, Mahan offered more elaborate plans for these blockhouses:

Some of these structures are built in the form of a cross, consisting of a square central chamber, twenty-four feet on a side, and of four wings of the same form and dimensions when the block-house is for cannon. An embrasure is pierced in each of the three sides of each wing to serve a single gun.  The cheeks of the embrasures are faced with logs, and the mouth is secured by a musket-proof shutter with a loop-hole in it.  The embrasures are below the level of the loop-hole, allowing these to be used whenever necessary.

Though not exclusively for the employment of artillery, the implication is that guns required more space within the fort and thus more elaborate arrangements were needed.  The addition of shutters is noteworthy and speaks to the need for crew protection in the era of muzzle-loading artillery.

But where you have artillery, you must also have a magazine:

Arrangements for magazines and store-rooms are made under the floor of the block-house in the most secure parts.

Ah… OK… answers that question.

And what about the entrance:

The entrance to the block-house may be either through a postern, the bottom of which is on the level of that of the ditch, a ramp leading from this level outwards, a door properly secured, and steps, forming the inner communications; or it may be arranged as shown in Fig. 51, 52, with a plank thrown across the ditch on the same level as the natural ground, the entrance to the door being masked by a double stoccade, leaving the same passage-way as that of the doorway.  Loop-holes in the door and sides of the building sweep this passage.

And here’s the referenced figures:


Also note in this set of figures, and that above, the berm built up against the blockhouse.  As discussed earlier, this improved the defensive quality of the work, particularly against artillery.  Furthermore it prevented the enemy from hiding under the loopholes.  We don’t see that in the wartime photo above.  I’m of a mind we are seeing a blockhouse in the final stages of construction, rather than some flaw in the engineer’s design.  Other wartime photos show earth banked up against the blockhouse:


This is another Tennessee blockhouse.  Wonderful details to consider – the loopholes and the entrance stand out nicely.  And consider the figure offered by Mahan (post-war) to illustrate the two story blockhouse:


Can you find a better match? That’s great stuff!

Another illustration Mahan provided in the post-war manual detailed arrangements for the loopholes.  So while he “said” such was “already given,” teaching after the war he saw need to elaborate.  We’ll look at that next.

(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 62-3.)