Last Friday, we considered the engineer’s tool for documenting a plan for fortifications – the profile.
This profile demonstrates Dennis Hart Mahan’s book definition of the parapet and ditch. It is the parapet that I want to focus upon today. So let us return to the words offered by Mahan to describe the nature of field fortifications. Let us first recall how Mahan described the functional requirement of those fortifications:
To enable troops to fight with advantage, the intrenchements should shelter them from the enemy’s fire; be an obstacle in themselves to the enemy’s progress; and afford the assailed the means of using their weapons with effect.
This is important. A timeless “truism” here – fortifications provide protection and thus offer advantage in battle. And Mahan elaborated to that point, noting how, in concept, the fortifications worked to provide that advantage… specifically how that part he defined as the parapet worked as part of those fortifications:
To satisfy these essential conditions, the component parts of every intrenchement should consist of a covering mass, or embankment, denominated the parapet, to intercept the enemy’s missiles, to enable the assailed to use their weapons with effect, and to present an obstacle to the enemy’s progress….
Again, we’ll return to the ditch in a post to follow. It’s the parapet we are interested in today. Three important components to the parapet under Mahan’s construct:
- Something that could stop musket and artillery fire.
- That protection should not be so constrictive to prevent the troops from using their weapons.
- The parapet itself would be an obstacle to enemy movement.
Looking back to Figure 1, Mahan noted the profile of an idea parapet as along the points A-B-C-D-E-F, highlighted here:
So far, none of this is stuff you didn’t figure out at a young age in the back yard playing soldier. But let’s consider that first bullet point in detail and put the “military science” behind it. How much parapet is needed to stop a musket ball or a cannon ball? Mahan answered:
The thickness of the parapet, which is always estimated by the horizontal distance between the interior and exterior crests, is regulated by the material used for the parapet; the kind of attack, and its probable duration.
The interior and exterior crests mentioned were points D and E on Figure 1, above. Mahan backed that instruction with a series of tables. First, the artillery from that time period against packed earth:
Please note the calibers cited. The weapons considered were all siege-caliber weapons. None of the field weapons commonly encountered in the Civil War… nor the rifled artillery. Still, if 6 ½ feet of packed earth could stop an 18-pdr at 110 yards (musket range), that was a fairly good planning factor.
For musketry against various materials:
Mahan noted that, “In order to insure perfect security, the thickness of parapets ought to be one-half greater than the depth of penetration furnished by this experiment.” From that, he derived what would be considered “musket proof” and proof against field pieces:
For practical purposes, Mahan desire 12 feet thick parapets when facing an enemy with field artillery. That sounds excessive, given the cited penetration of 18-pdr siege guns, even given Mahan’s “one-half greater” rule of thumb. But go back to the determining factors for thickness. The enemy would not fire one shot and then go home. So those parapets had to provide proof against multiple hits, particularly in close proximity along the wall. And note, Mahan did not define those 12-foot thick parapets as “bombproof.” That term denotes more improvement and resistance than a parapet could or should offer.
Lastly, Mahan offered comparisons of artillery against limestone (not brick) masonry:
And also against wood:
Mahan dismissed the use of shells against masonry as “trifling.” The cited experiments indicated the need for thicker pine than oak… by a factor of 75%.
What Mahan didn’t elaborate on, which would come into play at many times during the Civil War was the use of composite parapet structures. For instance, the reinforcement of old masonry with wood and earth, as done at Fort Moultrie. But we can draw some useful measures here – 6½ feet of earth was roughly equal in stopping power to 1 foot, 8 inches of limestone masonry. Three feet of oak was equal to the protection provided by 5½ feet of earth.
So before saying “I’ll build with stone masonry or wood” factor in the resource cost for those materials. Stone must be quarried, transported, shaped, and laid. Wood must be cut, sawed, transported, and fixed. But earth is most readily available anywhere, and may be moved with a strong back and a shovel. At that point, we must discuss how much earth can be moved over the time permitted within the tactical situation, and thus govern any expectation for elaborate, lengthy works.
All to easy to say “they threw up some works” in the history books… much harder to actually construct a parapet to meet those needs.
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 1-2, 4-8.)