Fortification Friday: Of planes, lines of fire, and regulated dimensions

Starting Chapter II of Field Fortifications, Mahan introduced a few more terms before advancing into what he termed the “plan of simple intrenchemnts.” These terms related to the profile as relating to the base ground level and, looking at the plan (on the horizontal plane), relative lines of fire against the attacker.

First those terms applicable to the vertical, or the profile:

The ground occupied by a work is denominated the site, or plane of site.

The command is the height of the interior crest above the site; and the relief is the height of the same line above the bottom of the ditch.

These terms are easy to depict on Mahan’s standard profile diagram:


From a practical standpoint, these terms defined useful measures for the engineer designing the defense.  These are also useful measures for us analyzing the fortifications today… speaking to the effective or defective arrangements, as the case may be. On the horizontal, with some emphasis on the angle of defense in the nine regulating principles, Mahan introduced terms defining the angle of fire from the line of defense:

A fire is said to be direct, slant, or enfilading, according to its direction is perpendicular to, makes an angle of 30º with, or is on the prolongation of the line at which it is aimed; when the line is taken in the rear the fire is denominated a reverse fire; and when a given space is defended by the fire from several points crossing over it, the defense is denominated a cross fire.

I don’t know about you, but this passage is painful to read.  It’s King James Old Testament stuff.  I prefer a passage from an updated version of the textbook, written by Colonel Junius B. Wheeler (a North Carolina-born officer who stayed with the Union after secession… but I’ll save that for another day, Robert Moore!) and published in 1882:

When the direction of the fire is perpendicular, or nearly so, to the line aimed at, the fire is a direct one; if this direction makes an angle with the line aimed at, it is oblique; if this angle is very slight, it is a slant fire; or if no angle is made but the direction coincides with the prolongation of the line aimed at, it is an enfilading fire; if it makes no angle, but is in front of the line, it is called a flanking fire.

Wheeler applied more terms to the angles of fires, but those spoke to considerations beyond the basic denominations that Mahan was presenting at the start of Chapter II.  But with both passages in mind, let’s lay them out on Mahan’s basic arrangements:


Goes without saying that those depictions are “notional” in relation to the works in the diagram.  To achieve reverse fire, the attacking force would need be well past a portion of the advanced works.  I’ve added the two types added by Wheeler – oblique and flanking – for complete coverage there.  And if we take the sum total of all those notional arrows, we see a cross fire.  Reaching back to the previous posts, we see a practical application of cross fire when examining the angles of defense:


These terms speak to how the officer planning the works hoped to defend it.  We also see these terms used in official reports to describe the types of fires placed upon an enemy… or received from the enemy.  So these “words” had “meanings” to the writer, and we should be mindful of such when interpreting those writings.  Next time you read of “flanking fire” recall there is a functional difference between that and “enfilading fire.”

Lastly, before starting into the wonderful descriptions of redans, lunettes, and priest-caps, Mahan touched upon some practical aspects of the line of works… a rule of thumb if you will:

In planning a work the interior crest is regarded as the directing line in regulating the dimensions of faces, flanks &c., because this line shows the column of fire for the defense. There exists a necessary subordination between the plan [plane?], relief, and command of works, which prevents the dimensions of the one being regulated independently of the others; but, without entering into a close examination of this necessary co-relation of the parts, it may be stated generally, that faces should very between 30 and 80 yards, flanks between 20 and 40 yards, and curtains should not be less than twelve times the relief.

This is raw “how it works” stuff applied to the lesson.  But one point of order here.  Mahan’s instructions relied upon the performance of the musket as a governing factor.  We, looking back at the fortifications built 150+ years ago, particularly those in the later phases of the war, notice the threshold of preferred dimensions let out somewhat.  Weapons technology drove that change to some extent. And we should consider that Mahan did not speak of vertical fires in this passage… a delivery method which was used more by the summer of 1864.

Another, less often considered, factor is the scale of combat.  When Mahan wrote his Treatise, in the American experience there were few examples of assaults larger than a division-sized formation.  And much of the treatise relied on the European method.  Some method changes were required as the force size scaled up – be that in the attack or defense.  Consider that in regard to field fortifications at some location such as… say… the Bloody Angle at Spotsylvania.

Rules of thumb are always subject to revision….

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 11; Junius B. Wheeler, The Elements of Field Fortification, New York: D. Van Nostrand, 1882, page 35.)


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