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This Palmerston fort built to defend Plymouth against
attack from the north was built between 1863 and 1872 in response to the
threat from Napoleon III of France. It is now in the care of the Landmark
Trust.
The seven sided "Polygonal" design incorporated some the the most advanced ideas in Victorian fort design with a dry ditch defended by caponiers that would rake the ditch with rifle fire and case shot. At the top of the inner ditch wall is the 'Chemin de Ronde', a parapeted walkway below the main rampart which circled the fort and was open to the east, south, and western sides of the fort but mostly covered to the northern side. Around 350 rifle loopholes are built into the Chemin de Ronde and the caponiers. Above this the steeply banked earthwork rampart protects the terreplein on which the forts main ordenance was stationed in a variety of enclosures with the heaviest firepower facing north. A northern section of the Chemin de Ronde - fully covered and casemented construction, note the rifle loopholes. |
| The defensive ditch is hewn from solid rock and required
the removal of some 200,000 tones of material. It measures about 30ft deep
and over 30ft wide and is protected by caponiers jutting out from the apexes
of the fort perpendicular to the inner ditch walls. A clear field of fire
was thus achieved along each stretch of ditch for both the riflemen and
artillery firing case shot to decimate any enemy attempting to cross the
ditch.
A interesting feature of this fort is the countermining gallery, a complex of tunnels running north from the counterscarp in which sentries would listen for the sounds enemy sappers digging to place explosives under the defences. Left - A typical view from a gun embrasure
looking along the ditch
Right - A rifle loophole
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Left - The lower rifle level of the northern caponier (note the steel joists supporting the gun floor) Right - An example of a double casement mounting two 32 pdr SBBL guns. |
The six caponiers are three storey structures, the first
floor at ditch level was for infantrymen with groups of three loopholes
in each casement. Above that supported on steel joists are the wooden floors
of the gun casements each housing a 32 pdr Smooth Bore Breech Loading (SBBL)
gun on a traversing carriage. The third storey connects with the Chemin
de Ronde and provides high angle fire into the ditch.
Flanking chambers built into the inner wall of the ditch provided covering fire across the face of each caponier to prevent the enemy laying charges to destroy them. These flanking galleries also contained a sally port in the middle level to allow the defenders access to the ditch. The northen caponier is double sided protecting the ditch on two sides and the countermining gallery on the counterscarp. This is a full caponier while the other five are demi-caponiers. |
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The earth ramparts protect the fighting platform
or terreplein on which various types of guns and emplacements are
sited. These guns were a mix of 64 pdr rifled muzzel loaders (RML),
7 inch Armstrong rifled breach loaders (RBL) and 6.6 inch rifled howitzers.
The 64 pdr guns were Palliser conversions from smooth bore weapons which involved the insertion of a rifled wrought iron tube into the existing bore. The 7 inch Armstrong RBL guns were housed in 'barbette' style emplacements or Haxo casements (see below) on various traversing carriages depending upon the circumstances. Howitzers because of their high angle of fire seem to have been mounted further back on the terreplein, the 6.6 inch rifled howitzer could throw a 100lb shell 5,400 yards at 35 degrees with just 5lb of powder. A 64 pdr RML gun on a 'C' pivot carriage in its barbette
emplacement
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| Haxo casements named after the German general
who proposed them were designed to provide the gun and crew with protection
from high angle fire. The downside to this added protection was the restricted
angle of fire although a mixture of Haxo and Barbette style emplacements,
as at Crownhill, could provide a compromise between protection and firepower.
A 7 inch Armstrong RBL housed in the
double Haxo casement
on the east face of the fort.
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Captain Colin Scott Moncrieff drew upon his
experiences in the Crimean war to improve upon a number of existing ideas
and designs for a gun carriage that would rise up over the parapet to fire
and drop down below for loading and aiming.
His method of using a counterweight to raise the gun and control the decent powered by the recoil, whilst storing that energy to raise the gun again was revolutionary but the idea was not well received by the Ordnance Select Committee. Interestingly he promoted his 'Protected Barbette System' as an inexpensive means of providing defence to a site, as the field works required were quite limited but the protection afforded was considerable. The Moncrieff pit as this type of emplacement became known could literally be a hole in the ground meaning that emplacements could be quickly constructed to suprise the enemy. A reconstruction of the Moncrieff Disappearing Carriage mounting a 7 inch RBL gun. It is important to note that the gun itself was standard ordnance at the time and only the carriage was unusual. |
| Links
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| Sources
Plymouth Papers No.1 Crownhill Fort - David Moore, Pub. David Moore ISBN 0 9513234 4 X The Moncrieff Story - David Moore, Pub. The Palmerston Fort Society ISBN 0 9523 634 1 0 |
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