Fortification Friday: Carefully place and construct Powder Magazines!

So you’ve built your works and improved it with a nice set of batteries.  Great!  Are those walls and obstacles all that is needed to scare away an attacker?  Probably not.  At some point, the defenders will need to do more than sit being the parapet.  They will need to do some shooting.  And shooting requires, among other things, gunpowder and projectiles.  Lots of gunpowder and projectiles.  But those are things one does not just have laying about in the open.  Not to mention the danger of explosions, gunpowder tends to deteriorate if not properly stored and maintained.  Thus the need for powder magazines.

Mahan registered the requirements of such powder magazines in his treatise:

Powder magazines. The main objects to be attended to in a powder magazine are, to place it in the position least exposed to the enemy’s fire; to make it shot proof; and to secure the powder from moisture.

Point of order here.  Mahan singled out powder magazines specifically as places where ammunition was kept. Defenders might build various protective structures for other uses, but the powder magazine’s arrangements were to directly address the needs of storing ammunition. Point to remember later when we look at other types of internal structures.

Also note the use of the word “shotproof” here.  Specifically that requirement is to prevent solid shot from battering the structure.  Bombproof would, of course, involve resisting enemy shells.  But from the text, it is not clear that Mahan made a distinction here… just food for thought.

Don’t know that I’d rank these three requirements, as all are important.  But I’d offer that the professor gave us his preferences in reverse order!  That is if he had such rankings.  Consider the next paragraph:

If there are traverses, such for example, as are used in defilement, the magazines may be made in them; or they may be placed at the foot of a barbette; or, in dry soils, be made partly under ground.

Egad!  A traverse, as we learned, is a structure designed to sit in the way of the enemy’s anticipated line of fire… so as to intercept those fires.  So much for “least exposed”….

But let us focus on the practical aspects of the magazines:

The magazines should be at least six feet high, and about the same width within; its length will depend on the quantity of ammunition. It may be constructed of facines, gabions, or cofferwork, or any means found at hand may be used which will effect the end in view.

I’ve not seen any justification for the six foot dimensions.  Perhaps just the average height of the men servicing the ammunition.  Hey, you need to save that back for throwing back the enemy’s assaulting troops!  And we see mention here of some revetment types in order to strengthen the magazine beyond that of plain soil.  But cofferwork is a new phrase, implying a more complex magazine arrangement.  Let us hold off details of that and focus on the basic work.

If [fascines] are used, the sides should slope outwards to resist the pressure of the earth; the fascines should be firmly secured by pickets and anchoring withes.  The top may be formed by a row of joists, of six-inch scantling, placed about two and-a-half feet apart; these should be covered by two layers of fascines laid side by side, and the whole be covered in by at least three feet thickness of earth.

Figure 34 illustrates these arrangements:

PlateVFig34

The figure shows a magazine buried at all sides.  So assume the placement is correct and sufficient earth is employed to make the structure shotproof as required.  Thus we focus on the internal arrangements.  As required, the fascines are secured and anchored.  Notice these are slanted (“sloped outward”) as necessary for support.  The floor is six feet wide.  Six feet above that is an eight feet wide ceiling, constructed with six-inch wide beams (scantling).  Those support two layers of fascines, laid in opposite orders.  And atop that, another three feet of earth.   Shotproof!

But let us look at details below the floor:

The bottom should be covered by a flooring of joists and boards; a shallow ditch being left under the flooring, with a pitch towards the door of the magazine, to allow any water that might leak through to be taken out.  A thatch of straw might be used on the inside, but it is somewhat dangerous, owing to its combustibility; hides or tarpaulins are better, and will keep out the moisture more effectually.

Thus, we see all three requirements addressed in this basic magazine. Nice notes here as to drainage.

Mahan was concerned mostly with construction of the magazine.  He did not address directly maintenance needed, which was of just as much importance.  Beyond just keeping earth on the magazine and the internal structure strengthened, the magazine need be tidy and organized.  Not only to reduce risks of accidental explosions, but also so that retrieval of ammunition was quick and easy.

And speaking to accidents, a good engineer would confront such risks.  To some degree the slope of the magazine wall would focus the force of an explosion upwards and out. The sides of the magazine should be thicker, or at least more resistant, than the roof, so as to allow the venting of such force.  But those were just mitigations against the risk.  The first line of defense against such risk was proper handling and maintenance of the ammunition.

With the basics of the magazine established, let us turn next to more elaborate arrangements.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 58-9.)

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