Fortification… Thursday: “Every point of the parapet should be guarded”

With Christmas falling on Friday, I’ll move the regular cycle of posting up a day… you get Fortification Thursday.

Let us close out the “lesson” on simple “intrenchments”, or basic field fortifications, by looking at the last passages from Mahan’s chapter on the subject:

The defense of enclosed works demands that every point of the parapet should be guarded, at the moment of assault, either by cannon or musketry. The troops may be drawn up for the defense either in one, two, or three ranks; and there should, moreover, be a reserve proportioned to the importance attached to the work.  The free interior space, denominated by the terre-parade plein, should be sufficiently great to lodge the troops, with the cannon and its accessories, and will therefore depend on the nature of the defense….

This paragraph takes us back to Mahan’s principles for the defense.  However, in the earlier lessons, Mahan had not discussed the interior structures at length.  Here he cites a feature by name – the terre-parade plein – and gives a requirement for that feature.  Goes to reason that important places, which would require large reserves, would thus need a large interior parade.

Mahan then began to provide some practical application of this advice:

Each man will occupy one yard, linear measure, along the interior crest, and each cannon from five to six yards.  The space requisite to lodge each man is one and a half square yards; and about sixty square yards should be allowed for each gun.

Think of that “frontage” as applied to Mahan’s rule of thumb about fort dimensions. So a thirty yard flank would need 30 men … or five or six cannon… or a combination of those.  Multiply that allocation against the similar sized opposite flank, then add in the manpower needed on the faces and curtain.  Very quickly we have need of a full strength company plus a battery of artillery (at least) to defend a single side of a bastion fort.  So project that math further… four sides, plus a need for reserve.  You see how we ring up the requirement for a full strength regiment plus three or four batteries of artillery.

Well… on the other side of the coin, at least there would be plenty of hands around to do the digging.

Mahan continued with details of the arrangements, and immediately brought up another interior structure for discussion:

Besides this space an allowance must be made for the traverses, which are mounds of earth thrown up in the work to cover an outlet, to screen the troops from a reverse, or an enfilading fire, &c.; and for powder magazines, when they are not placed in the traverses.  The area occupied by a traverse will depend on its dimensions, and cannot be fixed beforehand; that allowed for a magazine for three or four cannon may be estimated at fifteen or twenty square yards.

The traverse will be the focus of another post in time.  But for the moment, consider that Mahan basically instructed us to “bolt on” the traverse after building the fort.  Though he indicated a rough size for planning.

But wait… he said fifteen to twenty square yards for 3 to 4 guns.  Earlier he indicated we needed to factor in sixty square yards for the individual guns.  So for a four gun battery, we’d need 260 square yards.  A six gun battery (say holding back two guns for reserve), would need 380 to 400 square yards. And our math determining how many muskets and cannon were needed for a standard-size, by-the-book Mahan-approved fort was pointing to four of those batteries.  Better figure on at least 1500 square yards just to have space for the artillery… between a quarter and a third of an acre.  And that is just the artillery.  So if you have a fort that really needs to be defended properly, better plan on having a lot of space to dress out that fort.

Hold on… recall we have a preference for 250 yard exterior faces…. or a 62,500 yards square box in which to put the fort (including those areas that would lay outside the structures, mind you!).  Sounds like one would have ample space.  But when you subtract the area that lay outside the faces, flanks, and curtains; factor in the space that the works themselves will take up; and then figure on a regiment of infantry plus all those guns… well one has to work hard to avoid running out of space!  The engineer’s work was not simply drawing polygons on paper.

The last paragraph in this chapter provides the “soundbyte” for the philosophy of the defense:

As a field fort must rely entirely on its own strength, it should be constructed with such care that the enemy will be forced to abandon an attempt to storm it, and be obliged to resort to the method of regular approaches used in the attack of permanent works.  To effect this, all the ground around the fort, within the range of cannon, should offer no shelter to the enemy from its fire; the ditches should be flanked throughout; and the relief be so great as to preclude any attempt at scaling the work.

You see, if employed successfully by the defender, the fort would buy time for the defender.  That time would allow the defender to adjust to meet the attacker, or at least exact a toll upon him.

Apply this to one of the many episodes from the Civil War.  I think immediately of Morris Island.  After the Federals failed to carry Battery Wagner with a direct assault, they were “obliged” to lay a “regular approach” across a narrow beach in order to gain the Confederate works.  And that bought the Confederates a significant amount of time – in which they re-positioned forces (heavy cannon) out of Fort Sumter and to other locations in the harbor.  The Confederates could not stop the capture of Morris Island, but they could – and did – exact a toll for that prize.  Such was a “by the book” utilization of a field fortification… even if the fort itself were to fall.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 16-7.)

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