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FNG Fortified Line - Plate VII — Description
This is part of a series of nine drawings made for the non-fiction book "The Fortified Region Focsani-Namoloasa-Galati" published by the Romanian Military Museum about the eponymous Fortified Line (also known in English as the "Sereth Line" and by it's abbreviation "FNG") built by Romania for defence against a presumptive Russian invasion during 1888-90.

This is the eight drawing, showing the main artillery installations for the fortified position, the mixed 12cm battery of one gun in an armoured cupola and two spherical mortars.

Unlike the Fahrpanzer mobile turrets that were deployed in the first line and the disappearing cupolas which were located in the second line, the third line was dedicated to long range artillery support, and consisted of a mixed gun 12cm battery.

I've drawn the standard battery of the Focsani position, a design that was changed for the Namoloasa and Galati positions, which instead replaced it with a battery of two 12cm Gruson QF howitzers under cupolas, a much simpler and more flexible arrangement.

The armament of the third line position was an 12cm Krupp L/25 gun Md.1887 in a disappearing cupola and two 12cm Gruson "spherical mortars", so called because they were fitted with a spherical armoured shield that protruded through a horizontal armor plate and protected the crew no matter which way the mortar was pointing.

The 12cm disappearing cupola was simply a larger version of the 5.3cm disappearing one and made with the same methods and materials - the cupola proper was 12cm thick made out of laminated steel and recessed into a cast iron "voussoir" made out of segments placed in a steel circular rail and secured by concrete.
The top of the cupola also had a hole through which the gunner could pop his head out and look around , or even aim his guns directly through the use of a sliding rear sight and a fixed front sight mounted on the cupola (note: visible in the background are the second line positions for the 5.3cm disappearing cupolas ).

The cupola shaft was made out of 3mm steel plate embedded into concrete and formed two levels - the upper one for the loading and firing of the gun and turret traverse, while the lower one for the counterweight mechanism which, in the drawing, is rotated counter-clockwise in the horizontal plane by 100°, in order to bring it into view - normally it would have been located just behind the door on the right as you entered the barbette from the ventilator room.

The lower level was also protected by additional semicircular cast iron segments 12cm thick, also embedded in the concrete. On both levels the walls held shelves for either shells or powder, 76 in total, which meant that the maximum ammunition capacity was 1204 rounds.

The two spherical mortars were arranged just behind the cupola shaft and were protected by a 12cm thick plate of rolled armour secured on both sides into concrete. The walls and the door were also made out of 3mm plate and the former held 46 shelves for 576 rounds of ammunition. The floor of the mortar emplacement also held two recesses in which a spare 12cm gun and various accessories could be found.

Since firing the guns produced an ungodly amount of powder smoke, the emplacement was ventilated using a manual Roots centrifugal ventilator serviced by four men an located on the lower lever underneath the mortars. This was supposed to suck in fresh air from outside through an opening located under the stair and pump it, through cast iron pipes, into the fighting compartments, forcing the existing stale air along with the smoke out through holes in the cupola.

Needless to say, when tested in the first world war, this arrangement of manual ventilators, although popular, was found to be completely inadequate, and, indeed, some of the forts of Liege surrendered after their defenders almost suffocated inside the forts on a mixture of firing gasses and dust from bombardment.

Also, please note that none of the fortified positions had indoor sanitation facilities - in fact the IInd and IIIrd lines were provided with a single pit latrine located in the middle of the IInd line position just in front of the 12cm battery and accessible by walking over ground, a risky proposition if the position was under attack...
The soldiers in the Fahrpanzers had to make do without, I'm afraid.

When it comes to construction techniques, the emplacement is made out of the same poor quality concrete with a mix ratio of 1:2:4 by volume (one measure of Portland cement, two of sand and four of aggregate) while the voussoir and armoured plate over the mortars are secured in place with pourings of the theoretically tougher 1:1:2 mix ratio concrete. In practice however, both formulations proved to be pretty weak and crumbled easily, and the main mass of concrete formed into layers as soon as poured due to differences between the concrete batches, something which further weakened the structure.
Since unreinforced concrete is also notoriously weak under tension, the ceilings of the lower levels are vaults made out of "special" fortification brick.

The other drawing in this series can be found here:

Cover, showing cutaway through 5.3cm mobile gun turret
Plate I, the 3.7cm mobile gun turret and transport carriage.
Plate II, the 3.7cm gun and ammunition.

Plate III, the 5.3cm mobile gun turret in its concrete emplacement .
Plate IV, the 5.3cm mobile gun turret in transport configuration.

Plate V, the 5.3cm disappearing cupola.

Plate VI, the 5.3cm gun and ammunition.

Plate VII, the 12cm gun and mortar armoured battery.

Plate VIII, the 12cm gun and mortar and their ammunition.