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:


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.)